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/).

Our most recent report on Anatahan (BGVN 33:12) discussed sulfur dioxide emissions and steam plumes during 2008. This report covers activity between January and October 2009.

A team of research scientists from the University of Tokyo and Kyushu University visited the volcano during the week of 19 January. They worked with the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (CNMI) to perform seismic station maintenance. The team observed no unusual volcanic phenomena. Seismic levels remained low, and no anomalies were observed in satellite imagery.

The U.S. Geological Survey (USGS) reported that seismic activity at Anatahan during the first half of 2009 was generally at background levels. On 11 February a brief episode of tremor occurred. A low level plume was observed in satellite images on 13 June, but there was no evidence that it contained ash. Nothing unusual was observed in satellite images throughout the rest of the week. According to the USGS, Anatahan was quiet as of 6 November.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Dina Venezky, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025, USA (URL: http://volcanoes.usgs.gov/); Emergency Management Office, Commonwealth of the Northern Mariana Islands, PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/).

Deep volcanic earthquakes, seismic tremor, and five small explosions with corresponding ash emission were reported from Kaba in August 2000 (BGVN 25:11). Since then, Kaba has been quiet, but even in its normal state it almost always emits whitish plumes 25-100 m high.

On 20 October 2009, the Center of Volcanology and Geological Hazard Mitigation (CVGHM) reported that seismic activity from Kaba increased in August and remained elevated into September and October. Inflation was also detected. When weather permitted, diffuse white plumes were seen rising ~ 25-50 m above the summit crater complex and drifting E. Based on the deformation and increased seismicity, CVGHM raised the Alert Level to 2 (on a scale of 1-4).

From January through August 2009, the frequency of deep volcanic earthquakes averaged 85 events per month, but in August the number of events rose to 257 per month. During August-September, whitish plumes remained similar to September-October. In September seismicity fluctuated but tended to increase. Earthquakes and total tremors recorded at Kaba's monitoring post are shown in table 2.

Table 2. Summary of Kaba seismic data recorded during 12 September-20 October 2009. Courtesy of CVGHM.

Deformation measurements taken using an EDM (electronic distance measurement) method were as follows: Biring station, shorter by 10 cm; Voelsang station, longer by 0.4 cm; and Kaba station, shorter by 2 cm.

Measurements of the crater water temperature on 15 October showed a reading of 72°C, with a pH of 3.24. The sulfurous and associated solfatara areas recorded a temperature of around 106-107°C. There was no other activity in the area of the crater.

Geologic Background. Kaba, a twin volcano with Mount Hitam, has an elongated summit crater complex dominated by three large historically active craters trending ENE from the summit to the upper NE flank. The SW-most crater of Gunung Kaba, named Hidup (or Lama), is the largest. Most historical eruptions have affected only the summit area. They mostly originated from the central summit craters, although the upper-NE flank crater Kawah Vogelsang also produced explosions during the 19th and 20th centuries.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/).

During March-18 April 2009 (BGVN 34:03) seismicity and volcanism continued at Karymsky, and ash plumes were detected for hundreds of kilometers. This report discusses the interval May-October 2009, with some discussion of earlier seismicity. The key conclusion for this new interval is that seismicity has not stopped and occasional, though smaller ash plumes continued.

In the mid-May 2009 volcanic and seismic activity decreased considerably. The active fumarole on the NW slope produced gas-steam plumes, but no longer contained appreciable ash (figure 8).

Figure 8. Gas-steam emission seen on the NW side Koryaksky on 11 May 2009. The white snow on the volcano confirms the virtual absence of any ash in the plume. Photo by A. Socorenko.

On 2-3 June 2009, observers again saw small areas of snow that were dark gray, indicating some increased ash content. On 15-16 August 2009 instruments began to register a spasmodic volcanic tremor; seismicity increased, and on 17 August ash fell to the SW, forming a deposit 1 mm thick (figure 9).

Figure 9. Ash plume from a vent on NW slope of Koryaksky. Snow on the slope covered fresh ashfall. Dark area around vent is due to local heating and melting of snow cover. Photo taken on 18 August 2009 by S. Chirkov.

Seismic analysis. Seliverstov (2009) analyzed seismicity for May 2008 to 10 June 2009, looking at events larger than Ks 4 (Class 4 earthquakes, roughly those larger than M ~ 1.2) and found a spatial and temporal pattern to this stronger seismicity. During March 2008 a prominent swarm of earthquakes was often centered at about 5-10 km depth. Smaller earthquakes were also seen around that time, several at ~ 12 km depth, and some at 15 km depth. This stronger seismicity then waned for several months until late June. During August 2008 to 10 June 2009, earthquakes were numerous, often centered near 5 km depth.

