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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The Kamchatka Volcanic Eruptions Response Team (KVERT) characterized Bezymianny as having weak activity from mid-June 2014 through the end of 2015, including weak or moderate gas-and-steam emissions (figures 17 and 18) and, when not obscured by clouds, weak thermal anomalies (BGVN 41:01). Observations here through May 2017 come from KVERT reports and Tokyo Volcanic Ash Advisory Center (VAAC) advisories.

Figure 17. View of the summit showing fumarolic activity at Bezymianny on 16 September 2014. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS, KVERT.

Figure 18. Moderate gas-and-steam activity at Bezymianny on 15 April 2015. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS, KVERT.

Activity during 2016. KVERT reported that weak volcanic activity continued into January 2016, with moderate gas-and-steam activity through 12 December 2016. During this time, satellite data by KVERT showed a weak thermal anomaly over the volcano on most days, although on some days KVERT described the volcano as "quiet." Often the volcano was obscured by clouds.

The Tokyo VAAC reported that on 30 July an ash plume rose to an altitude of 3 km and drifted E, an observation based on information from the Yelizovo Airport (UHPP). Weak fumarolic activity continued in late August (figure 19).

Figure 19. A small, weak, fumarolic plume could be seen rising from Bezymianny on 24 August 2016. Photo by O. Girina; courtesy of IVS FEB RAS, KVERT.

Based on KB GS RAS (Kamchatka Branch of Geophysical Services, Russian Academy of Sciences) data, KVERT noted that seismicity began to increase on 18 November. The thermal anomaly temperature detected in satellite images also increased on 5 December, and then significantly increased on 13 December, probably caused by lava-dome extrusion. This activity prompted KVERT to raise the Aviation Color Code from Yellow, where it had been since 17 July 2014, to Orange (second highest level).

According to KVERT, a gas-and-steam plume containing a small amount of ash drifted about 118 km W on 15 December. The Tokyo VAAC noted that ash plumes rose as high as 6.1 km that same day. KVERT reported strong gas-and-steam emissions during 16-31 December (figure 20); a gas-and-steam plume drifted about 60 km SW on 18 December. A daily thermal anomaly was detected over the volcano.

Figure 20. A strong gas-and-steam plume was seen rising from Bezymianny on 19 December 2016. Photo by V. Buryi; courtesy of IVS FEB RAS, KVERT.

Activity during January-May 2017. According to KVERT, lava-dome extrusion likely continued into January 2017. Strong gas-and-steam emissions continued through 19 January 2017 and a thermal anomaly was detected over the volcano during most days. On 12 January, KVERT noted that activity had gradually decreased after an intensification during 5-24 December 2016, and thus the Aviation Color Code was lowered to Yellow. Thereafter, KVERT characterized the volcano as having moderate gas-steam activity. On 23 February, KVERT reported that the effusive eruption continued and that lava was flowing on the S flank of the lava dome.

On 9 March at about 1330, an explosive eruption occurred (figure 21). Based on webcam observations, at 1454 an ash plume rose to altitudes of 6-7 km and drifted 20 km NE. The Aviation Color Code was raised to Orange. About 30 minutes later, at 1523, an ash plume rose to altitudes of 7-8 km and drifted 60 km NW. KVERT raised the Aviation Color Code to Red, the highest level. Satellite data showed a 14-km-wide ash plume drifting 112 km NW at an altitude of 7 km. Later that day a 274-km-long ash plume identified in satellite images drifted NW at altitudes of 4-4.5 km; the majority of the leading part of the plume contained a significant amount of ash. Lava flowed down the NW part of the lava dome. The Aviation Color Code was lowered to Orange. Ash plumes drifted as far as 500 km NW.

Figure 21. The start of an explosive eruption from Bezymianny was captured in this image taken from a webcam video on 9 March 2017. Video from KB GS RAS; courtesy of IVS FEB RAS, KVERT.

KVERT reported that lava continued to advance down the NW flank of the lava dome during 10 March-21 April, and gas-and-steam plumes rose from the crater. A thermal anomaly was visible most days in satellite images. The Aviation Color Code was lowered to Yellow on 25 May. According to a KVERT report on 26 May, the volcano became quiet after the 9 March episode, although strong gas-and-steam emissions and daily thermal anomalies continued.

Thermal anomalies. Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were almost daily events during January through 2 November 2016, except none were reported in March through 19 May 2016. On many days, multiple pixels were reported (13 pixels on 1 September). The number of events diminished in December (only six days), and except for a brief period during 9-12 March 2017, none were reported after 20 December through at least 26 May 2017.

The Mirova (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, reported several hotspots each month during May-August 2016, with a significant increase in September through early November (figure 22). Numerous hotspots were again reported in December, but only a few in January and February, except for a narrow cluster during the middle of February. In contrast to the MODIS/MODVOLC data, numerous hotspots were reported in March, April, and May 2017. The vast majority of hotspots during the past 12 months were within 5 km of the volcano and were of low power.

Figure 22. Thermal anomalies at Bezymianny recorded by the MIROVA system (log radiative power) for the year ending 5 May 2017. Note stronger frequent activity in the second half of December 2016 and the stronger anomalies associated with the March 2017 activity. Courtesy of MIROVA.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS) (URL: http://www.emsd.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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/); 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/).

The remote island of Chirinkotan is in the Northern Kuril Islands at the southern end of the Sea of Okhotsk, about 320 km SW of the tip of Kamchatka, Russia. It is an outlier about 40 km NW of the main Kuril Islands Arc. There have been very few historical observations of activity at Chirinkotan, although there is at least one confirmed 19th century observation of lava flows. A short-lived event that resulted in a small, low-level ash plume-and-gas plume was seen in satellite imagery on 20 July 2004 (Neal et al., 2005). Volcanic activity resumed in mid-2013, with intermittent ash plumes, thermal anomalies, and block lava flows reported through April 2017. The volcano is monitored by the Sakhalin Volcanic Eruption Response Team (SVERT) of the Institute of Marine Geology and Geophysics (Far Eastern Branch, Russian Academy of Science), and aviation alerts are issued by the Tokyo Volcanic Ash Advisory Center (VAAC).

A new eruptive phase began with a likely ash emission on 11 June 2013. Intermittent thermal anomalies and gas-and-steam emissions were reported for the next 12 months, sometimes drifting up to 100 km, usually SE. Renewed thermal anomalies and gas emissions were recorded during clear weather beginning on 21 November 2014. Two ash plumes observed in late July 2015 were the likely sources of fresh ashfall and block lava flows sampled during a visit by Russian geoscientists on 9 August 2015. A gas-and-steam plume on 17 November 2015 was the last activity observed, except for low-level thermal anomalies, until a substantial ash plume was captured in satellite data at 8.8 km altitude over a year later on 29 November 2016. Additional ash plumes were observed in satellite data once in late January, and twice each in March and April 2017.

Activity during May 2013-June 2014. After no reports of activity since July 2004, SVERT observed gas-and-steam emissions in satellite imagery beginning in late May 2013. They raised the Alert Level from Green to Yellow (on the four level Green-Yellow-Orange-Red scale) sometime between 27 May and 10 June. The first likely ash emission was reported on 11 June, followed by a thermal anomaly detected on 13 June. Thermal anomalies continued to be detected by SVERT during June and July 2013. The first MODVOLC thermal alert was reported on 22 July; they were reported monthly after that through 11 December 2013, with several days of multiple-pixel alerts. SVERT also noted thermal anomalies and gas-and-steam emissions during August through December, including plumes drifting 30-60 km SE during 17-19 October, 55-100 km SE during 5-6 November, and more than 50 km SE on 25 November.

From the beginning of January 2014 through early June, persistent thermal anomalies were observed in clear imagery nearly every week by SVERT, along with intermittent steam-and-gas emissions. Several times during March, plumes were observed drifting 80-170 km SE. MODVOLC thermal alerts were reported on 8 February, 4 days in March (four pixels on 8 March), and twice on 27 May. SVERT reported that beginning on 24 May, gas emissions containing ash were detected in satellite images. A decrease in thermal anomalies observed by SVERT led them to lower the Alert Level to Green on 5 June 2014.

Activity during November 2014-July 2015. SVERT raised the Alert Level back to Yellow in late November 2014, citing new thermal anomalies beginning on 21 November followed by intermittent steam-and-gas emissions. A plume was observed drifting 40 km SE on 27 November. A new MODVOLC thermal alert appeared on 4 December. SVERT reported thermal anomalies and diffuse gas-and-steam plumes during December 2014 and January-February 2015. Emissions were detected 3 km above Chirinkotan drifting SE on 5 January 2015. MODVOLC reported two thermal alert pixels on 7 January and one on 10 January.

SVERT briefly lowered the Alert Level to Green between 4 and 20 March when no activity was detected. Thermal anomalies were reported again beginning on 19 March; they were noted weekly along with intermittent gas-and-steam emissions through mid-May when the Alert Level was lowered back to Green again on 19 May.

MODVOLC reported a three-pixel thermal alert on 20 July 2015 (local time). The Tokyo VAAC reported an eruption on 21 July (local time) with an ash plume rising to 3.7 km altitude drifting SE. The plume was observed in satellite imagery for about 2 hours before dissipating. SVERT reported a thermal anomaly and steam-and-gas emissions on 22 July, and the Alert Level was raised to Yellow. Another ash plume was reported by the Tokyo VAAC on 26 July rising to an altitude of 4.6 km and drifting NW for several hours before dissipating.

Expedition during August 2015. Scientists from the Institute of Marine Geology and Geophysics (IMGiG) of the Far Eastern Branch of the Russian Academy of Sciences visited Chirinkotan on 9 August 2015. While there, they observed steaming from a recent blocky lava flow near the coast (figure 3), hiked to the summit, and collected data about volcanic and biological activity on the island. A group of researchers climbed to the edge of the summit crater at 600 m elevation, where clouds prevented clear views of the crater (figure 4), however the strong odor of sulfur and noise from fumarolic activity was noted. The scientists sampled the fresh pyroclastic rocks. When the visibility improved, the depth of the crater was observed to be about 150 m; an extrusive dome in the center had a vent on the top emitting gas.

Figure 3. Steam rising from recent lava flow at Chirinkotan that reached the coastline, 9 August 2015. Courtesy of IMGiG (Diary of the Kurils 2015 Expedition, 7-9 August 2015, http://imgg.ru/ru/news/111 ).

Figure 4. Fieldwork at the summit crater rim of Chirinkotan, 9 August 2015. Courtesy of IMGiG. (Diary of the Kurils 2015 Expedition, 7-9 August 2015, http://imgg.ru/ru/news/111 ).

The upper flank of the volcano was strewn with ash and bombs (from 2-3 cm to several meters in diameter). Scientists observed recently buried and charred living vegetation, and nesting birds freshly killed by volcanic ash and bombs, indicating a very recent event (figure 5). The botanists in the research group noted that all of the vegetation on the upper and middle flanks had been killed 2-3 years ago in a major event, likely during the start of the 2013 eruptive cycle. Ash deposits ranged in thickness from a few centimeters near the coast to 8-15 cm near the summit. During a survey of a pyroclastic flow on the SW coast, scientists noted that it was still hot on the surface (40-60°?) and consisted of block lava, bombs, and volcanic ash (figure 6).

Figure 5. Evidence of recent explosive activity at Chirinkotan. Top: recently burned vegetation from a volcanic bomb on the flank. Bottom: living vegetation buried in recent volcanic ash, 9 August 2015. Courtesy of IMGiG (Diary of the Kurils 2015 Expedition, 7-9 August 2015, http://imgg.ru/ru/news/111 ).

Figure 6. Still-hot debris from a block lava flow on Chirinkotan, 9 August 2015. Courtesy of IMGiG (Diary of the Kurils 2015 Expedition, 7-9 August 2015, http://imgg.ru/ru/news/111 ).

Activity during November 2015-April 2017. As a result of the direct observations of the recent eruption on the island, SVERT raised the Alert Level to Orange on 11 August 2015. There were no further reports available from SVERT until 17 November when gas-and-steam emissions were detected, and the Aviation Color Code was reported as Yellow. SVERT reported on 7 December 2015 that the ACC had been lowered to Green. Although SVERT did not report renewed activity from Chirinkotan until it issued a VONA on 29 November 2016 and raised the Alert Level to Yellow, the MIROVA thermal anomaly detection system indicated intermittent low-level anomalies between late May and early October 2016 (figure 7), indicating a heat source on the island.

Figure 7. MIROVA data of Log Radiative Power at Chirinkotan for the year ending on 31 January 2017 showing a weak but persistent thermal anomaly between late May and early October 2016. Courtesy of MIROVA.

The Tokyo VAAC issued a report of a volcanic ash plume from an eruption on 29 November (local time) 2016. The plume rose to 8.8 km altitude and drifted N. It was observed in satellite imagery for about 9 hours before dissipating. SVERT briefly raised the ACC to Yellow between 29 November and 2 December. They noted that the ash plume was observed drifting 39 km N. A new report of ash emissions came from the Tokyo VAAC on 26 January 2017, with an ash plume at 3.7 km drifting SE observed in the Himawari-8 satellite imagery. SVERT raised the alert level to Yellow on 27 January (UTM) 2017 and also noted ash emissions on 29 January drifting SE to a maximum distance of 105 km. They lowered the Alert Level to Green on 1 February 2017.

A new ash plume was observed by the Tokyo VAAC on 1 March (local time) 2017 at an altitude of 5.5 km. When SVERT raised the Aviation Color Code to Yellow on 2 March, they noted that the plume had drifted 165 km E. They lowered the ACC back to Green on 6 March. The Tokyo VAAC reported a new ash plume at 6.1 km extending SE early on 21 March 2017. SVERT reported the emission at 15 km E of the volcano when they raised the ACC to Yellow a short while later. They noted on 24 March, when they lowered the ACC to Green, that the maximum extent of the ash cloud had been about 50 km SE.

On 31 March 2017, the Tokyo VAAC issued an advisory for an ash plume at 6.7 km altitude drifting E, and SVERT raised the Alert Level to Yellow the next day. They reported the ash plume drifting 165 km NE before dissipating. Another plume on 7 April was observed by the Tokyo VAAC at 3.7 km altitude drifting SE. SVERT reported the plume at 5 km altitude drifting NE. SVERT lowered the ACC to Green on 24 April 2017.

Reference: Neal C A, McGimsey R G, Dixon J, Melnikov D, 2005. 2004 volcanic activity in Alaska and Kamchatka: summary of events and response of the Alaska Volcano Observatory. U S Geol Surv, Open-File Rpt, 2005-1308: 1-67.

