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/).

The following Etna report from Sonia Calvari and Boris Behncke is based on daily observations by numerous staff members of the Istituto Nazionale di Geofisica e Vulcanologia (INGV). As previously reported here (BGVN 31:08), a 10-day-long eruption vented from the base of the Southeast Crater (SEC) in mid-July 2006. Eruptive activity then shifted to the crater's summit vent during 31 August-15 September, leading to lava overflows and repeated collapse on the SEC cone (BGVN 31:08).

This report discusses the time period 22 September to 4 November, an interval with multiple episodes of eruptive activity (roughly eight in all, seven of which involved a return of activity at the SEC summit). The activity typically included lava flows and Strombolian eruptions. In general, the eruptive episodes became increasingly brief and vigorous. Eruptions came from the SEC's summit as well as from multiple vents along fractures on the SEC's sides or adjacent to it that developed during the reporting interval.

As mentioned above, in general, during the reporting interval, the renowned SEC summit area was only episodically active. Since the SEC's collapse of September 2006, it has had a breached E wall. During this reporting interval, lava flows escaped the crater through the breach to form narrow rivulets down the steep upper SE flank. Ash from SEC fell on Catania on 30 October.

On 12 October a fissure opened at ~ 2,800-m elevation on the ESE base of the SEC cone, ~ 1 km from SEC's summit. Lava from this vent traveled SE, and a map showing the vents and pattern of flows through 20 November indicated lava extending ~ 2 km from the 2,800-m vent (Behncke and Neri, 2006). The 2,800-m vent also sits along the path of some of the SEC lavas from the summit crater. By late November, a complex flow field from both SEC summit and the 2,800-m vents lay on the SE side of the SEC. The field extended from the summit ~ 3 km, and its distal ends reached the W wall of the Valle del Bove.

Two other important vents began erupting in late October. One was on the SEC's upper S flank. The other, at 3,050 m elevation, stood ~ 1 km SW of the SEC's crater and at a spot ~ 0.5 km from the nearest margin of Bocca Nuova's crater. Although lava emissions from this vent at 3050-m elevation stopped, they later restarted and by 20 November the vent had created a large SW-trending field of lava flows roughly the size of those from the SEC summit and 2800-m vent.

Eruptive behavior 22 September-4 November. During this time interval, the seven episodes determined by eruptive activity at the SEC occurred as follows.

The first episode, which was five days long, started late 22 September from the summit of the SEC. Activity during the first two days was limited to mild Strombolian explosions, but lava began to overflow the SEC's crater on 24 September, spilling onto the cone's SE flank. This activity ceased sometime on 27 September.

The 2nd episode began late the afternoon of 3 October with Strombolian explosions from the SEC summit, which increased in vigor during the following hours. Late that evening lava began to spill down the SE side of the SEC cone adjacent to flows of the previous two episodes. Following a sharp decline in tremor amplitude on the afternoon of 5 October, the activity ended sometime between midnight and the early morning of 6 October.

The 3rd eruptive episode occurred between the evening of 10 October and the evening of the following day. The SEC's summit produced vigorous Strombolian activity and lava again descended the SEC cone's SE flank. A sharp drop in tremor amplitude on the afternoon of 11 October indicated the eruptions imminent cessation.

At the tail end of the 3rd episode, a short eruptive fissure opened with vents at ~ 2,800-m elevation. Monitoring cameras fixed the start of this activity at 2328 on 12 October. The SEC's summit was quiet throughout the following eight days, leaving this burst to be considered as activity late in the 3rd episode, rather than representing the start of the 4th episode in the SEC's sporadic on-and-off behavior.

Trending N90°E-N100°E, the new fissure resided on the ESE flank at the base of SEC, a spot also on the Valle del Bove's W wall. For the first few days, lava was emitted non-explosively, quietly spreading in the upper Valle del Bove and advancing a few hundred meters downslope. Mild spattering on 17 October resulted in the growth of three hornitos on the upper end of the eruptive fissure.

Summit SEC activity marked the 4th episode since 22 September. As lava effusion continued from the fissure vents at 2,800 m elevation, the SEC started a powerful eruption at 0600 on 20 October. Accompanied by a rapid increase in tremor amplitude, vigorous Strombolian eruptions occurred in the central portion of the SEC's summit. A vent near the E rim of the SEC's crater, in the notch created by the collapse events of early September, produced large explosions every few minutes and quickly built a new pyroclastic cone. Lava once more flowed down the SEC's SE side, stopping N of the 2,800-m fissure. At that fissure vent, lava emission continued but appeared reduced compared to the previous days. The SEC ceased issuing lava the same day it began, 20 October.

The 5th episode involving the SEC was preceded on 22 October with a few isolated bursts of ash from the SEC. The episode began with strong activity at 0700 the next day, when the SEC's summit generated vigorous Strombolian discharges and pulsating lava fountains from two vents. The new pyroclastic cone grew rapidly. Lava spilled down the ESE flank of the cone, to the N of the flows formed in the previous episodes.

Coincident with the above eruptions, INGV researchers noted an increased lava emission from the 2,800-m vents. This led to several lava overflows (in an area adjacent to the hornitos formed 17 October).

Although Strombolian activity and fountaining at the SEC diminished on the afternoon of 23 October, strong ash emissions began at around 1700, producing an ESE-drifting plume. Pulsating ash emissions and occasional bursts of glowing tephra continued and, at about 1750, the SEC cone's S flank fractured. Lava escaped from the fracture's lower end, forming two small lobes. The longer lobe reached the base of the cone and then traveled SE, ultimately to reach ~ 1 km from their source at the new fissure. The smaller lobe took a path down the cone slightly to the W, but halted before reaching the base of the cone. The new fissure's lava supply diminished early on 24 October, stopping around noon.

Coincident with the above events, effusive activity continued without significant variations at the 2,800-m vents. The farthest flow fronts reached an elevation of ~ 2,000 m to the NW of Monte Centenari, and extended ~ 2.5 km from their source.

Field observations made on 24 October revealed that part of the new pyroclastic cone had subsided and a new collapse pit, ~ 50 m wide, had opened on the SE flank of the SEC cone, roughly in the center of the largely obliterated collapse pit of 2004-2005.

The 6th episode of SEC activity began in the late afternoon on 25 October. Initially there was an increase in tremor amplitude, as well as both ash emissions and weak Strombolian activity from the SEC's summit. Both the tremor and Strombolian discharges decreased late that evening, but at 0054 on 26 November lava was emitted from a new fissure. This fissure, on the SEC cone's SSE flank, was active only for a few hours and produced a very small lava flow. As has often been the case during the reporting interval, the 2800-m vents continued to discharge lava toward the Valle del Bove.

What was to later become another important effusive vent opened at 0231 on 26 October. The vent developed at ~ 3,050 m elevation in an area ~ 700 m S of the center of Bocca Nuova's crater and ~ 500 m SW of the center of SEC's crater. This spot sits at the S base of the central summit cone below the Bocca Nuova, and ~ 700 m to the W of the fissure that had erupted 2 hours earlier.

Fieldwork carried out on 26 October by INGV researchers revealed that the vent at 3,050 m elevation had formed at the southern end of a fracture field. That field extended across the SE flank of Etna's central summit cone to the W flank of the SEC cone. Lava extruding at the 3050-m vent poured out at a decreasing rate before a pause began on the evening of 26 October.

