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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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/).

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/).

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/).

Persistent eruptive activity since April 2008 at Ibu, a stratovolcano on Indonesian’s Halmahera Island, has consisted of daily explosive ash emissions and plumes, along with observations of thermal anomalies (BGVN 47:04). The current eruption continued during October 2022-May 2023, described below, based on advisories issued by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), daily reports by MAGMA Indonesia (a PVMBG platform), and the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data. The Alert Level during the reporting period remained at 2 (on a scale of 1-4), except raised briefly to 3 on 27 May, and the public was warned to stay at least 2 km away from the active crater and 3.5 km away on the N side of the volcano.

According to MAGMA Indonesia, during October 2022-May 2023, daily gray-and-white ash plumes of variable densities rose 200-1,000 m above the summit and drifted in multiple directions. On 30 October and 11 November, plumes rose a maximum of 2 km and 1.5 km above the summit, respectively (figures 42 and 43). According to the Darwin VAAC, discrete ash emissions on 13 November rose to 2.1 km altitude, or 800 m above the summit, and drifted W, and multiple ash emissions on 15 November rose 1.4 km above the summit and drifted NE. Occasional larger ash explosions through May 2023 prompted PVMBG to issue Volcano Observatory Notice for Aviation (VONA) alerts (table 6); the Aviation Color Code remained at Orange throughout this period.

Figure 42. Larger explosion from Ibu’s summit crater on 30 October 2022 that generated a plume that rose 2 km above the summit. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Figure 43. Larger explosion from Ibu’s summit crater on 11 November 2022 that generated a plume that rose 1.5 km above the summit. Courtesy of MAGMA Indonesia.

Table 6. Volcano Observatory Notice for Aviation (VONA) ash plume alerts for Ibu issued by PVMBG during October 2022-May 2023. Maximum height above the summit was estimated by a ground observer. VONAs in January-May 2023 all described the ash plumes as dense.

Sentinel-2 L1C satellite images throughout the reporting period show two, sometimes three persistent thermal anomalies in the summit crater, with the most prominent hotspot from the top of a cone within the crater. Clear views were more common during March-April 2023, when a vent and lava flows on the NE flank of the intra-crater cone could be distinguished (figure 44). White-to-grayish emissions were also observed during brief periods when weather clouds allowed clear views.

Figure 44. Sentinel-2 L2A satellite images of Ibu on 10 April 2023. The central cone within the summit crater (1.3 km diameter) and lava flows (gray) can be seen in the true color image (left, bands 4, 3, 2). Thermal anomalies from the small crater of the intra-crater cone, a NE-flank vent, and the end of the lava flow are apparent in the infrared image (right, bands 12, 11, 8A). Courtesy of Copernicus Browser.

The MIROVA space-based volcano hotspot detection system recorded almost daily thermal anomalies throughout the reporting period, though cloud cover often interfered with detections. Data from imaging spectroradiometers aboard NASA’s Aqua and Terra satellites and processed using the MODVOLC algorithm (MODIS-MODVOLC) recorded hotspots on one day during October 2022 and December 2022, two days in April 2023, three days in November 2022 and May 2023, and four days in March 2023.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Dukono, a remote volcano on Indonesia’s Halmahera Island, has been erupting continuously since 1933, with frequent ash explosions and sulfur dioxide plumes (BGVN 46:11, 47:10). This activity continued during October 2022 through May 2023, based on reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG; also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data. During this period, the Alert Level remained at 2 (on a scale of 1-4) and the public was warned to remain outside of the 2-km exclusion zone. The highest reported plume of the period reached 9.4 km above the summit on 14 November 2022.

According to MAGMA Indonesia (a platform developed by PVMBG), white, gray, or dark plumes of variable densities were observed almost every day during the reporting period, except when fog obscured the volcano (figure 33). Plumes generally rose 25-450 m above the summit, but rose as high as 700-800 m on several days, somewhat lower than the maximum heights reached earlier in 2022 when plumes reached as high as 1 km. However, the Darwin VAAC reported that on 14 November 2022, a discrete ash plume rose 9.4 km above the summit (10.7 km altitude), accompanied by a strong hotspot and a sulfur dioxide signal observed in satellite imagery; a continuous ash plume that day and through the 15th rose to 2.1-2.4 km altitude and drifted NE.

Figure 33. Webcam photo of a gas-and-steam plume rising from Dukono on the morning of 28 January 2023. Courtesy of MAGMA Indonesia.

Sentinel-2 images were obscured by weather clouds almost every viewing day during the reporting period. However, the few reasonably clear images showed a hotspot and white or gray emissions and plumes. Strong SO₂ plumes from Dukono were present on many days during October 2022-May 2023, as detected using the TROPOMI instrument on the Sentinel-5P satellite (figure 34).

Figure 34. A strong SO2 signal from Dukono on 23 April 2023 was the most extensive plume detected during the reporting period. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

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/); MAGMA Indonesia (Multiplatform Application for Geohazard Mitigation and Assessment in 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sabancaya is located in Peru, NE of Ampato and SE of Hualca Hualca. Eruptions date back to 1750 and have been characterized by explosions, phreatic activity, ash plumes, and ashfall. The current eruption period began in November 2016 and has more recently consisted of daily explosions, gas-and-ash plumes, and thermal activity (BGVN 47:11). This report updates activity during November 2022 through April 2023 using information from Instituto Geophysico del Peru (IGP) that use weekly activity reports and various satellite data.

Intermittent low-to-moderate power thermal anomalies were reported by the MIROVA project during November 2022 through April 2023 (figure 119). There were few short gaps in thermal activity during mid-December 2022, late December-to-early January 2023, late January to mid-February, and late February. According to data recorded by the MODVOLC thermal algorithm, there were a total of eight thermal hotspots: three in November 2022, three in February 2023, one in March, and one in April. On clear weather days, some of this thermal anomaly was visible in infrared satellite imagery showing the active lava dome in the summit crater (figure 120). Almost daily moderate-to-strong sulfur dioxide plumes were recorded during the reporting period by the TROPOMI instrument on the Sentinel-5P satellite (figure 121). Many of these plumes exceeded 2 Dobson Units (DU) and drifted in multiple directions.

Figure 119. Intermittent low-to-moderate thermal anomalies were detected during November 2022 through April 2023 at Sabancaya, as shown in this MIROVA graph (Log Radiative Power). There were brief gaps in thermal activity during mid-December 2022, late December-to-early January 2023, late January to mid-February, and late February. Courtesy of MIROVA.

Figure 120. Infrared (bands 12, 11, 8A) satellite images showed a constant thermal anomaly in the summit crater of Sabancaya on 14 January 2023 (top left), 28 February 2023 (top right), 5 March 2023 (bottom left), and 19 April 2023 (bottom right), represented by the active lava dome. Sometimes gas-and-steam and ash emissions also accompanied this activity. Courtesy of Copernicus Browser.

Figure 121. Moderate-to-strong sulfur dioxide plumes were detected almost every day, rising from Sabancaya by the TROPOMI instrument on the Sentinel-5P satellite throughout the reporting period; the DU (Dobson Unit) density values were often greater than 2. Plumes from 23 November 2022 (top left), 26 December 2022 (top middle), 10 January 2023 (top right), 15 February 2023 (bottom left), 13 March 2023 (bottom middle), and 21 April 2023 (bottom right) that drifted SW, SW, W, SE, W, and SW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

IGP reported that moderate activity during November and December 2022 continued; during November, an average number of explosions were reported each week: 30, 33, 36, and 35, and during December, it was 32, 40, 47, 52, and 67. Gas-and-ash plumes in November rose 3-3.5 km above the summit and drifted E, NE, SE, S, N, W, and SW. During December the gas-and-ash plumes rose 2-4 km above the summit and drifted in different directions. There were 1,259 volcanic earthquakes recorded during November and 1,693 during December. Seismicity also included volcano-tectonic-type events that indicate rock fracturing events. Slight inflation was observed in the N part of the volcano near Hualca Hualca (4 km N). Thermal activity was frequently reported in the crater at the active lava dome (figure 120).

Explosive activity continued during January and February 2023. The average number of explosions were reported each week during January (51, 50, 60, and 59) and February (43, 54, 51, and 50). Gas-and-ash plumes rose 1.6-2.9 km above the summit and drifted NW, SW, and W during January and rose 1.4-2.8 above the summit and drifted W, SW, E, SE, N, S, NW, and NE during February. IGP also detected 1,881 volcanic earthquakes during January and 1,661 during February. VT-type earthquakes were also reported. Minor inflation persisted near Hualca Hualca. Satellite imagery showed continuous thermal activity in the crater at the lava dome (figure 120).

During March, the average number of explosions each week was 46, 48, 31, 35, and 22 and during April, it was 29, 41, 31, and 27. Accompanying gas-and-ash plumes rose 1.7-2.6 km above the summit crater and drifted W, SW, NW, S, and SE during March. According to a Buenos Aires Volcano Ash Advisory Center (VAAC) notice, on 22 March at 1800 through 23 March an ash plume rose to 7 km altitude and drifted NW. By 0430 an ash plume rose to 7.6 km altitude and drifted W. On 24 and 26 March continuous ash emissions rose to 7.3 km altitude and drifted SW and on 28 March ash emissions rose to 7.6 km altitude. During April, gas-and-ash plumes rose 1.6-2.5 km above the summit and drifted W, SW, S, NW, NE, and E. Frequent volcanic earthquakes were recorded, with 1,828 in March and 1,077 in April, in addition to VT-type events. Thermal activity continued to be reported in the summit crater at the lava dome (figure 120).

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: Instituto Geofisico del Peru (IGP), Centro Vulcanológico Nacional (CENVUL), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.igp.gob.pe/servicios/centro-vulcanologico-nacional/inicio); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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 MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Sheveluch (also spelled Shiveluch) in Kamchatka, has had at least 60 large eruptions during the last 10,000 years. The summit is truncated by a broad 9-km-wide caldera that is breached to the S, and many lava domes occur on the outer flanks. The lava dome complex was constructed within the large open caldera. Frequent collapses of the dome complex have produced debris avalanches; the resulting deposits cover much of the caldera floor. A major south-flank collapse during a 1964 Plinian explosion produced a scarp in which a “Young Sheveluch” dome began to form in 1980. Repeated episodes of dome formation and destruction since then have produced major and minor ash plumes, pyroclastic flows, block-and-ash flows, and “whaleback domes” of spine-like extrusions in 1993 and 2020 (BGVN 45:11). The current eruption period began in August 1999 and has more recently consisted of lava dome growth, explosions, ash plumes, and avalanches (BGVN 48:01). This report covers a significant explosive eruption during early-to-mid-April 2023 that generated a 20 km altitude ash plume, produced a strong sulfur dioxide plume, and destroyed part of the lava-dome complex; activity described during January through April 2023 use information primarily from the Kamchatka Volcanic Eruptions Response Team (KVERT) and various satellite data.