Geologic Background. The small, mostly unvegetated 3-km-wide island of Chirinkotan occupies the far end of an E-W volcanic chain that extends nearly 50 km W of the central part of the main Kuril Islands arc. It is the emergent summit of a volcano that rises 3000 m from the floor of the Kuril Basin. A small 1-km-wide caldera about 300-400 m deep is open to the SW. Lava flows from a cone within the breached crater reached the shore of the island. Historical eruptions have been recorded since the 18th century. Lava flows were observed by the English fur trader Captain Snow in the 1880s.

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Institute of Marine Geology and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, (FEB RAS IMGiG), 693 022 Russia, Yuzhno-Sakhalinsk, ul. Science 1B (URL: http://imgg.ru/ru); 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/); 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/).

Eruptive activity at Dukono has continued since 1933. As previously reported, ash explosions were frequently observed, and thermal anomalies were intermittent, from September 2011 through July 2014 (BGVN 39:06). Similar activity has continued through March 2017. Monitoring is conducted by the Indonesian Center for Volcanology and Geological Hazard (PVMBG, also known as CVGHM) from an observation post 11 km away. The Alert Level has remained at 2 (on a scale of 1-4), with residents and tourists advised to not approach the crater within a radius of 2 km.

PVMBG reported that in March-April 2015 seismicity remained high and consisted of explosion signals, volcanic earthquakes, and tremor, accompanied by roaring heard at the observation post. A powerful explosion on 23 May 2015 was followed by minor ashfall in areas to the E. During 1-5 July 2015 white-and-gray plumes rose as high as 600 m; minor ashfall was reported in northern areas on 1 July. Ashfall was reported in areas from the Galela District to Tobelo town (NNW) in August 2015 and at the observation post in September. Seismicity fluctuated at high levels, with elevated periods during 15-22 August, 28 August-5 September, and 15-25 October 2015.

As summarized by PVMBG, the period from 1 January to 19 December 2016 exhibited white-and-gray plumes rising as high as 1.2 km above the rim of the Malupang Warirang crater, accompanied by roaring heard at the observation post. The eruption plume height generally fluctuated though, was higher during periods in May and from late November into December; ashfall increased during the periods of higher plume heights, and was noted in villages within 11 km N, NE, and SW. Seismicity remained high.

Nearly daily aviation advisories from the Darwin VAAC (Volcanic Ash Advisory Centre) since July 2014 confirmed the PVMBG reports. As identified in satellite imagery, white and gray ash plumes were seen rising to altitudes of 1.5-4 km from the Malupang Warirang crater, and drifting in various directions for tens to hundreds of kilometers. Data compiled from VAAC reports and summarized by month for April 2016-March 2017 (table 15) reveal plume altitudes between 1.5 and 3.7 km with visible drift distances up to 300 km away.

Table 15. Monthly summary of reported ash plumes from Dukono for April 2016-March 2017. The direction of drift for the ash plume was highly variable. Data from Darwin VAAC and PVMBG.

Intermittent thermal anomalies, typically single pixels, were recorded by MODVOLC (table 16) in the months of April and June 2014, January-March 2015, December 2015, and November 2016. MODIS thermal data recorded by the MIROVA system during the year of April 2016-March 2016 (figure 6) showed intermittent low-power anomalies in May and August 2016, and then in every month from October 2016 through March 2017. It should be noted that the MODIS satellite thermal sensors cannot penetrate cloud cover, which is frequent over Dukono much of the year.

Table 16. Thermal anomalies at Dukono based on MODIS data processed by MODVOLC, August 2014-March 2017. Courtesy of Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System.

Figure 6. Thermal anomalies (Log Radiative Power) detected by MODIS and recorded by the MIROVA system for year ending 5 April 2017. Courtesy of MIROVA.

Vistors to the crater in March 2016 photographed ash rising form an incandescent vent (figure 7). Patrick Marcel reported that "the vents at the bottom of the crater emitted a sustained, extremely noisy jet of gas, steam and ash, and ejected incandescent bombs to up to 500 m height. Some of them landed outside the crater rim." The "You&MeTraveling2" blog posted a trip journal that described a late-August 2016 visit to Dukono, including photos and a video looking down into the crater that showed activity similar to that seen by Marcel in March 2016.

Figure 7. View into Dukono's crater on 12 March 2016. Photo by Patrick Marcel (color adjusted from original); courtesy of Volcano Discovery.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the N-flank Gunung Mamuya cone. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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/); 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/); 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/); Volcano Discovery (URL: http://www.volcanodiscovery.com/); You&MeTraveling2 (URL: http://youandmetraveling2.com/).

The existence of an anorthoclase phonolite lava lake in the summit crater of Mount Erebus was first reported in 1972, and it has been thought to be continuously active since that time. Antarctica's best known volcano is located on Ross Island, 90 km E of the continent, offshore of the Scott Coast. McMurdo station, run by the United States Antarctic Program, is about 40 km S on the tip of Ross Island (figure 16). During the history of observations, lava lake(s) have generally persisted, although changes in size and shape over time reflect variations in volcanic activity.

Figure 16. On 31 December 2013, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite acquired visible near-infrared images of the western end of Ross Island in austral mid-summer. McMurdo Station is about 40 km S of the summit of Mount Erebus. Courtesy of NASA Earth Observatory.

This report briefly summarizes research activity at Mount Erebus, and volcanic activity observed since 1972. Photographs from expeditions between 2010 and 2016 show more recent activity at the volcano. Observations from MODVOLC data collected from 2000 through 2016 are also discussed.

Summary of research activity. For most years since the 1970's, scientists have visited Erebus during the austral summer (November-February) and gathered samples, taken SO₂ and other geochemical measurements, collected GPS data, and made observations and overflights to evaluate the condition of the volcano.

Seismometers were initially installed by a joint project of United States, New Zealand, and Japanese scientists in 1980-1981. Between 1980 and 2016 as many as 10 seismic stations were recording activity at Erebus; they were monitored by the Mount Erebus Volcano Observatory (MEVO) run by the New Mexico Institute of Mining and Technology (New Mexico Tech). During the early 2000s MEVO also used infrasonic recordings to capture data on the frequency of eruptions. Researchers from New Mexico Tech, the University of Cambridge, and University College London made yearly expeditions there between 2003 and 2016.

The Mount Erebus Volcano Observatory closed in 2016. A final report was submitted to the National Science Foundation (NSF) on the past research and ideas for future research (Mattioli and LaFemina, 2016), and includes a comprehensive list of scientific publications about Erebus. One area of ongoing volcanology research relates to studying the behavior of the lava lake with a variety of on-site monitoring equipment (figure 17).

Figure 17. Radar altimeter installed at the crater rim of Erebus in December 2016. There are two dishes, to both transmit and receive data. Several other devices are seen in the background, all trained on the lava lake on the floor of the crater. Courtesy of the University of Cambridge Department of Geography.

Summary of activity, 1972-2009. During the 1970's, the lava lake was observed to be about 130 m long and oval shaped, producing occasional Strombolian explosions. Bombs up to 10 m in in diameter were ejected near the vent, and ones up to 30 cm in diameter were thrown out over the main crater. Oscillations of the lake level of up to 2 m were observed.

During a period of increased activity between September 1984 and January 1985, several large explosions were recorded by the seismic network, and there were reports of mushroom-shaped clouds rising as much as 2 km above the summit. During September 1984, numerous large explosions sent ejecta as high as 600 m above the summit, and incandescence was visible from 70 km away. Ash also covered the NW flank down to 3,400 m elevation. Observations in October 1984 indicated that much of the lava lake had solidified, and that the surface was covered with ejecta from the recent explosions. Seismicity remained above average through January 1985. During this period of increased activity, bombs averaging 2 m in diameter (but some as large as 10 m in diameter) were ejected up to 1.2 km from within the inner crater. The eruptions were witnessed from 60 km away and explosions could be heard up to 2 km from the volcano (SEAN 11:03). A small lava lake about 15 m in diameter reappeared late in 1985.

Two primary lakes of phonolitic lava, and a third transient lake, were present inside the crater during the late 1980s (see figure 9, SEAN 13:02), and infrequent Strombolian eruptions with small bombs were captured by a remote video camera mounted on the crater rim. Small ash eruptions were observed from an active vent near the lava lakes in January 1991. On 19 October 1993, two moderate phreatic eruptions created a new crater ~80 m in diameter on the main crater floor and ejected debris over the northern crater rim. These were the first known phreatic eruptions at Erebus, and probably resulted from steam build-up associated with melting snow in the crater (BGVN 20:11).

Vent and lava lake eruptions were recorded by MEVO during the late 1990s and early 2000s. The largest peaks in terms of numbers of eruptions were during 1995, 1997, 1998, 2000, and a broad peak beginning in late 2005 that continued into late 2006 (BGVN 31:12).

Activity during 2010-2016. The two primary lava lakes remained active at Erebus. The one in the NE sector of the inner crater has been persistent almost continuously since first reported in 1972. The second lake is more in the center of the main crater and is intermittently active. During a visit in 2010, only the NE sector lake was active (BGVN 36:09). During clear weather, a steady steam plume is often observed (figure 18).

Figure 18. Mount Erebus with a steam plume rising from the summit crater, viewed from the Lower Erebus Hut (LEH), 6 December 2010. Courtesy of Mount Erebus Volcano Observatory.

Visits during 2011-2016 have confirmed the ongoing Strombolian activity and convection at the lava lakes nearly every year. During 2011 the glowing lava lake emitted steam and magmatic gases from the bottom of a vent at the main crater (figure 19). An eruption on 2 January 2012 at the lava lake was captured by the remote video cameras managed by MEVO (figure 20). Several bombs were ejected on 18 December 2013 and landed close to monitoring equipment run by MEVO. Researchers were able to open a hot bomb and see the molten interior (figure 21).

Figure 19. The lava lake at Erebus, photographed in December 2011. Image by Clive Oppenheimer/Volcanofiles; courtesy of Erik Klemetti.

Figure 20. An eruption from the lava lake at Erebus, captured on the MEVO video cameras on 2 January 2012. Courtesy of MEVO and Volcano Discovery.

Figure 21. Several bombs erupted from Erebus on 18 December 2013 and landed close to monitoring equipment run by MEVO. Researchers were able to open a hot bomb and see the molten interior. Images courtesy of Aaron Curtis, MEVO, 18 December 2013 (posted on Facebook).

When UNAVCO (a non-profit university-governed consortium) flew over Erebus in December 2015, steam and magmatic gas plumes indicated that both lava lakes were active (figure 22). The two incandescent crater vents at were observed in greater detail during January 2016 by researchers associated with the University of Cambridge (figure 23).

Figure 22. The crater of Erebus, with active steam plumes from two lava lakes on 7 December 2015, photographed during an overflight by UNAVCO (a non-profit university-governed consortium). Photo by Annie Zaino, UNAVCO (posted on Facebook).

Figure 23. Two lava lakes at Erebus were observed on 14 January 2016 by researchers associated with the University of Cambridge. Lower image is a close-up of the right vent in the upper image. Courtesy of Kayla Iacovino and Tehnuka Ilanko (posted on Facebook).

MODVOLC data, 2000-2016. With the remoteness of Erebus, satellite imagery serves as one of the few year-round tools currently available to assess longer-term activity. The University of Hawaii's MODVOLC thermal alert system has been processing MODIS infrared satellite data since 2000. Mount Erebus has had a strong and nearly continuous MODVOLC signature throughout 2000-2016 (table 3), confirming its ongoing eruptive activity.

Table 3. Number of MODVOLC thermal alert pixels recorded per month from 1 January 2000 to 31 December 2016 by the University of Hawaii's thermal alert system for Erebus. Table compiled by GVP from data provided by MODVOLC. Spurious data from 25 October 2014 was omitted.

The MODVOLC thermal alert data show that thermal activity at Erebus has waxed and waned several times during the 2000-2016 interval (figure 24). Activity was very low during 2000, but increased steadily through mid-2005 to more than 20 times as many annual thermal alert pixels since 2000. Activity dropped off substantially from late 2005 and remained low through early 2007, when another increase began that peaked at an even higher level (2514 pixels during 2009) in mid-2009. Another drop in activity occurred during 2010, and since 2011 there have been fewer than 300 pixels per year, with numbers below 200 for 2015 and 2016.

Figure 24. The number of MODVOLC thermal alert pixels per year, colored by month, reported for Erebus from 2000 through 2016. Activity was very low during 2000, but increased steadily through mid-2005. Activity dropped off substantially from late 2005 through early 2007, when another increase began that peaked at an even higher level in mid-2009. Another drop in activity occurred during 2010, and since 2011, there have been fewer than 300 pixels per year. Data courtesy of MODVOLC.

Another trend in the MODVOLC data is also apparent when the number of pixels are plotted by month, as opposed to year, for this time period (figure 25). From November through February, during the austral summer, the number of pixels per month never exceeds 150 (see table 3, highest value is 125). From March through October, during the Austral winter, the number of pixels recorded per month can be much higher (the highest value is 458). The average number of 'summer' pixels per month (November-February, 2000-2016) is 30. The average number of 'winter' pixels per month for the same period (March-October) is 108, more than three times greater.

Figure 25. The number of MODVOLC thermal alert pixels per month for the period 2000-2016, colored by year. The total average number of pixels per month from 1 March through 31 October (1732) is three times the average total number of pixels per month from 1 November through 28 February (480). Data courtesy of MODVOLC.

References: Mattioli, G.S., and LaFemina, P.C., 2016, Final Report submitted to the National Science Foundation, Community Workshop: "Scientific Drivers and Future of Mount Erebus Volcano Observatory (MEVO)" (URL: https://www.unavco.org/community/meetings-events/2016/mevo/2016-MEVO-Final-Report.pdf)

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Mt. Erebus Volcano Observatory (MEVO), New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA; 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/); The University of Cambridge Department of Geography (URL: http://www.geog.cam.ac.uk/research/projects/lavalakes/); Erik Klemetti, Eruptions Blog, Wired (URL: https://www.wired.com/author/erikvolc/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); UNAVCO, 6350 Nautilus Drive, Boulder, CO 80301-5394 (URL: http://www.unavco.org/); Kayla Iacovino and Tehnuka Ilanko, The Volcanofiles (URL: http://www.volcanofiles.com/).