The 7th episode, 27 October and into early November, was first associated with a new increase in tremor amplitude and corresponding SEC ash emissions on the afternoon of the 27th. These emissions were followed at 0206 on the 28th by the reactivation of the vent at 3050-m elevation. Ash emissions and Strombolian activity occurred from the SEC between 0830 and 1100, but no lava overflows were produced. On the evening of 28 October, both effusive vents at 3,050 and 2,800 m were active.

29 October ash emissions from the SEC became more vigorous during the early morning of the 30th and fine ash fell over inhabited areas to the S, including Catania (27 km from the SEC). Intermittent bursts of glowing tephra were recorded by INGV-CT surveillance cameras, although later analysis revealed that most of the tephra was lithic rather than juvenile. Ash emissions gradually diminished and ceased at around 0800 on 29 October.

Ash was again emitted from the SEC shortly before 1300 on 31 October, and in minor quantities at least once per day through 5 November. No incandescent ejections occurred from this crater after 28 October until the evening of 4 November (during 1830-2005) when weak Strombolian explosions were recorded by the INGV-CT surveillance cameras.

The vent at 3050-m elevation continued to emit lava on 29 October. The effusion rate was estimated as 1 to 5 m³ per second. Emitted lava descended SW to ~ 2,400 m elevation.

Lava also continued to flow from the 2,800-m vents on the 29th, but the associated lava flow front advancing from these vents had moved little since 24 October. Lava continued to flow from both vents during the first days of November, but the effusion rate had clearly dropped by the 3rd when active flows had retreated upslope from the distal fronts. Similarly, a helicopter overflight on the morning of 5 November disclosed actively flowing lava confined to the uppermost parts of the lava flow fields.

References. Behncke, B., and Neri, M., 2006, Mappa delle colate laviche aggiornata al 20 Novembre 2006 (PDF file on the INGV website).

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Sonia Calvari and Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

An eruption from Home Reef in early August generated large volumes of pumice that floated to Fiji (over 700 km away) in the following two months; an island was also created (BGVN 31:09). Satellite data and imagery have been used to confirm these observations and provide additional information about this event.

Norman Kuring of the MODIS Ocean Color Team identified the earliest clear shot of the pumice raft in a Terra MODIS on 7 August at 2120 UTC (8 August at 1020 Tonga time). The image (figure 11) shows a circular patch of pumice over the eruption site with a small volcanic plume emerging from it. The first indication of pumice raft leaving the eruption site is in the Aqua MODIS image at 0132 UTC on 10 August, but the 9 August overpass was cloudy, so it could have happened earlier. Kuring also compiled other images showing the dispersion of the pumice through 22 August (figure 12).

Figure 11. Terra MODIS image taken on 8 August 2006 (local time) showing the early stages of the eruption at Home Reef. A steam plume is visible rising from the southern end of a mass of floating pumice covering an area larger than Late Island to the NW. Courtesy of the NASA Ocean Color Group.

Figure 12. Terra and Aqua MODIS satellite images showing the dispersion of the pumice raft generated by the eruption at Home Reef during 7-8 August. By 10 August a large raft was NE of Late Island. Most of the material stayed in that area through 12 August before breaking up into elongate pieces that began moving W towards Fiji. Courtesy of the NASA Ocean Color Group.

Kuring also made a preliminary estimate of the area of the pumice raft on 11 August (10 August at 2150 UTC), previously encountered by the Maiken (BGVN 31:09). A mask was created to cover identifiable areas of pumice, resulting in an area of 9,338 pixels. Each pixel in the image used covers an area of 0.0468 km². The calculated total area is approximately 440 km² for that time. Note that this estimate does not take into account errors caused by pumice being a high-contrast target (allowing linear patches less than the pixel width to be seen), small isolated patches of pumice that could not be recognized, material hidden by clouds, or fragments suspended in the water column under the surface. In the 8 August MODIS image, the circular area was determined by Bulletin editors to be at least 8 km in diameter, so the area covered was more than 50 km².

Simon Carn (UMBC) used the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite to constrain the timing of the eruption. OMI detected SO₂ emissions from the vicinity of Home Reef beginning on 8 August. Emissions appear to have peaked sometime during 8-9 August. The total SO₂ mass detected E of Tonga by OMI on 9 August was ~ 25 kilotons. By 12 August there were 3.3 kilotons of SO₂ in the area (figure 13). The emission episode was over by 15 August. HYSPLIT forward trajectories indicated that the SO₂ released on 8 August may have reached altitudes of 5 km or more. Carn also stated that "To our knowledge this is the first example of satellite detection of emissions from a submarine volcano. Significant scrubbing of SO₂ and other soluble volcanic gases is likely during such events."

Figure 13. Sulfur-dioxide emissions in the vicinity of Home Reef, 12 August 2006 at 0140 UTC. Data obtained from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite. Courtesy of Simon Carn.

Terra MODIS data from 4 September 2006 provided by Alain Bernard showed pumice rafts moving SE from Home Reef (figure 14). Pumice that previously followed a similar path was found on beaches in southern Vava'u (BGVN 31:09) by 2 September.

Figure 14. Terra MODIS data from 4 September 2006 showing pumice rafts moving SE from Home Reef. Data was obtained with a simple processing of bands 1 and 2; pixel size is 250 meters. Courtesy of Alain Bernard.

Island evolution. No data or reports are available to determine when the island built by the 1984 eruption (SEAN 09:02 and 09:04) eroded below the ocean surface. Recent reports from mariners and local fishermen noted that this current eruption had built a new island, implying the absence of an island at that location. An ASTER image inspected by Matt Patrick from 18 November 2005 did not show an island.

An ASTER image of the new island taken on 4 October 2006 (figure 15) has been studied by a number of scientists, including Greg Vaughan (JPL), Matt Patrick (Michigan Tech), and Alain Bernard (Univ. of Brussels). The image clearly shows the island (at 18.991°S, 174.762°W) with large ? and NE-directed anomalous areas that are likely caused by volcanic material suspended in the water. The new island is warmer than adjacent Late island. Greg Vaughan provided an annotated version of the image zoomed in on the new island, which he computed then had an area of 0.245 km². Vaughan also noted that the "daytime image shows considerable activity in the water around the new Home Reef island [and] a thermal plume in the same shape as the pink colored area in the attached VNIR images (ASTER channels 3-2-1 as R-G-B)." Work by Alain Bernard based on the ASTER thermal bands determined that the hot lake on the island had a maximum temperature of 64.7°C on 4 October. Bernard calculated the island area to be 0.230 km² on 4 October. Comparison with another ASTER image from 12 November showed that the island had changed shape and covered an area of 0.146 km², a decrease of 0.084 km² (figure 16).

Figure 15. ASTER VNIR image showing the new island at Home Reef on 4 October 2006. A volcanic lake is visible on the island, as are submarine plumes originating from the island. Some possible small pumice rafts can also be identified in this 15-m imagery. Modified from original provided courtesy of Greg Vaughan.

Figure 16. Comparison of the island at Home Reef on 4 October (left) and 12 November 2006 (right) using ASTER imagery. The size of the island decreased approximately 0.84 km2 over that time period. Courtesy of Alain Bernard.

Floating pumice observations. Additional pumice sightings have been reported that supplement those described earlier (BGVN 31:09). Areas known to have been impacted by the pumice now include Suva Point (where the capital of Fiji, Suva, is located) and Yasawa Island (N of Viti Levu and E of Vanua Levu). By early November pumice from Home Reef had reached Efate Island in Vanuatu.

