The lava lake in the summit crater of Erebus has been active since at least 1972. Located in Antarctica overlooking the McMurdo Station on Ross Island, it is the southernmost active volcano on the planet. Because of the remote location, activity is primarily monitored by satellites. This report covers activity during 2023.

The number of thermal alerts recorded by the Hawai'i Institute of Geophysics and Planetology’s MODVOLC Thermal Alerts System increased considerably in 2023 compared to the years 2020-2022 (table 9). In contrast to previous years, the MODIS instruments aboard the Aqua and Terra satellites captured data from Erebus every month during 2023. Consistent with previous years, the lowest number of anomalous pixels were recorded in January, November, and December.

Table 9. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2023. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP – MODVOLC Thermal Alerts System.

Sentinel-2 infrared images showed one or two prominent heat sources within the summit crater, accompanied by adjacent smaller sources, similar to recent years (see BGVN 46:01, 47:02, and 48:01). A unique image was obtained on 25 November 2023 by the OLI-2 (Operational Land Imager-2) on Landsat 9, showing the upper part of the volcano surrounded by clouds (figure 32).

Figure 32. Satellite view of Erebus with the summit and upper flanks visible above the surrounding weather clouds on 25 November 2023. Landsat 9 OLI-2 (Operational Land Imager-2) image with visible and infrared bands. Thermal anomalies are present in the summit crater. The edifice is visible from about 2,000 m elevation to the summit around 3,800 m. The summit crater is ~500 m in diameter, surrounded by a zone of darker snow-free deposits; the larger circular summit area is ~4.5 km diameter. NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the U.S. Geological Survey.

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: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://earthobservatory.nasa.gov/images/152134/erebus-breaks-through).

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). The current eruption period began June 2021. This report covers activity during July-December 2023 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Numerous weak phreatic explosions continued during July-December 2023, along with gas-and-steam emissions and plumes that rose as high as 3 km above the crater rim. Many weekly OVSICORI-UNA bulletins included the previous week's number of explosions and emissions (table 9). For many explosions, the time of explosion was given (table 10). Frequent seismic activity (long-period earthquakes, volcano-tectonic earthquakes, and tremor) accompanied the phreatic activity.

Table 9. Number of reported weekly phreatic explosions and gas-and-steam emissions at Rincón de la Vieja, July-December 2023. Counts are reported for the week before the Weekly Bulletin date; not all reports included these data. Courtesy of OVSICORI-UNA.

Table 10. Summary of activity at Rincón de la Vieja during July-December 2023. Weak phreatic explosions and gas emissions are noted where the time of explosion was indicated in the weekly or daily bulletins. Height of plumes or emissions are distance above the crater rim. Courtesy of OVSICORI-UNA.

According to OVSICORI-UNA, during July-October the average weekly sulfur dioxide (SO₂) flux ranged from 68 to 240 tonnes/day. However, in mid-November the flux increased to as high as 334 tonnes/day, the highest value measured in recent years. The high SO₂ flux in mid-November was also detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 43).

Figure 43. Sulfur dioxide (SO2) maps from Rincón de la Vieja recorded by the TROPOMI instrument aboard the Sentinel-5P satellite on 16 November (left) and 20 November (right) 2023. Mass estimates are consistent with measurements by OVSICORI-UNA near ground level. Some of the plume on 20 November may be from other volcanoes (triangle symbols) in Costa Rica and Nicaragua. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

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: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); 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/).

Bezymianny, located on Russia’s Kamchatka Peninsula, has had eruptions since 1955 characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. Activity during November 2022-April 2023 included gas-and-steam emissions, lava dome collapses generating avalanches, and persistent thermal activity. Similar eruptive activity continued from May through October 2023, described here based on information from weekly and daily reports of the Kamchatka Volcano Eruptions Response Team (KVERT), notices from Tokyo VAAC (Volcanic Ash Advisory Center), and from satellite data.

Overall activity decreased after the strong period of activity in late March through April 2023, which included ash explosions during 29 March and 7-8 April 2023 that sent plumes as high as 10-12 km altitude, along with dome growth and lava flows (BGVN 48:05). This reduced activity can be seen in the MIROVA thermal detection system graph (figure 56), which was consistent with data from the MODVOLC thermal detection system and with Sentinel-2 satellite images that showed persistent hotspots in the summit crater when conditions allowed observations. A renewed period of strong activity began in mid-October 2023.

Figure 56. The MIROVA (Log Radiative Power) thermal data for Bezymianny during 20 November 2022 through October 2023 shows heightened activity in the first half of April and second half of October 2023, with lower levels of thermal anomalies in between those times. Courtesy of MIROVA.

