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/).

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/).

Marapi in Sumatra, Indonesia, is a massive stratovolcano that rises 2 km above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera and trending ENE-WSW, with volcanism migrating to the west. Since the end of the 18th century, more than 50 eruptions, typically characterized by small-to-moderate explosive activity, have been recorded. The previous eruption consisted of two explosions during April-May 2018, which caused ashfall to the SE (BGVN 43:06). This report covers a new eruption during January-March 2023, which included explosive events and ash emissions, as reported by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and MAGMA Indonesia.

According to a press release issued by PVMBG and MAGMA Indonesia on 26 December, primary volcanic activity at Marapi consisted of white gas-and-steam puffs that rose 500-100 m above the summit during April-December 2022. On 25 December 2022 there was an increase in the number of deep volcanic earthquakes and summit inflation. White gas-and-steam emissions rose 80-158 m above the summit on 5 January. An explosive eruption began at 0611 on 7 January 2023, which generated white gas-and-steam emissions and gray ash emissions mixed with ejecta that rose 300 m above the summit and drifted SE (figure 10). According to ground observations, white-to-gray ash clouds during 0944-1034 rose 200-250 m above the summit and drifted SE and around 1451 emissions rose 200 m above the summit. Seismic signals indicated that eruptive events also occurred at 1135, 1144, 1230, 1715, and 1821, but no ash emissions were visually observed. On 8 January white-and-gray emissions rose 150-250 m above the summit that drifted E and SE. Seismic signals indicated eruptive events at 0447, 1038, and 1145, but again no ash emissions were visually observed on 8 January. White-to-gray ash plumes continued to be observed on clear weather days during 9-15, 18-21, 25, and 29-30 January, rising 100-1,000 m above the summit and drifted generally NE, SE, N, and E, based on ground observations (figure 11).

Figure 10. Webcam image of the start of the explosive eruption at Marapi at 0651 on 7 January 2023. White gas-and-steam emissions are visible to the left and gray ash emissions are visible on the right, drifting SE. Distinct ejecta was also visible mixed within the ash cloud. Courtesy of PVMBG and MAGMA Indonesia.

Figure 11. Webcam image showing thick, gray ash emissions rising 500 m above the summit of Marapi and drifting N and NE at 0953 on 11 January 2023. Courtesy of PVMBG and MAGMA Indonesia.

White-and-gray and brown emissions persisted in February, rising 50-500 m above the summit and drifting E, S, SW, N, NE, and W, though weather sometimes prevented clear views of the summit. An eruption at 1827 on 10 February produced a black ash plume that rose 400 m above the summit and drifted NE and E (figure 12). Similar activity was reported on clear weather days, with white gas-and-steam emissions rising 50 m above the summit on 9, 11-12, 20, and 27 March and drifted E, SE, SW, NE, E, and N. On 17 March white-and-gray emissions rose 400 m above the summit and drifted N and E.

Figure 12. Webcam image showing an eruptive event at 1829 on 10 February 2023 with an ash plume rising 400 m above the summit and drifting NE and E. Courtesy of PVMBG and MAGMA Indonesia.

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in 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, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1).

Kikai, located just S of the Ryukyu islands of Japan, contains a 19-km-wide mostly submarine caldera. The island of Satsuma Iwo Jima (also known as Satsuma-Iwo Jima and Tokara Iojima) is located at the NW caldera rim, as well as the island’s highest peak, Iodake. Its previous eruption period occurred on 6 October 2020 and was characterized by an explosion and thermal anomalies in the crater (BGVN 45:11). More recent activity has consisted of intermittent thermal activity and gas-and-steam plumes (BGVN 46:06). This report covers similar low-level activity including white gas-and-steam plumes, nighttime incandescence, seismicity, and discolored water during May 2021 through April 2023, using information from the Japan Meteorological Agency (JMA) and various satellite data. During this time, the Alert Level remained at a 2 (on a 5-level scale), according to JMA.

Activity was relatively low throughout the reporting period and has consisted of intermittent white gas-and-steam emissions that rose 200-1,400 m above the Iodake crater and nighttime incandescence was observed at the Iodake crater using a high-sensitivity surveillance camera. Each month, frequent volcanic earthquakes were detected, and sulfur dioxide masses were measured by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Mishima Village, and JMA (table 6).

Table 6. Summary of gas-and-steam plume heights, number of volcanic earthquakes detected, and amount of sulfur dioxide emissions in tons per day (t/d). Courtesy of JMA monthly reports.

Sentinel-2 satellite images show weak thermal anomalies at the Iodake crater on clear weather days, accompanied by white gas-and-steam emissions and occasional discolored water (figure 24). On 17 January 2022 JMA conducted an aerial overflight in cooperation with the Japan Maritime Self-Defense Force’s 1st Air Group, which confirmed a white gas-and-steam plume rising from the Iodake crater (figure 25). They also observed plumes from fumaroles rising from around the crater and on the E, SW, and N slopes. In addition, discolored water was reported near the coast around Iodake, which JMA stated was likely related to volcanic activity (figure 25). Similarly, an overflight taken on 11 January 2023 showed white gas-and-steam emissions rising from the Iodake crater, as well as discolored water that spread E from the coast around the island. On 14 February 2023 white fumaroles and discolored water were also captured during an overflight (figure 26).

Figure 24. Sentinel-2 satellite images of Satsuma Iwo Jima (Kikai) showing sets of visual (true color) and infrared (bands 12, 11, 8a) views on 7 December 2021 (top), 23 October 2022 (middle), and 11 January 2023 (bottom). Courtesy of Copernicus Browser.

Figure 25. Aerial image of Satsuma Iwo Jima (Kikai) showing a white gas-and-steam plume rising above the Iodake crater at 1119 on 17 January 2022. There was also green-yellow discolored water surrounding the coast of Mt. Iodake. Courtesy of JMSDF via JMA.

Figure 26. Aerial image of Satsuma Iwo Jima (Kikai) showing white gas-and-steam plumes rising above the Iodake crater on 14 February 2023. Green-yellow discolored water surrounded Mt. Iodake. Courtesy of JCG.

Geologic Background. Multiple eruption centers have exhibited recent activity at Kikai, a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake (or Iwo-dake) lava dome and Inamuradake scoria cone, as well as submarine lava domes. Recorded eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Satsuma-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Satsuma-Iojima.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/kaiikiDB/kaiyo30-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The current eruption at Lewotolok, in Indonesian’s Lesser Sunda Islands, began in late November 2020 and has included Strombolian explosions, occasional ash plumes, incandescent ejecta, intermittent thermal anomalies, and persistent white and white-and-gray emissions (BGVN 47:10). Similar activity continued during October 2022-April 2023, as described in this report based on information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

During most days in October 2022 white and white-gray emissions rose as high as 200-600 m above the summit. Webcam images often showed incandescence above the crater rim. At 0351 on 14 October, an explosion produced a dense ash plume that rose about 1.2 km above the summit and drifted SW (figure 43). After this event, activity subsided and remained low through the rest of the year, but with almost daily white emissions.

Figure 43. Webcam image of Lewotolok on 14 October 2022 showing a dense ash plume and incandescence above the crater. Courtesy of MAGMA Indonesia.

After more than two months of relative quiet, PVMBG reported that explosions at 0747 on 14 January 2023 and at 2055 on 16 January produced white-and-gray ash plumes that rose around 400 m above the summit and drifted E and SE (figure 44). During the latter half of January through April, almost daily white or white-and-gray emissions were observed rising 25-800 m above the summit, and nighttime webcam images often showed incandescent material being ejected above the summit crater. Strombolian activity was visible in webcam images at 2140 on 11 February, 0210 on 18 February, and during 22-28 March. Frequent hotspots were recorded by the MIROVA detection system starting in approximately the second week of March 2023 that progressively increased into April (figure 45).

Figure 44. Webcam image of an explosion at Lewotolok on 14 January 2023 ejecting a small ash plume along with white emissions. Courtesy of MAGMA Indonesia.

