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/).

An eruption on 16 May sent an ash plume 3,500 m above the summit, to ~4,600 m altitude (BGVN 21:05). This higher than usual ash plume was estimated at 4,880 m altitude in aviation notices from Tokyo. However, the eruption was not detected on GMS satellite imagery. Ths Volcano Research Center also noted that this explosion threw large cinders to 2 km NW of the crater.

According to the Sakura-jima Volcanological Observatory (SVO), Kyoto University, since March there has been a decrease in the amount of air-fall tephra, the frequency of explosions, and earthquakes (including BL, surface, and shallow types).

The Japanese Meterological Agency reported that Minami-dake crater produced four explosive eruptions during June. The highest ash plume of the month rose 900 m above the crater on 22 June. Ashfall measured at the Kagoshima Local Meteorological Station, 10 km from the crater, was 1 g/m². The seismic station 2.3 km NW of the crater recorded 118 earthquakes and 63 tremors through 29 June. High seismicity began around 0800 on 30 June; 349 earthquakes were recorded that day, bringing the monthly total to 467.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Volcanological Division, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

The daily number of recorded earthquakes continued to be low (at an average rate of a few events/day) in May and June, with some fluctuations. This rate was significantly lower than the rate measured during the seismic crisis of mid-March (BGVN 21:02-21:04).

Geologic Background. Akutan contains a 2-km-wide caldera with a large cinder cone in the NE part of the caldera that has been the source of frequent explosive eruptions and occasional lava effusion that covers the caldera floor. An older, largely buried caldera was formed during the late Pleistocene or early Holocene. Two volcanic centers are located on the NW flank. Lava Peak is of Pleistocene age, and a cinder cone lower on the flank produced a lava flow in 1852 that extended the shoreline of the island and forms Lava Point. The 60-365 m deep younger caldera was formed during a major explosive eruption about 1,600 years ago and contains at least three lakes. A lava flow in 1978 traveled through a narrow breach in the north caldera rim almost to the coast. Fumaroles occur at the base of the caldera cinder cone, and hot springs are located NE of the caldera at the head of Hot Springs Bay valley and along the shores of Hot Springs Bay.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys.

During June, Arenal continued its unbroken eruptive sequence lasting nearly 28 years. In the second half of the month, the active crater, Crater C, increased the volume of pyroclastic emissions above that seen in recent months. Some plumes ascended over 1 km above Crater C. Bombs and blocks landed as far down on the flanks as 1,100 m elevation.

Gerardo Soto (ICE) sketched how lava flows had advanced ~2 km into the N sector and then paused at this point on 17 June (figures 77 and 78). A new, parallel flow that developed on the W side of the older one reached 1,100 m elevation on 20 June. The estimated volume of these flows was 2.5 x 10⁶ m³. They spawned occasional avalanches off their margins. The lava's source vent was the scene of sub-continuous fountaining that formed a 30-m tall, horse-shoe shaped spatter cone open towards the escaping flows. W-flank ashfalls have increased in mass during the course of 1996 (table 16).

Figure 77. Sketch map of Arenal and vicinity showing distribution of major lava flows, 1968-96. Courtesy of G. Soto (ICE).

Figure 78. Sketch of Arenal in an oblique view from the N flank, mid-June 1996. Courtesy of G. Soto and F. Arias (ICE).

Table 16. The mass of Arenal's ash accumulating per day (collected 1.8 km W of the active vent). Courtesy of ICE.

OVSCICORI-UNA reported that during June low-frequency (<3.5 HZ) seismicity totalled 781 events; the majority of these events were associated with eruptions; some events were also recorded at a station 30 km SW of the active crater. Tremor durations of over 23 hours took place on 4, 6, 7, and 11 June; the total for the month was 375 hours. This tremor's dominant frequency was 2.1-3.4 hz.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

On 29 June, Japan Airlines reported volcanic ash erupting from Atka. In addition, GEOS-9 satellite images showed a possible small ash cloud in the immediate vicinity of Atka. In early May 1995 residents of Atka village observed a small plume-like cloud over Kliuchef and reported a strong sulfur smell (BGVN 20:05).

[The 1996 eruption described here was later discredited.]

Geologic Background. The Atka Volcanic Complex consists of a central shield and Pleistocene caldera with several post-caldera volcanoes. A major dacitic explosive eruption accompanied formation of the caldera about 500,000 to 300,000 years ago. The most prominent of the post-caldera stratovolcanoes are Kliuchef and Sarichef, both of which may have been active in historical time. Sarichef has a symmetrical profile, but the less eroded Kliuchef is the source of most if not all historical eruptions. Kliuchef may have been active on occasion simultaneously with Korovin volcano to the north. Hot springs and fumaroles are located on the flanks of Mount Kliuchef and in a glacial valley SW of Kliuchef. Korovin, at the NE tip of Atka Island, is the most frequently active volcano of the complex, and contains a double summit with two craters. The NW summit has a small crater, but the 1-km-wide crater of the SE cone has an open cylindrical vent of widely variable depth that sometimes contains a crater lake or a high magma column. A fresh-looking cinder cone lies on the flank of the partially dissected Konia volcano, located on the SE flank of the dominantly basaltic Korovin. Some late-stage dacitic lava flows are present on both Korovin and Konia.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys; NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Seismicity remained slightly above or at normal levels in June and the first half of July. Normal fumarolic activity was seen above the crater. Regular reports from KVERT (via AVO) resumed in June after funding problems in Russia halted communications in December 1994 (BGVN 19:11).

Geologic Background. Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. It began to form during the middle or late Pleistocene, and is flanked to the SE by Kozelsky volcano, which has a large crater breached to the NE. A large collapse scarp open to the SW was created when a major debris avalanche about 30,000-40,000 years ago buried an area of about 500 km2 to the south, underlying the city of Petropavlovsk. Reconstruction of the volcano took place in two stages, the first of which began about 18,000 years before present (BP), and the second 7,000 years BP. Most eruptions have been explosive, with pyroclastic flows and hot lahars being directed primarily to the SW by the collapse scarp, although there have also been relatively short lava flows. The frequent historical eruptions have been similar in style and magnitude to previous Holocene eruptions.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO); Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Russia.