Figure 10 shows a representative set of hypocenters during 3 January 2009-6 November 2009. The pattern shown was similar to that seen during various months during 2008 through mid-2009. Other patterns during 2008 to mid-2009 included intervals where epicenters dipped, as noted in the analysis presented by Seliverstov (2009). Despite decreased volcanic activity, the elevated seismicity remained until at least 10 June 2009.

Figure 10. Seismicity of Koryaksky (and Avachinsky, to the SE) recorded during May-October 2009. Map shows location and depths of earthquakes (white line is cross-section AB. Cross-section shows hypocenters within 20 km depth. Courtesy of the Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS).

The number of earthquakes recorded within 10-15 km of the summit during January through October 2009 by the KB GS RAS peaked in April with 422 events, and again in August with 245 events. Otherwise, the interval commonly had monthly totals of 100-200 with the lowest during January (59 events) and October (37 events).

References. Seliverstov, N., 2009, The activity Koryaksky volcano, Kamchatka, Vestnik KRAUNC, Earth Science Series; Petropavlovsk-Kamchatsky, 2009, v. 13, p.7-9 [ISSN 1816-5524]. In Russian.

Geologic Background. The large symmetrical Koryaksky stratovolcano is the most prominent landmark of the NW-trending Avachinskaya volcano group, which towers above Kamchatka's largest city, Petropavlovsk. Erosion has produced a ribbed surface on the eastern flanks of the 3430-m-high volcano; the youngest lava flows are found on the upper W flank and below SE-flank cinder cones. Extensive Holocene lava fields on the western flank were primarily fed by summit vents; those on the SW flank originated from flank vents. Lahars associated with a period of lava effusion from south- and SW-flank fissure vents about 3900-3500 years ago reached Avacha Bay. Only a few moderate explosive eruptions have occurred during historical time, but no strong explosive eruptions have been documented during the Holocene. Koryaksky's first historical eruption, in 1895, also produced a lava flow.

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/); Kamchatka Branch of the Geophysical Service of the Russian Academy of Sciences (KB GS RAS), Sergey Senukov, Russia (URL: http://www.emsd.ru/); Alexander Socorenko and Sergei Chirkov, IV&S FED RAS; Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).

Our most recent report on Pagan (BGVN 32:01) covered light ashfall and a small gas plume probably containing some ash during the first week of December 2006. We received no additional information regarding activity at Pagan until April 2009. The U.S. Geological Survey (USGS) does not currently have monitoring instruments on Pagan. Monitoring is by satellite and ground observers.

According to the Washington Volcanic Ash Advisory Center (VAAC), a plume from Pagan on 15 April consisting of intermittent puffs of steam rose to an altitude of 1.8 km and drifted about 37 km W. This observation was confirmed by a ship crew that noted a white plume "with some black" that same day.

On 16 April, the Washington VAAC reported that a narrow plume of unknown composition extended 85 km W from the volcano. According to the CNMI Emergency Management Office, fishermen reported that the plume was "thicker" on 15 April than on 16 April. Weather clouds obscured satellite views. The next day fishermen again reported a plume.

By 17 April, steaming had diminished. A passing pilot reported seeing no activity; however, the Washington VAAC noted a very faint plume extending 85 km NNW in satellite imagery.

Crew on a U.S. National Oceanic and Atmospheric Administration (NOAA) ship observed continuous emissions from the N crater during 21-22 April. Satellite imagery analyzed by the Washington VAAC showed a diffuse plume drifting 15 km W on 23 April. On 28 April, steam emissions had decreased.

Based on analyses of satellite imagery, the Washington VAAC reported that on 14 August a 2-hour-long thermal anomaly detected over Pagan was followed by a small emission. The emission, hotter than its surroundings, drifted NW and quickly dissipated.

No thermal hotspots on Pagan have been detected by MODIS during the last five years.

Geologic Background. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. North Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1,000 years ago. South Pagan is a stratovolcano with an elongated summit containing four distinct craters. Almost all of the recorded eruptions, which date back to the 17th century, have originated from North Pagan. The largest eruption during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: Dina Venezky, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025, USA (URL: http://volcanoes.usgs.gov/); Emergency Management Office, Commonwealth of the Northern Mariana Islands, PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Activity at Reventador between August 2008 and late April 2009 was a period of generally low seismicity (BGVN 34:03). During early November 2008 repeated small eruptions occurred with steam-and-ash plumes, Strombolian eruptions, and lava flows. This report continues coverage through October 2009, an interval that included new lava flows advancing ~ 500 m by mid-September 2009.