Crew on the SV Sandpiper encountered pumice during transit from Tonga to northern Fiji on 12 September. They went through "large patches" of pumice "all afternoon" while traveling about 200 km over the course of the day. The next evening, after sunset on 13 September, the boat suddenly slowed and the water "looked like a thick chocolate shake." Lights shining down from the rigging (spreader lights) showed that they were surrounded by pumice. The crew observed pumice again on 23 September at the southern end of Vanua Levu.

Wally Johnson was flying from Suva to Taveuni on 19 September and observed large amounts of pumice in the Koro Sea, drawn out into numerous parallel strings in the direction of the prevailing wind and heading towards Taveuni. A fair bit of the pumice had been washed up into ridges on beaches on the NW coast of Taveuni, and up and into pockets on some of the recent basaltic lava flows to the SW. Bernie Joyce forwarded additional reports from Fiji. On 27 October 2006, Rebekah Mue-Soko reported that the Suva Point area of Viti Levu was filled with pumice as of 27 October, and that it had appeared sometime before 8 October. About 6 November 2006 Lyn and Darcy Smith were on the Fijian island of Yasawa, N of Viti Levu, and reported "a heap of pumice on the beach" which apparently arrived during their one-week visit.

While pumice has persisted in Fiji, some reached Vanuatu. Sandrine Wallez reported pumice on the W coast of Efate Island during the night of 4-5 November. A deposit around 10 cm thick was observed along 40 km of coastline. The largest pumice fragments were the size of a tennis ball. Pumice was still on the beaches in early December (figure 17). Shane Cronin was in Vanuatu in early October when a new batch of fresh pumice washed up on northern Efate beaches. Pumice is commonly being deposited on beaches around Vanuatu, and local residents told Cronin that they thought it was coming from up around the Ambrym-Lopevi area. Douglas Charley (DGMWR - Vanuatu) recorded explosion earthquakes on a portable geophone from south Epi (BGVN 29:04) at the beginning of September.

Figure 17. Photograph showing beach deposits of pumice from the August eruption at Home Reef on western Efate, Vanuatu, on 3 December 2006. Courtesy of Sandrine Wallez.

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: Simon Carn, Joint Center for Earth Systems Technology, University of Maryland-Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: https://jcet.umbc.edu/); Norman Kuring, NASA/Goddard Space Flight Center, Code 970.2, Greenbelt, MD 20771, USA; Greg Vaughan, NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109-8099, USA; Alain Bernard, IAVCEI Commission on Volcanic Lakes (CVL), Université Libre de Bruxelles (ULB), CP160/02, avenue F.D. Roosevelt 50, Brussels, Belgium (URL: http://www.ulb.ac.be/sciences/cvl/homereef/homereef.html); Sandrine Wallez, Department of Geology, Mines, and Water Resources (DGMWR), Port-Vila, Vanuatu; R. Wally Johnson, 45 Alroy Circuit, Hawker, ACT 2614, Australia; Shane Cronin, Institute of Natural Resources, Massey University, Palmerston, New Zealand; Tom and Amy Larson, SV Sandpiper (URL: http://sandpiper38.blogspot.com/2006_09_01_sandpiper38_archive.html); E.B. Joyce, School of Earth Sciences, The University of Melbourne, VIC 3010, Australia (URL: http://earthsci.unimelb.edu.au/home).

-Lava from Kīlauea continued to flow through the PKK lava tube from its source at Pu`u `O`o to the ocean during this reporting period from late August to the end of November 2006. About 1 km S of Pu`u `O`o, the Campout lava flow branches off from the PKK tube. Through November, the PKK and Campout systems fed two widely separated ocean entries named East Lae`apuki and East Ka`ili`ili, respectively. Kīlauea's activity during this reporting period included numerous small breakouts from the Campout flow, new skylights along the PKK tube, and variable activity at the ocean entries, including small streams of lava crossing the coastal bench. Intermittant lava fountaining 15 m inland of the W edge of the East Lae`apuki bench was noted in late September-early October. Incandescence was also intermittently visible coming from the East Pond and January vents, the South Wall complex, and the Drainhole vent in Pu`u `O`o's crater. In general, during this reporting period the inflationary trend continued at the summit of Kīlauea, in areas S of Halema`uma`u crater and tremors remained at a very typical moderate level at Pu`u `O`o.

During 30 August-12 September, crews reported visible lava streams on the W side of the East Lae'apuki delta and occasionally from the East Ka'ili'ili entry. On 1 September, the East Lae'apuki lava bench was an estimated 22 hectares (54 acres) and East Ka`ili`ili was an estimated 2.3 hectares (5.8 acres). On 30 August, and 1 and 6 September, the Campout flow escaped from the PKK tube. On 11 September, Park Service field crews reported two lava flows visible down the entire length of the pali. Incandescence was intermittently visible from the East Pond and January vents, the South Wall complex, and the Drainhole vent in Pu`u `O`o's crater.

During 13-23 September, lava from the Campout and PKK systems continued to flow off of a lava delta into the ocean and breakout flows were visible on the pali. On 20 September, a tour pilot reported seeing three large lava flows from a breakout 10 m inland from the old sea cliff at East Lae`apuki (figures 180 and 181). On 23 September, incandescence from above Pulama pali in the direction of Pu`u`O`o was likely due to several new and reactivated skylights on the upper PKK tube.

Figure 180. Aerial view of the lava bench at East Lae`apuki, looking NE on 20 September 2006. An active lava flow is going over the sea cliff in roughly the center of the arcuate fault scarp in the widest part of the lava bench below it. White steam plumes from the ocean entry were blown towards along the coast towards the left. In the colored version of this shot the adjacent seawater contains a greenish hue. Courtesy of HVO.

Figure 181. A lava flow at Kīlauea breaks out to the surface 10 m inland from a sea cliff on 20 September 2006 . The lava pours over the cliff in places as thick curtains and elsewhere as smaller rivulets and dripping falls. After the fall the lava proceeded across the upper bench as a series of braided streams. Toward the left, some readers might claim they see a slender Pelé, dancing with arms upraised. Courtesy of HVO.

Littoral fountaining on 27 September was reported about 15 m inland of the W edge of the East Lae`apuki bench. Lava jetted about 30 m in the air accompanied by loud rumbling and jetting sounds. Observers reported ground shaking. Over the next couple of days, 3-4 lava streams were visible on the W side of East Lae`apuki entry, as were incidents of tephra jetting and lava fountaining 15-23 m (50-75 ft) high. Glow had been visible from the East Lae`apuki entry and the Campout flow breakout on the pali, but not from the Ka`ili`ili entry. The consistent lack of visible glow from the Ka`ili`ili entry was due to the absence of a very large bench, forcing lava to remain hidden at the base of the seacliff.

Observers reported that on 28 September the floor of the Drainhole vent had been replaced by an overturning lava pond. As of 29 September, a new tube and flow that formed on the E side of the Campout flow extended ~ 180 m. Another flow went W and butted up against the PKK tube. The USGS field crew also found a small stagnant breakout of lava at ~ 60 m elevation. It flowed E to cover a little more of the long-abandoned Royal Gardens subdivision. In the Pu'u O'o vicinity, a new collapse pit photographed in early October had engulfed pre-existing spatter cones (figure 182).

Figure 182. Two views of Kīlauea's W gap area illustrating morphologic changes there. (top) Aerial view of Pu`u `O`o taken in July 2006 shows two spatter cones.. Note helicopter above label for scale. (bottom) An aerial photo taken on 13 October 2006 shows a new collapse pit that grew to engulf the spatter cones. The bottom of the pit, which formed on the night of 10 October, is hidden by fume. Courtesy of HVO.