Volcán de Fuego has been erupting continuously since 2002. Historical observations of eruptions date back to 1531, and radiocarbon dates are confirmed back to 1580 BCE. These eruptions have resulted in major ashfalls, pyroclastic flows, lava flows, and damaging lahars. Fuego was continuously active from June 2014-December 2015. Ash plumes rose to 6 km altitude, ashfall was reported in communities as far as 90 km away, pyroclastic flows descended multiple drainages at least four times, Strombolian activity rose to 800 m above the summit, lava flows descended a few kilometers down five different drainages numerous times, and three different lahars damaged roadways (BGVN 42:05). This report continues with a summary of similar activity during January-June 2016. In addition to regular reports from INSIVUMEH, the Washington Volcanic Ash Advisory Center (VAAC) provides aviation alerts. Locations of towns and drainages are listed in table 12 (BGVN 42:05).

Daily weak and moderate explosions generating ash plumes to about 800 m above the summit (4.6 km altitude) that dissipated within about 10 km were typical activity for Fuego during January-June 2016. In addition, ten eruptive episodes were recorded during this time. Each episode lasted 24-72 hours, with all but one including incandescent material rising 200-400 m above the summit feeding lava flows down the larger drainages for several kilometers. Most also included pyroclastic flows down the larger drainages. One of the episodes consisted of only large pyroclastic eruptions (with an accompanying ash plume) that issued directly from the summit crater and down the ravines; all included ash plumes rising over 5 km in altitude. Several lahars were reported during late April-June.

Activity during 30 December 2015. INSIVUMEH reported a significant increase in activity on 30 December 2015. A series of pyroclastic flows descended the Las Lajas and El Jute drainages on the SE flank, and a dense ash plume rose to 5 km altitude and drifted 20 km W. Ashfall was reported in multiple communities on the flanks, including Panimache I and II (8 km SW), Morelia (9 km SW), and Santa Sofía (12 km SW).

Activity during January 2016. Two eruptive episodes with explosions that generated ash plumes, pyroclastic flows, Strombolian activity, lava flows, and ashfall were documented by INSIVUMEH during January 2016. The first eruption began with an increase in seismicity early in the morning of 3 January. Moderate to strong explosions were accompanied by an ash plume that rose to 4.8 km altitude (about 1 km above the summit) and drifted W and SW. Two lava flows emerged from the summit crater and traveled down the Las Lajas and Trinidad ravines. Moderate to strong explosions continued during 3 January. By the afternoon, dense plumes of ash were reported at 6 km altitude drifting SW and SE more than 40 km. Ashfall was reported in the villages of Panimaché I and II, Morelia, Santa Sofia, El Porvenir, La Rochela, Osuna, El Zapote and Rodeo. Also later in the day, incandescence was observed 400 m above the crater; it fed three lava flows in the Santa Teresa, Trinidad, and Las Lajas canyons that reached 2.5 km in length. Eruptive activity diminished after about 37 hours with weak bursts of ash rising to 4.6-4.7 km altitude on 5 January that drifted S, SW, and SE.

A smaller explosive event during 15-17 January produced block avalanches and created ash plumes that rose 450-750 m above the crater and drifted up to 12 km N and NE; four to five explosions per hour were detected. The second eruptive episode began with increased activity on 19 January; incandescent material was ejected 400-500 m above the summit, generating new lava flows to the same three canyons as the earlier eruption (Santa Teresa, Trinidad and Las Lajas) (figure 36). Ash emissions rose to 4.9 km altitude and drifted NE. Pyroclastic flows also descended the Las Lajas and El Jute canyons (figure 37).

Figure 36. Lava flows towards Las Lajas Canyon on 19 January 2016 as viewed from the SE flank. Courtesy of INSIVUMEH-OVFGO (Informe Mensual De La Actividad Del Volcán Fuego, January 2016).

Figure 37. A pyroclastic flow descends towards the Las Lajas and El Jute ravines on the SE flank of Fuego on 19 January 2016 in this thermal image captured by INSIVUMEH. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, January 2016).

The second episode continued throughout 20 January 2016 when the largest ash plume rose to 6.7 km altitude and drifted NE more than 90 km according to the Washington VAAC. Ashfall was reported in San Miguel, Las Dueñas, Alotenango, Acatenango, and Antigua. Ash plumes from the pyroclastic flows also generated ashfall on the S and SW flanks (figure 38). By the morning of 21 January, the lava flows had ceased advancing at about 3 km length, although a hot spot was still clearly visible in satellite imagery. Weak explosions generated ash plumes that rose only a few hundred meters above the summit and drifted NNE. During January, the Observatorio del Volcan de Fuego installed a second webcam on the SE side of Fuego at the Finca La Reunión, a resort about 8 km from the summit. The first webcam is located about 10 km SW of the summit at the Observatorio del Volcan de Fuego in the community of Panimache.

Figure 38. A pyroclastic flow on 20 January 2016 travels down the SE flank of Fuego, creating an ash cloud in the ravine. Additional ash emissions drifted in multiple directions. A recent lava flow is also visible in the ravine. View is from the La Reunión webcam, 8 km SE of the summit. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, January 2016).

Activity during February-March 2016. Explosions increased in number and energy on 5 February 2016, classified by INSIVUMEH as the 3rd episode of the year. Six moderate to strong explosions per hour were reported, sending ash emissions to 4.5 km altitude, drifting W, NW, and N more than 12 km, and avalanche blocks down the flanks to the base. The third eruptive episode of the year began with moderate explosions on 9 February 2016; it generated ash plumes which rose to 4.7 km altitude and dispersed up to 35 km NNW. Ashfall was reported in Chimaltenango, Zaragoza, Ciudad Vieja, San Pedro las Huertas, San Miguel Las Dueñas, San Juan Alotenango, Antigua Guatemala and the Capital City as far as 35 km N and NE. The explosions were accompanied by incandescent material rising to 300 m above the summit and feeding lava flows that traveled towards the Trinidad, Las Lajas, and Santa Teresa canyons, reaching lengths of 800 to 3,000 meters (figure 39).

Figure 39. Incandescence rises 300 m above the crater at Fuego, generating lava flows down the Trinidad, Las Lajas and Santa Teresa canyons on 9 February 2016. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Febrero 2016).

The following day (10 February 2016), pyroclastic flows descended the El Jute and Las Lajas ravines (figure 40) while ash plumes rose to 5.2 km altitude and incandescent material was ejected 400 m above the crater. Although activity decreased throughout the day, explosions continued to generate ash plumes to 4.9 km altitude that dispersed ash up to 45 km N and NE. Minor ash emissions were reported by the Washington VAAC on 17 February at 4.6-4.9 km altitude drifting SE about 40 km, and on 24 February at 4.6 km drifting about 25 km SW.

Figure 40. Pyroclastic flows descend the Las Lajas and El Jute ravines at Fuego on 10 February 2016 as viewed from the webcam at Finca la Reunión, 8 km SE. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Febrero 2016).

On 29 February 2016, moderate to strong explosions at a rate of 6-10 per hour were heard more than 14 km away. They were accompanied by an ash plume that rose to 4.8 km and drifted 12 km E, and a lava flow that traveled 500 m towards Las Lajas ravine. This 4th eruptive episode (according to INSIVUMEH) lasted more than 72 hours (figure 41). On 2 March, several ash plumes rose to different altitudes and dispersed in different directions. The largest ash plume, was observed by the Washington VAAC at 7.3 km altitude; it was visible 400 km N before it dissipated into weather clouds. Lower altitude plumes rose to 4.6 km and drifted 75 km SW before dissipating. Ash fell in the communities of Morelia, Santa Sofia, La Rochela, Panimaché I and II, Sangre de Cristo, La Soledad and Yepocapa. The incandescent activity fed two lava flows; the first in the direction of Las Lajas reached 3 km, the second flowed towards El Jute ravine and reached 2 km in length. Pyroclastic flows also travelled down these two canyons and block avalanches descended the Honda Canyon. Explosive activity diminished during 3-6 March; ash emissions rose to 550 m above the summit and drifted 8-10 km W, SE, and SE.

Figure 41. RSAM values spiked at Fuego during 29 February-3 March 2016 during eruptive episode 4. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Marzo 2016).

During 10 March 2016, moderate to strong Vulcanian explosions generated an ash plume that rose to 4.4 km altitude and drifted E. The Washington VAAC observed ash emissions in multispectral satellite imagery on 15 March at 4.3 km altitude extending about 80 km SW from the summit as well as hot spots and pyroclastic flows visible in the INSIVUMEH webcam. An increase in activity on 21 March generated weak and moderate explosions that produced ash plumes that rose to 4.3-4.7 km and drifted W. This activity was recorded as an increase in RSAM tremor amplitude and duration at the FG3 seismic station, but was not considered an eruptive episode by INSIVUMEH (figure 42).

Figure 42. Increases in RSAM tremor amplitude and duration at Fuego were recorded during 21 and 22 March, and eruptive episode 5 was recorded during 26 and 27 March 2016. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Marzo 2016).

Eruptive episode 5 began on 26 March 2016 and lasted more than 24 hours (figure 42). Strombolian eruptions rose up to 500 m above the crater (figure 43), feeding three lava flows that traveled 2 km down Las Lajas, 1.3 km down the Santa Theresa, and 1 km down the Trinidad ravines. Ash plumes rose to 6.1 km altitude and drifted up to 150 km W (figure 44); ash fell on the villages of Morelia, Santa Sofia, San Predro Yepocapa, Panimaché I and II. By the end of 27 March, eruptive activity had diminished to background conditions, which included weak and moderate explosions generating ash plumes to about 800 m above the summit (4.6 km altitude) that dissipated within about 10 km WSW. On 29 March ashfall was reported Sangre de Cristo and Panimaché I and II.

Figure 43. Strombolian activity rises 300 m above the crater at Fuego on 26 March 2016. Photo by Gustavo Chigna, courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Marzo 2016).

Figure 44. An ash plume at Fuego rose to over 6 km altitude on 26 March 2016 and drifted 150 km W before dissipating. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Marzo 2016).

Activity during April-May 2016. The Washington VAAC reported diffuse volcanic ash emissions in satellite and webcam imagery on 2 April 2016. The ash plume drifted W at 4.3 km altitude, and extended 75 km from the summit before dissipating. Increased eruptive activity during 6-7 April 2016 resulted in moderate and strong explosions which produced ash plumes rising to 4.6-4.8 km altitude that drifted W and SW 15 km. The explosions were audible more than 20 km from the volcano; roofs and windows vibrated within 12 km. INSIVUMEH received reports of ashfall from the villages of Morelia, Sangre de Cristo, and Panimche I and II.

An explosion on 8 April created an ash plume that rose to 5.8 km and drifted SSW about 35 km. Successive bursts of ash on 9 April rose to 4.9 km altitude and drifted W. Emissions on 11 April were reported at 4.3 km altitude about 15 km SW from the summit; the next day they rose to 4.9 km and drifted SW to a distance of 45 km. INSIVUMEH reported variable activity beginning on 11 April with high levels of explosive activity on 12 April marking the beginning of the sixth eruptive episode of the year, which lasted for three days. An incandescent fountain persisted 100-300 m above the crater and fed two lava flows during the event; one traveled 2 km down the Las Lajas ravine, and the other reached 1 km in length in the Santa Teresa ravine. Avalanches were constant along the flanks during this episode. Continuous ash emissions were observed as well; plumes generally rose no higher than 5.8 km (2 km above the summit). Ashfall was reported in La Rochela, Ceylon, Morelia, Hagia Sophia, Sangre de Cristo, Panimaché I and II. On 13 April the ash plume extended 185 km SW from the summit. A brilliant hotspot was observed in satellite imagery on 14 April after which no further VAAC reports were issued until early May. On 29 April, after more than a week of rain, a lahar descended the Las Lajas drainage but no damage was reported.

Activity at Fuego increased significantly during May 2016, and included three eruptive episodes that generated ash plumes, pyroclastic and lava flows, and increased rainfall that resulted in lahars. Ash plumes rose above 5.5 km altitude (more than 2 km above the summit) and dispersed to the S, SW, and SE. Seismic activity increased on 5 May in the form of internal vibrations caused by lava which flowed more than 1.2 km down the Las Lajas ravine, and moderate to strong explosions that produced ash plumes which rose to 4.8 km altitude and drifted S for 12 km. The Washington VAAC reported diffuse ash extending 65 km SE from the summit.

The 7th eruptive episode of the year began on 6 May 2016 with incandescent material rising 300 m above the summit crater, causing two lava flows. One traveled down Las Lajas ravine more than 3 km; the second descended the Trinidad ravine for 1.5 km. Block avalanches were constant around the crater rim. The episode lasted for more than 32 hours (figure 45); the moderate to strong explosions ejected ash to altitudes above 5.5 km that drifted S and SW. Ashfall was reported in Escuintla and its surroundings. There were no pyroclastic flows during this episode. The Washington VAAC reported emissions extending 65 km SE of the summit at 5 km altitude on 6 May.

Figure 45. RSAM values during 2 May-6 June 2016 helped INSIVUMEH to define eruptive episodes for 2016 at Fuego, along with observed activity. Eruptive episode 7, consisting of Strombolian activity, lava flows, and ash plumes, occurred during 6-7 May 2016. Episode 8 comprised ash plumes and several large pyroclastic flows that descended the S flank during 18 and 19 May, but no seismic explosive activity. Increases in explosive activity on 21 May marked the beginning of episode 9, which lasted through 23 May 2016 and included incandescent fountains, lava flows, and ash plumes. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Mayo 2016).

The next eruptive episode (8) did not involve seismic explosive activity (figure 45). Instead, several large pyroclastic flows overflowed the crater rim on 18 and 19 May 2016 and descended the flanks towards Las Lajas and Honda ravines (figure 46) resulting in ashfall reported to the S, SW, and W, in villages more than 30 km away. A large ash plume reached more than 5.5 km altitude and drifted 15 km SSW on 19 May (figure 47). Ashfall was reported in the villages of El Rodeo, La Rochela, Osuna, Panimaché, Morelia, Sangre de Cristo and Yepocapa. By late in the day, the Washington VAAC noted that the plume was centered about 90 km SW at 5.8 km altitude.

Figure 46. A pyroclastic flow descends Las Lajas ravine on the S flank of Fuego on 18 May 2016 in these images taken from Finca La Reunión. Lower photo by Basilo Sul, both images courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Mayo 2016).

Figure 47. An ash plume drifts SW from Fuego on 19 May 2016 after a series of pyroclastic flows and ash emissions sent ash plumes to over 5 km altitude. The Operational Land Imager instrument on Landsat 8 captured this image. Courtesy of NASA Earth Observatory.

