Aira caldera, located in the northern half of Kagoshima Bay, contains the active post-caldera Sakurajima volcano near the southern tip of Japan’s Kyushu Island. Eruptions date back to the 8th century and have deposited ash on Kagoshima, one of Kyushu’s largest cities, 10 km W from the summit. The Minamidake summit cone and crater has had persistent activity since 1955; the Showa crater on the E flank has also been intermittently active since 2006. The current eruption period began during late March 2017 and has more recently consisted of explosions, ash plumes, and ashfall (BGVN 48:01). This report covers activity during January through June 2023, characterized by intermittent explosions, eruption events, eruption plumes, and ashfall from both summit craters, according to monthly activity reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity remained at low levels during this reporting period; less than ten thermal anomalies were detected each month by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system (figure 139). Occasional thermal anomalies were visible in infrared satellite images mainly at the Minamidake crater (Vent A is located to the left and Vent B is located to the right) and during May, in the Showa crater on the E flank (figure 140).

Table 29. Number of monthly explosive events, days of ashfall, area of ash covered, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during January-June 2023. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory, and ashfall amounts represent material covering all the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 139. Thermal activity at Sakurajima in the Aira caldera was relatively low during January through June 2023, according to this MIROVA graph (Log Radiative Power). Three anomalies were detected during January, six during February, seven during March, nine during April, six during May, and none during June. Courtesy of MIROVA.

Figure 140. Infrared (bands 12, 11, 8A) satellite images showed occasional thermal anomalies mainly at the Minamidake crater at Aira’s Sakurajima volcano on 1 January 2023 (top left), 20 February 2023 (top right), 1 May 2023 (bottom left), and 16 May 2023 (bottom right). Vent A is located to the left and Vent B is to the right of Vent A; both vents are part of the Minamidake crater. On 16 May the image showed a weak anomaly in the Showa crater to the E of the Minamidake crater. Courtesy of Copernicus Browser.

JMA reported that during January 2023, there were 14 eruptions, nine of which were explosion events. Accompanying eruption plumes rose 2.4 km above the crater rim. Large blocks were ejected 800-1,100 m from the Minamidake crater. Nighttime incandescence was observed in the Minamidake crater using a high-sensitivity surveillance camera. No eruptions in the Showa crater were reported, though there was a gradual increase in the amount of white gas-and-steam emissions beginning around mid-January. Seismicity consisted of 121 volcanic earthquakes, which was higher than the 78 earthquakes in December. The Kagoshima Local Meteorological Observatory reported a total of 2 g/m² of ashfall was observed over the course of two days of the month. According to field surveys, daily sulfur dioxide emissions ranged from 1,000-2,800 tons/day (t/d); emissions have remained at comparable, elevated, levels since July 2022. Explosions were reported on 3 January at 1615, 8 January at 0642 and 1955, 18 January at 1215, 19 January at 0659, 21 January at 0307, and 28 January at 2342 where eruption plumes rose 1-2.4 km above the Minamidake crater and drifted SE and S. The explosion at 0307 on 21 January generated an eruption plume 1.6 km above the crater rim and ejected large blocks 800-1,100 m from the crater rim; crater incandescence was also visible (figure 141). On 28 January at 2342 an explosion produced an eruption plume that rose 2-2.2 km above the Minamidake summit crater and drifted SE.

Figure 141. Webcam image of the explosion at the Minamidake summit crater of Aira’s Sakurajima at 0307 on 21 January 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, January 2023).

There were 26 eruptions reported during February, 11 of which were explosion events. Eruption plumes rose 2.4 km above the crater rim. Large blocks were ejected 800-1,100 m from the Minamidake summit crater, and daily nighttime crater incandescence continued. Occasional eruptive activity was observed in the Showa crater starting on 8 February, which included four eruptions (figure 142). The last time activity was reported in the Showa crater was early April 2018, according to JMA. There were 130 volcanic earthquakes detected during the month. Sulfur dioxide emissions ranged from 1,900-3,500 t/d. On 8 February large blocks were ejected 300-500 m from the Showa crater and an accompanying eruption plume rose 1.5 km above the crater rim. Summit crater incandescence was also visible at night during 8 and 21-26 February at the Showa crater. Weak crater incandescence was also reported on 8 February at the Minamidake summit crater. Explosions were recorded at 1815 on 9 February, at 1007 on 11 February, at 1448 on 14 February, at 0851 on 16 February, at 0206 on 19 February, at 2025 on 20 February, at 0937 and at 1322 on 21 February, and at 0558 on 28 February. Volcanic plumes rose 300-2,000 m above the Minamidake crater and drifted N, E, S, SE, and NE. An explosion at 1448 on 14 February at the Minamidake summit crater ejected large blocks 800-1,100 m from the crater. The eruption plume rose 800-1,200 m above the crater and drifted S. A field survey conducted on 14 February showed that the ejected volcanic clasts measured up to 3 cm in diameter, though most were smaller in size, and were deposited in Arimura, Kagoshima City (3 km SE) (figure 143). An aerial survey conducted by the Japan Maritime Self-Defense Force Air Group (JMSDF) on 21 February confirmed white gas-and-steam plumes rising from the N side of the Showa crater and water was visible at the bottom of the crater. Ashfall measurements showed that a total of 6 g/m² fell over seven days during the month at the Kagoshima Local Metrological Observatory.

Figure 142. Webcam images showing the initial white gas-and-steam plume rising above the Showa summit crater of Aira’s Sakurajima at 0701 on 12 January 2023, at 0701 on 18 January (top left and right), and at 0708 on 5 February 2023 (bottom left). The amount of white gas-and-steam emissions gradually increased from mid-January leading up to the eruption at 1052 on 8 February 2023 (bottom right). Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, February 2023).

Figure 143. Photo showing the size of the deposits found in Arimura, Kagoshima City, after an eruption on 14 February 2023 at the Minamidake summit crater of Aira’s Sakurajima. The maximum diameter of these clasts was about 3 cm. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, February 2023).

During March, 22 eruptions were reported, eight of which were explosion events. Volcanic plumes rose 2.8 km above the crater rim. There were four eruptions recorded at the Showa crater, for a total of eight eruptions during February and March. Large volcanic blocks were ejected 1,000-1,300 m from the Minamidake crater and nighttime incandescence remained visible at night, based on webcam images. Blocks ejected from the Showa crater traveled 500-800 m and accompanying eruption plumes rose 2.7 km above the crater rim. Nighttime crater incandescence was reported during 4-5 March at the Showa crater, based on webcam images. Seismicity included 97 volcanic earthquakes detected throughout the month. According to the Kagoshima Local Meteorological Observatory, a total of 9 g/m² ashfall was observed over six days of the month. A field survey reported that 2,100-3,500 t/d of sulfur dioxide was released during the month. An eruption was detected at the Showa crater at 1404 on 6 March, that ejected blocks 500-800 m from the crater, accompanied by an eruption plume that rose 2.7 km above the crater rim (figure 144). Explosions were detected at 0116 on 3 March, at 2157 on 4 March, at 1322 on 8 March, at 2228 on 11 March, at 0418 on 14 March, and at 0035 on 22 March. Eruption plumes rose 1-2.8 km above the Minamidake crater and drifted SE, NE, NW, S, and SW. At 0035 on 22 March an explosion generated an eruption plume that rose 1.2 km above the Minamidake crater and drifted SW. Material was ejected 1-1.3 km from the Minamidake crater.

Figure 144. Webcam image of an eruption plume rising 2.7 km above the Showa crater rim of Aira’s Sakurajima at 1412 on 6 March 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, March 2023).

Two eruption events were reported in the Minamidake summit crater during April, neither of which were explosions; no eruptions occurred at the Showa crater. Eruption plumes rose 1.5 km above the crater rim and nighttime crater incandescence persisted nightly at the Minamidake crater. The number of volcanic earthquakes deceased to 38 and according to the Kagoshima Local Meteorological Observatory, a total of 3 g/m² of ash fell over a period of four days during the month. The amount of sulfur dioxide released during the month ranged 1,800-2,700 t/d. An eruption event at 0955 on 17 April generated an eruption plume that rose 1.5 km above the crater rim (figure 145).

Figure 145. Webcam image of an eruption plume rising 1.5 km above the Minamidake crater rim of Aira’s Sakurajima at 1004 on 17 April 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, April 2023).

Eruptive activity during May consisted of 17 eruptions, 10 of which were explosion events. Volcanic plumes rose 2.3 km above the crater rim and large ejecta traveled 800-1,100 m from the Minamidake summit crater. Activity at the Showa crater was characterized by 11 eruption events and material was ejected 300-500 m from the crater. Nighttime crater incandescence was observed at both summit craters. The number of monthly volcanic earthquakes increased to 88 and the amount of ashfall recorded was 10 g/m² over a period of 13 days during the month. According to a field survey, the amount of sulfur dioxide released ranged 1,800-3,900 t/d.

Explosions were recorded at 0422 on 2 May, at 0241 and at 1025 on 3 May, at 1315 on 9 May, at 2027 on 17 May, at 0610 on 24 May, at 1327 on 25 May, at 0647 and 1441 on 26 May, and at 1520 on 28 May. Resulting eruption plumes rose 400-1,800 m above the Minamidake crater and drifted SW, W, and N. On 14 May an eruption plume was visible above the Showa crater at 0859 that rose 1.7 km above the crater rim (figure 146). An eruption event at the Minamidake summit crater occurred at 1327 on 25 May; the eruption plume rose 2.3 km above the crater rim (figure 147).

Figure 146. Webcam image showing an eruption plume rising 1.7 km above the Showa crater rim of Aira’s Sakurajima at 0903 on 14 May 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, May 2023).

Figure 147. Webcam image showing an eruption plume rising 2.3 km above the Minamidake crater rim of Aira’s Sakurajima at 1331 on 25 May 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, May 2023).

JMA reported four eruptions occurred during June, two of which were explosion events. Eruption plumes rose as high as 2.5 km above the Minamidake crater rim and large volcanic blocks were ejected 500-700 m from the crater rim. At the Showa crater, seven eruptions occurred, one of which was an explosion event. Eruption plumes rose 1.5 km above the Showa crater rim and large material was ejected 500 m from the crater rim. Nighttime incandescence was reported for both summit craters. There were 73 volcanic earthquakes detected during the month and a total of 3 g/m² of ashfall during eight days of the month. According to a field survey, the amount of sulfur dioxide emissions released ranged 1,400-1,900 t/d. On 5 June at 0012 an explosion generated an eruption plume that rose 400-1,000 m above the Minamidake crater and drifted SE. An explosion at the Minamidake crater occurred at 1401 on 7 June that generated an eruption plume that rose 2.5 km above the crater and drifted SE (figure 148). A single explosion was reported at the Showa crater at 0438 on 22 June. The eruption plume rose 600 m above the crater rim and large blocks were ejected 500 m from the crater rim. This is the first report of an explosion at the Showa crater since October 2017, according to JMA.

Figure 148. Webcam image of an explosion and the accompanying plume that rose 2.5 km above the Minamidake crater rim of Aira’s Sakurajima at 1410 on 7 June 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, June 2023).

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Suwanosejima is located in the northern Ryukyu Islands, Japan, and is an 8-km-long island that consists of a stratovolcano and two active summit craters. Volcanism during the 20th century is characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has more recently consisted of intermittent explosions, eruption plumes, ashfall, and incandescent ejecta (BGVN 48:01). Similar activity continued during this reporting period of January through June 2023, based on monthly report from the Japan Meteorological Agency (JMA) and satellite data.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph of the MODIS thermal anomaly data showed low thermal activity throughout the reporting period (figure 76). Three anomalies were detected during February, four during March, three during April, one during late May, and two during early June. A single thermal hotspot was detected by the MODVOLC thermal alerts system on the NE flank on 7 February. There were only two clear weather days in infrared satellite imagery that showed a thermal anomaly on 7 March and 5 June (figure 77).

Figure 76. Low thermal activity was detected at Suwanosejima during January through June 2023, based on this MIROVA graph (Log Radiative Power). Three anomalies were detected during February, four during March, three during April, one during late May, and two during early June. Courtesy of MIROVA.

Figure 77. Infrared (bands B12, B11, B4) satellite imagery showing two thermal anomalies at the Otake crater of Suwanosejima on 7 March 2023 (left) and 5 June 2023 (right). Courtesy of Copernicus Browser.

Activity in the Otake crater during January 2023 was relatively low, which prompted JMA to lower the Volcano Alert Level (VAL) from 3 to 2 (on a 5-level scale) on 24 January. The number of explosions recorded during the month was 13. There were 50 volcanic earthquakes detected on the W side of the island, which was roughly comparable to December (44), although near the Otake crater, there were 188 earthquakes recorded, which excluded earthquakes associated with explosions. An aerial overflight conducted on 11 January by the Japan Maritime Self-Defense Force Air Group (JMSDF) reported a gray-white plume rising from the Otake crater. During 26-30 January there was a brief increase in the number of explosions. An eruption at 0331 on 26 January generated an eruption plume that rose 1.7 km above the crater rim and ejected large blocks 400 m S from the crater. Nighttime crater incandescence was visible in a highly sensitive surveillance camera starting on 26 January. According to the Toshima Village Office, Suwanosejima Branch Office, ashfall was occasionally observed in the village (3.5 km SSW). According to observations conducted by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Toshima Village, and JMA, the amount of sulfur dioxide emissions released during the month was 200-600 tons per day (t/d).

Eruptive activity in the Otake crater continued during February; the total number of explosions increased during this month from 13 to 56. There were 119 volcanic earthquakes detected on the W side of the island and 449 near the Otake crater, excluding earthquakes associated with explosions. During 15-21 February there was a brief increase in the number of explosions, and large blocks were ejected as far as 1 km from the crater. An explosion at 2131 on 15 March ejected material 900 m SE (figure 78). Eruptions on 18 and 27 February generated plumes that rose 2 km above the crater (figure 79). By 21 February the number of explosions reached 42, though no large-scale volcanic earthquakes were reported. Nighttime crater incandescence continued from late January through February. Ashfall was also occasionally observed in Toshima Village. The amount of sulfur dioxide emissions released during the month was 700 t/d.

Figure 78. Webcam image of the explosion at Suwanosejima’s Otake crater at 2131 on 15 February 2023. Crater incandescence was visible, and large blocks were ejected 900 m from the crater (white dashed line). Courtesy of JMA (Volcanic activity commentary for Suwanosejima, February 2023).

Figure 79. Webcam image of the explosion at Suwanosejima’s Otake crater at 1606 on 18 February 2023. The eruption plume rose 2 km above the crater rim. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, February 2023).

The number of explosions at the Otake crater increased during 2-5 March; 28 explosions were detected during this time. Large volcanic blocks were ejected 500 m from the crater. As a result, the VAL was increased to 3 on 5 March. There were 65 explosions recorded throughout the month. On the W side of the island, 63 volcanic earthquakes were reported, and closer to the Otake crater, 422 were detected, excluding earthquakes associated with explosions. Nighttime crater incandescence continued, as well as occasional ashfall in Toshima Village. On 16 March an eruption produced a volcanic plume that rose 2.4 km above the crater rim (figure 80). The amount of sulfur dioxide emissions released during the month was 200-1,100 t/d.

Figure 80. Webcam image of an eruption plume rising 2.4 km above the Otake crater at Suwanosejima at 0644 on 16 March 2023. Photo has been color corrected. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, March 2023).

Eruptive activity continued at the Otake crater during April. Eruption plumes rose as high as 2 km above the crater rim and large blocks were ejected as far as 500 m from the crater. The number of explosions decreased to one throughout the month, although nighttime crater incandescence remained visible in the surveillance camera. Rumbling and ashfall continued intermittently in Toshima Village. There were 32 volcanic earthquakes detected, and 129 volcanic earthquakes near the Otake crater, not including those associated with explosions. According to JMA, the amount of sulfur dioxide released during the month was 200-1,400 t/d. On 16 April at 0402 an eruption ejected incandescent material 500 m S from the crater.