Figure 45. MIROVA Log Radiative Power graph of thermal anomalies detected by the VIIRS satellite instrument at Lewotolok’s summit crater for the year beginning 24 July 2022. Clusters of mostly low-power hotspots occurred during August-October 2022, followed by a gap of more than four months before persistent and progressively stronger anomalies began in early March 2023. Courtesy of MIROVA.

Explosions that produced dense ash plumes as high as 750 m above the summit were described in Volcano Observatory Notices for Aviation (VONA) at 0517, 1623, and 2016 on 22 March, at 1744 on 24 March, at 0103 on 26 March, at 0845 and 1604 on 27 March (figure 46), and at 0538 on 28 March. According to the Darwin VAAC, on 6 April another ash plume rose to 1.8 km altitude (about 370 m above the summit) and drifted N.

Figure 46. Webcam image of Lewotolok at 0847 on 27 March 2023 showing a dense ash plume from an explosion along with clouds and white emissions. Courtesy of MAGMA-Indonesia.

Sentinel-2 images over the previous year recorded thermal anomalies as well as the development of a lava flow that descended the NE flank beginning in June 2022 (figure 47). The volcano was often obscured by weather clouds, which also often hampered ground observations. Ash emissions were reported in March 2022 (BGVN 47:10), and clear imagery from 4 March 2022 showed recent lava flows confined to the crater, two thermal anomaly spots in the eastern part of the crater, and mainly white emissions from the SE. Thermal anomalies became stronger and more frequent in mid-May 2022, followed by strong Strombolian activity through June and July (BGVN 47:10); Sentinel-2 images on 2 June 2022 showed active lava flows within the crater and overflowing onto the NE flank. Clear images from 23 April 2023 (figure 47) show the extent of the cooled NE-flank lava flow, more extensive intra-crater flows, and two hotspots in slightly different locations compared to the previous March.

Figure 47. Sentinel-2 satellite images of Lewotolok showing sets of visual (true color) and infrared (bands 12, 11, 8a) views on 4 March 2022, 2 June 2022, and 23 April 2023. Courtesy of Copernicus Browser.

Geologic Background. The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano's high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

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

Barren Island is part of a N-S-trending volcanic arc extending between Sumatra and Burma (Myanmar). The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic flow and surge deposits. Eruptions dating back to 1787, have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast. Previous activity was detected during mid-May 2022, consisting of intermittent thermal activity. This report covers June 2022 through March 2023, which included strong thermal activity beginning in late December 2022, based on various satellite data.

Activity was relatively quiet during June through late December 2022 and mostly consisted of low-power thermal anomalies, based on the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph. During late December, a spike in both power and frequency of thermal anomalies was detected (figure 58). There was another pulse in thermal activity in mid-March, which consisted of more frequent and relatively strong anomalies.

Figure 58. Occasional thermal anomalies were detected during June through late December 2022 at Barren Island, but by late December through early January 2023, there was a marked increase in thermal activity, both in power and frequency, according to this MIROVA graph (Log Radiative Power). After this spike in activity, anomalies occurred at a more frequent rate. In late March, another pulse in activity was detected, although the power was not as strong as that initial spike during December-January. Courtesy of MIROVA.

The Suomi NPP/VIIRS sensor data showed five thermal alerts on 29 December 2022. The number of alerts increased to 19 on 30 December. According to the Darwin VAAC, ash plumes identified in satellite images captured at 2340 on 30 December and at 0050 on 31 December rose to 1.5 km altitude and drifted SW. The ash emissions dissipated by 0940. On 31 December, a large thermal anomaly was detected; based on a Sentinel-2 infrared satellite image, the anomaly was relatively strong and extended to the N (figure 59).

Figure 59. Thermal anomalies of varying intensities were visible in the crater of Barren Island on 31 December 2022 (top left), 15 January 2023 (top right), 24 February 2023 (bottom left), and 31 March 2023 (bottom right), as seen in these Sentinel-2 infrared satellite images. The anomalies on 31 December and 31 March were notably strong and extended to the N and N-S, respectively. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Thermal activity continued during January through March. Sentinel-2 infrared satellite data showed some thermal anomalies of varying intensity on clear weather days on 5, 10, 15, 20, and 30 January 2023, 9, 14, 19, and 24 February 2023, and 21, 26, and 31 March (figure 59). According to Suomi NPP/VIIRS sensor data, a total of 30 thermal anomalies were detected over 18 days on 2-3, 7, 9-14, 16-17, 20, 23, 25, and 28-31 January. The sensor data showed a total of six hotspots detected over six days on 1, 4-5, and 10-12 February. During March, a total of 33 hotspots were visible over 11 days on 20-31 March. Four MODVOLC thermal alerts were issued on 25, 27, and 29 March.

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

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

S. Chandra, Fiji Meteorological Service, noted that Air Pacific FJ440 bound for Auckland from Nadi (Viti Levu) reported sighting pumice ~220-330 km out of Nadi at about 1530 on 25 November 1992.

On 2 December 1994, Mike Green of the Fiji Meteorological Service reported that the pilot of a flight from Nadi to Melbourne saw what he believed to be pumice ~130 km SSW of Nadi on a bearing of 200°. A lesser amount of pumice was seen to the left of the flight path ~240 km from the airport. The plane had been scheduled to depart at 1145, placing these observations around noon. Reply-paid telegrams were sent on 6 December to postal agents at Ono-i-Lau (southernmost Lau Group), Qalikarua (Matuku), and Daviqele (W Kadavu), asking if any pumice had arrived within the last few weeks. No reply had been received by the Fiji Mineral Resources Dept by 9 December, so it was assumed that none was seen.

Although no historical volcanism has been reported near these observation sites, the area is close to a spreading center.

Geologic Background. Reports of floating pumice from an unknown source, hydroacoustic signals, or possible eruption plumes seen in satellite imagery.

Information Contacts: P. Rodda and G. Wheller, Mineral Resources Dept, Suva, Fiji.

Explosive volcanism continued through November 1994; it caused no damage and was lower than last month in both the number of eruptions and the mass of ash fall collected. There were 21 eruptions from Minami-dake crater, including eight explosive ones. The highest ash plume in November rose to 2,700 m (at 1435 on 10 November). Seismic swarms were registered at a seismic station 2.3 km NW of Minamidake cone between 1900 on 30 November and 0700 on 1 December; earthquakes for the month numbered 427. During November, the mass of ash fall collected [at KLMO], was 60 g/m².

Volcano monitoring at Kagoshima airport. Recent papers discussed the challenge of operating aircraft in vicinity of active volcanoes, including Sakura-jima (Onodera and Kamo, 1994; Casadevall, 1994). In Japan, 19 out of 83 volcanoes are actively steaming and under constant surveillance by JMA headquarters or local observatories; the other volcanoes are regularly patrolled by "Mobile Volcanic Observation Teams" based in four cities. These surveillance groups disseminate critical eruption information to relevant organizations, for example, Aviation Weather Service Centers, Air Traffic Control Centers, and airlines.

The key components of the Sakura-jima monitoring system consist of a seismometer for detecting earthquakes and an infrasonic microphone for detecting air shocks produced by explosive eruptions. An additional prediction system includes other instruments, such as water tube tiltmeters and extensometers. Even though the monitoring system can detect volcanic emissions nearly instantaneously, a time delay of at least a couple of minutes allows volcanological officers to confirm the responses of the monitoring equipment. This time delay also allows for time to edit and dispatch outgoing SIGMET or notification messages. In general, a SIGMET (Significant Meteorological Event) gets issued when the volcanic ash cloud reaches cruising flight elevation or higher.

While in general the several-minute time delay may not cause serious aviation safety problems, it may be crucial when aircraft are close to volcanoes, as at Sakura-jima. In considering problems such as these, the investigators developed a working model to quantify hazards. They expressed the relationship between magnitude of danger (D), eruption magnitude (M), volcano-aircraft distance (L), and a constant that may be affected by wind and related atmospheric conditions (k): D = kM/L.