Based on analysis of satellite imagery, the Washington Volcanic Ash Advisory Center (VAAC) reported that on 1 May a thermal anomaly over Reventador occurred along with a possible low plume drifting W. The Instituto Geofísico-Escuela Politécnica Nacional (IG) reported to the VAAC the presence of lava and gas emissions and possible smoke from burning vegetation, but little to no ash.

On 15 May, the IG observed an ash emission, although neither an ash signature nor a thermal anomaly was detected in satellite imagery. On 26 May, a diffuse ash plume rose to an altitude of 6.4 km and drifted SW. Thermal anomalies were intermittently seen on satellite imagery.

On 21 July-3 August, tremor was sporadic. On 4 August, seismicity increased and periods of tremor frequently saturated the seismic stations. Thermal anomalies, detected in satellite imagery on 1 and 2 August, became more intense on 4, 5, and 10 August. On 6 August, a steam plume rose 1.2 km above the crater and drifted W. Incandescent blocks were ejected from the crater and fell onto the flanks. Thermal images taken from a location 7 km E of Reventador revealed a linear area of higher temperatures, confirming the presence of a new lava flow on the S flank. Incandescence in the crater was seen on 9 August. According to the Washington VAAC, based on information from the IG, an ash plume on 15 August rose to an altitude of 3.6 km and drifted NW.

Field observations on 16-17 September 2009. IG scientists visited Reventador during 16-17 September 2009; among their objectives was to map, sample, and collect thermal images of the new lava flows and to measure the sulfur dioxide (SO₂) concentrations with a mobile DOAS.

The team noted that recent lava flows had descended the flanks in a SE to E direction, continuing the same pattern that had begun with the 2005 eruption (figure 31). A dome within the crater showed constant growth (figure 32). Gas was emitted to a height of less than 200 m and drifted mainly W. A small lava flow originating in the dome area had descended ~ 500 m from the cone's S flank.

Figure 31. Panoramic view from the sequential camera of lava flows at Reventador. Courtesy of S. Vallejo.

Figure 32. At Reventador, a photo taken on 16-17 September 2009 of the actively growing dome in the summit crater. Courtesy of J. Bourquin.

Thermal images and SO₂ measurements were collected near the caldera, and lavas were sampled. SO₂ flux measurements (table 4) were collected both by helicopter and by car (figure 33). A telescope for the SO₂ measurements sat below the helicopter blades and those spinning blades may have interfered with the measurements. The values presented may thus underestimate the SO₂ fluxes.

Table 4. Reventador SO₂ data collected from helicopter on 16 and 17 September 2009. The transects in the first column are indicated on figure 33. Transect 34 measurements clearly indicated that the SO₂ gas plume had divided (bifurcated) and the two plumes appear as 34a and 34b. For this transect, the SO₂ fluxes were calculated separately for each lava plume segment, than added to get the total emission. Land-based measurements, transects 43 and 46, were collected S and W of the vent. Courtesy of IG.

Figure 33. Map of Reventador illustrating transects made (in colored version, each transect is in a different color). Transects 31 and 34 were conducted on 16 September; transects 35, 36, 43 and 46 were conducted on 17 September. Courtesy of IG.

Based on a pilot observation, the Washington VAAC reported that on 21 September a plume rose to an altitude of 7.6 km. An ash plume on 4 October drifted W. In both cases, ash was not seen in satellite imagery, although meteorological clouds were present. In the latter case, an occasional thermal anomaly was observed.

Thermal anomalies over the crater area were detected in MODIS satellite imagery on 6, 11, and 13 October. On 13 October, the OMI satellite sensor indicated that the SO₂ concentration in the atmosphere near the volcano had increased. On 14 October, seismicity increased and harmonic tremor was detected. A seismic station located at ~ 2,600 m elevation on the NE flank of the cone detected rockfalls. Several people living in the area reported roaring noises and had observed slight incandescence from the crater during the previous few nights.

During an overflight on 16 October, scientists saw the lava dome and a lava flow on the NE flank (figure 34). Bluish gases were being emitted. According to a thermal camera, the incandescent parts in the crater were about 300°C. Other observers heard roaring noises and sounds resembling "cannon shots." Incandescent blocks were ejected from the crater, and steam and gas rose 100 m and drifted SW. Incandescent material was seen on the S flank.

Figure 34. Aerial photo taken on the N side of Reventador on 16 October 2009 showing the lava dome amid weather clouds and some heavy steaming from the NE-flank lava flow. Courtesy of IG.

On 17 October, long period (LP) earthquakes and volcanic explosions lasting up to 10 hours were registered, incandescence on the S flank was noted, and noises similar to the previous day were again heard. A small gray plume was seen the next day. On 19 October, thermal anomalies were again detected on satellite imagery. During an overflight, blue gas plumes containing SO₂ were seen (figure 35). The lava flow on the S flank occupied a large area and was divided into two branches.

