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).

Semisopochnoi is located in the western Aleutians, is 20-km-wide at sea level, and contains an 8-km-wide caldera. The three-peaked Mount Young (formerly Cerberus) was constructed within the caldera during the Holocene. Each of these peaks contains a summit crater; the lava flows on the N flank appear younger than those on the S side. The current eruption period began in early February 2021 and has more recently consisted of intermittent explosions and ash emissions (BGVN 47:12). This report updates activity during December 2022 through May 2023 using daily, weekly, and special reports from the Alaska Volcano Observatory (AVO). AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Activity during most of December 2022 was relatively quiet; according to AVO no eruptive or explosive activity was observed since 7 November 2022. Intermittent tremor and occasional small earthquakes were observed in geophysical data. Continuous gas-and-steam emissions were observed from the N crater of Mount Young in webcam images on clear weather days (figure 25). On 24 December, there was a slight increase in earthquake activity and several small possible explosion signals were detected in infrasound data. Eruptive activity resumed on 27 December at the N crater of Mount Young; AVO issued a Volcano Activity Notice (VAN) that reported minor ash deposits on the flanks of Mount Young that extended as far as 1 km from the vent, according to webcam images taken during 27-28 December (figure 26). No ash plumes were observed in webcam or satellite imagery, but a persistent gas-and-steam plume that might have contained some ash rose to 1.5 km altitude. As a result, AVO raised the Aviation Color Code (ACC) to Orange (the second highest level on a four-color scale) and the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale). Possible explosions were detected during 21 December 2022 through 1 January 2023 and seismic tremor was recorded during 30-31 December.

Figure 25. Webcam image of a gas-and-steam plume rising above Semisopochnoi from Mount Young on 21 December 2022. Courtesy of AVO.

Figure 26. Webcam image showing fresh ash deposits (black color) at the summit and on the flanks of Mount Young at Semisopochnoi, extending up to 1 km from the N crater. Image was taken on 27 December 2022. Image has been color corrected. Courtesy of AVO.

During January 2023 eruptive activity continued at the active N crater of Mount Young. Minor ash deposits were observed on the flanks, extending about 2 km SSW, based on webcam images from 1 and 3 January. A possible explosion occurred during 1-2 January based on elevated seismicity recorded on local seismometers and an infrasound signal recorded minutes later by an array at Adak. Though no ash plumes were observed in webcam or satellite imagery, a persistent gas-and-steam plume rose to 1.5 km altitude that might have carried minor traces of ash. Ash deposits were accompanied by periods of elevated seismicity and infrasound signals from the local geophysical network, which AVO reported were likely due to weak explosive activity. Low-level explosive activity was also detected during 2-3 January, with minor gas-and-steam emissions and a new ash deposit that was visible in webcam images. Low-level explosive activity was detected in geophysical data during 4-5 January, with elevated seismicity and infrasound signals observed on local stations. Volcanic tremor was detected during 7-9 January and very weak explosive activity was detected in seismic and infrasound data on 9 January. Weak seismic and infrasound signals were recorded on 17 January, which indicated minor explosive activity, but no ash emissions were observed in clear webcam images; a gas-and-steam plume continued to rise to 1.5 km altitude. During 29-30 January, ash deposits near the summit were observed on fresh snow, according to webcam images.

The active N cone at Mount Young continued to produce a gas-and-steam plume during February, but no ash emissions or explosive events were detected. Seismicity remained elevated with faint tremor during early February. Gas-and-steam emissions from the N crater were observed in clear webcam images on 11-13 and 16 February; no explosive activity was detected in seismic, infrasound, or satellite data. Seismicity has also decreased, with no significant seismic tremor observed since 25 January. Therefore, the ACC was lowered to Yellow (the second lowest level on a four-color scale) and the VAL was lowered to Advisory (the second lowest level on a four-color scale) on 22 February.

Gas-and-steam emissions persisted during March from the N cone of Mount Young, based on clear webcam images. A few brief episodes of weak tremor were detected in seismic data, although seismicity decreased over the month. A gas-and-steam plume detected in satellite data extended 150 km on 18 March. Low-level ash emissions from the N cone at Mount Young were observed in several webcam images during 18-19 March, in addition to small explosions and volcanic tremor. The ACC was raised to Orange and the VAL increased to Watch on 19 March. A small explosion was detected in seismic and infrasound data on 21 March.

Low-level unrest continued during April, although cloudy weather often obscured views of the summit; periods of seismic tremor and local earthquakes were recorded. During 3-4 April a gas-and-steam plume was visible traveling more than 200 km overnight; no ash was evident in the plume, according to AVO. A gas-and-steam plume was observed during 4-6 April that extended 400 km but did not seem to contain ash. Small explosions were detected in seismic and infrasound data on 5 April. Occasional clear webcam images showed continuing gas-and-steam emissions rose from Mount Young, but no ash deposits were observed on the snow. On 19 April small explosions and tremor were detected in seismic and infrasound data. A period of seismic tremor was detected during 22-25 April, with possible weak explosions on 25 April. Ash deposits were visible near the crater rim, but it was unclear if these deposits were recent or due to older deposits.

Occasional small earthquakes were recorded during May, but there were no signs of explosive activity seen in geophysical data. Gas-and-steam emissions continued from the N crater of Mount Young, based on webcam images, and seismicity remained slightly elevated. A new, light ash deposit was visible during the morning of 5 May on fresh snow on the NW flank of Mount Young. During 10 May periods of volcanic tremor were observed. The ACC was lowered to Yellow and the VAL to Advisory on 17 May due to no additional evidence of activity.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).

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/).

Home Reef is a submarine volcano located in the central Tonga islands between Lateiki (Metis Shoal) and Late Island. The first recorded eruption occurred in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, a large volume of floating pumice, and an ephemeral island 500 x 1,500 m wide, with cliffs 30-50 m high that enclosed a water-filled crater. Another island-forming eruption in 2006 produced widespread pumice rafts that drifted as far as Australia; by 2008 the island had eroded below sea level. The previous eruption occurred during October 2022 and was characterized by a new island-forming eruption, lava effusion, ash plumes, discolored water, and gas-and-steam plumes (BGVN 47:11). This report covers discolored water plumes during November 2022 through April 2023 using satellite data.

Discolored plumes continued during the reporting period and were observed in true color satellite images on clear weather days. Satellite images show light green-yellow discolored water extending W on 8 and 28 November 2022 (figure 31), and SW on 18 November. Light green-yellow plumes extended W on 3 December, S on 13 December, SW on 18 December, and W and S on 23 December (figure 31). On 12 January 2023 discolored green-yellow plumes extended to the NE, E, SE, and N. The plume moved SE on 17 January and NW on 22 January. Faint discolored water in February was visible moving NE on 1 February. A discolored plume extended NW on 8 and 28 March and NW on 13 March (figure 31). During April, clear weather showed green-blue discolored plumes moving S on 2 April, W on 7 April, and NE and S on 12 April. A strong green-yellow discolored plume extended E and NE on 22 April for several kilometers (figure 31).

Figure 31. Visual (true color) satellite images showing continued green-yellow discolored plumes at Home Reef (black circle) that extended W on 28 November 2022 (top left), W and S on 23 December 2022 (top right), NW on 13 March 2023 (bottom left), and E and NE on 22 April 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Home Reef, a submarine volcano midway between Metis Shoal and Late Island in the central Tonga islands, was first reported active in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, large amounts of floating pumice, and an ephemeral 500 x 1,500 m island, with cliffs 30-50 m high that enclosed a water-filled crater. In 2006 an island-forming eruption produced widespread dacitic pumice rafts that drifted as far as Australia. Another island was built during a September-October 2022 eruption.

Information Contacts: Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ambae, also known as Aoba, is a large basaltic shield volcano in Vanuatu. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas. Periodic phreatic and pyroclastic explosions have been reported since the 16th century. A large eruption more than 400 years ago resulted in a volcanic cone within the summit crater that is now filled by Lake Voui; the similarly sized Lake Manaro fills the western third of the caldera. The previous eruption ended in August 2022 that was characterized by gas-and-steam and ash emissions and explosions of wet tephra (BGVN 47:10). This report covers a new eruption during February through May 2023 that consisted of a new lava flow, ash plumes, and sulfur dioxide emissions, using information from the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and satellite data.