Activity continued at the Otake crater in May. An eruption plume rose 1.8 km above the crater rim and large volcanic blocks were ejected 300 m from the crater. The number of explosions remained low throughout the month (7) and nighttime crater incandescence persisted. Occasional ashfall was reported in Toshima Village. As many as 44 volcanic earthquakes were recorded on the W side of the island, and 205 were recorded closer to the Otake crater, which was higher compared to the previous month. Generally, the amount of sulfur dioxide released during the month ranged 400-700 t/d, but on 19 May the amount increased to 2,600 t/d. On 16 May an eruption produced a volcanic plume that rose 1.8 km above the crater rim.

Eruptive activity was relatively low in June; the number of explosions generally decreased and on 9 June the VAL was lowered to 2. Nighttime crater incandescence continued, and according to the Toshima Village Office, rumbling and ashfall were also noted occasionally. There were 31 explosions throughout the month and 28 volcanic earthquakes detected on the W side of the island and as many as 722 volcanic earthquakes were recorded near the Otake crater. During 13-19 June, JMA reported a brief increase in the number of explosions. On 15 June at 2200 an eruption generated a volcanic plume that rose 2 km above the crater rim. An eruption on 16 June at 2147 ejected material 400 m SE from the crater. The amount of sulfur dioxide emitted was relatively low, at 100 t/d on 27 June.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Semeru contains the active Jonggring-Seloko vent at the Mahameru summit and is located in East Java, Indonesia. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano. The current eruption began in June 2017 and more recently has been characterized by intermittent gas-and-ash plumes and incandescent avalanches (BGVN 48:01). This report updates activity such as ash plumes, incandescent avalanches, and pyroclastic flows from January through June 2023, based on information from daily, VONA, and special reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, and various satellite data.

Activity during January and February mainly consisted of frequent ash plumes and white-and-gray emissions. The ash plumes during January rose 200-1,000 m above the crater and drifted in different directions. The white-and-gray emissions rose 200-1,000 m above the crater. A photo was posted on social media that showed an incandescent lava flow extending 500 m from the summit crater on the SE flank at 0027 on 8 January (figure 83). Video posted to social media on 5 February showed a pyroclastic flow descending the SE flank and ash plumes rising along the path and drifting N. Ash plumes rose 1 km above the crater at 0802 on 13 January, at 0536 on 17 January, at 0628 on 19 January and drifted SW, W, and SE, respectively. White, gray, and brown emissions were reported on 15 and 17 January that rose 300-1,000 m above the crater. During February, ash plumes rose 200-1,500 m above the crater and drifted mainly N and NE. White-and-gray emissions rose 100-1,000 m above the crater.

Figure 83. Photo showing an incandescent lava flow descending 500 m on the SE flank of Semeru at 0027 on 8 January 2023. Photo has been color corrected. Courtesy of Info Semeru.

Similar activity consisting of frequent ash plumes and gas-and-steam emissions continued through March and April. During March, ash plumes rose 300-1,200 m above the crater and drifted in multiple directions. On 25 March at 0738 an ash plume rose 1.2 km above the crater and drifted SE. Occasional white-and-gray emissions rose 50-1,000 m above the crater. Ash plumes in April rose 400-1,200 m above the crater and drifted in different directions. An ash plume on 3 April rose 1.2 km above the crater and drifted SE and S at 0538. On 8 April a photo and videos were posted on social media showing a pyroclastic flow moving 1.5 km down the SE flank, accompanied by an ash plume (figure 84). New material was deposited along the crater, according to a local news source. Another pyroclastic flow occurred at 0710 on 18 April that descended up to 2 km from the crater to the SE (figure 85). White-and-gray emissions rose 100-800 m above the crater during April.

Figure 84. Photo showing a pyroclastic flow descending the SE flank of Semeru on 8 April 2023. Courtesy of Info Semeru.

Figure 85. Photo showing a pyroclastic flow descending 2 km on the SE flank of Semeru on 18 April 2023. Photo has been color corrected. Courtesy of Info Semeru.

Ash plumes and white-and-gray emissions persisted during May and June. During May, ash plumes rose 300-1,200 m above the crater and drifted generally N and S. On 13 May around 1012 a pyroclastic flow was observed moving 1.5 km down the SE flank, accompanied by an ash plume (figure 86). On 27 May an ash plume rose 1.2 km above the crater and drifted S and SW at 0819. White-and-gray emissions rose 100-800 m above the crater. Ash plumes during June rose 200-1,500 m above the crater and generally drifted N and SW. A webcam image showed incandescent material at the summit and on the flanks at 0143 on 23 June that traveled 3.5 km. According to a local news source, a pyroclastic flow traveled 5 km down the SE flank at 1910 on 26 June; the accompanying an ash plume rose as high as 1.5 km above the crater and drifted NE and E. Dominantly white gas-and-steam emissions rose 50-300 m above the crater.

Figure 86. Photo of a pyroclastic flow descending the SE flank of Semeru as far as 1.5 km at 1012 on 13 May 2023. Photo has been color corrected. Courtesy of Info Semeru.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent and moderate-power thermal anomalies during January through June 2023 (figure 87). There was a short gap in activity during late January through late February, followed by low-power and less frequent anomalies through April. During mid-May, there was an increase in both power and frequency of the anomalies. A total of 73 thermal hotspots were detected, based on data from the MODVOLC thermal algorithm. There were 10 detected in January, four in March, two in April, 17 in May, and 40 in June. Infrared satellite images showed persistent thermal activity at the summit crater during the reporting period; strong incandescent avalanches of material were occasionally captured in these images and affected the SE flank (figure 88).

Figure 87. Frequent, moderate-power thermal anomalies were detected at Semeru during January through June 2023, according to this MIROVA graph (Log Radiative Power). There was a short gap in activity during late January through late February, and lower-power anomalies were registered during late February through April; during mid-May there was an increase in both power and frequency of the anomalies. Courtesy of MIROVA.

Figure 88. Infrared (bands B12, B11, B4) satellite images showed strong thermal activity at Semeru on 10 January 2023 (top left), 19 February 2023 (top right), 11 March 2023 (middle left), 20 April 2023 (middle right), 30 May 2023 (bottom left), and 14 June 2023 (bottom right). Incandescent material mainly affected the SE flank from the summit crater, as shown in each of these images. Courtesy of Copernicus Browser.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Info Semeru (Twitter: @info_semeru, https://twitter.com/info_semeru).

Manam is a 10-km-wide island that consists of two active summit craters: the Main summit crater and the South summit crater and is located 13 km off the northern coast of mainland Papua New Guinea. Frequent mild-to-moderate eruptions have been recorded since 1616. The current eruption period began during June 2014 and has more recently been characterized by intermittent ash plumes and thermal activity (BGVN 47:11). This report updates activity that occurred from November 2022 through May 2023 based on information from the Darwin Volcanic Ash Advisory Center (VAAC) and various satellite data.

Ash plumes were reported during November and December 2022 by the Darwin VAAC. On 7 November an ash plume rose to 2.1 km altitude and drifted NE based on satellite images and weather models. On 14 November an ash plume rose to 2.1 km altitude and drifted W based on RVO webcam images. On 20 November ash plumes rose to 1.8 km altitude and drifted NW. On 26 December an ash plume rose to 3 km altitude and drifted S and SSE.

Intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which exceeded at least two Dobson Units (DU) and drifted in different directions (figure 93). Occasional low-to-moderate power thermal anomalies were recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system; less than five anomalies were recorded each month during November 2022 through May 2023 (figure 94). Two thermal hotspots were detected by the MODVOLC thermal alerts system on 10 December 2022. On clear weather days, thermal activity was also captured in infrared satellite imagery in both the Main and South summit craters, accompanied by gas-and-steam emissions (figure 95).

Figure 93. Distinct sulfur dioxide plumes were captured, rising from Manam based on data from the TROPOMI instrument on the Sentinel-5P satellite on 16 November 2022 (top left), 6 December 2022 (top right), 14 January 2023 (bottom left), and 23 March 2023 (bottom right). Plumes generally drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 94. Occasional low-to-moderate power thermal anomalies were detected at Manam during November 2022 through May 2023, as shown in this MIROVA graph (Log Radiative Power). Only three anomalies were detected during late November, one in early December, two during January 2023, one in late March, four during April, and one during late May. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite images show a consistent thermal anomaly (bright yellow-orange) in both the Main (the northern crater) and South summit craters on 10 November 2022 (top left), 15 December 2022 (top right), 3 February 2023 (bottom left), and 24 April 2023 (bottom right). Gas-and-steam emissions occasionally accompanied the thermal activity. Courtesy of Copernicus Browser.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 2023 based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Activity was relatively low during November and December 2022. Daily white gas-and-steam plumes rose 25-100 m above the summit and drifted in different directions. Gray ash plumes rose 200 m above the summit and drifted NE at 1047 and at 2343 on 11 November. On 14 November at 0933 ash plumes rose 300 m above the summit and drifted E. An ash plume was reported at 0935 on 15 December that rose 100 m above the summit and drifted NE. An eruptive event at 1031 later that day generated an ash plume that rose 700 m above the summit and drifted NE. A gray ash plume at 1910 rose 100 m above the summit and drifted E. Incandescent material was ejected above the vent based on an image taken at 1936.

During January 2023 daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions. Gray-to-brown ash plumes were reported at 1638 on 3 January, at 1410 and 1509 on 4 January, and at 0013 on 5 January that rose 100-750 m above the summit and drifted NE and E; the gray-to-black ash plume at 1509 on 4 January rose as high as 3 km above the summit and drifted E. Gray ash plumes were recorded at 1754, 2241, and 2325 on 11 January and at 0046 on 12 January and rose 200-300 m above the summit and drifted NE. Toward the end of January, PVMBG reported that activity had intensified; Strombolian activity was visible in webcam images taken at 0041, 0043, and 0450 on 23 January. Multiple gray ash plumes throughout the day rose 200-500 m above the summit and drifted E and SE (figure 135). Webcam images showed progressively intensifying Strombolian activity at 1919, 1958, and 2113 on 24 January; a gray ash plume at 1957 rose 300 m above the summit and drifted E (figure 135). Eruptive events at 0231 and 2256 on 25 January and at 0003 on 26 January ejected incandescent material from the vent, based on webcam images. Gray ash plumes observed during 26-27 January rose 300-500 m above the summit and drifted NE, E, and SE.

Figure 135. Webcam images of a strong, gray ash plume (left) and Strombolian activity (right) captured at Krakatau at 0802 on 23 January 2023 (left) and at 2116 on 24 January 2023 (right). Courtesy of PVMBG and MAGMA Indonesia.

Low levels of activity were reported during February and March. Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in different directions. The Darwin VAAC reported that continuous ash emissions rose to 1.5-1.8 km altitude and drifted W and NW during 1240-1300 on 10 March, based on satellite images, weather models, and PVMBG webcams. White-and-gray ash plumes rose 500 m and 300 m above the summit and drifted SW at 1446 and 1846 on 18 March, respectively. An eruptive event was recorded at 2143, though it was not visible due to darkness. Multiple ash plumes were reported during 27-29 March that rose as high as 2.5 km above the summit and drifted NE, W, and SW (figure 136). Webcam images captured incandescent ejecta above the vent at 0415 and around the summit area at 2003 on 28 March and at 0047 above the vent on 29 March.

Figure 136. Webcam image of a strong ash plume rising above Krakatau at 1522 on 28 March 2023. Courtesy of PVMBG and MAGMA Indonesia.

Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions during April and May. White-and-gray and black plumes rose 50-300 m above the summit on 2 and 9 April. On 11 May at 1241 a gray ash plume rose 1-3 km above the summit and drifted SW. On 12 May at 0920 a gray ash plume rose 2.5 km above the summit and drifted SW and at 2320 an ash plume rose 1.5 km above the summit and drifted SW. An accompanying webcam image showed incandescent ejecta. On 13 May at 0710 a gray ash plume rose 2 km above the summit and drifted SW (figure 137).

Figure 137. Webcam image of an ash plume rising 2 km above the summit of Krakatau at 0715 on 13 May 2023. Courtesy of PVMBG and MAGMA Indonesia.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during November 2022 through April 2023 (figure 138). Some of this thermal activity was also visible in infrared satellite imagery at the crater, accompanied by gas-and-steam and ash plumes that drifted in different directions (figure 139).

Figure 138. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during November 2022 through April 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 139. A thermal anomaly (bright yellow-orange) was visible at Krakatau in infrared (bands B12, B11, B4) satellite images on clear weather days during November 2022 through May 2023. Occasional gas-and-steam and ash plumes accompanied the thermal activity, which drifted in different directions. Images were captured on 25 November 2022 (top left), 15 December 2022 (top right), 27 January 2023 (bottom left), and 12 May 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Nishinoshima is a small island located about 1,000 km S of Tokyo in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. Eruptions date back to 1973; the most recent eruption period began in October 2022 and was characterized by ash plumes and fumarolic activity (BGVN 47:12). This report describes ash plumes and fumarolic activity during November 2022 through April 2023 based on monthly reports from the Japan Meteorological Agency (JMA) monthly reports and satellite data.

The most recent eruptive activity prior to the reporting internal occurred on 12 October 2022, when an ash plume rose 3.5 km above the crater rim. An aerial observation conducted by the Japan Coast Guard (JCG) on 25 November reported that white fumaroles rose approximately 200 m above the central crater of a pyroclastic cone (figure 119), and multiple plumes were observed on the ESE flank of the cone. Discolored water ranging from reddish-brown to brown and yellowish-green were visible around the perimeter of the island (figure 119). No significant activity was reported in December.

Figure 119. Aerial photo of gas-and-steam plumes rising 200 m above Nishinoshima on 25 November 2022. Reddish brown to brown and yellowish-green discolored water was visible around the perimeter of the island. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, November 2022).

During an overflight conducted by JCG on 25 January 2023 intermittent activity and small, blackish-gray plumes rose 900 m above the central part of the crater were observed (figure 120). The fumarolic zone of the E flank and base of the cone had expanded and emissions had intensified. Dark brown discolored water was visible around the perimeter of the island.

Figure 120. Aerial photo of a black-gray ash plume rising approximately 900 m above the crater rim of Nishinoshima on 25 January 2023. White fumaroles were visible on the E slope of the pyroclastic cone. Dense brown to brown discolored water was observed surrounding the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, January, 2023).

No significant activity was reported during February through March. Ash plumes at 1050 and 1420 on 11 April rose 1.9 km above the crater rim and drifted NW and N. These were the first ash plumes observed since 12 October 2022. On 14 April JCG carried out an overflight and reported that no further eruptive activity was visible, although white gas-and-steam plumes were visible from the central crater and rose 900 m high (figure 121). Brownish and yellow-green discolored water surrounded the island.

Figure 121. Aerial photo of white gas-and-steam plumes rising 900 m above Nishinoshima on 14 April 2023. Brown and yellow-green discolored water is visible around the perimeter of the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, April, 2023).

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during November 2022 through April 2023 (figure 123). A cluster of six to eight anomalies were detected during November while a smaller number were detected during the following months: two to three during December, one during mid-January 2023, one during February, five during March, and two during April. Thermal activity was also reflected in infrared satellite data at the summit crater, accompanied by occasional gas-and-steam plumes (figure 124).

Figure 123. Intermittent low-to-moderate thermal anomalies were detected at Nishinoshima during November 2022 through April 2023, according to this MIROVA graph (Log Radiative Power). A cluster of anomalies occurred throughout November, while fewer anomalies were detected during the following months. Courtesy of MIROVA.

Figure 124. Infrared (bands B12, B11, B4) satellite images show a small thermal anomaly at the summit crater of Nishinoshima on 9 January 2023 (left) and 8 February 2023 (right). Gas-and-steam plumes accompanied this activity and extended S and SE, respectively. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in late November 2018 and has more recently consisted of weak thermal activity and gas-and-steam emissions (BGVN 48:01). This report updates activity characterized by lava flows, incandescent avalanches, and ash plumes during January through June 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Activity during January was relatively low and mainly consisted of white gas-and-steam emissions that rose 25-150 m above Main Crater (S crater) and drifted in different directions. Incandescence was visible from the lava dome in Kawah Dua (the N crater). Weather conditions often prevented clear views of the summit. On 18 January the number of seismic signals that indicated avalanches of material began to increase. In addition, there were a total of 71 earthquakes detected during the month.