Satellite data. Activity during the majority of this reporting period was characterized by continued lava dome growth, strong fumarole activity, explosions, and hot avalanches. According to the MODVOLC Thermal Alerts System, 140 hotspots were detected through the reporting period, with 33 recorded in January 2023, 29 in February, 44 in March, and 34 in April. Frequent strong thermal activity was recorded during January 2023 through April, according to the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph and resulted from the continuously growing lava dome (figure 94). A slightly stronger pulse in thermal activity was detected in early-to-mid-April, which represented the significant eruption that destroyed part of the lava-dome complex. Thermal anomalies were also visible in infrared satellite imagery at the summit crater (figure 95).

Figure 94. Strong and frequent thermal activity was detected at Sheveluch during January through April 2023, according to this MIROVA graph (Log Radiative Power). These thermal anomalies represented the continuously growing lava dome and frequent hot avalanches that affected the flanks. During early-to-mid-April a slightly stronger pulse represented the notable explosive eruption. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite imagery showed persistent thermal anomalies at the lava dome of Sheveluch on 14 January 2023 (top left), 26 February 2023 (top right), and 15 March 2023 (bottom left). The true color image on 12 April 2023 (bottom right) showed a strong ash plume that drifted SW; this activity was a result of the strong explosive eruption during 11-12 April 2023. Courtesy of Copernicus Browser.

During January 2023 KVERT reported continued growth of the lava dome, accompanied by strong fumarolic activity, incandescence from the lava dome, explosions, ash plumes, and avalanches. Satellite data showed a daily thermal anomaly over the volcano. Video data showed ash plumes associated with collapses at the dome that generated avalanches that in turn produced ash plumes rising to 3.5 km altitude and drifting 40 km W on 4 January and rising to 7-7.5 km altitude and drifting 15 km SW on 5 January. A gas-and-steam plume containing some ash that was associated with avalanches rose to 5-6 km altitude and extended 52-92 km W on 7 January. Explosions that same day produced ash plumes that rose to 7-7.5 km altitude and drifted 10 km W. According to a Volcano Observatory Notice for Aviation (VONA) issued at 1344 on 19 January, explosions produced an ash cloud that was 15 x 25 km in size and rose to 9.6-10 km altitude, drifting 21-25 km W; as a result, the Aviation Color Code (ACC) was raised to Red (the highest level on a four-color scale). Another VONA issued at 1635 reported that no more ash plumes were observed, and the ACC was lowered to Orange (the second highest level on a four-color scale). On 22 January an ash plume from collapses and avalanches rose to 5 km altitude and drifted 25 km NE and SW; ash plumes associated with collapses extended 70 km NE on 27 and 31 January.

Lava dome growth, fumarolic activity, dome incandescence, and occasional explosions and avalanches continued during February and March. A daily thermal anomaly was visible in satellite data. Explosions on 1 February generated ash plumes that rose to 6.3-6.5 km altitude and extended 15 km NE. Video data showed an ash cloud from avalanches rising to 5.5 km altitude and drifting 5 km SE on 2 February. Satellite data showed gas-and-steam plumes containing some ash rose to 5-5.5 km altitude and drifted 68-110 km ENE and NE on 6 February, to 4.5-5 km altitude and drifted 35 km WNW on 22 February, and to 3.7-4 km altitude and drifted 47 km NE on 28 February. Scientists from the Kamchatka Volcanological Station (KVS) went on a field excursion on 25 February to document the growing lava dome, and although it was cloudy most of the day, nighttime incandescence was visible. Satellite data showed an ash plume extending up to 118 km E during 4-5 March. Video data from 1150 showed an ash cloud from avalanches rose to 3.7-5.5 km altitude and drifted 5-10 km ENE and E on 5 March. On 11 March an ash plume drifted 62 km E. On 27 March ash plumes rose to 3.5 km altitude and drifted 100 km E. Avalanches and constant incandescence at the lava dome was focused on the E and NE slopes on 28 March. A gas-and-steam plume containing some ash rose to 3.5 km altitude and moved 40 km E on 29 March. Ash plumes on 30 March rose to 3.5-3.7 km altitude and drifted 70 km NE.

Similar activity continued during April, with lava dome growth, strong fumarolic activity, incandescence in the dome, occasional explosions, and avalanches. A thermal anomaly persisted throughout the month. During 1-4 April weak ash plumes rose to 2.5-3 km altitude and extended 13-65 km SE and E.

Activity during 11 April 2023. The Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS) reported a significant increase in seismicity around 0054 on 11 April, as reported by strong explosions detected on 11 April beginning at 0110 that sent ash plumes up to 7-10 km altitude and extended 100-435 km W, WNW, NNW, WSW, and SW. According to a Tokyo VAAC report the ash plume rose to 15.8 km altitude. By 0158 the plume extended over a 75 x 100 km area. According to an IVS FEB RAS report, the eruptive column was not vertical: the initial plume at 0120 on 11 April deviated to the NNE, at 0000 on 12 April, it drifted NW, and by 1900 it drifted SW. KVS reported that significant pulses of activity occurred at around 0200, 0320, and then a stronger phase around 0600. Levin Dmitry took a video from near Békés (3 km away) at around 0600 showing a rising plume; he also reported that a pyroclastic flow traveled across the road behind him as he left the area. According to IVS FEB RAS, the pyroclastic flow traveled several kilometers SSE, stopping a few hundred meters from a bridge on the road between Klyuchi and Petropavlovsk-Kamchatsky.

Ashfall was first observed in Klyuchi (45 km SW) at 0630, and a large, black ash plume blocked light by 0700. At 0729 KVERT issued a Volcano Observatory Notice for Aviation (VONA) raising the Aviation Color Code to Red (the highest level on a four-color scale). It also stated that a large ash plume had risen to 10 km altitude and drifted 100 km W. Near-constant lightning strikes were reported in the plume and sounds like thunderclaps were heard until about 1000. According to IVS FEB RAS the cloud was 200 km long and 76 km wide by 0830, and was spreading W at altitudes of 6-12 km. In the Klyuchi Village, the layer of both ash and snow reached 8.5 cm (figure 96); ashfall was also reported in Kozyrevsk (112 km SW) at 0930, Mayskoye, Anavgay, Atlasovo, Lazo, and Esso. Residents in Klyuchi reported continued darkness and ashfall at 1100. In some areas, ashfall was 6 cm deep and some residents reported dirty water coming from their plumbing. According to IVS FEB RAS, an ash cloud at 1150 rose to 5-20 km altitude and was 400 km long and 250 km wide, extending W. A VONA issued at 1155 reported that ash had risen to 10 km and drifted 340 km NNW and 240 km WSW. According to Simon Carn (Michigan Technological University), about 0.2 Tg of sulfur dioxide in the plume was measured in a satellite image from the TROPOMI instrument on the Sentinel-5P satellite acquired at 1343 that covered an area of about 189,000 km² (figure 97). Satellite data at 1748 showed an ash plume that rose to 8 km altitude and drifted 430 km WSW and S, according to a VONA.

Figure 96. Photo of ash deposited in Klyuchi village on 11 April 2023 by the eruption of Sheveluch. About 8.5 cm of ash was measured. Courtesy of Kam 24 News Agency.

Figure 97. A strong sulfur dioxide plume from the 11 April 2023 eruption at Sheveluch was visible in satellite data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of Simon Carn, MTU.

Activity during 12-15 April 2023. On 12 April at 0730 satellite images showed ash plumes rose to 7-8 km altitude and extended 600 km SW, 1,050 km ESE, and 1,300-3,000 km E. By 1710 that day, the explosions weakened. According to news sources, the ash-and-gas plumes drifted E toward the Aleutian Islands and reached the Gulf of Alaska by 13 April, causing flight disruptions. More than 100 flights involving Alaska airspace were cancelled due to the plume. Satellite data showed ash plumes rising to 4-5.5 km altitude and drifted 400-415 km SE and ESE on 13 April. KVS volcanologists observed the pyroclastic flow deposits and noted that steam rose from downed, smoldering trees. They also noted that the deposits were thin with very few large fragments, which differed from previous flows. The ash clouds traveled across the Pacific Ocean. Flight cancellations were also reported in NW Canada (British Columbia) during 13-14 April. During 14-15 April ash plumes rose to 6 km altitude and drifted 700 km NW.

Alaskan flight schedules were mostly back to normal by 15 April, with only minor delays and far less cancellations; a few cancellations continued to be reported in Canada. Clear weather on 15 April showed that most of the previous lava-dome complex was gone and a new crater roughly 1 km in diameter was observed (figure 98); gas-and-steam emissions were rising from this crater. Evidence suggested that there had been a directed blast to the SE, and pyroclastic flows traveled more than 20 km. An ash plume rose to 4.5-5.2 km altitude and drifted 93-870 km NW on 15 April.

Figure 98. A comparison of the crater at Sheveluch showing the previous lava dome (top) taken on 29 November 2022 and a large crater in place of the dome (bottom) due to strong explosions during 10-13 April 2023, accompanied by gas-and-ash plumes. The bottom photo was taken on 15 April 2023. Photos has been color corrected. Both photos are courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Activity during 16-30 April 2023. Resuspended ash was lifted by the wind from the slopes and rose to 4 km altitude and drifted 224 km NW on 17 April. KVERT reported a plume of resuspended ash from the activity during 10-13 April on 19 April that rose to 3.5-4 km altitude and drifted 146-204 km WNW. During 21-22 April a plume stretched over the Scandinavian Peninsula. A gas-and-steam plume containing some ash rose to 3-3.5 km altitude and drifted 60 km SE on 30 April. A possible new lava dome was visible on the W slope of the volcano on 29-30 April (figure 99); satellite data showed two thermal anomalies, a bright one over the existing lava dome and a weaker one over the possible new one.

Figure 99. Photo showing new lava dome growth at Sheveluch after a previous explosion destroyed much of the complex, accompanied by a white gas-and-steam plume. Photo has been color corrected. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

References. Girina, O., Loupian, E., Horvath, A., Melnikov, D., Manevich, A., Nuzhdaev, A., Bril, A., Ozerov, A., Kramareva, L., Sorokin, A., 2023, Analysis of the development of the paroxysmal eruption of Sheveluch volcano on April 10–13, 2023, based on data from various satellite systems, ??????????? ???????? ??? ?? ???????, 20(2).

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), 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 Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Kam 24 News Agency, 683032, Kamchatka Territory, Petropavlovsk-Kamchatsky, Vysotnaya St., 2A (URL: https://kam24.ru/news/main/20230411/96657.html#.Cj5Jrky6.dpuf); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn).

Bezymianny is located on the Kamchatka Peninsula of Russia as part of the Klyuchevskoy volcano group. Historic eruptions began in 1955 and have been characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. During the 1955-56 eruption a large open crater 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. The current eruption period began in December 2016 and more recent activity has consisted of strong explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 2023, based on weekly and daily reports from the Kamchatka Volcano Eruptions Response Team (KVERT) and satellite data.