Figure 35. Photograph of the E side of Revantador's cone taken the morning of 19 October 2009. Note steam rising from dome summit and lava flows on volcano's flanks. Courtesy of IG.

According to the IG, on 21 October, steam-and-gas plumes with little to no ash rose 2-4 km above the crater and drifted in various directions. An explosion that day ejected incandescent material from the crater and blocks rolled down the flanks. On 22 October, a few explosions generated ash-and-steam plumes that rose 4 km. Observations during an overflight revealed a small lava flow on the N flank and a larger flow with four branches on the S flank (figure 36). Part of the lava dome base had disappeared and small spines were present, especially on the S side of the dome. Thermal images revealed that material in the crater was 400°C and the lava-flow fronts were 250°C. Cloudy weather prevented visual observations during 23-26 October. Roaring noises were heard on 25 October.

Figure 36. Aerial photograph displaying the distribution of lava flows on the N side of Reventador's caldera on 22 October 2009. Courtesy of S. Vallejo.

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

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

A series of eruptions at Rinjani began on 2 May 2009 (BGVN 34:06). The current report, provided by Alain Bernard, presents additional data regarding these eruptions, which continued through 31 August 2009.

Studies of Rinjani volcanic lake are part of a cooperative agreement between Indonesia and Belgium, a collaboration funded by the Commission Universitaire pour le Développement (CUD, the main Belgian development cooperation agency for universities). Geochemical and physical studies of the Segara Anak lake started in the framework of this collaborative effort during the summer of 2006. During the summer of 2008, investigators installed a monitoring station for continuous measurements of the lake's level and temperature, and for meteorological parameters.

The scientific teams involved in this study were (Indonesia) Akhmad Solikhin, Devy Syabahna and Syegi Kunrat of the Center of Volcanology and Geological Hazards Mitigation (CVGHM); (Belgium) Alain Bernard, Benjamin Barbier, Robin Campion, and Corentin Caudron of the Université Libre de Bruxelles (ULB), and Vincent Hallet and David Lemadec of the Facultés Universitaires Notre Dame de la Paix (FUNDP).

Segara Anak lake. The majestic Segara Anak lake filling the caldera of Rinjani covers an area of 11 km². Prior to the 2009 eruption, the lake's volume was 1.02 km³. The lake-surface elevation is ~ 2 km (figures 11 and 12).

Figure 11. Bathymetric map of Rinjani's Segara Anak lake made from 65 km of echo-sounder surveys conducted in 2007 and 2008. Maximum depth of the lake is 205 m. Courtesy of the CVGHM, ULB, and FUNDP study team.

Figure 12. Topographic map of Rinjani caldera from Bakosurtanal-Indonesia (National Coordinator for Survey and Mapping Agency, Indonesia). Margins defining squares are 1 km long. "CTD" and "CTD-B" are locations of conductivity-temperature-depth profiles. "Meteo" is the site of the meteorological station monitoring air temperature, humidity, wind velocity, and net solar flux. The labels 51-54 are locations of hot springs discussed below. Courtesy of the CVGHM, ULB, and FUNDP study team.

The lake is neutral (pH: 7-8) and its chemistry dominated by chlorides and sulfates with a relatively high concentration of total-dissolved solids (TDS: 2,640 mg/L). This unusually high TDS value and lake surface temperatures (20-22°C), well above ambient temperatures (14-15°C) for this altitude, reflects a strong input of hydrothermal fluids. Numerous hot springs are located along the shore at the foot of the Barujari cone. Bathymetric profiles showed several areas with columns of gas bubbles escaping from the lake's floor.

Precursory signals of the May 2009 eruption. Changes in Segara Anak lake and the hot springs before the first 2 May 2009 eruption included significant anomalies in the temperature and chemistry of the hot springs.

During field work 10-14 April 2009, the researchers noted an increase in temperature and acidity of hot springs 53 and 54 (figures 13-14) compared to July. This increasing acidity was confirmed in the lab as the consequence of an increase in sulfate levels not observed during studies since 2004.

Figure 13. Geochemistry of Rinjani's Segara Anak lake (from CTD locations, figure 12) and hot springs. NA signifies not analyzed. Courtesy of the CVGHM, ULB, and FUNDP study team.

Figure 14. A plot of 2004 to 2009 sulfate (SO42-) ion concentration versus pH. Note the April 2009 increase in the acidity and sulfate contents of Rinjani's hot springs 53 and 54 (upper left). Courtesy of the CVGHM, ULB, and FUNDP study team.