Activity continued to increase during the first week of February. Material from Main Crater traveled as far as 800 m down the Batuawang (S) and Batang (W) drainages and as far as 1 km W down the Beha (W) drainage on 4 February. On 6 February 43 earthquake events were recorded, and on 7 February, 62 events were recorded. White gas-and-steam emissions rose 25-250 m above both summit craters throughout the month. PVMBG reported an eruption began during the evening of 8 February around 1700. Photos showed incandescent material at Main Crater. Incandescent material had also descended the flank in at least two unconfirmed directions as far as 2 km from Main Crater, accompanied by ash plumes (figure 60). As a result, PVMBG increased the Volcano Alert Level (VAL) to 3 (the second highest level on a 1-4 scale).

Figure 60. Photos of the eruption at Karangetang on 8 February 2023 that consisted of incandescent material descending the flanks (top left), ash plumes (top right and bottom left), and summit crater incandescence (bottom right). Courtesy of IDN Times.

Occasional nighttime webcam images showed three main incandescent lava flows of differing lengths traveling down the S, SW, and W flanks (figure 61). Incandescent rocks were visible on the upper flanks, possibly from ejected or collapsed material from the crater, and incandescence was the most intense at the summit. Based on analyses of satellite imagery and weather models, the Darwin VAAC reported that daily ash plumes during 16-20 February rose to 2.1-3 km altitude and drifted NNE, E, and SE. BNPB reported on 16 February that as many as 77 people were evacuated and relocated to the East Siau Museum. A webcam image taken at 2156 on 17 February possibly showed incandescent material descending the SE flank. Ash plumes rose to 2.1 km altitude and drifted SE during 22-23 February, according to the Darwin VAAC.

Figure 61. Webcam image of summit incandescence and lava flows descending the S, SW, and W flanks of Karangetang on 13 February 2023. Courtesy of MAGMA Indonesia.

Incandescent avalanches of material and summit incandescence at Main Crater continued during March. White gas-and-steam emissions during March generally rose 25-150 m above the summit crater; on 31 March gas-and-steam emissions rose 200-400 m high. An ash plume rose to 2.4 km altitude and drifted S at 1710 on 9 March and a large thermal anomaly was visible in images taken at 0550 and 0930 on 10 March. Incandescent material was visible at the summit and on the flanks based on webcam images taken at 0007 and 2345 on 16 March, at 1828 on 17 March, at 1940 on 18 March, at 2311 on 19 March, and at 2351 on 20 March. Incandescence was most intense on 18 and 20 March and webcam images showed possible Strombolian explosions (figure 62). An ash plume rose to 2.4 km altitude and drifted SW on 18 March, accompanied by a thermal anomaly.

Figure 62. Webcam image of intense summit incandescence and incandescent avalanches descending the flanks of Karangetang on 18 March 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Summit crater incandescence at Main Crater and on the flanks persisted during April. Incandescent material at the S crater and on the flanks was reported at 0016 on 1 April. The lava flows had stopped by 1 April according to PVMBG, although incandescence was still visible up to 10 m high. Seismic signals indicating effusion decreased and by 6 April they were no longer detected. Incandescence was visible from both summit craters. On 26 April the VAL was lowered to 2 (the second lowest level on a 1-4 scale). White gas-and-steam emissions rose 25-200 m above the summit crater.

During May white gas-and-steam emissions generally rose 50-250 m above the summit, though it was often cloudy, which prevented clear views; on 21 May gas-and-steam emissions rose 50-400 m high. Nighttime N summit crater incandescence rose 10-25 m above the lava dome, and less intense incandescence was noted above Main Crater, which reached about 10 m above the dome. Sounds of falling rocks at Main Crater were heard on 15 May and the seismic network recorded 32 rockfall events in the crater on 17 May. Avalanches traveled as far as 1.5 km down the SW and S flanks, accompanied by rumbling sounds on 18 May. Incandescent material descending the flanks was captured in a webcam image at 2025 on 19 May (figure 63) and on 29 May; summit crater incandescence was observed in webcam images at 2332 on 26 May and at 2304 on 29 May. On 19 May the VAL was again raised to 3.

Figure 63. Webcam image showing incandescent material descending the flanks of Karangetang on 19 May 2023. Courtesy of MAGMA Indonesia.

Occasional Main Crater incandescence was reported during June, as well as incandescent material on the flanks. White gas-and-steam emissions rose 10-200 m above the summit crater. Ash plumes rose to 2.1 km altitude and drifted SE and E during 2-4 June, according to the Darwin VAAC. Material on the flanks of Main Crater were observed at 2225 on 7 June, at 2051 on 9 June, at 0007 on 17 June, and at 0440 on 18 June. Webcam images taken on 21, 25, and 27 June showed incandescence at Main Crater and from material on the flanks.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during mid-February through March and mid-May through June, which represented incandescent avalanches and lava flows (figure 64). During April through mid-May the power of the anomalies decreased but frequent anomalies were still detected. Brief gaps in activity occurred during late March through early April and during mid-June. Infrared satellite images showed strong lava flows mainly affecting the SW and S flanks, accompanied by gas-and-steam emissions (figure 65). According to data recorded by the MODVOLC thermal algorithm, there were a total of 79 thermal hotspots detected: 28 during February, 24 during March, one during April, five during May, and 21 during June.

Figure 64. Strong thermal activity was detected during mid-February 2023 through March and mid-May through June at Karangetang during January through June 2023, as recorded by this MIROVA graph (Log Radiative Power). During April through mid-May the power of the anomalies decreased, but the frequency at which they occurred was still relatively high. A brief gap in activity was shown during mid-June. Courtesy of MIROVA.

Figure 65. Incandescent avalanches of material and summit crater incandescence was visible in infrared satellite images (bands 12, 11, 8A) at both the N and S summit crater of Karangetang on 17 February 2023 (top left), 13 April 2023 (top right), 28 May 2023 (bottom left), and 7 June 2023 (bottom right), as shown in these infrared (bands 12, 11, 8A) satellite images. The incandescent avalanches mainly affected the SW and S flanks. Sometimes gas-and-steam plumes accompanied the thermal activity. Courtesy of Copernicus Browser.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); IDN Times, Jl. Jend. Gatot Subroto Kav. 27 3rd Floor Kuningan, Jakarta, Indonesia 12950, Status of Karangetang Volcano in Sitaro Islands Increases (URL: https://sulsel.idntimes.com/news/indonesia/savi/status-gunung-api-karangetang-di-kepulauan-sitaro-meningkat?page=all).

Ahyi seamount is a large, conical submarine volcano that rises to within 75 m of the ocean surface about 18 km SE of the island of Farallon de Pajaros in the Northern Marianas. The remote location of the seamount has made eruptions difficult to document, but seismic stations installed in the region confirmed an eruption in the vicinity in 2001. No new activity was detected until April-May 2014 when an eruption was detected by NOAA (National Oceanic and Atmospheric Administration) divers, hydroacoustic sensors, and seismic stations (BGVN 42:04). New activity was first detected on 15 November by hydroacoustic sensors that were consistent with submarine volcanic activity. This report covers activity during November 2022 through June 2023 based on daily and weekly reports from the US Geological Survey.

Starting in mid-October, hydroacoustic sensors at Wake Island (2.2 km E) recorded signals consistent with submarine volcanic activity, according to a report from the USGS issued on 15 November 2022. A combined analysis of the hydroacoustic signals and seismic stations located at Guam and Chichijima Island, Japan, suggested that the source of this activity was at or near the Ahyi seamount. After a re-analysis of a satellite image of the area that was captured on 6 November, USGS confirmed that there was no evidence of discoloration at the ocean surface. Few hydroacoustic and seismic signals continued through November, including on 18 November, which USGS suggested signified a decline or pause in unrest. A VONA (Volcano Observatory Notice for Aviation) reported that a discolored water plume was persistently visible in satellite data starting on 18 November (figure 6). Though clouds often obscured clear views of the volcano, another discolored water plume was captured in a satellite image on 26 November. The Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level (VAL) was raised to Advisory (the second lowest level on a four-level scale) on 29 November.

Figure 6. A clear, true color satellite image showed a yellow-green discolored water plume extending NW from the Ahyi seamount (white arrow) on 21 November 2022. Courtesy of Copernicus Browser.

During December, occasional detections were recorded on the Wake Island hydrophone sensors and discolored water over the seamount remained visible. During 2-7, 10-12, and 16-31 December possible explosion signals were detected. A small area of discolored water was observed in high-resolution Sentinel-2 satellite images during 1-6 December (figure 7). High-resolution satellite images recorded discolored water plumes on 13 December that originated from the summit region; no observations indicated that activity breached the ocean surface. A possible underwater plume was visible in satellite images on 18 December, and during 19-20 December a definite but diffuse underwater plume located SSE from the main vent was reported. An underwater plume was visible in a satellite image taken on 26 December (figure 7).

Figure 7. Clear, true color satellite images showed yellow-green discolored water plumes extending NE and W from Ahyi (white arrows) on 1 (left) and 26 (right) December 2022, respectively. Courtesy of Copernicus Browser.

Hydrophone sensors continued to detect signals consistent with possible explosions during 1-8 January 2023. USGS reported that the number of detections decreased during 4-5 January. The hydrophone sensors experienced a data outage that started at 0118 on 8 January and continued through 10 January, though according to USGS, possible explosions were recorded prior to the data outage and likely continued during the outage. A discolored water plume originating from the summit region was detected in a partly cloudy satellite image on 8 January. On 11-12 and 15-17 January possible explosion signals were recorded again. One small signal was detected during 22-23 January and several signals were recorded on 25 and 31 January. During 27-31 January a plume of discolored water was observed above the seamount in satellite imagery (figure 8).

Figure 8. True color satellite images showed intermittent yellow-green discolored water plumes of various sizes extending N on 5 January 2023 (top left), SE on 30 January 2023 (top right), W on 4 February 2023 (bottom left), and SW on 1 March 2023 (bottom right) from Ahyi (white arrows). Courtesy of Copernicus Browser.

Low levels of activity continued during February and March, based on data from pressure sensors on Wake Island. During 1 and 4-6 February activity was reported, and a submarine plume was observed on 4 February (figure 8). Possible explosion signals were detected during 7-8, 10, 13-14, and 24 February. During 1-2 and 3-5 March a plume of discolored water was observed in satellite imagery (figure 8). Almost continuous hydroacoustic signals were detected in remote pressure sensor data on Wake Island 2,270 km E from the volcano during 7-13 March. During 12-13 March water discoloration around the seamount was observed in satellite imagery, despite cloudy weather. By 14 March discolored water extended about 35 km, but no direction was noted. USGS reported that the continuous hydroacoustic signals detected during 13-14 March stopped abruptly on 14 March and no new detections were observed. Three 30 second hydroacoustic detections were reported during 17-19 March, but no activity was visible due to cloudy weather. A data outage was reported during 21-22 March, making pressure sensor data unavailable; a discolored water plume was, however, visible in satellite data. A possible underwater explosion signal was detected by pressure sensors at Wake Island on 26, 29, and 31 March, though the cause and origin of these events were unclear.

Similar low activity continued during April, May, and June. Several signals were detected during 1-3 April in pressure sensors at Wake Island. USGS suggested that these may be related to underwater explosions or earthquakes at the volcano, but no underwater plumes were visible in clear satellite images. The pressure sensors had data outages during 12-13 April and no data were recorded; no underwater plumes were visible in satellite images, although cloudy weather obscured most clear views. Eruptive activity was reported starting at 2210 on 21 May. On 22 May a discolored water plume that extended 4 km was visible in satellite images, though no direction was recorded. During 23-24 May some signals were detected by the underwater pressure sensors. Possible hydroacoustic signals were detected during 2-3 and 6-8 June. Multiple hydroacoustic signals were detected during 9-11 and 16-17 June, although no activity was visible in satellite images. One hydroacoustic signal was detected during 23-24 June, but there was some uncertainty about its association with volcanic activity. A single possible hydroacoustic signal was detected during 30 June to 1 July.

Geologic Background. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the ocean surface ~18 km SE of the island of Farallon de Pajaros in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: US Geological Survey, Volcano Hazards Program (USGS-VHP), 12201 Sunrise Valley Drive, Reston, VA, USA, https://volcanoes.usgs.gov/index.html; Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kadovar is a 2-km-wide island that is the emergent summit of a Bismarck Sea stratovolcano. It lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the volcano, filling an arcuate landslide scarp open to the S. Submarine debris-avalanche deposits occur to the S of the island. The current eruption began in January 2018 and has comprised lava effusion from vents at the summit and at the E coast; more recent activity has consisted of ash plumes, weak thermal activity, and gas-and-steam plumes (BGVN 48:02). This report covers activity during February through May 2023 using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

Activity during the reporting period was relatively low and mainly consisted of white gas-and-steam plumes that were visible in natural color satellite images on clear weather days (figure 67). According to a Darwin VAAC report, at 2040 on 6 May an ash plume rose to 4.6 km altitude and drifted W; by 2300 the plume had dissipated. MODIS satellite instruments using the MODVOLC thermal algorithm detected a single thermal hotspot on the SE side of the island on 7 May. Weak thermal activity was also detected in a satellite image on the E side of the island on 14 May, accompanied by a white gas-and-steam plume that drifted SE (figure 68).

Figure 67. True color satellite images showing a white gas-and-steam plume rising from Kadovar on 28 February 2023 (left) and 30 March 2023 (right) and drifting SE and S, respectively. Courtesy of Copernicus Browser.

Figure 68. Infrared (bands B12, B11, B4) image showing weak thermal activity on the E side of the island, accompanied by a gas-and-steam plume that drifted SE from Kadovar on 14 May 2023. Courtesy of Copernicus Browser.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Geologic Background. The symmetrical cone of San Miguel, one of the most active volcanoes in El Salvador, rises from near sea level to form one of the country's most prominent landmarks. A broad, deep, crater complex that has been frequently modified by eruptions recorded since the early 16th century caps the truncated unvegetated summit, also known locally as Chaparrastique. Flanks eruptions of the basaltic-andesitic volcano have produced many lava flows, including several during the 17th-19th centuries that extended to the N, NE, and SE. The SE-flank flows are the largest and form broad, sparsely vegetated lava fields crossed by highways and a railroad skirting the base of the volcano. Flank vent locations have migrated higher on the edifice during historical time, and the most recent activity has consisted of minor ash eruptions from the summit crater.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).

Ebeko, located on the N end of Paramushir Island in the Kuril Islands, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruption period began in June 2022 and has recently consisted of frequent explosions, ash plumes, and thermal activity (BGVN 47:10). This report covers similar activity during October 2022 through May 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Activity during October consisted of explosive activity, ash plumes, and occasional thermal anomalies. Visual data by volcanologists from Severo-Kurilsk showed explosions producing ash clouds up to 2.1-3 km altitude which drifted E, N, NE, and SE during 1-8, 10, 16, and 18 October. KVERT issued several Volcano Observatory Notices for Aviation (VONA) on 7, 13-15, and 27 October 2022, stating that explosions generated ash plumes that rose to 2.3-4 km altitude and drifted 5 km E, NE, and SE. Ashfall was reported in Severo-Kurilsk (Paramushir Island, about 7 km E) on 7 and 13 October. Satellite data showed a thermal anomaly over the volcano on 15-16 October. Visual data showed ash plumes rising to 2.5-3.6 km altitude on 22, 25-29, and 31 October and moving NE due to constant explosions.

Similar activity continued during November, with explosions, ash plumes, and ashfall occurring. KVERT issued VONAs on 1-2, 4, 6-7, 9, 13, and 16 November that reported explosions and resulting ash plumes that rose to 1.7-3.6 km altitude and drifted 3-5 km SE, ESE, E, and NE. On 1 November ash plumes extended as far as 110 km SE. On 5, 8, 12, and 24-25 November explosions and ash plumes rose to 2-3.1 km altitude and drifted N and E. Ashfall was observed in Severo-Kurilsk on 7 and 16 November. A thermal anomaly was visible during 1-4, 16, and 20 November. Explosions during 26 November rose as high as 2.7 km altitude and drifted NE (figure 45).