Activity during November and March 2023 was relatively low and mostly consisted of gas-and-steam emissions, occasional small collapses that generated avalanches along the lava dome slopes, and a persistent thermal anomaly over the volcano that was observed in satellite data on clear weather days. According to the Tokyo VAAC and KVERT, an explosion produced an ash plume that rose to 6 km altitude and drifted 25 km NE at 1825 on 29 March.

Gas-and-steam emissions, collapses generating avalanches, and thermal activity continued during April. According to two Volcano Observatory Notice for Aviation (VONA) issued on 2 and 6 April (local time) ash plumes rose to 3 km and 3.5-3.8 km altitude and drifted 35 km E and 140 km E, respectively. Satellite data from KVERT showed weak ash plumes extending up to 550 km E on 2 and 5-6 April.

A VONA issued at 0843 on 7 April described an ash plume that rose to 4.5-5 km altitude and drifted 250 km ESE. Later that day at 1326 satellite data showed an ash plume that rose to 5.5-6 km altitude and drifted 150 km ESE. A satellite image from 1600 showed an ash plume extending as far as 230 km ESE; KVERT noted that ash emissions were intensifying, likely due to avalanches from the growing lava dome. The Aviation Color Code (ACC) was raised to Red (the highest level on a four-color scale). At 1520 satellite data showed an ash plume rising to 5-5.5 km altitude and drifting 230 km ESE. That same day, Kamchatka Volcanological Station (KVS) volcanologists traveled to Ambon to collect ash; they reported that a notable eruption began at 1730, and within 20 minutes a large ash plume rose to 10 km altitude and drifted NW. KVERT reported that the strong explosive phase began at 1738. Video and satellite data taken at 1738 showed an ash plume that rose to 10-12 km altitude and drifted up to 2,800 km SE and E. Explosions were clearly audible 20 km away for 90 minutes, according to KVS. Significant amounts of ash fell at the Apakhonchich station, which turned the snow gray; ash continued to fall until the morning of 8 April. In a VONA issued at 0906 on 8 April, KVERT stated that the explosive eruption had ended; ash plumes had drifted 2,000 km E. The ACC was lowered to Orange (the third highest level on a four-color scale). The KVS team saw a lava flow on the active dome once the conditions were clear that same day (figure 53). On 20 April lava dome extrusion was reported; lava flows were noted on the flanks of the dome, and according to KVERT satellite data, a thermal anomaly was observed in the area. The ACC was lowered to Yellow (the second lowest on a four-color scale).

Figure 53. Photo showing an active lava flow descending the SE flank of Bezymianny from the lava dome on 8 April 2023. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Satellite data showed an increase in thermal activity beginning in early April 2023. A total of 31 thermal hotspots were detected by the MODVOLC thermal algorithm on 4, 5, 7, and 12 April 2023. The elevated thermal activity resulted from an increase in explosive activity and the start of an active lava flow. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on the analysis of MODIS data also showed a pulse in thermal activity during the same time (figure 54). Infrared satellite imagery captured a continuous thermal anomaly at the summit crater, often accompanied by white gas-and-steam emissions (figure 55). On 4 April 2023 an active lava flow was observed descending the SE flank.

Figure 54. Intermittent and low-power thermal anomalies were detected at Bezymianny during December 2022 through mid-March 2023, according to this MIROVA graph (Log Radiative Power). In early April 2023, an increase in explosive activity and eruption of a lava flow resulted in a marked increase in thermal activity. Courtesy of MIROVA.

Figure 55. Infrared satellite images of Bezymianny showed a persistent thermal anomaly over the lava dome on 18 November 2022 (top left), 28 December 2022 (top right), 15 March 2023 (bottom left), and 4 April 2023 (bottom right), often accompanied by white gas-and-steam plumes. On 4 April a lava flow was active and descending the SE flank. Images using infrared (bands 12, 11, 8a). Courtesy of Copernicus Browser.

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/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Chikurachki, located on Paramushir Island in the northern Kuriles, has had Plinian eruptions during the Holocene. Lava flows have reached the sea and formed capes on the NW coast; several young lava flows are also present on the E flank beneath a scoria deposit. Reported eruptions date back to 1690, with the most recent eruption period occurring during January through October 2022, characterized by occasional explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers a new eruptive period during January through February 2023 that consisted of ash explosions and ash plumes, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

According to reports from KVERT, an explosive eruption began around 0630 on 29 January. Explosions generated ash plumes that rose to 3-3.5 km altitude and drifted 6-75 km SE and E, based on satellite data. As a result, the Aviation Color Code (ACC) was raised to Orange (the second highest level on a four-color scale). At 1406 and 1720 ash plumes were identified in satellite images that rose to 4.3 km altitude and extended 70 km E. By 2320 the ash plume had dissipated. A thermal anomaly was visible at the volcano on 31 January, according to a satellite image, and an ash plume was observed drifting 66 km NE.

Occasional explosions and ash plumes continued during early February. At 0850 on 1 February an ash plume rose to 3.5 km altitude and drifted 35 km NE. Satellite data showed an ash plume that rose to 3.2-3.5 km altitude and drifted 50 km NE at 1222 later that day (figure 22). A thermal anomaly was detected over the volcano during 5-6 February and ash plumes drifted as far as 125 km SE, E, and NE. Explosive events were reported at 0330 on 6 February that produced ash plumes rising to 4-4.5 km altitude and drifting 72-90 km N, NE, and ENE. KVERT noted that the last gas-and steam plume that contained some ash was observed on 8 February and drifted 55 km NE before the explosive eruption ended. The ACC was lowered to Yellow and then Green (the lowest level on a four-color scale) on 18 February.

Figure 22. Satellite image showing a true color view of a strong ash plume rising above Chikurachki on 1 February 2023. The plume drifted NE and ash deposits (dark brown-to-gray) are visible on the NE flank due to explosive activity. Courtesy of Copernicus Browser.

Geologic Background. Chikurachki, the highest volcano on Paramushir Island in the northern Kuriles, is a relatively small cone constructed on a high Pleistocene edifice. Oxidized basaltic-to-andesitic scoria deposits covering the upper part of the young cone give it a distinctive red color. Frequent basaltic Plinian eruptions have occurred during the Holocene. Lava flows have reached the sea and formed capes on the NW coast; several young lava flows are also present on the E flank beneath a scoria deposit. The Tatarinov group of six volcanic centers is located immediately to the south, and the Lomonosov cinder cone group, the source of an early Holocene lava flow that reached the saddle between it and Fuss Peak to the west, lies at the southern end of the N-S-trending Chikurachki-Tatarinov complex. In contrast to the frequently active Chikurachki, the Tatarinov centers are extensively modified by erosion and have a more complex structure. Tephrochronology gives evidence of an eruption around 1690 CE from Tatarinov, although its southern cone contains a sulfur-encrusted crater with fumaroles that were active along the margin of a crater lake until 1959.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far East Division, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The latest eruption of Barren Island began about 28 May 2005 (BGVN 30:05 and 30:07). The following additional information regarding this eruption was provided by Dhanapati Haldar (Presidency College). A photograph (figure 11) taken on 21 July by the Indian Coast Guard indicated that the lava pouring from the main crater had cascaded down to arrive at two points on the W shore. The seawater boiled profusely.

Figure 11. Photo taken on 21 July 2005 showing the Barren Island eruption continuing unabated. Lava cascaded down and into the sea along the island's W shore. It entered the sea at two points following the pre-existing lava routes of the 1991 and 1994-95 eruptions. Courtesy of the Indian Coast Guard.

On 28 August, a senior geologist with the Central Ground Water Board (CGWB), A. Kar, made observations from a ship (figure 12). Kar noted that Strombolian eruptive activity had increased, and was both explosive and effusive in nature. The main crater and a newly created vent on the N flank were active. Streams of hot lava flowed down the slope of the cinder cone at the main crater. This cinder cone was built during the eruption in 1787-1832 and modified during subsequent eruptive pulses in 1991, in 1994-95, and (the current episode) in 2005. Kar's observations on 28 August 2005 noted that the descending lava flows traveled to the W shore, entering the sea near the lone preexisting landing site and ~ 250 m S of it. The latter was where the lava stream had met the sea during the 1994-95 eruption. Gas columns rose to more than 2 km, and fire fountains attained a height of around 300 m.

Figure 12. Photo taken 26 August 2005 showing Barren Island in Strombolian eruption. The main crater was active, and both explosive and effusive activity had shifted N. Hot lava (seen as incandescent strips) was flowing down the slope of the cinder cone. As before, lava entered the sea at two points on the W shore. Courtesy of A. Kar.

Kar visited the island again on 2 September and noted that eruptive activity was continuing unabated. As before, a thick gas plume hovered over the N part of the island, and hot lava still flowed down into the sea. The lava coming in contact with sea water was immediately broken into fine particles that were forcefully thrown into the air to a height of nearly 100 m. Accompanying steam rose to a height of about 300-400 m before being drawn away by the prevailing wind. The eruption column's top formed a spectacular mushroom of gas and smoke, blowing to the N. Subsequent reports received from the Indian Coast Guard indicated that the eruption was continuous until at least 25 September.

All the active vents so far observed during 2005 eruption, including the S footwall vent, lie in a zone trending almost N-S. This zone conforms with a pre-existing surficial fracture. This alignment of the active vents had been noted during the 1991 and 1994-95 eruptions, and, as previously mentioned, the lava streams of the current eruption retraced the 1991 and 1994-95 lava routes.

According to Haldar, recent lava samples show large (to 5 mm) megacrysts and phenocrysts of plagioclase (An 93-57), olivine (Fo 85-70), and diopside (Mg 47-44, Fe 16-10). The samples also included a groundmass of glass charged with microlites of plagioclase (An 50-45), augite, olivine, titanomagnetite, and rare orthopyroxene. The 2005 lavas contain relatively few olivine megacrysts, but are rich in plagioclase megacrysts, similar to the 1994-95 lavas.

The bulk chemical composition of the lava falls within the basalt field (table 2), which was comparable with the compositions of the 1994-95 lava. In comparison, the 2005 lava is slightly richer in both MgO and Na₂O and slightly lower in SiO₂.

Table 2. Analysis of one lava sample (number B1/05) erupted in June 2005 from Barren Island volcano. EMPA stands for electron microprobe analysis. Courtesy of Dhanapati Haldar.

As this issue went to press Haldar noted that Barren Island continued to vigorously spew lava, gas, and ash at least as late as 10 November 2005. The eruption was unabated since the last week of May 2005.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

Information Contacts: Dhanapati Haldar, Presidency College, Kolkata, 4/3K/2 Ho-Chi-Min Sarani, Shakuntala Park, Biren Roy Road (West), Kolkata-700 061, India; Geological Survey of India, 27 Jawaharlal Nehru road, Kolkata 700 016, India (URL: https://www.gsi.gov.in/); Indian Coast Guard, National Stadium Complex, New Delhi 110 001, India (URL: http://indiancoastguard.nic.in/indiancoastguard/).