During October and November, breakout flows were intermittently visible on the Pulama pali, at the base of the pali, or on the sea cliff and incandescence from vents in Pu`u`O`o was visible. For example, on 25 October, two separate break-out lava flows were visible on pali. The upper flow at about 320 m (1,050 ft) elevation consisted of 'a'a and pahoehoe and the lower flow at 114 m (375 ft) was solely pahoehoe. On 3 and 4 November, tephra jetted at the tip of the East Lae`apuki bench. On 15 November, breakouts resumed on top of the seacliff after a few weeks without activity. On 18 November, the Drainhole vent twice ejected spatter as high as 25 m above its rim. On 19 November, observers saw small explosions at East Lae`apuki ocean entry as well as well-defined streams of lava entering the ocean. The next evening, six rivers of lava flowed over the bench and into the ocean at the W entry. When weather permitted, incandescence was visible from the East Pond, the South Wall complex, the January vents, and Drainhole vent.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

Although Likuruanga volcano in West New Britain is thought to be of Pleistocene age (Johnson, 1971, 1970a, b), a boy died of carbon-dioxide (CO₂) asphyxiation in a hole at Bakada village on the volcano's N flank on 21 September 2006. Details of the follow-up investigation came out in a report of the Rabaul Volcano Observatory (Mulina and Taranu, 2006). This report is a condensation of that work. The event serves as a reminder of threats from gas release in volcanic regions, even those areas in repose or unlikely to erupt again. In this case, the linkage to biogenic versus volcanogenic origins of the gas remains equivocal. Likuruanga's summit is ~ 13 km NNE of Ulawun's summit (figure 1). The volcano and Bakada village appear in several Google Earth images (figures 2 and 3).

Figure 1. Likuruanga sits along the N coast of New Britain island (inset map). The larger figure comprises a sketch map of important features along the coast from Likuruanga to the Sulu Range. By far the most frequently active and reported-on volcano on the map is Ulawun, although recent reports have discussed unrest at both Bamus and the Sulu Range (BGVN 31:09) and regional seismicity has been high in 2006. This figure was scanned from Johnson (1970b) and modified.

Figure 2. The coastal village Bakada on Likuruanga's N flanks. The village, which has few permanent residents but is used as a safe haven by nearby coastal villagers when Ulawun becomes restless. The linked line segments across the image crudely approximate the boundary between East and West New Britain. Courtesy of Google Earth.

Figure 3. A closer view of Likuruanga's N-flank village Bakada. Courtesy of Google Earth.

In 2004, a logging company dug a number of holes to build latrines but ceased after finding water at shallow depths. The company ultimately left the area without refilling the holes, which are behind some of the remaining buildings (figure 4). A conspicuous disturbed area corresponded with the reported coordinates of the hole on the zoomed-in image of the village ("hole," figure 4).

Figure 4. Although somewhat fuzzy, this zoomed-in view of Bakada shows the Likuruanga hole, which was labeled based on coordinates provided in the RVO report. Courtesy of Google Earth.

Background on gas hazards. Natural sources of CO₂ include volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms. CO₂ gas is ~ 1.5 times heavier than air at the same temperature and can collect in depressions, and confined spaces such as caves and buildings. Without wind to ventilate an area, the denser CO₂ displaces the typical atmosphere, causing an oxygen deficiency. For adult occupational exposure, one US agency recommends a ceiling limit of 3 percent CO₂ for up to 10 minutes. Watanabe and Moritea (1998) studied responses of rats to various gases, including CO₂. They discuss various types of asphyxia and the related diagnoses of causes of death.

Although the main component of volcanic gas is usually water vapor, other common volcanic gases can endanger life and property. These can include, as in this case, carbon dioxide (CO₂); and, in an elevated temperature environment, a multitude of other gasses such as sulfur dioxide (SO₂), hydrogen (H₂), hydrogen sulfide (H₂S), carbon monoxide (CO), and hydrogen fluoride (HF). The main dangers to health and life results from the effects of the acids and ammonia compounds on eyes and respiratory systems. The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride.

Tragedy at Bakada village. On 21 September 2006, an 8-year-old boy, with his mother nearby, went down a large (2.6 m deep and 3.4 m wide) hole. He entered the hole trying to rescue his dog, which had fallen in. Witnesses recalled that the boy soon started shaking and screamed for help. A nearby woman went down the hole to rescue the boy and she, too, fell unconscious. Both were pulled from the hole by people at the rim with the aid of a long stick and knotted rope. The boy was dead; his hands a pale color. The woman was still breathing but vomited blood. She was rushed to a health center where she soon recovered. It was estimated that the woman was in the hole for 15 minutes and the boy somewhat longer (though this estimation remains crude as it could not be confirmed by anyone with a watch during the incident).

The following day, 22 September, villagers threw five small animals into the hole and noted that they all died immediately. The villagers also recalled that the previous year an employee of the logging company attempted to burn dried vegetation in the same hole and failed, even after adding waste diesel fuel to assist the process.

Investigation and conclusions. On 25 September RVO scientists arrived in Bakada to investigate the incident. It should be noted that for two days before their arrival there was moderate rainfall in the area. The scientists found that a frog and a dog were moving freely in the hole alongside the remains of the original five animals. The RVO report did not indicate when the frog and dog were put into the hole. In addition, a burning paper lowered into the hole continued to burn on the bottom surface.

On 27 September a return visit by RVO with instruments permitted the measurement of CO₂ emissions from adjacent soil. The results listed in tables 1 and 2 show that the rate of CO₂ emission varied, but generally increased as they approached the hole.

Table 1. Preliminary CO₂ soil flux made with approach to the hole where the child died at Bakada village, Likuruanga volcano. The measurements were made ~ 6 days after the tragedy, on 27 September 2006. After Mulina and Taranu (2006).

Table 2. Preliminary CO₂ soil flux analyses at various distances from the hole; as measured on 27 September 2006. After Mulina and Taranu (2006).

The investigators concluded that CO₂ in the 2.6-m-deep hole caused the boy to die of asphyxiation and the woman attempting to rescue him to enter a semi-conscious state. It was also noted that whereas air currents may keep CO₂ concentrations acceptably low on the land surface, the same does not hold true for deep holes. A final conclusion was that external factors such as rain may be able to wash out trapped CO₂ from the air, but the continuing emission of the gas from the soil may lead to further accumulations during dry spells.

The authors recommended that the logging company refill all the holes and that knowledge of this tragedy be made more-widely known to cope with the dangers of toxic gases in volcanic areas. The authors also suggests that carbon isotopic analyses be carried out on the CO₂ released at Bakada to determine if it is of magmatic or biogenic origin.

References. Johnson, R.W., 1971, Bamus Volcano, Lake Hargy Area, and Sulu Range, New Britain: Volcanic Geology and Petrology: Bur. Miner. Resour. Aust. Rec. 1971/55.

Johnson, R.W., 1970a, Ulawan Volcano, New Britain: geology,petrology and eruptive history between 1915 and 1967: Bur. Miner. Resour. Aust. Rec. 1970/21.

Johnson, R.W., 1970b, Likuruanga volcano, Lolobau Island, and associated volcanic centres, New Britain: geology and petrology: Bur. Miner. Resour. Aust. Rec. 1970/42.

Mulina, K., and Taranu, F., 2006, Gas related deaths at Bakada village inside Likuruanga volcano, West New Britain on 21st September 2006, report of Rabaul Volcano Observatory.

Watanabe, T. and Morita, M., 1998, Asphyxia due to oxygen deficiency by gaseous substances: Forensic Science International, v. 96, no. 1, p. 47-59.