During the reporting period, the Alert Level remained at a 2 (on a scale of 0-5), which has been in place since December 2021. Activity during October 2022 through March 2023 remained relatively low and mostly consisted of gas-and-steam emissions in Lake Voui. VMGD reported that at 1300 on 15 November a satellite image captured a strong amount of sulfur dioxide rising above the volcano (figure 99), and that seismicity slightly increased. The southern and northern part of the island reported a strong sulfur dioxide smell and heard explosions. On 20 February 2023 a gas-and-ash plume rose 1.3 km above the summit and drifted SSW, according to a webcam image (figure 100). Gas-and-steam and possibly ash emissions continued on 23 February and volcanic earthquakes were recorded by the seismic network.

Figure 99. Satellite image of the strong sulfur dioxide plume above Ambae taken on 15 November 2022. The Dobson Units (DU) exceeded 12. Courtesy of VMGD.

Figure 100. Webcam image of a gas-and-ash plume rising above Ambae at 1745 on 20 February 2023. The plume drifted SSW. Courtesy of VMGD.

During April, volcanic earthquakes and gas-and-steam and ash emissions were reported from the cone in Lake Voui. VMGD reported that activity increased during 5-7 April; high gas-and-steam and ash plumes were visible, accompanied by nighttime incandescence. According to a Wellington VAAC report, a low-level ash plume rose as high as 2.5 km above the summit and drifted W and SW on 5 April, based on satellite imagery. Reports in Saratamata stated that a dark ash plume drifted to the WSW, but no loud explosion was heard. Webcam images from 2100 showed incandescence above the crater and reflected in the clouds. According to an aerial survey, field observations, and satellite data, water was no longer present in the lake. A lava flow was reported effusing from the vent and traveling N into the dry Lake Voui, which lasted three days. The next morning at 0745 on 6 April a gas-and-steam and ash plume rose 5.4 km above the summit and drifted ESE, based on information from VMGD (figure 101). The Wellington VAAC also reported that light ashfall was observed on the island. Intermittent gas-and-steam and ash emissions were visible on 7 April, some of which rose to an estimated 3 km above the summit and drifted E. Webcam images during 0107-0730 on 7 April showed continuing ash emissions. A gas-and-steam and ash plume rose 695 m above the summit crater at 0730 on 19 April and drifted ESE, based on a webcam image (figure 102).

Figure 101. Webcam image showing a gas-and-ash plume rising 5.4 km above the summit of Ambae at 0745 on 6 April 2023. Courtesy of VMGD.

Figure 102. Webcam image showing a gas-and-ash plume rising 695 m above the summit of Ambae at 0730 on 19 April 2023. Courtesy of VMGD.

According to visual and infrared satellite data, water was visible in Lake Voui as late as 24 March 2023 (figure 103). The vent in the caldera showed a gas-and-steam plume drifted SE. On 3 April thermal activity was first detected, accompanied by a gas-and-ash plume that drifted W (figure 103). The lava flow moved N within the dry lake and was shown cooling by 8 April. By 23 April much of the water in the lake had returned. Occasional sulfur dioxide plumes were detected by the TROPOMI instrument on the Sentinel-5P satellite that exceeded 2 Dobson Units (DU) and drifted in different directions (figure 104).

Figure 103. Satellite images showing both visual (true color) and infrared (bands B12, B11, B4) views on 24 March 2023 (top left), 3 April 2023 (top left), 8 April 2023 (bottom left), and 23 April 2023 (bottom right). In the image on 24 March, water filled Lake Voui around the small northern lake. A gas-and-steam plume drifted SE. Thermal activity (bright yellow-orange) was first detected in infrared data on 3 April 2023, accompanied by a gas-and-ash plume that drifted W. The lava flow slowly filled the northern part of the then-dry lake and remained hot on 8 April. By 23 April, the water in Lake Voui had returned. Courtesy of Copernicus Browser.

Figure 104. Images showing sulfur dioxide plumes rising from Ambae on 26 December 2022 (top left), 25 February 2023 (top right), 23 March 2023 (bottom left), and 5 April 2023 (bottom right), as detected by the TROPOMI instrument on the Sentinel-5P satellite. These plumes exceeded at least 2 Dobson Units (DU) and drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone with numerous scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.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/); 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/).

Rogers and others (2012) reported on the presence of black smokers, diffuse venting, and associated chemosynthetically-driven ecosystems along the East Scotia Ridge (ESR), a geographically isolated back-arc spreading center in the Atlantic sector of the Southern Ocean, near Antarctica (figure 1). To their best knowledge, this was the first time that these features were observed at this location. Rogers and others (2012) noted that, since the discovery of hydrothermal vents along the Galápagos Ridge in 1977 (Corliss and others, 1979), scientists have detected “numerous vent sites and faunal assemblages at many mid-ocean ridges and back-arc basins...an apparent global biogeography of vent organisms with separate provinces.”

Figure 1. (Inset) Map of the Scotia Sea, showing the ESR in relation to the Scotia plate (SCO), the South Sandwich plate (SAN), the South American plate (SAM), the Antarctic plate (ANT), the Antarctic Peninsula (AP), and the South Sandwich trench (SST). Oceanographic features shown include the Polar front (PF), the Sub-Antarctic front (SAF), and the southern Antarctic Circumpolar Current front (SACCF). The vents sites E2 and E9, locations of the detailed studies discussed here, are indicated by red arrows. (Larger image) Map of the South Sandwich islands showing known active island volcanoes (red triangles) relative to the South Sandwich trench, and the East Scotia ridge (ESR) and submarine vents (E9) and fissure (E2, Dog’s Head) discussed here. Index map after Rogers and others (2012); ocean-floor base map from GEBCO, NOAA National Graphic, DeLorme, and ESRI.

Vent sites E2 and E9. The vent site E2 lies just S of the segment axial high (called the Mermaid’s Purse), between 56°5.2’ and 56°5.4’S and between 30°19’ and 30°19.35’W at ~2,600 m depth (figures 2A and 2B). Prominent N-trending structural fabric seen on the seafloor defines a series of staircased, terraced features that are divided by W-facing scarps (figures 2B and 2C). A major steep-sided fissure runs N-S through the center of the site, between longitude 30°19.10’W and 30°19.15’W (figure 2C). The main hydrothermal vents are located at the intersection between this main fissure and a W-striking fault or scarp, consistent with the expected location of active venting on back-arc spreading ridges such as the case at hand.

Figure 2. Swath maps of the location and setting of ESR vents. (A) Ship-based swath bathymetry at the location of site E2 showing the axial summit graben. The black circle indicates the sites of main venting. (B) and (C) ROV-based 3-D swath bathymetry of site E2 and high-resolution swath bathymetry of the major steep-sided fissure that runs N-S through the center of the site, between longitude 30°19.10’W and 30°19.15’W. Dog’s Head vent is indicated in panel C. White arrows indicate vents not mentioned in text. (D) Ship-based swath bathymetry at the location of site E9 showing the axial fissures and the collapsed crater called the Devil’s Punchbowl. The black spot indicates the sites of main venting. (E) ROV-based 3-D swath bathymetry of the site E9. The vents Ivory Tower, Car Wash, and Black and White are indicated. Other vents are indicated by white arrows. From Rogers and others (2012).

Relict (extinct) and actively venting chimneys were both resolvable in the high-resolution multibeam bathymetry obtained by the ROV (remotely operated vehicle) Isis, clustered in a band running approximately NW-SE. Numerous volcanic cones and small volcanic craters are also apparent around the vent field. Chimneys of variable morphology were up to 15 m tall and venting clear fluid with a maximum measured temperature of 352.6°C. These formed focused black smokers on contact with cold seawater (figure 3A).

Figure 3. Photographs of vents and associated biological communities. (A) Active black smoker chimneys at vents site E2 (2,602 m depth). Note the chimneys emitting dark-colored chemical-laden water into the seabed through vents, hitting cold seawater and causing metallic sulfides to precipitate. (B) Vent flange at E2 with trapped high-temperature reflective hydrothermal fluid (2,621 m depth). (C) Microbial mat covering rock surfaces on vent periphery at E2 (2,604 m depth). (D) Active vent chimney at vents site E9 supporting the new species of the anomuran yeti crab Kiwa (2,396 m depth). (E) Dense mass of the anomuran crab (Kiwa n. sp.) at E9 with the stalked barnacle (cf. Vulcanolepas) attached to nearby chimney (2,397 m depth). Scale bars: 10 cm for foreground. Courtesy of Rogers and others (2012).

Some of the chimneys have expanded tops with hot (above 300°C) vent fluid emanating from the underside (figure 3B), similar to the flanges found at North East Pacific vents. Diffuse vent flow was observed at a variety of locations, with temperatures varying from 3.5 to 19.9°C, compared with a background temperature of ~0.0°C. Around the periphery of the active high-temperature vents and diffuse flow sites are microbial mats that form a halo around the venting area at E2 (figure 3C).

