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/).

As reported by Sistema Poseidon, activity at Etna during 9 April-13 May 2001 was chiefly characterized by typical episodic Strombolian blasts, ash emissions, and modest lava flows. The larger lava flows that emerged from new vents and grew during June and July will be discussed in later reports.

Activity during mid- to late-April 2001. During this time interval ash escaped at the Bocca Nuova (BN) vent. The weather thwarted direct observations of summit activity; however, later information was obtained through outings to intermediate elevations and from La Montagnola surveillance camera.

Lava continued to flow from a vent low on the NNE flank of the Southeast Crater (SEC) cone, as it has since approximately 20 January 2001. This lava flowed down the SEC's NE flank. During the nights of 18 and 21 April observers noted that the SEC produced flashing, denoting effusive activity. The SEC also continued to give off gray-colored gas from both the fumarole on the crater's edge and from the pit-crater in the crater's interior. Later in April the SEC's N flank vent continued to emit lava variably, but generally weakly, and beginning 26 April, the flow became visible principally from the volcano's NE quadrant. During 26-28 April degassing increased at SEC, yielding abundant clouds of white steam that diminished on 29 April.

Observations on 27 April revealed two hornitos (at 3,085 m, ~3 m high, and aligned N-S). They produced steady emissions, sounds of pressurized gas, and discontinuous expulsion of vitreous and blistering lava fragments which fell within a few meters of the vents. The more northerly hornito produced a lava flow within a confined channel. At about 3,000 m elevation, this lava river divided into two branches before rejoining just above 2,900 m. In late April, the flow rate was estimated at 2-3 m³/s.

A party viewing the base of BN's crater saw two prominent, steep-sided fissures that were ~100 m in length and at least 30-50 m deep. At a shelf inside the N fissure a small pyroclastic cone gave off dense brown and reddish clouds visible from the slopes of the volcano. The fissure in the SW quadrant also degassed intensely, and both fissures gave off almost continuous noise associated with magma inferred to reside at depth. A field of semi-circular fissures was observed nearby running S and W from this depression. Observers also noted fumaroles emitting bluish gas. Until at least early May, Voragine and Northeast craters continued weak degassing.

When seen on 3 May SEC's N hornitos had grown by almost 1.5 m compared with the preceding week. The lava canal had also widened to about 2 m, corresponding to a significantly increased flow rate, 5-10 m³/s. Two small lava flows developed on the E and W sides of the hornitos.

Strombolian eruptions starting on 7 May. Strombolian activity began again at the SEC late on the morning of 7 May. When seen on 9 May these eruptions were almost continuous, as frequent as about 45-50 explosions per minute, including some strong ones that sent lava fragments 20-30 m above the crater. Lava fragments as big as a meter in diameter were thrown up to 50 m above the crater rim.

Beginning at 1400, along with a new increase in tremor, the Strombolian activity evolved into a more violent phase at 1520-1540. Ballistics landed at elevations as low as ~3,000 m, reaching the spatter rampart at the S base of the cone. At about 1630 modest lava fountaining was observed from the fracture on the N flank of the SEC. Jets of magma reached ~100 m high. The fragments emitted from the lava fountain fell mostly in the SW sector of the volcano.

At the same time, the Montagnola camera began to register frequent ash emissions from the cone's summit; Strombolian activity and ash emissions continued until midnight in a discontinuous manner and with variable intensity. Observations on 10 May showed a substantial decrease in the activity at the SEC summit. Weak explosive activity was observed from the N fracture.

The lava emission from the fracture cutting the N flank of SEC continued with more or less intense phases. On 9 May, the cessation of lava fountaining was followed by a repeat of effusive activity, still within the same area of emission, which gave rise to finger-like flows ~1.5-2 km long. On 10 and 13 May, short lengths of the active branches of the flows were observed. The outburst led to a considerable plume that impacted local air traffic.

Bocca Nuova continued to issue brown-reddish ash emissions, presumably ongoing ash-bearing eruptions from one of the fissures described above. On 9 May a new fumarolic field was seen in the S part of the Bocca Nuova, extending from the rim to half way down the cone.

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: Sistema Poseidon, a cooperative project supported by both the Italian and the Sicilian regional governments, and operated by several scientific institutions (URL: http://www.ct.ingv.it/en/chi-siamo/la-sezione.html).

After the earthquake swarms in January 1999 (BGVN 24:01), two reports of anomalous activity at Hood were received; in September and October of 2000 landslides and debris flows traveled down the flanks of the volcano, and in January 2001 small earthquake swarms occurred.

The Cascades Volcano Observatory (CVO) reported that intense rainfall during 30 September to 1 October 2000 triggered a series of landslides and debris flows in several of Hood's drainages. The largest flows occurred in White River Valley on the S flank and Newton Creek Valley on the E flank. Both streams were diverted from their channels and severely damaged two sections of Oregon Highway 35; one section is an important link between I-84 and US 26, and the other is a recreational highway that provides access to Mount Hood Meadows Ski Area. The landslides and debris flows caused more than $1 million in damage. The Oregon Department of Transportation reopened the highway on 27 October.

According to CVO, a small earthquake swarm occurred at Hood during 10-19 January 2001. During this period a swarm of 13 earthquakes, with magnitudes ranging from 0.2-2.0, occurred in an area ~4-8 km SSE of the summit at a depth of 4-7 km. This area is frequently a source of earthquake swarms, but this swarm consisted of fewer and smaller events than is typical. The last similar type of swarm occurred in May 2000. On average, 1-2 swarms of small earthquakes occur at Hood each year.

Geologic Background. Mount Hood, Oregon's highest peak, forms a prominent backdrop to the city of Portland. The eroded summit area consists of several andesitic or dacitic lava domes. Major Pleistocene edifice collapse produced a debris avalanche and lahar that traveled north down the Hood River valley and crossed the Columbia River. The glacially eroded volcano has had at least three major eruptive periods during the past 15,000 years. The last two occurred within the past 1,800 years from the central vent high on the SW flank and produced deposits that were distributed primarily to the south and west along the Sandy and Zigzag rivers. The last major eruptive period took place beginning in 1781 CE, when growth of the Crater Rock lava dome was accompanied by pyroclastic flows and lahars down the White and Sandy rivers. The Sandy River lahar deposits extended to the west as far as the Columbia River and were observed by members of the 1804-1805 Lewis and Clark expedition shortly after their emplacement. Minor 19th-century eruptions were witnessed from Portland.