Geologic Background. Likuruanga is a dissected, low stratovolcano with a large crater breached to the north. In September 2006, a boy died of carbon dioxide asphyxiation in a hole at Bakada village on the volcano's N flank.

Information Contacts: Rabaul Volcanological Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea.

The Rabaul Volcano Observatory (RVO) reported that a large, sustained Vulcanian eruption began at Rabaul at about 0845 on 7 October 2006 (BGVN 31:09). A further point regarding that eruption, absent from our previous report, was that some members of the Volcanic Clouds Group (a listserv discussion group) conducted significant observations and initial modeling of the 7 October eruption clouds, including mapping the cloud's sulfur dioxide content and making forecasts of their dispersion. In the Volcaniccloud listserv discussions of the 7 October clouds, Andrew Tupper noted the following: "The cloud was at 16 km (upper troposphere/lower stratosphere) when it passed over Manus on its way NW . . . . However, the north/northeastern parts were initially higher . . . , with the eastward bit clearly stratospheric. There were multiple flights under the cloud over Micronesia for [sic] that reported that was no ash or smell?this puts a lower boundary (~ 10 km) on the cloud, consistent with our view that the bits at cruising levels had gone to the SE."

Since that event and in reference to the time interval for this report, 4 November to early December 2006, RVO has noted that activity continued at Tavurvur at varying intensities. The largest event in the reporting interval took place at 0715 on 14 November 2006; Tavurvur produced a large explosion that rose several kilometers above the cone.

During 4-13 November, mild eruptive activity continued at Tavurvur, with occasional small-to-moderate ash emissions continuing and blowing to the SE. An emission on 11 November consisted of thick white vapor accompanied by occasional small-to-moderate ash clouds that drifted variably to the SE, S, and NW and resulting in fine ash fall downwind. On 12 November the emission was blown W and NW, and on the morning of 13 November the ash cloud drifted N of the volcano.

An explosion occurred at Tavurvur at 0715 on 14 November 2006, accompanied by a thick ash cloud that rose to about 2 km above the summit before drifting NW. The explosion showered the flanks of the volcano with lava fragments, some of which fell into the sea. Fine ash fall occurred at Rabaul Town areas and downwind to the Ratavul and Nonga areas. Continuous ash emission followed the explosion. Seismic activity continued at low levels; however, high-frequency earthquakes continued to occur within the Rabaul caldera. After the large explosion on 14 November, mild eruptive activity continued at Tavurvur, consisting of continuous thick white vapor accompanied by pale gray to gray ash clouds that rose ~ 1.5 km above the summit before drifting variably S and E of Tavurvur. During 16-17 November, continuous thick white vapor accompanied by pale gray ash clouds rose to about 2.5 km above the summit before drifting variably to the NW and E with fine ash falling on settlements downwind, including Rabaul Town. One high-frequency earthquake occurred on 16 November.

Mild eruptive activity continued at Tavurvur during 18-20 November. On 18 November and on the morning of 20 November continuous gray ash clouds rose less than 200 m above the summit before being blown N and NW. Fine ash continued to fall on villages downwind including Rabaul Town. Activity on 19 November consisted of emission of thick white vapor only, accompanied by roaring noises heard between 1130 and 1400.

Quiet generally prevailed at Tavurvur during 20-23 November. Emissions then consisted of thick white vapor accompanied by a small amount of pale gray ash clouds. On 21 November the emissions accumulated in the atmosphere around the caldera causing haze, and on 22 November the emissions rose less than 1,000 m above the summit before drifting W. Fine ash fell on villages downwind. On the morning of 23 November the emission consisted of white vapor rising more than a kilometer above the summit before drifting E.

On 26 and 27 November the activity consisted of gentle sporadic emission of subcontinuous, gray to pale gray ash clouds of varying thickness. The ash clouds drifted NW to W resulting in fine ash fall downwind. From November to 1 December the emission consisted of pale gray to dark gray ash clouds being released more forcefully. The ash clouds rose less than 200 m above the summit before drifting E. On the morning of 2 December the emission consisted thick white vapor and pale gray ash clouds that rose about 2 km before being blown ENE. On 3 December thick pale gray ash clouds that rose about 1 km above the summit were emitted. The ash clouds drifted NE in the morning and then slightly to the W in the afternoon. On the morning of 4 December the ash cloud rose about 2 km before drifting E. Fine ash fall occurred in downwind areas. There was no glow from the volcano visible at night. From late morning to the afternoon of 4 December the activity consisted of emission of thick pale gray ash clouds that rose about 500m above the summit before drifting NW. In the morning of 5 December the ash cloud rose 200 m before drifting E. By mid-morning the ash clouds were rising about 1 km above the summit before drifting NNW, and during the early afternoon the ash clouds drifted briefly to the E and then S before going back to the E by late afternoon. On the morning of 6 December the ash cloud rose about a km before drifting N-NW. The emission was accompanied by loud roaring noises. Fine ash fall occurred in downwind areas including Rabaul..

There was no significant deformation until 10 December. The RVO reported that loud and continual roaring was present from 8 December 2006 until the morning of 9 December, when the roaring became intermittent. The roaring ceased on 10 December and at that time parts of the caldera underwent a rapid ~ 1 cm uplift. On 11 December the volcano was quiet with very little fume. At 0400 on 12 December, a loud explosion occurred with an airwave which shook houses in Rabaul. This event generated a billowing gray column that rose to a maximum of 1,000 m before being blown to the E. Following the 12 December explosion subsidence returned the site's level to that of 9 December. Seismic activity continued at low levels. No high frequency earthquake was recorded.

Table 5 shows the MODIS thermal anomalies observed during 22 October-12 December 2006 (see BGVN 31:09 for earlier October anomalies).

Table 5. MODIS thermal Anomalies for Rabaul volcano for 24 October through 12 December 2006. Courtesy of the Hawai'i Institute of Geophysics and Planetology.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Steve Saunders and Herman Patia, Rabaul Volcanological Observatory (RVO), Department of Mining, Private Mail Bag, Port Moresby Post Office, National Capitol District, Papua, New Guinea; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Darwin, Australia; National Aeronautics and Space Administration Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); HIGP MODIS Thermal Alert System, Hawai'i Institute of Geophysics and Planetology (HIGP), University of Hawaii at Manoa, 168 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Volcanic Clouds Group (URL: http://groups.yahoo.com/group/volcanicclouds/).

According to El Salvador's Servicio Nacional de Estudios Territoriales (SNET) activity levels at San Miguel have generally remained similar to those during January 2002 when a minor plume rose above the summit crater (BGVN 27:02). The volcano's vigor continued into at least October 2006 at a level slightly at or above the base line of normal activity.

Recent publications have discussed the volcano and its lahar-hazard potential (Escobar, 2003; Chesner and others, 2003; Major and others, 2001). Figures 1 and 2 are taken from the latter publication.

Figure 1. Index map indicating El Salvador's volcanic front and the location of volcan San Miguel. Major cities are also shown (circles). From Major and others (2001).

Figure 2. The lahar hazard map of San Miguel depicts likely lahar paths, which are shown as colored or shaded areas. The contour interval is 20 m; the urban center ~ 11 km NE of the summit is San Miguel. From Major and others (2001); their plate 1, cropped, highly reduced, and excluding the key.

In January 2005 observers saw new fumaroles as well as small landslides on the N and SW wall of the crater. The accumulation of mass-wasted material in the crater led to a rise in the elevation of the crater floor.