The ninth eruptive episode of 2016 generated incandescent fountains 200-300 m above the summit; they fed a 2-km-long lava flow down the Las Lajas ravine (figure 48). Seismic activity began to increase on 21 May and lasted through 23 May (see figure 45). Moderate and strong explosions created an ash plume that rose to 5.5 km altitude and drifted SW and W. The Observatory reported ashfall in Morelia, El Porvenir, Santa Sofia, Los Yucales, Panimaché I and II. The Washington VAAC reported an ash plume visible in satellite imagery at 5.5 km altitude, drifting 75 km S beyond the coast on 23 May 2016. A lahar descended the Las Lajas ravine on 20 May and was recorded by the seismic station FG3, but no damage was reported.

Figure 48. Landsat band 7 (top) and band 10 (bottom) images of the still-cooling lava flow in Las Lajas ravine at Fuego on 26 May 2016. Courtesy of Rudiger Escobar, Michigan Technological University and INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Mayo 2016).

Activity during June 2016. A significant rainfall combined with the plentiful ash from recent pyroclastic flows, resulted in lahars descending Las Lajas and El Jute ravines on 5 June 2016. They transported blocks, branches, and tree trunks, and a strong sulfur smell was reported by nearby residents. Another lahar was reported on 18 June that was 15 m wide and had a 1.5-m-high front. An increase in seismic activity during the afternoon of 24 June signaled the beginning of eruptive episode 10. This was followed by about 30 hours of moderate to strong explosive activity that could be heard and felt as far as 12 km away. A dense ash plume on 25 June rose to 5.5 km altitude and drifted S, SW, and W more than 40 km. Ashfall was reported in San Pedro Yepocapa, Sangre de Cristo, Morelia, Santa Sofia, Panimaché I and II. The Washington VAAC observed the ash plume in multispectral imagery on 25 June extending 120 km WSW from the summit. NASA Goddard Space Flight Center captured a small but distinct SO₂ plume from Fuego on 25 June as well (figure 49). Incandescent material rose 300 m above the summit crater during this episode and fed three lava flows; the first descended Las Lajas ravine 2.5 km, the second traveled 2.3 km down El Jute ravine, and the third flowed down Taniluyá ravine for 600 meters. Seismic activity from episode 10 decreased on 26 June.

Figure 49. A small but distinct SO2 anomaly was measured from Fuego on 25 June 2016. INSIVUMEH reported the 10th eruptive episode of the year during that time with a dense ash plume and lava flows emerging from the summit crater. Courtesy of NASA Goddard Space Flight Center.

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous historical eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

The Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo (DRC) is part of the western branch of the East African Rift System (EARS). Nyamuragira (or Nyamulagira), a high-potassium basaltic shield volcano on the W edge of VVP, includes a lava field that covers over 1,100 km² and contains more than 100 flank cones in addition to a large central crater (see figure 47, BGVN 40:01). A large lava lake that had been active for many years emptied from the central crater in 1938. Numerous flank eruptions have been observed since that time, the last during November 2011-March 2012 on the NE flank. This report covers the substantial SO₂ emissions from both Nyamuragira and nearby Nyiragongo (15 km SE) between November 2011 and April 2016, and the onset of eruptive activity, including a new lava lake, at the summit crater beginning in May 2014. Activity is described through April 2017.

On-the-ground information about Nyamuragira is intermittent due to the unstable political climate in the region, but some information is available from the Observatoire Volcanologique de Goma (OVG), MONUSCO (the United Nations Organization working in the area), geoscientists who study Nyamuragira, and travelers who visit the site. The most consistent data comes from satellite – thermal data from the MODIS instrument processed by the MODVOLC and MIROVA systems, SO₂ data from the AURA instrument on NASA's OMI satellite, and NASA Earth Observatory images from a variety of satellites.

A substantial flank eruption took place from November 2011 through March 2012. This was followed by a period of degassing with SO₂-rich plumes, but no observed thermal activity, from April 2012 through April 2014. Increased seismicity and minor thermal activity was observed at the central crater during April 2014; lava fountains first seen in early July 2014 continued through September. A lava lake in the crater was confirmed on 6 November 2014, and it produced a consistent and strengthening thermal anomaly through the first week of April 2016, when it stopped abruptly. Thermal activity suggesting reappearance of the lava lake began again in early November 2016, and strengthened in both frequency and magnitude into early January 2017, continuing with a strong signal through April 2017.

Activity during November 2011-March 2012. Nyamuragira erupted from cones and fissures on the NE flank between early November 2011 and mid-March 2012 (BGVN 39:03). The vent area, 12 km ENE of the central crater, was an E-W fissure 500-1,000 m long. Lava fountains up to 300 m high produced flows that advanced nearly 12 km N in the first 10 days. Three scoria cones formed adjacent to the fissure during the eruption, and a small lava lake appeared in the center of the largest cone. During January 2012, lava flowed from the vent area and from numerous small breakouts within 2 km of the cones (figures 51 and 52). Dario Tedesco reported that the eruptions ceased in March 2012 after a series of explosion earthquakes recorded by the OVG had ended; the last MODVOLC thermal alert in the area of the eruption was captured on 14 March 2012, and none were reported again until 2014.

Figure 51. Lava fountain and active lava flow emerging from the breach of the erupting flank cone of Nyamuragira volcano on 8 January 2012. Courtesy of Volcano Discovery.

Figure 52. Lava fountains around 150 m high erupt on 8 January 2012 from the active flank vent during the 2011-2012 eruption of Nyamuragira. Photo by Lorraine Field, courtesy of Volcano Discovery.

Activity during April 2012-May 2014. Periodic field surveys at Nyamuragira have been carried out since 2009 by helicopter, thanks to the support of the United Nations Organization Stabilization Mission in the DR Congo (MONUSCO). Since 2013, observations of the crater have also been done once or twice a month by helicopter. The team has included researchers from the OVG, Dario Tedesco, and other international scientists. This area is a high-risk sector due to the presence of armed groups, and it is impossible, due to the lack of security, to make detailed field surveys (Coppola et al., 2016).

Dario Tedesco reported SO₂-rich fumaroles in Nyamuragira's central crater beginning in early March 2012, shortly before the NE-flank fissure eruptions ended (BGVN 40:01). A progressive collapse of the 400-m-wide, 50-80 m deep pit crater located in the NE part of the caldera began as soon as the eruptions ended. They noted that during the second half of April, large SO₂ plumes continuously emerged from the pit crater.

NASA's Global Sulfur Dioxide Monitoring program captured major SO₂ plumes from the area for an extended period between November 2011 and February 2014. The plumes represent combined emissions from both Nyamuragira and Nyiragongo, which are too close together to distinguish the source in the satellite data. Campion (2014), however, noted that SO₂ emissions from the VVG increased several fold after the end of the 2011-2012 Nyamuragira eruption; they interpreted that 60-90 % of these emissions should be attributed to Nyamuragira.

Significant areas of SO₂ plumes with DU > 2 (shown as red pixels on the Aura/OMI images, figure 53) were captured by the OMI instrument at the beginning of the November 2011 eruption and continued through February 2012. Beginning in April 2012 elevated values occurred more than 20 days per month through December 2012. Values were more variable in both frequency and magnitude during 2013 with a notable surge of activity during 6-19 June 2013 that resulted in daily SO₂ plumes. Details of monthly SO₂ values are given in the last section of this report (see table 3).

Figure 53. Large SO2 plumes from Nyamuragira and Nyiragongo between November 2011 and December 2013. Four of the dates correspond to the Maximum DU days for that month (see table 3), and two represent other days of the month with substantial plumes. Courtesy of NASA/GSFC.

Activity during June 2014-April 2017. Incandescence at the summit and increased seismicity was reported again in April 2014, along with increasing SO₂ values. A strong MODVOLC thermal alert signal appeared on 22 June 2014, and a satellite image from 30 June showed clear hotspots at both Nyamuragira and Nyiragongo (figure 54).

Figure 54. Hot spots from both Nyamuragira and Nyiragongo on 30 June 2014. This false-color image combines shortwave-infrared, near-infrared, and green light as red, green, and blue, respectively. Since shortwave- and near- infrared light penetrates hazy skies better than visible light, more surface detail is visible in this image than would be in natural-color. Because very hot surfaces glow in shortwave-infrared, the lava within both summit craters appear bright red. The dark lava flows spreading from Nyamuragira were erupted within the past 50 years, some as recently as 2012. Vegetation is bright green. The image was collected by Landsat 8. Courtesy of NASA Earth Observatory.

An extended series of MIROVA thermal anomaly data beginning in May 2014 clearly shows the episodic periods of active heat flow at Nyamuragira from late May 2014 through April 2017 (figure 55). During the first episode, from late May to early September 2014, lava fountains were observed in early July, and reported to be active through September (BGVN 40.01). Campion (2014) and Smets and others (2014) debated whether the lava lake first appeared in April or not until November. On 6 November 2014 a small lava lake was confirmed at the base of the summit pit when sighted during an OVG helicopter survey. Both MODVOLC and MIROVA thermal anomalies appeared again in early November and persisted through the end of the year.

Figure 55. MIROVA thermal anomaly data from Nyamuragira from May 2014 through April 2017. Vertical black bar on each chart show the ending date of the previous chart. Chart "A" was previously published (BGVN 40:01, figure 50); other charts were captured via Volcano Discovery, Erik Klemetti, and Culture Volcan. Courtesy of MIROVA.

Thermal anomalies were persistent throughout 2015, with a noted increase in both frequency and magnitude during July (figure 55 C). A NASA Earth Observatory image from 9 February 2015 clearly shows active plumes venting from both Nyamuragira and Nyiragongo (figure 56). MONUSCO-supported summit crater visits by researchers on 2 April 2015, and photographer Oliver Grunwald on 10 July 2015, confirmed the presence of an active lava lake during both visits (figure 57, and video link in Information Contacts).

Figure 56. On 9 February 2015, clear skies afforded an unobstructed view from space of plumes venting from both Nyamuragira (north) and Nyiragongo (south) volcanoes in the Democratic Republic of the Congo. The lower image shows a close-up view of Nyamuragira, which is topped with a small caldera with walls about 100 m high. In 1938, a lava lake within the caldera drained during a large, long-lasting fissure eruption that sent lava flows all the way to Lake Kivu. Satellite observations and helicopter overflights in 2014 confirmed that the caldera again contained a small but vigorous lava lake. Courtesy of NASA Earth Observatory.

Figure 57. An active lava lake at Nyamuragira crater on 2 April 2015. Courtesy of MONUSCO/Abel Kavanagh.

The MIROVA and MODVOLC thermal anomaly data suggest that the lava lake at Nyamuragira was active until 4 April 2016 when the signals abruptly ended (figure 55 D). This also corresponds closely in time to when the major SO₂ emissions captured by NASA also ceased. Observations by Dario Tedesco at the summit on 6 April 2016, during a UNICEF and MONUSCO-sponsored helicopter overflight, showed only an incandescent vent releasing hot gases, and no active lava lake. A small lava lake was again visible in the pit crater on 27 April 2016 when observed by Sebastien Valade of the University of Florence on another MONUSCO-sponsored flight (figure 58).

Figure 58. Nyamuragira's pit crater with a small lava lake observed on 27 April 2016; volcanologist Sebastien Valade takes thermal measurements from the rim. Photo by Abel Kayanagh/MONUSCO. Courtesy of MONUSCO via Culture Volcan.

Thermal anomaly data from MIROVA suggest a pulse of activity during late April through early June 2016 (figure 55 D). This was followed by a period from early June through early November 2016 with no record of activity at Nyamuragira. The MIROVA signal reappeared in early November, followed by intermittent MODVOLC thermal alerts beginning on 27 November. A new pulse of thermal activity, with values similar to those observed during July 2015-April 2016, reappeared in early January 2017 (figure 55 E) and continued through April 2017. On an OVG-sponsored visit to the summit crater on 11 March 2017, independent journalist Charly Kasereka photographed the summit crater with incandescent lava covering the crater floor (figure 59).

Figure 59. Effusive activity at the bottom of the summit crater of Nyamuragira on 11 March 2017. Additional photos show minor spattering of molten lava near the vent on the crater floor. Photo by Charly Kasereka; courtesy of Cultur Volcan.

Sulfur dioxide and thermal anomaly data. Abundant sulfur dioxide emissions at Nyamuragira during November 2011-April 2017 show large variations in both magnitude and frequency during the period (table 3). A plot of the SO₂ data (figure 60) reveals a sharp increase in both the number of days per month with DU greater than 2 and the actual maximum DU value during the active flank eruption between November 2011 and February 2012. After lower values during March 2012, they rise steadily and remain significantly elevated for all of 2013. Values drop briefly in early 2014 and then rise again during April 2014, remaining elevated through February 2016 before dropping off significantly.

Figure 60. Sulfur dioxide data for Nyamuragira and Nyiragongo, October 2011 through April 2017. Blue bars represent the number of days each month where DU > 2 was captured in the Aura/OMI data (left axis). The orange points represent the highest DU value for the months where SO2 emissions had DU values > 2 for at least one day. See table 3 for details of Dobson Units (DU), and text for discussion of values. The two volcanos are less than 20 km apart, and thus the individual sources of SO2 cannot be distinguished in the satellite data.

A similar plot of the number of monthly MODVOLC thermal alert pixels for Nyamuragira from November 2011 through April 2017 (figure 61) shows that there were no thermal alerts for the period from April 2012-February 2014 when SO₂ emissions were large and frequent. In contrast, there were frequent thermal alerts from June 2014-April 2016 when SO₂ emissions were also high.

Figure 61. Number of MODVOLC thermal alert pixels per month at Nyamuragira from October 2011 through April 2017. Data courtesy of MODVOLC.

Table 3. Days per month that SO₂ values over the Nyamuragira and Nyiragongo area exceeded 2 Dobson Units (DU), October 2011-April 2017, and maximum DU values for each month. Data represent minimum values due to OMI row anomaly missing data (gray stripes), and missing days. SO₂ is measured over the entire earth using NASA's Ozone Monitoring Instrument (OMI) on the AURA spacecraft. The gas is measured in Dobson Units (DU), the number of molecules in a square centimeter of the atmosphere. If you were to compress all of the sulfur dioxide in a column of the atmosphere into a flat layer at standard temperature and pressure (0 C and 1013.25 hPa), one Dobson Unit would be 0.01 millimeters thick and would contain 0.0285 grams of SO₂ per square meter.