Information Contacts: Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

The instrumented, yet now-quiet Mount Lamington resides on the SE peninsula of the main island of Papua New Guinea. It lies roughly across that peninsula from the capital city of Port Moresby and 40 km inland from the Solomon Sea. Lamington's summit contains ragged peaks and a U-shaped crater open to the N. The volcano is ~21 km SSW of Popondetta Town, the provincial center for Oro Province. Lamington does not erupt frequently like Manam and Ulawun, but had a single historical eruption of such magnitude that, if repeated, could be catastrophic for the more than 30,000 people who live nearby.

About fifty years ago, on 21 January 1951, a major explosive eruption at Lamington killed ~3,000 people, the most of all historical volcanic eruptions in Papua New Guinea. Before the 1951 eruption, Lamington was not known to be a volcano. The group of mountains where the volcano stands was covered in thick jungle and there were no stories to suggest that eruptions had occurred before. As documented in a classic study by Taylor (1958), the paroxysmal eruption was not a sudden happening, but had begun several days earlier when nearby residents started to see changes in the summit area. The pyroclastic flow from the eruption devastated an area of ~200 km², forming a radial pattern around the volcano that extended slightly farther on the N side. Two photos illustrating aspects of the eruption appear in figures 1 and 2. One of the hallmarks of Taylor's study was his well-developed timelines that clearly stated the sequence of events.

Figure 1. In an area devastated by a Lamington nuée ardente (pyroclastic flow) on 21 January 1951; this motor vehicle was left suspended in two truncated trees. The person shown for scale is staff member Leslie ToPue, who worked at RVO until 1992. The spot shown lies on the N flank, 9-10 km from the summit dome (in the N end of the settlement of Higaturu), an area directly in front of the summit crater's prominent opening. This photo is cropped from one included in Taylor (1958, 1983) as his figure 69 (page 56). Courtesy of RVO.

Figure 2. Photograph of Lamington taken on 8 February 1951 looking northward into the summit crater's prominent opening and onto the adjacent area immediately downslope of the crater, called Avalanche Valley. The crater contains the steaming dome that grew after the paroxysmal eruption. The mid- to fore-ground shows the ash-mantled NNE slopes (the subject of most of this part of the photo) and mudflow deposits (dark zones, sweeping across limited areas in the right center). This photo came from Taylor (1958, 1983 figure 118 on page 84).

Hastily arranged monitoring commenced immediately after the 1951 eruption but only operated during the active phase of the eruption. A more permanent monitoring program began in 1970 with the installation of a seismograph. In October 1996, a modern seismic station and an electronic tiltmeter were installed on Lamington.

Currently RVO has permanent, smaller observatories at Lamington, as well as at Ulawun, Langila, Karkar, Manam, and Esa'ala. Each is equipped with a recording seismograph. In addition, the stations at Lamington, Ulawun, Karkar, and Manam contain real-time high-frequency data-transmission systems that allow RVO volcanologists to remotely monitor those sites.

Since the 1951 eruption, seismic activity has been absent to rare. Seismic records on 21 December 2000 and 17 February 2001 showed several hours of very high seismicity, but it was difficult to ascertain the cause.

Reference. Taylor, G.A.M., 1958 (2nd ed., 1983), The 1951 eruption of Mount Lamington, Papua: BMR (Australia) Bulletin 38, Australian Government publishing service, Canberra (ISBN 0 644 01969 7; ISSN 0084-7089).

Geologic Background. Lamington is an andesitic stratovolcano with a 1.3-km-wide breached summit crater containing a lava dome that rises above the coastal plain of the Papuan Peninsula of New Guinea north of the Owen Stanley Range. A summit complex of lava domes and crater remnants tops a low-angle base of volcaniclastic deposits dissected by radial valleys. A prominent broad "avalanche valley" extends northward from the breached crater. Ash layers from two early Holocene eruptions have been identified. In 1951 a powerful explosive eruption produced pyroclastic flows and surges that swept all sides of the volcano, killing nearly 3,000 people. The eruption concluded with growth of a 560-m-high lava dome in the summit crater.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

From 1963 to 1982 ash emissions, lava flows, lava fountains, and Strombolian explosions were intermittent. Eruptive activity resumed in July 1998. In December 1998, lava extruded but remained confined to the W-flank craters (BGVN 24:07). Sporadic eruptive activity again took place in March and October 1999. Ash clouds were noted through the end of April 2000 (BGVN 25:04).

This report focuses on field observations of activity during 2000. In mid-February 2000 a pyroclastic flow from the NW-flank crater traveled towards the W and was followed by a smaller debris avalanche that only extended ~250 m in length (BGVN 24:07).

July 2000 visit. A group visited Lopevi on 18-21 July 2000. The following was derived from reports provided by Sandrine Wallez, Douglas Charley, Roberto Carniel, Marco Fulle, and student Esline Garaebiti. Wallez and Charley's sketch map summarizes year 2000 activity (figure 11).

Figure 11. A sketch map of Lopevi emphasizing deposits during 1939-2000. Produced from an original map by A-J. Warden including observations by A-J. Warden and R. Priam (Archive Service de Mines); revised and updated by S. Wallez and D. Charley; drafted by A. Mabonlala. Courtesy of IRD.

The July visitors observed significant deposits on the WSW flank (heavy slash pattern, figure 6) from the February 2000 activity. These visitors found few clear remnants of the pyroclastic-flow deposit. Instead the entire swath was overlain by a debris avalanche and possibly other mass-wasting deposits (figure 6).

Two lava flows came down the W-flank zone impacted by the pyroclastic flow and the debris avalanche. The area lies constrained by a near-vertical topographic discontinuity that reaches 800 m elevation.

The longer lava flow (N2) vented at the SE boundary of a 1963 crater. Overlying one of these lavas, the group found a field overlain by large bombs. The flow accumulated over the intracrater flow of December 1998, and moved in a westerly direction. Another smaller lava flow erupted nearer to the sea on the NW flank. Judging from the map, it reached the sea along a front ~1 km wide.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Sandrine Wallez and Douglas Charley, Département de la Géologie, des Mines et des Resources en eau (IRD), Vanuatu; Roberto Carniel, Dipartmento di Georisorse e Territorio, Università di Udine, Via Cotonificio 114, 33100 Udine, Italy; Marco Fulle, Osservatorio Astronomico, Vai Tiepolo 11, 34131 Trieste, Italy.