Seismicity remained at or a little above normal background levels from 26 May to 22 July. Gas-and-steam plumes rose 100-300 m above the crater and extended ~2-7 km downwind. On 30 June, seismicity increased slightly, possibly associated with processes inside the extrusive lava dome. Regular reports from KVERT (via AVO) resumed in June after funding problems in Russia halted communications in December 1994 (BGVN 19:11).

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: Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, 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; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

Visiting scientists made observations of eruptive activity during late May and June. The observations revealed continued activity similar to that previously described for this eruptive phase (BGVN 20:06-20:11/12 and 21:02-20:03).

Observations during 26 May-3 June. Activity at the summit craters was described by Marco Fulle following visits on 26 and 30-31 May and on 1 and 3 June.

Bocca Nuova was filled by thick steam on the afternoon of 26 May, but there were many strong explosions. A vent on the SE side of the crater was emitting steam jets. A vent with a lava pond on the crater floor was ejecting meter-sized lava clots 20 m high. When the pond level was close to the bottom, Strombolian explosions rose 50 m. Northeast Crater (NEC) erupted thin ash and black bombs, but later produced Strombolian explosions every 10-50 seconds that sent a few bombs 50 m above the rim directed towards the E. No bombs fell outside the crater.

Observations of NEC beginning in the late afternoon of 30 May were made for 18 hours from Pizzi Deneri and the W rim of NEC. Strombolian explosions ejected black bombs 100-200 m above the rim at 1-40 second intervals. On the morning of 31 May many meter-sized incandescent bombs were ejected well beyond the SW crater rim due to a strong wind. After 30 minutes, this activity changed to predominantly ash eruptions. Eruption intensity soon increased again, ejecting lava clots and dark bombs well above the rim.

For two hours on the evening o f 1 June observers watched from the W rim of Bocca Nuova and Voragine. Bocca Nuova contained two vents with active lava ponds aligned N-S. The N vent produced Strombolian explosions 50 m high every 10-30 minutes. The S vent produced Strombolian explosions 10-30 m high every 5-20 seconds. A third vent in the SE side of the crater produced steam eruptions every 2-10 minutes with red glow during the steam ejections. Voragine produced steam jets when NEC was inactive. During five hours of observations at NEC from the W rim on 3 June, Strombolian explosions every 2-50 seconds rose 100-200 m.

Observations during 1-20 June. While making GPS measurement of a deformation network on the volcano's upper S flanks on 1-20 June, J.L. Moss and co-workers observed summit activity.

During 1-5 June, no explosions or ash clouds were observed, but the summit vents were vigorously steaming. On 6 June, local guides reported explosive activity at NEC. On 9 June steam degassed strongly from the summit craters, and yellowish fumes escaped from Southeast Crater.

On the night of 10 June Bocca Nuova exhibited two strongly glowing vents on the crater floor, each producing mild Strombolian explosions every 5-10 seconds and ejecting material to heights of a few meters. Larger explosions took place about every minute, but no material was ejected above the crater rim. At La Voragine crater (the Chasm), a single glowing vent on the crater floor produced mild, audible, Strombolian explosions every 5-30 seconds that ejected material a few meters high.

At NEC Moss's group felt radiant heat and saw intense heat shimmering above radial fractures around the crater rim. Very strong gas emissions prompted them to wear gas masks. The crater was filled with dark (non-glowing) solidified lava; it formed a fractured dome from which a dense gas/water mixture escaped. No Strombolian activity was observed.

During 13-17 June, loud explosions were heard in the Valle del Bove, up to 3 km from the summit. Black ash clouds periodically rose 100 m above NEC.

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

Information Contacts: Marco Fulle, Osservatorio Astronomico, Via Tiepolo 11, I-34131 Trieste, Italy; J.L. Moss, Centre for Volcanic Research, Cheltenham & Gloucester College, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, United Kingdom; S.J. Saunders and V.A. Buck, Brunel University, Department of Geography & Earth Science, Borough Road, Isleworth, Middlesex TW7 5DU, United Kingdom.

Seismicity during May and June remained low, similar to previous months, and was characterized by fracture events centered 2-8 km NNE of the main crater, generally at 5-10 km depths. Surface degassing was still concentrated in craters, and fumaroles were located in the W sector of the active cone, mainly at the Chavas and La Joya sites. Correlation spectrophotometer (COSPEC) data indicated that SO₂ was emitted around the volcano at rates of <100 tons/day. The deformation network did not show significant changes.

On 1 May, a M 1.6 event was felt. Moreover, on 20 May, an M 3.7 event occurred 10 km SE of the volcano that was felt by inhabitants of Pasto and in the epicentral zone.

In June, the coda magnitudes for seismic events had M < 1.7. High-frequency events with M < 2.3 were located near Rio Bobo, ~15 km SW of Galeras, at depths <14 km. In addition, during June the seismicity interpreted as related to dynamic fluids remained low, and the seismic network recorded 35 long-period events.

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

Information Contacts: Pablo Chamorro, INGEOMINAS Observatorio Vulcanologico y Sismologico de Pasto (OVP), A.A. 1795, San Juan de Pasto, Nariño, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

Seismicity remained above background in June and the first half of July, and was indicative of continued low-level Strombolian eruptive activity. Gas-and-ash explosions occurred about every 5-20 minutes, generating ash-and-steam plumes to an altitude of 500-3,000 m. Regular reports from KVERT (via AVO) resumed in June after funding problems in Russia halted communications in December 1994 (BGVN 19:11).

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Tom Miller, Alaska Volcano Observatory; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry.

During 26 May-22 July, seismicity remained at normal background levels. Gas and steam plumes rose 50-300 m above the crater and extended up to 15 km downwind. Regular reports from KVERT (via AVO) resumed in June after funding problems in Russia halted communications in December 1994 (BGVN 19:11).