References: Campion, R., 2014, New lava lake at Nyamuragira volcano revealed by combined ASTER and OMI SO₂ measurements, 7 November 2014, Geophysical Research Letters (URL: http://onlinelibrary.wiley.com/doi/10.1002/2014GL061808/full).

Coppola, D., Campion, R., Laiolo, M., Cuoco, E., Balagizi, C., Ripepe, M., Cigolini, C., Tedesco, D., 2016, Birth of a lava lake:Nyamulagira volcano 2011-2015. Bull Volcanol (2016) 78: 20. doi:10.1007/s00445-016-1014-7.

Smets, B., d'Oreye, N., Kervyn, F., 2014, Toward Another Lava Lake in the Virunga Volcanic Field?, 21 October 2014, EOS, Transactions American Geophysical Union (URL: http://onlinelibrary.wiley.com/doi/10.1002/2014EO420001/pdf).

Smets, B., d'Oreye, N., Kervyn, F., Kervyn, M., Albino, F., Arellano, S., Bagalwa, M., Balagizi, C., Carn, S.A., Darrah, T.H., Fernández, J., Galle, B., González, P.J., Head, E., Karume, K., Kavotha, D., Lukaya, F., Mashagiro, N., Mavonga, G., Norman, P., Osodundu, E., Pallero, J.L.G., Prieto, J.F., Samsonov, S., Syauswa, M., Tedesco, D., Tiampo, K., Wauthier, C., Yalire, M.M., 2014. Detailed multidisciplinary monitoring reveals pre- and co-eruptive signals at Nyamulagira volcano (North Kivu, Democratic Republic of Congo). Bull Volcanol 76 (787): 35 pp.

Smets, B., Kervyn, M., Kervyn, F., d'Oreye, N., 2015. Spatio-temporal dynamics of eruptions in a youthful extensional setting: Insights from Nyamulagira volcano (D.R. Congo), in the western branch of the East African Rift. Earth-Science Review 150, 305-328. doi:10.1016/j.earscirev.2015.08.008

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); 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/); 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/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Erik Klemetti, Eruptions Blog, Wired (URL: https://www.wired.com/author/erikvolc/); Cultur Volcan, Journal d'un volcanophile (URL: https://laculturevolcan.blogspot.com/); MONUSCO, United Nations Organization Stabilization Mission in the DR Congo (URL: https://monusco.unmissions.org/en/); Oliver Grunewald, Video filmed on 10 July 2015 (URL: https://laculturevolcan.blogspot.fr/2015/07/le-lac-de-lave-du-volcan-nyamuragira.html).

The andesitic Volcán El Reventador lies well east of the main volcanic axis of the Cordillera Real in Ecuador and has historical observations of eruptions of numerous lava flows and explosive events going back to the 16th century. The largest historical eruption took place in November 2002 and generated a 17-km-high eruption cloud, pyroclastic flows that traveled 8 km, and several lava flows. This report briefly summarizes activity between 2002 and June 2014, and covers details of activity from July 2014 through December 2015. The volcano is monitored by the Instituto Geofisico-Escuela Politecnicia Nacional (IG) of Ecuador, and the Washington Volcanic Ash Advisory Center (VAAC).

Summary of 2002-2014 activity. Intermittent activity including pyroclastic flows, ash plumes, lava flows and explosive events took place between 2003 and 2008. Since July 2008 there have been persistent gas-and-ash plumes, dome growth, and both pyroclastic and lava flows. Lahars are also very common in this high-rainfall area, and cause damage to infrastructure on a regular basis. A lava dome was first observed growing in September 2009 within the crater that formed during the 2002 eruption. By July 2011, it had reached the height of the highest part of the crater rim; by January 2013 it filled the crater and formed a new summit, 100 m above the E rim. This led to lava blocks travelling down the flanks, in addition to the lava flows and pyroclastic flows traveling down the flanks of the cone inside the crater during 2012-2014. A summary of thermal anomalies compiled from MIROVA data (figure 46) demonstrates the ongoing but intermittent nature of heat flow between 2002 and 2014.

Figure 46. Thermal activity detected by the MIROVA system at Reventador, January 2002-January 2014. Courtesy of IG (Informe Especial del Volcan Reventador No. 3, 7 July 2014).

Summary of June 2014-December 2015 activity. Activity was very consistent throughout the period of June 2014 through December 2015. The thermal webcam captured images of lava flows, pyroclastic flows and ejected incandescent blocks nearly every month. MODVOLC thermal alerts were reported every month except March 2015. Satellite imagery of hot spots were common as well. The Washington VAAC reported observations of ash plumes every month, although they generally rose only to altitudes below 5.6 km (2 km above the summit). IG reported seismicity as varying between moderate and high during the period.

Activity during June-December 2014. Activity during June 2014 was characterized by numerous explosions and small pyroclastic flows that descended the flanks of the cone. The Washington VAAC issued two series of reports on 11-12 and 19-20 June. A pilot reported an ash plume on 11 June rising 2.8 km above summit at 6.4 km altitude and drifting W, and the next day ash was observed 1.8 km above the summit. Weather generally obscured satellite views. On 19 June, multiple small emissions of volcanic ash were seen in the observatory webcam along with incandescent material on the flanks. MODVOLC thermal alerts were issued on 5, 21, and 30 June.

IG reported a new lava flow on 2 July 2014 descending 400 m on the SSW flank. A pyroclastic flow was also reported on 2 July (figure 45, BGVN 39:07) extending 1,500 m down the S flank. IG noted ash emissions on 2, 4, 9-12, 18, 22-24, and 27 July rising 800 m to 2 km above the summit. MODVOLC reported multi-pixel thermal alerts on 2, 16, and 27 July, and single pixel alerts on 10 and 25 July. In addition to the ash plumes reported by IG, the Washington VAAC reported on-going ash emissions and detected hotspots at the crater on 31 July.

The Washington VAAC issued a report of hot spots visible in satellite imagery on 1 August 2014 and a pilot report of an ash plume at 6.1 km altitude (2.5 km above the summit) on 25 August. The only MODVOLC thermal alerts were issued on 31 August. IG reported lower level plumes (300-800 m above the summit) with minor ash on 6 other days during the month.

Activity increased during September 2014. The Washington VAAC issued reports during 2-4, 18, and 23 September. On 2 September, ash plumes were observed extending about 45 km W of the summit at 5.5 km altitude. Another faint plume of volcanic ash was observed within 20 km of the summit the next day. An ongoing hotspot with possible small ash emissions was noted on 4 September. IG reported an explosion on the morning of 5 September that generated a plume and ejected blocks from the crater that fell ~500 m below the summit on the W flank. A thermal camera detected an explosion on the following day that also included ballistics. MODVOLC thermal alerts were issued on eight days during September. Steam plumes with minor ash rose to around 1 km above the summit and dispersed generally W several times during the month.

A single MODVOLC thermal alert was reported on 6 October 2014. The Washington VAAC reported short 2-3 minute bursts of minor volcanic ash on 19 October which was seen drifting WNW and dispersing within 16 km of the summit below 5.8 km altitude. An additional single pixel thermal alert was issued on 25 October, and a three-pixel alert appeared on 29 October.

IG reported steam-and-ash plumes rising up to 1 km above the summit a few times during the month, which were visible on the rare clear-weather days (figure 47). Only two days in November, 5 and 21, had MODVOLC thermal alerts. The Washington VAAC, however, issued reports during 11-12, 18-19, and 27 November of possible low-level ash-bearing plumes. The IG webcam LAVA on the SE flank captured images of pyroclastic flows on 20 and 25 November (figure 48).

Figure 47. The active cone at Reventador on 9 November 2014 with a low-level steam plume. Image taken from the IG Webcam LAVA on the SE flank. Courtesy of IG via La Culture Volcan.

Figure 48. Pyroclastic flows at Reventador, 20 (left) and 25 (right) November 2014 taken from the IG LAVA webcam on the SE flank. Courtesy of IG via Culture Volcan.

On 5 December 2014 a webcam recorded a steam-and-gas emission associated with an incandescent lava flow on the E flank. MODVOLC thermal alert pixels appeared on four days in December 2014 (3, 7, 14, and 23), and VAAC reports of ash plumes were issued on 5, 13-14, 21-22, and 30 December. The largest plume, on 14 December, rose to 6.1 km (2.5 km above the summit) and drifted NE. IG reported moderate seismicity and low-level steam plumes with minor ash content on several occasions.

Activity during 2015. Moderate seismic activity continued during January 2015 with low-level steam-and-ash plumes from explosions rising a few hundred meters above the summit, according to IG. A larger explosion reported by IG on 16 January generated an ash plume that rose 2 km and drifted SE. The Washington VAAC reported activity from 14-18 January, and again on 26 January. Their reports were of small puffs of ash within a kilometer of the summit drifting for a few hours before dissipating. MODVOLC thermal alerts were issued on 15 and 29 January.

Steam plumes containing minor amounts of ash were recorded a few times during February 2015 during periods of moderate seismicity. The Washington VAAC issued several reports, during 7-9, 13-17, 19-21, 24, and 26-28 February, noting occasional plumes with ash rising to less than one km above the summit, and hot-spots seen in satellite imagery on 13-14, 17, 19, and 27 February. An aircraft reported volcanic ash on 19 February at 6.1 km altitude. A new lava flow first observed on the SW flank on 11 February had advanced 1 km by 19 February. This is consistent with the four-pixel MODVOLC thermal alert issued on 18 February. Single pixel alerts were issued on 7, 19, and 23 February as well.

No MODVOLC thermal alerts were issued during March 2015, but the Washington VAAC continued to note low-level small bursts of ash emissions several times a week within 15 km of the summit, as reported by IG. The webcam captured a hotspot at the summit on 11 March. A thermal camera image of a lava flow taken on 13 March showed the visible part of it to be over 500 m long (figure 49), and IG noted in their 13 March report that is was actually about 1.5 km long that day.

Figure 49. Annotated thermal camera image at Reventador of an 11 March 2015 lava flow. Camera is located SE of the volcano. Courtesy of IG (Informe especial del Volcan Reventador No. 1, 13 March 2015).

Activity during April 2015 included moderate seismicity and incandescence at the crater reported by IG. A lava flow on the SW flank was visible with the infrared camera during the first week; this agrees with the 5-pixel MODVOLC thermal alert recorded on 5 April and the bright hotspot observed in both satellite imagery and the webcam during 3-5 April. Hot spots were observed via satellite and webcam several additional times during the month. Additional thermal alerts also appeared on 10 and 21 April. Steam-and-ash plumes rising to 1 km above the summit were intermittent throughout the month, mostly observed from the webcam.

Multi-pixel MODVOLC thermal alerts appeared during 2-3, 20, and 30 May, indicating continued sources of heat from lava flows. In a special report issued on 19 May, IG noted a new lava flow during the previous week that descended the S flank, forming a fan with three lobes on the SE and SW flanks. The length was greater than 1,000 m from the summit on 19 May, although the flows remained on the flanks of the summit cone within the caldera (figure 50). IG noted an increase in emission tremor on 17 May which may have been related to the extrusion of the lava, but weather conditions prevented visual confirmation. During 17-30 May, intermittent low-level gas-and-ash plumes within 15 km of the summit were reported on most days.

Figure 50. Annotated thermal image of the summit cone of Reventador on 19 May 2015 showing a 3-lobed lava flow descending the S flank of the cone for more than 1 km. Courtesy of IG (Informe especial del Volcan Reventador No. 2, 19 May 2015).

MODVOLC thermal alerts diminished during June 2015, occurring only on 8 and 15 June. Nonetheless, thermal images showed lava flows down the SW and S flanks of the cone several times, and hot spots were observed in satellite images and on the webcam when the weather permitted. Steam-and-ash plumes were generally reported to rise to 1 km or less above the summit and drift usually NW or SW within 15 km of the volcano. A pilot reported volcanic ash on 30 June at 6.7 km, but no ash was seen in satellite imagery under cloudy conditions. IG issued a special report on 24 June noting increased seismicity in the form of increased tremor signal and explosions on 23 June. The thermal camera located in the area of El Copete, 5 km S of the crater, showed an increase in surface activity characterized by several lava flows on the SW, S, and SE flanks exceeding one km in length (figure 51).

Figure 51. Thermal image of Reventador taken on 23 June at 1950 by the webcam near El Copete. Courtesy of IG (Informe especial del Volcan Reventador No. 3, 24 June 2015).

Seismic activity was reported as high during July 2015 by IG, and included explosions, tremor, long-period earthquakes, harmonic tremor, and emission signals. During the first week, incandescent material was visible more than 1 km down the SE flank in thermal images. On 17 July, light gray deposits possibly from a pyroclastic flow were observed; on 21 July explosions again ejected incandescent material onto the flanks. Steam and ash emissions were intermittent and generally remained below 5.1 km altitude. MODVOLC thermal alerts appeared on 1, 3, 15, and 17 July.

High levels of seismic activity continued during August 2015. The Washington VAAC reported possible ash plumes on 14 days during the month, and MODVOLC thermal alerts were issued on six dates, including four-pixel alerts on 4 and 27 August suggestive of lava flows and/or incandescent material on the flanks of the cone. A discrete volcanic ash emission on 6 August was reported by the Washington VAAC at 7 km altitude (3.4 km above the summit) with a plume extending about 25 km NW of the summit. Other plumes that were reported by pilots (on 25 August at 8.8 km altitude moving NW, and on 26 August at 6.7 km moving W) were not observed in cloudy satellite imagery.

Ash-and-gas emissions were reported by the Washington VAAC during 14 days in September 2015, generally drifting N and W at altitudes less than 2 km above the crater (5.6 km altitude); high levels of seismicity also continued, according to IG. The Guayaquil MWO reported volcanic ash at 6.1 km on 19 September. Puffs of ash seen in the webcam were reported at 7.3 km altitude on 25 September and thought to have quickly dissipated. MODVOLC thermal alerts appeared on seven days during the month; five of them were two- or three-pixel alerts. An SO₂ plume drifting WNW from Reventador was captured by NASA's OMI instrument on 22 September (figure 52).

Figure 52. An SO2 plume drifting W from Reventador on 22 September 2015. Reventador is represented by the triangle south of the NW-SE trending Ecuador/Colombia border in the bottom center of the image near longitude 78 W just south of the equator. A small plume in the top half of the image is likely SO2 from Nevado del Ruiz. Courtesy of NASA/GSFC.