The last eruption of Makushin occurred on 30 January 1995 and produced an ash cloud that rose to ~2.5 km altitude (BGVN 20:01). The Alaska Volcano Observatory reported that during July 2000 to June 2001 they detected a slight increase in the number of small earthquakes beneath Makushin. The volcano is located 25 km W of the city of Unalaska/Dutch Harbor in the eastern Aleutian Islands. Hypocenters of the earthquakes generally ranged between 0 and 8 km depth. The events had magnitudes of 0-1.5, so they were too small to be felt by humans. The earthquakes were not thought to be immediate precursors to eruptive activity because similar fluctuations in seismic activity have been observed at a number of Aleutian volcanoes and were not followed by eruptions. The level of concern color code remained at Green.

Geologic Background. The ice-covered Makushin volcano on northern Unalaska Island is capped by a 2.5 km caldera. Its broad, dome-like structure contrasts with the steep-sided profiles of most other Aleutian stratovolcanoes. Much of the edifice was formed during the Pleistocene, but the caldera (which formed about 8,000 years ago), Sugarloaf cone on the ENE flank, and a cluster of about a dozen explosion pits and cinder cones at Point Kadin on the WNW flank, are of Holocene age. A broad band of NE-SW-trending vents cuts across the volcano. The composite Pakushin cone, with multiple summit craters, lies 8 km SW. Table Top (Pleistocene, 68 +/- 14 ka) and Wide Bay (Holocene) cinder cones are about 20 km ENE on the peninsula across the bay from the City of Unalaska. Frequent explosive eruptions have occurred during the past 4,000 years, sometimes accompanied by pyroclastic flows and surges. Geothermal areas are found in the summit caldera and on the SE and E flanks. Small-to-moderate explosive eruptions have been recorded since 1786.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.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.

Activity remained low following the 4 June 2000 eruption of Southern Crater. A pilot's report of multiple lava flows traveling from Manam on 25 June along with an ash cloud to 4.5 km was determined to be false. The Rabaul Volcano Observatory reported that the volcano had been quiet for many months and that the only observed activity occurred on 14 June when fine ash was produced from a small emission, and on 26 June when weak roaring/rumbling noises were heard. After 26 June only occasional low-level ash emissions took place. There have been no instrumental recordings since 16 January 2001.

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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

The following report covers activity during 28 May through most of June 2001, and discusses the high-energy event that began 24 June. This report was compiled from those posted on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) website. Until the evening of 23 June the five-step PHIVOLCS hazard status system for Mayon stood at Alert Level 3, a status that implies a rapid rate of magma supply and that an explosive eruption may occur within weeks. This projection proved true as both the monitored parameters and the vigor or eruptive events rose significantly in late June. A pyroclastic flow on 24 June stimulated the rise to Alert Level 5, and this status remained for all or most of the month. Tables 3 and 5 summarize SO₂ flux and seismic data; table 4 describes the qualitative scale of crater glow intensity.

Table 3. SO₂ fluxes for Mayon during 28 May through June 2001; questionable values that were ambiguously referred to in the daily report appear in parentheses. Mayon's stated baseline values have been ~ 500 metric tons per day (tons/day). Values were measured by COSPEC. Taken from reports posted on the PHIVOLCS website.

Table 4. Qualitative scale of the intensity of crater glow used at Mayon. Through mid-June, crater glow fell into one of the first three categories; heightened activity led to stronger glow and Intensity IV was introduced; it was first reported for the evening of 23 June. Crater glow was often mentioned in daily reports, sometimes with descriptions of the incandescent part(s) of the dome or lava flows. Courtesy of PHIVOLCS.

Table 5. Mayon seismic data at Upper Anoling station as posted on daily reports in June, with the relative amplitudes shown in parentheses where clearly stated. Dashes are used to represent undisclosed values. "Tremor" refers to short-duration high-frequency harmonic tremor linked to rockfalls. Some intervals of continuous tremor appeared in late June as noted in the comments. Courtesy of PHIVOLCS.

Activity during 1-8 June 2001. During this time period, seismic instruments registered generally increasing numbers of tremors (table 5). Many of these tremors were of high frequency but short duration and inferred to be associated with mass-wasting of lava-dome fragments that descended from the volcano's SE rim. Other kinds of tremor were seen later in the month (see table 5).

The summit lava dome glowed brightly (Intensity III, table 4) during cloud breaks on the night of 1 June. During 2-8 June crater glow held steady at a Level II intensity except for 4 and 6 June when it varied between Level II and Level III. Incandescent materials occasionally rolled down from Mayon's summit, traveling along the SE slopes in the upper Bonga Gully. Glow came from detached zones of extruding, pasty lava at the dome's W base and SE face. On 3 and 6 June moderate to weak steaming issued from the summit crater.

Activity during 9-16 June 2001. As observed from Legazpi City and vicinity, lava fragments frequently detached from the summit dome and slid or rolled into the Bonga Gully to the SE and deposited a pyroclastic fan on Mayon's middle to upper slopes. Nearly continuous rockfalls produced distinct ground tremor with high-frequency spectra. PHIVOLCS noted that recordings of these multiple rockfall events from the reference station in Upper Anoling graded into each other, indicating more vigorous extrusions and rockfall events than those recorded by the station.

Ground-deformation surveys using EDM (Electronic Distance Meter) instruments were unable to make readings due to weather during 2-8 June. The previous reading, made on 28-29 May 2001, found universal inflation (i.e. displacements along the line LHO-Lower Slope measured -9 mm and the line Buan-MRHO, -6 mm). Ground deformation recorded on 10 June again indicated a minor degree of inflation (the line Buang-MRHO, -1 mm).

At 1819 on 12 June, part of the summit lava dome collapsed and heralded a period of vigorous rockfalls from the lava dome; however, no lava flow formed. Bright glow (Intensity III) occurred at a point in the mid-portion of the dome where extruding pasty lava squeezed out.

On 10 June moderate steam emission at the summit correlated with an SO₂ flux of 4,115 metric tons/day (t/d) (table 3). At this point in time, Mayon was still considered to be in a mild state of eruption with magma only slowly intruding the summit. On 11 June PHIVOLCS noticed an increase in the overall tempo of unrest, including days with elevated numbers of rockfall-induced tremor.

At 1347 on 11 June the dome partially collapsed and produced a small pyroclastic flow that descended along the Bonga Gully. The flow reached about 1,480 m elevation and produced a thin ash cloud, which drifted E. Similarly, on 12 June at about 1819 the summit lava dome again partly collapsed, spawning vigorous, continuous emissions of lava fragments until about 1930.