Figure 45. Photo of an ash plume rising to 2.7 km altitude above Ebeko on 26 November 2022. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

Explosions and ash plumes continued to occur in December. During 1-2 and 4 December volcanologists from Severo-Kurilsk observed explosions that sent ash to 1.9-2.5 km altitude and drifted NE and SE (figure 46). VONAs were issued on 5, 9, and 16 December reporting that explosions generated ash plumes rising to 1.9 km, 2.6 km, and 2.4 km altitude and drifted 5 km SE, E, and NE, respectively. A thermal anomaly was visible in satellite imagery on 16 December. On 18 and 27-28 December explosions produced ash plumes that rose to 2.5 km altitude and drifted NE and SE. On 31 December an ash plume rose to 2 km altitude and drifted NE.

Figure 46. Photo of an explosive event at Ebeko at 1109 on 2 December 2022. Photo has been color corrected. Photo by S. Lakomov, IVS FEB RAS.

Explosions continued during January 2023, based on visual observations by volcanologists from Severo-Kurilsk. During 1-7 January explosions generated ash plumes that rose to 4 km altitude and drifted NE, E, W, and SE. According to VONAs issued by KVERT on 2, 4, 10, and 23 January, explosions produced ash plumes that rose to 2-4 km altitude and drifted 5 km N, NE, E, and ENE; the ash plume that rose to 4 km altitude occurred on 10 January (figure 47). Satellite data showed a thermal anomaly during 3-4, 10, 13, 16, 21, 22, and 31 January. KVERT reported that an ash cloud on 4 January moved 12 km NE. On 6 and 9-11 January explosions sent ash plumes to 4.5 km altitude and drifted W and ESE. On 13 January an ash plume rose to 3 km altitude and drifted SE. During 20-24 January ash plumes from explosions rose to 3.7 km altitude and drifted SE, N, and NE. On 21 January the ash plume drifted as far as 40 km NE. During 28-29 and 31 January and 1 February ash plumes rose to 4 km altitude and drifted NE.

Figure 47. Photo of a strong ash plume rising to 4 km altitude from an explosive event on 10 January 2023 (local time). Photo by L. Kotenko, IVS FEB RAS.

During February, explosions, ash plumes, and ashfall were reported. During 1, 4-5 and 7-8 February explosions generated ash plumes that rose to 4.5 km altitude and drifted E and NE; ashfall was observed on 5 and 8 February. On 6 February an explosion produced an ash plume that rose to 3 km altitude and drifted 7 km E, causing ashfall in Severo-Kurilsk. A thermal anomaly was visible in satellite data on 8, 9, 13, and 21 February. Explosions on 9 and 12-13 February produced ash plumes that rose to 4 km altitude and drifted E and NE; the ash cloud on 12 February extended as far as 45 km E. On 22 February explosions sent ash to 3 km altitude that drifted E. During 24 and 26-27 February ash plumes rose to 4 km altitude and drifted E. On 28 February an explosion sent ash to 2.5-3 km altitude and drifted 5 km E; ashfall was observed in Severo-Kurilsk.

Activity continued during March; visual observations showed that explosions generated ash plumes that rose to 3.6 km altitude on 3, 5-7, and 9-12 March and drifted E, NE, and NW. Thermal anomalies were visible on 10, 13, and 29-30 March in satellite imagery. On 18, 21-23, 26, and 29-30 March explosions produced ash plumes that rose to 2.8 km altitude and drifted NE and E; the ash plumes during 22-23 March extended up to 76 km E. A VONA issued on 21 March reported an explosion that produced an ash plume that rose to 2.8 km altitude and drifted 5 km E. Another VONA issued on 23 March reported that satellite data showed an ash plume rising to 3 km altitude and drifted 14 km E.

Explosions during April continued to generate ash plumes. On 1 and 4 April an ash plume rose to 2.8-3.5 km altitude and drifted SE and NE. A thermal anomaly was visible in satellite imagery during 1-6 April. Satellite data showed ash plumes and clouds rising to 2-3 km altitude and drifting up to 12 km SW and E on 3 and 6 April (figure 48). KVERT issued VONAs on 3, 5, 14, 16 April describing explosions that produced ash plumes rising to 3 km, 3.5 km, 3.5 km, and 3 km altitude and drifting 5 km S, 5 km NE and SE, 72 km NNE, and 5 km NE, respectively. According to satellite data, the resulting ash cloud from the explosion on 14 April was 25 x 7 km in size and drifted 72-104 km NNE during 14-15 April. According to visual data by volcanologists from Severo-Kurilsk explosions sent ash up to 3.5 km altitude that drifted NE and E during 15-16, 22, 25-26, and 29 April.

Figure 48. Photo of an ash cloud rising to 3.5 km altitude at Ebeko on 6 April 2023. The cloud extended up to 12 km SW and E. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

The explosive eruption continued during May. Explosions during 3-4, 6-7, and 9-10 May generated ash plumes that rose to 4 km altitude and drifted SW and E. Satellite data showed a thermal anomaly on 3, 9, 13-14, and 24 May. During 12-16, 23-25, and 27-28 May ash plumes rose to 3.5 km altitude and drifted in different directions due to explosions. Two VONA notices were issued on 16 and 25 May, describing explosions that generated ash plumes rising to 3 km and 3.5 km altitude, respectively and extending 5 km E. The ash cloud on 25 May drifted 75 km SE.

Thermal activity in the summit crater, occasionally accompanied by ash plumes and ash deposits on the SE and E flanks due to frequent explosions, were visible in infrared and true color satellite images (figure 49).

Figure 49. Infrared (bands B12, B11, B4) and true color satellite images of Ebeko showing occasional small thermal anomalies at the summit crater on 4 October 2022 (top left), 30 April 2023 (bottom left), and 27 May 2023 (bottom right). On 1 November (top right) ash deposits (light-to-dark gray) were visible on the SE flank. An ash plume drifted NE on 30 April, and ash deposits were also visible to the E on both 30 April and 27 May. Courtesy of Copernicus Browser.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Our previous report about Arenal discussed ongoing sporadic eruptive behavior, preliminary information about the 24 May 2010 dome collapse, and the higher frequency of rockfalls through September 2010 (BGVN 35:07). Since October 2010, volcanic activity at Arenal appears to be decreasing. Events like the explosion on 24 July 2010, discussed below (see figure 110) have become rare. Reports from Costa Rica's Volcanological and Seismological Observatory and National University (OVSICORI-UNA) include direct observations of summit activity, seismic analysis, and acid-rain data and provide the basis for this report covering the 24 May, 2010 event in addition to activity from October 2010 to May 2011.

Figure 110. At 0538 on 24 July 2010 (local time) an ash explosion at Arenal was recorded seismically and its resulting cloud was photographed. In the lower left-hand corner is the seismic trace of the event, which began suddenly and saturated the record (seismic station VACR; OVSICORI-UNA). Courtesy of Phil Slosberg (OVSICORI-UNA).

Incandescent avalanche of 24 May 2010. Sudden activity down Arenal's SW flank on 24 May 2010 produced long, incandescent avalanches and pyroclastic flows, forcing the National Park to evacuate visitors on this day. No injuries or damage to infrastructure had been reported during Arenal's activity in May 2010. Previous pyroclastic events had also caused evacuations in June 2009, June 2008, and September 2007.

Beginning at noon on 24 May, incandescent avalanches descended from the summit dome. They affected a sector that has been subject to avalanches in the last 3 years (see figure 111). A field investigation by OVSICORI on 31 May found that material fell from the summit down to 1,200 m elevation and accumulating in a toe 400 m x 80 m. The majority of blocks surpassed 2 m in diameter. Deposits from the dome collapse were still hot when they arrived at the forest that borders Río Agua Caliente. The OVSICORI-UNA field report of 31 May 2010 contains photos and additional details. Several sections of the river scarp show signs of being struck and eroded by direct impact of the incandescent blocks that arrived with high speed. The dome that supplied the block-and-ash flows became visibly deflated but activity culminated through the week with the formation of a new dome toward the E side of the summit. The formation and destruction of domes at the top of Crater C is very common. These domes reach ten's of meters in size and frequently collapse violently, especially when they are destabilized at the crater rim.

Figure 111. Changes in morphology at Arenal's Crater C are visible owing to the 24 May 2010 dome collapse. Located on the eastern side of the summit, the point of failure was attributed to the "Unstable area." Courtesy of E. Duarte (OVSICORI-UNA).

Decreasing activity. The number of explosive events peaked in February 2010, became regular up to October, but since mid-October they have become sporadic. No lava flows or night-time incandescence was observed on the flanks. Gas emission continued at the active Crater C and fumarolic activity was continuous at Crater D, the pre-1968 summit crater.

Acid-rain affected Arenal's flanks and the NE, E, and SE flanks showed a loss of vegetation. These conditions plus the high amounts of rainfall aggravated erosion on the steep slopes; rockfalls and landslides continued to occur in these valleys: Calle de Arenas, Manolo, Guillermina, and Río Agua Caliente. OVSICORI-UNA released a report on acid-rain measurements that began on 9 April 2003 and ended on 30 November 2010; data from four stations showed generally decreasing acidity with time (figure 112). The trend steadily increased from pH ~4 to ~4.5 for all stations. Although irregular spikes are recorded, the low outliers were generally less acidic with time.

Figure 112. Variation of the pH (level of acidity) of rain-water collected from four stations on Arenal. Data points represent measurements from 9 April 2003 to 30 November 2010. Courtesy of OVSICORI-UNA.

Waldo Taylor assessed seismic data from the local network. The 2010 mid-year ICE report discussed seismicity and the general trend shown in table 26. The large spike in seismic events from 2009 dropped off abruptly the following year.

Table 26. Earthquakes counted at Arenal during 2005-2010. Courtesy of ICE.

Gerardo J. Soto discussed Arenal seismicity. "In general terms, the average magnitude increased from 2.0 in 2006 to 2.3 in 2010. The biggest was M 4.1 in 1 November 2009. Mean [focal] depth deepened from 5.5 km in 2006 to about 2 km in 2010. Most of them were between 2 and 5 km deep in 2009-2010, and down to 9 km deep in 2010.

"The number of [respective] earthquakes from September through December 2010 decreased monthly [in the sequence] 24, 12, 9, 3. Epicenters shifted from SE to NW quadrangle of the volcano through time.

"We preliminarily interpret this as a possible withdrawal of magma below the volcano, [on the basis of] focal mechanisms."

Secondary hazards. With Arenal's decrease in explosive activity, no ash collection has been possible this year (2011). A network of seven stations exists for regular sampling. The most effusive event occurred in 1968 when roughly 2 x 10⁵ metric tons of ash fell on the flanks. Later, a hydroelectric project was completed in the 1970s and filled the basin below the volcano with 2.416 x 10⁶ m³ of water (the maximum storage capacity), forming Lake Arenal. From 1992 to 1997, the annual sediment load into the lake contained 1.4% remobilized material from Arenal.

Future activity at Arenal within the next 100 years may include large eruptions with the potential to produce 10 million metric tons of volcanic sediments; within the next 200 years an extreme event could contribute 10⁷ metric tons of volcaniclastics to Lake Arenal (Soto, 1998). The distribution of volcaniclastic sediments is largely controlled by the Río Agua Caliente, a drainage connecting tributaries from Arenal's southern flank. Roughly every 2-5 years there are relatively large debris flows along this river. As recently as the first week of May 2011, intense flooding damaged a bridge by severely undermining the concrete abutments (G.J. Soto, personal communication).

Satellite thermal alerts. Since 15 September 2010 there have been no MODVOLC satellite thermal alerts through February 2011.

References. Soto, G.J., 1998, Cálculo de ceniza eyectada por el Volcán Arenal y ceniza caída en el embalse durante el período 1992-1997; Informe OSV.98.05.ICE, 18 pp. (in Spanish)

OVSICORI-UNA, 2010, Cambios Morfológicos y Avalanchas Incandescentes del 24 de Mayo en el Volcán Arenal. (in Spanish) (URL: http://www.ovsicori.una.ac.cr/vulcanologia/informeDeCampo/2010/InfcampAremayo10.pdf)

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: Phil Slosberg and Eliecer Duarte, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Gerardo J. Soto, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Waldo Taylor, Sismológico y Vulcanológico de Arenal y Miravalles (OSIVAM), Oficina de Sismología y Vulcanología (OSV), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

The Grotto vent cluster contains an assemblage of black smoker vents that lie within the Main Endeavour Field on the northern Juan de Fuca ridge (Bemis, 2001; Rona and others, 2001, 2010a; Bobbitt, 2007) (figure 4). New imagery of submarine plume behavior and properties was achieved with a new acoustic system that extends underwater observational distances beyond those of light to image buoyant plumes of submarine black smokers in 3-dimensions and image areas of diffuse flow seeping from the sea floor in 2-dimensions (Rona, 2011; Rona and others, 2010a, 2010b, and 2011).

Figure 4. Map of Main Endeavour Field, Juan de Fuca Ridge (grid system in meters), showing the location of the Grotto Vent at grid coordinates of about 6115 and 4920. Note scale-the entire Endeavour Field is only ~400 m long. According to Merle (2006) Grotto vent resides at 47.95°N latitude, 129.10°W longitude, and at a depth of ~2,196 m.

The Cabled Observatory Vent Imaging Sonar (COVIS) was installed in September 2010 (Light, 2011). Operations were initiated with in situ sensors in the NEPTUNE (North-East Pacific Time-Series Underwater Networked Experiments) Canada Program cabled observatory on the Main Endeavour Field (MEF) of the Juan de Fuca Ridge, nearly 370 km (200 nautical miles) off British Columbia, Canada, in the NE Pacific Ocean (figures 5 and 6). NEPTUNE is a Canadian research facility designed for regional-scale underwater ocean investigations focusing on continuous monitoring of temperature, chemistry, biodiversity, and motion. This data will be broadcast via the Internet for scientists, students, educators and the public to collaborate and promote investigations into: underwater volcanic processes; earthquakes and tsunamis; minerals, metals, and hydrocarbons; ocean-atmosphere interactions; climate change; greenhouse gas cycling in the ocean; marine ecosystems; long-term changes in ocean productivity; marine mammals; fish stocks; pollution and toxic blooms. The public can gain a more in-depth understanding of the seafloor, while ocean scientists can run deep-water experiments from labs and universities anywhere around the world.

Figure 5. Map of NEPTUNE Canada Program's six submarine sites with multiple sensors connected to a high-speed optical cable linked with University of Victoria in British Columbia, Canada. The Main Endeavour Field, labeled as Endeavor (in red), one of the instrumented sites, is ~350 km WSW from Port Alberni. Over the project's 25-year lifespan, Endeavor will collect data for underwater volcanic processes, seismicity, plate tectonics, hydrothermal vent systems, and deep sea ecosystems. Courtesy of NEPTUNE Canada (2011).

During a research cruise in September-October 2010, scientists from the University of Washington and Rutgers University connected COVIS to the NEPTUNE Canada cable system for the first time and initiated data acquisition on 29 September 2010. COVIS, equipped with a customized multibeam sonar, 400/200 kHz projectors, and a rotator system to orient acoustic transducers, was positioned to acquire acoustic data from a fixed site on the floor of the ridge's axial valley at a range of tens of meters from the Grotto vent cluster in the MEF (figure 6).

Figure 6. COVIS acoustic image, oriented NE on the left to NW on the right, made at 0600 UTC on 11 October 2010, looking S at black smoker plumes and areas of diffuse flow draped over bathymetry of the Grotto vent cluster (Jackson and others, 2003) in the Main Endeavour Field, Juan de Fuca Ridge. The image was made when tidal currents were minimal (e.g., near slack tide). The larger plume is from the N tower edifice at the NW end, and the smaller plumes are from the NE end of Grotto vent at the in-situ experiments. The legend (at the upper left) specifies isosurfaces of plume volume scattering strengths (in decibels per meter) related to particle content and temperature-density discontinuities. The vertical color bar (at the far right) gives normalized decorrelation of backscatter (0-1) due to diffuse flow from the sea floor at 0.8-sec lag. The plumes decrease in acoustic backscatter intensity as they mix with surrounding seawater with height (in meters) above vents. From Rona (2011).