A series of VAAC reports of low-level minor ash emissions were issued during 1-5 October 2015. After two weeks of no activity, multi-pixel MODVOLC thermal alerts and VAAC reports increased during 20-30 October. The peak MODVOLC activity included 4-6 daily pixels during 26-28 October, and the VAAC reports noted a bright hotspot on the satellite images beginning on 20 October and present for most of the rest of the month. Continuous emissions were observed in the webcam during 22-26 October, generally below 4.6 km, moving NW, and extending up to 40 km from the summit. Continuous emissions appeared again on 30 October at 5.1 km moving W.

During the last two weeks of November 2015, steam, gas, and ash emissions rose to less than 2 km above the summit and incandescent blocks rolled 500 m down the flanks of the cone. MODVOLC thermal alerts were reported for five days between 15 and 29 November. Similar activity was reported during December, although the Washington VAAC only issued reports on four different days, and MODVOLC thermal alerts were recorded only on 6 and 24 December. VAAC reports noted hotspots in satellite imagery on 7 December. The VAAC reports on 11 and 16 December indicated ash plumes at 5.5 km moving W and SW.

Geologic Background. Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Culture Volcan, Journal d'un volcanophile (URL: https://laculturevolcan.blogspot.fr/).

A February 2012 ash explosion of Colombia's Nevado del Ruiz volcano was the first confirmed ash emission in over 20 years. The broad, glacier-capped volcano has an eruption history documented back 8,600 years, and historical observations since 1570. Notably, a large explosion at night in heavy rain on 13 November 1985 generated large lahars that washed down 11 flank valleys, inundating most severely the town of Armero where over 20,000 residents were killed. It remains the second deadliest volcanic eruption of the 20th century after Mt. Pelee in 1902 killed 28,000.

This report summarizes and concludes the February 2012-April 2014 eruption (BGVN 37:08, 39:07), and then describes details of new activity beginning in November 2014, through December 2015. The volcano is monitored by the Servicio Geologico Colombiano (SGC) and aviation reports are provided by the Washington Volcanic Ash Advisory Center (VAAC).

Summary of activity, November 1985-June 2012. After the large explosions and deadly lahars of November 1985, activity at Ruiz continued with intermittent ash emissions and significant seismic activity through July 1991. Seismicity, deformation, and SO₂ emissions have been closely monitored since the 1985 eruption. Between 1991 and February 2012 intermittent high-frequency seismic events (earthquake swarms) were recorded, but no ash emissions were observed. In September 2010, seismicity notably increased in frequency and diversity of event type until early 2012 when fresh ashfall was observed. INGEOMINAS (Instituto Colombiano de Geología y Minería, precursor to SGC) also noted an inflationary trend in the geodetic data from October 2010 through 2011.

A March 2012 overflight by INGEOMINAS noted minor amounts of ash-covered snow on the E flank, which they surmised came from an explosion on 22 February (BGVN 37:08). During March, long-period seismicity underwent a 20-fold increase. SO₂ emissions also dramatically increased between March and June 2012. Several ash emissions from the summit were observed during April-June 2012 (BGVN 37:08). An ash plume that rose to 11 km altitude on 29 May caused ashfall in over 20 communities to the NW and closures at three nearby airports. Widespread ashfall during June covered solar panels on field equipment. An EO-1 satellite image from 6 June 2012 shows a plume and significant ashfall around the summit (figure 71).

Figure 71. Satellite image of Nevado del Ruiz taken on 6 June 2012 showing an active ash plume from the Arenas crater and ash deposits NW of the summit. It was acquired by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite. Courtesy NASA Earth Observatory.

Summary of activity, July 2012-December 2015. Explosions and seismic tremor with ash emissions continued during July and August 2012. Ashfall was reported within 30 km on numerous occasions. From September 2012 through early July 2013 minor amounts of ashfall were reported a few times each month, mostly in the immediate vicinity of the volcano. After a larger explosion on 11 July 2013, sparse and intermittent ash emissions were reported between August 2013 and April 2014. Between May and October 2014 there were no reports of ash emissions or thermal anomalies.

A significant increase in seismicity occurred during the second week of November 2014, and ash was seen at the summit during an overflight on 19 November. Ash fell in communities within 30 km several times each month through December 2015. Seismic evidence suggesting possible lava dome extrusion first appeared in August 2015, and stronger signals were recorded on 22 October. Thermal anomalies around the summit crater increased in frequency and magnitude during the last three months of 2015.

Activity during July 2012-October 2014. A large ash plume on 30 June 2012 prompted evacuation warnings to several communities within 30 km and closed three nearby airports for the second time within 30 days. On 2 July the Washington VAAC reported a 7.5-km-wide ash plume at 6.1 km altitude drifting 75 km W (BGVN 37:08). Additional VAAC reports were issued on 8, 9, and 10 July for SO₂ emissions containing minor volcanic ash. SGC noted that explosions and ash emissions continued throughout the month in spite of a decrease in seismicity. Ashfall was reported near the volcano, and in municipalities in the departments of Caldas (W) and Risaralda (SW), steadily throughout the month.

Tremors associated with continuing gas and ash emissions occurred throughout August 2012; ash plumes were observed rising 200-800 m above the summit crater. During 3-6 August, gas and ash emissions were seen from Manizales (30 km NW) and Chinchiná (30 km WNW). On 12 August, a gas-and-ash plume observed with a webcam rose 1 km above the crater and drifted W, and ashfall was reported in Brisas (50 km SW). A layer of ash was deposited at the Observatorio Vulcanológico y Sismológico de Manizales (OVSM) on 13 August; they also reported ash emissions associated with seismic signals the next evening. Webcams showed gas-and-ash plumes rising 400 m and drifting W and NW during 15-16 August.

Minor amounts of ashfall were reported by SGC in areas around the volcano each month during September 2012 through 11 July 2013 (table 4), when a larger ash emission occurred. A noted increase in seismicity beginning on 13 April 2013 was also reported by SGC. The ash emission on 11 July was captured by the webcam in the Parque Nacional Natural Los Nevados (PNNN) (figure 72), and fine ash fell in Manizales. The Washington VAAC reported the ash plume at 6.1 km altitude. Multispectral imagery showed the plume extending 55 km NW. After 12 July 2013 there were no further reports from the Washington VAAC until December 2014.

Table 4. Ash emission events at Ruiz during September 2012-July 2013. Data compiled from various sources as shown.

Figure 72. Ash emission at Ruiz on 11 July 2013 at 1143. The column of gases and gray ash stands out among the white clouds. Photo by Julián Peña, courtesy of SGC (Informe-Technico, July 2013).

Evidence for ash emissions between August 2013 and April 2014 is sparse and intermittent. The SGC Monthly reports during this time mention pulses of low-energy tremor associated with emissions of gases, steam, and small amounts of ash every month except November, when they reported only steam and gas, but no specific dates are given. SGC's Technical Information Monthly reports mention occasional grayish coloration, suggesting ash in the gas-and-steam plumes during August-October 2013. Tremors associated with small amounts of ash and grayish coloration in the plumes are again noted from January through April 2014 without describing specific events.

The weekly activity reports issued by SGC make no mention of ash from August through November 2013. They note in weekly reports for 2-8 and 9-15 December that gray emissions possibly associated with ash in plumes of mostly water vapor and gases were observed. During the week of 16-23 December they recorded low-energy tremors associated with the output of small amounts of ash, which were reported in trace quantities in Manizales. In their 31 December 2013-6 January 2014 and 10-16 February 2014 weekly reports they noted the occurrence of tremors associated with ash and gas. There is no mention of ash in their March or April 2014 weekly reports. There is also no mention of ash emission in SGC monthly reports during May-October 2014. The MIROVA thermal anomaly data do show minor thermal anomalies in latest August and more persistent anomalies at the beginning of October 2014 (figure 73) prior to the reports of ash emissions during November.

Figure 73. MIROVA signal of MODIS data for the year ending on 15 May 2015. Persistent thermal anomalies are present between late October 2014 and mid-April 2015. Courtesy of the MIROVA project supported by the Centre for Volcanic Risk of the Italian Civil Protection Department via SGC (Informe de Actividad, April 2015).

Activity during November 2014-December 2015. A significant change in seismicity occurred beginning in the second week of November 2014. There was an increase in the number of long-period (LP) earthquakes, pulses of volcanic tremor, and several periods of continuous tremor (lasting for hours or even days) associated with fluid movement, and with emissions of gas and ash (table 5). Several of these periods were preceded by an LP event. The first significant pulse of volcanic tremor began on the evening of 18 November following an LP event and lasted more than 12 hours.

Table 5. Periods of continuous tremor associated with ash emissions at Ruiz during November 2014. Some of the tremor episodes were preceded by long-period (LP) events. Courtesy of SGC (Informe de Actividad, November 2014).

The Unidad Nacional de Gestion de Riesgo de Desastres (UNGRD, National Disaster Risk Management Unit) coordinated an overflight during 19-21 November 2014 and observed fresh ash deposits on the S flank. Ash emissions were also verified in satellite imagery (figure 74) and by reports from nearby communities. The ash dispersed generally SE and SW during 18-21 November. Ash was again observed on the N side of the Arenas crater on 29 November in the early morning after a lengthy period of continuous tremor was recorded the previous day (see table 5).

Figure 74. Image of Ruiz on 24 November 2014 taken by the OLI-TIRS sensor on the Landsat 8 Satellite at 1018 local time. Ash deposits are dispersed SE and SW of the summit crater, and the steam plume is drifting W. Courtesy of SGC (Informe de Actividad, November 2014).

During the second half of December 2014, SGC reported significant concentrations of ash in the emissions that were associated with continuous tremor episodes. On 15 December seismic signals indicating ash emissions were detected, and then confirmed by a local webcam and nearby residents. The Washington VAAC also noted an ash emission based on a pilot observation extending 16 km S at 7.6 km altitude. The next day they reported a narrow plume of minor volcanic ash extending 22 km SW of the summit at 6.1 km altitude. On 18 and 19 December the Washington VAAC reported ash plumes to altitudes of 7.9 and 9.1 km, respectively, that drifted SSW and dissipated within a few hours. A faint thermal anomaly was also detected. A satellite image taken on 26 December 2014 clearly shows ash deposits in nearly all directions from the Arenas crater (figure 75). Ashfall was reported during this time in the Caldas (W) and Risaralda (SW) departments.

Figure 75. An ASTER image from the OLI-TIRS Sensor on the Landsat 8 satellite taken on 26 December 2014 of Ruiz (N is to the top) showing fresh ash deposits covering the summit glacier in nearly all directions. Courtesy of SGC (Informe de Actividad, December 2014).

According to the news source Prensa Latina, increased ash emissions at Ruiz prompted closure of the La Nubia airport (22 km NW) on 7 January 2015. On 14 January, the Washington VAAC reported an ash plume visible in satellite imagery extending 16 km SW of the summit at 6.7 km altitude. SGC reported seven episodes of continuous tremor on 4, 7, 14, 24, 26, 28, and 29 January, almost all of which were associated with ash emissions (figures 76). Ashfall was reported several times after these episodes in the Eje Cafetero area to the W of Ruiz.

Figure 76. Ash emissions on six different dates during January 2015 at Ruiz. Photographs taken by the webcam located in the Azufrado sector (NW). Courtesy of SGC (Informe de Actividad, January 2015).

Occasional minor ash emissions were reported during February 2015 during periods of continuous tremor, but most of the emissions were steam and gas. On 9 February, ashfall was reported in El Libano (29 km E), El Oso (10 km SE), and Murillo (17 km E). Although seismic tremors were diminished during March from the previous month, emissions associated with these tremors contained gases and minor amounts of ash from 8 March through the end of the month. Ashfall was reported after a tremor in the evening on 8 March by personnel from the Parque Nacional Natural Los Nevados (PNNN), the Observatorio Vulcanológico y Sismológico de Manizales (OVSM), and from the municipalities of Manizales and Villamaria (27 km NW).

An increase in several types of seismicity was observed by SGC during April 2015. Volcanic tremor, associated with gas and ash emissions, were confirmed through photographs taken by the webcams (figure 77), and by officials at PNNN and SGC. Ashfall was reported on 20 April in the municipalities of Manizales and Villamaría. The Washington VAAC reported a small puff of gas and minor amounts of ash visible in satellite imagery on 22 April at 7.3 km altitude drifting W about 40 km before dissipating. The MIROVA signal from the MODIS thermal anomaly data shows persistent thermal activity from late October 2014 through mid-April 2015 (figure 73).

Figure 77. Plumes of ash-and-gas from Ruiz during April 2015. Confirmed ash emissions were observed on 9, 22, 27, and 29 April. Courtesy of SGC (Informe de Actividad, April 2015).

Ash emissions were photographed by the webcams located in the Azufrado and Cerro Guali regions on at least eleven dates during May 2015. The Washington VAAC reported possible emissions on 19 and 26 May, but extensive weather clouds prevented satellite observations. Most of the frequent episodes of volcanic tremor during June were also associated with ash emissions which were photographed at least six times during the month. The Observatory at Manizales reported ash moving WNW on 6 June at about 800 m above the summit; weather clouds obscured satellite observations by the Washington VAAC.

A significant increase in ashfall was reported during July 2015 (figure 78), including in the regions of Caldas, Tolima, and Risaralda, as well as by officials in the Park (PNNN). The Observatory at Manizales (OVSM) reported an ash plume on 6 July at about 7.3 km altitude, but it was not observed in satellite data due to weather. The Washington VAAC noted ash emissions visible in satellite data and the webcam on 13 July, with a plume at 7 km altitude drifting NW a few tens of kilometers before dissipating. OVSM reported plumes at about 6 km moving S and W during 18-20 July. Seismic signals indicating emissions were reported on 23 July and observed in the webcam, according to the Washington VAAC. SGC noted seismic tremors and a plume on the morning of 26 July that rose to 3 km above the summit (8.2 km altitude) (figure 79); near summit-level emissions were also observed via the webcam on 26 and 27 July. Seismic data indicated continued occasional bursts of ash drifting W to WSW during the next few days. Ashfall was reported downwind in the municipalities of Chinchina (33 km NW), Palestina (35 km NW), Santa Rosa de Cabal (33 km W), Dosquebradas (40 km WSW), and Pereira (40 km WSW). A bright thermal anomaly was reported in satellite imagery on 31 July, but no ash was observed.

Figure 78. Gas, steam, and ash plumes from the Arenas crater at Ruiz during July 2015. Photographs captured by the cameras located in the area of Azufrado, Cerro Gualí, and in the OVSM. Courtesy of SGC (Informe de Actividad, July 2015).