Mount Cleveland produced significant ash plumes during March 2001 (BGVN 26:04). Volcanic unrest continued through 4 May 2001, and signals consistent with volcanic seismicity were detected by an Alaska Volcano Observatory (AVO) seismic network 230 km E. By the end of May, neither eruptive activity nor thermal anomalies were observed. Until July 2005, no alert level was assigned, and AVO monitoring produced no reports on Cleveland.

Cleveland lacks a real-time seismic network. Accordingly, even during times of perceived quiet there is an absence of definitive information that activity level is at background. AVO's policy for volcanoes without seismic networks is to not get assigned a color code of Green.

Satellite imagery of Cleveland taken during 24 June to 1 July 2005 showed increased heat flow from the volcano and a possible debris flow. AVO stated that although observations were inhibited by cloudy weather, they indicated the possibility of increased volcanic activity. AVO did not assign a Concern Color Code to Cleveland due to the lack of seismic monitoring and limited satellite observations.

Satellite images during 1-8 July showed increased heat flow, thin ash deposits, and possible debris flows extending ~ 1 km down the flanks from the summit crater. AVO assigned a Concern Color Code of Yellow on 7 July. On 18 July satellite imagery showed steam emanating from Cleveland's summit and evidence of minor ash emissions. Meteorological clouds obscured Cleveland during the third week of July. During 22-29 July satellite images showed minor steaming from the summit, possible fresh localized ash deposits, and a weak thermal anomaly.

On 4 August satellite images showed a thermal anomaly. On 27 August AVO reduced the Concern Color Code at Cleveland from Yellow to "Not Assigned" because there had been no evidence of activity since a thermal feature was observed on satellite imagery from 11 August. A thermal feature was detected on several satellite images obtained on 31 August, and one on 19 September, but there was no evidence of eruptive activity.

On 7 October, AVO raised the Concern Color Code to Orange after detecting a small drifting volcanic ash cloud. The cloud was seen in satellite data at a spot ~ 150 km ESE of Dutch Harbor at 1700 UTC. Based on data from a regional seismometer at Nikolski, AVO concluded that the ash came from a small Cleveland eruption at approximately 0145. AVO, in consultation with the National Weather Service, estimated the top of the ash cloud to be no more than 4,600 m altitude. The ash cloud dissipated and was not detected via satellite after 1800 UTC. Three days passed during which there were no new observations of eruptive activity at Cleveland from satellite data, pilots, or ground-based observers. Accordingly, on 10 October the Concern Color Code was reduced to Yellow.

Geologic Background. The beautifully symmetrical Mount Cleveland stratovolcano is situated at the western end of the uninhabited Chuginadak Island. It lies SE across Carlisle Pass strait from Carlisle volcano and NE across Chuginadak Pass strait from Herbert volcano. Joined to the rest of Chuginadak Island by a low isthmus, Cleveland is the highest of the Islands of the Four Mountains group and is one of the most active of the Aleutian Islands. The native name, Chuginadak, refers to the Aleut goddess of fire, who was thought to reside on the volcano. Numerous large lava flows descend the steep-sided flanks. It is possible that some 18th-to-19th century eruptions attributed to Carlisle should be ascribed to Cleveland (Miller et al., 1998). In 1944 it produced the only known fatality from an Aleutian eruption. Recent eruptions have been characterized by short-lived explosive ash emissions, at times accompanied by lava fountaining and lava flows down the flanks.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA; Washington Volcanic Ash Advisory Center (VAAC), Washington, DC, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

An eruption began on 26 September 2005 in the Afar triangle region of NW Ethiopia, near the Afar's W topographic margin, a spot ~ 330 km E of Lake Tan'a (the source of the Blue Nile river) and ~ 320 km NNW of the city of Djibouti. The venting took place on the flanks of Dabbahu (Boina), a volcano without previous historical eruptions. What follows is a brief synopsis of seismicity available from the USGS and some field observations from Gezahegn Yirgu, Dereje Ayalew, Asfawossen Asrat, and Atalay Ayele of Addis Ababa University (AAU). Shortly after the Bulletin editors received the AAU report, normal lines of communication were temporarily halted due to civil unrest. Consequently, this report was reviewed and augmented by Anthony Philpotts of the University of Connecticut, who had flown to Erta Ale and Dabbahu with them and other scientists on 16 October 2005.

Dabbahu, a stratovolcano, also goes by several other names, including Mount Dabbahu, Boina, Moina, and Boyna. The eruption occurred at least 5 km NE of Dabbahu's summit area, at a flat spot referred to by the names Da'Ure and Teru Boyna. The profusion of names and spellings for this region of Africa partly stems from widely dissimilar alphabets; the one used in the region has over 100 letters, complicating conversion into languages having only 26.

The Dabbahu eruption has been confusing. Initial news reports shed little light on the eruption's source, size, or impact. Several news reports stated that nearby earthquakes had caused an eruption at Erta Ale, which is 113 km N of Dabbahu, but that was not the critical eruption in this region during late September. (Seismicity, however, did appear associated with an elevated level of unrest at Erta Ale in October-see report in this issue.) The confusion propagated into the Smithsonian-USGS Weekly Report of 5-11 October 2005, which incorrectly attributed some details of the Dabbahu eruption to Erta Ale. A correction was issued and the report was withdrawn. Official sources and news reports also seem to have initially overstated the impact (e.g., statements like 50,000 nomads evacuated, almost 500 goats killed, etc.).

In a later, more measured report, The Ethiopian Herald posted a 6 October article on the web that noted the following.

"... the [Disaster Prevention and Preparedness Commission] has sent relief aid, household utensils and a tanker truck to areas affected by the natural disaster. A regional committee set up in charge of studying the magnitude of the disaster has already sent its report to the commission. According to the report, 1,215 quintals [121,500 kg] of food aid has been dispatched to 6,384 citizens displaced from Boya and Debawo ... and resettled in Debabo locality, 20 km from Teru. Some 18,234 various household utensils, 1,280 blankets as well as 119 roles of plastic sheets were being transported to the area."

According to faculty at Addis Ababa University, prior to the eruption and in addition to the earthquake swarm there was also volcanic tremor, as well as faulting, fracturing, and possible local landslides.

Earthquake swarm. During September-4 October 2005, an earthquake swarm consisting of 131 events occurred at and immediately surrounding Dabbahu (figure 1 and table 1). The swarm was sudden and comparatively intense, with magnitudes ranging from body-wave magnitude (mb) 3.9 to 5.2. Instruments registered earthquakes of both the highest number and magnitude during 24-26 September, just prior to the 26 September eruption. Seismicity in the area declined sharply on 27 September and stopped on 4 October. According to another data set, earthquakes occurred in the region during the 5 years prior to this swarm at an average rate of ~ 12 per year.

Figure 1. A map showing Dabbahu volcano in the Afar triangle, along with epicenters from the earthquake swarm of 14 September to 4 October 2005 The solid triangles indicate Holocene volcanoes, although the one for Dabbahu is swamped by the pattern of epicenters. The Alayta shield volcano (labeled "A") sits 32.7 km NNE of Dabbahu's summit and erupted several times in the early 1900's. Epicenters were compiled from the U.S. Geological Survey (USGS) National Earthquake Information Center website.

Table 1. Daily number and maximum magnitude of earthquakes located in the Dabbahu region during 14 September-4 October 2005 (up to 42 per day, with a total of 131 earthquakes). Mw stands for moment magnitude; mb stands for body-wave magnitude. Data courtesy of National Earthquake Information Center, USGS.

First-hand observations. Gezahegn Yirgu of AAU submitted a preliminary description of the eruption. He reported that people in the area noted that on 26 September at about 1300 a very strong earthquake occurred. That was followed by a dark column of "smoke" that rose high into the atmosphere and spread out to form an umbrella-shaped cloud. Emissions darkened the area for 3 days and 3 nights. On their first visit, provoked by the abnormal seismicity, his team departed the site just two hours before the 26 September eruption. He went back to Dabbahu for several more visits, some of which included geologists from overseas.

The visitors found that a minor explosive eruption had taken place from a fissure-vent system, producing a light-colored ash layer that extended over 500 m from the vent (figure 2). The eruption threw out pre-existing near-surface pyroclastic deposits (sediments) and felsic lavas, and redeposited them near the vent (figure 3). Some of the rocks that were thrown 20 m from the vent measured 2-3 m across. Fine white ash fell in the surrounding region as far as Teru village, 40 km SW of the eruption site.

Figure 2. An aerial view of the fissure vent at Da'Ure (Dabbahu) taken around 4-5 October 2005, showing the post-eruptive scene captured by a camera that was aimed down and toward the NW. The fissure vent, which extends ~500 m and trends nearly N-S, cuts across the photo diagonally (for sense of scale, see people in figure 4). The deepest part, ~100 m below the surface, lies along the vent's base at its widest point. It exposes dark material at the bottom (see figure 3). N of that wide segment lies a cauliflower-shaped pumice dome, a feature ~30 m in diameter. What appears as a short, narrow segment of the fissure vent continuing in the distance behind (to the N of) the dome is actually longer and more prominent than it appears, owing to foreshortening due to camera angle, surface topography, and perspective to the more distant location. This northernmost segment of the vent is roughly one-third as long as the segment in front of the dome. Photo taken by Asfawossen Asrat.

Figure 3. An aerial view of the fissure vent at Da'Ure (Dabbahu) taken around 4-5 October 2005, showing the post-eruptive scene captured by a camera aimed down and approximately NE. This image presents enlarged views of both the pumice dome and the fissure vent's lower portions. (For sense of scale, see figure 4). Photo taken by Asfawossen Asrat.

Roughly two-thirds of the way from the S end of the fissure vent, a 30-m-diameter pumice dome formed. From within the fractures in this dome, the team heard a sound from below resembling the sound of a helicopter engine or a boiling liquid.

The bulk of Yirgu's report on the second visit to the eruption site, on 4-5 October, follows.

"A team of three geologists and one geophysicist (Gezahegn Yirgu, Dereje Ayalew, Asfawossen Asrat, and Atalay Ayele) revisited the Da'Ure locality (at approximately 120° 43' 37" N, 40° 32' 55" E) immediately adjacent to the NE flank of the Quaternary Boina felsic complex. This locality is the southwestern extension of the area we visited a week earlier and where we observed a number of newly opened parallel fissures and a major reactivated normal fault.

"We first investigated the area where a volcanic eruption had been reported. Here we observed the presence of a wide and elongate fissure more than 500 m long and about 60 m deep [(figures 2-4)]. The elongate fissure attains a maximum width of about 100 m where a semi-circular pit has formed and from where the explosive eruption appears to have taken place. This elongate vent is oriented almost N-S [trending N10W] and has broken through felsic pyroclastic deposits and lavas. Two smaller pits were also observed farther N along the fissure [situated] to the N of the major pit. A very fine and light grey ash has been deposited on both sides of the elongate fissure with the ash cover extending more than 500 m away from the vent. Beneath the ash deposit lies a sequence of loose layers consisting of mixed volcanic ash and ejecta from pre-existing fissure wall rocks. These layers have a total thickness of about 20 m near the large pit."