Site E9 is situated between 60°02.5’ and 60°03.00’S and between 29°59’ and 29°58.6’W, at ~2,400 m depth, amongst relatively flat sheet lavas to the N of a major collapse crater named the Devil’s Punchbowl (figure 2D). The ridge axis is heavily crevassed and fissured, with numerous collapse features, lava drain-back features, and broken pillow lava ridges. Major fissures run NNW-SSE through the site, breaking up an otherwise flat and unvaried terrain (figure 2E).

Topographic highs in the center of the study site lack hydrothermal activity and thus are possibly inactive magma domes. Most active venting appears to lie along one of the smaller fissures, W of a main N-trending feature. Diffuse flow and black smokers line the feature intermittently, but activity becomes reduced and dies away farther S, towards the “Punchbowl.” The chimneys were either emitting high-temperature fluids with a maximum temperature of 382.8°C (Ivory Tower; figure 3E) or had lower temperature diffuse flow, between 5 and 19.9°C (Car Wash vent; figure 3E). Low-temperature diffuse flow was associated with fissures and fine cracks in the sheet lava; the background temperature at E9 varied from -0.11 to -1.3°C.

Deep-sea hydrothermal vents. The ESR vents can be seen in the broader context of deep-sea hydrothermal vents. Hydrothermal vents are essentially hot springs on the ocean floor.

Figure 4 shows the locations of many of the Earth’s known deep-sea hydrothermal vent systems. International Cooperation in Ridge-Crest Studies (InterRidge - a non-profit international organization promoting mid-ocean ridge research) created this map for the International Seabed Authority to show locations of vents that should be protected from exploitation.

Figure 4. Deep-sea hydrothermal vent systems that require protection from exploitation, according to InterRidge (Chown, 2012). Vent biogeographic provinces identified by Bachraty and others (2009) are displayed using color, and the two East Scotia Ridge vents sites described by Rogers and others (2012) are indicated with diamonds, just to the east of the Antarctic Peninsula. A full list of vent sites can be found on InterRidge’s web pages (see Information Contacts, below). The base map is the NOAA global relief model. Modified from Chown (2012); figure compiled by Aleks Terauds.

References. Bachraty C., Legendre, P., and Desbruyères, D., 2009, Biogeographic relationships among deep-sea hydrothermal vent faunas at global scale, Deep Sea Research, Part I, v. 56, no. 8, p. 1371-1378.

Chown, S.L., 2012, Antarctic marine biodiversity and deep-sea hydrothermal vents, PLoS Biology, v. 10, no. 1, e1001232. doi:10.1371/journal.pbio.1001232 (URL: http://www.plosbiology.org/article).

Corliss, J.B.. Dymond, J., Gordon, L.I., Edmond, J.M., von Herzen, R.P., Ballard, R.D., Green, K., Williams, D., Bainbridge, A., Crane, K., and van Andel, T.H., 1979, Submarine thermal springs on the Galapagos Rift, Science, v. 203, no. 4385, p. 1073-1083. doi: 10.1126/science.203.4385.1073.

InterRidge, 2012, InterRidge Vents Database (URL: http://www.interridge.org/irvents).

Rogers, A.D., Tyler, P.A., Connelly, D.P., Copley, J.T., James, R., Larter, R.D., Linse, K., Mills, R.A., Garabato, A.N., Pancost, R.D., Pearce, D.A., Polunin, N.V.C., German, C.R., Shank, T., Boersch-Supan, P.H., Alker, B.J., Aquilina, A., Bennett, S.A., Clarke, A., Dinley, R.J.J., Graham, A.G.C., Green, D.R.H., Hawkes, J.A., Hepburn, L., Hilario, A., Huvenne, V.A.I., Marsh, L., Ramirez-Llodra, E., Reid, W.D.K., Roterman, C.N., Sweeting, C.J., Thatje, S., and Zwirglmaier, K., 2012, The discovery of new deep-sea hydrothermal vent communities in the Southern Ocean and implications for biogeography, PloS Biology, v. 10, no. 1, e1001234. doi: 10.1371/journal.pbio.1001234 (URL: http://www.plosbiology.org/article).

Geologic Background. Reports of floating pumice from an unknown source, hydroacoustic signals, or possible eruption plumes seen in satellite imagery.

Information Contacts: International Cooperation in Ridge-Crest Studies (InterRidge) (URLs: http://www.interridge.org; http://www.interridge.org/irvents); VENTS Program, Pacific Marine Environmental Laboratory (PMEL), National Oceanographic and Atmospheric Administration (NOAA) (URL: http://www.pmel.noaa.gov/vents/).

Gamalama volcano, Indonesia, erupted on 4 December 2011, following precursory gas emissions and an increase in seismicity. Lahars killed at least four people, injured dozens, and thousands evacuated. Gamalama had remained at Alert Level 2 (on a scale from 1-4) since 11 May 2008 (BGVN 33:10). Coincident with the beginning of the eruption at 2300 on 4 December, CVGHM raised the Alert Level from 2 to 3, prohibiting access to areas within 2.5 km of the summit. In late January seismicity stabilized and the hazard status fell.

Precursory activity. The Center for Volcanology and Geological Hazard Mitigation (CVGHM) reported white plumes reaching 25 and 150 m above the summit of Gamalama on 1 and 4 December, respectively (figure 1). Clouds obscured the view on 2-3 December. Seismicity also increased during 1-4 December, with a sharp increase in the occurrence of shallow volcanic earthquakes, from one on 3 December to 47 on 4 December (table 2). Tremor was recorded continuously after 2258 on 4 December. At 2300, the Alert Level was raised to 3, and access to Hazard Zone II (areas within 2.5 km of the summit) was prohibited.

Figure 1. Reported plume heights at Gamalama during 1-14 December 2011. No plumes were reported by the Center for Volcanology and Geological Hazard Mitigation (CVGHM) or the Darwin Volcanic Ash Advisory Centre (VAAC) on 2-3 and 10-12 December. Plumes heights indicated in white were ash-free emissions, while those in black indicate plumes that contained ash. The Alert Level was raised from 2 (yellow) to 3 (orange) at 2300 on 4 December. Data courtesy of CVGHM and Darwin VAAC.

Table 2. Precursory seismicity during 1-4 December 2011 at Gamalama. Note the sharp increase of shallow volcanic earthquakes on 4 December 2011; that day, tremor amplitude also increased by at least an order of magnitude. The symbol '--' indicates data not reported. Data courtesy of CVGHM.

Eruption. According to the Jakarta Post, most residents living on Gamalama's slopes evacuated, although some insisted on staying in their homes. Most of Ternate and its surrounding villages were covered in ash (figure 2), and ash fall caused the loss of electricity in some areas around the slopes of the volcano. No fatalities were reported.

Figure 2. Residents in the Tubo district (3-4 km from the summit) walking on recently deposited (and most likely reworked) volcanic material that fell or was remobilized after an eruption of Gamalama. Photograph dated 5 December 2011; courtesy of Associated Press.

Over the next 10 days (into mid-December) the Darwin Volcanic Ash Advisory Centre (VAAC) reported ash plumes that rose to 2.1-6.1 km altitude (figures 1 and 4). Some plumes drifted up to 140 km to the S, SE, and E. Three photos of plumes on 12 December appear in figure 3.

Figure 3. Photos of ash-bearing eruptive plumes from Gamalama taken on 12 December 2011. Courtesy of Andi Rosadi, Volcano Discovery.

Fatal lahar. The Jakarta Post reported that heavy rainfall mobilized fresh ash deposits, spawning a lahar on 27 December 2011 that killed at least four people and injured dozens; many homes were destroyed in the Tubo and Tofure districts, and in locations along the Togorara and Marikurubu rivers (figure 4). On 1 January 2012, the Jakarta Post reported that up to 3,490 people were still being housed in ten different emergency shelters. It also reported that the National Disaster Mitigation Agency (Badan Nasional Penanggulangan Bencana, BNPB) had allocated 1.1 billion Indonesian Rupiah (US$121,000) in emergency funds for the residents affected by the eruption. The Jakarta Globe reported that thousands of farmers had their crops destroyed by ash erupted during December 2011. Agricultural losses are especially devastating, as the island has historically been a major producer of spices such as cloves.

Figure 4. Combined Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery of Gamalama (Ternate Island) on 17 April 2005 and 30 November 2006. Ternate City, the districts of Tubo and Tofure, and the Togorara and Marikurubu rivers are indicated. Index map shows regional location. ASTER imagery courtesy of the Geological Survey of Japan; index map modified from MapsOf.net.