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, 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; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

A March 1996 EDM survey of the active Cumbre Vieja rift volcano indicated no significant surface deformation since installation of the network in October 1994. The network contains 11 benchmarks (incorporating two Spanish survey triangulation pillars) and was measured using the infrared EDM method. Together with one 3-component seismic station NE of the main rift, the network provides the only current means of monitoring activity on the island.

The deformation network covers the area affected by faulting associated with the July 1949 eruption (figure 1), a zone where W-facing normal faults showed a maximum vertical displacement of ~4 m. The Cumbre Vieja ridge lies between the two 1949 eruptive centers (Duraznero and San Juan). Eyewitness accounts (Bonnelli, 1950) and detailed mapping of the eruptive products showed that during the 1949 eruption, fault displacements also had westward components with downslope movement on the volcano's flanks. La Palma is comparable in form and structure to other Canary Islands that have undergone large-scale slope failure. Steep topography, together with the prospect of a future magma intrusion, cause concern for the long-term stability of the Cumbre Vieja ridge.

Figure 1. Sketch map of the Cumbre Vieja rift volcano showing the distribution of benchmarks, vents, and faults associated with the July 1949 eruption at La Palma. Courtesy of J.L. Moss and W.J. McGuire.

The wedge-shaped island of La Palma contains two large volcanic centers. The northern one is cut by the massive Caldera Taburiente. The southern Cumbre Vieja rift volcano, oriented N-S, has been the site of historical eruptions recorded since the 15th century. An eruption from the S tip of La Palma in 1971 produced the Teneguia cinder cone. Fissure-fed eruptions from vents ~1 km S of the 1677 San Antonio cone produced lava flows that reached the SW coast.

Reference. Bonnelli, R., 1950, Contribucion al estudio de la erupcion del volcan del Nambroque o San Juan (Isla de la Palma), 24 de Junio - Agosto de 1949: Instituto Geografico y Catastral, Madrid, Spain.

Geologic Background. The 47-km-long wedge-shaped island of La Palma, the NW-most of the Canary Islands, is composed of two large volcanic centers. The older northern one is cut by the steep-walled Caldera Taburiente, one of several massive collapse scarps produced by edifice failure to the SW. On the south, the younger Cumbre Vieja volcano is one of the most active in the Canaries. The elongated volcano dates back to about 125,000 years ago and is oriented N-S. Eruptions during the past 7,000 years have formed abundant cinder cones and craters along the axis, producing fissure-fed lava flows that descend steeply to the sea. Eruptions recorded since the 15th century have produced mild explosive activity and lava flows that damaged populated areas. The southern tip of the island is mantled by a broad lava field emplaced during the 1677-1678 eruption. Lava flows also reached the sea in 1585, 1646, 1712, 1949, 1971, and 2021.

Information Contacts: J.L. Moss, W.J. McGuire, and S.J. Day, Center for Volcanic Research, Cheltenham & Gloucester College, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, United Kingdom; S.J. Saunders, Brunel University, Department of Geography & Earth Science, Borough Road, Isleworth, Middlesex TW7 5DU, United Kingdom; J-C. Carracedo, Estacion Volcanologica de las Canarias, Tigua Carretera de la Esperanza 3, Apartado de Correos 195, 38206 La Laguna, Tenerife, Canary Islands, Spain.

During June, white-gray or brown ash and vapor clouds emitted from Crater 2 rose to several hundred meters above the crater rim. Fine ash was mostly blown to the NW and occasionally to the SE and NE. Rumbling noises were heard throughout the month. During the first half of June, night glow was seen only on 5 June, but weak red glow was observed on most nights of the second half of the month. Weak to moderate projections of glowing lava fragments were observed on the nights of 17, 18, 19, and 21 June. Crater 3 remained quiet during June.

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: D. Lolok, and C. McKee, RVO.

The following report summarizes morphological changes in the summit crater seen in a series of visits between July 1995 and April 1996 (figures 38-41).

On 19 July 1995 a group of cones observed W of T5/T9 included T23, T20, T36, T35 (a small cone joined to the SE side of T20), and T34 (BGVN 20:10). The upper parts of T8, T14, and T15 were engulfed by younger flows. T37, a low mound at the base of the NW slope of T5/T9, was not active. Lavas escaped from small vents at the base of T36. At the same time two hornitos in the T36 cluster were emitting low sprays of gassy lava. Lava, coming probably from T23 and T36, had almost filled the depression W of T5/T9 described by Dawson in late 1993 (BGVN 17:10).

On 27 September 1995, Andy Johnson observed a slight growth of T37, which remained a broad open feature, 1.5 m tall (figure 38). T36 showed a more pointed summit, but it hadn't changed significantly. There was no other sign of activity. A visit on 17 October 1995 by Jorg Keller revealed that T37 had grown into a broad cone to half the height of T5/T9. Small clots of lava were erupted S of T37 (at T37S). T37S soon became taller that the rest of the cone.

Figure 38. Ol Doinyo Lengai crater looking SW across the crater floor, 27 September 1995. Sketched by C. Nyamweru from a photo by A. Johnson.

Burra Ami Gadiye visited on 21 November 1995 and noted no drastic change for T37, but reported that the N and S parts were distinguishable. T37S was clearly higher and darker than the N part, with fresh lava on its W slope. Activity was observed from a vent on T36 (figure 39). On 1 December 1995, Gadiye reported that continued growth of T37 had resulted in a large cone (T37S) and an open vent on a low dome (T37N). Lava flowing copiously and rapidly N traveled past T15. The source, located W of T5/T9, was probably at the base of T37S.