Figure 4. A view taken from Da'Ure's (Dabbahu's) new pumice dome looking S down the fissure vent on 16 October 2005, with people for scale. Part of the outer flank of Dabbahu is visible on the right side of the photo; Dabbahu's central area lies farther to the right off the margin of the photo. Courtesy of Anthony Philpotts.

At the pumice dome Yirgu noted "... intense degassing is occurring with the production of SO₂ as evidenced by its smell as far as 500 m away. Degassing is also visible along the length of the vent as well as through nearby fissures. The local people have reported that on 26 September 2005 at about 1300 local time a very strong earthquake shook the area. This was followed by a dark column of 'smoke' that rose high into the atmosphere and spread out to form a cloud, which darkened the area for three days and three nights. Our field observations were consistent with . . . [a minor ejection] of volcanic ash from a small vent or vents along the opened fissure."

"In the same locality, we also studied the newly formed second-order fractures and fissures, most of which were located on the eastern side of the main eruptive fissure/vent. Here, the [roughly N- to S-trending] fractures and fissures were all parallel to each other .... They were better developed on unconsolidated pyroclastic deposits and sediments; they affected an area nearly 700 m away from the main eruptive vent/fissure; spacing is commonly between 10 and 20 m; some extend discontinuously along strike for over 500 m, as observed from the helicopter; open fissures in the pyroclastic deposits measure up to 20 cm wide with common elliptical pits or collapse structures between fissures up to 4 m wide and up to 4 m deep.

"We have also observed a major reactivation on a N- to S-trending normal fault located some 500 m to the E of the elongate eruptive vent/opening. This fault breaks through felsic lavas and unwelded pyroclastic deposits and has a reactivated displacement (down thrown to the W) reaching half a meter in places. This reactivated fault extends . . . discontinuously for at least three kilometers as observed from the helicopter. Degassing is occurring along some parts of this fault."

Yirgu also said that, according to the AAU Geophysical Observatory, seismicity continued in early October in the area affected by the eruption, faulting, and fissuring.

Other data from a 16 October visit. Anthony Philpotts accompanied a team who, along with AAU colleagues, were helicoptered to the eruption site, which had completely ceased by this time. At the eruption site and on the helicopter trip to and from it, he saw no dead nor injured livestock. The team also visited a refugee camp for displaced nomads.

In discussions with AAU colleagues who saw the fissure vent during multiple visits, and in comparing photographs, it appeared that material exposed at depth in the wall of the vent changed to a lighter color. Presumably, these color changes were linked to water, initially present but that had evaporated in the intense heat of the Afar day. Philpotts suggested that if the vent did provide a window into the water table, groundwater may have added to the explosive activity.

Philpotts said that when they arrived, on 16 October, the pumice dome (shown in close-up in figure 5) still yielded temperatures of 400°C in cracks. The pumice dome lacked any deposits on top of its upper surfaces, and thus clearly represented the last volcanic feature to form. Some post-eruptive faulting was noticed with offsets on the order of 10 m.

Figure 5. Curving fractures in the top of the new Da'Ure (Dabbahu) pumice dome; view looking N. Two people are visible in the photo, one immediately behind the large central fracture. It was from these fractures the boiling noise had been heard the previous week. No sound was heard during the visit on 16 October. Courtesy of Anthony Philpotts.

Philpotts made several thin sections of pumice dome samples, and found it to be almost totally aphyric. It contains a very few rounded (resorbed) sanidine phenocrysts (figure 6) and needle-shaped microlites with high refractive index (pyroxene?). He noted that "The microlites undoubtedly formed during emplacement of the dome, but the resorption of the sanidine phenocrysts must have occurred at depth prior to eruption and probably indicates heating of the source magma chamber with an influx of hotter (basaltic?) magma."

Figure 6. A rounded, twinned phenocryst of sanidine feldspar in pumice from the Da'Ure (Dabbahu) dome in the center of the vent. Dark circles are air bubbles trapped during preparation of the thin section. The photo was taken with partly crossed polarizing filters; the width of the entire field is 1.62 mm. Courtesy of Anthony Philpotts.

Geologic Background. Dabbahu (also known as Boina, Boyna, or Moina) is a Pleistocene-to-Holocene volcanic massif forming an axial range of the Afar depression SSW of the Alayta massif. Late-stage pantelleritic obsidian flows, lava domes, and pumice cones form the summit and upper flanks. The volcano rises above the Teru Plain and was built over a volumetrically dominant base of basaltic-to-trachyandesitic lava flows of a shield volcano. Late-stage basaltic fissure eruptions also occurred at the NW base of the volcano. Abundant fumaroles are located along the crest of the volcano and extend NE towards Alayta. The first historical eruption took place from a fissure vent on the NE flank in September 2005, producing ashfall deposits and a small pumice dome. More than 6000 people were evacuated from neighboring villages.

Information Contacts: Gezahegn Yirgu, Dereje Ayalew, Asfawossen Asrat, and Atalay Ayele, Department of Earth Sciences, Addis Ababa University, PO Box: 1176, Addis Ababa, Ethiopia; Anthony Philpotts, University of Connecticut, U-45, Beach Hall, Storres, CT 06269, USA; National Earthquake Information Center (NEIC), US Geological Survey, Geologic Hazards Team Office, Colorado School of Mines, 1711 Illinois St., Golden, CO 80401, USA (URL: https://earthquake.usgs.gov/); The Ethiopian Herald, Addis Ababa, Ethiopia.

In conjunction with their investigation of eruptive activity related to a swarm of earthquakes at Dabbahu/Boina, a team of geologists from Addis Ababa University (AAU) also undertook field observations at Erta Ale, with the aid of a military helicopter (see map in this issue of BGVN, the report on Dabbahu/Boina). What follows is their report combined with other information they gathered.

Between 21 and 24 September 2005, the local people saw, from a distance, red and glowing light shooting and rising into the air above Erta Ale. This was an indication that a Strombolian eruption probably occurred, emitting a significant volume of fresh magma within and possibly out of the pit.

The AAU team surveyed Erta Ale's craters at about 0930 on 26 September from the helicopter, as landing was not possible. Within the small southern pit crater of the main crater, they observed a new cone-shaped construct and the presence of an actively convecting lava lake in the center of the new cone. The lava lake occupied the entire lower/inner pit with hot red lava visibly overturning at the edges of the pit. Molten lava was breaking through the lake's solidified black crust. In the northern pit crater, there was a conspicuous solidified lava bulge with dark emissions along the crater walls. No incandescent lava was visible in this pit.

In addition to their direct observations, the AAU team studied videos taken by Walta Information Center of the southern pit on November 2004 and 26 September 2005. The comparison revealed significant changes, particularly in the morphology and activity of the southern pit crater. In the later videos the main crater/pit had widened significantly, with portions of the earlier crater walls having collapsed into the lava lake. There was a new cone-shaped construct within the crater in place of the previous platform that existed between the rim of the outer crater/pit and the lower pit. The new cone was estimated to be some 20 to 30 m from the top of the crater rim. The new cone apparently contained layers of basaltic scoria covered by fresh lava flows. The combined thickness of tephra and lava was estimated to be 20 to 30 m. The lava lake occupied the entire width of the inner crater/pit and was then bounded by steep sides. The lake's surface stood 20 to 30 m below the cone's top.

Anthony Philpotts accompanied Gezahegn Yirgu and colleagues from Addis Ababa University faculty on a helicopter visit to Erta Ale on 15 October. They found the lava lake incredibly active, much more so than when filmed by earlier visitors in March 2005.

A brief review of satellite thermal anomaly data from MODIS/MODVOLC revealed an absence of thermal activity between 12 October 2004 and 31 March 2005, with a renewal beginning on 31 March 2005, increasing substantially in mid-2005 and continuing vigorously through at least 2 November 2005.

Geologic Background. The Erta Ale basaltic shield volcano in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The volcano includes a 0.7 x 1.6 km summit crater hosting steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera usually also holds at least one long-term lava lake that has been active since at least 1967, and possibly since 1906. Recent fissure eruptions have occurred on the N flank.

Information Contacts: Gezahegn Yirgu, Department of Earth Sciences, Addis Ababa University, P.O. Box: 1176, Addis Ababa, Ethiopia; Walta Information Centre, Woreda Kirkos, Kebele 05, House No. 095, PO Box 12918, Addis Ababa, Ethiopia (URL: http://www.waltainfo.com/); Anthony Philpotts, University of Connecticut, U-45, Beach Hall, Storrs, CT 06269, USA; MODIS/MODVOLC Thermal Alerts Team, Hawaii Institute of Geophysics and Planetology (HIGP), University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

In a meeting abstract Gomez and others (2004) wrote, "Following 11 years of relatively low activity, Galeras . . . produced a sequence of ash eruptions in July and August 2004. . .. Initial evidence of the activity transition appeared in the gas measurements [of] early June, followed by a strong increase in the shallow seismic activity below the active cone on 27 June. As in many cases at other volcanoes, the most clear evidence for the transition came in the form of seismic swarms and tremor. The current activity has culminated in two brief episodes of ash emission, on 16 July and 21 July, followed by two longer episodes, [during] 27 July-8 August and 11-19 August. This last episode began with a large explosion and released more ash than any individual episode from 1989 to 1993. Sudden deformation, as well as changes in the electric and magnetic [EM] fields at the crater EM station, and [in] gas parameters such as CO₂ concentration and fumarole temperature accompanied the [16 and 21 July] ash emissions. Unfortunately, the EM and gas instruments were lost to ashfall shortly afterward."

The same abstract also noted that "Starting in March 1996, a multiparameter real-time monitoring system was installed at Galeras, as a part of a cooperative program between INGEOMINAS [Instituto Colombiano de Geología y Minería] (Colombia) and the BGR [Bundesanstalt f?r Geowissenschaften und Rohstoffe] (Germany). Broadband seismometers were installed first, with electromagnetic (EM) sensors, sensors for the chemistry and physics of the fumarole gases, and a weather station following later. The data from these instruments augment the short-period seismic network and tiltmeters of Observatorio Vulcanológico de Pasto (OVP). Additional spot measurements [relied upon] visual inspection from the ground or helicopter, a thermal camera[,] and regular geological forays onto Galeras' slopes."

Our previous report covered events through late July 2004 (BGVN 29:07); this report discusses events through mid-October 2005. Besides the eruptions of July through August 2004, another month with vigorous activity was November 2004. A sudden explosion on 21 November drove a plume to 9-10 km altitude. The latter portion of this report interval (June to mid-October 2005) typically involved ongoing though diminished intensity of volcanism and seismicity.

In response to the crisis, authorities produced engaging graphics, reminiscent of landscape paintings, but also containing risk assessments (figures 108 and 109), in the originals as color-coded lines. These graphics accompanied explanatory text. The authorities also produced a colorful poster. In addition, articles on Galeras hazards appeared in local papers, many along with clear graphics. The distance from the summit to central Pasto is only ~ 9 km.