Activity increased significantly on 17 October 2023 when large collapses began during 0700-0830 on the E flanks of the lava dome and continued to after 0930 the next day (figure 57). Ash plumes rose to an altitude of 4.5-5 km, extending 220 km NNE by 18 October. A large explosion at 1630 on 18 October produced an ash plume that rose to an altitude of 11 km (8 km above the summit) and drifted NNE and then NW, extending 900 km NW within two days at an altitude of 8 km. Minor ashfall was noted in Kozyrevsk (45 km WNW). At 0820 on 20 October an ash plume was identified in satellite images drifting 100 km ENE at altitudes of 4-4.5 km.

Figure 57. Sentinel-2 satellite images of Bezymianny from 1159 on 17 October 2023 (2359 on 16 October UTC) showing a snow-free S and SE flank along with thermal anomalies in the crater and down the SE flank. Left image is in false color (bands 8, 4, 3); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Lava flows and hot avalanches from the dome down the SE flank continued over the next few days, including 23 October when clear conditions allowed good observations (figures 58 and 59). A large thermal anomaly was observed over the volcano through 24 October, and in the summit crater on 30 October (figure 60). Strong fumarolic activity continued, with numerous avalanches and occasional incandescence. By the last week of October, volcanic activity had decreased to a level consistent with that earlier in the reporting period.

Figure 58. Daytime photo of Bezymianny under clear conditions on 23 October 2023 showing a lava flow and avalanches descending the SE flank, incandescence from the summit crater, and a small ash plume. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 59. Night photo of Bezymianny under cloudy conditions on 23 October 2023 showing an incandescent lava flow and avalanches descending the SE flank. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 60. Sentinel-2 satellite images of Bezymianny from 1159 on 30 October 2023 (2359 on 29 October UTC) showing a plume drifting SE and thermal anomalies in the summit crater and down multiple flanks. Left image is in true color (bands 4, 3, 2); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Aviation warnings were frequently updated during 17-20 October. KVERT issued a Volcano Observatory Notice for Aviation (VONA) on 17 October at 1419 and 1727 (0219 and 0527 UTC) raising the Aviation Color Code (ACC) from Yellow to Orange (second highest level). The next day, KVERT issued a VONA at 1705 (0505 UTC) raising the ACC to Red (highest level) but lowered it back to Orange at 2117 (0917 UTC). After another decrease to Yellow and back to Orange, the ACC was reduced to Yellow on 20 October at 1204 (0004 UTC). In addition, the Tokyo VAAC issued a series of Volcanic Ash Advisories beginning on 16 October and continuing through 30 October.

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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has recently been characterized by lava effusions, spatter, and sulfur dioxide emissions in the active Halema’uma’u lava lake (BGVN 47:08). Lava effusions, some spatter, and sulfur dioxide emissions have continued during this reporting period of July through December 2022 using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Summary of activity during July-December 2022. Low-level effusions have continued at the western vent of the Halema’uma’u crater during July through early December 2022. Occasional weak ooze-outs (also called lava break outs) would occur along the margins of the crater floor. The overall level of the active lava lake throughout the reporting period gradually increased due to infilling, however it stagnated in mid-September (table 13). During September through November, activity began to decline, though lava effusions persisted at the western vent. By 9 December, the active part of the lava lake had completely crusted over, and incandescence was no longer visible.

Table 13. Summary of measurements taken during overflights at Kīlauea that show a gradual increase in the active lava lake level and the volume of lava effused since 29 September 2021. Lower activity was reported during September-October. Data collected during July-December 2022. Courtesy of HVO.

Activity during July 2022. Lava effusions were reported from the western vent in the Halema’uma’u crater, along with occasional weak ooze-outs along the margins of the crater floor. The height of the lava lake was variable due to deflation-inflation tilt events; for example, the lake level dropped approximately 3-4 m during a summit deflation-inflation event reported on 1 July. Webcam images taken during the night of 6-12 July showed intermittent low-level spattering at the western vent that rose less than 10 m above the vent (figure 519). Measurements made during an overflight on 7 July indicated that the crater floor was infilled about 130 m and that 95 million cubic meters of lava had been effused since 29 September 2021. A single, relatively small lava ooze-out was active to the S of the lava lake. Around midnight on 8 July there were two brief periods of lava overflow onto the lake margins. On 9 July lava ooze-outs were reported near the SE and NE edges of the crater floor and during 10-11 July they occurred near the E, NE, and NW edges. On 16 July crater incandescence was reported, though the ooze-outs and spattering were not visible. On 18 July overnight webcam images showed incandescence in the western vent complex and two ooze-outs were reported around 0000 and 0200 on 19 July. By 0900 there were active ooze-outs along the SW edge of the crater floor. Measurements made from an overflight on 19 July indicated that the crater floor was infilled about 133 m and 98 million cubic meters of lava had erupted since 29 September 2021 (figure 520). On 20 July around 1600 active ooze-outs were visible along the N edge of the crater, which continued through the next day. Extensive ooze-outs occurred along the W margin during 24 July until 1900; on 26 July minor ooze-outs were noted along the N margin. Minor spattering was visible on 29 July along the E margin of the lake. The sulfur dioxide emission rates ranged 650-2,800 tons per day (t/d), the higher of which was measured on 8 July (figure 519).