The purpose of the COVIS experiment was to acoustically image, quantify, and monitor seafloor hydrothermal flow on time scales of hours (response to ocean tides) to weeks-months-years (response to volcanic and tectonic events); this advances our understanding of these interrelated processes. According to Rona and others (2003), net volume flux of a plume can be calculated by integrating the vertical flux through a plume cross-section, which can then be converted to heat and particle flux if coordinated with in-situ measurements of temperature and particle properties (concentration, size distribution, density). To achieve this, COVIS acquired acoustic data from a projector mounted on a tripod ~4 m above the seafloor at a fixed position. A computer controlled, 3- degrees-of-freedom (yaw, pitch, and roll), positioning system was used to point the sonar transducers providing a large coverage area at the site. Sonar data is collected at ranges of tens of meters from targets to make three types of measurements: 1) volume backscatter intensity from suspended particulate matter and temperature fluctuations in black smoker plumes which was used to reconstruct the size and shape of the buoyant portion of a plume; 2) Doppler phase shift which was used to obtain the flow rise velocity at various levels in a buoyant plume; 3) scintillation which was used to image the area of diffuse flow seeping from the seafloor.

References. Bemis, K.G., Rona, P.A., Jackson, D.R., Jones, C., Mitsuzawa, K., Palmer, D., Silver, D., and Gudlavalletti, R., 2001, Time-averaged images and quantifications of seafloor hydrothermal plumes from acoustic imaging data: a case study at Grotto Vent, Endeavour Segment Seafloor Observatory, Abstract OS21B-0446 presented at American Geophysical Union, Fall Meeting 2001, San Francisco, CA, December.

Bobbitt, A., 2007, NeMO 2007 Cruise Report: Axial Volcano, Endeavour Segment, and Cobb Segment, Juan de Fuca Ridge, R/V Atlantis Cruise AT 15-21, August 3-20, 2007, Astoria, Oregon, to Astoria Oregon, Jason dives J2-286 to J2-295, unpublished report (URL: http://www.pmel.noaa.gov/vents/nemo/NeMO2007-cruise-report.pdf)

Jackson, D.R., Jones, C.D., Rona, P.A., and Bemis, K.G., 2003, A method for Doppler acoustic measurement of black smoker flow fields, Geochemistry Geophysics Geosystems (G3), v. 4, no. 11, p. 1095 (DOI: 10.1029/2003GC000509, 2003).

Light, R., Miller, V., Rona, P., and Bemis, K., 2010, Acoustic Instrumentation for Imaging and Quantifying Hydrothermal Flow in the NEPTUNE Canada Regional Cabled Observatory at Main Endeavour Field (unpublished paper - URL: http://www.apl.washington.edu/projects/apl_presents/topics/covis/covis.php).

Light, R., Miller, V., Jackson, D.R., Rona, P.A., and Bemis, K.G., 2011, Cabled observatory vent imaging sonar (abstract of presentation), Journal of the Acoustical Society of America, v. 129, no. 4, p. 2373.

Merle, S. (compiler), 2006, NeMO 2006 Cruise Report, NOAA Vents Program, Axial Volcano and the Endeavour Segment, Juan de Fuca Ridge, R/V THOMPSON Cruise TN-199, August 22 - September 7, 2006. Seattle WA to Seattle WA; ROPOS dives R1008 - R1014 (URL: http://www.pmel.noaa.gov/vents/nemo2006/nemo06-crrpt-final.pdf).

NEPTUNE Canada, 2011, Transforming Ocean Science; Ocean Networks Canada. (URL: http://www.neptunecanada.ca/about-neptune-canada/neptune-canada-101/)

Rona, P.A., Bemis, K.G., Jackson, D.R., Jones, C.D., Mitsuzawa, K., Palmer, D.R., and Silver, D., 2001, Acoustic Imaging Time Series of Plume Behavior at Grotto Vent, Endeavour Observatory, Juan de Fuca Ridge, Abstract OS21B-0445 presented at American Geophysical Union, Fall Meeting 2001, San Francisco, CA, December.

Rona, P.A., Jackson, D.J., Bemis, K.G., Jones, C.D., Mitsuzawa, K., Palmer, D.R., and Silver, D., 2003, A New Dimension in Investigation of Seafloor Hydrothermal Flows, Ridge 2000 Events, v. 1, no. 1, p. 26 (URL: http://ridge2000.bio.psu.edu).

Rona, P.A., Bemis, K.G., Jones, C., Jackson, D. R., Mitsuzawa, K, and Palmer, D. R., 2010a, Partitioning Between Plume and Diffuse Flow at the Grotto Vent Cluster, Main Endeavour Vent Field, Juan de Fuca Ridge: Past and Present, Abstract OS21C-1519 presented at American Geophysical Union, Fall Meeting 2010, San Francisco, Calif., December.

Rona, P., Light, R., Miller, V., Jackson, D., Bemis, K., Jones, C., and KenneyM., 2010b, Cabled Observatory Vent Imaging Sonar (COVIS) Connected to NEPTUNE Canada Cabled Observatory (poster abstract), 2010 R2K (Ridge 2000) Community Meeting, Portland, OR, 29-31 October 2010 (URL: http://ridge2000.marine-geo.org/community-meeting/october-2010/2010-r2k-community-meeting).

Rona, P., 2011, Sonar images hydrothermal vents in seafloor observatory, EOS Transactions, American Geophysical Union, v. 92, no., 20, p. 169-170.

Rona, P.A., Benis, K.G., Jones, C.D., and Jackson, D.R., 2011, Multibeam sonar observations of hydrothermal flows at the Main Endeavour Field (abstract of presentation), Journal of the Acoustical Society of America, v. 129, no. 4, p. 2373.

Geologic Background. The Endeavour Segment (or Ridge) lies near the northern end of the Juan de Fuca Ridge, W of the coast of Washington and SW of Vancouver Island. The northern end is offset to the east with respect to the West Valley Segment, which extends north to the triple junction with the Sovanco Fracture Zone and the Nootka Fault. The 90-km-long, NNE-SSW-trending segment lies at a depth of more than 2,000 m and is the site of vigorous high-temperature hydrothermal vent systems that were discovered in 1981. Five major vent fields that include sulfide chimneys and black smoker vents are spaced at about 2-km intervals in a 1-km-wide axial valley at the center of the ridge. Preliminary uranium-series dates of Holocene age were obtained on basaltic lava flows, and other younger "zero-age" flows were sampled. Seismic swarms were detected in 1991 and 2005.

Information Contacts: Peter Rona, Institute of Marine and Coastal Sciences and Department of Earth and Planetary Sciences, Rutgers University, New Brunswick, NJ; NEPTUNE Canada (URL: http://www.oceannetworks.ca/).

Gudmundsson and others (2010a) noted that the last day of sustained activity at Eyjafjallajökull took place on 22 May 2010. By 23 June 2010, the Iceland Meteorological Office (IMO) and the University of Iceland Institute of Earth Sciences (IES) ceased issuing regular status reports. In addition to discussing the eruption and its final stages, this report also cites a small sample of abstracts and papers from the numerous conferences, sessions, and publications that have thus far emerged on the eruption.

The eruption's initial phase, 20 March-12 April 2010, occurred at Fimmvörðuháls, a spot on the E flanks of Eyjafjallajökull (figure 16, and "F" and "E" on figure 17). Venting at Fimmvörðuháls took place on an exposed ridge cropping out in a region with extensive glaciers to the E and W. Eruptions began in the initially ice-capped summit crater of Eyjafjallajökull on 14 April 2010 (BGVN 35:03 and 35:04). After melting overlying portions of the icecap, the summit crater then emitted clouds of fine-grained ash that remained suspended in the atmosphere for long distances. The ash blew both over the Atlantic and for considerable intervals passed directly over Europe, halting flights of most commercial aircraft for nearly a week in a controversial shutdown with economic impacts in the billions.

Figure 16. Index map showing Iceland, some major plate-tectonic features and generalized spreading directions, and the location of Eyjafjallajökull volcano. Note proximity of Eyjafjallajökull to Katla and to the volcanoes of the Vestmann island area (Vestmannaeyjar), Surtsey and Heimaey. Courtesy of USGS.

Figure 17. A shaded-relief map showing Eyjafjallajökull (E), and 9 km to its E, the flank vent Fimmvörðuháls (F). Stars indicate 2010 eruptive sites (map scale at top left). Glaciers cover extensive portions of both Eyjafjallajökull and Katla volcanoes (light pattern). During 14-29 April 2010 many earthquakes struck with epicenters along the N-S axis of Eyjafjallajökull (black dots). The map includes a small slice of the Atlantic ocean along the lower left-hand margin. Two of four geodetic (GPS) stations are shown (STE2 and THEY). Revised from a map by Sigmundsson and others (2010).

In terms of satellite thermal data on the overall eruption, the MODVOLC system measured extensive (multi-pixel) daily alerts during 21 March-21 May 2010, but the alerts became absent thereafter.

Venting at Fimmvörðuháls. At a 15-19 September 2010 conference on the eruption, Höskuldsson and others (2010a) characterized the course of events during the 20 March to 12 April basaltic Fimmvörðuháls flank eruption at Eyjafjallajökull as follows: "At the beginning the eruption featured as many as 15 lava fountains with maximum height of 150 m. On March 24 only four vents were active with fountains reaching to heights of 100 m. On March 31 and April 1 the activity was characterized by relatively weak fountaining through a forcefully stirring pool of lava. The vents were surrounded by 60-80 m high ramparts and the level of lava stood at approximately 40 m. This high stand led to opening of a new fissure trending northwest from the central segment of the original fissure. As activity on the new fissure intensified, the discharge from the original fissure declined and stopped on April 7.

"The intensity of the lava fountains varied significantly on the time scale of hours and was strongly influenced the level of the lava pond in the vents, producing narrow, gas-charged, piston-like fountains during periods of low lava levels, but spray-like fountains when the lava level was high . . ..

"The eruption produced a fountain-fed lava flow field with an area of about 1.3 km². Initially (20-25 March), the lava advanced towards northeast, but on March 26 the lava began advancing to the west and northwest, especially after April 1 when the activity became concentrated on the new fissure. The flow field morphology is dominantly 'a'a, but domains of pahoehoe and slabby pahoehoe are present, particularly in the western sector of the flow field. The advance of the lava from the vents was episodic; when the lava stood high the lava surged out of the vents, but at low stand there was a lull in the advance. The lava discharged from the vents through open channels as well as internal pathways. The open channels were the most visible part of the transport system, feeding lava to active 'a'a flow fronts and producing spectacular lava falls when cascading into deep gullies just north of the vents. The role of internal pathways was much less noticeable, yet an important contribution to the overall growth of the flow field as it fed significant surface breakouts emerging on the surface of what otherwise looked like stagnant lava. When activity stopped on April 12 the fissure had issued about 0.025 km³ of magma, giving a mean discharge of 13 m³/s."

Summit eruption. The second eruption occurred within the initially ice-covered caldera of Eyjafjallajökull. Opening of the ice cover and explosivity into the atmosphere was amplified by magma-ice interaction that produced a fine ash capable of suspension in the atmosphere for prolonged periods.

Höskuldsson and others (2010b) described the eruption at Eyjafjallajökull's summit (beginning 14 April 2010) as consisting of three phases (table 2). They also stated that at the summit the "Total amount of tephra produced in the eruption is about 0.11 km³ and that of lava 0.025 km³ DRE [dense-rock equivalent]. Average discharge rate in the eruption was about 40 m³/s DRE or about 4 times that of Fimmvörðuháls eruption."

Table 2. Three phases of the eruption at Eyjafjallajökull volcano's summit beginning 14 April 2010 as summarized and condensed by Höskuldsson and others (2010b).

The summit area was still steaming and geothermally active, and the eruption channel was still very hot in October 2010 (figure 18). Investigators expected that cooling to ambient temperatures would take a few years . As noted below, during June 2010, hot lava could still be seen in cracks in the cooled rock on Fimmvörðuháls, and inside craters, but that was not the case at the ice-engulfed summit caldera.

Figure 18. The summit crater complex of Eyjafjallajökull taken after the first winter snow, as seen from the air at 0810 on 9 October 2010. The scene helps explain the high degree of water and ice interaction with the erupting lavas. Snow had melted from numerous ash and lava-covered surfaces (black areas). Although portions of the crater emitted steam, evidence of substantial ongoing lava emissions were absent at this point in time. Photo courtesy of Ólafur Sigurjónsson, IMO.

According to Gudmundsson and others (2010b) the summit eruption produced 0.1-0.2 km³ (dense rock equivalent) of tephra. IES reported that by 11 June 2010 a lake about 300 m in diameter had formed in the large summit crater, and by 23 June water was slowly accumulating in the crater because ice was no longer in contact with hot material.

Intrusion triggering. Sigmundsson and others (2010) noted that the 2010 eruptions came after 18 years of intermittent volcanic unrest. The deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single source. Deformation was rapid before the flank eruption (0.5 mm per day after 4 March 2010), but negligible during it.

During the summit eruption (beginning 14 April 2010) gradual contraction of a source, distinct from the pre-eruptive inflation sources, was evident from geodetic data. Thus, clear signals of volcanic unrest may occur over years to weeks, indicating reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Figure 19 shows a cross-sectional model of the shallow crust by Sigmundsson and others (2010) based deformation and seismic analyses of the 2010 event. A previous issue of the Bulletin (BGVN 35:03) contained an alternate model by Paul Einarsson.

Figure 19. Schematic E-W cross-section across the Eyjafjallajökull summit area, with deformation sources plotted at their best-fit depth (vertical exaggeration of 2). Gray shaded background indicates source-depth uncertainties (95% confidence interval), which overlap. Courtesy of Sigmundsson and others (2010).

Processed satellite image. Vincent J. Realmuto created two composite figures generated from the MODIS-Terra satellite data acquired 15 April 2010 at 1135 UTC (figure 20). Outlined in black in each image are Iceland on the upper left side (W), Faroe Islands in the center, Scotland and N Ireland in the lower center, and part of the Scandinavian peninsula on the right side (E). An ash plume can be seen in each image extending from Iceland SW toward Europe. The left-hand image is the true-color RGB (red-green-blue) composite and the right-hand image is a false-color composite; in the right-hand rendition the ash plume appears red and the ice-rich clouds appear blue. The right-hand image puts obvious emphasis on the ash plume and shows it streaming and more or less intact for several hundreds of kilometers E of Iceland.

Figure 20. Graphics generated from the MODIS-Terra satellite data acquired 15 April 2010 at 1135 UTC. The left-hand graphic is a true-color RGB (red-green-blue) composite, and the right-hand image is a false-color composite of Bands 32, 31, and 29 (12, 11, and 8.5 um, respectively) displayed in red, green, and blue, respectively. These data were processed with the decorrelation stretch (D-stretch), a technique for enhancing spectral contrast based on principal components analysis. In this rendition the ash plume appears red and the ice-rich clouds appear blue. The D-stretch was based on scene statistics and was intended to be a quick method for discriminating material that may be volcanic in origin. Courtesy of Vincent J. Realmuto, Jet Propulsion Laboratory, California Institute of Technology.

Conference field trip. Following The Atlantic Conference on Eyjafjallajökull and Aviation in Iceland, 15-16 September 2010 (discussed below), a field trip brought scientists to accessible areas on the volcano, including the flank vent on Fimmvörðuháls ridge where the eruption began. John and Liudmila Eichelberger provided some photographs from this trip (figure 21). The same base map appeared in BGVN 35:03, with the key and other data. The horseshoe shape of the lava distribution in this figure is the feature imaged by an ASTER satellite thermal signature as active lava flows on 19 April 2010 in BGVN 35:03.