Figure 108. A hazards map prepared for Galeras. The key (upper left) shows symbols for risk zones (high, medium, and low). The settlement Genoy is also spelled Jenoy. This map is slightly modified from one by Observatorio Vulcanológico y Sismológico de Pasto, INGEOMINAS.

Figure 109. A sample of Galeras hazards graphics available during the 2004-2005 crisis. Galeras and surroundings appear in a series of perspectives that also illustrate likely paths of destructive processes. The artwork emphasizes important geography, labels many settlements, and portrays familiar buildings and skylines. Hazard zones are shown by symbols (see key at the bottom). Views are as follows: A: NW-looking view (S-SE flanks, "Pasto"), B: NE-looking view (SW flanks, "Consaca-Yacuanquer"), C: SE-looking view, (NW flanks, "La Florida-Sandona"), and D: SW-looking view, (NE flanks, "Genoy-Nariño"). Copyrighted images courtesy of Observatorio Vulcanológico y Sismológico de Pasto, INGEOMINAS.

July-December 2004. INGEOMINAS noted that the July 2004 emissions came from El Pinta crater and from Deformes fumarolic field. Field observations on 19 July disclosed ash freshly vented from El Pinta crater, forming a deposit that ranged in thickness from 3 mm at the base of the cone to ~ 20 cm near the point of emission.

During the latter half of July 2004 INGEOMINAS noted that emissions rose ~ 600 m above the volcano's summit. Ash was not then visible on satellite imagery. On 21 July 2004 a seismic signal corresponded with a visible plume rising ~ 500 m above the volcano and seen from Pasto. According to a news report, a wide area around the volcano had been declared off limits to visitors. Several higher plumes followed.

According to the Washington VAAC, several ash plumes emitted were visible on satellite imagery during 7-10 August 2004. The highest rising plume reached ~ 6 km altitude.

INGEOMINAS reported that on 11 August at 2349 an eruption sent an ash-and-gas cloud to an unknown height and generated visible incandescence. According to the Washington VAAC, satellite imagery showed an ash plume that rose to ~ 10.7 km altitude. This plume spread in all directions, but mainly to the NE, E, and SW. Later, a thin plume reached a height of ~ 7.3 km altitude and drifted SW into northern Ecuador. A distinctly separate plume also occurred, drifting NW at an altitude of ~ 6.1 km.

Figure 110 shows a graphical depiction of the two plumes issued by the Washington VAAC, which incorporated GOES-12 satellite imagery as part of an advisory sent out at 0807 on 12 August 2004. The observations were from about an hour earlier. This following message was in the 'remarks' part of the advisory."Ash heading [NE] earlier in the night can no longer be seen in satellite imagery. A faint plume of ash is heading SW into northern Ecuador but is slowly becoming diffused in satellite imagery. The ash heading SW is estimated to FL240 [~ 7.3 km altitude]. An ash plume moving NW from the summit is estimated to FL200 [~ 6.1 km altitude]. We will continue to closely monitor and advise earlier than normal if needed."

Figure 110. The Galeras ash plumes were distributed in a Volcanic Ash Advisory (VAA) issued at 1307 UTC on 12 August 2004. When this image was taken at 1215 UTC, the two visible plumes had separated widely; one lingering slightly N of the volcano, the other, larger, reached a higher altitude and drifted over Ecuador. Plume top altitude estimates were 'flight level' (FL) 200 and 240, equivalent to 20,000 and 24,000 feet, ~ 6.1 and ~ 7.3 km altitude. Information sources listed included the GUAYAQUIL Meteorological Watch Office (MWO) and the GOES-12 satellite. (This VAA was issued under the header FVXX20 KNES 121307). Courtesy of the Washington VAAC; analysis by Jamie Kibler.

The next advisory noted that ash had ceased to be visible in the imagery after 0715 (1215 UTC) on 12 August 2004 (in other words, after the image associated with the graphic in figure 110).

Fine ash from the 11 August eruption was deposited in villages near the volcano, including La Florida (~ 10 km NW of the volcano), Nariño, Sandoná, and Consacá, and farther afield in Ancuya, Linares, and Sotomayor (~ 40 km NW of the volcano). News articles reported that during these episodes ~ 230 families were evacuated, mainly from the volcano's N flank. The village of La Florida on the volcano's NW flank was most strongly impacted by the eruption. Ash contaminated potable water in some villages, impacted farm animal's health, and left hundreds of dead fish floating in rivers. On 16 August, ash emissions continued, depositing ash in several villages.

INGEOMINAS reported that gas-and-ash emissions continued at Galeras as of 18 August. Ash fell in villages near the volcano, including La Florida, Sandoná, El Ingenio (within 15 km of the volcano), and farther afield in Samaniego and Sotomayor (between 20 and 40 km from the volcano). During 19 August to 1 September, there was a decrease in the level of seismicity and the number of ash emissions. But, gas-and-steam emissions continued.

During September 2004, tremor associated with ash-and-gas emissions was recorded at Galeras. On the 23rd, ash deposits were seen on the upper N flank. By the 27th, the amount of tremor had decreased significantly, a change that coincided with a decrease in ash emissions. During most of October 2004, emissions of gas and fine ash continued at Galeras. Plumes rose to a maximum height of ~ 1.5 km above the volcano. Instruments recorded small-amplitude tremor associated with gas-and-ash emissions.

INGEOMINAS reported that at 1544 on 21 November 2004 Galeras erupted explosively. A resulting shock wave was felt as far away as Cimarrones (18 km N of the volcano), Chachagui (17 km N of the volcano), and Laguna de La Cocha (20 km SW of the volcano). Effects of the shock wave varied from a loud roar, to the vibration of large windows, to the vibrating sensation of an earthquake. Hot ballistic blocks fell nearly 3 km from the volcano on its eastern flank, producing short-lived forest fires. The eruption produced an ash-and-gas column that rose to an estimated 9-10 km altitude and drifted to the S and W. The Washington VAAC reported that satellite imagery of 21 November at 1815 (i.e., 22 November at 0015 UTC) revealed two separate plumes, a situation somewhat analogous to 11-12 August (figure 110). One set of plumes from the 21 November eruption were estimated to reach 9 km altitude, and they blew to the W. Other plumes interpreted as low-level ash were estimated to be near 4-5 km altitude; these remained in the vicinity of the volcano and showed little motion.

January-September 2005. During January 2005, low-level relatively shallow seismicity continued, and a small amount of deformation towards the W portion of the volcanic cone occurred. On 30 January an emission of gas and ash rose ~ 800 m above the volcano. During the first week of February 2005, small gas-and-ash emissions continued. Ash was deposited in the sectors of Consacá (~ 15 km W of the volcano) and La Florida, and in the city of Pasto (~ 10 km E). Low-level seismicity and a small amount of deformation were recorded.

According to a news article, on 24 May 2005 the Colombian government ordered the evacuation of ~ 9,000 people living near Galeras due to an increase in volcanic activity. INGEOMINAS reported that during 16-23 May, small shallow earthquakes occurred beneath the volcano. Earthquakes believed associated with fracturing within the volcano increased during the night of 21 May to the morning of 22 May. Deformation continued to be recorded at the volcano's summit. There were no ash emissions. Galeras remained at alert level II ('probable eruption in terms of days or weeks') as it has since 19 April 2005.

During early June 2005, seismicity and deformation decreased in comparison to the previous week. On 6 June the alert level was decreased from II to III ('changes in the behavior of volcanic activity have been noted'). During July and August 2005, seismicity chiefly remained low. One exception, a M 2.5 volcano-tectonic earthquake on 4 July 2005, was felt in sections of some towns near the volcano. Generally, observers also noted small amounts of deformation and low rates of gas discharge, with continued emissions from the main and secondary craters. Thirty volcano-tectonic earthquakes were recorded at Galeras during 19-21 August 2005. The earthquakes occurred 3-4 km NW of the volcano's active cone, near the towns of Santa Bárbara, Nariño, and La Florida. About five earthquakes felt by nearby populations occurred at depths of 6-8 km, with the largest (M 4.7) occurring at a depth of 6 km on 21 August.

During September 2005, minor seismicity and minor deformation continued. Seismic signals included 365 minor events near the volcano at less than 6 km depth. The larger September record consisted of 179 volcano-tectonic events, 291 long-period events, 258 hybrid events, and 96 tremor episodes. Some of these earthquakes correlated with gas and fine ash discharges. Flyovers at the end of September confirmed that gas emissions were significantly reduced compared to August 2005.

October INGEOMINAS reports noted occasional steam plumes visible from Pasto, often correlated with and presumably related to increases in rainfall and infiltration of water into hot portions of the volcano. A 5 October 2005 overflight revealed a small increase in gas emissions compared to similar flights during September 2005. Seismicity fluctuated and some instrumentally measured deformation continued.

Reference. Gomez, D., Hellweg, M., Buttkus, B., Boker, F., Calvache, M. L., Cortes, Faber, E., Gil Cruz, F., Greinwald, S., Laverde, C , Narváez, L., Ortega, A., Rademacher, H., Sandmann, Seidl, D., Silva, B., and Torres, R., 2004, A Volcano Reawakens: Multiparameter Observations of Activity Transition at Galeras Volcano (Colombia), Transactions, American Geophysical Union, Fall meeting (session entitled "Sources of Oscillatory Phenomena in Volcanic Systems I; Posters"), December 2004, San Francisco, CA

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: Diego Gomez Martinez, Observatorio Vulcanológico y Sismológico de Pasto (OVSP), INGEOMINAS, Carrera 31, 1807 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html; Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); El Spectador; El Pais (URL: http://elpais-cali.terra.com.co/paisonline/); Reuters.

The first recorded eruption of Mt. Belinda volcano (Montagu Island), which began around 20 October 2001, continued (as reported in BGVN 28:02, 29:01, 29:09, 29:10) until at least the latter part of 2005. Information for the following report was prepared and submitted by Matt Patrick of the Hawai'i Institute of Geophysics and Planetology (HIGP) and John Smelie of the British Antarctic Survey, with the assistance of the HIGP Thermal Alerts Team.

This eruption was detected by the MODVOLC automated satellite detection system, which scans for anomalous thermal activity in MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data over the entire Earth approximately twice per day (Wright and others, 2004). Investigators acquired a recent, 23 Sept 2005, cloud-free ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) image (15-30 m pixel size), which provided valuable information on a new phase of activity. It revealed a larger effusive eruption than previously identified in satellite imagery of Montagu Island (figure 9).

Figure 9. ASTER image showing Montagu Island's Mount Belinda on 23 September 2005. Courtesy of HIGP Thermal Alerts Team.