Figure 519. Minor spattering rising less than 10 m was visible at the E end of the lava lake within Halema‘uma‘u, at the summit of Kīlauea on 8 July 2022. Sulfur dioxide is visible rising from the lake surface (bluish-colored fume). A sulfur dioxide emission rate of approximately 2,800 t/d was measured on 8 July. Courtesy of K. Mulliken, USGS.

Figure 520. A helicopter overflight on 19 July 2022 allowed for aerial visible and thermal imagery to be taken of the Halema’uma’u crater at Kīlauea’s summit crater. The active part of the lava lake is confined to the western part of the crater. The scale of the thermal map ranges from blue to red, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

Activity during August 2022. The eruption continued in the Halema’uma’u crater at the western vent. According to HVO the lava in the active lake remained at the level of the bounding levees. Occasional minor ooze-outs were observed along the margins of the crater floor. Strong nighttime crater incandescence was visible after midnight on 6 August over the western vent cone. During 6-7 August scattered small lava lobes were active along the crater floor and incandescence persisted above the western vent through 9 August. During 7-9 August HVO reported a single lava effusion source was active along the NW margin of the crater floor. Measurements from an overflight on 4 August indicated that the crater floor was infilled about 136 m total and that 102 million cubic meters of lava had been erupted since the start of the eruption. Lava breakouts were reported along the N, NE, E, S, and W margins of the crater during 10-16 August. Another overflight survey conducted on 16 August indicated that the crater floor infilled about 137 m and 104 million cubic meters of lava had been erupted since September 2021. Measured sulfur dioxide emissions rates ranged 1,150-2,450 t/d, the higher of which occurred on 8 August.

Activity during September 2022. During September, lava effusion continued from the western vent into the active lava lake and onto the crater floor. Intermittent minor ooze-outs were reported through the month. A small ooze-out was visible on the W crater floor margin at 0220 on 2 September, which showed decreasing surface activity throughout the day, but remained active through 3 September. On 3 September around 1900 a lava outbreak occurred along the NW margin of the crater floor but had stopped by the evening of 4 September. Field crews monitoring the summit lava lake on 9 September observed spattering on the NE margin of the lake that rose no higher than 10 m, before falling back onto the lava lake crust (figure 521). Overflight measurements on 12 September indicated that the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had been erupted since September 2021. Extensive breakouts in the W and N part of the crater floor were reported at 1600 on 20 September and continued into 26 September. The active part of the lava lake dropped by 10 m while other parts of the crater floor dropped by several meters. Summit tiltmeters recorded a summit seismic swarm of more than 80 earthquakes during 1500-1800 on 21 September, which occurred about 1.5 km below Halema’uma’u; a majority of these were less than Mw 2. By 22 September the active part of the lava lake was infilled about 2 m. On 23 September the western vent areas exhibited several small spatter cones with incandescent openings, along with weak, sporadic spattering (figure 522). The sulfur dioxide emission rate ranged from 930 t/d to 2,000 t/d, the higher of which was measured on 6 September.

Figure 521. Photo of spattering occurring at Kīlauea's Halema’uma’u crater during the morning of 9 September 2022 on the NE margin of the active lava lake. The spatter material rose 10 m into the air before being deposited back on the lava lake crust. Courtesy of C. Parcheta, USGS.

Figure 522.The active western vent area at Kīlauea's Halema’uma’u crater consisted of several small spatter cones with incandescent openings and weak, sporadic spattering. Courtesy of M. Patrick, USGS.

Activity during October 2022. Activity during October declined slightly compared to previous months, though lava effusions persisted from the western vent into the active lava lake and onto the crater floor during October (figure 523). Slight variations in the lava lake were noted throughout the month. HVO reported that around 0600 on 3 October the level of the lava lake has lowered slightly. Overflight measurements taken on 5 October indicated that the crater floor was infilled a total of about 143 m and that 111 million cubic meters of lava had been effused since September 2021. During 6-7 October the lake gradually rose 0.5 m. Sulfur dioxide measurements made on 22 October had an emission rate of 700 t/d. Another overflight taken on 28 October showed that there was little to no change in the elevation of the crater floor: the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had erupted since the start of the eruption.

Figure 523. Photo of the Halema’uma’u crater at Kīlauea looking east from the crater rim showing the active lava lake, with active lava ponds to the SE (top) and west (bottom middle) taken on 5 October 2022. The western vent complex is visible through the gas at the bottom center of the photo. Courtesy of N. Deligne, USGS.

Activity during November 2022. Activity remained low during November, though HVO reported that lava from the western vent continued to effuse into the active lava lake and onto the crater floor throughout the month. The rate of sulfur dioxide emissions during November ranged from 300-600 t/d, the higher amount of which occurred on 9 November.