During a 15-19 December 1995 visit, C. Oppenheimer and P. Vetsch observed emission of small amounts of carbonatitic lava coming from the cone cluster T36 (BGVN 20:11/12). On 18 December the E-flank of T37S collapsed followed by a NE-directed lava flow that covered a large part of the F35 flow and surrounded the E slope of T5/T9. Continued activity at T37S built a large cone with a vent high on its SE slope (figure 40). T37N remained small and formed a partial cone on the W side of an open vent.

Figure 39. Ol Doinyo Lengai crater looking E across the crater floor, 21 November 1995. Sketched by C. Nyamweru from a photo by B.A. Gadiye.

Figure 40. Ol Doinyo Lengai crater looking NNW from SE rim, 19 December 1995. Sketched by C. Nyamweru from a photo by B.A. Gadiye.

On 20 December Iris Saxer saw that the vent on the SE slope of T37S had been filled with lava and built a rim of spatter on its lip. This cone became almost as high as T5/T9 but much broader. A small summit vent was observed emitting puffs of steam and occasional clots of lava.

On 18 January 1996, H. Schabel observed small amounts of pahoehoe lava flowing N from vents near the base of T37S; the lavas surrounded T15. Few changes were noted in the shape of T37S but lava still spattered from its summit vent.

On 8 February, B. Saunders found that the top of T37S had collapsed (figure 41). Several flows originating from T37 had gone N and NE to reach the base of the crater wall. Some of these flows had crossed F35, around the E side of T5/T9. Other flows had reached farther E to separate F35 from the base of the E crater wall. Bubbling lava issued from a small vent at the SE base of T37S.

Figure 41. Ol Doinyo Lengai crater looking SW from the E rim, 8 February 1996. Sketched by C. Nyamweru from a photo by B. Saunders.

During 4-6 April, J.M. Bardintzeff observed a lava platform on the E side of T37S and a lava lake with asymmetrical overhang facing W. At that time lava was escaping the lake to the S and SW. Saunders also noted that a vent on the W side of the T36 cluster was a source of intermittent lava spattering.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617, USA

Moderate earthquake swarms on 14 and 19-21 June triggered a "D" alert status (moderate unrest) for the caldera. The D-status for the second swarm expired at 0600 on 28 June. The activity associated with these swarms is common in the caldera and poses little or no threat. Focal depths of both swarms centered around 5-6 km, and there was no significant ground deformation.

The first swarm, which began on the morning of 14 June, was centered at the SW margin of the resurgent dome (near the Highway 203/395 junction 5 km E of Mammoth Lakes). It included 17 M > 2 earthquakes and more than 150 events large enough to be located (M >0.5). The two largest events in this swarm were M 3.0 and 3.3 earthquakes at 0943 and 2154, respectively. This swarm gradually died out early the next morning.

The second swarm, centered 5 km to the E along the SE margin of the resurgent dome, began late in the evening of 18 June and gradually tailed off through the early morning of 21 June. This swarm included a M 3.3 earthquake at 0513 on 19 June, and over 33 events of M >2. The number of M >0.5 events detected and located through the morning of 21 June exceeded 400. This swarm was centered 2-3 km N of the 29 March-10 April S-moat swarm, which included three M > 4 earthquakes (BGVN 21:04).

This 17 x 32 km caldera formed about 730,000 years ago as a result of the Bishop Tuff eruption. Resurgent doming was followed by eruptions until ~50,000 years ago. Since then the caldera has remained thermally active, and in recent years, has undergone significant deformation. Although distinct from Long Valley Caldera, both Inyo Craters and Mammoth Mountain sit adjacent to it.

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

Low-level activity persisted during June as in the previous months (BGVN 21:04 and 21:05). Both craters gently released white vapor with occasional whitish gray ash clouds from Southern Crater. There were no audible noises or night glow from either crater. Seismicity was low during the first half of June with daily totals of 350-850 small low-frequency earthquakes. Seismicity increased to 1,090-1,700 events per day after 17 June.

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

Information Contacts: D. Lolok, and C. McKee, RVO.

The following report describes preliminary results of investigations on the eruptive activity that began on 28 February along the Gorda Ridge (BGVN 21:02). On 10-11 March 1996 NOAA's RV MacArthur carried out a series of conductivity-temperature-depth (CTD) casts to study the plume(s) activity.

Figure 2 shows a N-S cross section of the temperature anomaly through the event plume(s) discovered above the ridge. The temperature anomaly was defined as the increase of the water temperature above that expected for a given density horizon. The contours are based on five vertical casts evenly spaced between 42°36' and 42°43'N. A similar pattern was observed on an E-W transect. Since previous event plumes were characterized by symmetry about a central core, the structure in this anomaly suggested an agglomeration of two or more separate plumes. The events that caused the plumes could have been separated in space and time or both.

Figure 2. N-S cross section of the temperature anomaly through the Gorda Ridge event plume(s), 18 March 1996. Courtesy of E. Baker, NOAA/PMEL.

Water samples taken from station 7 (42°37.9'N, 126°47.8'W) in mid-March showed a very high enrichment in Helium-3 (figure 3). The presence of this isotope, enriched in fresh oceanic volcanic rocks and in submarine hydrothermal fluids, suggested a hydrothermal input into the ocean. Station 7 was also characterized by an increase in temperature and in suspended particles (nephels).

Figure 3. Concentration of Helium-3 for station 7 along the Gorda Ridge. Courtesy of J. Lupton, R. Greene, L. Evans, and R. Kovar NOAA/PMEL.

The plot of Helium-3 concentration versus temperature anomaly at stations 7 and 13 (42°45.7'N, 126°44.8'W) suggested that each had sampled water columns with different plume characteristics (figure 4). The Helium-3/heat trend for station 7 (the event plume) had a flat slope of 0.34 x 10-12 cm³/cal, similar to other event plumes detected in 1986 and 1993 over the Juan de Fuca Ridge. In contrast, the plume detected at station 13 had a much higher helium-3/heat trend of 2.27 x 10-12 cm³/cal.