Figure 79. Seismic and visual images of tremors that produced ash emissions at Ruiz between 0800 and 1559 on 26 July 2015. The digital seismogram and spectrogram are from station BIS (2 km W of Arenas Crater) and show a characteristic spasmodic tremor (1, 2, and 3) that was associated with ash emissions recorded on the Piranha-Azufrado webcamera in the lower images. Courtesy of SGC (Informe de Actividad, July 2015).

SGC reported greater instability at Ruiz compared with previous months during August 2015. Seismicity related to fracturing and fluid flow both increased during the month. Energy levels for spasmodic tremor related to gas and ash emissions were also generally higher. The Washington VAAC reported ash visible in satellite imagery on 6 August at 7.3 km altitude moving NW as far as 20 km for about 10 hours before dissipating. They noted another possible plume with minor ash on 12 August at 6.7 km drifting 55 km NW from the summit. Ashfall was reported on 23 August from officials of PNNN and residents of Pereira. A brief emission containing minor ash on 28 August, observed in a webcam, was reported by the Washington VAAC as extending about 35 km W. Ongoing emissions rising a few hundred meters above the summit with occasional small bursts of ash continued for the next two days.

The tremor event on 31 August 2015 was the largest since 18 November 2014; ashfall affected numerous cities and municipalities, including Manizales (30 km NW) (with the largest particle sizes towards the E side of the city), La Linda, La Cabaña (36 km NW), and trace amounts in Santagueda (40 km NW), Arauca (48 km NW), Kilómetro 41, Villamaría (27 km NW), Chinchiná, Palestina, and Neira (36 km NW) (figure 80). A news article reported that the La Nubia airport closed that day due to ash emissions. Most ash emissions during the month affected the regions of Caldas and Risaralda NW of the volcano.

Figure 80. Ashfall was recorded in a number of cities during the 31 August 2015 emission event at Ruiz. The four left images are from the city of Manizales. The six right images are from different towns in the department of Caldas. Courtesy of SGC (Informe de Actividad, August 2015).

The Washington VAAC issued advisory reports on 3, 12-15, 17, 23-24, 27, and 29-30 September 2015. Most reports were based on observations from the webcams near the volcano and/or seismic activity, but many events were not visible in satellite imagery due to weather clouds. Plume altitudes ranged from 5.5 to 7.9 km. Incandescence observed in a webcam on 4 September was followed by a high-energy tremor. The ash plumes reported by the Washington VAAC on 12 and 13 September rose to 7.9 km and drifted in several directions. Ash was moving to the NW below 5.2 km and extended for over 90 km; between 5.2 and 7.9 km altitude it extended about 80 km SW. Ongoing emissions with small bursts of ash continued through 15 September with a new emission to 7.6 km around 1600 that day.

The OVSM reported a strong seismic signal at 0728 on 17 September, but weather clouds blocked observation from satellite imagery of the potential ash plume. The largest tremor of the month occurred in the afternoon of 18 September and ash emissions were verified in the webcams as well as by SGO officials doing fieldwork in the area; ash emissions were also observed in the webcam on 19 September at 1556. SGO reported a seismic event on 22 September that produced water-vapor, gas, and ash plumes that rose 2 km above the crater and drifted mainly NW. An ash plume was confirmed by the Washington VAAC in a satellite image on 27 September extending about 70 km WNW at 6.1 km altitude. An advisory issued on 29 September noted ash to 8.5 km within 16 km of the summit. SGO noted that the 29 September emissions were observed both E and W of the volcano.

The Washington VAAC confirmed continuous ash emissions on 5 October 2015 at 7 km altitude extending about 25 km W of the summit. A gas, steam, and ash plume rose 1.7 km and drifted NW on 8 October. Another report of volcanic ash early on 9 October was not visible in satellite imagery, although a thermal anomaly persisted and seismicity was elevated. A small ash emission was spotted in imagery data drifting WNW late on 9 October. A gas, steam, and ash plume rose 1.8 km and drifted NW on 17 October. A discrete emission of ash rose to 9.1 km altitude on 22 October and drifted N. SGO reported ash emissions observed in webcams on 26 October, but weather clouds prevented satellite observation by the Washington VAAC. A gas, steam, and ash plume rose 1.7 km and drifted NW on 30 October.

SGC first noticed an unusual pattern of seismicity known as a "drumbeat" signal, for which they issued a special report on 20 August 2015. The "drumbeat" signal is characterized by discrete episodes of short duration (about 30 minutes each) that repeat at regular time intervals and show similar waveforms and energy. They are interpreted by volcanologists to represent phenomena associated with the ascent of high-viscosity magma to the surface and thus are an indicator of near-surface extrusion or dome building. SGC recorded the same signal on 8 September, and then again on 22 October (figure 81). Thermal anomalies near the Arenas crater were observed by SGO on 26, 28, and 30 September, and were again recorded on 7, 9, and 10 October 2015.

Figure 81. Episodes of seismic "drumbeats" at Ruiz recorded on 22 October 2015. The top box is the vertical component seismic record from station BIS, the larger yellow shaded box highlights the entire 'drumbeat' episode. The seismogram from the OLLETA station (lower left) shows a clearer view of the first episode (1). The lower right images show details of the signal at three different time intervals highlighted in smaller boxes in the top image. This signal is interpreted to represent phenomena associated with the ascent of high-viscosity magma to the surface and thus are an indicator of near-surface extrusion or dome building over the emission conduit. Courtesy of SGC (Informe de Actividad, October 2015).

Seismic activity decreased slightly during November 2015, but there still were episodes of volcanic tremor associated with gas and ash emissions that were recorded by the webcams and personnel at PNNN. Continuous tremor signal was recorded on 1 and 4 November. The "drumbeat" signal was again briefly recorded on 13 November. Thermal anomalies increased in frequency and were observed on 4, 18, 20, 22, 26, and 27 November. SGC confirmed ash emissions on 5, 10, 14, 27, and 29 November. The Washington VAAC reported an ash plume on 14 November at 6.4 km altitude moving SW. SGC captured images of the ash plume from two different webcams (figure 82).

Figure 82. Photographs of the ash emission at Ruiz of 14 November 2015 at 0537 from two different webcams. Top image is from the Azufrado webcam (5 km NE) and the lower image is from the Pitayo webcam. Courtesy of SGC (Informe de Actividad, November 2015).

Thermal alerts captured by the University of Hawai'i's MODVOLC system appeared in December 2015 for the first time in several years. They were recorded on 3, 22, 26, and 31 December. Additionally, the MIROVA thermal anomaly system showed significant increases in anomalies at Ruiz during the last three months of 2015 (figure 83).

Figure 83. MIROVA data for the year ending 2 January 2016 showing the substantial increase in frequency and magnitude of thermal anomalies at Ruiz during the last three months of 2015. Courtesy of MIROVA via SGC (Informe de Actividad, December 2015).

Minor episodes of volcanic tremor with ash emissions were reported by SGC during the first two weeks of December 2015. A significant volcanic tremor with ash emissions occurred on 20 December, and ashfall was reported by SGC officials, PNNN personnel, and residents near the volcano and in the city of Manizales. The Washington VAAC noted the ash plume at 6.1 km altitude with 25 km of the summit. A gas, steam and ash plume rose 1.7 km and drifted NW on 28 December.

Sulfur Dioxide emissions, June 2012-2015. Persistent, large SO₂ plumes were captured from Ruiz many times during June 2012-December 2015 (figure 84 and 85). Every month during this period the OMI (Ozone Measuring Instrument) on the Aura satellite recorded days with SO₂ emissions exceeding 2 DU (Dobson Units); many months had more than half of the recording days with values > 2 DU. Dobson Units are the number of molecules in a square centimeter of the atmosphere. If you were to compress all of the sulfur dioxide in a column of the atmosphere into a flat layer at standard temperature and pressure, one Dobson Unit would be 0.01 millimeters thick and contain 0.0285 grams of SO₂ per square meter.

Figure 84. Select Aura/OMI images of SO2 plumes from Ruiz, 2012-2013. Top left: 14 June 2012, the SO2 plume drifts NW. Top right: 18 August 2012, the SO2 plume from Ruiz drifts W. An SO2 plume is also visible drifting W from Ecuador's Cotopaxi in the lower left corner of the image. Bottom left: A 10.26 DU (Dobson Unit) SO2 plume sits directly over Ruiz on 7 December 2012. Bottom right: The SO2 plume drifts south on 19 December 2013. See text above for description of Dobson Units. Courtesy of NASA Goddard Space Flight Center (NASA/GSFC).

Figure 85. Select Aura/OMI images of SO2 plumes from Ruiz, 2014-2015. Top left: On 3 February 2014 an SO2 plume from Ruiz drifts due W while another plume drifts NE from Guagua Pichincha in northern Ecuador. Top right: A 24 September 2014 SO2 plume drifts NW from Ruiz as far as the coastline. Bottom left: On 5 March 2015, a plume drifts slightly W from Ruiz. Bottom right: A W-drifting SO2 plume from Ruiz on 4 October 2015 is visible along with W-drifting plumes from both Cotopaxi and Tungurahua in Ecuador. Courtesy of NASA/GSFC.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: Servicio Geologico Colombiano (SGC), Observatorio Vulcanologico Y Sismologico Manizales, Diagonal 53 N0. 34 - 53 - Bogotá D.C. Colombia (URL: http://www2.sgc.gov.co/Manizales.aspx); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); 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 Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); 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/); Prensa Latina, Agencia Informativa Latinoamericana (URL: http://www.plenglish.com/).

Strong fumarolic activity characterized activity at Costa Rica's Turrialba for several decades before a phreatic eruption in January 2010 resulted in ashfall tens of kilometers from the volcano. Since the January-March 2010 eruption, there have been one or two brief eruptive episodes with ash emissions each year, generally lasting days to weeks. An episode from 29 October through 8 December 2014 began with an ash explosion, followed by continuous emissions on 30 and 31 October. Several additional explosions with ash emissions occurred during November, followed by a strong Strombolian explosion on 8 December that included ashfall up to 1 cm thick in places, and ballistics deposited 300 m from the vent (BGVN 40:04). This report covers the increasing ash-emission activity during 2015 and 2016. Information comes primarily from the Observatorio Vulcanologico y Sysmologico de Costa Rica-Universidad Nacional (OVSICORI-UNA). Aviation alerts are issued by the Washington Volcanic Ash Advisory Center (VAAC).

Turrialba began a new eruptive episode with an ash plume on 8 March 2015. Frequent, intermittent ash-bearing events continued through mid-May, and tapered off during June, with a final event reported on 22 June 2015. The larger plumes rose 2-2.5 km above the vent rim and drifted in many different directions, leading to ashfall throughout the region as far as 40 km from the volcano. A 'bubble of magmatic gas' dispersed accumulated ash from the vent on 15 August 2015. An eruption on 16 October 2015 was the largest in a year, and the start of a new series of emissions that persisted through the end of October, dispersing ash for tens of kilometers in most directions. A brief period of ash emissions between 2 and 8 February 2016 deposited ash within a few kilometers of the summit crater. Ash emissions and frequent small explosions between 28 April and 7 May preceded a longer series of emissions that began with a significant explosion on 16 May, included significant ashfall in regions within 30 km, and lasted until late July 2016. Strombolian activity and pyroclastic flows were also reported during late May; ashfall was reported up to 100 km SW. A new series of explosions and ash emissions began on 13 September that continued nearly uninterrupted through the end of the year, although ashfall reports were greatest in October 2016.

Activity during 2015. Little activity was reported during January and February 2015. Seismicity slowly increased from short-duration, low-amplitude, higher-frequency events in January to more lower-frequency events in February. Very-long-period earthquakes (VLP's) began to register in February and became more pronounced during March, when some were associated with explosions and ash emissions. The first, short, effusive emissions with low ash content occurred on 8 March. The largest events with prolonged ash emissions occurred on 12 (figure 43) and 15 March.

Figure 43. Eruption at Turrialba on 12 March 2015. Webcam image courtesy of OVSICORI (Boletín de Vulcanología Estado de los Volcanes de Costa Rica, January, February, March 2015).

Based on webcam views, weather models, and OVSICORI-UNA updates, the Washington VAAC reported that on 8 March diffuse ash emissions rose from the Cráter Oeste (West Crater) and seismicity increased. OVSICORI-UNA reported more ash emissions on 11 and 12 March. Almost continuous ash emissions were observed in the afternoon of 12 March punctuated by two noticeable explosions. Ash plumes rose as high as 2 km above the crater and drifted NW. Ashfall also occurred in the Valle Central and in the capital of San José (30 km WSW), and caused the closure of the Juan Santamaria International Airport (48 km W), which reopened during the evening on 13 March. The local Tobias Bolanos airport (40 km WSW) closed intermittently. On 13 March three short-duration explosions were reported. According to the Washington VAAC, ash plumes that day drifted 45 km NE at an altitude of 9.1 km, and drifted over 35 km W at an altitude of 6.1 km.

On 18 March, OVSICORI-UNA reported that gas, vapor, and ash plumes rose from Cráter Oeste and seismicity remained high. Observers in Finca La Central (2 km SW) noted gas-and-steam emissions. On 19 March two gas-and-water-vapor emissions were observed; one from Cráter Central contained a small amount of ash. At 1400 the webcam recorded strong emissions of gas, vapor, and tephra from Cráter Oeste. On 23 March a gas, vapor, and ash plume rose from Cráter Oeste, causing ashfall in areas E and SE of the crater including in the Cráter Central and El Mirador. In addition, a dense and vigorous gas-and-vapor plume caused Parque Nacional Volcán Turrialba authorities to recommend masks for protection against gas inhalation.

There were 11 gas-and-ash eruptions and 10 additional smaller ash emissions during April 2015. OVSICORI-UNA reported that a small ash eruption occurred on 3 April, causing ashfall in nearby areas including Silvia and La Central. On 5 April, an eruption generated a plume that rose 500 m and caused ashfall in Curridabat (31 km WSW), Granadilla (29 km WSW), San Pedro, Desamparados (35 km WSW), Aserrí (40 km SW), San Sebastián (37 km WSW), and Escazú (42 km WSW). The eruption of 7 April was the largest of the month (figure 44), and although it occurred at night, the visible ash plume rose to about 2.5 km above the summit. Ash and sulfur odors were reported in many areas of the city of San José (30-40 km WSW). The largest quantities of ash fell in the La Picada and La Silvia communities a few kilometers NNE of the volcano, and affected several hundred cows and other animals at dairy farms. Small ash emissions occurred on 8, 16, and 18 April, and every day during 20-24 April. The ash on 20 April dispersed N and affected Guápiles (20 km N). On 23 and 24 April, ash dispersed NW and affected the inhabitants of the Valle Central, and was reported at Tobias Bolanos and San Juan Santamaria international airports.