Activity during December 2022. Similar low activity was reported during December, with lava effusing from the western vent into the active lava lake and onto the crater floor. During 4-5 December the active part of the lava lake was slightly variable in elevation and fluctuated within 1 m. On 9 December HVO reported that lava was no longer erupting from the western vent in the Halema’uma’u crater and that sulfur dioxide emissions had returned to near pre-eruption background levels; during 10-11 December, the lava lake had completely crusted over, and no incandescence was visible (figure 524). Time lapse camera images covering the 4-10 December showed that the crater floor showed weak deflation and no inflation. Some passive events of crustal overturning were reported during 14-15 December, which brought fresh incandescent lava to the lake surface. The sulfur dioxide emission rate was approximately 200 t/d on 14 December. A smaller overturn event on 17 December and another that occurred around 0000 and into the morning of 20 December were also detected. A small seismic swarm was later detected on 30 December.

Figure 524. Photo of Halema’uma’u crater at Kīlauea showing a mostly solidified lake surface during the early morning of 10 December 2022. Courtesy of J. Bard, USGS.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Nyamulagira (also known as Nyamuragira) is a shield volcano in the Democratic Republic of Congo with the summit truncated by a small 2 x 2.3 km caldera with walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from numerous flank fissures and cinder cones. The current eruption period began in April 2018 and has more recently been characterized by summit crater lava flows and thermal activity (BGVN 48:05). This report describes lava flows and variable thermal activity during May through October 2023, based on information from the Observatoire Volcanologique de Goma (OVG) and various satellite data.

Lava lake activity continued during May. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system recorded moderate-to-strong thermal activity throughout the reporting period; activity was more intense during May and October and relatively weaker from June through September (figure 95). The MODVOLC thermal algorithm, detected a total of 209 thermal alerts. There were 143 hotspots detected during May, eight during June, nine during September, and 49 during October. This activity was also reflected in infrared satellite images, where a lava flow was visible in the NW part of the crater on 7 May and strong activity was seen in the center of the crater on 4 October (figure 96). Another infrared satellite image taken on 12 May showed still active lava flows along the NW margin of the crater. According to OVG lava effusions were active during 7-29 May and moved to the N and NW parts of the crater beginning on 9 May. Strong summit crater incandescence was visible from Goma (27 km S) during the nights of 17, 19, and 20 May (figure 97). On 17 May there was an increase in eruptive activity, which peaked at 0100 on 20 May. Notable sulfur dioxide plumes drifted NW and W during 19-20 May (figure 98). Drone footage acquired in partnership with the USGS (United States Geological Survey) on 20 May captured images of narrow lava flows that traveled about 100 m down the W flank (figure 99). Data from the Rumangabo seismic station indicated a decreasing trend in activity during 17-21 May. Although weather clouds prevented clear views of the summit, a strong thermal signature on the NW flank was visible in an infrared satellite image on 22 May, based on an infrared satellite image. On 28 May the lava flows on the upper W flank began to cool and solidify. By 29 May seismicity returned to levels similar to those recorded before the 17 May increase. Lava effusion continued but was confined to the summit crater; periodic crater incandescence was observed.

Figure 95. Moderate-to-strong thermal anomalies were detected at Nyamulagira during May through October 2023, as shown on this MIROVA graph (Log Radiative Power). During late May, the intensity of the anomalies gradually decreased and remained at relatively lower levels during mid-June through mid-September. During mid-September, the power of the anomalies gradually increased again. The stronger activity is reflective of active lava effusions. Courtesy of MIROVA.

Figure 96. Infrared (bands B12, B11, B4) satellite images showing a constant thermal anomaly of variable intensities in the summit crater of Nyamulagira on 7 May 2023 (top left), 21 June 2023 (top right), 21 July 2023 (bottom left), and 4 October 2023 (bottom right). Although much of the crater was obscured by weather clouds on 7 May, a possible lava flow was visible in the NW part of the crater. Courtesy of Copernicus Browser.

Figure 97. Photo of intense nighttime crater incandescence at Nyamulagira as seen from Goma (27 km S) on the evening of 19 May 2023. Courtesy of Charles Balagizi, OVG.

Figure 98. Two strong sulfur dioxide plumes were detected at Nyamulagira and drifted W on 19 (left) and 20 (right) May 2023. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 99. A map (top) showing the active vents (yellow pins) and direction of active lava flows (W) at Nyamulagira at Virunga National Park on 20 May 2023. Drone footage (bottom) also shows the fresh lava flows traveling downslope to the W on 20 May 2023. Courtesy of USGS via OVG.

Low-level activity was noted during June through October. On 1 June OVG reported that seismicity remained at lower levels and that crater incandescence had been absent for three days, though infrared satellite imagery showed continued lava effusion in the summit crater. The lava flows on the flanks covered an estimated 0.6 km². Satellite imagery continued to show thermal activity confined to the lava lake through October (figure 96), although no lava flows or significant sulfur dioxide emissions were reported.