Figure 21. (Central panel) Map showing fissures at Fimmvörðuháls (thin red lines) and the distribution of new scoria and lava deposited at various points in time (shaded areas) during 21 March-7 April 2010. Marked arrows on the map give locations of labeled photos (A-E) taken 18 September 2010. (A) Fresh lava (darker) seen looking N. In the distance appear fresh black lava flows, some portions of which formed the lava falls down the valley walls. (B) View showing the elongate ridge as seen from the upslope perspective (people in the distance for scale). (C, looking down) Glowing lava (~1.5 m long and ~0.3 m wide) at the bottom of a fissure. This photo was taken with a flash, otherwise the fissure walls would have been very dark. (D) The fracture indicated on the map as it appeared near the rim of the ridge of newly erupted lava. (E) The same fracture seen in D from another perspective. Courtesy of John and Ludmilla Eichelberger.

More on conferences and publications. Recently, several conferences have been held and many publications have been issued relevant to the eruption. What follows is a mere sample of the available resources, many of which emphasized plume research. At the American Geophysical Union (AGU) 2010 Fall Meeting, several sessions focused on the 2010 eruption (eg., Carn and others, 2010; see References for the link to abstracts volume).

The Workshop on Ash Dispersal Forecast and Civil Aviation held in Geneva, 18-20 October 2010, addressed the characteristics and range of application of different volcanic ash transport and dispersal models (VATDM), identifying the needs of the modeling community, investigating new data acquisition strategies, and discussing how to improve communication between the volcanology community and operational agencies (eg., Bonadonna and others, 2011).

The Cities on Volcanoes conference (COV-6; Tenerife, Canary Islands, Spain, 31 May-4 June 2010) included both papers (eg. Fischer and others, 2010) and a forum on the "Assessment of volcanic ash threat: learning and considerations from the Eyjafjallajökull eruption."

In addition, several other papers relevant to the eruption were presented during this meeting, as well as at the Annual Meeting of the American Meteorological Society (AMS) in Seattle, WA, in January 2011, and at the European Geosciences Union (EGU) 2011 General Assembly in Vienna, Austria.

The journal Atmospheric Chemisrty and Physics published multiple issues with a section entitled "Atmospheric implications of the volcanic eruptions of Eyjafjallajökull, Iceland 2010." These and other papers discussed various means of plume detection, and in some cases, sampling, including on the ground, in ultralight aircraft, and on satellites; models of plume dispersion were evaluated (Flentje and others, 2010; Emeis and others, 2011; Vogel and others, 2011; Fischer and others, 2010).

According to Loughlin (2010), scientists from the British Geological Survey found large ash particles from the eruption in the United Kingdom. Most of the very small ash particles in volcanic plumes fell as clusters of particles known as aggregates. The aggregation could have resulted from a number of mechanisms, including electrostatic attraction, particle collisions, condensation of liquid films and secondary mineralization. The process of aggregation effectively removed very small particles from the plume and was therefore one variable on how long ash particles stay in the atmosphere. Ripley (2010) and Chivers (2010) published articles on the U.K. Met Office's tracking and prediction of movements of volcanic ash based on observations from the Eyjafjallajökull eruption.

Gislason and others (2011) reported on analyses of two sets of fresh, comparatively dry ash samples that fell in Iceland and were collected rapidly on 15 and 27 April, during more and less explosive phases, respectively. Both sets of samples were kept dry and analyzed swiftly to minimize issues with hydration and alteration, particularly to salts on the ash surfaces. The ash was dominantly glass of andesitic composition (57-58% SiO₂). They found the ash particles especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers.

References. Bonadonna, C., Folch, A., and Loughlin, S., 2011, Future Developments in Modeling and Monitoring of Volcanic Ash Clouds, Eos, Transactions of the American Geophysical Union (AGU), v. 92, no. 10; pp. 85-86, DOI: 10.1029/2011EO100008 (URL: http://www.agu.org/pub/eos/).

Carn, S.A., Karlsdottir, S., and Prata, F., 2010, The 2010 Eruption of Eyjafjallajokull: A Landmark Event for Volcanic Cloud Hazards I, II, and III, Abstracts V41E, V53F, and V54C presented at 2010 Fall Meeting, American Geophysical Union, San Francisco, CA, 13-17 December 2010 (URL: http://www.agu.org/meetings/fm10/program/index.php).

Chivers, H., 2010, Dark Cloud: VAAC and predicting the movement of volcanic ash, Meterological Technology International, June 2010, pp. 62-65.

Emeis, S., Forkel, R., Junkermann, W., Schäfer, K., Flentje, H., Gilge, S., Fricke, W., Wiegner, M., Freudenthaler, V., Groß, S., Ries, L., Meinhardt, F., Birmili, W., Münkel, C., Obleitner, F., and Suppan, P., 2011, Measurement and simulation of the 16/17 April 2010 Eyjafjallajökull volcanic ash layer dispersion in the northern Alpine region, Atmospheric Chemistry and Physics, v. 11, pp. 2689-2701.

Fischer, C., van Haren, G., Pohl, T., Vogel, A., and Weber, K., 2010, Airborne in-situ measurements of the volcanic ash dust plume over a part of Germany caused by the volcano eruption of the Eyjafjallajökull (Iceland) by means of an optical particle counter and a light

sport aircraft, Abstract, Session 1.3, p. 229, Cities on Volcanoes 6 Conference (URL: http://www.citiesonvolcanoes6.com/ver.php).

Flentje, H., Claude, H., Elste, T., Gilge, S., Köhler, U., Plass-Dülmer, C., Steinbrecht, W., Thomas, W., Werner, A., and Fricke W., 2010, The Eyjafjallajökull eruption in April 2010 - detection of volcanic plume using in-situ measurements, ozone sondes and lidar-ceilometer profiles, Atmospheric Chemistry and Physics, v. 10, pp. 10085-10092, DOI: 10.5194.

Gasteiger, J., Groß, S., Freudenthaler, V., and Wiegner, M., 2011, Volcanic ash from Iceland over Munich: mass concentration retrieved from ground-based remote sensing measurements, Atmospheric Chemistry and Physics, v. 11, pp. 2209-2223.

Gislason, S.R., Hassenkam, T., Nedel, S., Bovet, N., Eiriksdottir, E.S., Alfredsson, H.A., Hem, C.P., Balogh, Z.I., Dideriksen, K., Oskarsson, N., Sigfusson, B., Larsen, G., and Stipp, S.L.S., 2011, Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment, Proceedings of the National Academy of Sciences, v. 108, no. 18, p. 7303-7312.

Gudmundsson, M. T., Pedersen, R., Vogfjörd, K., Thorbjarnardóttir, B., Jakobsdóttir, S., and Roberts, M.J., 2010a, Eruptions of Eyjafjallajökull Volcano, Iceland, Eos, Transactions of the American Geophysical Union (AGU), v. 91, no. 21, p. 190, DOI: 10.1029/2010EO210002.

Gudmundsson, M.T., Thordarson, T., Hoskuldsson, A., Larsen, G., Jónsdóttir, I., Oddsson, B., Magnusson, E., Hognadottir, T., Sverrisdottir, G., Oskarsson, N., Thorsteinsson, T., Vogfjord, K., Bjornsson, H., Pedersen, G.N., Jakobsdottir, S., Hjaltadottir, S., Roberts, M.J., Gudmundsson, G.B., Zophoniasson, S., and Hoskuldsson, F., 2010b, The Eyjafjallajökull eruption in April-May 2010; course of events, ash generation and ash dispersal, EOS, Transactions of the American Geophysical Union (AGU), V. 91, no. 21, Abstract V53F-01, 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 December (URL: http://www.agu.org/cgi-bin).

Heue, K.-P., Brenninkmeijer,C.A.M., Baker, A. K., Rauthe-Schöch, A., Walter, D., Wagner, T., Hörmann, C., Sihler, H., Dix, B., Frieß, U., Platt, U., Martinsson, B. G., van Velthoven, P.F.J., Zahn, A., and Ebinghaus, R., 2011, SO₂ and BrO observation in the plume of the Eyjafjallajökull volcano 2010: CARIBIC and GOME-2 retrievals, Atmospheric Chemistry and Physics, v. 11, pp. 2973-2989.

Höskuldsson, A., Magnusson, E., Guðmundsson, M.T., Sigmundsson, F., and Sigmarsson, O., 2010a, The 20 March to 12 April basaltic Fimmvörðuháls flank eruption at Eyjafjallajökull volcano, Iceland: Course of events, abstract of presentation in Program of the Eyjafjallajökull and Aviation Conference (15-16 September 2010) and associated Eyjafjallajökull Eruption Workshop (Hotel Hvolsvellir, 17-19 September 2010); (URL: http://en.keilir.net/keilir/conferences/eyjafjallajokull/volcanological-workshop).

Höskuldsson, Á., Larsen, G., Gudmundsson, M.T., Oddsson, B., Magnússon, E., Sigmarsson, O., Óskarsson, N., Jónsdóttir, I., Sigmundsson, F., Einarsson, P., Hreinsdóttir, S., Pedersen, R., Högnadóttir, Þ., Thordarson, T., Hayward, C., Hartley, M., Meara, R., Arason, Þ., Karlsdóttir, S., and Petersen, G.N., 2010b, The Eyjafjallajökull eruption April to May 2010: Magma fragmentation, plume and tephra transport, and course of events, abstract of presentation in Program of the Eyjafjallajökull and Aviation Conference (15-16 September 2010) and associated Eyjafjallajökull Eruption Workshop (17-19 September 2010); (URL: http://en.keilir.net/keilir/conferences/eyjafjallajokull/volcanological-workshop).

Laursen, L., 2010, Iceland eruptions fuel interest in volcanic gas monitoring, Science, v. 328, no. 5977, p. 410-411.

Loughlin, S., 2010, Modelling of Iceland volcanic ash particles, news item from British Geological Survey (URL: http://www.bgs.ac.uk/research/highlights/IcelandAshParticles.html?src=sfb).

Ripley, T., 2010, Cloud Busting: How the UK is tracking the volcanic ash cloud, Meterological Technology International, June 2010, pp. 6-10.

Schumann, U., Weinzierl, B., Reitebuch, O., Schlager, H., Minikin, A., Forster, C., Baumann, R., Sailer, T., Graf, K., Mannstein, H., Voigt, C., Rahm, S., Simmet, R., Scheibe, M., Lichtenstern, M., Stock, P., Rüba, H., Schäuble, D., Tafferner, A., Rautenhaus, M., Gerz, T., Ziereis, H., Krautstrunk, M., Mallaun, C., Gayet, J.-F., Lieke, K., Kandler, K., Ebert, M., Weinbruch, S., Stohl, A., Gasteiger, J., Groß, S., Freudenthaler, V., Wiegner, M., Ansmann, A., Tesche, M., Olafsson, H., and Sturm, K., 2011, Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010, Atmospheric Chemistry and Physics, v. 11, pp. 2245-2279.

Sigmundsson, F., Hreinsdóttir, S., Hooper, A., Árnadóttir, T., Pedersen, R., Roberts, M.J., Óskarsson, N., Auriac, A., Decriem, J., Einarsson, P., Geirsson, H., Hensch, M., Ófeigsson, B.G., Sturkell, E., Sveinbjörnsson, H., and Feigl, K.L., 2010, Letter: Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption, Nature, v. 468, pp. 426-430.

Stohl, A., Prata, A.J., Eckhardt, S., Clarisse, L., Durant, A., Henne, S., Kristiansen, N.I., Minikin, A., Schumann, U., Seibert, P., Stebel, K., Thomas, H.E., Thorsteinsson, T., Tørseth, K., and Weinzierl, B., 2011, Determination of time- and height-resolved volcanic ash emissions and their use for quantitative ash dispersion modeling: the 2010 Eyjafjallajökull eruption, Atmospheric Chemistry and Physics, v. 11, pp. 4333-4351.

Vogel, A., Weber, K., Fischer, C., van Haren, G., Pohl, T., Grobety, B., and Meier, M., 2011, Airborne in-situ measurements of the Eyjafjallojökull ash plume with a small aircraft and optical particle spectrometers over north-western Germany - comparison between the aircraft measurements and the VAAC-model calculations, European Geophysical Union General Assembly, Geophysical Research Abstracts, v. 13, p. EGU2011-13253.

Geologic Background. Eyjafjallajökull (also known as Eyjafjöll) is located west of Katla volcano. It consists of an elongated ice-covered stratovolcano with a 2.5-km-wide summit caldera. Fissure-fed lava flows occur on both the E and W flanks, but are more prominent on the western side. Although the volcano has erupted during historical time, it has been less active than other volcanoes of Iceland's eastern volcanic zone, and relatively few Holocene lava flows are known. An intrusion beneath the S flank from July-December 1999 was accompanied by increased seismic activity. The last historical activity prior to an eruption in 2010 produced intermediate-to-silicic tephra from the central caldera during December 1821 to January 1823.

Information Contacts: Institute of Earth Sciences (IES), University of Iceland, Sturlugata 7, Askja , 101 Reykjavík (URL: http://www.earthice.hi.is/); Icelandic Meteorological Office (IMO) (URL: http://en.vedur.is/earthquakes-and-volcanism/articles/nr/1884); U.K. Meteorological Office (URL: http://www.metoffice.gov.uk); ármann Höskuldsson, Institute of Earth Sciences (IES), University of Iceland, Sturlugata 7, Askja , 101 Reykjavík (URL: http://www.earthice.hi.is); Hawai'i Institute of Geophysics and Planetology (HIGP) MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sue C. Loughlin, The British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland, UK (URL: http://www.bgs.ac.uk/); Vincent J. Realmuto, Jet Propulsion Laboratory, California Institute of Technology, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109 USA; John Eichelberger, U.S. Geological Survey, Volcano Hazards Program, Reston, VA (URL: http://volcanoes.usgs.gov/); Ludmilla Eichelberger, Global Volcanism Program, National Museum of Natural History, 10^th and Constitution Ave., NW, Washington, DC 20560 USA; Iceland Review (URL: http://icelandreview.com/icelandreview/daily_news/).

In April 2010 the lake within Irazú's crater dwindled to only a few centimeters depth and from May to August the lake was dry enough to allow plants to grow up to 10 cm high. Water began to accumulate in September 2010 but disappeared again during the following month. Since November 2010 water returned to the crater and as late as April 2011, a shallow turquoise-blue lake was maintained. Continuous monitoring of acid rain on Irazú's flanks reflected contributions from Turrialba. Often called Irazú's "twin volcano," Turrialba is less than 10 km to the ENE and during the past 4 years it has caused a region-wide increase in acid rain. Covering January 2004 through September 2007, the last Bulletin report on Irazú (BGVN 32:11) highlighted decreasing lake levels, fumarolic changes, and minor mass wasting on the crater walls during January 2004 to March 2007 (see table 8 for a summary of lake changes).

Table 8. Changing lake conditions based on observations of Irazú's crater. Double asterisks indicate times when the lake disappeared; "--" fills cells where no data is available; lake levels are reported qualitatively except for the 7 October to 12 March 2010 time interval when absolute values were measured. This summary is based on ICE data and OVSICORI Monthly Reports.

On 22 July 2010 a team of investigators from Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) descended to the dry crater floor. They documented changes in vegetation, fumaroles, and clay deposition on the crater floor. Photos taken during prior trips provided comparisons with previous conditions (figure 14). Rockfalls and minor mass wasting had been occurring regularly and the long runout of debris across the crater floor was visible during this investigation. Most of the debris fell from the E and SW walls. On the NE side of the dry crater a rocky area emitted low temperature (24°C) sulfur-smelling gases from three aligned vents. Higher temperatures (86°C) were measured from fumaroles on the N side of the crater but they appeared to be releasing gas with less energy than observed in the past years when bubbles were visible within the lake. Another interesting finding was a waterfall on the inside of the crater on the SW wall; this small waterfall did not have sufficient volume to pool on the crater floor and instead soaked directly into the surrounding clay.