During February 2005, weak fumaroles and small rock landslides persisted in the central crater. Digital sensors installed there recorded fumarolic temperatures in real time. On the outer portions of the cone the terrain is steeply sloping and contains prominent gullies (figure 3).

Figure 3. A photo of San Miguel taken from the N on 22 February 2005 showing the steep sides of the upper slopes and the incised drainages there. Although much of the area on the volcano is rural, hazards could easily affect 40,000 residents living nearby. Courtesy of Servicio Nacional de Estudios Territoriales (SNET).

The SNET reports for March and April 2005 noted that the crater was structurally weak due to the fumarolic activity, ongoing rock alteration, occasional landslides, and fractures on the western plateau. Microseismicity had increased; but it did not exceed typical base-line levels. Workers at the Santa Isabel farm (finca) noted N-flank lahars after heavy rains during March. The N flank contains abundant fine-grained volcanic deposits of the sort easily swept away during times of heavy rain.

Intense rains during May 2005 were associated with tropical storm Adrian (over an unstated interval the meteorological station near the volcano, San Miguel UES, recorded 428 mm of rainfall). As a result of the deluge, fumarolic activity from the crater increased. The crater walls remained intact, but eroded material previously deposited in the central crater that was poorly consolidated had to some degree stabilized. Substantial further compaction, settling, or collapse in the central crater seemed to have ceased by July 2005. During August 2005 the crisis at volcan Santa Ana forestalled visits to San Miguel.

A spike in seismic activity occurred during August 2005, with 7,048 long-period earthquakes, compared to July 2005, with 2,239 long-period earthquakes. SNET reports noted that based on monitoring, San Miguel generally remained within its base-line of normal behavior during the reporting interval. Figure 4 shows a histogram of long-period and volcano-tectonic events from the SNET reports for the interval September 2005-June 2006.

Figure 4. A plot of seismicity at San Miguel during September 2005-June 2006. Courtesy of SNET.

On 14 September 2005 a visiting group (OIKOS- Soliradaridad Internacional) made a trek to the summit and videotaped the scene there. SNET said the video disclosed a lack of significant changes in the crater; however, they saw debris-flow deposits in summit drainages on the volcano's outboard flanks. The visitors described both sounds of degassing and moderately intense odors of H₂S. During the course of September the seismic system recorded several minutes of tremor.

The October 2005 SNET report noted that workers at the plantation Santa Isabel noted N-slope lahars associated with rainfall. The lahars were also described as small debris flows; they descended from the high-elevation headwater areas, which are steep sided and narrow. The November report commented about the quantity of debris-flow material accumulating at the base of some N-flank channels. The same report also mentioned that moderate degassing was seen in the crater leaving areas of abundant sulfur, which appeared as yellow zones in one or more fumarolic areas.

The November 2005 report of SNET also discussed substantial landslides inside the crater that were followed by widening of the funnel-shaped area of collapse in the central crater. The landslides had left three distinct perched remnants of the crater floor (small terraces) at various elevations on the crater walls. The crater's western plain (one such terrace of the sort mentioned above) was stable but showed areas of subsidence (figure 5).

Figure 5. (top) A photo of San Miguel taken on 16 November 2005 showing the 'western plain' of San Miguel's crater (a terrace representing a remnant of a former crater floor). A considerable portion of the remaining terrace is in the process of subsidence (slumping). (bottom) A photo of the same area taken on 15 February 2006 (looking S). A zone of local subsidence, a pit along the head scarp, appears in the foreground but the subsidence also includes the region to the left of the large arcuate area extending well beyond the pit and still conspicuous in the upper left edge of the photograph. Courtesy of SNET.

Lahar monitoring during December 2005 disclosed erosion of easily mobilized cinders and scoria material on the N to NW flanks during the previous wet season. December seismicity was elevated, but cracks in the crater changed little compared to previous measurements. A field team visited the summit on 11 January 2006 and again in February and found few substantive changes in the crater. On the ascent route during January, the team saw a small recent "fall of material" reaching 40 cm thick. Some fumaroles discharged yellowish gases. During February the team conducted measurements of cracks on the western plain but found few changes, suggesting the headscarp had moved little if at all. February and March tremor episodes were centered at ~ 5 Hz and lasted 1-3 minutes.

The March 2006 SNET report noted small rockslides on the crater's N and S sides and, with the beginning of the rainy season in March 2006, there was a potential for the development of lahars. During the March visit the team found abundant granular material in the gullies on the NW flank, judged to be the result of debris flows. Monitored cracks remained stable.

With the arrival of the wet season in April, lahars and enhanced fumarolic output became apparent. One debris flow intersected a highway. On 23-24 April, 105 mm of rain was recorded at plantation (finca) Santa Isabel. Figure 6 shows the results of one lahar which left a trail of debris during the rainy interval. Earlier in the month on the 16th, a tremor or multi-phase episode lasted over an hour.

Figure 6. A San Miguel photo showing a part of the freshly scoured upslope channel in the Gato erosional gully. The material deposited in the channel consisted of reworked volcanic rocks and must have descended as a small lahar or debris flow. Several such flows occurred during heavy late-April rains at the start of the rainy season, a few days before this picture was taken. Courtesy of SNET (from their April 2006 report).

In April 2006, an increase in fumarole degassing within the crater and small landslides contributed to the instability of the deposits on the NW flanks of the volcano. Steam emanated from the fumaroles occasionally forming a weak column that reached the edge of the crater. There was a slight increase in seismicity throughout the month. Seismic activity increased in March and April 2006 (figure 4). Rocks in the crater show intense hydrothermal alteration with a yellowish reddish color. Small rock landslides were observed in the N and S zone of the crater.

During June 2006, the temperature of the fumaroles, opening of cracks and the gas discharge by the crater of the volcano, remained stable. There was an increase of small landslides within the crater. The analysis of the seismicity indicates that the volcano is slightly above its base line of normal behavior. New landslides and cracked rock were observed in the walls of the crater (figure 7). Rains have transferred volcanic material down the NW flank. Seismicity gradually increased in both frequency and magnitude beginning on 16 June. 47 VT earthquakes and 7,505 LP earthquakes were recorded, an amount that surpasses those registered in May; but smaller than those registered in March and April (figure 4).

Figure 7. San Miguel's S crater wall exposes zones of altered and fractured rocks. A planar zone of structural weakness appears towards the right. Photo taken on 22 June 2006. Courtesy of SNET.

During July 2006, stability continued with respect to fumarole temperatures, crack openings, and gas emissions around the crater. However, the seismicity increased by ~ 70%. Small and sporadic landslides took place inside the crater off the SE to SW walls. Intense hydrothermal alteration in the NW wall was also observed. SNET did not report any lahars during July 2006; however intense rains have continued to remove volcanic material from the NW flanks. The fumarolic field gave off weak emissions.

In August 2006, the monitored parameters such as fumarole temperature, crack opening, and visual estimates of gas discharge maintained normal levels. The seismicity diminished significantly in relation to July.

During September 2006, San Miguel reached a low level of activity. There were no significant changes in the morphology of the volcano as reported in previous months. At the S wall, there were evidence of small rock slides.

A sudden increase in seismicity occurred on 9 October 2006. Contact was made with other observatories and it was determined there were no landslides or rock falls associated with the event. Seismic increases such as 9 October had previously occurred, particularly on 19 June 2003 and from 2-6 May 2004. The 9 October increases were attributed to gas emission from the crater.

References. Chesner, C. A., Pullinger, C., Escobar, C. D., 2003, Physical and chemical evolution of San Miguel Volcano, El Salvador. GSA Special Paper 375.