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

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

The remote volcano of Bagana is located in central Bougainville Island, Papua New Guinea. Recorded eruptions date back to 1842 and activity has consisted of effusive activity that has built a small lava dome in the summit crater and occasional explosions that produced pyroclastic flows. The most recent eruption has been ongoing since February 2000 and has produced occasional explosions, ash plumes, and lava flows. More recently, activity has been characterized by ongoing effusive activity and ash emissions (BGVN 48:04). This report updates activity from April through September 2023 that has consisted of explosions, ash plumes, ashfall, and lava flows, using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

An explosive eruption was reported on 7 July that generated a large gas-and-ash plume to high altitudes and caused significant ashfall in local communities; the eruption plume had reached upper tropospheric (16-18 km altitude) altitudes by 2200, according to satellite images. Sulfur dioxide plumes were detected in satellite images on 8 July and indicated that the plume was likely a mixture of gas, ice, and ash. A report issued by the Autonomous Bougainville Government (ABG) (Torokina District, Education Section) on 10 July noted that significant ash began falling during 2000-2100 on 7 July and covered most areas in the Vuakovi, Gotana (9 km SW), Koromaketo, Laruma (25 km W) and Atsilima (27 km NW) villages. Pyroclastic flows also occurred, according to ground-based reports; small deposits confined to one drainage were inspected by RVO during an overflight on 17 July and were confirmed to be from the 7 July event. Ashfall continued until 10 July and covered vegetation, which destroyed bushes and gardens and contaminated rivers and streams.

RVO reported another eruption on 14 July. The Darwin VAAC stated that an explosive event started around 0830 on 15 July and produced an ash plume that rose to 16.5 km altitude by 1000 and drifted N, according to satellite images. The plume continued to drift N and remained visible through 1900, and by 2150 it had dissipated.

Ashfall likely from both the 7 and 15 July events impacted about 8,111 people in Torokina (20 km SW), including Tsito/Vuakovi, Gotana, Koromaketo, Kenaia, Longkogari, Kenbaki, Piva (13 km SW), and Atsinima, and in the Tsitovi district, according to ABG. Significant ashfall was also reported in Ruruvu (22 km N) in the Wakunai District of Central Bougainville, though the thickness of these deposits could not be confirmed. An evacuation was called for the villages in Wakunai, where heavy ashfall had contaminated water sources; the communities of Ruruvu, Togarau, Kakarapaia, Karauturi, Atao, and Kuritaturi were asked to evacuate to a disaster center at the Wakunai District Station, and communities in Torokina were asked to evacuate to the Piva District station. According to a news article, more than 7,000 people needed temporary accommodations, with about 1,000 people in evacuation shelters. Ashfall had deposited over a broad area, contaminating water supplies, affecting crops, and collapsing some roofs and houses in rural areas. Schools were temporarily shut down. Intermittent ash emissions continued through the end of July and drifted NNW, NW, and SW. Fine ashfall was reported on the coast of Torokina, and ash plumes also drifted toward Laruma and Atsilima.

A small explosive eruption occurred at 2130 on 28 July that ejected material from the crater vents, according to reports from Torokina, in addition to a lava flow that contained two lobes. A second explosion was detected at 2157. Incandescence from the lava flow was visible from Piva as it descended the W flank around 2000 on 29 July (figure 47). The Darwin VAAC reported that a strong thermal anomaly was visible in satellite images during 30-31 July and that ash emissions rose to 2.4 km altitude and drifted WSW on 30 July. A ground report from RVO described localized emissions at 0900 on 31 July.

Figure 47. Infrared (bands B12, B11, B4) satellite images showed weak thermal anomalies at the summit crater of Bagana on 12 April 2023 (top left), 27 May 2023 (top right), 31 July 2023 (bottom left), and 19 September 2023 (bottom right). A strong thermal anomaly was detected through weather clouds on 31 July and extended W from the summit crater. Courtesy of Copernicus Browser.

The Darwin VAAC reported that ash plumes were identified in satellite imagery at 0800 and 1220 on 12 August and rose to 2.1 km and 3 km altitude and drifted NW and W, respectively. A news report stated that aid was sent to more than 6,300 people that were adversely affected by the eruption. Photos taken during 17-19 August showed ash emissions rising no higher than 1 km above the summit and drifting SE. A small explosion generated an ash plume during the morning of 19 August. Deposits from small pyroclastic flows were also captured in the photos. Satellite images captured lava flows and pyroclastic flow deposits. Two temporary seismic stations were installed near Bagana on 17 August at distances of 7 km WSW (Vakovi station) and 11 km SW (Kepox station). The Kepox station immediately started to record continuous, low-frequency background seismicity.