Figure 14. Views taken from Irazú's S rim. (top) The crater on 24 April 2004 contained a turquoise lake. (bottom) A repeat photo taken on 22 July 2010 shows the lake had disappeared; the former lake level and the clay base on the crater floor are marked. Since November 2010 water had accumulated and as of April 2011, was several meters deep. Courtesy of Eliecer Duarte, OVSICORI-UNA.

The water level in Irazú's crater has been variable throughout time; the Bulletin recorded a dry crater during February 1977 and June 1987 (SEAN 12:07), and April 1990 (BGVN 15:04). Factors highlighted during the IAVCEI CVL-7 ("Commission of Volcanic Lakes" Costa Rica, 10-19 March 2010) included complex connections with Turrialba, seasonal effects, infiltration within the crater, and the role of mass wasting. The mechanism for the recent disappearance of the lake is still under investigation by OVSICORI-UNA and ICE investigators (Guillermo Alvarado, personal communication).

Erosion. Mass wasting had been an ongoing process for at least 10 years. Material is primarily shed from the E and SW walls and the lake contained islands of black and red material formed from the debris. In February 2003 a major rockslide into the lake caused the water color to change from green to shades of red. An analysis of seismicity during that month showed no correlation to these slope failures (BGVN 28:12). Cracks along the NW rim formed and widened since December 2007; these cracks caused blocks up to 3 x 20 m to fall from the rim in March 2008.

Local gas measurements. Since the large phreatic explosion in December 1994 (BGVN 19:12), the NW fumarole has been releasing low gas emissions regularly. Different temperature measurements recorded since June 2010 ranged between 90°C to 86°C. To monitor changes in sulfur dioxide output from Irazú, a network of three stations collected rain samples from sites along the volcano's flanks.

The pH data from September 2004 through July 2010 were plotted in the OVSICORI-UNA July 2010 monthly report. The results correlate pH changes to much larger degassing events occurring at Turrialba, a neighboring volcano that began major degassing in 2007. Only the "Borde Sur" station was sampling continuously but the other two stations reflected similar trends in acidity. Despite irregular fluctuations, a decreasing pH trend began in 2007. The lowest point of the trend was measured by "Borde Este" at approximately pH 3.25. Where there "Pacayas" station data began, the trend appeared to have stabilized between pH 3.25 and 4.75.

References. D. Rouwet, R.A. Mora-Amador, C.J. Ramírez-Umaña, G. González, Seepage of "aggressive" fluids reduce volcano flank stability: the Irazú and Turrialba case, Costa Rica, Abstract, CVL 7 Workshop Costa Rica, IAVCEI-Commission of Volcanic Lakes, March 2010.

Geologic Background. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the main crater, which contains a small lake. The first well-documented eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: E. Duarte, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); G. Alvarado and G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San Jose, Costa Rica.

This is the first Bulletin report on Cerro Machín volcano, the site of seismic unrest for many years, most recently, 1992, 1999, 2002, 2004, 2008, 2009, and 2010. This activity did not lead to eruptions. Instrumental monitoring by INGEOMINAS began in 1987 and has determined Machín's background seismicity ranged from 1 to10 earthquakes/day, but during intervals of unrest, seismicity sometimes reached several hundred earthquakes per day.

This is a small but explosive volcano located at the S end of the Ruiz-Tolima massif, 185 km NNE of the Nevado del Huila volcano and 147 km WSW of Bogotá, the capital (figure 1). (Tolima volcano, not shown, lies ~22 km NNE of Machín.)

Figure 1. Map of Colombia showing the location of the Machín volcano. Note the Departments (states) of Tolima (1) and Huila (2) are shaded regions. Courtesy of the IFRC and Relief Web.

Machín caldera contains three dacitic domes; the 3-km-wide caldera is breached to the S. According to Mendez and others (2002), there have been six eruptions within the past 10,000 years. In the same report, the authors noted geomorphological similarities between Machín and Pinatubo prior to its large 1991 eruption. The seismic events have drawn increased attention to Machín from the Volcanic and Seismological Observatory of Manizales, Colombia Institute of Geology and Mining (INGEOMINAS).

According to news articles published in mid-May 2004, INGEOMINAS reported that there had been an increase in seismicity at Machín in April. About 60 earthquakes were recorded daily (in comparison to the 1-10 earthquakes normally recorded); however, no surface changes were seen at that time at the volcano.

There was no further significant seismic activity until the first week of January 2008 when INGEOMINAS reported unusual seismicity at Machín during 6-8 January. Long-period earthquakes were detected S of the main lava dome. On 7 January, the volcano-tectonic seismic signals were occasionally felt and reported by nearby residents. The simultaneous occurrence of both types of seismic signals was unusual for Machín. Again, the activity diminished to the previous background levels until 9 November when INGEOMINAS reported a cluster of ~375 earthquakes, the majority of which were located towards the E sector and below the dome of the volcano with depths between 2.5 and 5 km. The earthquake activity occurred underneath the central and E parts of the lava dome complex in the summit caldera and fumarolic activity in the area increased. During 8-10 November 2008, Machín registered 1,210 volcano-tectonic earthquakes, 9 of which were M 2.5. According to news articles, approximately 400-450 people evacuated to shelters or other safe areas. There were also reports of landslides that blocked a highway.

Table 1 and figure 2 detail the local villages in proximity to Machín.

Table 1. Villages in proximity to Machín and the respective distances from the caldera (approximate). Taken from web sources such as Google Earth.

Figure 2. A regional map showing population centers and paved and unpaved roads. Courtesy of INGEOMINAS.

On 10 November the seismic activity of the volcano diminished to background conditions. On 17 December INGEOMINAS reported that a swarm of 98 earthquakes occurred at Machín SE of the lava domes at depths of 2-6 km. The largest earthquake was M 2.6 at a depth of ~4 km.

There were two significant seismic events at Machín during 2009. On 31 July there was in increase in seismic activity, which consisted of ~200 events. Initially the increase was gradual, however, during the last hour the activity increased abruptly and included an earthquake of M 2.7. This subsided to a background level until early December when INGEOMINAS detected 54 earthquakes, some M ~ 1.3. Authorities issued a "Yellow" alert (Yellow; "changes in the behavior of volcanic activity") for Machín. The Tolima Regional Emergency Committee conducted evacuation training with local communities as a precaution.

INGEOMINAS reported that on 24 July 2010 a seismic crisis at Machín was characterized by volcano-tectonic earthquakes. An M 2.6 earthquake was located S of the main lava dome at a depth of ~4 km. The next day an M 4.1 volcano-tectonic earthquake occurred 0.8 km S of the main dome at a depth of ~3.9 km. The Yellow alert remained in effect following the increase in registered seismic activity in the area. On 29 July the number of volcano-tectonic events again increased; the earthquakes were a maximum M 1.7 and between 3 and 4 km depth, S of the main dome.

On 17 September 2010, INGEOMINAS again reported increased seismicity. About 140 volcano-tectonic earthquakes as large as M 1.85 were located S and SW of the main lava dome at depths of 2-4 km. On 4 October there was an M 3.5 tectonic earthquake located 0.37 km S of the main dome at a depth of ~4.14 km. Residents near the volcano felt this earthquake. The Alert Level remained at Yellow.

On 3 December 2010 about 340 volcano-tectonic earthquakes with low magnitudes were located SW of the main lava dome, at an average depth of 4 km. The largest event, a M 3.7 earthquake located SW of the dome at a depth of about 3.5 km, was felt by local residents. On 31 December INGEOMINAS reported a period of increased seismicity. A total of 346 volcano-tectonic events no stronger than M 2.1 were located S and SW of the main lava dome.

On 1 January 2011 seismicity again increased, and at the time of the report, 367 events had been detected. The low-magnitude events were located S and SW of the main dome at depths between 2.5 and 4.5 km. The largest event, M 2.3, was located S of the dome at a depth of about 3.3 km and felt by residents near the volcano and in the municipality of Cajamarca, 8 km SSW. On 13 January an increased number of earthquakes were located to the W and SW of the main dome at depth of 2.5-3.5 km. The largest event registered M ~2.6 and was reported to have been felt by residents near the volcano.

Since 1989, INGEOMINAS noted a gradual increase in seismicity has been following the events closely in order to report any changes on the volcano's activities (figure 3). All the local emergency committees were activated in the area near Machín volcano in addition to the regional emergency committees in Tolima District.

Figure 3. Map showing potential hazards from hypothetical future activity at Machín. Thicknesses of potential ash fall to the W are shown (in cm) as modeled by computer-aided dispersion modeling (VAFTAD); PF stands for pyroclastic flow deposits. Adaped from INGEOMINAS (2007).

References. Méndez, RA; Cortés, GP; and Cepeda, H; [Calvache, ML, Project Chief], 2002, Evaluacíon de la Amenaza Volcánica Potencial del Cerro Machín (Departamento del Tolima, Colombia), Manizales, Sept. 2002, INGEOMINAS, 66 p. (in Spanish).

Méndez, RA, Cortés, GP, and Cepeda, H., 2007, Evaluacíon amenazas potencial de volcan Cerro Machín [Large map in Spanish taken from 2002 report of same name. Name in English, 'Evaluation of potenial hazards from volcan Cerro Machín'] Mapa Amenaza Volcán Machín, INGEOMINAS (URL: http://intranet.ingeominas.gov.co/manizales/images/5/55/MAPA_AMENAZA_VOLCAN_MACHIN.jpg)

Geologic Background. The small Cerro Machín stratovolcano lies at the southern end of the Ruiz-Tolima massif about 20 km WNW of the city of Ibagué. A 3-km-wide caldera is breached to the south and contains three forested dacitic lava domes. Voluminous pyroclastic flows traveled up to 40 km away during eruptions in the mid-to-late Holocene, perhaps associated with formation of the caldera. Late-Holocene eruptions produced dacitic block-and-ash flows that traveled through the breach in the caldera rim to the west and south. The latest known eruption of took place about 800 years ago.

Information Contacts: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Manizales, Manizales, Colombia; Relief Web (URL: https://reliefweb.int/); International Federation of Red Cross And Red Crescent Societies (IFRC) (URL: http://www.ifrc.org/); Caracol Radio; El Tiempo:Portafolio (URL: http://columbiareports.com).

Occasional, typically minor phreatic eruptions occurred at Poás through at least early February 2011 (BGVN 35:12). They emerged from the active crater lake, Lago Caliente. The Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) illuminated intervals of phreatic eruptions and relations on the chemistry of Lago Caliente's waters over a period of more than 30 years (figure 94). This report includes photos of phreatic eruptions in 2009, 2010, and early 2011, and reviews events through March 2011.

Figure 94. Plots of the sulfur, chlorine, and fluorine concentrations, as well as the temperature, pH, and gas volumes in the Lago Caliente waters at Poás, with respect to time. The data on the time axis extends from early 1978 to late 2009. Arrows along the top indicate periods with frequent phreatic eruptions. Notice the low pH, often well below pH 1.5. Courtesy of OVSICORI-UNA.

Volcanic gases and associated condensate and rainfall led to increasing areal extent and degree of damage to vegetation in nearby areas. In studying the Lago Caliente's waters, Martinez and others (2011) found in solution a variety of oxo-anions of sulfur called polythionates (S_nO₆^-2, where n can be 20 or larger), which they found to vary in concentration from undetectable to 8,000 mg/L. They considered polythionates to be "highly relevant for monitoring purposes at Poás, in particular because they may signal impending phreatic eruptions."

More on the 25 December 2009 phreatic eruption. A previous report (BGVN 35:12) discussed a phreatic eruption on 25 December 2009 but some further comments are worth adding. As previously noted (BGVN 35:12), "Steam and lake water mixed with sediment and blocks were ejected 550-600 m above Laguna Caliente and fell in the vicinity of the lake, within the crater." No mention was previously made of a 24 December 2009 phreatic eruption discussed by OVSICORI-UNA. It took place in the morning at 0808 and all erupted material fell back in the crater.

Photos taken on 25 December 2009 and recently posted on the Picasa website have come to our attention. The four photos on figure 95 come from a set of nine taken from the S rim. The earliest of the set depict a very tranquil lake with steaming at or near the dome (not shown here). The next photo, taken 129 seconds after that tranquil scene, portrays the advancing eruption (figure 95a). The subsequent two photos (figure 95b and c) captured the interval closest to the peak of the eruptive vigor.

Figure 95. Four sequential photos taken looking N at Poás of a phreatic eruption from the center of Lago Caliente on 25 December 2009. The time intervals between the four photos was as follows: photos (a) to (b), 5 sec; photos (b) to (c), 5 sec; and photos (c) to (d), 11 sec. Photo descriptions below: (a) The earliest available photo of the eruption cloud, which, based on the next photo in this set, was clearly still emerging energetically. It advanced with the leading portions of the plume chiefly dark. At the plume's base, white steam clouds mask the lake. (b and c) The shots taken closest to the maximum point of the eruption's thrust phase, with dark material still conspicuous. White tufts expanded and began to cap most of the advancing jets. The clouds engulfing the base of the plume now contain more discolored zones. (d) As the plume evolves and the vigorous exhalative part of the eruption ends or wanes, a steam-rich cloud envelops the eruption cloud. Note the gray-colored rain falling out of the plume. Taken from Cindy and JM's Gallery (undated) on the Picasa photo sharing website (see References and Information Contacts below).

An exact assessment of the photos is complicated by several factors. There were shifts in the focal length of the lens (documented in camera metadata found on the website). Also, in detail, the camera's time record indicated 0252 hrs, clearly incorrect for this daylight scene. That problem is reconciled by a photo featured in the OVSICORI-UNA report, which showed a plume photo by another photographer at a stage nearly identical to figure 95b and the text indicated the eruption occurred at 0952 hrs local time.

An email response from Cindy Doire provided these comments about witnessing the phreatic eruption.

"We arrived at the volcano early in the morning. We were one of the first to arrive that day. Our group and a few other tourists were looking at it and NOTHING was happening. The people finished looking and started leaving that spot. It was just about 4 of us still there, when suddenly the volcano started to erupt. There was NO warning at all. Even the rangers were surprised. At the beginning, white steam (gas?) shot up, then black rock and dirt started exploding out. I believe that everything that shot up, fell back into the crater . . . the gas could be smelled and was strong . . .."

In an email to GVP regarding the 25 December 2009 eruption, Eliecer Duarte commented: "It seems that this [25 December 2009] eruption opened a more permanent vent at the bottom of the lake. Since that event the frequency of phreatic ones increased and remained like this for [a] year and a half. We still have dozens of smaller ones daily.

More on crater degassing. Field visits during 2010 and 2011 allowed scientists to see the expanding effects of Poás volcanic gases on vegetation (figures 96 and 97). Dry conditions resulted in winds carrying the gases considerable distances from the volcano. The area most affected was an elongate zone downwind of the active crater and extending ~4 km SW. Figure 97 portrays transitional zones with intermediate effects.

Figure 96. A commercial airline pilot and amateur photographer took this and other photos of Poás on 28 April 2010. The active crater and its discolored lake (Lago Caliente) reside at the right-hand side of this shot. It is part of an elongate zone of barren rock stretching ~4 km across the otherwise lushly vegetated landscape. As is typical, the plume's orientation on this day lies directly over the barren zone. From "Len" (undated), (see Reference below).

Figure 97. Oblique view highlighting the area to the S of Poás (note volcano's crater lakes, including the active "Lago Caliente") On color versions of this figure, the pink rhombuses show sites for collecting acid rain. Providencia is shown in the lower left. The crater lake at upper right, "Botos" is ~0.5 km across in the long direction but the scale on this image varies with distance towards the foreground. Courtesy E. Duarte, OVSICORI-UNA.

Starting just beyond the elongate zone of harsh effects, the areas of discolored vegetation had increased impact and areal extent. One such impacted area was a nature preserve called Providencia, which is seen in figure 97 to the left of Poás. Farther from the volcano lies Cerro Pelón (2.5 km distance and direction SW of the crater) , which also showed the effects of chemical burning from volcanic gases (figure 97).