Aircraft operations adjacent Sakura-jima. Figure 18 shows Kagoshima airport, at the S tip of Kyushu Island, sitting 22 km N of Sakura-jima's active crater. One of Japan's busiest airports, it has about 130 large transport takeoffs and landings a day; aircraft on the lowest category approach (ILS RWY34) pass a point 17 km NE of Sakura-jima's crater. Meanwhile, Sakura-jima produces over 100 explosive eruptions a year on average, but over 400 eruptions on some years (figure 19). Ash production has also been measured for the years 1978-93 (figure 20). It varied by a factor of about 5.5. At Sakura-jima there were 12 encounters between aircraft and volcanic ash during the years 1975-91 (Onodera and Kamo, 1994).

Figure 18. Sakura-jima airport showing both normal and ash avoidance air routes (top). More detailed map of the volcano and airport showing an air route and the JAL observation site (bottom). Courtesy of Onadera, Iguchi, and Ishihara.

Figure 19. Annual number of explosions and mass of ashfall from Sakura-jima (1978-1993, with 1994 annual total up to July also shown). Courtesy of Onadera, Iguchi, and Ishihara.

Figure 20. Annual number of explosions from Sakura-jima (1955 to July 1994). Arrows indicate small pyroclastic-flow episodes. Courtesy of Onadera and others (1994).

References. Onodera, S., Iguchi, M., and Ishihara, K., Recent advances in Japan, Volcano monitoring system of Japan Airlines at Kagoshima Airport: 9th Annual International Oceanic Airspace Conference, 9 November 1994.

Casadevall, T.J., 1994, Volcanic ash and aviation safety: Proceedings of the first International Symposium on Volcanic Ash and Aviation Safety, July 1991, Seattle, Washington, USGS Bulletin 2047, 450 p.

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: JMA; S. Onodera, Japan Airlines; K. Kamo, M. Iguchi, and K. Ishihara, Sakurajima Volcano Observatory, Kyoto Univ.

Strombolian eruptions and lava output from Crater C continued in November with columns reaching as high as 1 km above the Crater. OVSICORI reported that during 1994 the following accumulated deflations took place: a) the W-flank leveling line, 7.8 µrad; b) the inclination network, 7.7 µrad; and c) the distance network, 28.6 and 18.5 ppm (SW- and S-flanks, respectively). ICE reported that seismicity for November 1994 was comparatively low (table 8).

Table 8. ICE reported seismicity for Arenal, fall 1994. Their seismometer sits 1.5 km from Crater C. * November seismicity extrapolated based on 15 days of data. Courtesy of G. Soto.

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: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE.

A photograph taken from the Space Shuttle in mid-November 1994 showed a possible steam plume originating from the summit of Arjuno (figure 2).

Figure 2. This is a striking, oblique view to the south of the Indonesian islands of Java (right), Bali and Lombok (upper left). The linear array of dark regions across the photo is a chain of volcanoes. Plumes of steam can be seen rising from the summits of Arjuno (eastern Java, near the center of the photo) and Merapi (central Java, near the right of the photo). The region appears hazy due to an extended drought over Indonesia, New Guinea, and Australia resulting in huge fires and a regional smoke pall. NASA Photo ID: STS066-154-157. Approximate date: 14 November 1994.

Geologic Background. The Arjuno and Welirang volcanoes anchor the SE and NW ends, respectively, of a 6-km-long line of volcanic cones and craters. The complex overlies most of the Gunung Ringgit edifice, whose summit is about 3 km NE from the main ridge. Pyroclastic cones are located on the north flank of Gunung Welirang and along an E-W line cutting across the southern side of Gunung Arjuno that extends to the lower SE flank. Fumarolic areas with sulfur deposition occur at several locations on Welirang.

Information Contacts: NASA JSC Digital Image Collection (URL: http://images.jsc.nasa.gov/).

During November, no eruptive activity took place at Crater 1. Water and gas ejection from a pool of water on the crater floor was observed on 5 days in November (specifically, 2, 3, 6, 27 and 28 November). Tremor amplitude registered at a seismic station 800 m W of the crater was not greater than 0.5 µm, but in December the amplitude began to rise.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

A phreatic eruption at 2043 on 27 November sent an ash plume 1.5 km high that drifted W and SW, causing ashfall in six villages, and was accompanied by 14 minutes of felt tremor. Following this event, PHIVOLCS declared the area within 4 km of the crater off-limits. A second ash explosion on 3 December at 2348 was accompanied by rumbling, but details are sketchy owing to heavy cloud cover. The third ash ejection, on 4 December, deposited traces of ash ~7 km downwind; no other observations were possible. The next day, another explosion at 1227 sent ash 1.5 km high that caused ashfall 5 km WSW and was noticed in two villages.

A phreatic explosion at 0650 on 12 December was also the strongest so far. The cauliflower-shaped eruption column, accompanied by a loud "pop," rose 3 km and deposited ash as far as 16 km SW. The main eruption column, light gray in color, rose vertically, and a smaller dark-gray surge cloud seemed to emanate from the base of the main eruption cloud. However, the runout was still within 4 km of the vent and no evacuation was recommended.

Five additional small explosions occurred through 28 December. Observations of an ash explosion at 0155 on 18 December was hampered by clouds, but was inferred from the seismogram and ash deposits at 5 villages, all SW of the volcano. A minor ash explosion at 0807 on 20 December produced an ash cloud not directly observed due to rain clouds, but ash fell ~7 km SW of the vent. A brief cloud break enabled volcanologists to make a COSPEC measurement of ~370 metric tons/day. At 1525 on 23 December, a slightly stronger ash ejection lasted 4 minutes, causing light ashfall in 6 villages, also in the SW. Light ashfall 7 km from the summit was noted again the next day following a 3-minute ash ejection at 2153 on 24 December. Ash output from a 7-minute eruption at 1253 on 27 December seemed to be larger than other events and spread to a wider area, despite calmer winds, depositing small amounts of ash in nine villages.

The onset of all ash emissions had a corresponding explosion-type earthquake recorded on the seismogram. This became diagnostic during heavy cloud cover when ash plumes could not be observed directly. Based on the earthquake amplitudes, the 27 November and 12 December events were the biggest explosions, although ash emission was greater on 27 December. In nearly each case, the ash deposit was <=2 mm thick at ~7 km downwind. Hazard maps had been prepared before the 27 November event. PHIVOLCS is planning to pull the telemetered seismic network installed on Mindoro for aftershock monitoring, and move it to Bulusan.

Geologic Background. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Information Contacts: R. Punongbayan, E. Corpuz, and E. Listanco, PHIVOLCS; Reuters.

The fumarole at 1,550 m elevation directly N of the crater, observed in January and April 1993, remained active in November 1994. The fumarole was located on a crescentic fault with a downthrow to the N, which is probably related to outward/downward movement on the N flank. Clouds obscured most of the fumarole sites during a crater visit in April 1994; those seen had not changed since 1993. A 20-point deformation survey network was installed from 13 November to 27 December 1994 to measure spreading rates (van Wyk de Vries and others, 1993). The network will also be used for general monitoring.

Reference. van Wyk de Vries, B., Brown, G.C., and Borgia, A., 1993, Spreading at Concepción volcano, Nicaragua (abs.), in EOS, Abstracts of the American Geophysical Union, 1993 Fall Meeting, San Francisco.

Geologic Background. Volcán Concepción is one of Nicaragua's highest and most active volcanoes. The symmetrical basaltic-to-dacitic stratovolcano forms the NW half of the dumbbell-shaped island of Ometepe in Lake Nicaragua and is connected to neighboring Madera volcano by a narrow isthmus. A steep-walled summit crater is 250 m deep and has a higher western rim. N-S-trending fractures on the flanks have produced chains of spatter cones, cinder cones, lava domes, and maars located on the NW, NE, SE, and southern sides extending in some cases down to Lake Nicaragua. Concepción was constructed above a basement of lake sediments, and the modern cone grew above a largely buried caldera, a small remnant of which forms a break in slope about halfway up the N flank. Frequent explosive eruptions during the past half century have increased the height of the summit significantly above that shown on current topographic maps and have kept the upper part of the volcano unvegetated.