Satellite data. Little to no thermal activity was detected during April through mid-July 2023; only one anomaly was recorded during early April and one during early June, according to MIROVA (Middle InfraRed Observation of Volcanic Activity) data (figure 48). Thermal activity increased in both power and frequency during mid-July through September, although there were still some short gaps in detected activity. MODVOLC also detected increased thermal activity during August; thermal hotspots were detected a total of five times on 19, 20, and 27 August. Weak thermal anomalies were also captured in infrared satellite images on clear weather days throughout the reporting period on 7, 12, and 17 April, 27 May, 1, 6, 16, and 31 July, and 19 September (figure 48); a strong thermal anomaly was visible on 31 July. Distinct sulfur dioxide plumes that drifted generally NW were intermittently captured by the TROPOMI instrument on the Sentinel-5P satellite and sometimes exceeded two Dobson Units (DUs) (figure 49).

Figure 48. Low thermal activity was detected at Bagana during April through mid-July 2023, as shown on this MIROVA graph. In mid-July, activity began to increase in both frequency and power, which continued through September. There were still some pauses in activity during late July, early August, and late September, but a cluster of thermal activity was detected during late August. Courtesy of MIROVA.

Figure 49. Distinct sulfur dioxide plumes rising from Bagana on 15 July 2023 (top left), 16 July 2023 (top right), 17 July 2023 (bottom left), and 17 August 2023 (bottom right). These plumes all generally drifted NW; a particularly notable plume exceeded 2 Dobson Units (DUs) on 15 July. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.0

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); 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); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

Mayon is located in the Philippines and has steep upper slopes capped by a small summit crater. Historical eruptions date back to 1616 CE that have been characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during June through October 2022 and consisted of lava dome growth and gas-and-steam emissions (BGVN 47:12). A new eruption was reported during late April 2023 and has included lava flows, pyroclastic density currents, ash emissions, and seismicity. This report covers activity during April through September 2023 based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During April through September 2023, PHIVOLCS reported near-daily rockfall events, frequent volcanic earthquakes, and sulfur dioxide measurements. Gas-and-steam emissions rose 100-900 m above the crater and drifted in different directions. Nighttime crater incandescence was often visible during clear weather and was accompanied by incandescent avalanches of material. Activity notably increased during June when lava flows were reported on the S, SE, and E flanks (figure 52). The MIROVA graph (Middle InfraRed Observation of Volcanic Activity) showed strong thermal activity coincident with these lava flows, which remained active through September (figure 53). According to the MODVOLC thermal algorithm, a total of 110 thermal alerts were detected during the reporting period: 17 during June, 40 during July, 27 during August, and 26 during September. During early June, pyroclastic density currents (PDCs) started to occur more frequently.

Figure 52. Infrared (bands B12, B11, B4) satellite images show strong lava flows descending the S, SE, and E flanks of Mayon on 13 June 2023 (top left), 23 June 2023 (top right), 8 July 2023 (bottom left), and 7 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 53. Strong thermal activity was detected at Mayon during early June through September, according to this MIROVA graph (Log Radiative Power) due to the presence of active lava flows on the SE, S, and E flanks. Courtesy of MIROVA.

Low activity was reported during much of April and May; gas-and-steam emissions rose 100-900 m above the crater and generally drifted in different directions. A total of 52 rockfall events and 18 volcanic earthquakes were detected during April and 147 rockfall events and 13 volcanic events during May. Sulfur dioxide flux measurements ranged between 400-576 tons per day (t/d) during April, the latter of which was measured on 29 April and between 162-343 t/d during May, the latter of which was measured on 13 May.

Activity during June increased, characterized by lava flows, pyroclastic density currents (PDCs), crater incandescence and incandescent rockfall events, gas-and-steam emissions, and continued seismicity. Weather clouds often prevented clear views of the summit, but during clear days, moderate gas-and-steam emissions rose 100-2,500 m above the crater and drifted in multiple directions. A total of 6,237 rockfall events and 288 volcanic earthquakes were detected. The rockfall events often deposited material on the S and SE flanks within 700-1,500 m of the summit crater and ash from the events drifted SW, S, SE, NE, and E. Sulfur dioxide emissions ranged between 149-1,205 t/d, the latter of which was measured on 10 June. Short-term observations from EDM and electronic tiltmeter monitoring indicated that the upper slopes were inflating since February 2023. Longer-term ground deformation parameters based on EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano remained inflated, especially on the NW and SE flanks. At 1000 on 5 June the Volcano Alert Level (VAL) was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity.

A total of 151 dome-collapse PDCs occurred during 8-9 and 11-30 June, traveled 500-2,000 m, and deposited material on the S flank within 2 km of the summit crater. During 8-9 June the VAL was raised to 3. At approximately 1947 on 11 June lava flow activity was reported; two lobes traveled within 500 m from the crater and deposited material on the S (Mi-isi), SE (Bonga), and E (Basud) flanks. Weak seismicity accompanied the lava flow and slight inflation on the upper flanks. This lava flow remained active through 30 June, moving down the S and SE flank as far as 2.5 km and 1.8 km, respectively and depositing material up to 3.3 km from the crater. During 15-16 June traces of ashfall from the PDCs were reported in Sitio Buga, Nabonton, City of Ligao and Purok, and San Francisco, Municipality of Guinobatan. During 28-29 June there were two PDCs generated by the collapse of the lava flow front, which generated a light-brown ash plume 1 km high. Satellite monitors detected significant concentrations of sulfur dioxide beginning on 29 June. On 30 June PDCs primarily affected the Basud Gully on the E flank, the largest of which occurred at 1301 and lasted eight minutes, based on the seismic record. Four PDCs generated between 1800 and 2000 that lasted approximately four minutes each traveled 3-4 km on the E flank and generated an ash plume that rose 1 km above the crater and drifted N and NW. Ashfall was recorded in Tabaco City.