Eruption wanes. Following a month of decreasing activity, CVGHM decreased the Alert Level from 3 to 2 on 24 January 2012. The Alert Level notification cited that, since 23 December 2011, seismicity was dominated by tremor with relatively stable amplitude (0.5-2 mm) and hot air blasts that tended to decrease in occurrence (table 3). During the same period, observed plumes from Gamalama reached 25-100 m above the summit, none of which contained observable ash. In consequence of the lowered Alert Level, access to the summit craters of Gamalama was prohibited, and residents living along rivers descending the flanks of the volcano were advised to be aware of the dangers of lahars. In addition, the North Maluku Province Local Government was asked to prepare evacuation procedures in the case of an increase in activity.

Table 3. Seismicity at Gamalama from 24 December 2011 through 23 January 2012. CVGHM lowered the Alert Level from 3-2 on 24 January. Data courtesy of CVGHM.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the extensive documentation of activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano; the S-flank Ngade maar formed after about 14,500–13,000 cal. BP (Faral et al., 2022). Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.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/); The Jakarta Post, Jl. Palmerah Barat 142-143, Jakarta 10270, Indonesia (URL: http://www.thejakartapost.com/); Associated Press (AP) (URL: http://www.apimages.com/); Andi Rosadi, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Erik Klemetti/Wired (URL: http://www.wired.com/wiredscience/eruptions); Geological Survey of Japan (URL: http://www.gsj.jp/); MapsOf.net (URL: http://mapsof.net/); The Jarkarta Globe, Citra Graha Building, 11th Floor, Suite 1102, Jl. Jend. Gatot Subroto Kav 35-36, Jakarta 12950, Indonesia (URL: http://www.thejakartaglobe.com/).

This report mainly summarizes information on Guagua Pichincha conveyed in 2008 to 2010 yearly reports by the IG-EPN (Instituto Geofísico Escuela Politécnica Nacional). In broad terms, and with the exceptions of an anomalously high number of emission and explosion signals in 2009, Guagua Pichincha volcanic activity continued to decline since the eruptions during September 1999 to June 2001. Further, the volcano has cooled and crater morphology, as stated in IG-EPN yearly reports, has remained relatively unchanged since 2002 (Samaniego,P, 2006, and 2007-2010 yearly reports). Nevertheless, it is possible for further emissions and explosions to occur as potential hazards to life and property. Especially since Guagua Pichincha (figures 22 and 23) is 11 km from the capital, Quito, a city with a population of over 2.5 million (as estimated by the Metropolitan District of Quito population projection, Directorate of Territorial Planning and Public Services). Our previous report on the volcano (BGVN 32:12) discussed phreatic explosions that occurred in early 2008. This report includes seismic data plots, locations of events on topographic maps and a multi-year seismic table beginning in the year 2005.

Figure 22. Map showing proximity of Quito to Guagua Pichincha. Courtesy of Google Earth.

Figure 23. Photograph of Guagua Pichincha's crater taken in May 2008, showing the still-active year 1660 dome and adjacent crater floor. The area is heavily pockmarked with explosion craters (labeled). Note sampled fumarole (bottom left). Photo courtesy of J. Bustillos (IG-EPN 2008 annual report).

During the 2008-2010 reporting interval, the IG yearly reports cited fumarolic emissions, surfurous odors, and noise at various locations within the crater, including the 1660 dome, and the 1981 and 2002 craters. As discussed below, rainfall often correlated with phreatic eruptions during 2008 and 2009.

Seismicity is monitored using five short-period (1 Hz) seismic stations, of which three are single-component stations (GGP, JUA2, YANA) and two are three-components stations (PINE, TERV).

Low seismicity generally prevailed during 2003-2010, with few long-period (LP) and hybrid (hb) earthquake occurrences (figure 24). Compared to 2003 to 2005 the number of volcano-tectonic (VT) earthquakes increased during 2006 to 2010 (figure 24).

Figure 24. Guagua Pichincha volcano seismic event data from 2002 to 2010, shown in the number of events. Above the plot, earthquakes and periods of emission are indicated by arrows. Multiple events that happened closely spaced in time are shown by a single arrow. Data courtesy of IG-EPN (2008-2010 annual reports).

During the period from 2005 to 2010 (table 11) the annual number of total seismic events generally remained in the range of several hundred to over 1,700. Seismically detected emission signals (phreatic outbursts) were recorded less than 25 times per year. The number of emissions in 2008 and 2009 were the largest in the years in discussion, 20 and 24 events respectively. At most, several explosions (producing non-juvenile ash found in vicinity of the crater) were recognized each year but three years had zero. More details on the 2008, 2009, and 2010 reports follows.

Table 11. Seismic data for Guagua Pichincha from IG-EPN 2005 to 2010 yearly summaries. Note the explosion column, which was often low, under three per year. IG-EPN attributed the emission cases to phreatic eruptions, in the explosion cases they recognized non-juvenile ash at the crater. The value for emissions in 2009 corrects those in the 2009 IG-EPN report. Data courtesy of IG-EPN.

2008 seismicity. The three explosion events in 2008 took place on 27 January (two events) and on 5 May (one event). 2008 seismicity remained at a similar level as in 2007, with increased earthquakes in January and May, 326 and 299, respectively (figure 24). These two months had appreciable numbers of located events compared to other months. The locations of events tended to fall along trends to the WNW and NE. The WNW group is distributed in a line that runs from the N of the caldera to the foothills of Pichincha, following the Rumipamba gorge (figure 25a), which deepens towards the E. Epicenters of the NE group fall in a line on and near the caldera (figure 25a).

Figure 25. Located earthquakes (colored dots) at Guagua Pichincha presented as a series of annual maps: 2008 (4a), 2009 (4b), and 2010 (4c). The colors indicate accuracy and are listed as follows from highest to lowest accuracy: pink, red, blue, green. Courtesy of IG-EPN.

2009 seismicity. The first half of the year was the most seismically active and ~77% of the total earthquakes occurred then (figure 24). Of the hundreds of events recorded for 2009, only 63 could be located. Their foci occurred below the crater around 7 km depth. Vapor-associated emissions mainly occurred during the first several months of the year (figure 24), coinciding with the rainy season. The highest number of emission events were on 16 February, 7 March, and 11 March.

2010 seismicity. No explosions occurred in 2010. Of the events recorded, 161 were localized near the crater (figure 25c). These recorded events were mainly grouped under the crater and to the NE with a majority of near depths of 7 km. Another group, fewer in number, was located and aligned E of the caldera (figure 25c). IG related emission events to existing heat inside the volcano interacting with groundwater.

Correlation of phreatic explosions and the rainy season. The occurrence of phreatic explosions and emissions appears to be related to the rainy season at the beginning of the year (SEAN 07:06, BGVN 18:02, 24:02, 24:11, 29:06, and 32:12). This behavior was most-recently reported on by the IG in 2008 and 2009. A possible model for the interaction of rain water with the volcanic system can be found in BGVN 24:11.

2008-2010 cooling and morphologic stability. Continued cooling of the dome was indicated by the temperatures recorded in situ from November 2000 to 2005 in the IG 2005 report. It was concluded the dome shows no thermal anomalies. IG 2010 ASTER TIR images are consistent with information from previous years and show continued cooling. In addition to undergoing continual cooling, the crater morphology has remained relatively unchanged since the formation of an additional crater in 2002. The IG concluded that Guagua Pichincha was generally becoming less active over time. However, they noted that it is possible for further emissions and explosions to occur that could possibly threaten Quito.

Reference. Samaniego, P; Robin, C; Monzier, M; Mothes,P; Beate; B; Garcia, 2006, Guagua Pichincha Volcano Holocene and Late Pleistocine Activity, Cities on Volcanoes, Fourth Conference; IAVCEI, Quito Equador, (URL: http://www.igepn.edu.ec/images/collector/collection/biblioteca/guaguapichincha_ field_guide.pdf).

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Information Contacts: Instituto Geofísico Escuela Politécnica Nacional (IG-EPN), Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Observatorio Vulcanológico Pichincha (OVGGP) (URL: http://www.igepn.edu.ec/index.php/nuestro-blog/item/158).

Ijen, which hosts both the world's largest highly acidic lake and intensive sulfur mining operations, showed increased seismicity and SO₂ emissions during October-December 2011. The increased activity caused the Center for Volcanology and Geological Hazard Mitigation (CVGHM) to raise the Alert Level from 1-2 (on a scale from 1-4) on 15 December. The Alert Level was then raised from 2-3 on 18 December following further increases in activity.