Escobar, C.D., 2003, San Miguel Volcano and its Volcanic Hazards; MS thesis, Michigan Technological University, December 2003. 163 p.

Major, J.J.; Schilling, S.P., Pullinger, C.R., Escobar, C.D., Chesner, C.A, and Howell, M.M., 2001, Lahar-Hazard Zonation for San Miguel Volcano, El Salvador: U.S. Geological Survey Open-File Report 01-395. (Available on-line.)

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: Carlos Pullinger, Seccion Vulcanologia, Servicio Geológico de El Salvador, c/o Servicio Nacional de Estudios Territoriales, Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv/Geologia/Vulcanologia/).

Matt Patrick sent a new Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image, collected 28 October 2006 over Saunders Island . In his opinion this is the best image collected to date owing to the lack of a plume obscuring the summit crater, which was a problem in all previous images. The improved image provides a clear view of the crater (figures 5 and 6).

Figure 5. An ASTER image of Mt. Michael created using energy in the visible near-infrared wavelength ("VNIR"; bands 3-2-1, RGB), with the inset showing a closer view of the summit crater. There are two small near-IR anomalies (band 3, 0.807 microns wavelength) in the otherwise dark center of the crater, shown as red spots in the colored image. The two anomalies suggest very high temperatures and support the idea that fresh lava may reside at the surface or a shallow level in the crater. Courtesy of Matt Patrick.

Figure 6. The ASTER Short Wave Infrared (SWIR; band 9, 2.4 microns) image with a conspicuous anomaly at the summit, with numerous saturated pixels. Courtesy of Matt Patrick.

Analyzing the VNIR, SWIR, and Thermal Infrared (TIR) (not shown in figures 5 or 6) images together shows that the outer crater is 500-600 m wide, with a 180m high-temperature crater interior. The latter shows up as an SWIR anomaly and may indicate the rough extent of active lava flow being ~ 180 m wide. Matt Patrick chose Villarrica volcano in Chile for comparison to Mt. Michael (figure 7) since it presents a potentially good analogue in terms of morphology and activity style. Maximum radiant heat flux values were similar (up to ~ 150 MW), suggesting that the maximum intensity of activity may be similar. Mt. Michael shows a much lower frequency of thermal alerts, which may be the result of more frequent cloud cover in the South Sandwich Islands or a greater depth to molten lava in the Mt. Michael crater.

Figure 7. The real-time satellite thermal monitoring (MODVOLC) radiant heat flux values for Michael and Villarrica volcanoes during the period 2000-11 November 2006. Courtesy of Matt Patrick.

Table 1 shows a summary of thermal anomalies and possible eruptions from Moderate Resolution Imagine Spectroradiometer (MODIS) satellites since November 2005. The last reported activity of Mount Michael was noted in the SI/USGS (Smithsonian Institution/U.S. Geological Survey) Weekly Volcanic Activity Report of 12-18 October 2005 (see BGVN 31:04). At that time the first MODVOLC alerts for the volcano since May 2003 indicated an increased level of activity in the island's summit crater and a presumed semi-permanent lava lake that appeared confined to the summit crater. Those alerts occurred on 3, 5, and 6 October 2005.

Table 1. Thermal anomalies measured by MODIS satellites for Mount Michael for the period 3 October 2005 to 1 November 2006. All of the anomalies appeared on the SW side of the volcano. Courtesy of Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts Team.

References. Lachlan-Cope, T., Smellie, J.L., and Ladkin, R., 2001, Discovery of a recurrent lava lake on Saunders island (South Sandwich Islands) using AVHRR imagery: Journal of Volcanology and Geothermal Research, vol. 112, no. 1-4, p. 105-116 (authors are members of the British Antarctic Survey).

LeMasurier, W.E., and Thomson, J.W. (eds), 1990, Volcanoes of the Antarctic Plate and Southern Oceans: American Geophysical Union, Washington, D.C., AGU Monograph, Antarctic Research Series, v. 48.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

Information Contacts: Matt Patrick, Michigan Technological University, Houghton, MI; Thermal Alerts Team, Hawai'i Institute of Geophysics and Planetology (HIGP), School of Ocean and Earth Science and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); John Smellie, British Antarctic Survey, Natural Environment Research Council, High Cross, Madingly Road, Cambridge CB3 0ET, United Kingdom (URL: https://www.bas.ac.uk/); ASTER Science Project Teams, United States and Japan (URL: https://asterweb.jpl.nasa.gov/).

Ubinas began erupting ash on 25 March 2006 (BGVN 31:03 and 31:05); ash eruptions and steam emissions continued through at least 31 October 2006. Eruptive benchmarks during that period included a lava dome in the crater on 19 April. Ashfall in late April forced the evacuation of Querapi residents, who resided ~ 4.5 km SE of the crater's active vent, to Anascapa (S of the summit). Ash columns rose to almost 8 km altitude during May.

This report discusses ongoing eruptions through 31 October 2006 as drawn from Buenos Aires Volcanic Ash Advisory Center (VAAC) reports and especially from an enlightening 26-page report published in Péru during September 2006 by the Institutio Geológico Minero y Metalúrgico?INGEMMET (Salazar and others, 2006). It includes a detailed digital elevation map with hazard zones.

Background. Ubinas lies 90 km N of the city of Moquegua and 65 km E of the city of Arequipa (figure 4). The bulk of adjacent settlements reside to the SE, and generally at more distance, towards the E. Figure 5 shows a shaded region where airfall deposits took place during the span 1550-1969. The zone of deposits includes some modern settlements.

Figure 4. Map indicating the geographic setting of the Perúvian volcanic front (inset) and the area around Ubinas. From Salazar and others (2006).

Figure 5. The boundary of identified Ubinas ashfall from the years 1550 to 1969 appears as a curve across the S portion of this map, 10-12 km from the summit crater. Note the SE-sector settlements (and their respective distances from the summit crater) for the district capital Ubinas (6.5 km), Tonohaya (7 km), San Miguel (10 km), and Santa Cruz de Anascapa (~ 11 km), and Huarina (15 km). Map taken from Rivera (1998).

The geologic map on figure 6 shows the area of the settlements SE of the summit includes large Holocene deposits, including those from debris avalanche(s) at ~ 3.7 ka, and units containing pyroclastic flows. The map also indicates deposits of volcaniclasics, glacial moraines, airfall-ash layers, and lava flows. Extensive Miocene deposits envelope both the NE flanks (Pampa de Para) and SW flanks.

Figure 6. Geologic map of Ubinas shown here without the key, which is available in the original report. From Salazar and others (2006).

The map of hazard zones (figure 7) indicates a nested, tear-drop shaped set of zones, with comparatively lower inferred hazard to the NE and NW. The SE-trending, elongate area of hazards follows the key drainage in that direction. Elevated hazard zones also follow many of the roads passing through the region.

Figure 7. The SE corner of the Ubinas hazard map, showing the central crater, and the hazard zones that follow the main drainage (Rio Ubinas) leading SE through the most populated region close to the volcano. Map key is omitted. Margins of the map note that its construction was a partnership of numerous groups, including French collaborators at Blaise Pascal University and IRD. Taken from Salazar and others (2006).

Eruptions during 2006. Salazar and others (2006) reported that the current eruptive crisis could be divided into three stages. During July 2005-27 March 2006, the eruption was primarily gas discharge rising 100-300 m above the crater. During 27 March-8 April the eruptions consisted of ash emissions and gas produced by phreatic activity (figure 8). After a moderate explosion on 19 April, Ubinas produced ash and gas, and explosions ejected volcanic bombs. Several views into the crater appear on figure 9.