Activity during 17-23 June 2001. On 23 June mild explosive activity and lava fountaining took place. Prior to that, a significant change in the pace of unrest was indicated by the appearance of tremor at 0405 on 19 June. A lava flow spotted during a cloud break from 1008-0152 enabled observers to see an intense glow emitted by the dome and the margins of a newly emplaced lava flow, which extended to about 500 m below the summit dome (to ~1,800-1,900 m elevation). The tremor so dominated the seismic record that discrete rockfall counts dropped. Only 76 rockfall-related tremors were registered, although extrusive activity had clearly increased. The lava flow signified that hotter, more fluid, and more voluminous lavas were being extruded. The new lava corresponded to a sudden increase in sulfur dioxide emissions from 1,700 metric tons/day (t/d) the previous week to nearly 6,000 t/d on 19 June.

By 20 June the volcanic edifice had inflated slightly as recorded by ground-deformation surveys. Tiltmeters midway up on the NE edifice, at the Buan-Mayon Resthouse station, registered accelerating inflation. During 1209-1218 on 20 June a portion of the lava dome collapsed, generating brownish dust clouds along the Bonga Gully.

On 21 June lavas were seen exiting from two points of the dome. Two lobes descended, both on the SE side (in the general direction of the settlements of Buyuan and Mabinit). Magma ascent through the uppermost levels of the volcano's conduit appeared to be associated with high-frequency harmonic tremor at all five seismic stations in the vicinity of the volcano. Magma intruding the summit area also exerted pressure on the edifice and influenced ground tiltmeters. The COSPEC instrument measured the highest SO₂ flux of the June episode: ~9,000 t/d.

The main lava flow moved SE in the general direction of Mabinit on 21 June, and the lowermost toe of the lava flow descended 300 m farther, to ~1,500 m elevation. On 22 June the lava flow reached 1,200 m elevation; by 23 June, it had descended 3.4 km from the summit to reach 600 m elevation.

At 1909 on 23 June, lava fountaining in the summit crater ejected material at least 50 m above the rim, with the bulk of pyroclasts falling to the SE (into the upper Bonga Gully). As lava flows continued to travel SE they generated high-frequency tremor. Activity was still dominated by relatively rapid but quiet effusion of lava. At this point the seismicity lacked clear explosion signals and deformation measurements lacked inflation signals; it was believed that such signals would presumably accompany a major explosive eruption (if one were to occur).

Activity during 24-30 June 2001. At 2000 on 23 June the Alert Level was raised from 3 to 4 when the already substantial lava extrusions changed from quiet effusions to more explosive, but nonetheless non-destructive, Strombolian outbursts. The latter were first observed in the crater at 1909 on 23 June. Small explosions in the crater sent molten lava up to 50 m above the rim.

At 0317 on 24 June, a series of strong explosions were audible as far as Lignon Hill Observatory, 12 km SSE of the volcano. Accompanying ash columns reached 1 km above the summit. Visible molten lava fragments were thrown to 300 m in height. Lofted ash blew N and ash fell in the barangays (settlements) Amtic and Tambo of Ligao City and barangays San Vicente, San Antonio, Quinastillojan, Bantayan, Tabiguian, and Buang of Tabaco City.

At 1245 on 24 June a pyroclastic flow descended the Bonga and Buyuan Gullies to ~600 m elevation, about 4 km from the summit. An explosion from the crater also produced a 5-km-high column. Ash associated with the pyroclastic flow ascended to ~2.4 km altitude. The two ash-laden clouds then drifted NE, in the general direction of Malilipot (a town 10 km away on the coast).

The 24 June pyroclastic flows signaled the start of explosive eruptions with tall columns. At 1300 the hazard status was raised from 4 ("Hazardous Eruption Possible Within Days") to 5 ("Hazardous Eruption in Progress"). Concomitant with Alert Level 5, the previously delineated 7-km-radius Extended Danger Zone in the SE sector was extended to a radius of 8 km. People within these new zones evacuated. Areas to the E and NE of the volcano were considered prone to heavy ashfall due to prevailing winds.

Another major eruption sequence began at 1444 on 24 June, characterized by strong explosions, multiple pyroclastic flows around the volcano, and lava flows into SE-flank gullies. Following drainages, the pyroclastic flows passed the settlements of Basud, Buyuan, Mabinit-Bonga, Miisi, Anoling, Maninila, Nabonton, and Buang, all within the 6-km-radius Permanent Danger Zone (PDZ).

The main eruption cloud discharged from the crater rose to about 10 km altitude and moderate-to-heavy ash blew mainly NE towards Malilipot. Residents ~5 km N of Malipot (in Tabaco) along the coast also experienced light ashfalls. Lava flows and dilute ash clouds dominated activity after 1541. Activity waned in the early morning of 25 June. Beginning at 0037 on 25 June seismicity diminished from continuous tremors into discrete events.

On 26 June Mayon lapsed into an apparently quiet state; however, SO₂ flux remained high at 4,640 t/d and reflected active degassing from both the crater as well as from newly extruded lavas covering the summit area. Lava still flowed SE from the summit area along Bonga Gully on the 26th, but its lowermost portions moved slowly. The lava by then extended ~4.3 km from the summit. Its flow front constantly shed incandescent boulders that released gases and ash, burning vegetation along its path. However, the crater's diminished extrusion rate led PHIVOLCS scientists to conclude that the lava flow was unlikely to reach populated areas.

Although outward quiet prevailed for most of 24-30 June, several explosion signals occurred during 26-27 June. One explosion sent an ash cloud to about a kilometer above the summit and caused small lava avalanches in the upper Bonga Gully. Lava continued to trickle from the summit towards the SE along the Bonga Gully. From this time through at least 29 June crater glow stood at Intensity II and lava continued to descend from the summit crater.

Heavy rains fell on the night of 27 June. A team dispatched to the Padang area watched the river channel for lahars. Only a muddy stream flow was observed and rains eventually abated after about an hour. The swollen, muddy streams after this time meant that smaller amplitude volcanic earthquakes were often obscured by the seismic noise produced by the streams. Ground deformation measurements employing EDM instruments and electronic tiltmeters continued to indicate inflation of the edifice. Observers also noticed small rockfalls, and vigorous steaming of the hot lava deposits.

At 1605 and 1702 on 30 June, explosions generated pyroclastic flows that swept the upper and middle slopes within the Bonga Gully and produced billowing ash clouds to about 4 km altitude. Their runout distance reached ~3 km from the summit (in the general direction of Matanag). During the eruption an undisclosed portion of the volcano's E sector also collapsed along the Upper Basud Gully.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Avenue, U.P. Diliman, 1101 Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

This report covers the period from November 2000 through May 2001. Activity at Rabaul was relatively low through this period until 14 March, when low-frequency earthquakes resumed and continued to increase in number and amplitude throughout that month. These earthquakes were apparently precursors to an ash eruption at Tavurvur on 2 April after several months of relative quiet.