1 October-15 December 2011 activity. CVGHM reported increased seismicity beginning in October 2011. Seismicity remained increased, yet more-or-less constant, through 15 December (figure 12a). Shallow volcanic earthquakes showed the greatest increase. The onset of harmonic tremor was reported during the first week of December, and increased tremor amplitude was reported beginning on 5 December.

Figure 12. Reported seismicity (a) and crater lake temperatures (b) at Ijen during 1 October-17 December 2011. The Alert Level remained at 1 (green) until 15 December when it was raised to 2 (yellow); it was further increased to 3 (orange) on 18 December. Data courtesy of the Center for Volcanology and Geological Hazard Mitigation (CVGHM).

Measured temperatures of the crater lake waters were mostly stable during October (ranging from 30.6-31.2°C), but showed significant variation and increased maximum temperatures during November and December 2011 (figure 12b). The measured pH of the crater lake waters also showed an increase during October-November, rising from 0.7±0.1 in October to 0.83±0.04 in November.

CVGHM also reported blasts of hot air and smoke that generated small plumes rising to 50-100 m above the peak in October, 50-150 m above the peak in November, and 50-200 m above the peak in December, outlining an increasing trend in the energy of the blasts. Plumes in October and November were reported to be sparse to medium white, while those in December were reported to be white to brown, indicating possible ash content in plumes generated during December.

During 1 October-15 December 2011, the color of the crater lake water remained whitish light green, and bubbling water was observed in the center of the lake. The area of bubbling water measured approximately 5 m in diameter. Clumps of sulphur were reported to coalesce in the center and on the shores of the crater lake. Vegetation in areas around the crater remained healthy.

On 15 December, CVGHM raised the Alert Level to 2, citing increased shallow and deep volcanic seismicity, the onset and increased amplitude of harmonic tremor 10 days prior, and visual observations as cause for concern. The CVGHM report expressed concern about possible phreatic, mud, or ash eruptions, and prohibited access to within 1 km of the crater lake.

Increased SO₂ emissions. During the next few days, a sharp increase in shallow and deep volcanic seismicity (figure 12a) was accompanied by increased SO₂ emissions. Observation on 17 December revealed the strong smell of sulphurous gases in the vicinity of the crater; so strong, in fact, that the CVGHM reported that measurements of lake water temperatures had become difficult without wearing a mask. The lake waters had changed color from whitish light green to completely white. All observations indicated an increased concentration of SO₂ in the crater lake.

On 18 December, CVGHM raised the Alert Level to 3, and prohibited access to within 1.5 km of the crater lake. The Jakarta Post reported that the National Disaster Mitigation Agency (Badan Nasional Penanggulangan Bencana, BNPB) had prepared 466 million Indonesian Rupiah (US$51,260) in disaster-relief funds for the basic needs of evacuees for a two week period in the case that an evacuation occurred.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the rim was buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Kawah Ijen is the site of a labor-intensive mining operation in which baskets of sulfur are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor; nearby waterfalls and hot springs are tourist destinations.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.go.id/); The Jakarta Post, Jl. Palmerah Barat 142-143, Jakarta 10270 (URL: http://www.thejakartapost.com/).

Plumes and seismic activity at Lewotolo volcano, Indonesia, increased during December 2011 and early January 2012. Lewotolo has erupted potassic calc-alkaline lavas containing as an accessary phase in vessicle fillings, the rare, complex zirconium-titanium-oxide mineral zirconolite (Ca_0.8 Ce_0.2 Zr Ti_1.5 Fe²⁺_0.3 Nb_0.1 Al_0.1 O₇; de Hoog and van Bergen, 2000). Lewotolo last erupted in 1951. All historical eruptions were small (Volcanic Explosivity Index, VEI 2) with the exception of the first recorded eruption, which took place in 1660 and was as large as VEI 3. According to de Hoog and van Bergen (2000), strong fumarolic activity at the summit of Lewotolo indicates the presence and degassing of a shallow magma chamber.

December 2011-January 2012 activity increase. According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), Lewotolo produced thick white plumes reaching 50-250 m above the summit during December 2011. Seismicity increased on 31 December, and intensified on 2 January 2012 with tremor commencing at 1400. Accordingly, CVGHM raised the Alert Level from 1 to 2 (on a scale from 1-4) at 1800 on 2 January. Between 1800 and 2300 the same day, the maximum amplitude of recorded seismicity increased, and at 2000, incandescence was noticed at the summit.

At 2330 on 2 January, CVGHM increased the Alert Level to 3. Under the recommendation of CVGHM, access was prohibited within 2 km of Lewotolo (Hazard Zone III, figure 1), and residents in villages SE of the volcano were advised to keep vigilant and secure a safe place to flee to one of the towns to the N, W, or S in the event of an eruption.

Figure 1. Map of areas around Lewotolo showing Hazards Zones I-III. Hazard Zone I includes areas possibly threatened by ash fall and incandescent bombs (within 7 km of Lewotolo, yellow dashed circle) and areas possibly affected by lahars (shaded yellow). Hazard Zone II includes areas possibly threatened by heavy ash-fall and incandescent bombs (within 4 km of Lewotolo, dark pink dashed circle) and areas possibly affected by pyroclastic flows, lava flows, and lava avalanches (shaded light pink). Hazard Zone III includes areas very likely to be threatened by heavy ash fall and incandescent bombs (within 2 km of Lewotolo, light pink dashed circle) and areas very likely to be affected by pyroclastic flows, lava flows, lava avalanches, and volcanic gases (shaded dark pink). Other symbols are explained in the legend at the right. Authorities prohibited access to Hazard Zone III on 2 January 2012. Modified from CVGHM.

Residents decide to evacuate. According to Antara News, evacuations began on 4 January spurred by increased activity of the previous few days, as well as minor ash falling in the villages. Antara News stated that most of the residents went to Lewoleba, the closest city to the volcano (~15 km to the SW of the summit). Of the evacuees in Lewoleba, all but about 50 people were reported to have found temporary housing with other residents of the city.

On 5 January, Channel 6 News reported that around 500 residents had evacuated leaving their homes in villages surrounding Lewotolo. They noted that residents who evacuated did so on their own accord, as the government had not yet called for evacuation. The Deputy District Chief of Lembata, Viktor Mado Watun, said "Black smoke columns are coming out of the mountain's crater, the air is filled with the smell of sulfur while rumbling sounds are heard around the mountain."

According to UCA News on 9 January, the health of the evacuees was cause for concern. Father Philipus da Gomez stated that "there are many refugees who have started suffering from acute respiratory infections."

Alert Level lowered. On 25 January 2012, CVGHM lowered the Alert Level of Lewotolo from 3 to 2 following decreased activity after 2 January. The lowered Alert Level restricted access to the summit craters only. CVGHM stated that the observed seismicity (table 1) showed a declining trend, tending towards normal conditions after 23 January. Visual observation revealed thick, white plumes reaching 400 m above the summit during 2-14 January (and a dim crater glow), and thin white plumes reaching no more than 50 m above the summit during 16-24 January (with no accompanying crater glow).

Table 1. Seismicity at Lewotolo during 3-24 January 2012, showing a declining trend in seismicity prior to CVGHM's lowering of the Alert Level from 3-2 on 25 January. Data courtesy of CVGHM.

On 15 January, direct observation of the crater was made, and revealed incandescence in solfataras, a weak sulfur smell, and hissing sounds in both the N and S side of the crater. CVGHM especially noted that the N side of the crater was quite different than when it was last observed in June 2010, when no solfataras were present. Differential Optical Absorption Spectroscopy (DOAS) measurements revealed fluctuating and increasing SO₂ flux between 11-90 tons/day during 8-16 January.

References. de Hoog, J.C.M. and van Bergen, M.J., 2000, Volatile-induced transport of HFSE, REE, Th, and U in arc magmas: evidence from zirconolite-bearing vesicles in potassic lavas of Lewotolo volcano (Indonesia), Contributions to Mineralogy and Petrology, v. 139, no. 4, p. 485-502 (DOI: 10.1007/s004100000146).

Geologic Background. The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano's high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.go.id/); Channel 6 News (URL: http://channel6newsonline.com/); Antara News, Wisma ANTARA 19th Floor, Jalan Merdeka Selatan No. 17, Jakarta Pusat (URL: http://www.antaranews.com/); UCA News, Yayasan UCINDO, Gedung Usayana Holding, Lt.3, Jl. Matraman Raya No.87, Jakarta Timur 13140 (URL: http://www.ucanews.com/).