Figure 4. Plot of Helium-3 concentration vs. temperature difference for stations 5 and 7 along the Gorda Ridge. The y-axis shows 3He concentrations in units of cubic centimeters of gas at standard temperature and pressure per gram of seawater. Courtesy of J. Lupton, R. Greene, L. Evans, and R. Kovar NOAA/PMEL.

SEM analysis of the first sample from the megaplume site at GR-14 revealed the presence of Fe-oxides, Zn-sulfides, and bacterial aggregates. The Fe-oxides were found with and without phosphorus. It was suggested that Fe-oxides formed beneath the seafloor lacked phosphorus, whereas Fe-oxides formed within the megaplume were enriched in phosphorus. The Zn-sulfides were very pure (i.e., no Fe). This sample appeared similar to the plume samples collected over the flow site in 1993. Preliminary results from the dissolved concentrations of Mn and Fe suggested that the event plume had formed recently.

In April the RV Wecoma surveyed a new lava flow with five camera tows. Figure 5 is a digital camera image that shows the contact between the new lava flow at the ridge and the surrounding older lava. The eruption site was at least 3.5 km long and only ~100-200 m wide, based mainly on the distribution of near-bottom temperature anomalies above the cooling flow. With this shape, the flow was clearly the product of a brief fissure eruption, where a dike reached the surface. The lava flow was located directly under the event plume mapped by CTD casts during the RV MacArthur cruise. Figure 6 shows the new lava flow, the camera tows, the CTD casts, the event plume(s), and the epicenters detected in the area since 28 February (BGVN 21:02).

Figure 5. Contact between the new lava flow and the surrounding older lava. The image was taken by a Benthos digital camera developed for Woods Hole Oceanographic Institution (Dan Fornari, P.I.) (scale of image was undisclosed).

Figure 6. Map showing an overview of the investigated area with the general location of the event plume. Courtesy of B. Embley and B. Chadwick. The T-wave epicenters appear as triangles; the RV McArthur CTD casts are shown as crosses.

Geologic Background. The northernmost of five segments of the Gorda Ridge lies immediately south of the Blanco Transform Fault that offsets the Gorda and Juan de Fuca oceanic spreading ridges. The 65-km-long segment is located about 200 km W of the southern Oregon coast and has deep 5- 10-km-wide valleys at either ends with a shallower narrow axial valley at the center. This morphology, which in plan view resembles an hourglass, is typical of magmatically active spreading segments. A submarine lava flow was erupted in late February and early March 1996, near the center of the segment. The eruption was initially detected through acoustic T-waves from a seismic swarm and the emission of large thermal plumes. In April submarine cameras revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. A seismic swarm of uncertain origin also occurred at this location in January 1998.

Information Contacts: Chris Fox, Bob Dziak, Bob Embley, Bill Chadwick, Ed Baker, John Lupton, Dick Feely, and Gary Massoth, NOAA Pacific Marine Environmental Laboratory, 2115 SE Osu Drive, Newport, OR 97365 USA (URL: https://www.pmel.noaa.gov/eoi/).

During June the lake at Poás had a greenish-turquoise color, a temperature of 44°C, and bubbled constantly from points on its S, SW, and W sides. A gas plume rose to between 400 and 500 m. Small rockfalls continued along the crater's N and W walls and gas escape rates in the latter area appeared low. The main source of fumarolic discharge came from the pyroclastic cone; at an accessible point on this region the temperature measured 94°C. Fumaroles on the SE, S, and SW walls maintained temperatures of 90-95°C.

OVSCICORI-UNA reported that June seismicity totalled 2,043 events, chiefly of low frequency. During June, totals for medium- and high-frequency events were 115 and 11, respectively. The low-frequency events occurred 20-157 times/day, with the highest number of events at mid-month. The high- and medium-frequency events also both peaked mid-month (with daily highs of about 3 and 13 events, respectively). The peak in high-frequency daily events took place on the 15th and coincided with the appearance of new fumaroles in the active crater.

Mauricio Mora (UCR) described six months of seismicity at Poás (table 7). A seismic peak in January was characterized by harmonic tremor and low-frequency events (both below 2 Hz). Tremor decreased during the first half of the year but a small peak in seismicity appeared in June. Mora also discussed January-June activity in the three primary areas of fumaroles, including: 1) the S crater wall, which appeared to be growing SW (91°C); 2) the dome and crater's S border, which issued plumes to 30 m height (~90°C); and 3) the crater's W border, which in March was covered by a landslide of hydrothermally altered wall rock.

Table 7. Number of low-frequency and A-type seismic events at Poás (recorded at station VPS2, 1 km SW of the active crater), January-June 1996. Courtesy of Mauricio Mora.

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Mauricio Mora F., Sección de Sismologia, Volcanologia y Exploración Geofisica, Escuala Centroamericana de Geología, Universidad de Costa Rica, Apdo. 35-2060, San José, Costa Rica

From December to April, Tavurvur continued fairly steady eruptive activity. During May emissions became more variable. In June Tavurvur's summit emissions waned, stopped for several days, then resumed.

On 1-2 June Tavurvur produced weak-to-moderate explosions at irregular intervals, but these became more frequent on 4-5 June. The explosions generated pale- to dark-gray ash-and-vapor clouds that rose 400-2,000 m above the crater rim. The clouds blew N and NW and produced ashfalls. Between 3 and 5 June, explosions were accompanied by roaring noises.

On 5 June, the number of earthquakes dropped rapidly, reaching the lowest level in six months and remaining low until 29 June (figure 27). During 5-29 June observers saw white- to pale-gray vapor clouds rising ~1 km; these clouds blew N, NW, and W. Moderate explosions 1-2 times/day caused regular warnings to aircraft in the vicinity. For example, on 10 June a SIGMET from Port Moresby noted volcanic ash to below 5,600 m altitude. For a period of six days (11-16 June), no emissions took place from the summit, but explosions resumed on 17 June.

Figure 27. Rabaul seismicity for the period November 1995-June 1996. Courtesy of RVO.