Occasional ash-laden clouds resulting from mild explosions occurred in January and February. White vapors were released in varying amounts from Tavurvur. Two large explosions occurred on 12 and 26 January producing a dark gray, billowing ash cloud that rose to ~1,000-2,000 m above the summit before dispersing W and NW. The explosions showered the flank of the volcano with rock fragments and deposited significant amounts of ash on Rabaul Town. For short periods during these months H₂S was smelled downwind of Tavurvur.

Seventeen high-frequency earthquakes were recorded in March, only five of which were determined as having originated from NE and ESE of the caldera. No high-frequency earthquakes have been recorded on the once-active ring-fault seismic zone since 1995. Between February and the end of March, GPS recorded ~1.5 cm of uplift in the central part of the caldera, while an electronic tiltmeter measured ~3-4 µrad of inflation.

The caldera had previously subsided about 4 cm on 16 November 2000, associated with earthquakes N of Rabaul. According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), two earthquakes, M 7-8, occurred in Papua New Guinea about 3 hours apart on 16 November. The first earthquake was ~50 km N of Rabaul and just S of New Ireland. The second earthquake struck ~100-150 km from Rabaul and N of New Ireland, near the Lihir, Tabar, and Tanga Islands. Both earthquakes occurred about 50 km below sea level. Tsunami of 1-2.5 m height caused damage on New Britain, New Ireland, and Bougainville, leaving thousands homeless; no casualties were reported. At least four other M ~6.5 aftershocks were reported in the following days. According to the BBC, recent tectonic activity has caused subsidence of coral islands between New Ireland and New Britain. As many as 40,000 people may need to be evacuated.

At 1300 on 2 April the number and amplitude of the low-frequency earthquakes increased again, culminating in the first ash clouds between 2100 and 2200. Figure 36 shows an ash eruption on 4 April 2001. Similar low-frequency earthquakes were noted a few days before the 28 November 1995 eruption. High-frequency earthquakes, another good indicator of eruptive activity, continued to occur on the NE side of the volcano during April 2001. Other parameters indicating signs of likely renewed eruptive activity were 3-4 months of slow inflation in the central part of Rabaul Caldera, GPS measurements that showed ~3-4 cm of uplift, and tiltmeter measurements near the GPS benchmark and ~2 km from Tavurvur that also indicted inflation. The smell of sulfuric gas was noted occasionally.

Figure 36. Ash eruption on 4 April 2001 at the Tavurvur cone. This photo was taken looking from the NW and shows the SE side of the cone. Courtesy of RVO.

From 2 to 24 April Tavurvur's ash emissions fluctuated between white to pale-gray ash clouds and sub-continuous ejection of pale- to dark-gray ash clouds. Beginning at about 1400 on 25 April, activity changed to short explosions that produced white to pale-gray mushroom-shaped ash columns and were usually accompanied by roaring noises. During the month ash clouds rose from a few hundred to ~1,000 m above the summit area. Variable winds blew the ash N and NW. Similar eruptive activity continued through the end of April.

During April, 1,089 low-frequency (LF) earthquakes were registered by the trigger system. Daily LF totals ranged between 0 and 291. High LF totals occurred on the 25th (172), 26th (291), 27th (228), and 28th (212). This period corresponded to the time when the mode of Tavurvur's eruptive activity changed from occasional sub-continuous ash cloud emissions to frequent, short-duration ash cloud expulsions. The totals for April 2001 were substantially higher than for the previous months of January (22), February (31), and March (13). During April, short duration, non-harmonic volcanic tremors were also recorded and were usually associated with the sub-continuous ash cloud emissions. On the other hand, during April the system recorded only six high-frequency earthquakes, fewer than in January (15), February (8), and March (17). Moreover, in April, half of the high-frequency earthquakes struck to the NE and outside the caldera.

During May, Tavurvur emitted pale gray to white ash clouds, sometimes accompanied by 0.5-2 minute periods of roaring. The ash clouds typically reached as high as several hundred meters above the vent. During the first half of May incandescent explosions were observed at night, but towards the end of May these explosions lessened in frequency and vigor. The roaring noises also lessened. On 30 May the roaring noises were replaced by stronger, discrete explosions. These produced dark ash clouds that rose to 1-1.5 km above the vent. In general, intra-caldera seismicity was low in frequency and associated with explosions. Almost 2,000 seismic events were recorded.

The unambiguous inflationary trend observed over the previous six months slowed in early May, and a period of relative stability occurred until the end of the month. The start of the darker emissions heralded a period of small-scale rapidly fluctuating vertical movements, but no overall inflationary or deflationary trend predominated.

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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Shiveluch erupted at 0209 on 22 May (BGVN 26:04) and produced a mushroom-shaped ash column to an estimated altitude of ~20 km. According to reports from Klyuchi, the event destroyed both the new dome (first observed on 12 May) and the W part of the old dome. GMS satellite imagery at 1432 on 22 May showed the eruption cloud as it continued to diffuse over the Kliuchevskoi volcanoes; at that time the estimated plume area reached ~50,000 km². The hazard status remained at Red as of 22 May.

On May 23, an approximately 10-pixel anomaly with temperatures at 30-49°C was observed on satellite images. The anomaly was large and elongated to the S. It may signify a new pyroclastic-flow deposit.

By 24 May the hazard status had been lowered to Orange and, by 31 May, to Yellow. The hazard status was unchanged until 29 June, when a short-lived explosion sent an ash plume to a height of 1,200 m above the dome; associated pyroclastic flows had runouts of ~2.5-3.0 km. During the period from the end of May to the end of June, gas-and-steam plumes were observed rising 500-1,200 m above the dome. Seismic activity remained above background with earthquakes of M 2-3, and many small earthquakes within the edifice. On 8 June a short-lived explosion sent an ash plume 2,000 m above the dome accompanied by 2- and 3-minute-long, shallow seismic events.

During the week of 22-28 June, instruments registered seven M 2 earthquakes, many small earthquakes within the volcano's edifice, local seismic signals (explosions, avalanches, collapses), and episodes of weak spasmodic volcanic tremor. Based on seismicity, a possible increase in eruptive vigor occurred at 1500 on 28 June, a time when tremor and the number of shallow earthquakes increased.

At 1150 on 29 June, the aforementioned short-lived explosion occurred. The hazard status was again raised to Orange. Seismic data recorded on 29 June suggested possible explosion plumes that ascended to ~6 km above the dome (~8.5 km altitude). According to a Tokyo VAAC report, at 0300 on 30 June the ash plume attained 7.3 km altitude.