Information Contacts: B. van Wyk de Vries, Open Univ; Pedro Hernandez, INETER.

Since 1974 several expeditions have been organized to evaluate the mass and energy transfer from the magma in the lava lake to the atmosphere. Results have been in the range of 3-230 tons/day (t/d) of SO₂. During this time, both the volcanic activity and the methods used to evaluate the gas output have changed. For the 1993-94 campaign both the COSPEC method and the SF₆ tracer-gas method were used. A bottle of SF₆ gas was driven into the crater and injected into the volcanic plume at a rate of 1.2 l/min. Seventeen sampling bottles installed downwind on the crater rim each sampled the plume for ~1 hour. Analyzing the SF₆ concentration in each bottle allowed calculation of the atmospheric transfer coefficient: equal to the ratio of the concentration in the flask to the source SF₆ flow rate. By analyzing the concentration of gas or aerosols collected at the same time and place it has been possible to determine the volcanic source output, assuming that the diffusion laws are the same for the artificial and the natural products.

The following results were obtained using the SF₆ method (in tons/day): S, 50-80; Cl, 150-240; F, 50-80; Pb, 0.35; Zn, 0.53; As, 0.009; Bi, 0.0011; Cd, 0.01; Mo, 0.003; Cu, 0.19; Au, 0.002. COSPEC results obtained from a distance gave a SO₂ flux of 120-150 t/d; an average of 60-75 t/d of sulfur.

CO was analyzed automatically during the cruise between Australia, Antarctica, and New Zealand, at the same time that samples were collected using a metallic cylinder on the crater rim and in the ice cave on the outer slopes of the volcano. The gas samples were analyzed using a trace analytical reduction gas detector connected with a gas chromatograph containing a 2-m molecular sieve column. Reduction gas detection occurs as a result of the passage of certain species through a heated bed of mercuric oxide (HgO); this method allows detection of reducing gases from the low parts per billion (ppb) to low percentages. The average concentration of CO varied between 80 and 120 ppb on the sea between Australia and Antarctica, but in the ice cave the CO concentration reached 152-456 ppb, and in the volcanic plume on the crater rim it reached 1,000-3,000 ppb.

Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. The summit has been modified by several generations of caldera formation. The glacier-covered volcano was erupting when first sighted in 1841 and has had an active lava lake in its summit crater since late 1972.

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

Information Contacts: R. Faivre-Pierret, Institut de Protection et de Surete Nucleaire, Grenoble, France; F. LeGuern, B. Bonsang, E. Demont, M. Le Cloarec, E. Nho, and B. Ardouin, CNRS Centre des Faibles Radioactivites, Gif sur Yvette, France.

. . . Galeras displayed weak seismicity and deformation during November. Both tremor and long-period screw-type events (monochromatic and with a slow coda decay) continued. In addition to these signals, earthquakes took place. Some were located in the volcano's W sector at superficial depths. Others were located on the NW flank 3.5-4 km from the crater at 2-3 km depth. A third group struck on the NE flank in an area activated on previous occasions. Tiltmeters showed no significant change during November.

Tremor on 4 November lasted for 16 minutes (starting at 1638), on 5 November, for 43 minutes (starting at 1942). Coincident with the tremor, increased rain fell and a rise in mud flows was noted on the Azufral river in the W sector.

Airborne observers flying over the main crater noted a migration and an increase in the release of fumarolic gases. The escaping gases had migrated toward the external western wall of the cone and they concentrated along a tangentially oriented crevice and in some key fumaroles of this area. Nevertheless, the monthly SO₂ measurements yielded low flux values for November.

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: INGEOMINAS, Pasto.

After the Paez earthquake (6 June 1994) tremor pulses began appearing on local seismic records. Such pulses were previously unseen since seismic monitoring began in 1986. On 7 August, a 75-minute interval of banded tremor took place over a 4-hour time span. On 27 September continuous tremor prevailed for up to 9.5 hours; the dominant frequency was in the 1-2 Hz range.

Geologic Background. Nevado del Huila, the highest peak in the Colombian Andes, is an elongated N-S-trending volcanic chain mantled by a glacier icecap. The andesitic-dacitic volcano was constructed within a 10-km-wide caldera. Volcanism at Nevado del Huila has produced six volcanic cones whose ages in general migrated from south to north. The high point of the complex is Pico Central. Two glacier-free lava domes lie at the southern end of the volcanic complex. The first historical activity was an explosive eruption in the mid-16th century. Long-term, persistent steam columns had risen from Pico Central prior to the next eruption in 2007, when explosive activity was accompanied by damaging mudflows.

Information Contacts: H. Cepeda, INGOMINAS, Popayan.

During November, Irazú produced [no explosions, but] was shaken by a seismic event. In the interval 7-18 November a seismic swarm took place during which the OVSICORI seismic station registered a total of 255 seismic events. There were 42 locatable events that fell on a 10-km-long segment of the NW- to SE-trending Irazú fault (figure 6).  The earthquakes ranged in magnitude, M 2.0-3.4, and some had focal depths of 27-29 km, though others had depths of <8 km. Similar alignments of epicenters have been seen on the fault since 1991. These epicenters suggest that the fault extends across Irazú. The seismic swarm terminated at 1337 on 18 November when a M 3.4 event occurred. Its epicenter fell 3 km SSE of the active crater. During this time, deformation detected via the inclinometer network failed to show significant changes. But, in contrast, around this time a leveling line 4 km S of the active crater did show a pulse of inflation: 32 µrad.

Figure 6. Irazú earthquake epicenters, 7-18 November 1994. Courtesy of OVSICORI-UNA.

ICE reported that at 2248 on 8 December there was a phreatic explosion vented from a well-established fumarole on the upper NW-flank. They suggested that based on the response of the seismic station in San José, the released energy was similar to a M 4.4 earthquake. They further suggested that the explosion traveled toward the NW and destroyed forest on the upper slopes of the Rio Sucio, down to 2,500 m elevation. Explosion-triggered landslides and mudflows also followed along that drainage, but no lives were lost due to the absence of inhabitants in that area. The ash was composed of particles that appeared to be hydrothermally altered lithic fragments. The ash distribution pattern trended W (at an azimuth of 250°) and reached <= 30 km from the vent.

After the 8 December explosion, several tectonic earthquakes took place adjacent to Irazú, the largest, M 3.2 (at 0519 on 14 December) had a focal depth of 7 km. The explosion was also followed by many low-frequency and tremor-like signals. These were possibly triggered by the explosion.

Geologic Background. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the main crater, which contains a small lake. The first well-documented eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE.

Observers in Adak . . . reported little activity during the first half of October, when clouds obscured Kanaga. Minor ash fall was noted 3-5 km S of the volcano on 12 October. A white steam cloud was observed from Adak the next day rising 1,200-1,500 m above the summit, and no new ash deposits were seen on the flanks of the volcano, covered by fresh snowfall. AVHRR satellite imagery on 13 October revealed a "hot spot" at the summit, but no eruption cloud was observed. During the following week, a white steam cloud rose 900-1,200 m above the summit. The volcano was obscured by cloudy weather conditions from 21 October through 25 November.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: AVO.

Although clouds obscured the volcano in early November, continuous tremor (maximum amplitude 0.1-0.3 Nm) was recorded, and 4-11 earthquakes/day were detected under the volcano except on 7 November, when 23 events occurred. On 10 November, a gas-and-steam plume seen from Kliuchi (30 km NNE) was directed ESE for ~1 km. An observer in Kliuchi saw a gas-and-steam plume on 12 November rising 1 km above the summit that extended ~10 km ENE. On 18 November, observers in Kozirevsk (50 km W) saw a gas-and-steam column rising 50 m above the summit crater. Seismicity on the 18th consisted of continuous tremor (maximum amplitude 0.24 µm), one weak deep earthquake, and 9 shallow events.