Ground deformation changes were low for the first half of June. After 13 June (figure 28), however, rapid radial inflation (to the N of Tavurvur) was recorded by the Matupit (MPT) electronic tiltmeter 2 km W of Tavurvur. Inflation was also recorded by a water-tube tiltmeter at Sulphur Creek (3.3 km NW). Sea-shore leveling measurements near Tavurvur have showed slow uplift since September 1995. The total inflation recorded at MPT was >52 µrad. Forty-two µrad were recorded in four days, the highest recorded rate since electronic tiltmeters were installed in October 1994.

Figure 28. Inflationary tilt at Rabaul in June 1996, recorded by the Matupit (MPT) electronic tiltmeter 2 km W of Tavurvur. Courtesy of RVO.

Following the rapid inflation indicated by the electronic tiltmeter, it was expected that a higher level of eruptive activity would commence. It did so later in the month when activity similar to that in the past six months resumed. Frequent explosions sent ash clouds to 400-1,000 m above the crater rim, some accompanied by loud roaring noises. These explosions continued through the end of June.

Seismicity in June consisted of 1,575 explosion earthquakes, 10 volcanic tremors, and 11 high-frequency earthquakes. The number of explosion earthquakes was the lowest since December 1995. Four high-frequency earthquakes were located to the NE of the caldera, four struck in the W part of the area delineated by the pre-1994 caldera seismicity, and the rest were scattered elsewhere.

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

Information Contacts: D. Lolok, and C. McKee, Rabaul Volcano Observatory (RVO), P.O. Box 385, Rabaul, Papua New Guinea; Jim Lynch, NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

The local seismic station (RIN3, located 5 km SW of the active crater) registered a total of 50 events, a 6-fold increase over any previous month in 1996. These events were only detected at this seismic station.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Variable intensity eruptions continued at Ruapehu. Although during 15-16 June volcanic tremor reached the highest levels seen in the past six months (BGVN 21:05), tremor dropped to background levels early on 20 June. Tremor and local earthquakes both remained low until 26 June when they increased slightly. A more definite increase in the intensity of seismicity took place on 27 June. Ruapehu then began larger discrete eruptions (around 0400) followed by ash-bearing eruptions (around 1000).

Airborne observers saw the volcano at 1120-1140 on 27 June and reported weak emissions that rose ~100 m above the crater. There appeared to be two source vents in an area N of the former crater lake's outlet area (on the crater's E side). The westernmost vent produced vivid white fumes; the easternmost vent produced dark gray ash. Ashfall had accumulated on Mitre Peak. Variable eruptions continued and after 1310 they grew larger. A ~6-km-tall ash column developed and ash fell to the E and SE.

The eruption continued the next day (the 28th), but appeared in conjunction with less seismicity and as a relatively gas-rich plume carrying minor ash. Tremor then was described as 5-10% of that recorded during the mid- June peak. Later, at 1144 on 28 June, a dark gray plume rose to over 600 m. Associated seismicity at the Dome station was weak. Weak to moderate ash emissions were detected 100-150 km downwind. Concern was raised about the mobility of near-source ash deposits.

A substantial amount of SO₂ escaped from the volcano. Correlation spectrometer (COSPEC) measurements of SO₂ flux rose from 4,100 metric tons/day (t/d) on 19 June to 5,100 t/d on 28 June (revised from an original estimate of 3,800 t/d), and remained high on 10 July at 6,000 ± 1,500 t/d.

Predominantly gas-rich plumes were seen from the ground on 1 July. Low seismicity, similar to that of 28 June, prevailed until 0400 on 2 July. At that time there were increases in both low-amplitude, high-frequency emergent events (the sort previously correlated with small ash eruptions) and larger high-frequency impulsive (A-type) events (interpreted as located at shallow depths beneath the summit). Bad weather on 2 July prevented detailed ground observations, but commercial aircraft reported ash-bearing plumes rising up to 600 m above the summit and blowing E.

The next day (2 July) white plumes were seen to 300 m above the summit. The day after (4 July) pilots reported a dark ash plume at ~3 km altitude extending 40 km downwind. At 1520 on 4 July, clouds cleared from the summit and observers saw a dark ash column rising up to 300 m above the summit; it deposited ash on snow in the ski area. On the Dome seismograph a large number of impulsive seismic events (larger than seen in the past few days) appeared to coincide with the ash column.

A report on 5 July stated that only relatively minor emissions had taken place since the last moderate-sized ash eruption (the event of 27 June). Intermittent minor eruptions still continued on 5 and 6 July, but on the later day, calm weather conditions allowed relatively high ash-bearing columns to develop.

An interval of increased seismicity, reaching levels seen in mid-July, began at 2030 on 7 July and preceded discrete inferred eruptions at 0115-0730 on 8 July. These eruptions took place at 2 to 3 minute intervals. An hour-long, intense burst of shallow seismicity ending about 0830 was followed by another interval of discrete explosions that diminished after about 1300. A helicopter flight at 0950-1030 documented an ash-poor plume to about 4.6 km altitude; an afternoon flight at 1330 also took place. Together, these flights confirmed that Strombolian eruptions ejected lava bombs up to 500 m above the vent.

Although seismicity dropped and then fluctuated in the last hours of 8 July, it resumed for intervals on 9 July. Eruptions continued on 9 July but bad weather and decreased, typically low seismicity prevailed during most of the next week.

At 1435-1735 on 16 July the volcano discharged an ash column to over 6-km altitude. Other eruptions followed; from 1735 to 0516 the next day there were 15 large explosive events, most with well-recorded air-wave phases. Tremor of variable amplitude and several hours of elevated seismicity prevailed. Aviation reports noted plumes at 3.3-4.6 km.

On 17 [July] seismicity dropped beginning at about 0330 and remained low for the next several days. Despite the lull in seismicity, in the afternoon on 18 and 19 [July] plumes rose 100-300 m above the summit.