At 1250 on June 30 another short-lived explosion sent an ash plume to ~8.0 km altitude. The top part of a mushroom-like plume slowly extended to the E. Pyroclastic flows passed 5 km down the Baidarnaya River. Weak volcanic tremor and local seismic signals (avalanches) continued. Starting at 0100 on 2 July, earthquakes occurred in greater number, larger magnitudes, and at greater depth (~5 km). By 6 July the hazard status was returned to Yellow.

Subsequently, seismic activity continued above background levels. A magnitude 2 earthquake accompanied many smaller ones within the edifice, some 3-minute-long shallow seismic events, a variety of local seismic signals, and episodes of weak tremor. In mid-July this spasmodic tremor increased. At 1900 on 14 July it reached velocity-characterized amplitudes of 1.7 x 10^-6 m/s; at 2020 that day it reached 2.0 x 10^-6 m/s; at 0300 on 16 July it increased to 2.5 x 10^-6 m/s and finally, after 2300 on July 15, it attained 4.0 x 10^-6 m/s. Accordingly, the hazard status was set to Orange and visual observations from Klyuchi at 2100 on 15 July indicated that a gas plume rose 1,500 m above the dome. Seismic data suggested the plume was accompanied by explosions.

An AVHRR image (number 12.01196.05:03) at 1803 on 15 July revealed a 3-pixel thermal anomaly near the SW flank of Shiveluch. The maximum band-3 temperature was 44°C within a background near 22°C. No associated ash was observed in the imagery.

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.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; Anchorage Volcanic Ash Advisory Center (VAAC), NOAA Alaska Aviation Weather Unit, 6930 Sand Lake Road, Anchorage, AK 99502-1845, USA (URL: http://vaac.arh.noaa.gov/); Tokyo Volcanic Ash Advisory Center, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).

A previous report about the eruption plumes of late April-early May was based on information received from satellites (e.g., TOMS, which disclosed 5 ktons of SO₂) and the Darwin VAAC (BGVN 26:05). This follow-up recounts ground-based reports from the Rabaul Volcano Observatory (RVO). It covers the new flank-vent eruption and its preceding events. Ulawun's prior eruption was about 7 months earlier (BGVN 25:11).

On 25 April, Ulawun began what appeared to ground-based observers as a relatively small eruption that lasted about 6 days (ending the 30th). Activity had been low from the beginning of April until the 24th, with the summit venting mainly small, occasionally moderate volumes of steam. Seismicity consisted mainly of low-frequency earthquakes, which had been present for many months, even before the September 2000 eruption. The low-frequency earthquakes were slightly larger than the usual earthquakes recorded when Ulawun is quiet, but no particular pattern indicated that these earthquakes were forerunners to an eruption. Earthquakes such as these were rare before the build-up to the September eruption, but they have continued since then.

Ashfall from the 25-30 April eruption blew mainly N and NW during the second and third Strombolian episodes, 27-29 April. Most ash fell along a NW-trending axis (270-300° from the summit). Nearby residents were evacuated, and as of 14 June were allowed to return home. No damage or casualties were reported.

Behavior in months prior to the 25 April eruption. Ongoing sporadic tremors followed the 28 September 2000 eruption for most of October-January. A swarm of earthquakes occurred between 31 January and 1 February 2001. The only break in activity was in February, March, and the first part of April.

The high seismicity on 31 January was followed the next day by occasional deep roaring and rumbling noises. On 2 February thick dark-gray and gray-brown emissions caused ashfall to the NW around Ubili village. Poor visibility after 0800 prevented further observations. The next day weak-to-moderate thin white vapor was observed. Similar summit activity was reported on 4 February with occasional booming noises between 1300 and 1400. After the 5th, thin white vapor was present on most days in February.

Seismicity during 31 January-2 February was characterized by B-type volcanic events, which occurred at irregular intervals. During the last week of January, continuous background volcanic tremor was recorded. On the morning of 31 January the seismicity suddenly changed to distinct B-type events. Within a few hours the events intensified and became hard to distinguish due to signal overlap on the analog records. The intense seismic activity lasted for several hours and then declined to a low level. It remained relatively low, with distinct B-type events, until the morning of 2 February, when the B-type events intensified again. Afterwards, seismicity declined to a very low level. Distinct B-type events continued, but in very low numbers. A-type volcanic events also occurred throughout February, but the month was generally quiet.

Most of March was also quiet, characterized by thin white vapor emission, except on 2-4 March when occasional weak puffs of gray-brown ash were produced. Villagers on the N, NW, and SW sides of the volcano reported rumbling and booming noises associated with the ash puffs. A weak, steady glow was observed on 27 March. Low-frequency earthquakes continued throughout the month with an average of 60 per day. Some high-frequency earthquakes also occurred, but no volcanic tremors were recorded during March.

The highest seismicity outside of the eruption took place between 31 January and 1 February. It was followed by a rapid inflation of 3-4 µrad in a few days. This was followed by deflation of about 10 times less. The September 2000 and April 2001 eruptions occurred during deflationary periods preceded by a few months of inflation. In retrospect one might speculate that the seismic swarm and inflation were signs of rapid intrusion of significant volumes of magma to a shallow depth.

Behavior in the days prior to the 25 April eruption. The eruption was preceded by volcanic tremors commencing at about 0600 on 22 April. The tremors were initially small, but at about 2100 the they increased in amplitude and became sub-continuous. On 24 April at 1400 the tremors increased again, making it hard to detect patterns in the analog records.

This was when RSAM (Real-time Seismic Amplitude Measurement) data became useful. According to the RSAM, after 1400 tremor levels increased exponentially until about 1800 on the 25th, when it began to fluctuate. The start of the fluctuations coincided with the beginning of a steady weak glow from the summit vent. Earlier, occasional forceful emissions of weak to moderate gray ash clouds had begun at about 0600 on the 25th, and occasional low rumbling noises began at about 1600. Activation of Stage 1 of the Ulawun Volcano Stage of Alert system was recommended to authorities at 0200 on the 25th.

Phases of the 25-30 April eruption. Volcanism on 25 April consisted of a steady weak red glow, occasional rumbling noises, and thick ash clouds. This lasted until about 0530 on 26 April, when a small Strombolian eruption began. Glowing lava fragments ejected by frequent explosions were restricted to the summit's N and NE sides. Small pyroclastic flows occurred, but also failed to progress beyond the summit area. Ash clouds blew NW dropping very fine ash. The Strombolian activity lasted about an hour. Activity then subsided and noises became infrequent; but forceful ash-bearing emissions continued.