A moderate explosive eruption occurred beginning about 0400 on 23 November, based on interpretations of seismicity. The volcano was completely obscured by clouds, but as much as 0.5 mm of ash fell in Kliuchi. Thirteen strong and shallow earthquakes beneath the volcano between 0400 and 1200 had maximum amplitudes of 14.25 µm at a seismic station 14 km from the volcano, and were recorded at stations up to 70 km away; persistent volcanic tremor had a maximum amplitude of ~0.33 µm. Comparing the seismicity to that of 30 September-1 October, the ash plume may have reached an altitude of ~7 km.

On 24 November, observers in Kliuchi noted a vigorous gas-and-steam plume containing minor ash rising 1 km above the volcano and extending >30 km NE. Weak volcanic tremor (amplitude ~0.15 µm) and 22 shallow earthquakes were registered beneath the crater area. The next day, observers in Kozirevsk reported a gas-and-steam plume above the volcano. Continuous tremor was recorded ~32 km from the volcano, and 12 shallow earthquakes were recorded beneath the crater area. On 28 November, a gas-and-steam plume seen from Kliuchi rose 2 km above summit and extended 3 km SW. A vigorous gas-and-steam plume of unknown height was also seen from Kliuchi on the 30th, continuous tremor (0.4 µm) was recorded 11 km away, and 73 shallow earthquakes were detected as far as 70 km away.

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: V. Kirianov, IVGG; AVO.

"Continuing the trend of previous months, eruptive activity consisted of moderate and intermittent Vulcanian explosions from Crater 2. During most of November, activity at Crater 2 consisted of noiseless emission of thin white vapour. Occasionally (on 4, 6-8, 15, 18, and 27-29 November), weak explosions were heard and accompanied the rise of dark-grey ash-laden columns to a few hundred meters above the crater. Some of these explosions were large enough to be recorded by a seismometer 9 km away. Fine ashfall was reported in downwind coastal areas. Between 14 and 27 November, weak night glow was seen and the activity was accompanied by low to loud rumblings. Crater 3 released only fumarolic emissions, occasionally accompanied by blue vapour."

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: B. Talai, R. Stewart, and P. de Saint-Ours, RVO.

Observations during 11-23 November revealed a plume of variable strength, indicating continuing instability, and the volcano was not climbed. The fumarole on the N rim was visible and appeared to be stronger than in February. A small phreatic eruption at 1720 on 13 November ejected a brownish column ~700 m above the crater which was then blown SE. This event was preceded by a weak, diffuse vapor plume which reached 300-500 m above the crater. Following the eruption, the plume gradually strengthened, reaching altitudes of 2-2.5 km above the summit . . . by 16 November (figure 23). The plume became more dense, yellowish to brownish in color, and pulsed, ejecting "ashy slugs" every 5-15 minutes. A second phreatic eruption observed at 1720 on 19 November emitted a dense white plume to 3 km above the crater. Although sheared by wind to the SE, it retained its form for ~20 minutes.

Figure 23. Plume altitudes and phreatic eruptions at Lascar, 11-23 November 1994. Courtesy of S. Matthews.

Similar activity was observed by Matthews in February, and was related to continuing collapse of the crater floor. In this interpretation, blockage of the degassing system leads to a weak plume and buildup of pressure beneath the crater floor. Periodic phreatic eruptions clear the conduit and allow the gas to vent freely, causing the plume to strengthen; the reason for the strong pulsing is not clear.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: S. Matthews, Univ of Bristol.

"During November, the background level of activity consisted of noiseless weak emissions of white and blue vapour, with weak glow at night. Two short eruptions occurred at South Crater in November. A lava flow was produced on 12-13 November and pyroclastic flows on the 28th.

"On the evening of the 10th, weak incandescent projections were seen just above the crater rim. Nothing could be seen on the 11th, although weak rumbling noises were heard. On the morning of the 12th, white-grey, ash-laden emissions were rising 600-700 m every 3-5 minutes. By night time, moderately strong Strombolian explosions accompanied a forceful dark-brown ash column rising 1-2 km above the crater, with loud rumbling and explosion sounds. Glowing lava fragments rolled down into the SE and SW valleys, and thick ashfall was reported in coastal areas on the ESE side of the island. Lava started to flow out of South Crater into the SE valley at 1900 on 12 November and the flow later stopped with the front at ~700 m elev. The strength of the eruption decreased after 0200 on the 13th, and for the next day and a half, the crater produced high, loud, bright explosions at progressively longer time intervals (from 1-15 minutes apart).

"Weak rumbling sounds and fluctuating glow were reported on the 25th. Intermittent (3-5 minute intervals) forceful emissions of dark ash-laden vapour, accompanied by weak-to-loud rumbling or explosion sounds, were noted on the 26th at 1730. Emissions became sub-continuous by 1900. A period of sub-Plinian activity with high projections of incandescent fragments lasted until the next morning. During 27-28 November, forceful dark emissions occurred at 1-2 minute intervals. The strength of the eruption seemed to increase again after 1030 on the 28th and there were pyroclastic flows in the SE valley at 1330. The eruption waned after ~0400 on the 29th, becoming intermittent, with forceful grey-brown explosions to 1-2 km above the crater and glowing lava fragments to 100-200 m. Unstable products around the vent tumbled into the SE and SW valleys as scoria avalanches.

"Main Crater activity was apparently unaffected by these eruptions. It continued to release white vapour in weak to moderate volumes throughout November. The water-tube tiltmeter at Tabele Observatory showed no significant deflection. No seismograph was operating."

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: B. Talai, R. Stewart, and P. de Saint-Ours, RVO.

When observed during November, the vent in Santiago crater was the same shape as in April 1994. It was possible to see ~20 m down into the hole, which was 10-20 m wide. During daylight a red glow could be seen from the lip of the vent inwards, but no lava or ejecta were observed. Pulses of gas emission occurred every 3-5 seconds.

Geologic Background. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: B. van Wyk de Vries, Open Univ; Pedro Hernandez, INETER.

The fumarole that has been active since at least 1986 continued to vent vapor in November and December 1993. A strong sulfur odor was detected even when the wind was blowing towards the fumarole. This observation led to the discovery of two other previously unreported fumarole fields (figure 1). Vapor was seen rising from both, but they were not approached closely; neither appeared to be a new feature.

Figure 1. Map of the Mombacho summit area, showing locations of reported and previously unreported fumarole areas. Courtesy of B. van Wyk de Vries and P. Hernandez.

Geologic Background. Mombacho is an andesitic and basaltic stratovolcano on the shores of Lake Nicaragua south of the city of Granada that has undergone edifice collapse on several occasions. Two large breached craters formed by edifice failure cut the summit on the NE and S flanks. The NE-flank scarp was the source of a large debris avalanche that produced an arcuate peninsula and a cluster of small islands (Las Isletas) in Lake Nicaragua. Two small, well-preserved cinder cones are located on the lower N flank. The only reported activity was in 1570, when a debris avalanche destroyed a village on the south side of the volcano. Although there were contemporary reports of an explosion, there is no direct evidence that the avalanche was accompanied by an eruption. Fumarolic fields and hot springs are found within the two collapse scarps and on the upper N flank.

Information Contacts: B. van Wyk de Vries, Open Univ; Pedro Hernandez, INETER.

Fumarolic activity continued at Poás in the re-established crater lake. OVSICORI reported the lake level remained the same in both October and November. ICE reported that due to heavy rains in November the lake had attained a diameter of ~220 m and its surface reached 8 m above the minimum level seen in August.

The turquoise-green colored lake hosted subaqueous fumarolic activity, leading to bubbling and minor phreatic eruption columns to 100 m height. In the NE part of the lake there existed a spot with sporadic phreatic eruptions. These reached 1-m height and had a dark-gray color. The area adjacent to the crater continues to recuperate from acidic conditions found earlier this year.