Previously reported activity at San Cristóbal, from April 2006 to June 2010, included ash plumes and degassing (BGVN 35:04). Here we describe several substantial explosions during 2010, in addition to ash plumes that occurred without precursory activity (in 2010 and 2011). Based on Instituto Nicaragüense de Estudios Territoriales (INETER) reports, we compiled significant located seismic events for January 2010 through October 2011 and also present gas monitoring results for May 2010 through September 2011.

INETER prepared an additional report along with their monthly review of volcanic activity in December 2010. They highlighted five distinct explosive episodes at San Cristóbal's summit in April, July, September, and December 2010 and also characterized long-term unrest. During the last few decades, activity at San Cristóbal had been dominated by constant gas emissions, small ash and gas explosions, high seismicity, and specifically tremor. Prior to activity in 2010, large explosions and elevated seismicity had occurred in November 1999 (BGVN 25:02) and more recently in April 2006 (BGVN 31:09 and 35:04). Since that time, there have been smaller explosions and regular degassing.

Earthquake followed by explosion signals in April 2010. In early 2010, San Cristóbal produced increasing amounts of gas. From January through March, temperatures measured from fumaroles within the crater generally increased (figure 18). In April, seismicity was similar to the previous months: frequent tremor episodes, occasional volcanic-tectonic events with low amplitudes, and rare long-period events. On 8 April two earthquakes, M_L 3.1 and 2.9, suddenly occurred beneath the S side of the volcano and local residents reported shaking in nearby towns (table 3). Following the largest, shallow earthquake a small explosion was recorded. Another explosion occurred on 18 April but the seismic record was incomplete due to problems with the station. By 27 April, reports from field investigators described quiescence within the crater (BGVN 35:04).

Figure 18. Fumarole temperatures from San Cristóbal measured throughout 2010 by INETER scientists. Note some data gaps for Fumarole 5 and Fumarole 3. Courtesy of INETER.

Table 3. The date, local magnitude (M_L), and depth to epicenters are listed for significant earthquakes located near San Cristóbal. No locations were determined for January and February 2010 or November and December 2011. Courtesy of INETER.

In May and June 2010 San Cristóbal was relatively quiet. Field measurements determined that fumarole temperatures were variable. The 3-station Mini-DOAS array detected relatively low levels of sulfur dioxide; INETER reported 274 tons/day (table 4). Visual observations determined that degassing was more vigorous in June and, while banded tremor had been recorded in May, seismicity was also higher in June. On 15 June, more than 12 hours of tremor were recorded.

Table 4. The average SO₂ flux per sampling period in metric tons per day from San Cristóbal measured with Mini-DOAS from May 2010 to September 2011. Courtesy of INETER.

Significant ashfall from 2 July explosions. Elevated seismicity continued into July 2010 and was dominated by low-amplitude events. On 2 July an explosion from the summit crater released a low-altitude plume of ash (described as a "mushroom cloud" in news reports) that drifted over villages located W of the volcano. Local residents heard explosions and observed a dense ash plume sustained for ~20 minutes. Ash was accompanied by ejected incandescent blocks (reporters noted that block sizes were up to 10 meters in diameter) that scattered across the summit area and started grass fires. Field investigations by INETER on 24 July found that light ash had remained on foliage and grass and there were charred trees below the summit area. Civil Protection noted that ashfall had reached these towns and districts within a 10 km radius of the crater: Las Grecias, El Piloto, El Chonco, Mokorón, and Villa. Comarca Las Grecias is located WSW of San Cristóbal (figure 19).

Figure 19. The extent of ashfall from San Cristóbal frequently reached towns W and SW of the volcanic edifice in 2010 and 2011. Light ash from the 2 July 2010 event fell on Comarca Las Grecias (~12 km SW of the summit) and other locations not marked on this map. The explosive event from 23 October 2011 caused ashfall at four sites marked here: Comarca Las Grecias, El Viejo, Chinandega (regional capital), and El Realejo (~25 km from the summit). Courtesy of INETER.

Plumes and advisories. On 20 August 2010, a volcanic ash advisory was released for the N sector of San Cristóbal (table 5). The GOES-13 satellite detected a plume of gas and potentially light ash drifting from the summit over 35 km N. No associated activity was detected by local instrumentation that day although 10 minutes of tremor and several volcanic-tectonic (VT) events were recorded on 6 August. INETER field investigators visiting the summit on 22 August 2010 reported strong degassing and frequent rockfalls from the crater rim.

Table 5. Ash plumes from San Cristóbal reported by the Washington Volcanic Ash Advisory Center (VAAC) for June 2010 through August 2011. The 9 June event was the first to occur in 2010 and no additional reports were issued in 2011 after 21 August.

Late 2010-early 2011 observations. Seismic activity in September 2010 was sparsely recorded due to intermittent equipment errors (local GPS malfunctioned) but seismicity from 21 September corroborated observations of activity from San Cristóbal. A series of small explosions occurred, beginning early on 21 September. Reports from Civil Defense based in Chinandega described rumbling sounds from the crater (lasting up to 20 minutes). Ashfall reached the regional capital as well as the town of El Viejo to the NW (figure 19).

INETER teams visited San Cristóbal in October and November 2010 and measured fumarole temperatures (figure 18). The team also observed strong gas emissions from the summit. Numerous rockfalls from the crater walls had occurred in October. Some tremor was recorded in October and sporadic seismicity continued into November. On 6 November, one hour of tremor was recorded. Earthquakes occurred more frequently toward the end of the month. Interesting sequences of VT events were recorded that lasted 15-20 minutes with frequencies of 3-5 Hz.

In early December 2010, seismicity gradually increased. Long-period events (LP) dominated the record and some VTs were recorded with frequencies of 1-3 Hz. Without any apparent precursory activity, a small explosion was recorded on 13 December at 0638 (figure 20).

Figure 20. Seismicity on 13 December 2010 from San Cristóbal. The impulsive explosion was recorded at ~0638 from seismic station CRIN. Courtesy of INETER.

An ash plume was reported by a local pilot at the time of the seismic signature. Elevated seismicity did not occur until after the explosion, when low-frequency tremor appeared in the records. Three subsequent volcanic ash advisories were issued by the Washington VAAC for the area on 15, 17, and 23 December (table 5).

Dense plumes of gas were emitted in early January 2011 and reported by Washington VAAC (table 5). Low-altitude plumes (2.1 km) and cloudless days provided excellent conditions for INETER scientists to detect SO₂ flux on 21 January 2011. Traverses under the plume with a mobile Mini-DOAS collected data along points between Chinandega (SW of San Cristóbal) and Las Grecias (to the NW). INETER discussed the slight increase (~200 tons/day since December 2010, table 4) in SO₂ in their monthly report and attributed elevated emissions to the general increase in seismicity during the last few months (table 3) and to changes in the volcano's structure.

Throughout 2011, field investigations by INETER included monitoring fumarole temperatures within the summit crater (figure 21). During 2011, temperatures from five separate fumaroles ranged between 50 and 90°C. Similar to measurements taken in 2010, intermittent values were recorded for Fumarole 5 (Fumarole 4 was also intermittent, no measurable value in June). Data collection was not possible in November and measurements in December clustered at comparatively elevated temperatures of 80 and 90°C.

Figure 21. Fumarole temperatures from San Cristóbal measured throughout 2011 by INETER scientists. Some data gaps for Fumaroles 4 and 5; no measurements were taken in November. Courtesy of INETER.

Within the summit crater during 2011, investigators found evidence of rockfalls as well as ground cracks at the crater rim. INETER described gradual accumulation of debris on the crater floor from February through April. During a field visit in May, two small pools of water had appeared within the crater. These features persisted from May through July.

Ash event without unrest. A sudden ash explosion was reported by Chinandega Civil Defense at 1900 on 23 October 2011. Ash fell over Chinandega (the regional capital) as well as El Viejo, El Realejo, and the district of Las Grecias (figure 19). Minor tremor events occurred during the day but signals suggesting explosions were absent. Tremor continued to appear in the seismic record during November through the end of December.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); La Prensa (URL: http://www.laprensa.com.ni/2010/07/04/nacionales/30240); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/nacionales/78105).

Seismicity at Seulawah Agam volcano, Indonesia, caused the Center for Volcanology and Geological Hazard Mitigation (CVGHM) to raise the Alert Level from 1 to 2 (on a scale from 1-4) from 1 September 2010 through 11 July 2011. According to historical records, Seulawah Agam last erupted in 1839, although the likelihood and character of that eruption is in debate.

The summit of Seulawah Agam hosts a forested crater ~400 m wide (figure 1). The volcano also hosts several active fumarole fields, such as those in the van Heutsz crater, which sits on the NNE flank at ~650 m elevation (figure 2).