Based on frequent MODVOLC alerts (figure 10) and occasional high-resolution satellite data (ASTER, IKONOS, and Quickbird), Mount Belinda has maintained persistent activity since the start of the eruption. Activity has consisted of continuous steaming and low-intensity explosive events at the summit (presumably Strombolian), producing low-level ash plumes and ubiquitous tephra deposits on the island's ice cover, and at least three distinct effusive events. Several satellite images were posted by HIGP on the National Aeronautics and Space Administration (NASA) Earth Observer website, 13 October 2004 and 19 October 2005.

Figure 10. (A) Chronological graph of radiant heat output from Mount Belinda measured from satellite sensors. The date range depicted along the x-axis of this graph is from late 2001 to September 2005. (B) A plot showing the distance of satellite-measured thermal anomaly pixels from the Mount Belinda vent during the period 2001 to September 2005. Courtesy of HIGP Thermal Alerts Team.

Scientists noted an intense shortwave-IR anomaly at the summit of Mt. Belinda in all cloud-free ASTER images acquired throughout the eruption. This suggested the presence of a lava lake in the summit crater (see Patrick and others, 2005, for more detailed information on the eruption).

Far from slowing down, the activity throughout 2005 marked the highest levels yet registered by MODVOLC (figure 10a). For the first time in 2005, radiant heat output exceeded 150 MW (see Wright and Flynn, 2004, and Wright and others, 2005, for calculation details).

By plotting the position of each anomalous MODVOLC pixel relative to the central vent (figure 10b) one can see that most pixels are within 1 km of the vent. This reflects the approximate scale of MODIS pixels and thus the inherent level of location ambiguity (note, however, these results fail to show the 2-km-long lava flow emplaced in mid-2003 — see BGVN 29:01).

For the first time during this eruption, anomalous pixels began appearing more than 2 km away from the central vent on the satellite image for 0100 UTC on 15 September 2005, some up to 3.3 km away (figure 10b). This suggested the presence of a ~ 3 km long lava flow. Corroborating this was the ASTER image from 23 September 2005 (figure 9), which indicated heightened activity and a 3.5-km long lava flow extending from the summit cone of Mt. Belinda into the sea. A steam plume originated in the vicinity of the ocean entry. Note that the steam plume appears to drift W from its origin (where the plume is whitest), while the ash plume from the summit of Mt. Belinda (1,370 m elev.) drifts E, indicating varying wind directions at different elevations.

The lava flow initially traveled NE from the vent, but farther on it ran into a rocky arete, which diverted its path to due N. A 90-m-wide lava channel is visible at a distance of 1 km from the summit. The flow appears to be covered (perhaps entering a tube) within its first kilometer, where no anomalous shortwave IR pixels exist. It is unlikely that the flow is subglacial in this first kilometer, as its path is coincident with emplacement of the previously mentioned lava flow of mid-2003, which was 2 km long and had already melted ice along this route.

At the request of the British Antarctic Survey, the Royal Air Force sent an airplane from the Falkland Islands on 11 October 2005. The plane encountered cloudy conditions but those on board recognized steam rising from the sea. This flight took place prior to study of the 23 September ASTER image and thus it marked the first observation that lava reached the sea.

References. Patrick, M., Smellie, J.L., Harris, A.J.L., Wright, R., Dean, K., Izbekov, P., Garbeil, H., and Pilger, E., 2005, First recorded eruption of Mount Belinda volcano (Montagu Island), South Sandwich Islands: Bulletin of Volcanology, v. 67, p. 415-422.

Wright, R., and Flynn, L.P., 2004, A space-based estimate of the volcanic heat flux into the atmosphere during 2001 and 2002: Geology, v. 32, p. 189-192.

Wright, R., Flynn, L.P., Garbeil, H., Harris, A.J.L., and Pilger, E., 2004, MODVOLC: near-real-time thermal monitoring of global volcanism: Journal of Volcanology and Geothermal Research, v. 135, p. 29-49.

Wright, R., Carn, S., and Flynn, L.P., 2005, A satellite chronology of the May-June 2003 eruption of Anatahan volcano: Journal of Volcanology and Geothermal Research, v. 146, p. 102-116.

Geologic Background. The largest of the South Sandwich Islands, Montagu consists of a massive shield volcano cut by a 6-km-wide ice-filled summit caldera. The summit of the 11 x 15 km island rises about 3,000 m from the sea floor between Bristol and Saunders Islands. Around 90% of the island is ice-covered; glaciers extending to the sea typically form vertical ice cliffs. The name Mount Belinda has been applied both to the high point at the southern end of the summit caldera and to the young central cone. Mount Oceanite, an isolated peak at the SE tip of the island, was the source of lava flows exposed at Mathias Point and Allen Point. There was no record of Holocene activity until MODIS satellite data, beginning in late 2001, revealed thermal anomalies consistent with lava lake activity. Apparent plumes and single anomalous pixels were observed intermittently on AVHRR images from March 1995 to February 1998, possibly indicating earlier volcanic activity.

Information Contacts: Matt Patrick, University of Hawaii, Hawaii Institute of Geophysics and Planetology (HIGP) Thermal Alerts Team, 2525 Correa Road, Honolulu, HI 96822 (URL: http://modis.higp.hawaii.edu/); John Smelie, British Antarctic Survey, Natural Environment Research Council, High Cross, Madingly Road, Cambridge CB3 0ET, United Kingdom (URL: https://www.bas.ac.uk/); NASA Earth Observer (URL: http://earthobservatory.nasa.gov/NaturalHazards/).

At about 1730 on 22 October 2005 Sierra Negra began erupting. This shield volcano with a large oval-shaped caldera is located at the S end of Isabela Island. Circumferential fractures define the northern edge of the caldera. Volcán Chico, noted for its 1963 and 1979 eruptions, is comprised of a series of scoria cones and other vents aligned along several prominent fractures on the outer slope of the N caldera rim. The present activity is not related to the Volcán Chico fracture system, but is venting from fractures along the N inner caldera wall. The most prominent fracture can be traced westward ~3 km where it lies along the rim. This initial report was provided by a scientific team from the Instituto Geofísico.

The eruption was preceded by a seismic event at 1438 on 22 October, felt in the coastal village of Villamil (20 km SE of the caldera border) and by Park Wardens on Cerro Azul. Others reported single earthquakes on 19 October and two weeks earlier. At 1730 the eruption began with an explosion heard by many people in the Villamil area. Hikers in the area of the subsequent lava emission in the mid-afternoon of both 21 and 22 October witnessed no unusual activity. By 1745 the eruption column had reached an estimated altitude of 5 km, and the setting sun illuminated the light gray eruption column. At 1815 the team observed the column after sunset from Point (Punto) Ayora, Santa Cruz Island (80 km E) and estimated its height at 10 km. The still-rising column was 4-6 km wide, not spreading laterally, and a small lenticular cloud was beginning to form a cap over the column. As night fell, the western sky above the caldera was a burgundy red, suggesting that lava had covered an extensive area of the caldera floor. Satellite imagery of the eruption at 1745 showed an eruption cloud at an estimated altitude of at least 15 km moving SW. A very large hotspot in the multispectral imagery was also observed and continued on 27 October.

Observations at 1945 from the Santa Cruz highlands (75 km away) employing a camcorder with night vision capabilities confirmed extensive lava fountaining estimated to be 200-300 m high along a segment of the caldera rim, as well as the incandescence from a lava flow several kilometers long descending the NW outer flank. Although the complete eruption column was not visible, it may have reached an altitude close to 20 km and had spread out. Tourist boats between Isabela and Fernandina Island reported seeing two lava flows descending the N flank.

During an overflight between 0715 and 0900 on 23 October the team did not witness active lava flows or evidence of lava having entered the sea. A thin khaki-colored ash cloud layer was observed, between about 1,200 and 1,500 m altitude, that had spread out laterally and extended E as far as St. Cruz Island and N to Santiago Island. Later in the day the plume was directed NNW in agreement with satellite information. From the plane the team confirmed that the main eruption was venting from four craters along a 500-m-long fracture at the base of the NNE inner caldera wall. The highest lava fountaining (up to 200 m high) was being generated at the two middle vents, while the end vents were feeding many lava flows S onto the caldera floor. The fracture apparently extended W along the inner wall, but then climbed to the caldera rim where its trace was not obvious. However, small vents with fountaining and incandescent lava were observed on the rim along this general fracture system, implying that the active fracture extended ~ 2 km W of the main vents.

During the mid-day hours of 23 October the team ascended the S flank, followed the E rim of the caldera, and reached a point ~ 800 m from the active vents, from which the following description was made. From the four principal vents the lava flowed S with exceptional force, volume, and speed downslope in three main channels (figures 3 and 4). Based on the apparent speed of the lava, and the more than 10-m height of the waves in the stream of passing lava, the team estimated that the main lava flow was traveling nearly 20 m/second as it left its vent. The W channels, some 30-50 m wide, maintained their red incandescent color and high speeds, albeit less than that near the vent.

Figure 3. View looking W from the NE rim of Sierra Negra's caldera (right) on 23 October 2005. The caldera floor is to the left. The four active vents are superimposed in this photo, aligned along the E-W fracture that lies at the base of the inner caldera wall. Numerous lava flows descended southwards to the left where they joined to form one single flow of a'a lava ~ 1 km wide and 7 km long that had already reached the southern inner wall of the caldera on 23 October. Courtesy of M. Hall.

Figure 4. A 150-m-high lava fountain rises on 23 October 2005 from one of four active vents that define the active fracture system at the base of the northern inner wall of Sierra Negra's caldera. From these four principal vents lava flows moved southwards at velocities estimated at close to 20 m/second on 23 October. Courtesy of M. Hall.

By 1500 the E channel was slowing and cooling to a gray surface color; this thin solid veneer was subsequently fragmented when the flow went over the edge of the bench and cascaded to the caldera floor. On the caldera floor the incandescent lavas of all three channels disappeared under the black solidified a'a lava that already covered about 12% of the caldera. In the 22 hours since the eruption had begun, the lavas had formed one large flow 1-1.5 km wide that traveled SE along the base of the E interior caldera wall, then W along the S wall reaching a point almost halfway across the caldera. As such it had traveled a total distance of 7 km and had started small brush fires on the floor and interior walls of the caldera. With an estimated thickness of no more than 3 m, the volume of the lava ejected by 1530 on 23 October was calculated at about 25 million cubic meters.

Along the trail leading to the vent area an increasing amount of scoria fragments was observed on the rim's edge. Fragments ~ 1 cm in size were first observed ~ 4 km SE of the active vents, and they increased in size (up to 15 cm) and abundance towards the vents. Very little fine ash was in the air or on the ground along the E caldera rim. The scoria was black, exceedingly vesiculated, with vesicles from millimeter to many centimeters in diameter; it seemed comparable in density to popcorn. No crystals were observed in the glassy scoria material. At their closest approach to the vent, scoria fragments formed a deposit 3-5 cm thick.

An explosion heard at 1900 on 25 October was accompanied by a dark eruptive column and minor ashfall along the E rim of the caldera and probably elsewhere. By early 26 October the Park Wardens were reporting that one of the four principal vents had shut down. Observations made late on 26 October indicated that the a'a flow on the caldera floor had slowed and was still several kilometers from the sulfur mine area. Civil Defense officials also reported that apparently less lava was leaving the vents and that lava extrusion might have shifted to the outer N flank, possibly to the Volcán Chico fracture system.