On 20 July Ruapehu produced the largest outbursts since those in mid-June. Early on 20 July the seismicity and tremor again increased significantly; by about 0700 that day the tremor was replaced by small eruption earthquakes with airwave signals. Although pilots noted ash rising to 6.1-7.5 km altitude, some sustained moderate eruptions rose as high as 10.7 km and included bombs thrown 1.4 km above the crater and fire fountaining. Continuing through the next day, the seismic signals included both episodes with 2 to 7 discrete events per hour and episodes with continuous tremor and fewer discrete events. On 21 July the eruption's E-blown plumes, which were brown in color and thought to contain ash but not be ash-rich, ascended as high as 4.3 km.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: Colin Wilson, B.J. Scott, B.F. Houghton, C.J. Bryan, S. Sherburn, T. Thordarson, and I. Nairn, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.

A pilot report from Qantas Airlines on 5 May noted activity around 1800 generating a plume to ~10.5 km altitude. Eruptive activity was again observed from the same flight at 1745 on 7 May, but the ash cloud was only ~1 km above the summit and drifting NE.

Based on a report from Japan Air Lines, another aviation notice of volcanic ash from Semeru was posted at 0335 on 11 May. The report indicated that ash extended ~300 m above the peak and was moving SE at 18.5 km/hour. Lauda Air reported a low-level ash cloud around 1,300 m altitude on the early morning of 12 May, and a Qantas Airlines flight observed periodic emissions later that evening that did not rise above 6 km altitude. Satellite imagery throughout 5-12 May showed no ash plumes.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801, Australia; Jim Lynch, NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

During the period of 26 May-22 July, seismicity remained at normal background levels. Gas and steam plumes were observed above the volcano, rising to heights of 50-300 m above the crater and extending 2-8 km downwind. Regular reports from KVERT (via AVO) resumed in June after funding problems in Russia halted communications in December 1994 (BGVN 19:11).

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: Tom Miller, Alaska Volcano Observatory (URL: https://www.avo.alaska.edu/); Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

The current eruption, which began on 18 July 1995 (BGVN 20:06), started extruding a lava dome on about 16 November 1995 (BGVN 20:11/12). What follows condenses Montserrat Volcano Observatory (MVO) Scientific Reports for the weeks ending 5 and 12 June and Daily Reports for the rest of June.

On 31 May, and 1 and 15 June, pyroclastic flows progressed several kilometers E; one to within 300 m of the sea. Associated plumes reached up to 3 km altitude. Persistent dome growth continued in June. Its talus filled the W moat, allowing rockfalls to begin escaping the crater, but none reached farther than 100 m beyond the rim. Summaries are included for visual observations, seismic observations, daily seismic event counts, and SO₂ flux (tables 5, 6, 7, and 8).

Table 5. Chronology of visual observations at Soufriere Hills, Montserrat, late May through early June 1996. Poor weather conditions often prevented observations. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Table 6. Chronology of seismically derived observations of volcanic activity at Soufriere Hills, Montserrat, 30 May through July 1996. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Table 7. Daily counts of seismic events at Soufriere Hills, Montserrat, 30 May-1 July 1996. The amount of tremor is described qualitatively (high-low) or using analysis from the Daily Reports (particularly after 12 June). Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Table 8. COSPEC measurements of SO₂ flux at Soufriere Hills. Courtesy of MVO.

During the week ending 5 June gas measurements using a Fourier Transform Infrared Spectrometer showed substantial errors (50-100%), but did establish that at ground level on the lower slopes of the volcano (excluding Upper Amersham), the ambient concentrations of HCl and SO₂ in the air were well below 100 ppb. The SO₂:HCl ratio was generally well below 1.0. The only exception to this, on 24 May, was when the measurement errors were large. The SO₂:HCl ratios could be used to make an argument about status of the magma chamber. Assuming that this ground-level SO₂:HCl ratio was the same as in the plume, then the low ratios measured would indicate a moderately degassed magma chamber.

During 6-12 June, ash generation was generally low and few ash clouds emerged from the crater area; seismically detected rockfalls decreased with respect to the previous week and although no swarm of hybrid earthquakes occurred as in the previous week, the abundance of long-period earthquakes did appear similar to the previous few weeks.

EDM deformation measurements often detected an overall shortening rate along survey lines of ~1 mm/day during the interval from the beginning of December through the end of April. During 1 May-4 June there was a shortening rate of 2.4 and 2.1 mm/day in the volcano's N region (White's and Long Ground respectively); however, the shortening rate subsequently returned to ~1 mm/day.

On 12 June it was noted that during the previous 7-day interval, a ~12 mm/day shortening rate occurred on the N line (Long Ground to Castle Peak). In contrast, during this interval the W and N triangles continued to show no changes in line length above the error of the method.

On 13 June the E triangle was remeasured; its previous measurement was on 11 June: the Long Ground to Castle Peak line shortened by 9 mm and the Whites to Castle Peak line shortened by 6 mm. On 15 June it was reported that the W triangle's line lengths had recently shortened by ~1 mm/day. On 19 June it was found that NE sector (White's and Long Ground to Castle Peak) line lengths shortened by 2.5 cm over 5 days; this result extended a 3-week trend of 3-5 mm/day shortening here. Despite these larger than typical deformations in the NE sector during June, during the same month it was reported that the tiltmeters at Long Ground had remained stable for the past 10 months.

Scientists also noted that by June considerable new dome lava and talus had piled against the crater's W wall. Still, the June EDM surveys failed to show corresponding movement in this portion of the older edifice.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).

No eruptive activity occurred during the first half of 1996, and a trend of declining seismicity since October 1995 continued. Thus far in 1996 monthly earthquake totals have been: January, 14; February, 13; March, 17; April, 16; May, 11; and June, 10 (figure 11). At 1651 on 21 February, an M 2.4 earthquake occurred ~4 km beneath the crater floor; this was followed by four locatable aftershocks and then by ~20 very small seismic events that resembled signals typical of rock or snow avalanches. These later events were shallow, apparently triggered by the M 2.4 earthquake that preceded them. Activity returned to normal within a few hours. Except for that on 21 February, the rest of the earthquakes from January to June did not exceed M 2.0. During the night of 9 June, a large seismic event from the volcano triggered an automated, 24-hour alarm system. The character of the signal suggested that the source was a rockfall from the crater wall. This interpretation was confirmed when a USGS crew working in the crater on 11 June observed a large fresh rockfall deposit that originated from the S crater wall. Rockfalls are a common occurrence in the crater during the summer, and generally do not indicate any increase in volcanic activity.