Figure 44. Nightime eruption of ash and hot volcanic blocks from Turrialba on 7 April 2015 that began at 0205 and lasted until 0241. Webcam image courtesy of OVSICORI-UNA, (Boletín de Vulcanología, Estado de los Volcanes de Costa Rica, April 2015)

During May 2015, OVSICORI-UNA recorded 39 eruptions with ash emissions. In general, the plumes did not rise more than 500 m above the crater, and a few were accompanied by small pyroclastic flows. The largest events were on 1 and 4 May when emissions lasted for 4 and 23 minutes, respectively. The 4 May event produced an ash plume that rose 2.5 km and drifted SW. The eruption ejected ballistics 1 km from the crater. Most of the ashfall occurred around the crater. Reports of minor ashfall and sulfur odors came from communities 30-40 km WSW around the city of San José (Moravia, Coronado, Mata de Plátano, La Uruca, Guadalupe, Tibás, Calle Blancos, San Pedro Montes de Oca, Sabanilla Montes de Oca, Pavas, Zapote, Escazú, Paso Ancho, Curridabat, Santa Ana), and a few localities in the eastern region of Heredia (40 km W). Additional ash emissions were reported on 6, 11, 14, and 18 May. Although the multiple emissions on 18 May lasted as long or longer than earlier events (23 and 25 minutes), they were lower energy, and the plumes rose only 400-500 m above the summit crater.

OVSICORI-UNA reported that ash emissions occurred on 1, 4, 7, and 22 June 2015. The eruption on 1 June was the largest, and the small ash eruption on the afternoon of 22 June deposited ash mainly in the vicinity of the volcano to the SW (figure 45). They also reported a significant decrease in the seismic activity, such that by late June, the RSAM values had returned to levels similar to October 2014, prior to the start of the most recent eruptive events. Significant rains after April 2015 led to a shallow lake forming in the Cráter Oeste. Images taken in July of the Cráter Central showed deposits of eruptive material more than 2 m thick compared with May 2014.

Figure 45. Eruption at Turrialba on 22 June 2015. Webcam image courtesy of OVSICORI-UNA (Boletín de Vulcanología, Estado de los Volcanes de Costa Rica, June 2015).

Seismicity continued to decrease during August 2015. However, an event on 15 August comprised nine hours of tremor associated with the ascent and escape of a bubble of magmatic gas, according to OVSICORI-UNA. The resulting ash ejection was believed to be material that had accumulated at the bottom of the crater. Seismicity remained low during September, with no reported ash emissions.

An increase in seismicity began on 1 October 2015, and until a large eruption on 16 October (figure 46). This was followed on 23 October by a lengthy sequence of ash emissions that continued until 31 October. The 16 October eruption was the largest in terms of energy since the 30 October 2014 eruption. Most of the ash fell on the summit, but a plume headed NW and minor ashfall was reported in parts of the Valle Central such as la Unión, Concepción de Tres Ríos, Montes de Oca (30 km WSW), San Rafael de Coronado (26 km WSW), and Moravia (27 km W). A strong odor of sulfur was reported in Tierra Blanca (18 km SW), Pacayas (12 km SSW), Moravia, and Guadalupe (32 km WSW).

Figure 46. A Google Earth image of Turrialba annotated with images from the 16 and 26 October 2015 eruptions. a) 20-cm- diameter impact from volcanic ejecta. b) Solar panel destroyed by impacts. c) Ash deposit. d) Pyroclastic flow deposit. e) Hot material deposited by the pyroclastic flow. f) Thermal image of an eruption on 26 Of October (Photos: G.Avard). Courtesy of OVSICORI-UNA (Boletín de Vulcanología, Estado de los Volcanes de Costa Rica, October 2015).

Seismicity increased between 16 and 23 October, when new ash emissions began and were accompanied by pyroclastic flows. Between 23 and 31 October, OVSICORI-UNA reported 57 small emissions and 120 explosions of varying size and characteristics. The Washington VAAC was unable to see most of the emissions in satellite imagery due to weather clouds, however the plumes on 31 October were reported at 4.3 km altitude moving W. Both seismic and eruptive activity declined considerably during November 2015. OVSICORI-UNA reported one small eruption on 27 November and a small explosion on 30 November; they did not mention ash related to either event.

Activity during 2016. OVSICORI-UNA reported a brief emission of gases and volcanic ash to 500 m above the crater on 2 February 2016. Residents of La Silva (2 km NW) reported a sulfur odor and ashfall on 5 February, and additional emissions above Cráter Oeste on 6 February. The Washington VAAC noted gray emissions on 8 February. The next report, on 3 April, described an explosion lasting less than one minute that generated a small gas-and-ash plume. Seismicity increased on 28 April, followed by ash emissions and frequent small explosions on 30 April and 1 May from Cráter Oeste. Gas-and-tephra emissions increased on 1 May with minor amounts of ash deposited in La Central (4 km SW) and La Pastora (6 km SSE). A larger ash plume on 2 May rose 2 km above the summit, and was followed by frequent explosions producing 1-km-high ash plumes the next day. Frequent explosions were again recorded during 3-5 May with ash plumes rising up to 1 km above Cráter Oeste. Small lahars were reported on 7 May, and small, frequent ash emissions accompanied spasmodic tremor on 8 May.

A significant explosion on 16 May 2016, that caused abundant ashfall on farms 2.5 km WNW, was the start of a new episode that lasted for more than two months. Frequent ash emissions continued the next day, although seismic tremor amplitude decreased substantially from the initial explosion. Numerous gas-and-ash emissions were reported during 17-19 May. Ashfall was reported in areas of Valle Central (30-40 km W), including Coronado, Guadalupe, and Heredia (38 km W). On 20 May a Strombolian phase began, producing an ash-and-gas plume that rose 3 km and drifted W. The eruptive column collapsed, generating pyroclastic flows that reached the nearby ranches of La Silva and La Picada, and the Cráter Central. According to a news article, some airlines canceled or delayed flights into the Juan Santamaría International Airport (48 km W).

Gas-and-ash emissions continued during 21-22 May; plumes rose as high as 600 m above the summit. Villagers reported ashfall in areas of San José (40 km WSW), Cartago (25 km SW), Alajuela (49 km W), Heredia (38 km W), Puriscal (65 km WSW), and Jaco (100 km SW). Ash plumes rose as high as 1 km and drifted W and SW on 23 May, causing ashfall in areas downwind including Tapezco (Zarcero-Alfaro Ruíz, 70 km WNW), Guácima de Alajuela (55 km WSW), Barva (39 km W), Finca Lara (17 km W), Finca Laguna (23 km WNW), Grecia, and Naranjo. A strong explosion on 24 May generated new ash plumes that rose 3.5 km and drifted SW. This event ejected large rocks around the crater and led to ashfall in multiple areas including Santa Rosa de Oreamuno, Santa Cecilia de Heredia, and San Francisco de Heredia, tens of kilometers to the W. Large amounts of ash (deposits 2-7 mm thick) fell in Carthage, Heredia (38 km W), San José (40 km W), and Alajuela (49 km W) from more explosions on 25 May that also ejected incandescent material.

A small explosion on 1 June 2016 began a new sequence of ash emissions, with plumes rising 1-2 km, that lasted until 4 June. Ashfall was reported in a number of communities including San Rafael de Moravia (31 km WSW), Sabana (38 km WSW), Buenos Aires (17 km N), and Pococí (45 km N) during 2-3 June. Ash emissions and explosions on 10 June caused ashfall and/or a sulfur odor in multiple areas of Valle Central including San Luis, Santo Domingo, Moravia, San Francisco, and Coronado. OVSICORI-UNA reported increased seismic activity on 16 June; the webcam showed areas of incandescence. Morning satellite imagery showed a diffuse ash plume extending 45 km WNW of the summit that dissipated by mid-afternoon. Tremor increased on 23 June, followed by a lengthy sequence of tremor episodes and ash emissions that lasted through 26 June; ashfall was reported in several neighborhoods in San José and Heredia. Increased tremor on 28 June was likely accompanied by ash emissions, but darkness and clouds obscured views from the webcam.

Strong tremor on 7 July 2016 was followed by an ash plume that rose 1 km above the crater and likely drifted WNW and WSW. Ashfall was recorded in many neighborhoods downwind, in San José, Heredia, and Turrubares. Emissions of large amounts of ash were visible in the webcam the next day, and ashfall was reported in many of the same areas as the day before. The Washington VAAC issued daily reports from 7 to 15 July of diffuse ash emissions observed in the webcam, generally rising less than 500 m above the summit. A new series of explosions during 22-25 July were recorded seismically, but visual observations were difficult due to fog. Hot rock fragments, gas, and ash were noted as high as 500 m above the crater on 24 July. Ash plumes rose to 3 km above the crater and drifted NW, W, and SW the next day. OVSICORI-UNA reported possible volcanic ash again on 29 July and 1 August, but weather clouds prevented views in satellite imagery.

Another new series of explosions and ash emissions began on 13 September 2016. They were reported daily from 15 September to the end of the month. Most plumes rose less than 1 km above the crater, but explosions on 19 September generated ash plumes that rose as high as 4 km and resulted in ashfall in many communities in the Valle Central, including those in San José (35 km WSW), Heredia (38 km W), Alajuela, and Cartago (25 km SW). According to news articles, flights in and out of the Juan Santamaría International Airport were canceled; the airport remained closed at least through the morning of 20 September. The Pavas San José Tobías Bolaños Airport in San José was also temporarily closed. Plumes that rose as high as 2 km were reported on 22, 26, and 27 September.

During a 22-24 September field visit OVSICORI-UNA scientists observed a significant lahar in the Rio Toro Amarillo which flows NW from Turrialba, that mobilized logs and large rocks in a 1.5-m-deep flow (figure 47). They had observed 3 cm of fresh ash in the drainage prior to the start of the rainfall on 22 September.

Figure 47. The abrupt change in flow conditions was observed by OVSICORI-UNA scientists on 22 September 2016 when heavy rains generated a lahar in the Rio Toro Amarillo at Turrialba. The inset photo shows the same area about an hour before the flooding. Photo by E. Duarte, courtesy of OVSICORI-UNA (Algunos Efectos Proximales y Distales por Acumulación de la Ceniza: Volcán Turrialba, Reporte de campo: 22-24 de setiembre de 2016).

From 26 September through 24 November 2016 multiple reports were issued by the Washington VAAC virtually every day, usually reporting minor emissions of gas and ash. OVSICORI reported intermittent steam, gas, and ash emissions rising 500-1,000 m during all of October 2016. Ashfall was reported in Guadeloupe on 11 October. On 16 October OVSICORI-UNA noted that the almost constant ash emission in the previous few days affected the operation and communication of various scientific instruments installed at the top of the volcano and surrounding areas; communication with two seismic stations located near the summit was lost. Webcams showed continuing ash emissions rising as high as 1 km during 16-18 October. During 18-25 October, passive ash emissions continued, causing ashfall in Siquirres (30 ENE), Guacimo (23 km NNE), Guapiles (21 km N), Moravia (27 km W), San José (36 km WSW), Tibás (35 km WSW), Guadalupe (32 km WSW), Curridabat (32 km WSW), Tres Ríos (27 km SW), San Pedro (32 km WSW), and various areas of the Valle Central. Ashfall was reported in Nubes de Coronado (25 km W) on 28 October.

There were fewer reports of ashfall during November, although many areas of the Valle Central reported ashfall during 9-13 November. A small quantity of ash fell in Cartago and Paraiso de Cartago (25 km SE) on 20 November. The Washington VAAC again issued near-daily reports of ash and gas plumes between 6 December and the end of 2016. The weak and sporadic emissions generally rose only a few hundred meters, drifting in multiple directions, and there were few reports of ashfall in the surrounding communities.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

Murray Ford, a coastal geomorphologist from New Zealand's Auckland University, reported in a Radio New Zealand story on 1 February 2017 that satellite imagery showed a large plume of discolored water between Tongatapu and the volcanic Hunga Tonga-Hunga Ha'apai islands. The activity seen by Murray was on a Landsat 8 OLI (Operational Land Imager) satellite image acquired on 27 January 2017 (figure 2). which showed a bright area of discolored water above the summit and a broader area of discolored water immediately NW, likely from previous events. According to volcanologist Brad Scott (GNS Science) there are additional satellite images from 23, 26, 28, and 29 January 2017, indicating that the eruption had been ongoing for over a week. His colleagues in Tonga indicated a possible associated steam plume, but cloud cover made observations uncertain.

Figure 2. Landsat 8 OLI satellite image a submarine plume from an unnamed seamount in Tonga on 27 January 2017, about 33 km NW of Tongatapu island. A small bright area of discolored water is directly over the summit (bottom center), with a small plume immediately N, and a broad area of discolored water to the NW, likely from previous eruptive events. The larger plume to the NW measures 30 km long and 20 km wide. Courtesy of NASA Earth Observatory.

A report prepared by Taylor (2000) noted that there had been four previous reports of activity from this location: submarine activity in August 1911, a steam plume in July 1923, discolored water in 1970, and an ephemeral island near the end of an eruptive episode during 27 December 1998-14 January 1999 (also see BGVN 24:03). In a blog post about the latest eruption, Brad Scott (GNS Science) also stated that there had been discolored water and felt earthquakes sometime in 2007.

Reference: Taylor, P., 2000, A volcanic hazards assessment following the January 1999 eruption of Submarine Volcano III, Tofua Volcanic Arc, Kingdom of Tonga, Australian Volcanological Investigations (AVI) Occasional Report No. 99/01, 5 August 2000, 7 p.

Geologic Background. An unnamed submarine volcano is located along the Tofus volcanic arc 35 km NW of the Niu Aunofo lighthouse on Tongatapu Island. The first documented eruptions took place in 1911 and 1923; an ephemeral island was formed in 1999. A submarine eruption in January 2017 produced discolored water visible in satellite imagery.

Information Contacts: NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/, https://earthobservatory.nasa.gov/images/89565/underwater-eruption-near-tongatapu); Brad Scott, New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/, http://www.geonet.org.nz/news/1usjOmF4LqaI64qScMocuW); Radio New Zealand (URL: http://www.radionz.co.nz/international/pacific-news/323569/scientist-discovers-underwater-eruption-in-tonga).