Figure 1. (Index map) The location of Seulawah Agam at the NW end of Sumatra island. (photo) Annotated aerial photograph of Seulawah Agam taken on 19 November 2007 looking SE, showing the ~400-m-wide, vegetated summit crater (white dashed outline). Photograph courtesy of Michael Thirnbeck; index map modified from MapsOf.net.

Figure 2. Hazard map of Seulawah Agam. Hazard Zones I-III (from outer to innermost) consist of both circular areas (indicating hazards from material dispersed through the air) and irregularly shaped areas (funneled by topography along the ground). Courtesy of the Center for Volcanology and Geological Hazard Mitigation (CVGHM).

The hazard zones, as with all other monitored Indonesian volcanoes, concern airborne ejected/explosive material (circular zones delineating areas prone to ash fall and/or pyroclastic bombs) and ground-traveling, topographically controlled processes (irregular shaped zones delineating areas prone to lava flows, pyroclastic flows, and/or lahars); each Hazard Zone level (I-III) thus delineates a circular and an irregular area. At Seulawah Agam, the hazard zones are centered at the summit of the volcano. The van Heutsz crater, however, is located outside of the 2 km radius of Hazard Zone III, but within the topographically prone area of Hazard Zone III.

Seismicity increase. Beginning in April through September 2010 seismicity fluctuated at Seulawah Agam, although increased overall, indicating increased activity of the volcano. The Jakarta Post reported that CVGHM recorded 80 volcanic earthquakes during August 2010, the equivalent of nearly 3 volcanic earthquakes per day. On 1 September, CVGHM raised the Alert Level to 2, and restricted access to areas within 3 km of the summit crater (figure 2).

According to CVGHM, seismicity fluctuated at elevated levels from October 2010 through June 2011. In July, seismicity was still elevated above the baseline during October 2010-June 2011. However, the occurrence of shallow volcanic earthquakes was reduced compared to recent trends (table 2).

Table 2. Seismicity at Seulawah Agam during 1 October 2010-10 July 2011. The Alert Level was lowered from 2 to 1 (on a scale from 1-4) on 11 July 2011. Data courtesy of the Center for Volcanology and Geological Hazard Mitigation (CVGHM).

CVGHM also reported that comparison of data from October 2010 and February 2011 indicated a decline in the emission of volcanic gases, a stabilization of the pH of crater waters, and a decrease in the measured temperature of fumaroles. On 11 July 2011, CVGHM lowered the Alert Level to 1, restricting access only to the summit crater.

Geologic Background. Seulawah Agam at the NW tip of Sumatra is an extensively forested volcano of Pleistocene-Holocene age constructed within the large Pleistocene Lam Teuba caldera. A smaller 8 x 6 km caldera lies within Lam Teuba caldera. The summit contains a forested, 400-m-wide crater. The active van Heutsz crater, located at 650 m on the NNE flank of Suelawah Agam, is one of several areas containing active fumarole fields. Sapper (1927) and the Catalog of Active Volcanoes of the World (CAVW) reported an explosive eruption in the early 16th century, and the CAVW also listed an eruption from the van Heutsz crater in 1839. Rock et al. (1982) found no evidence for historical eruptions. However the Volcanological Survey of Indonesia noted that although no historical eruptions have occurred from the main cone, the reported NNE-flank explosive activity may have been hydrothermal and not have involved new magmatic activity.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.go.id/); TheJakarta Post, Jl. Palmerah Barat 142-143, Jakarta 10270 (URL: http://www.thejakartapost.com/); Michael Thirnbeck (URL: http://www.flickr.com/photos/thirnbeck/); MapsOf.net (URL: http://mapsof.net/).

Scientists first detected signs of eruptions at West Mata, a small active seamount ~200 km SW of Samoa, in 2008 when a particle-rich plume was identified ~175 m above the volcano's summit (BGVN 34:06). An eruption site was located in May 2009 (Resing and others, 2011; BGVN 34:12), and found to be still active in March 2010 (Clague and others, 2011). Thus, as of the beginning of 2012, the W Mata eruption has been ongoing for at least 3 years (since November 2008). This report provides an updated version of the one that first appeared in BGVN 36:12 about W Mata volcano (figure 6).

Figure 6. Location maps of West Mata volcano. (a) Regional map showing features of the NE Lau basin; inset shows the volcano's location at the N end of the Tonga trench. (b) Detailed bathymetric map produced by the autonomous underwater vehicle D. Allan B during the May 2009 cruise. Remotely operated vehicle (ROV) Jason2 dive tracks along which observations and measurements were made and samples recovered are shown by colored lines. Two eruptive vents, Hades and Prometheus, are located by red dots. Relative lava age assessments are based on visual observations. The line T08C17 was a towed hydrocast with samples taken along the line, and point V08C26 was a stationary hydrocast with samples taken over a range of depths at a single location. These hydrocasts collected temperature data and samples of the plume for chemical analyses. From Resing and others (2011).

Baker and others (2012) noted that W Mata volcano, a low effusion rate eruption, was the deepest active submarine eruption ever observed [as of 2011] and had both explosive and effusive phases. Hydrophones moored for two 5-month deployment periods before and after the 2009 seafloor observations recorded variable but continuous explosions, proof that W Mata, like Northwest Rota-1 (in the Mariana islands), is undergoing a lengthy eruption episode. Rubin and others (2012) reported that W Mata represented the deepest witnessed violent submarine eruption to this time (~700 m deeper than currently-erupting NW Rota-1 in the Mariana Islands, BGVN 29:03, 31:05, 33:12, 34:06, and 35:07).

It was previously thought that explosive eruptions, which involve expanding bubbles, shouldn't occur below a depth of ~1 km. Basically, as water pressure increases with depth in the ocean, the ability of gas to come out of solution in the magma and cause eruption is diminished. The suppression of bubbles thus limits explosions, but the depth at which this occurs is called into question. Clague and others (2011) suggest that pyroclastic activity at West Mata occurred to at least 2.2 km depth.

Presenting a list of ocean depths and locations where explosive processes have been documented, Clague and others (2011) gave the following information (presented here omitting their cited references): "...fine clastic debris formed during pyroclastic eruptions along [West Mata's] rift zones, and coarser talus shed from the lava flows, plateaus, and cones, can be traced upslope perpendicular to contours to the rift zones at depths as great as 2,350 m, suggesting that explosive pyroclastic activity on West Mata is common at least this deep, and much deeper than most theoretical models suggest without extraordinary initial volatile contents or accumulation of volatiles. Previous studies suggest that strombolian bubble-burst basalt eruptions occur along the mid-ocean ridge system for volatile-poor mid-ocean ridge basalt at least as deep as 1,600 m deep on Axial Seamount on the Juan de Fuca Ridge, 1,750 m on the mid-Atlantic Ridge near the Azores platform, 3,800 m on the Gorda Ridge, and 4,000-4,116 m deep on the Gakkel Ridge. Deep water strombolian activity of more volatile rich lavas has also been observed at 550-560 m depth on NW Rota-1 in the Marianas arc for basaltic-andesitic lava, and inferred at least as deep as 590 m depth off shore Oahu, 1,300 m at Lōʻihi Seamount, and 4,300 m for volatile-rich strongly alkalic lavas in the North Arch volcanic field. The distribution of clastic debris on West Mata suggests that boninite eruptions can also be pyroclastic much deeper than the activity observed at the active vents near the summit at 1,175-1,200 m depth."

Resing and others (2011) made the following introductory comments (quoted here without most of the references they cited): "Submarine eruptions account for ~75% of Earth's volcanism [White and others, 2006], but the overlying ocean makes their detection and observation difficult. The scientific community has made a concerted effort to study active submarine eruptions since the mid-1980s. Despite these efforts only two active submarine eruptions have been witnessed and studied: NW-1, a much shallower submarine volcano in the Mariana arc, and now West Mata, at 1,200 m depth. Here we describe sampling and video observations of an explosive eruption driven by the release of slab-derived gaseous H₂O, CO₂ and SO₂. The generation of fine-sized clastic materials provides direct evidence for eruptive styles that produce similar materials deeper in the ocean."

Boninites. Resing and others (2011) and Rubin and others (2009) noted that among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. A boninite sample was collected at W Mata by the ROV Jason during the 2009 cruise (see figures 10 and 11, BGVN 34:12). Boninite is a mafic extrusive rock, an olivine- and bronzite-bearing andesite with little to no feldspar, containing high levels of both magnesium and silica. The rock is typically composed of large crystals of bronzite (pyroxenes) and olivine in a crystallite-rich glassy matrix. These lavas are considered diagnostic of the early stages of subduction, yet, because most preserved and observable subduction systems on continents are old and well-established, boninite lavas had previously only been observed in the ancient geological record.