The only inhabited areas include the small town of Villamil, located 20 km SE of the caldera's border on the S coast, plus several other small populated areas about halfway between the caldera and Villamil. There was no immediate threat to those residents, given the fact that in order to spill out of the caldera and descend the S flanks the entire 100-m depth of the caldera would have to fill with lava. The southern caldera border has not been active in the recent geologic past.

Geologic Background. The broad shield volcano of Sierra Negra at the southern end of Isabela Island contains a shallow 7 x 10.5 km caldera that is the largest in the Galápagos Islands. Flank vents abound, including cinder cones and spatter cones concentrated along an ENE-trending rift system and tuff cones along the coast and forming offshore islands. Although it is the largest of the five major Isabela volcanoes, it has the flattest slopes, averaging less than 5° and diminishing to 2° near the coast. A sinuous 14-km-long, N-S-trending ridge occupies the west part of the caldera floor, which lies only about 100 m below its rim. The Volcán de Azufre fumarolic area lies within a graben between this ridge and the west caldera wall. Lava flows from a major eruption in 1979 extend all the way to the north coast from circumferential fissure vents on the upper northern flank.

Information Contacts: Minard Hall and Patricio Ramón, Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Tapia and Oscar Caravajal, Parque Nacional Galápagos, Pto. Ayora, Santa Cruz Island, Ecuador.

This report discusses a 1 October 2005 eruption at Santa Ana (also called Ilamatepec) that sent a plume to 14 km altitude and led to initial estimates cited in the press of two deaths (perhaps from landslides), several injuries, and the evacuation of over 2,000 people. Observations of glowing fumaroles and release of magmatic gas during 2000-2001 were previously reported at Santa Ana (BGVN 26:04). Servicio Nacional de Estudios Territoriales (SNET) scientists noticed that between the summer of 2000 and April 2001 there was increased venting of a well-developed hydrothermal system through the crater lake, hot springs, and fumaroles, but these changes were not accompanied by detected seismicity, which was then taken to suggest that the increase in hydrothermal activity was not driven by the arrival of new magma beneath the crater. An ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) image from 3 February 2001 shows the volcano's setting well before the eruption (figure 1).

Figure 1. An ASTER image of Santa Ana from 2001 featured in one of several Earth Observatory reports. N is to the top left of the image and Santa Ana is the large, blunt-topped edifice closest to the left side of the image. In the color version of this image can be seen a tiny blue spot in the center of the inner-most crater?a crater lake (often called the lagoon). Behind Santa Ana is a large (7-km-diameter) lake inside the Coatepeque caldera. In the center is Izalco volcano, with dark-colored historical lava flows. Courtesy of NASA's Earth Observatory.

SNET reported that a sudden eruption at Santa Ana took place around 0820 on 1 October 2005. They estimated that it produced an ash-and-gas plume to a height of ~ 10 km above the volcano. According to the Washington VAAC, ash was visible on satellite imagery at an altitude of ~ 14 km. The US Air Force Weather agency provided images of the plume (figure 2).

Figure 2. Two images of a Santa Ana eruptive plume on 1 October 2005. (top) The plume at 1516 UTC; (bottom) the plume at 1650 UTC. Note that the label 'FL 460,' stands for 'flight level 460,' which is equivalent to an altitude of 46,000 feet or 14 km. Courtesy of the US Air Force Weather Agency.

Ash fell in towns W of the volcano, including in Naranjos, Nahuizalco, Juayúa, Ahuachapán (NW), and La Hachadura (at the border, ~ 40 km W, figure 3). SNET produced a graphic similar to an isopach map that showed near-source thicknesses provisionally to over 10 cm. The 10 cm isopach stretched ~ 5 km W; the 1 mm isopach, ~ 20 km W. The outermost isopach, presumably where measurable ash fell, was not closed; instead it was cut off along the Guatemalan border (~ 40 km to W of Santa Ana) and the caption said that ash would fall into valleys in Guatemala and to the sea. Volcanic blocks up to a meter in diameter fell as far as 2 km S of the volcano's crater. Lahar deposits were seen SE of the volcano. The alert level within a 4-km radius around the volcano's central crater was raised to Red, the highest level.

Figure 3. Graphic from SNET showing ashfall distribution from Santa Ana that appeared in the newspaper, La Prensa Grafica, following the 1 October eruption. N is upwards; Santa Ana lies ~ 40 km E of the Guatemalan border. This clearly transmitted the message that the ashfall was variable and W-directed over parts of El Salvador and neighboring Guatemala. The bottom of the graphic discussed the impact of the ash fall, including damage to specialty coffee farms. Credit: Ricardo Orellana, La Prensa Grafica.

According to news reports, two people were killed by landslides (possibly caused by heavy rain in the area) in the town of Palo Campana, and thousands of residents near the volcano were evacuated. As many as 1,400 hectares of crops were damaged by ash (1 hectare = 10,000 m²). News also mentioned other processes such as a flood of boiling mud and water, and molten rocks, some the size of small automobiles, that will be discussed in later reports. A several-minute-long video from the LPG Television website appears as both a hyperlink and an active file on our website. In addition to numerous interviews with evacuees, it shows a host of features including what appear to be the swaths left by previously inflated mudflows passed down steep-sided valleys.

Prior to the eruption, no significant change in seismicity was observed. On 3 October, after the eruption, seismicity fluctuated and small explosions occasionally occurred. Earthquakes associated with explosions were recorded. In addition, there was a decrease in the amount of SO₂ emitted from the volcano.

Strong degassing had been measured at the volcano since June 2004. An ash emission occurred on 16 June 2005, and a slight increase in seismicity and a significant increase in gas emission were measured from 27 July until at least 30 August. SNET also reported a significant increase in seismic activity at Santa Ana on the night of 27 August. A cluster of 17 volcano-tectonic earthquakes were recorded, with four located S of the volcano. Afterwards, continuous high-frequency tremor was recorded until at least 30 August. Observations made on 29 August revealed incandescent rocks in the fumarole field, effects attributed to hot gases heating the rocks to sufficient temperature to glow. A significant increase in SO₂ emission was recorded, and gas-and-steam plumes rose 500-1,000 m above the volcano's crater. As a safety measure, access to the volcano's crater was restricted.

From 27 July until the eruption on 1 October, seismicity and gas emissions were above normal levels, and Santa Ana was at alert level yellow. During the first week of September, tremor continued to be recorded, and on 2 September a cluster of at least eight small earthquakes occurred, which were not felt by local residents. Gas plumes rose to ~ 500 m above the volcano, and the SO₂ flux was over 1,000 metric tons per day during the first two weeks of September. Satellite imagery from 5 September showed a thermal anomaly.

Microseismicity increased significantly on 12 September. During a visit to the volcano on 8 September, larger areas of incandescence were visible at a field of fumaroles than during a visit on 29 August. Satellite imagery showed a thermal anomaly at the volcano on several days during the second week of September.

During 15-19 September gas plumes rose to ~ 500 m above the volcano, and the SO₂ flux reached a maximum of 3,320 metric tons per day on 16 September. Microseismicity remained at relatively high levels. No significant changes were seen at the volcano's crater when observed on 19 September in comparison to 13 September. Intense degassing continued and the crater lake (lagoon) remained a dark coffee color. Incandescence was visible inside some cracks.

During a visit to the crater on 21 September, observers noted that the lagoon had become greener and small rock landslides occurred in the field of fumaroles. Gas plumes rose to ~ 1 km above the volcano on 26 September.

Following the eruption of 1 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana during 5-11 October. Inclement weather prohibited ground and satellite observations, and sulfur-dioxide (SO₂) measurements during much of the report period. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater. Around 11 October, SO₂ measurements were around 600-700 metric tons per day. The alert level within a 5-km radius around the volcano's central crater remained at Red.

Geologic Background. Santa Ana (also known as Ilamatepec), is a massive, dominantly andesitic-to-trachyandesitic stratovolcano in El Salvador immediately W of Coatepeque Caldera. Collapse during the late Pleistocene produced a voluminous debris avalanche that swept into the Pacific Ocean, forming the Acajutla Peninsula. Reconstruction of the volcano subsequently filled most of the collapse scarp. The broad summit is cut by several crescentic craters, and a series of vents and cones have formed along a 20-km-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro la Olla cinder cones on the SE flank. Small to moderate explosive eruptions from both summit and flank vents have been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 13 km E.

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv); Washington Volcanic Ash Advisory Center (VAAC), NOAA/NESDIS Satellite Analysis Branch (SAB), 5200 Auth Road, Camp Springs, MD 20746, USA; Charles Holliday and Jenifer E. Piatt, U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); La Prensa Grafica and La Prensa Grafica Television, Final bulevar Santa Elena, frente a embajada de EUA, Antiguo Cuscatlán, La Libertad, San Salvador, El Salvador.

During the week of 22-28 August 2005, Ulawun often remained quiet but also displayed continued restlessness. People from Tauke, on the S side of the volcano reported occasional low roaring, rumbling, and booming noises on 21-22 and 26-28 August. Emissions from the summit crater consisted of moderate volumes of thick grayish vapor released forcefully. Some traces of blue vapor were also visible, but no glow was observed. Seismicity fluctuated between low and moderate, marked by small low-frequency earthquakes and small sporadic volcanic tremors. Only one high-frequency earthquake was recorded. An earthquake was felt on 22 August by people from Tauke. Apparently the earthquake was not reported by the observer at Ulamona, NW of the volcano, suggesting it was local and focused on the S side of the volcano.

Ulawun remained quiet through mid-September 2005, with the summit crater releasing weak to moderate volumes of thick white vapor.

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

Information Contacts: Rabaul Volcano Observatory, Papua New Guinea.

During the observation interval 12-18 September 2005, Pago continued to be quiet. Very small volumes of thin white vapor were released from all vents. No noises were heard and no glow was observed. Seismic activity was low, with some small, high frequency earthquakes being recorded. The highest number of high frequency events on any given day was 3, recorded on 18 September.

Geologic Background. The active Pago cone has grown within the Witori caldera (5.5 x 7.5 km) on the northern coast of central New Britain contains the active Pago cone. The gently sloping outer caldera flanks consist primarily of dacitic pyroclastic-flow and airfall deposits produced during a series of five major explosive eruptions from about 5,600 to 1,200 years ago, many of which may have been associated with caldera formation. Pago cone may have formed less than 350 years ago; it has grown to a height above the caldera rim, and a series of ten dacitic lava flows from it covers much of the caldera floor. The youngest of these was erupted during 2002-2003 from vents extending from the summit nearly to the NW caldera wall. The Buru caldera cuts the SW flank.

Information Contacts: Ima Itikarai and Herman Patia, Rabaul Volcano Observatory (RVO), PO Box 386, Rabaul, Papua New Guinea.