Results from the OVSICORI seismic system appear in table 6. The day of the month with the greatest number of seismic events was 7 November. Compared to earlier in 1994, the number of seismic events in November was low to moderate.

Deformation, measured by dry-tilt, failed to show significant changes in November. The four distance-measuring lines inside and across the active crater showed changes of less than 8 ppm in a deflationary direction. The two precision leveling lines at the summit changed less than 6 and 12 µrad. These leveling-line changes were interpreted as tending toward slow deflation after a brief pulse of inflation registered during the eruptive activity of August 1994.

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: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE.

A new episode of explosive activity began at Popocatépetl volcano on 21 December 1994 (figure 5). The eruption followed increases in seismicity, SO₂ flux, and fumarolic activity seen during the last 13 months. Although in the last year seismicity rose and fell several times, during late-October there was a sudden, prominent (roughly 1.6- to 10-fold) increase in daily earthquakes compared to previous months. Measurements of the volcano's total SO₂ flux were consistently large (some airborne measurements averaged over 1,000 tons/day). During October-November 1993 a cluster of steam vents in the summit crater produced clouds that reached 6,000 m elevation, several-hundred meters above the 5,465 m summit. These clouds sometimes stretched for 50 km.

Figure 5. Base map of Popocatépetl and vicinity (elevations taken from the 1986 México City 1:250,000 topographic sheet).

Eruptive activity. Near midnight on 22 December 1994, Servando De la Cruz sent the following report.

"The fumarolic activity that has been developing during the last two years or so culminated on early 21 December 1994, when a series of volcanic earthquakes, probably associated with phreatic explosions, marked the beginning of a new stage of eruptive activity. The seismic events, detected at 0131, 0132, 0138, 0140, and 0148, were very impulsive, high-frequency, short-duration signals, and were followed by a major, lower-frequency event at 0153. The events were recorded by four telemetric stations within 11 km of the volcano operated jointly by CENAPRED and the Institutes of Geophysics and Engineering of UNAM. As the day cleared an ash plume was observed for the first time in decades emerging from the volcano's crater. The ash emission was moderate and produced an almost horizontal plume causing a light ashfall over the city of Puebla, about 45 km ENE of the volcano's summit. A helicopter flight at 1030 showed that most of the ash issued from near the lower NE rim of the inclined crater. A radial fissure on the NE flank of the cone displayed some steam-producing vents, though the cloudy conditions make this interpretation equivocal. Old cracks in the glacier appeared to have extended a significant amount towards the W. A second flight at 1430 the same day revealed a substantial increase in ash production (about 3-4 times the amount observed in the morning). The light-gray ash appeared to be emitted episodically, with "puffs" every few minutes.

"The seismicity consisted of mostly low-amplitude B-type earthquakes and concurrent high-frequency A-type events. Though this seismicity remained lower than during night of 21 December, during the next day the seismicity again increased. At this stage and after several consultations between the scientific group and the Civil Protection authorities, an evacuation of the 19 most vulnerable towns and villages on the E sector of the volcano was started around 2100 of 21 December, and about 31,000 persons were moved during the night to shelters in safer areas. Since then the situation has remained fairly stable, though long-duration, low-amplitude tremors appeared in the night of 21-22 December, and continue."

Claus Siebe reported that climbers at Popocatépetl reached the summit, which lies along the W margin of the gaping summit crater's rim, both on the day before the eruption, and hours after the 21 December eruption started. On the day before the eruption visiting climbers could see the crater lake and sparse fumaroles. They reportedly heard no hissing sounds and they smelled less odor from sulfur-bearing gases than in previous months.

Curiously, the six volcanic earthquakes that took place between 0130 and 0200 on 21 December were not felt, and the presumably associated phreatic summit explosions were not heard by any of about 25 mountain climbers at Tlamacas, 4 km N of the summit (figure 6). The climbers, who said they started ascending the mountain around 0400 on 21 December, did not notice anything unusual until they neared the crater rim. Just prior to reaching the rim, a few minutes before 0800, climbers were stunned by what they thought was the sound of jet engines. At the crater rim they saw new bombs as large as 40 cm that had been thrown out of the 250-m-deep crater and had burrowed deep impact-pits in the snow. According to Siebe: "Most climbers who reached the summit that morning thought that the activity was normal, because they had never visited Popocatépetl before." At the summit, the climbers said they could not see the crater floor even though a strong wind was blowing. They descended back down the mountain without incident.

Siebe was at Tlamacas at 0900 on 21 December during clear weather. He observed a continuous ash plume rising 100-500 m above the crater with pulses at intervals of 1-5 minutes. The plume was carried at least 60 km E. Enough silt- and sand-sized material reached Puebla to produce a thin coating on cars. The ejecta appeared to be non-juvenile, and it contained pyrite, sulfur, and Ca-sulfate.

A report from Steve McNutt indicated that the volcano began to quiet down on the afternoon of 25 December. During the night of 27-28 December a M 2 earthquake took place; for reference the largest prior event in the recent past was M 2.9. On 27 December tremor was barely perceptible and a few small low-frequency events took place. During the 24-hour period ending about midday on 28 December there were ~30 low-frequency events. Tremor roughly doubled between 23 and 24 December, but then during 25-28 December it dropped and became barely detectible. No specific seismic data were available for dates after that, though seismicity did increase again and an audible explosion was heard roughly 10 km from the summit at about 1300 on 31 December. Investigators planned to install about four new seismic stations to improve spatial and azimuthal coverage, and to add one station close in.

By 27 December all but three of the previously evacuated towns had been reoccupied; those towns not reoccupied were subject to lahar hazard. A glaciologist made an initial helicopter inspection of the glacier looking especially for signs of abnormal melting. No report was available at the time of this publication, but steps to monitor the glacier included both a daily inspection flight and a video camera aimed at it from 5 km away. The last of the three previously evacuated towns was reoccupied by 28 December.

News reports. A 21 December Associated Press story said Popocatépetl, "spewed a column of roiling black ash Wednesday, dusting villages and farmland but causing no injuries" and that "television footage from traffic helicopters showed a dense column of ash belching from the summit."

As of 23 December, an Associated Press report noted that the Puebla state government said 75,000 people would be evacuated from the countryside around the volcano. Some other news reports put the number of evacuees at about 50,000. One of the evacuated towns, Santiago Xalitzintla, is located about 13 km NE of the summit. The town sits along the road over "Paso de Cortez," the pass between Popocatépetl and the adjacent Quaternary stratovolcano to the N, Iztaccihuatl (figure 6).

A 26 December United Press International news report noted that "Jorge Martinez Soto, a researcher at the Univ of Puebla, said the amount of smoke and ash being emitted from the volcano . . . diminished by about 75 percent since last week . . . ."

Plume imagery and transport modeling. Although the 21 December eruption plume may appear on satellite imagery, to our knowledge no investigator has yet announced having found it. There is an AVHRR (channel 1) image of a Popocatépetl plume on 22 December at 0818 (1418 GMT). That image shows a SE-directed plume tens of kilometers long. There are also three other AVHRR images for plumes on 26, 27, and 28 December. All four images are available via e-mail from Melissa Seymour. We learned of these images at press time and although we have not had time to see them first-hand and tabulate plume orientations, the imaged plumes reportedly trailed southward.

The Synoptic Analysis Branch (SAB) of NOAA/NESDIS first reported Popocatépetl activity at 1530 (2130 GMT) on 26 December for an eruption that took place at around 1300. A SIGMET (Significant Meteorological Event) notice was posted from México City announcing that a new eruption had taken place and that the plume from this eruption reached an altitude of about 6.7 km (22,000 feet). SAB later continued to describe the shape of the plume associated with this eruption based on GOES-7 and -8 data (table 2 and figure 6). A report later that day (26 December) indicated that the volcano had continued to erupt, creating a visible plume that at 1745 extended to 50 km E. At 0745 the next day (27 December), a GOES-8 visible satellite image of the plume suggested a gently curving, funnel-shaped mass tracking NE (figure 6). Based on the lack of infrared signatures and on their visible signatures, all the plumes reported in table 2 and figure 6 were thought to be of low density.