Similar strong activity continued during July; slow lava effusion remained active on the S and SE flanks and traveled as far as 2.8 km and 2.8 km, respectively and material was deposited as far as 4 km from the crater. There was a total of 6,983 rockfall events and 189 PDCs that affected the S, SE, and E flanks. The volcano network detected a total of 2,124 volcanic earthquakes. Continuous gas-and-steam emissions rose 200-2,000 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 792-4,113 t/d, the latter of which was measured on 28 July. During 2-4 July three PDCs were generated from the collapse of the lava flow and resulting light brown plumes rose 200-300 m above the crater. Continuous tremor pulses were reported beginning at 1547 on 3 July through 7 July at 1200, at 2300 on 8 July and going through 0300 on 10 July, and at 2300 on 16 July, as recorded by the seismic network. During 6-9 July there were 10 lava flow-collapse-related PDCs that generated light brown plumes 300-500 m above the crater. During 10-11 July light ashfall was reported in some areas of Mabinit, Legazpi City, Budiao and Salvacion, Daraga, and Camalig, Albay. By 18 July the lava flow advanced 600 m on the E flank as well.

During 1733 on 18 July and 0434 on 19 July PHIVOLCS reported 30 “ashing” events, which are degassing events accompanied by audible thunder-like sounds and entrained ash at the crater, which produced short, dark plumes that drifted SW. These events each lasted 20-40 seconds, and plume heights ranged from 150-300 m above the crater, as recorded by seismic, infrasound, visual, and thermal monitors. Three more ashing events occurred during 19-20 July. Short-term observations from electronic tilt and GPS monitoring indicate deflation on the E lower flanks in early July and inflation on the NW middle flanks during the third week of July. Longer-term ground deformation parameters from EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano was still generally inflated relative to baseline levels. A short-lived lava pulse lasted 28 seconds at 1956 on 21 July, which was accompanied by seismic and infrasound signals. By 22 July, the only lava flow that remained active was on the SE flank, and continued to extend 3.4 km, while those on the S and E flanks weakened markedly. One ashing event was detected during 30-31 July, whereas there were 57 detected during 31 July-1 August; according to PHIVOLCS beginning at approximately 1800 on 31 July eruptive activity was dominated by phases of intermittent ashing, as well as increased in the apparent rates of lava effusion from the summit crater. The ashing phases consisted of discrete events recorded as low-frequency volcanic earthquakes (LFVQ) typically 30 seconds in duration, based on seismic and infrasound signals. Gray ash plume rose 100 m above the crater and generally drifted NE. Shortly after these ashing events began, new lava began to effuse rapidly from the crater, feeding the established flowed on the SE, E, and E flanks and generating frequent rockfall events.

Intensified unrest persisted during August. There was a total of 4,141 rockfall events, 2,881 volcanic earthquakes, which included volcanic tremor events, 32 ashing events, and 101 PDCs detected throughout the month. On clear weather days, gas-and-steam emissions rose 300-1,500 m above the crater and drifted in different directions (figure 54). Sulfur dioxide emissions averaged 735-4,756 t/d, the higher value of which was measured on 16 August. During 1-2 August the rate of lava effusion decreased, but continued to feed the flows on the SE, S, and E flanks, maintaining their advances to 3.4 km, 2.8 km, and 1.1 km from the crater, respectively (figure 55). Rockfall and PDCs generated by collapses at the lava flow margins and from the summit dome deposited material within 4 km of the crater. During 3-4 August there were 10 tremor events detected that lasted 1-4 minutes. Short-lived lava pulse lasted 35 seconds and was accompanied by seismic and infrasound signals at 0442 on 6 August. Seven collapses were recorded at the front of the lava flow during 12-14 August.

Figure 54. Photo of Mayon showing a white gas-and-steam plume rising 800-1,500 m above the crater at 0645 on 25 August. Courtesy of William Rogers.

Figure 55. Photo of Mayon facing N showing incandescent lava flows and summit crater incandescence taken at 1830 on 25 August 2023. Courtesy of William Rogers.