In the past, activity centers have migrated within the crater. OVSICORI-UNA reported that, for at least the past year (ending March 2011), the points of degassing have been concentrated in the hot crater lake and dome (figure 98). The emanating steam and gases, often carried by wind, have affected areas up to several hundred meters around the crater (figures 96-98).

Figure 98. The active crater at Poás, showing pronounced steam release both from fractures in the dome as well as from the lake's surface. Conditions like this (with more or less steam) often prevailed in recent times (including just a few seconds prior to the eruption sequence shown in figure 95). The crater lake (Lago Caliente) rests behind (N of) the dome and steam clouds. Courtesy E. Duarte, OVSICORI-UNA.

OVSICORI-UNA reported that through at least March 2011 small phreatic eruptions occurred daily at Lago Caliente. These eruptions sometimes only reached the lake's surface, but at other times reached a few meters above the lake, and occasionally, tens of meters above the lake. The majority of the erupted sediments fell back into the lake. The fine sediments sometimes remained suspended in the lake water and caused its gray color. The majority of eruptions occurred in the central part of the crater, with a few originating slightly more to the N or S of the center. Because of the phreatic activity and high temperature of the lake (57°C), strong evaporation occurred and plumes traveled long distances in the wind (figure 99).

Figure 99. At Poás, a phreatic eruption at Lago Caliente reaching several meters high, in a manner typical of daily activity during recent months. View from the active crater's N side (opposite the viewpoint). Photo taken sometime in January 2011. Courtesy E. Duarte, OVSICORI-UNA.

A comparison of vegetation in the area between Cerro Pelón and Providencia (designated "F1" in figure 97) made during August 2010 to January 2011 found that most plant species were resistant at certain levels of acidification. However, when their tolerance thresholds were reached, the affected species decayed quickly and were sometimes unable to recover. Certain species, including eucalyptus, pine, alder, and cypress, were particularly sensitive to the volcanic gases. Minor effects from gases were observed on Cypress trees as far as 9 km SW of the emission source. OVSICORI-UNA reports contained several photos showing more details on the effects of acidic gases on vegetation. One of their later reports, from April 2011, discussed ongoing phreatic eruptions and dome temperature of 560°C.

References. Cindy and JM's Gallery, undated, "Poas volcano eruption, December 25th, 2009" [9 photos] Picassa (URL: https://picasaweb.google.com/cjmdoire); [includes camera-related metadata].

Len (Barfbag), undated, "Wednesday, April 28, 2010, Mt Poas, Costa Rica" ; in Viewsfrom the left seat, A look at the airline world ... ride along in the cockpit (URL: http://viewsfromtheleftseat.blogspot.com/2010/04/mt-poas-costa-rica.html)

Martínez, M., van Bergen, M.J., Fernández, E., and Takano, B., 2011, Polythionates monitoring at the acid crater lake of Poás Volcano, IAVCEI-COMMISSION OF VOLCANIC LAKES, CVL7 Workshop, Costa Rica, 10-19 March 2010, Online Abstracts volume (May 2011), p. 12 (URL: http://www.ulb.ac.be/sciences/cvl/)

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Duarte and E. Fernández, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Cindy Doire (address withheld by request).

This report on Ranau, a Pleistocene caldera that lies along the Great Sumatran fault, is based on accounts of fish kills, including one on 4 April 2011. The fish died near hot springs in Lake Ranau, a large caldera lake, and their deaths were attributed to seismically induced H₂S releases by the Center of Volcanology and Geological Hazard Mitigation (CVGHM). CVGHM reported the surface area of Lake Ranau to be ~127 km², and noted that the Lake Ranau complex is geothermally active, with hot springs that emerge at the foot of Mount Seminung on the banks of Lake Ranau. In addition to the 2011 event, fish kills have been recorded in Lake Ranau (figure 1) for the past five decades (table 1).

Figure 1. Photo of Lake Ranau with Mount Seminung in the background. Posted by blogger "masternewstoday" in May 2011.

Table 1. Previous fish kills in Lake Ranau reported during the past five decades. (Note that there is no mention of any correlation between seismicity and geochemical anomalies.) Courtesy of CVGHM.

Reports stated that the 4 April 2011 fish kill was large in scale. According to the head of a nearby village, Sugih Sane, the event began with turbulent water in Lake Ranau that lasted for approximately 30 minutes. Local residents reported that the fish kill occurred during a relatively short time in portions of the lake surrounding hot springs. At the time of the incident, the water in the affected areas appeared milky white, and wind spread the smell of sulfur to surrounding areas.

Geochemistry. Scientists conducted field work near the three hot springs Kota Batu, Ujung, and Way Wahid during 16-19 April 2011. At that time they reported the following: No dead algae were found on the lake's surface. There was no smell of sulfur, the water was clear, and the water around the hot springs was bubbling and warm. * Dead fish were no longer present. The pH of the lake water was 7.74, and the temperature was 26.1°C. The water near the hot springs had a pH of 6.32-7.06, with a temperature of 47.8-62°C. The water of the river that empties into Lake Ranau (input) had a pH of 8.07-8.10, and the lake water discharge (output) had a pH of 7.86. The result of ambient gas examination showed no gases associated with magmatic gases, such as CH₄, CO₂, CO, and H₂S, in the vicinity of the hot springs discharge. The degree to which the above measurements were anomalous was unstated.

Seismicity. Seismic data recorded during 16-20 April 2011 showed microearthquake activity around Lake Ranau. The earthquakes were located along a fault line oriented in the SE-NW direction along Lake Ranau, at depths of 0.6 and 10 km below the surface of the lake. The Berkelulusan location coincides with the location of the Kota Batu hot springs. Prior to the fish kill at Lake Ranau on 4 April, an M 5.1 earthquake was recorded on 29 March 2011 in Bengkulu, ~160 km W of Lake Ranau.

Cause of the fish kill. CVGHM concluded that, based on the results of the field work (location of dead fish near hot springs, sulfur smell carried by wind up to 3 km away, absence of dead algae, and changing color of the lake water to milky white during the event), the fish kill in Lake Ranau was caused by the release of H₂S gas into the lake water, which caused imbalances in lake water chemistry. They said that hydrothermal gas was trapped over time and escaped to the surface after the pressure due to tectonic disturbances. CVGHM concluded that the M 5.1 earthquake in Bengkulu on 29 March 2011 led to increased pressure on the fault in the vicinity of Lake Ranau; then, H₂S gas was released to the surface in the vicinity of the hot springs. According to CVGHM, the occurrence of microearthquakes is a result of the fault in the vicinity of Lake Ranau, and are neither dangerous nor destructive. However, CVGHM asked residents to report future fish kills to the local government.

Geologic Background. Ranau is an 8 x 13 km Pleistocene caldera partially filled by the crescent-shaped Lake Ranau. The caldera lies along the Great Sumatran Fault that extends the length of Sumatra. Incremental formation of the caldera culminated in the eruption of the voluminous Ranau Tuff about 0.55 million years ago. A morphologically young post-caldera stratovolcano, Gunung Semuning, was constructed within the SE side of the caldera to a height of more than 1,200 m above the lake surface. The volcano has not been mapped in sufficient detail to determine the age of its latest eruptions, although fish kills and sulfur smells in the late 19th and early 20th centuries may be related to volcanism.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Masternewstoday (URL: http://hot-breaking-news-masternewstoday.blogspot.com).

Low-frequency earthquakes and tremor were reported at Rincón de la Vieja during the first half of 2008 (BGVN 33:07). Since then, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) had issued intermittent reports of activity through January 2011. Those reports are summarized in the following sections, with much of the discussion centered around fumaroles and behavior of the geothermally warmed lake in the active crater. Occasional, typically small phreatic eruptions had occurred here in past years, for example in the 1990s (eg., BGVN 21:02, 21:03, 22:01, and 23:03) but were absent in the current reporting interval (last half of 2008 through January 2011).

August 2008. OVSICORI-UNA reported that the level of the lake was at a high level, with a bluish color, generated convection cells with evaporation, and had sulfur particles visible on it's surface. Sulfur deposition and fumarolic activity continued along the SW wall.

March 2009. In mid-March 2009, scientists visited the S and SW flank, collected samples, and noted some temperatures of 75-78°C. Because the visit occurred during the dry season, most areas encountered were dry. The scientists examined an area of acidification to the W of Von Seebach crater, ~3 km SW of the active crater. Strong winds common in that direction sometimes carried volcanic gases. Consequently, most of this narrow expanse only contained patches of grassland and shrubs that barely covered the rocky surface.

October 2009. OVSICORI-UNA reported that seismographic station RIN3, located ~5 km SW of the main crater, registered volcano-tectonic events and tremor lasting for minutes.

Weak ongoing fumarolic activity during 2010 through January 2011. OVSICORI-UNA reported that the level of the crater lake remained high during 2010, with constant evaporation. Geochemical, seismic, and deformation data did not show significant changes in physico-chemical parameters during 2010. The changing color of the lake, from blue to gray, was attributed to intense rains and fumarolic activity in the crater.

Later reporting. Reports during 2010 through at least January 2011 described fumarolic activity along the S and SW walls of the crater, with sulfur deposition and moderate gas discharge. The lake remained a gray color, with sulfur particles in suspension. Figure 15 shows a photo taken in April of the crater looking at the SW wall with fumarolic activity along with sulfur deposition. In April 2010, OVSICORI-UNA reported that the temperature of the lake was 49°C. A fumarole sometimes seen active along the N flank had stopped discharging gas.

Figure 15. Photo of the active crater lake of Rincón de la Vieja on 29 April 2010 showing yellow sulfur deposits and fumarolic activity along the SW wall of the crater. This kind of activity was typical throughout the reporting interval (last half of 2008 through January 2011). Photo by E. Fernandez, OVSICORI-UNA.

OVSICORI-UNA reported that 2010 was unusual in that four domestic volcanoes were active: Arenal, Poás, Turrialba, and Rincón de la Vieja. Irazú was comparatively inactive (see separate report in this issue of the Bulletin).

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernández, W. Sáenz, E. Duarte, M. Martínez, S. Miranda, F. Robichaud, T. Marino, M. Villegas, and J. Barquero, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

This report first describes activity seen at Shiveluch during December 2010-March 2011. Data from that interval included several ash plumes visible as they blew to over 100 km from the volcano. Thermal imagery analysis showed the character of the dome and the path of pyroclastic-flow deposits during that interval. After that, we provide a follow-up to the 27 October 2010 eruption (BGVN 35:11), adding some previously unmentioned details. That eruption destroyed the dome's SE sector and generated pyroclastic flows.

During December 2010-March 2011, KVERT reported that Shiveluch both underwent moderate seismicity and emitted bright thermal anomalies conspicuous in satellite imagery (figure 27). Details of significant explosions and ash plumes during that time appear on table 10. Figure 28 shows a photo with the distant skyline dominated by a long Shiveluch ash plume.

Table 10. An inexhaustive synopsis of significant plumes at Shiveluch visible on satellite imagery from December 2010 through 26 March 2011 (times and dates are UTC). Courtesy KVERT.

Figure 27. Satellite thermal anomalies recorded at Shiveluch during December 2010-March 2011. Data from KB GS RAS, with cooperation from Alaska Volcano Observatory (AVO).

Figure 28. A panoramic photo showing a long ash plume from Shiveluch, seen in the distant parts of the photo (volcano is on the left). Photo taken on 24 February 2011 from N slope of Kliuchevskoi volcano by Yuri Demyanchuk.

More on the 27 October 2010PFs. As previously reported, an explosive eruption on 27 October 2010 (BGVN 35:11) vented at the dome and destroyed its SE portion, generating pyroclastic flows laden with many fragments of dome material (figure 29). The associated eruptive plume extended more than 1,500 km from the volcano. The pyroclastic flows traveled SSE in a radial direction, as far as 20 km from the source.

Figure 29. Two images showing the lava dome of Shiveluch. Photo (a) was taken before the eruption, on 7 October 2010. Photo (b) was taken a few days after the eruption, on 2 November 2010 and discloses enormous losses to the mass of the dome toward the SE (free face). The large ash clouds from the dome document ongoing explosions, processes associated with continued rebuilding of the lava dome. Both photos courtesy of Yuri Demyanchuk.

Near the dome, visiting scientists found agglomerate deposits of fragmental dome material spread widely down the SE slope. The character of the deposits was similar to debris avalanches, since so much dome material suddenly traveled down slope. The pyroclastic flow deposits retraced numerous upslope tributaries along the Kabeku River. The deposits filled small valleys and other low-lying areas, leveling landscapes that had prior to the eruption been rough (figure 30).

Figure 30. Photo showing the fresh pyroclastic flow deposits filling Bekesh river valley to the point where the valley had become nearly flat in transverse profile. In the background appears the steaming, Shiveluch with its recently broken lava dome. Photo taken 2 November 2010 by Alexander Ovsyannikov.

Figures 31a and b, satellite images, illustrate the trail of hot material descending to the S. They formed a large, complex, and widely distributed deposit following the recent collapse of the lava dome. A sub-circular area about ~4 km in diameter at about 9-14 km distance from the dome may reflect denser deposition (figure 31a). The images make clear that pyroclastic flow deposits descended yet farther, leaving dense, thermally radiant tracks over narrower valleys trending to the SE. The images are from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Figure 31b shows the flow's heat signature as measured in thermal infrared energy. The white area at the lava dome was very hot, while the red areas on the edge of the flow were merely warmer than the surrounding snow.

Figure 31. (a) False-color ASTER satellite image of Shiveluch showing the visible-wavelength information that discloses the remnants of the 27 October 2010 pyroclastic flow. Image taken 25 February 2011. (b) The hot pyroclastic flow appears in this ASTER image made using thermal infrared wave lengths. The white area at the lava dome is very hot, while the red areas on the edge of the flow are simply warmer than the surrounding snow. Image taken on 25 January 2011. Courtesy of NASA Earth Observatory.

Fieldwork in the distal area revealed that the most powerful pyroclastic flow went into the headwaters of two narrow valleys, then merged into a single stream down into the Kabeku Valley river almost to its confluence with the Bekesh river (5 km N of the Kluchi-Ust'-Kamchatsk road, figures 32 and 33).

Figure 32. Images (a) and (b) show Shiveluch deposits of pyroclastic flows in the Bekesh river valley. Note person in distance in center of photo for scale. Courtesy Yuri Demyanchuk and Alexander Manevich.

Figure 33. Results of pyroclastic surges, with small trees and shrubs knocked over and stripped of bark. Trees and shrubs showed signs of scorching up to 3-4 m high. Deposits of pyroclastic surges were found on the sides of the Bekesh river valley. Image taken 2 November 2010. Courtesy of Yuri Demyanchuk.

Water in the bed of the Bekesh river ran down the same path as thick pyroclastic flows and continued to be fed by melting snow on the upper slopes. Water also seeped through the loose pyroclastic flow deposit, resulting in large amounts of steam escaping at the surface in the form of fumaroles, degassing pipes, and zones of jetting emissions. This created the impression that the river water was boiling; on its surface rose a wall of steam (figure 34). Walking over the pyroclastic flow deposit was difficult and potentially dangerous, since the deposit's upper portion remained hot and gas saturated (figure 34b).

Figure 34. At Shiveluch, fresh pyroclastic-flow deposits occurring on the Bekesh river. (a) Steam and gas pervade the atmosphere as the river makes its way across the fresh pyroclastic-flow deposits. (b) The still-hot deposits emitting abundant steam and gas. Photos courtesy of Yuri Demyanchuk.

Reference. Ovsyannikov, A., Manevich, A., 2010, Eruption Shiveluch in October 2010, Bulletin of Kamchatka Regional Association (Educational-Scientific Center); Earth Sciences (in Russian), IV&S FEB RAS, Petropavlovsk-Kamchatsky, 2010, vol. 2, no. 16, ISSN 1816-5532 (Online).

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Institute Volcanolohy and Seismology Far East Division, Russian Academy of Sciences (IVS FED RAS), Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS) (URL: http://www.emsd.iks.ru/index-e.php). 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/); Y. Demyanchuk, A. Ovsyannikov, A. Manevich (IVS FED RAS); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/); Tokyo Volcanic Ash Advisory Centre (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).