Resing and others (2011) found that large volumes of gaseous H₂O, CO₂, and SO₂ were emitted, which they suggested are derived from the subducting slab. The volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. Some examples of various eruptive modes observed in West Mata are shown in figure 7. As at other submarine volcanoes, the volatile-rich fluids found at West Mata fuel chemosynthetic biological activity (figures 7g and 7h).

Figure 7. ROV Jason2 photographs depicting West Mata's Hades and Prometheus vents (shown in figure 6(b). (a) Discovery of the eruption at Hades vent seen here with the field of view (FOV) ~4 m across. (b) Active degassing and explosive clast formation at Prometheus vent; white particles are primarily elemental sulphur (FOV is ~3 m). (c) Magma bubble and active degassing at Hades vent, with degassed lava progressing downhill, forming pillow flows (FOV is ~3.5 m). (d) Quenched lava being collected from an active flow; the active pillow is ~0.3 m wide; iset is the quenched sample being stored on the ROV. (e) Pillow lava extruding (~0.2 m wide). (f) At Hades vent, double magma bubble emerging from the vent before breaking apart; the base of the bubble is ~0.5?0.8 m (most of the observed bubbles ranged in size from 0.25?1 m in diameter, with occasional larger bubbles). (g) Microbial flock near diffuse venting between Prometheus and Hades vents. (h) Colony of shrimp near diffuse venting; warm water was collected here; the two red dots are 0.1 m apart. This set of images came from Resing and others (2011); others may be found in Rubin and others (2012).

In May 2009, scientists using ROV Jason 2 discovered two sites of active explosive eruption (vents) on the summit of W Mata (Resing and others, 2011). The first vent, Hades, was located on the S end of the summit ridge at ~1,200 m depth, and the second vent, Prometheus, was found ~100 m NE of Hades at 1,174 m depth (located in figure 6b). Figure 7 shows some newly published images from these vents. During a one-week study in 2009, explosive eruptions at both vents were almost continuous with only occasional quiet episodes. Several modes of magmatic gas-driven eruptions were identified and some may have contained significant trapped water. They produced pyroclasts (i.e., spatter, ash and tephra) and abundant fine-grained particulate material composed predominantly of sulfur.

The most spectacular eruptive mode observed during the week occurred when erupting gases stretched molten lava to create incandescent bubbles of ~0.2? to 1-m diameter (figure 7c and 7f ). As the lava bubbles burst they produced fine-grained particle clouds devoid of visible gas bubbles. A hydrophone placed nearby recorded distinctive low-frequency sounds.

In a less explosive eruptive mode, pulses of gas emitted pebble- and sand-size clastics (figure 7b). These formed mounds of debris through which magmatic gases escaped. Observers also saw pyroclasts and fine-grained sulfur (figures 7a-c and 7f).

Another eruptive mode occurred following quiet episodes, when cap rock was pushed aside and incandescent, degassing, molten lava emerged accompanied by low-frequency sound. At other times, the gas passing through the incandescent lava was flame-like in appearance. In both these cases, escaping hot volatiles insulated the incandescent lava from surrounding seawater for prolonged intervals.

The general absence of free gas bubbles at West Mata markedly contrasts with the abundance of bubbles observed at the much shallower (520 m) eruption at NW-Rota. This fits with the diminished ability to form bubbles at depth.

Clague and others (2011) reported that the autonomous underwater vehicle (AUV) D. Allan B conducted high-resolution (1.5-m scale) mapping during the May 2009 expedition to W Mata that helped identify the processes that construct and modify the volcano. In addition, ship-based multibeam sonar bathymetry had been collected over West Mata during expeditions in 1996, 2008, 2009, and 2010, with the results enabling comparisons over a 14-year period.

According to Baker and others (2012), a significant drawback to existing moored arrays is the absence of realtime information, precluding a prompt response to a detected event. This deficiency led to the addition of hydrophones to profiling floats and underwater ocean acoustic gliders. The QUEphone, or Quasi-Eulerian hydrophone, is a new-generation free-floating autonomous hydrophone with a built-in satellite modem and a GPS receiver (Matsumoto and others, 2006). Because it does not have station-holding capability, its main value to response efforts is its potential for rapid deployment by aircraft. Underwater ocean gliders offer a more structured monitoring strategy, as they can be preprogrammed to follow, and repeat, a horizontal and vertical course. Low instrument noise and buoyancy-based drive systems make gliders ideal acoustic monitoring tools, able to navigate around seafloor obstacles and resurface every few hours to transmit data. Matsumoto and others (2011) demonstrated this capability by driving a glider around W Mata volcano and recording the broadband volcanic explosion sounds.

References. Baker, E.T., Chadwick Jr., W.W., Cowen, J.P., Dziak, R.P., Rubin, K.H., and Fornari, D.J., 2012, Hydrothermal discharge during submarine eruptions: The importance of detection, response, and new technology, Oceanography, v. 25, no. 1, pp.128?141 [http://dx.doi.org/10.5670/oceanog.2012.11].

Clague, D.A., Paduan, J.B., Caress, D.W., Thomas, H., Chadwick Jr., W.W., and Merle, S.G., 2011, Volcanic morphology of West Mata Volcano, NE Lau Basin, based on high-resolution bathymetry and depth changes, Geochemistry, Geophysics, Geosystems (G3), v. 12, QOAF03, 21 pp, doi:10.1029/2011GC003791.

Matsumoto, H., Dziak,, R.P., Mellinger, D.K., Fowler, M., Lau, A., Meinig, C., Bumgardner, J., and W. Hannah, 2006, Autonomous hydrophones at NOAA/OSU and a new seafloor sentry system for real-time detection of acoustic events, Oceans 2006, MTS/IEEE?Boston, September 18?21, 2006, IEEE Oceanic Engineering Society, pp. 1-4, doi:.10.1109/OCEANS.2006.307041.

Matsumoto, H., Bohnenstiehl, D.R., Haxel, J.H., Dziak, R.P., and Embley, R.W., 2011, Mapping the sound field of an erupting submarine volcano using an acoustic glider, Journal of the Acoustical Society of America, v. 129, no. 3, pp. EL94?EL99, doi: 10.1121/1.3547720.

Resing, J.A., Rubin, K.H., Embley, R.W., Lupton, J.E., Baker, E.T., Dziak, R.P., Baumberger, T., Lilley, M.D., Huber, J.A., Shank, T.M., Butterfield, D.A., Clague, D.A., Keller, N.S., Merle, S.G., Buck, N.J., Michael, P.J., Soule, A., Caress, D.W., Walker, S.L., Davis, R., Cowen, J.P., Reysenbach, A-L., and Thomas, T., 2011, Active submarine eruption of boninite in the northeastern Lau Basin, Nature Geoscience, v. 4, 9 October 2011, pp. 799?806, doi:10.1038/ngeo1275.

Rubin, K.H., Soule, S.A., Chadwick Jr., W.W., Fornari, D.J., Clague, D.A., Embley, R.W., Baker, E.T., Perfit, M.R., Caress, D.W., and Dziak, R.P., 2012, Volcanic eruptions in the deep sea, Oceanography, v. 25, no. 1.p. 142?157 [http://dx.doi.org/10.5670/oceanog.2012.12].

Geologic Background. West Mata, a submarine volcano rising to within 1,174 m of the ocean surface, is located in the northeastern Lau Basin at the northern end of the Tofua arc, about 200 km SW of Samoa and north of the Curacoa submarine volcano. Discovered during a November 2008 NOAA Vents Program expedition it was found to be producing submarine hydrothermal plumes consistent with recent lava effusion. A return visit in May 2009 documented explosive and effusive activity from two closely spaced vents, one at the summit, and the other on the SW rift zone.

Information Contacts: Joseph A. Resing, NOAA PMEL and Joint Institute for the Study of the Atmosphere and Ocean (JISAO), The University of Washington, 7600 Sand Point Way, NE, Seattle, WA, USA (URL: http://www.pmel.noaa.gov and http://jisao.washington.edu); David A. Clague, Jennifer B. Paduan, David W. Caress, and Hans Thomas, Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, California, USA (URL: http://www.mbari.org); William W. Chadwick Jr., Robert W. Embley, and Susan G. Merle, Hatfield Marine Science Center, Oregon State University and NOAA, Newport, OR, USA (URL: http://www.pmel.noaa.gov); Kenneth H. Rubin, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology (SOEST), University of Hawaii at Monoa, HI, USA (URL: http://www.soest.hawaii.edu/).