The Fe-ion concentrations in spring 54, usually below detection limits, peaked at 120 mg/L. This change in chemistry produced a yellowish-brown coloration of the lake waters because of the precipitation of ferric hydroxide, Fe(OH)₃ (figures 15-16). An ASTER image from 21 August 2009, processed to enhance the Fe(OH)3 precipitates, revealed a chemical plume close to where hot springs 53 and 54 injected water into the lake.

Figure 15. The Rinjani lake shoreline seen at a point close to the hot spring 54 on 12 April 2009. The water has a brown color and the coating on the rocks is an amorphous ferric hydroxide that precipitated when hydrothermal fluids oxidized by mixing with the lake waters (yellow-brown in color). Rock discoloration reaches the height of the mans lower hand. Changes in lake level were a consequence of the rainy season. Photo by A. Bernard.

Figure 16. A photo of Rinjani and Segara Anak Lake thought to have been taken on 26 April 2009 but certainly before the start of the eruption. A spectacular yellow-brown chemical precipitate floated on the lake's surface (at left). Copyrighted photo by Jim Chow.

A chemical plume of low pH and dissolved oxygen was observed at the lake surface extending up to several hundred meters away from hot spring 54. pH profiles as a function of depth recorded at several locations showed a clear acidification of Segara Anak lake especially at shallow depths (15-20 m) (figure 17). Rainfall in April 2009 caused a shallow zone of higher pH.

Figure 17. Plot showing shifts of pH in Rinjani's caldera lake waters with depth. The 2009 profile was recorded in April 2009, and the 2008 profile, in July 2008. Line 2009b was drawn as an estimate of the curve without the rainfall event. Measurements made with an SBE Seacat 19-Plus profiler. Courtesy of the CVGHM, ULB, and FUNDP study team.

A slight lake surface temperature increase from 20°C in July 2008 to 22°C in early April 2009 was mostly attributed to meteorological effects. Large increases seen in lake level in January and February 2009 were the consequence of heavy rainfalls. Heating of the lake between August 2008 and April 2009 occurred mainly during periods with reduced heat loss to the atmosphere due to less wind.

A report of these field observations was made on 17 April to CVGHM, which prompted them to send another team to the volcano. The new team arrived at the summit of the volcano on 2 May 2009, the day the eruptions started.

Eruptive activity May-August 2009. The 2009 eruptions originated from the same vent of the October 2004 activity (figure 18), and was characterized by mild eruptions that produced a small lava flow and low altitude, ash-poor gas plumes (figure 19). Contrary to that reported in some newspapers, the 2009 eruption did not open a new vent.

Figure 18. This July 2006 photo of Rinjani illustrates the new vent that opened on the NE slope of Gunung Baru in 2004 and then produced a short lava flow. The 2009 eruptive activity at Rinjani started from this same vent, producing a significant lava flow that entered the crater lake and built a delta. Photo by A. Bernard (July 2006).

Figure 19. Eruption of Rinjani seen on 10 June 2009. The plume at this point was relatively small and lava proceeded N to enter the lake. Photo by R. Campion.

Alain Bernard sent a report by Robin Campion (ULB) who was on site June 2009. According to Campion, mild activity was observed from the SE rim during 9-11 June. Pressurized incandescent gas was released at a 1-2 second intervals from a vent located in the 2004 crater, on the S flank of Barujari. At variable intervals (10 seconds to 10 minutes), stronger gas jets ejected lava fragments to heights up to 100 m. Occasional ash ejections occurred from a second vent in the same crater. A third lower vent emitted a viscous lava flow that reached Segara Anak Lake. Contact between the lake and the lava delta resulted in a warm surface current (figure 20).

Figure 20. A 10 June 2009 FLIR thermal camera image of Rinjani's Barujari cone and Segara Anak lake. A thermal plume of hot lake water was drifting from the lava entry points. Temperature scale is for lake waters. Photo by R. Campion.

Figure 21 shows an ASTER satellite image of Rinjani lake-surface temperatures. Increased discharge of the hot springs on the S flank of Barujari produced a distinct plume with an orange-red color. Weak winds carried the steam-and-gas plume (with low ash content) N and W at an altitude of 3-4 km. Activity did not vary during the 3-day observation period.

Figure 21. ASTER satellite image of the N part of Rinjani's Segara Anak lake taken on 29 July 2009 (at 1446 UTC). Thermal infrared bands 13 and 14 processed with a split-window algorithm. Contours are in degrees C. Maximum temperature in the lake water was 57°C. Courtesy of the CVGHM, ULB, and FUNDP study team.