Table 2. Visible (GOES-7 and -8) satellite images reported for Popocatépetl. The time of initial eruption for all these plumes was around 1300 (1900 GMT) on 26 December. The third and fifth plumes listed are shown graphically on figure 6. Courtesy of SAB.

Figure 6. Popocatépetl ash plume at a) 0745 (1345 GMT) on 26 December 1994 (black) and b) 0815 (1415 GMT) on 28 December 1994 (stipple) as seen on satellite imagery. The northern edge of the longer plume just touched the Gulf Coast near Tampico. Courtesy of Nick Heffter.

A modeling program called "VAFTAD" was used to forecast the transport and dispersion of the plume from the 26 December eruption (see references and description of VAFTAD in the report for Rinjani, 19:06). VAFTAD produced a series of visual ash cloud forecasts such as those on figure 7, which showed the plume initially covering both quadrants in the E half of the volcano and then traveling NE along about the same path taken by actual plumes seen in the GOES imagery (table 2 and figure 6). The models forecasted that after about 24 hours the plume would travel NE over the Gulf of Mexico.

Figure 7. Examples of forecasts of the Popocatépetl plume after a large eruption. Both of these forecasts were for an initial erupted plume height of 7.6 km (25,000 feet) and an eruption duration of 24 hours. They both portray the elevation range from 6 to 10 km (20,000-35,000 feet). The forecasts were based on an eruption beginning at 1300 (1900 GMT) on 26 December. The map on the left shows the forecast plume 12 hours after the eruption began, the map on the right, 24 hours after the eruption began. Courtesy of Nick Heffter.

VAFTAD uses wind and pressure data updated twice daily on grids with spacings of 91 km in the USA and 1 degree over the rest of the globe. The model assumes the eruption delivers a mass load to the atmosphere. The mass load is not scaled to the actual mass of the eruption, but rather the load is assumed to be 1 gram (composed of spherical particles with a density of 2.5 x 10⁶ grams/m^-3 in a size range of 0.3-30 µm in diameter). VAFTAD computes transport and dispersion assuming particles are carried by advection both horizontally and vertically, diffuse with a bivariate normal distribution, and fall according to Stoke's law with a slip correction. Calculated ash concentrations have been correlated with satellite imagery for defining the visual ash cloud forecasts.

One noteworthy aspect of the Popocatépetl plumes is the relatively large height of the summit crater (elevation ~5,215 m). Even small, low-energy eruptions from this high altitude vent can erupt material to 6 km (~20,000 feet) elevation.

So in essence, these ash cloud forecasts serve best for hazards planning purposes. A key use, in fact, is to warn airline pilots of the airspace most likely to contain volcanic ash particles. Besides the other hazards discussed in Boudal and Robin (1989), a large eruption from Popocatépetl could affect air travel in routes over parts of NE México and much of the Gulf of Mexico.

Eruptive history. In the Holocene Popocatépetl has produced both effusive and pyroclastic activity. The latter has ranged from mild steam-and-ash emissions to Plinian eruptions accompanied by pyroclastic flows and surges. Vigorous Holocene explosive activity took place in three periods (in years before present, ybp): a) 10,000 to 8,000, b) 5,000 to 3,800, and c) 1,200 to present (Boudal and Robin, 1989). An effusive period from 3,800 to 1,200 ybp ended with a vigorous explosive eruption that both enlarged the summit crater and generated St. Vincent-type pyroclastic flows. Another large explosive eruption, about 1,000 ybp, produced pyroclastic flows that descended the N flank.

Historical eruptions depicted on Aztec codices date back to 1345 AD. About 30 eruptions have been reported since then, although documentation is poor. Most historical eruptions were apparently mild-to-moderate Vulcanian steam and ash emissions. Lava flows restricted to the summit area may also have occurred in historical time, but cannot be attributed to specific eruptions. Larger explosive eruptions, possibly Plinian in character, were recorded in 1519 and possibly 1663. The last significant activity took place from 1920-22. Then, intermittent explosive eruptions produced 6.6-km-tall columns and extruded a small lava plug onto the floor of the summit crater. Ash clouds were also reported in 1923-24, 1933, 1942-43, and 1947.

Reference. Boudal, C., and C. Robin, 1989, Volcan Popocatépetl: Recent eruptive history, and potential hazards and risks in future eruptions, IAVCEI Proceedings in Volcanology 1; J.H. Latter (Ed.), Volcanic Hazards, Springer-Verlag Berlin Heidelberg, pp. 110-128.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Servando de la Cruz-Reyna, Instituto de Geofísica, UNAM, Ciudad Universitaria; Claus Siebe, Instituto de Geofísica, UNAM, Coyoacán; Steve McNutt, Alaska Volcano Observatory, Univ. Alaska Fairbanks, USA; Melissa Seymour, LSU Earth Scan Lab, Coastal Studies Institute, USA; Nick Heffter, National Oceanic and Atmospheric Administration (NOAA), Air Resources Laboratory, USA; Jim Lynch, Synoptic Analysis Branch, NOAA/NESDIS, USA.

"The eruption . . . continued through November. Tavurvur exhibited moderate Vulcanian activity that declined slowly in strength, while Vulcan remained quiet. Vulcan exhibited only weak fumarolic activity from four small vents filled with bubbling water at the base of the new crater.

"Activity at Tavurvur consisted mainly of discrete explosive pulses. The ash content was generally low, producing a pale-grey emission column. The size of, and timing between, explosions was variable, but there was a general decline in activity during November. At the beginning of the month, explosions were 1-4 minutes apart and the emission columns rose forcefully to ~1.5 km. By the 6th, the intervals were 1-10 minutes and the crater was sometimes clear of emissions. Blue vapours were seen around the active vent at the bottom of a 50-m-high tephra cone. There were, however, large explosions on the 5th, 6th, and 9th which showered the flanks of Tavurvur with blocks and bombs, and produced a large billowing cloud up to 2 km high. From 9-19 November, emissions were mainly of white vapour with occasional explosion clouds up to 1 km. The eruption was mainly silent, except for rumbling and roaring noises on the 10th and 11th.

"The Tavurvur crater was never freely open during this phase of the eruption, but was clogged up with a mass of rubble, welded together and sometimes glowing. The dark ash-laden billowing clouds that suddenly rushed out of the vent every few minutes seemed to percolate through the rubble. A lava mound, 10 m in diameter and 2 m thick, formed over the vent on the 15th but was destroyed by a large explosion the next day. A new lava mound had formed by the 18th, this time 20 m across and 4 m thick, possibly consisting of two lobes and fractured into four main blocks. The intermittent ash-laden emissions were then hissing out from under the sides of the mound. Details of the crater could not be seen again until the 25th, when all traces of the lava mound had disappeared from the base of the bowl-shaped crater, presumably blown out by the large explosions heard at intervals of 1-4 hours on the 19th.

"From the 19th until the end of the month explosions were generally mild. Large explosions, however, occurred on 20-22, 26, and 29 November. At night, these explosions resulted in a shower of incandescent blocks on the flanks of the volcano. Sizeable blocks were occasionally found in the Talwat road that goes around the base of the cone.

"Seismic activity in the caldera was lower in November than in October. It was dominated by shallow explosive and low-frequency earthquakes associated with the eruptive activity at Tavurvur. RSAM amplitudes and event counts showed a marked decline between 29 October and 2 November (figure 22). Throughout the rest of the month, the data were dominated by diurnal meteorological effects, although a gradual decline could still be seen. Data captured on the seismic data-acquisition system showed an average of ~6.5 low-frequency and explosive events per day, compared to almost 26 per day in the second half of October.

Figure 22. Seismicity at Rabaul (station KPTH), October-November 1994. Courtesy of RVO.