Activity reached a low at about 0300 on the 27th before another phase of Strombolian eruption began at about 0530. The build-up to the second phase was very rapid. Stage 2 hazard status was recommended at 1630 on the 27th. Activity was sustained at an intense level for about 30 hours from 0530 on the 27th to about 1130 on the 28th. Incandescent lava fragments (visible in the early morning) and other rock material from the intense activity rolled almost a third of the way down the slopes. Eruptive material was seen on all sides of the volcano, but most went N and NE, suggesting emissions came from near Vent B (BGVN 25:11) at 1,600-1,800 m elevation. In this interval a pyroclastic flow traveled N-NE following the path of the pyroclastic flow of 28-29 September 2000. The run-out distance of the pyroclastic flow exceeded that of the flow from the September eruption. A lava flow also followed the same path. The distal end of the lava flow reached about 500-600 m elevation.

Another period of slightly lower activity followed the second phase of the eruption. The third phase of Strombolian eruption began at about 0600 on the 29th. This phase was slower and more gradual, peaking at about 1800-2000 on the 29th.

Early in this phase, local people reported ash emissions from a site in a gully where the pyroclastic and lava flows had passed. It was later confirmed that a dike had reached the surface, resulting in a fissure where ash emissions were released. A lava lobe protruded from the new vent and extended about 20 m downslope. Figure 4 shows a mild explosion from this vent on 3 May. Dike intrusions were also observed during the 1978 eruption at Ulawun, and resulted in surface fissure activity on the higher SE slopes and farther down on the E slope, which produced a lava flow.

Figure 4. A mild explosion on 3 May 2001 from the new vent on Ulawun's NNE flank. The photo was taken just three days after the 25-30 April eruption ended. This fortuitous view of the small ash cloud helped fixed the new vent's location. Courtesy of Ima Itikara, RVO.

The last phase of this Strombolian eruption fluctuated before it began to decline at about 1130 on 30 April; the eruption stopped at about 2400. Although the 25 April eruption was comparatively small, the development of radial fissures from dike intrusions in the upper interior of the volcanic system might contribute to weaknesses in the structure of the volcano (figure 5).

Figure 5. The summit and NNE flanks of Ulawun taken 23 May 2001 showing the whereabouts of the new vent near the head of a ravine and a notch in the summit crater's wall at a point upslope from the ravine and vent. The new fissure-shaped vent is not directly visible in this shot; it lies in shadow at the ravine's bottom, and it is not degassing. Courtesy of Ima Itikara, RVO.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Recent work by Hart and others (2000) has described this volcano and identified it as the source of acoustic signals noted in July 1973 and an earthquake swarm during January 1995 (BGVN 20:01 and 20:02). The following is from Hart and others (2000) except where noted.

Vailulu'u Seamount is located 45 km east of Ta'u island, the easternmost island of the Samoan chain, and defines the leading edge of the Samoan swell (figure 2). Mapped in March 1999 with SeaBeam aboard the RV Melville during AVON cruises 2 and 3 (figures 2 and 3), Vailulu'u rises from an ocean depth of 4,800 m to its crater rim within 590 m of the sea surface, with a total volume of ~1,050 km³. The summit includes a 400-m-deep, 2-km-wide crater (figure 4). These cruises were motivated by the 1973 and 1995 acoustic and seismic events in this region, and were a direct attempt to find the current location of the Samoan hotspot.

Figure 2. Bathymetry of Vailulu'u and nearby Ta'u Island, based on a SeaBeam bathymetric survey performed during R/V Melville's AVON 2 and 3 cruises, augmented with satellite-derived bathymetry from Smith and Sandwell (1996). The inset shows the general location of Vailulu'u with respect to the Samoan Archipelago; two other newly mapped and dredged seamounts (Malumalu and Muli, AVON 3 cruise) are shown as well. Scale: 10' = 18 km. From Hart and others (2000).

Figure 3. Perspective view of Vailulu'u seamount looking NW, displaying three major rifts toward the E, SE, and W. The lower slopes of Vailulu'u and Ta'u merge along the western ridge, with a saddle at 3,200 m. Vailulu'u is ~ 35 km in diameter at its base. Scale: 10' = 18 km. From Hart and others (2000).

Figure 4. SeaBeam bathymetry map of the summit crater of Vailulu'u, showing the crater rim with three peaks and three breaches, the location of CTDO (conductivity, temperature, depth, optical) casts 1 and 4, and the tow-yo track circumnavigated around the summit. Dotted azimuth lines are given every 30° along the track. Scale: 1' = 1.8 km. From Hart and others (2000).

The overall shape of Vailulu'u is dominated by two rift zones extending E and W from the summit, defining a lineament parallel to the Samoan hotspot track. A third, slightly less well-developed rift extends SE from the summit, and several minor ridges extend out from the lower slopes, making an overall asymmetric, star-like pattern. Rift zones and ridges in the southern sector are more strongly developed than those on the N flank, giving Vailulu'u a stunning similarity to a juvenile Ta'u island (figure 2). The three major rift zones define three high points of the crater rim. The crater and rim are oval-shaped (figure 3), with two well-developed pit craters defining the northern two-thirds of the crater and two minor depressions on a bench in the southern third of the crater.

Several historical events suggest volcanic activity. There was a series of acoustically detected explosions on 10 July 1973 (Johnson, 1984), and during 9-29 January 1995 the global seismic network recorded a strong (M 4.2-4.9) earthquake swarm in the vicinity (BGVN 20:01 and 20:02). While most of the 1995 earthquakes were formally located NW of the volcano, their uncertainty ellipses include Vailulu'u; a SeaBeam survey within the apparent earthquake area did not reveal any volcano-tectonic features. Dredges, especially those from the summit area, are dominated by fresh volcanic rock, with pristine volcanic glass, many original glassy surfaces, unaltered olivine phenocrysts, and a virtual lack of vesicle fillings. Extremely "bright" SeaBeam sidescan returns suggest that fresh volcanic rocks occur ubiquitously throughout the slopes of Vailulu'u and that sediment cover is largely absent.

A detailed nephelometry survey of the water column shows clear evidence for hydrothermal plume activity in the summit crater. The water inside the crater is very turbid, and a halo of "smog" several hundred meters thick encircles and extends away from the summit for at least 7 km (see Hart and others, 2000, for details).