As of 31 August 2009, the eruption was still underway. At that point, the new lava flow covered an area of 0.65 km² (figure 22). The shoreline had been significantly modified by the entry of lava into Segara Anak lake, and the lake surface area had been reduced by 0.46 km² (figures 22 and 23).

Figure 22. Rinjani seen in ASTER false color on 21 August 2009 (0235 UTC). The new lava covers an area of 650,000 m2 and significantly changed the shoreline. The inset shows the pre-eruption shoreline and the new lava margin (in red on colored versions). Courtesy of the CVGHM, ULB, and FUNDP study team.

Figure 23. Rinjani, in a photo apparently taken on 4 August 2009, showing a new delta built into the lake. Brown waters from hot springs were still visible at the end of the lake and drifted over a large area of the lake. Courtesy of Arjo Vanderjagt.

According to Alain Bernard, updates on Rinjani's activity will be posted on the website of the Commission of Volcanic Lakes (CVL, see information contacts). Bernard sent additional figures describing Rinjani behavior as late as 27 September 2009.

Geologic Background. Rinjani volcano on the island of Lombok rises to 3726 m, second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the west side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak (Samalas) caldera. The caldera formed during one of the largest Holocene eruptions globally in 1257 CE, which truncated Samalas stratovolcano. The western half of the caldera contains a 230-m-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the east end of the caldera. Historical eruptions dating back to 1847 have been restricted to Barujari cone and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake.

Information Contacts: Alain Bernard, Benjamin Barbier, Robin Campion, and Corentin Caudron, Université Libre de Bruxelles (ULB), Belgium (Commission of Volcanic lakes, URL: http://www.ulb.ac.be/sciences/cvl/rinjani/rinjani.html); Akhmad Solikhin, Devy Syabahna, and Syegi Kunrat, Center of Volcanology and Geological Hazards Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Vincent Hallet and David Lemadec, Facultés Universitaires Notre Dame de la Paix (FUNDP, URL: http://www.fundp.ac.be/); Bakosurtanal, Badan Koordinasi Survei dan Pemetaan Nasional (URL: http://www.bakosurtanal.go.id/); Jim Chow (URL: http://www.flickr.com/photos/11668976@N06/3486237730); Arjo Vanderjagt, Oude Boteringestraat 52, 9712 GL Groningen, The Netherlands (URL: http://www.flickr.com/photos/87453322@N00/3794836615).

The eruptive episode that began with the volcano reawakening in October 2004 (BGVN 29:09) ended in late January or early February 2008. The activity included explosions containing ash that rose up to ~ 3 km above the crater and lava dome growth. Sherrod and others (2008) provide a comprehensive discussion of the 2004-2006 portion of the eruption. This report spans 28 November 2007 through October 2009.

A GPS receiver on the W part of the active spine recorded continued SW advance at a rate of 3-4 mm per day during September through November 2007. During 28 November-4 December 2007, small inflation-deflation events occurred, which the USGS Cascades Volcano Observatory (CVO) interpreted as dome-growth pulses. On 31 December 2007 aerial observers saw a new small, snow-free spine on top of the active lobe.

On 25 January 2008, a steam plume rose from the dome slightly above the crater rim. Though seismicity had persisted at low levels through mid-February 2008, very few earthquakes were recorded after late January. Locatable earthquakes were fewer than one per day, all under M 2.0. Ground tilt measurements showed an overall subsidence in the area of the new dome. A GPS receiver on the previously active spine settled about 2 cm per day on a southward path. During February, the daily ground-tilt events stopped and gas emissions were barely detectable.

Comparison of photographs taken by remote cameras during late January to mid-February 2008 showed no evidence of extrusion. Cynthia Gardner (CVO), in a personal communication, noted that dome growth stopped in late January or early February (January 27 ± 10 days).

During March 2008, the most significant developments were a small, M 2.0 earthquake on 4 March and a very small earthquake swarm on 6 March. The latter started with a roughly M 1.2 event, followed by several smaller tremors over a seven-minute period. No tilt changes were associated with the swarm. On 14 March, the Pacific Northwest Seismic Network recorded four very small earthquakes located near the volcano. There were no tilt changes associated with this activity.

Radar imagery analyzed by Jet Propulsion Laboratory staff during late March 2008 showed that the E and W arms of Crater Glacier were touching, or close to touching, just N of the 1980s lava dome. From 30 May 2008 (figure 71) to 8 July 2008, the W arm of the glacier advanced ~ 20 m. By 8 July, the old and new lava domes in the crater were encircled by ice (figure 72). Further down slope glacier ice descended into the gullies that had been carved by erosion into the Pumice Plain. On 10 July, after nearly 5 months without signs of renewed activity, CVO lowered the Alert Level to Normal and the Aviation Color Code to Green.