"Before the eruption, seismic activity . . . was dominated by high-frequency earthquakes located on the caldera ring-fault system. Since the eruption, there have been few high-frequency earthquakes detected (58 in October and 37 in November, compared to normal pre-eruption levels of 200-300/month) and most of these were located away from the ring fault or in previously inactive regions of it. The level of seismicity cannot be easily compared to earlier pre-eruption levels because totally different seismic detection systems were used. However, it is believed that the level is much lower than before the eruption. This, and the fact that the majority of the epicenters are away from the ring-fault system that previously contained almost all of the seismicity, suggest that the caldera is no longer in a highly pressurized state.

"Ground deformation determined from electronic tilt meters and dry-tilt measurements indicate a reduction in the rate of deflation of the caldera since the onset of the eruption. This change is illustrated by an offshore pylon near the centre of deformation, 2 km S of Tavurvur, which subsided by 8 cm in November, compared to 18 cm in October and at least 45 cm in the last 10 days of September."

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: B. Talai, R. Stewart, and P. de Saint-Ours, RVO.

The fumarolic activity in the main crater that remained vigorous during August and September, continued in November. A seismic record made by ICE in November suggested seismo-volcanic activity of low frequency and magnitude located at very shallow depths beneath the crater.

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: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE.

[Following notice in early December that seismic stations at Shiveluch and Tolbachik had closed, on 22 December the following message was sent from the Alaska Volcano Observatory (AVO): "KVERT [Kamchatka Volcanic Eruptions Response Team] has informed AVO that, because of a long delay in promised funding from the Ministry of Transportation in Moscow, KVERT must suspend transmittal of information on volcanic activity in Kamchatka. The length of the suspension is unknown at this time.]

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: V. Kirianov, IVGG; T. Miller, AVO.

Following notice in early December that seismic stations at Shiveluch and Tolbachik had closed, on 22 December the following message was sent from the Alaska Volcano Observatory (AVO): "KVERT [Kamchatka Volcanic Eruptions Response Team] has informed AVO that, because of a long delay in promised funding from the Ministry of Transportation in Moscow, KVERT must suspend transmittal of information on volcanic activity in Kamchatka. The length of the suspension is unknown at this time. Expressions of concern and support... by interested parties would be appreciated."

An AVO Information Release on 9 January 1995 suggested that "Letters of concern might mention the Kamchatka Volcanic Eruptions Response Team under the leadership of Vladimir Kirianov, its value in monitoring and reporting of volcanic eruptions, the suspension of KVERT activities because of the delay in funding, the need for rapid transfer of funds, etc." Letters should be sent to the Russian Department of Air Transport official handling the KVERT funds [outdated contact information removed].

KVERT began regularly sending reports to AVO for further distribution in April 1993. Since then, KVERT has provided the overwhelming bulk of information for GVN reports about Kamchatkan volcanic activity, the first steady stream of information from this important region. For example, information provided by KVERT has described significant eruptions at Shiveluch (22 April 1993), Bezymianny (21 October 1993), and Kliuchevskoi (1-3 October 1994). Continuous activity at Shiveluch (gas-and-steam plumes, growth of extrusive lava dome) and Kliuchevskoi (minor ash explosions, gas-and-steam plumes, lava fountaining, lava flows) has also been consistently reported. Prompt notification of Kamchatkan eruptions is especially critical because of the large volume of international air traffic in the vicinity.

Geologic Background. Special announcements or information of general interest not linked to any specific volcano.

Information Contacts: Vladimir Yu. Kirianov, Institute of Volcanic Geology & Geochemistry (see Kliuchevskoi); Thomas P. 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.

On about 15 January 1994, Bolivar Miranda, a SERNAGEOMIN chemical engineer, observed a 5-km-high explosive column rising above Tinguiririca from a location 65 km W. A photograph taken by his son, Matías, showed a distinct white cauliflower-shaped column on a clear day. Based on the shape and growth of the column, this eruption was most likely phreatic.

Geologic Background. Tinguiririca is composed of at least seven Holocene scoria cones W of the Chile-Argentina border constructed along a NNE-SSW fissure over an eroded Pleistocene stratovolcano. The complex was constructed during three eruptive cycles dating back to the middle Pleistocene. The latest activity produced a series of youthful small stratovolcanoes and craters, of which the youngest appear to be Tinguiririca and Fray Carlos. Constant fumarolic activity occurs within and on the NW wall of the summit crater. Hot springs and fumaroles with sulfur deposits are found on the W flanks of the summit cones. A single historical eruption was recorded in 1917.

Information Contacts: J. Naranjo, SERNAGEOMIN, Santiago.

[Following notice in early December that seismic stations at Shiveluch and Tolbachik had closed, on 22 December the following message was sent from the Alaska Volcano Observatory (AVO): "KVERT [Kamchatka Volcanic Eruptions Response Team] has informed AVO that, because of a long delay in promised funding from the Ministry of Transportation in Moscow, KVERT must suspend transmittal of information on volcanic activity in Kamchatka. The length of the suspension is unknown at this time.]

Geologic Background. The massive Tolbachik volcano is located at the southern end of the Kliuchevskaya volcano group. The massif is composed of two overlapping, but morphologically distinct, volcanoes. The flat-topped Plosky Tolbachik shield volcano with its nested Holocene calderas up to 3 km in diameter is located east of the older and higher sharp-topped Ostry Tolbachik stratovolcano. The summit caldera at Plosky Tolbachik was formed in association with major lava effusion about 6,500 years ago and simultaneously with a major southward-directed sector collapse of Ostry Tolbachik. Long rift zones extending NE and SSW of the volcano have erupted voluminous basaltic lava flows during the Holocene, with activity during the past two thousand years being confined to the narrow axial zone of the rifts. The 1975-76 eruption originating from the SSW-flank fissure system and the summit was the largest historical basaltic eruption in Kamchatka.

Information Contacts: V. Kirianov, IVGG; T. Miller, AVO.

The period from mid-November through mid-December was characterized by a low eruption rate (~10⁴ m³/d) and low frequency of pyroclastic-flow events. A theodolite survey indicated that lava blocks (a spine and the surrounding area) in the center of the endogenous dome had moved upward at a rate of <0.5 m/day. Movement of talus slopes on the dome was hardly detected during this period. Some geophysicists proposed that the upward movement of the spine and the surrounding area was related directly to microearthquakes, which occurred periodically within the dome in recent months. It is difficult to test this hypothesis because of the slow movement and poor weather conditions. The endogenous dome was the highest point in early December, reaching ~220 m above the former Jigokuato Crater. The height of the dome has varied but generally increased with time, and had reached 245 m in April 1994.

Oxidized lava blocks (several meters across) on the dome surface tumbled NE and SE due to inclination of the surface around the uplifting part; some developed into pyroclastic flows. During October, eight pyroclastic flows were observed to travel <=2 km SE. The Geological Survey of Japan reported that the average volume of pyroclastic-flow deposits in November was ~100 m³/day, which is the lowest since May 1991. Volume estimates were made using pyroclastic-flow seismic records (amplitude and duration of signal).

During November, microearthquakes detected 3.6 km W of the dome (station A) totaled 436, roughly half the number seen in October (993). Since mid-October, the number of hourly earthquakes has been periodic, with 38-40 hours between cycles. A corresponding periodic character was also found in tilt data at the N caldera rim, but the mechanism remains unknown. COSPEC analysis by the Tokyo Institute of Technology in late November showed that SO₂ flux from the dome was ~20 t/d; half of the value in late September. The value of SO₂ flux . . . is roughly concordant with the lava eruption rate throughout the last 3.5 years.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: S. Nakada, Kyushu Univ; JMA.

Cloudy conditions throughout October and the first half of November prevented observations on most days. On 13 October AVHRR satellite imagery revealed a "hot spot" in the same location as during the past few months, but no eruption cloud was observed. By October 18, when clear skies allowed good views, no "hot spot" or eruption cloud was detected. Satellite imagery on 17 November again revealed a possible "hot spot" within the caldera, indicating probable continuing low-level activity. No activity was observed from Perryville . . . during clear conditions on 24 November.

Geologic Background. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: AVO.