During the DeepFreeze 2000 cruise in March 2000, aboard the U.S. Coast Guard Icebreaker Polar Star, conductivity temperature depth optical (CTDO)/Niskin stations were occupied at three places within the summit crater and two outside the crater; in addition, the summit area was circumnavigated in tow-yo mode along the ~1,000-m contour (figure 4). Particulate distribution in the water column was studied using a light backscattering sensor (LBSS) attached to a CTD/Niskin water sampling rosette. At 600-m depth in the crater turbidity increased sharply and continued to do so in a stepwise fashion to the bottom of the crater at 996 m. Turbidity near the bottom was greater than that associated with active venting and plume formation on ridge crests. At station 1, outside the crater, the LBSS "smog" layer starts at about the same depth (610 m) but returns to background values at 850 m. This depth interval is comparable to the elevation range of the crater rim, which has peaks at 590 m and a deepest breach at ~780 m (figure 4). At station 5, 7.5 km E of the crater rim, a small turbidity anomaly was observed at a depth of 600-720 m.

During a complete 360° circumnavigation of the summit crater, the plume was mapped from 500 to 900 m depth in tow-yo mode (figure 4). Overall, the hydrothermal plume was confined to a narrow depth interval bracketed between the breaches and summits of the crater wall. Its upper, neutral buoyancy, level corresponds closely with the heights of the peaks on the crater rim. Virtually no particulate matter appears to be ejected from the crater to heights above the peaks on the crater rim nor does any settle below the breach depth during its dispersion laterally away from the summit. Particulates are being generated within the crater and are subsequently carried away by ocean currents.

Vailulu'u is clearly a young and active submarine volcano. Its activity is reflected in acoustic/seismic events in 1973 and 1995, the lack of any sediment cover, fresh basalt and pristine glass in dredges from all levels, and radiometric ages ranging from 5 to 50 years. The summit is marked by a sharply delineated crater over 400 m deep, filled with highly turbid water. This smog layer extends out as a halo for many kilometers in all directions, in a narrow depth interval defined by the range in depths of the rim of the summit crater.

During another cruise to Vailulu'u in April 2001, on the USCG Icebreaker Polar Sea, Hart and colleagues retrieved five hydrophones and temperature loggers that had been deployed the year before. A lot of minor seismic activity was still occurring, but detailed analyses have not been completed. The crater was still full of "smog," indicating that the crater remains hydrothermally active.

Previous work by Rockne Johnson. This seamount was discovered on 18 October 1975 by Rockne Johnson (Johnson, 1984) using an echosounder and a proton magnetometer aboard the 19-m ketch Kawamee while searching for the source of explosions detected on 10 July 1973. Those explosions, 26 within a 30-minute period, were identified in records from SOFAR (sound-fixing and ranging) stations at Wake and Midway Islands. The signals were calculated to have been from a source along a line that fell 15 km E of Ta'u Island, and were distinct from signals recorded a few hours later caused by a submarine eruption south of Curacao Reef 500 km W at the north end of the Tonga Ridge (CSLP Cards 1679, 1685, and 1694). Depths near 600 m were found around the summit, and a large magnetic anomaly was centered 4 km NW of the summit. Johnson (1984) believed that the seamount, which he named "Rockne Volcano," was the most likely source for the July 1973 activity, but noted that there was some doubt because of its distance from the line of position calculated from the acoustic data.

Selection of a volcano name. As reported by the Samoa News, the Samoa Department of Education's Science Department held a "Name that Volcano" contest in the high schools to come up with a permanent name for this volcano. Previously the volcano had been catalogued as "Unnamed" (Simkin and Siebert, 1994), and named "Rockne" (Johnson, 1984) and "Fa'afafine" (Hart and others, 1999). Woods Hole Oceanographic Institution scientist Stan Hart urged that the name endorsed by American Samoa be adopted by the scientific community. The winning entry, announced on 8 May 2000, came from Taulealo Vaofusi, a sophomore at Samoana High School. "Because of the location of the volcano being very close to the Manu'a Islands village of Ta'u," Vaofusi explained to the Samoa News, "I would like to rename that volcano 'Vailulu'u Volcano.' According to legend, Vailulu'u was the sacred sprinkling of gentle rain that fell just before the gatherings of the great King Tuimanu'a. The Manu'a group is also call the sacred islands or the Motu Sa, and the name 'Vailulu'u' is given to the fountain owned by King Tuimanu'a," said Vaofusi in his entry form.

References. Hart, S.R., Staudigel, H., Koppers, A.A.P, Blusztajn, J., Baker, E.T., Workman, R., Jackson, M., Hauri, E., Kurz, M., Sims, K., Fornari, D., Saal., A., and Lyons, S., 2000, Vailulu'u undersea volcano: The New Samoa: Geochemistry, Geophysics, Geosystems (G³), American Geophysical Union, v. 1, December 8, 2000.

Hart, S.R., Staudigel, H., Kurz, M.D., Blusztajn, J., Workman, R., Saal, A., Koppers, A., Hauri, E.H., and Lyons, S., 1999, Fa'afafine volcano: The active Samoan hotspot: EOS Transactions, American Geophysical Union, v. 80, 1999 Fall Meeting Supplement, p. F1102.

Johnson, R.H., 1984, Exploration of three submarine volcanos in the South Pacific: National Geographic Society Research Reports, National Geographic Society, v. 16, p. 405-420.

Simkin, T., and Siebert, L., 1994, Volcanoes of the World, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Smith, W.H.F., and Sandwell, D., 1996, Predicted bathymetry, new global seafloor topography from satellite altimetry: EOS Transactions, American Geophysical Union, v. 77, no. 46, p. 315.

Stice, G.D., and McCoy, F.W., Jr., 1968, The geology of the Manu'a Islands, Samoa: Pacific Science, v. 22, p. 427-457.

Geologic Background. Vailulu'u, a massive basaltic seamount discovered in 1975, rises 4,200 m from the sea floor to a depth of 590 m. Located about one-third of the way between Ta'u and Rose islands at the E end of the American Samoas, it is considered to mark the current location of the Samoan hotspot. The summit contains an oval-shaped crater that is 2 km wide and 400 m deep. Two principal rift zones extend E and W from the summit, parallel to the trend of the hotspot; a third rift extends SE. The rift zones and escarpments produced by mass wasting phenomena give the seamount a star-shaped pattern. On 10 July 1973 explosions were recorded by SOFAR (hydrophone records of underwater acoustic signals). An earthquake swarm in 1995 may have been related to an eruption. Eruptive activity between April 2001 and April 2005 formed a cone almost 300 m high, named Nafanua. Repeated bathymetric mapping surveys showed depth changes, including height and width increases of Nafanua after 2005, that suggest at least intermittent activity during 1999-2017; a 2019 survey showed no further changes since 2017.

Information Contacts: Stanley R. Hart, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA (URL: http://www.whoi.edu/); Samoa News, P.O. Box 909, Pago Pago, AS 96799 (URL: http://www.samoanews.com/).