Figure 45. Plot of focal depth versus time for earthquakes at Mount St. Helens, July 1995-June 1996. Courtesy of the Cascades Volcano Observatory, U.S. Geological Survey.

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: Dan Dzurisin, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Steve Malone, Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

On 2 June, an aviation notice to airmen (NOTAM) indicated a volcanic ash cloud to 4,600 m emanating from Suwanose-jima. A second NOTAM at 1515 on 2 June noted that the ash cloud top was at 2,100 m. No discernible ash plume was evident in GMS satellite data from the Japanese Meteorological Agency (JMA) through 1732 on 2 June.

The Kagoshima Prefectural Government confirmed to JMA that emissions on 1-2 June caused ashfall. Ashfall was also observed on the island on 4 June.

The Sakura-jima Volcanological Observatory of Kyushu University reported that activity has continued at the same level since 1994, with nearly constant A-type earthquakes. Ash emissions have occurred this year on 10-13 January, 23 February, 5-6 March, and 14 April. The eruption column in March rose 500 m above the volcano.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Jim Lynch, NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Volcanological Division, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

After 23 May Turrialba's seismic station (VTU, located 0.5 km E of the active crater) registered a sudden increase in microseismicity. During the first four months of 1996 nearly no events were registered. In late May there were over 50 events; in June, 246 events. The dominant frequency of these events varied between 2.5 and 4.0 Hz.

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: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

The low-level activity of recent months persisted through June. Emissions consisted mainly of small to moderate volumes of white vapor. Seismic recording resumed on 28 June and showed low-level seismicity.

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

Information Contacts: D. Lolok, and C. McKee, RVO.

The Somma-Vesuvius volcanic complex is a central composite volcano formed by an older stratovolcano (Monte Somma) with a summit caldera partially filled by the composite cone of Vesuvius. The most noted eruption, in 79 A.D., destroyed the ancient cities of Pompeii and Herculaneum. Since the explosive sub-Plinian eruption of 1631, Vesuvius has erupted with both Strombolian and mixed effusive-explosive styles. For the past three centuries the volcanic activity has mainly focused inside the Somma caldera but occasionally lava issued outside it (i.e., 1760 eruption). The last cycle of activity ended with the 1944 eruption. Since then, the volcano has been characterized by moderate seismicity and intra-crater fumarolic activity.

The Osservatorio Vesuviano maintains an array of short-period seismographs (eight three-component and nine vertical-component instruments). Seismicity was monitored during 1995 and March-April 1996. The 1995-96 period was the most active of the past fifty years. Several hundred microearthqukes (M < 3.2) were recorded during 1995, many from sources within the volcanic edifice above sea level. An increase in strain release and in frequency of earthquakes was observed from August to October 1995. During this period 217 events were recorded. Three of these earthquakes had M > 3.0 and were felt by the local population (~600,000 people): the first event (M 3.1, focal depth 3.1 km) occurred on 2 August; the second (M 3.2, focal depth 4.2 km) on 16 September, and the third (M 3.1, focal depth 3.3 km) on 24 September.

During November 1995-February 1996 the seismicity decreased to less than 10 events/month, and the strongest earthquake was M 2.5. During March-April 1996, however, 296 earthquakes were recorded. A M 3.4 event at 2 km depth was the strongest recorded during the past fifty years. Seismic activity at Vesuvius decreased again after this event.

Hypocenter locations for the past two years have clustered in a small volume below the crater area, no deeper than 6 km below sea level (figure 1). Focal mechanisms of relevant events suggested that the cause of seismicity was crustal rupture. Harmonic tremor and monochromatic low-frequency events were not observed. No changes in ground deformation or fumarolic gas compositions were reported.

Figure 1. Locations of seismic events at Vesuvius (1995-May 1996). All events have at least five P and one S picks (RMS less than or equal to 0.15 seconds). Bars in cross sections represent the errors on focal depths. Circle dimensions are proportional to event magnitudes.

A Reuters news story noted that a geophysical experiment is planned at the end of June to obtain a tomographic image of the volcano. The report said that the experiment, a joint-venture of Swiss, French, and Italian scientists, includes a series of controlled explosions at 14 boreholes on the volcano's slopes and as far away as the Sorrento peninsula. The explosions will be monitored by a network of 250 seismic stations. In addition, a marine seismic prospecting survey will be carried out in the Bay of Naples to investigate the volcano's submarine flanks.

Geologic Background. One of the world's most noted volcanoes, Vesuvius (Vesuvio) forms a dramatic backdrop to the Bay of Naples. The active cone was constructed within a large caldera of the older Monte Somma edifice, thought to have formed incrementally beginning about 17,000 years ago. The Monte Somma caldera wall has channeled lava flows and pyroclastic flows primarily to the south and west. Eight major explosive eruptions have taken place in the last 17,000 years, often accompanied by large pyroclastic flows and surges, such as during the 79 CE Pompeii eruption. Intermittent eruptions since 79 CE were followed by a period of frequent long-term explosive and effusive eruptions between 1631 and 1944. The large 1631 eruption produced pyroclastic flows that reached as far as the coast and caused great destruction. Many towns are located on the flanks, and several million people live within areas that could be affected by eruptions.

Information Contacts: Lucia Civetta, Francesca Bianco, Giuseppe Vilardo, and Mario Castellano, Osservatorio Vesuviano, Via Manzoni 249, 80123 Napoli, Italy; Paul Holmes, Reuters News Service.