Figure 71. Aerial view of the St. Helens crater, as seen from the N. The two arms of the Crater Glacier had by 30 May 2008 fully encircled the dome. USGS photograph by Steve Schilling.

Figure 72. Old and new lava domes (center and upper right respectively) in the St. Helens crater encircled by ice, as seen from the NW. USGS photograph taken on 5 August 2009 by Steve Schilling.

As of October 2009, earthquakes, volcanic gas emissions, and ground deformation had all fallen to levels observed prior to the onset of the eruption.

References. Lahusen, R.G., 2005, Acoustic flow monitor system?user manual: U.S. Geological Survey Open-File Report 02-429, 22 p.

Sherrod, D.R., Scott, W.E., and Stauffer, P.H., eds., 2008, A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006: U.S. Geological Survey, Professional Paper 1750, 856 p.

Geologic Background. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismic Network, University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

The Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA) continued monitoring the Turrialba non-eruptive interval of February 2008 through August 2009. As during the previous four months (BGVN 33:01), Turrialba continued to emit sulfurous gas from its central and W craters, and elsewhere, including some new cracks.

Activity during February-December 2008. During February 2008, the area around Turrialba affected by acid rain increased due to degassing. The degassing vents on the N, NW, W, and SW walls were rich in sublimated native sulfur. Gas-emission temperatures ranged from 72 to 132°C. Owing to prevailing winds, the vegetation most affected was on the N, NW, and SW flanks. The effects ranged from discoloration to death of various plant species. Residents in the area reported occasional nausea and irritation of the skin and eyes. On 22 February, local observers reported a gas plume up to ~ 2 km in height.

On the SE and SW walls of the central crater two cracks 2-3 cm wide and 100 m long continued to emit gases at ~ 90°C and produced sulfur deposits (figure 13). In stable atmospheric conditions gas columns often rose ~ 500 m above the crater. Rockslides sometimes covered emitting fumaroles, and new sulfur deposits tended to develop in these areas.

Figure 13. Elongated cracks (red lines on colored version) mapped at Turrialba during in August 2009. From first being noticed in mid-2009 to being measured in August 2009, some cracks opened by as much as 12 cm. Courtesy of OVSICORI-UNA.

During 7-8 March 2008, gas sampling at the summit fumaroles determined the maximum temperature at the largest W wall vent was 278°C. Degassing vents were also noted at spots in the middle of the forest. In some cases emissions had killed all local vegetation.

On 7-8 March 2008, Erick Fernandez and Eliécer Duarte of OVSICORI, and the National University (UNA) took gas samples. The analysis, done by Jorge Andrés Diaz and Sergio Achí of the University of Costa Rica, revealed the presence of He at 80,000 ppm (parts per million), whereas the typical He concentration in the neighborhood of a volcano is 25 ppm.

OVSICORI-UNA reported continued degassing during August and September 2008. Multiple fumaroles and areas of sulfur deposition were noted in both the central and W craters. Fumarolic emissions on the S and SE flanks of the W crater continued to damage vegetation in that area.

On 23 September 2008, OVSICORI fieldwork confirmed a severe impact of acid-rain on areas that had been only mildly affected during the preceding 3 years of degassing. At least three sectors showed new impacts on vegetation and infrastructure, from the summit downhill ~ 3 km along the S and SE flanks. The upper sector, which includes the entire caldera and lower sectors to the E, S, and SE near the summit, had been severely burned during August and September. This area goes from the summit down to an elevation of ~ 2,900 m. By 23 September, weeds, dwarf vegetation, and trees had been completely burned; however in these areas some resistant species maintained some green and appeared seemingly viable. Along the external walls to the S of the W crater, plants had been burned down to the soil. Due to the removal of that natural coverage, erosion had cut extended radial gullies.

Between the elevations of 2,900 and 2,600 m, significant forest patches have been partially seared by extreme acidification, particularly the dense birch forests. Below 2,600 m elevation mild burns to the tree canopy and pasture areas were evident. The evidence of chemical burns due to the heavy gases are amplified along canyons and depressions. These conditions caused residents to voluntarily leave their farms in 2007.

Monitored SO₂ emissions during the early part of 2008 had been ~ 750 metric tons per day (t/d). At the end of April 2008, an increase to ~ 1,000 t/d was noted, which then increased to ~ 2,000 t/d well into July. During the end of July the emissions declined to ~ 1,100 t/d. The increase in SO₂ flux corresponded to increases in vegetation damage.