Figure 8. Ubinas gas emissions as seen from unstated direction on 4 April 2006. From Salazar and others (2006).

Figure 9. Views looking down into the Ubinas crater on 31 March, 19 April, and 26 May 2006. The former was taken in comparatively mild conditions. The 19 April photo was taken when a 60-m-diameter lava body was first seen on the crater floor (the color version of this photo shows faint red incandescence penetrating the steamy scene). The 26 May captured a relatively clear view of the steaming dome on the crater floor. March and April photos from and Salazar and others (2006); May photo from the INGEMMET website.

On 7 May 2006 a moderate explosion sent ash to ~ 3 km above the summit. Although the situation calmed in the following days, an impressive bomb fell 200 m from the crater on 24 May 2006 (figure 10). Larger outbursts occurred on 29 May and 2 June, prompting the civil defense decision to evacuate residents in the S-flank Ubinas valley, including the settlements of Ubinas, Tonohaya, San Miguel, Huatahua, and Escacha. Residents evacuated were lodged in refugee camps (figure 11).

Figure 10. Ubinas eruptions in May 2006 ejected volcanic bombs, seen here in their impact craters. A 2-m-diameter bomb (top), struck ~ 200 m from the crater. A crater containing a large, partly buried, smooth-faced bomb is seen in the bottom photo. Numerous bucket-sized angular blocks appear on the far side of the impact crater. Two geologists stand adjacent a ~ 2-m-long block that ended up on the impact crater's rim. The bomb fragments were of andesitic composition. Top photo from Salazar and others (2006); bottom photo from INGEMMET website.

Figure 11. Settlement camp housing families taking refuge from Ubinas ash. This camp, named Chacchagen, housed people from the S-flank settlements of Ubinas, Tonohaya, San Miguel, Huatahua, and Escacha. Inset shows the ash-dusted face of a local child. Courtesy of Salazar and others (2006).

On 18 June instruments recorded two explosions. Ash clouds discharged; the second one also ejected incandescent blocks ~ 1 km SE of the crater. The early stages of a rising plume seen at 0822 on 18 July appears on figure 12. Similar magnitude ash emissions were noted on 23, 24, and 30 June 2006, and incandescent rocks fell up to 1.2 km from the summit crater. During 10, 17-19, 22, 27 July, and 7 August 2006 there were various explosions (figure 12). Resulting ash clouds extended more than 70 km SE or SW.

Figure 12. A moderate Ubinas explosion on 18 July 2006 generated this rising ash plume. Courtesy of Salazar and others (2006).

In August 2006, ash plumes reached 4.6-7.6 km altitude and were occasionally visible on satellite imagery. The direction of drift of the ash varied widely. On 12 August, ash dispersed more than 100 km to the SE and S. On 14 August an astronaut on the International Space Station took a picture of the ash plume from Ubinas (figure 13).

Figure 13. This image taken from the International Space Station (ISS) captures Ubinas discharging a light-colored ash cloud roughly to the S (N is up on this photo). The cloud had been observed earlier on satellite imagery at 0600 local (1100 UTC) on 14 August 2006. One-hour and 45-minutes later (at 1245 UTC), an ISS astronaut took this picture at non-vertical (oblique) angle to the Earth. Pumice and ash blanket the volcanic cone and surrounding area, giving this image an overall gray appearance. Shadows on the N flank throw several older lava flows into sharp relief. (Photograph ISS013-E-66488 acquired with a Kodak 760C digital camera using an 800 mm lens). Photo provided by the ISS Crew.

The most significant effect on people and the environment has come from ashfall (figure 14). GOES satellite images indicate visible airborne ash for distances greater than 60 km from the vent. Figure 14 indicates net ash accumulation through about August 2006, extrapolating sampling points with concentric circles. The report specifically noted ash thicknesses of 1.5 cm at ~ 4.5 km SE in Querapi, 0.1-0.8 cm in Sacoaya, 0.5-0.8 cm in Ubinas, 0.3-0.4 cm in Anascapa, 0.15 mm in Huatahua, and less than 0.1 cm in Chacchagén. The accumulation has apparently been due to ongoing ashfalls On 13 April, several millimeters of ash dusted all surfaces in Querapi, ~ 4.5 km from the center of the summit crater.

Figure 14. Net ash accumulation around Ubinas from start of eruption in March through about August 2006. Ash has covered agricultural fields in the valley and pastures in the highlands, seriously affecting the two main economic activities in the area, agriculture and cattle ranching; and has caused respiratory and skin problems. Courtesy of Salazar and others (2006).

Aviation reports of ash plumes. As summarized in table 2, ash clouds were reported by the Buenos Aires Volcanic Ash Advisory Center (VAAC) on 2 May and then during 2 August through October on a nearly daily basis. The observation sources were usually pilot's reports (AIREPs) and/or satellite images (GOES 12). After 8 August, ash emissions were essentially continuous to 31 October. During the later interval, the aviation color code was generally Red. Plumes rose to 10 km and higher during 23-26 October.

Table 2.Compilation of aviation reports (specifically, 195 Volcanic Ash Advisories, VAAs) on Ubinas and its plumes during May through 31 October. The second column shows some contractions used in the table (eg., "VA CLD FL 160" means "Volcanic ash cloud at Flight Level 160"). Flight Level is an aviation term for altitude in feet divided by 100 (eg., FL 200 = 20,000 feet = ~ 7 km altitude). Courtesy of the Buenos Aires VAAC.

References. Rivera, M., 1998, El volcán Ubinas (sur del Perú): geología, historia eruptiva y evaluación de las amenazas volcánicas actuales: Tesis Geólogo, UNMSM, 132 p.

Rivera, M., Thouret, J.C., Gourgaud, A., 1998, Ubinas, el volcán mas activo del sur del Perú desde 1550: Geología y evaluación de las amenazas volcánicas. Boletin de la Sociedad Geológica del Perú, v. 88, p. 53-71.

Salazar, J.M., Porras, M.R., Lourdes, C.D., and Pauccara, V.C., 2006, Evaluación de seguridad físca de áreas aledañas al volcán Ubinas: INGEMMET (Instituto Geológico Minero y Metalúrgico Dirección de Geología Ambiental, September 2006), 26 p.

Thouret, J.C., Rivera, M., Worner, G., Gerbe, M.C., Finizola, A., Fornari, M., and Gonzales, K., 2005, Ubinas: the evolution of the historically most active volcano in southern Perú: Bull. Volc., v. 67, p. 557-589.

Geologic Background. The truncated appearance of Ubinas, Perú's most active volcano, is a result of a 1.4-km-wide crater at the summit. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45°. The steep-walled, 150-m-deep summit crater contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one from about 1,000 years ago. Holocene lava flows are visible on the flanks, but activity documented since the 16th century has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Jersy Mariño Salazar, Marco Rivera Porras, Lourdes Cacya Dueñas, Vicentina Cruz Pauccara, Instituto Geológico Minero y Metalúrgico (INGEMMET), Av. Canadá No 1470, Lima, Perú (URL: http://www.ingemmet.gob.pe/); Buenos Aires Volcanic Ash Advisory Center, Servicio Meteorológico Nacional, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php); ISS Crew, Earth Observations Experiment and the Image Science & Analysis Group, NASA Johnson Space Center, 2101 NASA Parkway Houston, TX 77058, USA (URL: http://www.nasa.gov/centers/johnson/home/); National Aeronautics and Space Administration (NASA) Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/).