Activity during January-June 2009. In May 2009 OVSICORI reported ongoing fumarolic degassing during the preceding months from the central crater, from the N, NW, W, SW and S walls, from new vents on the S and SW walls, and other locations. Some locations continued to form sublimated sulfur deposits. The two cracks in the SE and SW walls had temperatures of ~ 87°C. The emissions in the W wall registered ~ 91°C and displayed sulfur deposits. In meteorologically quiet conditions, gas plumes were noted up to 500-600 m above the crater floor. All of these areas had experienced small landslides that occasionally covered some vents.

SO₂ flux was variable during early 2009 (figure 14). The flux data were collected with a roughly consistent sun angle, between 0900 and 1100 in the morning on the SW flank. In the graph the SO₂ flux varies between ~ 0 and ~ 2,000 t/d, the maximum flux occurred on 23 April.

Figure 14. SO2 fluxes measured at Turrialba during April 2009 (with y-axis showing SO2 flux in metric tons per day and x-axis dates in the format, month/day/year). Courtesy of OVSICORI-UNA.

On 14 June 2009, OVSICORI-UNA reported that fumarolic activity from Turrialba had been observed all around the upper flanks of the active W crater. During the previous two months, the fumarolic activity was also accompanied by widening radial cracks (1.5 cm on average), 1-2 km tall gas-and-vapor plumes, and one sustained seismic swarm. Temperatures of fumarolic vents in the lower parts of the crater were between 120 and 160°C. The temperature of summit cracks was 94°C. By mid-June, dairy pastures and forests had been chemically burned as far away as 3.5 km NW and W. During the last week of August 2009, the W and NW lower flanks, sectors previously reported with moderate effects, showed acute burns, and yellow pastures within 3 to 4 km radius (figures 15, 16, and 17).

Figure 15. Vegetation damage as of late August 2009 at Turrialba plotted on a shaded relief map by F. Robichaud. E. Duarte and others found that damage was generally within several kilometers of the volcano and in broader areas on the W flanks. The large dotted line indicates the boundary of detectible damage. Severe damage covered an irregular area, a strip both directly W of the active crater and a lobe to its SW as well. Courtesy of OVSICORI-UNA.

Figure 16. Newly emerging fumaroles on Turrialba's upper NW flank and burns on vegetation, August 2009. Courtesy of OVSICORI-UNA.

Figure 17. A view from Turrialba's seismic station PICA on the NW flank, showing active degassing from a variety of locations in August 2009. Left mid-ground shows plumes from the lowest fumaroles yet developed on this flank. Green grass is in the foreground, but most of the other foliage is brown to orange. Courtesy of OVSICORI-UNA.

Near the Toro Amarillo river (4 km E of the crater) chemical burning surrounds stands of trees. Such whitening effect had been previously reported at the end of 2007 for areas closer to the active crater, 1.5 km W (figure 18).

Figure 18. An example of a zone with intense burns on grass at the foot of injured trees, damage attributed to acidic gases from Turrialba. The spot is near the Toro Amarillo river in late August 2009. Courtesy of OVSICORI-UNA.

Several elongated cracks were mapped just south of the W crater as well as 1 km downslope NW. One main crack, noticed during mid-2009 due to sulfur depositions on the surface, was opened by August in places as much as 12 cm. In late August 2009 it emitted gasses at 90°C. The crack trends E-W, in places intersecting a trail used to reach the summit's SW and W sides.

The last two years have caused residents to leave owing to the burned and dead pastures. Some commented on their apprehension related to the emergence of the lower fumaroles. Along the S side of the Irazú summit located 10 km SW of Turrialba's summit, mild burns have been observed on patches of birch, eucalyptus and pine. Lesser impact was reported last year in that same area.

False eruption report. The Washington VAAC received surface observations from an airport near the volcano erroneously indicating an eruption on the morning of 23 September 2009.

The VAAC decided to initially describe the activity as an eruption because it was the first time the airport had reported emissions, the volcano was known to have been degassing for some time, and early morning satellite imagery showed cloud cover, preventing good analysis. In addition, attempts to reach local volcanologists by telephone using contact numbers from the ICAO [International Civil Aviation Organization] handbook and the OVSICORI webpage were not successful. OVSICORI-UNA personnel reported a few hours later that the volcano had not erupted. As a result of the incident, the VAAC has obtained current contact numbers, including personal cell phones, for future use.

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

Information Contacts: Eliécer Duarte, Erick Fernández, Vilma Barboza, S. Miranda, L. Ortiz, G. Chavez, Jorge Brenes, Thomás Marino, Javier Pacheco, Juan Segura, and Rodolfo van der Laat, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apdo. 2346-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Francois Robichaud, Université de Sherbrooke, 2500 boul. de l'Université, Sherbrooke, Québec J1K 2R1, Canada; Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).