During September, similar activity of slow lava effusion, PDCs, gas-and-steam emissions, and seismicity continued. There was a total of 4,452 rockfall events, 329 volcanic earthquakes, which included volcanic tremor events, two ashing events, and 85 PDCs recorded throughout the month. On clear weather days, gas-and-steam emissions rose 100-1,500 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 609-2,252 t/d, the higher average of which was measured on 6 September. Slow lava effusion continued advancing on the SE, S, and E flanks, maintaining lengths of 3.4 km, 2.8 km, and 1.1 km, respectively. Rockfall and PDC events generated by collapses along the lava flow margins and at the summit dome deposited material within 4 km of the crater.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); William Rogers, Legazpi City, Albay Province, Philippines.

Nishinoshima, located about 1,000 km S of Tokyo, is a small island in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent submarine peaks to the S, W, and NE. Eruptions date back to 1973 and the current eruption period began in October 2022. Recent activity has consisted of small ash plumes and fumarolic activity (BGVN 48:07). This report covers activity during May through August 2023, using information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports and satellite data.

Activity during May through June was relatively low. The Japan Coast Guard (JCG) did overflights on 14 and 22 June and reported white gas-and-steam emissions rising 600 m and 1,200 m from the central crater of the pyroclastic cone, respectively (figure 125). In addition, multiple white gas-and-steam emissions rose from the inner rim of the W side of the crater and from the SE flank of the pyroclastic cone. Discolored brown-to-green water was observed around almost the entire perimeter of the island; on 22 June light green discolored water was observed off the S coast of the island.

Figure 125. A white gas-and-steam plume rising 600 m above the crater of Nishinoshima at 1404 on 14 June 2023 (left) and 1,200 m above the crater at 1249 on 22 June 2023 (right). Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, June, 2023).

Observations from the Himawari meteorological satellite confirmed an eruption on 9 and 10 July. An eruption plume rose 1.6 km above the crater and drifted N around 1300 on 9 July. Satellite images acquired at 1420 and 2020 on 9 July and at 0220 on 10 July showed continuing emissions that rose 1.3-1.6 km above the crater and drifted NE and N. The Tokyo VAAC reported that an ash plume seen by a pilot and identified in a satellite image at 0630 on 21 July rose to 3 km altitude and drifted S.

Aerial observations conducted by JCG on 8 August showed a white-and-gray plume rising from the central crater of the pyroclastic cone, and multiple white gas-and-steam emissions were rising from the inner edge of the western crater and along the NW-SE flanks of the island (figure 126). Brown-to-green discolored water was also noted around the perimeter of the island.

Figure 126. Aerial photo of Nishinoshima showing a white-and-gray plume rising from the central crater taken at 1350 on 8 August 2023.

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity), showing an increase in both frequency and power beginning in July (figure 127). This increase in activity coincides with eruptive activity on 9 and 10 July, characterized by eruption plumes. According to the MODVOLC thermal alert algorithm, one thermal hotspot was recorded on 20 July. Weak thermal anomalies were also detected in infrared satellite imagery, accompanied by strong gas-and-steam plumes (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Nishinoshima during May through August 2023, showing an increase in both frequency and power in July, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 128. Infrared (bands B12, B11, B4) satellite images showing a small thermal anomaly at the crater of Nishinoshima on 30 June 2023 (top left), 3 July 2023 (top right), 7 August 2023 (bottom left), and 27 August 2023 (bottom right). Strong gas-and-steam plumes accompanied this activity, extending NW, NE, and SW. Courtesy of Copernicus Browser.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of Strombolian eruptions and ash plumes (BGVN 48:07). This report describes lower levels of activity consisting of ash and white gas-and-steam plumes during May through August 2023, based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, and satellite data.

Activity was relatively low during May and June. Daily white gas-and-steam emissions rose 25-200 m above the crater and drifted in different directions. Five ash plumes were detected at 0519 on 10 May, 1241 on 11 May, 0920 on 12 May, 2320 on 12 May, and at 0710 on 13 May, and rose 1-2.5 km above the crater and drifted SW. A webcam image taken on 12 May showed ejection of incandescent material above the vent. A total of nine ash plumes were detected during 6-11 June: at 1434 and 00220 on 6 and 7 June the ash plumes rose 500 m above the crater and drifted NW, at 1537 on 8 June the ash plume rose 1 km above the crater and drifted SW, at 0746 and at 0846 on 9 June the ash plumes rose 800 m and 3 km above the crater and drifted SW, respectively, at 0423, 1431, and 1750 on 10 June the ash plumes rose 2 km, 1.5 km, and 3.5 km above the crater and drifted NW, respectively, and at 0030 on 11 June an ash plume rose 2 km above the crater and drifted NW. Webcam images taken on 10 and 11 June at 0455 and 0102, respectively, showed incandescent material ejected above the vent. On 19 June an ash plume at 0822 rose 1.5 km above the crater and drifted SE.

Similar low activity of white gas-and-steam emissions and few ash plumes were reported during July and August. Daily white gas-and-steam emissions rose 25-300 m above the crater and drifted in multiple directions. Three ash plumes were reported at 0843, 0851, and 0852 on 20 July that rose 500-2,000 m above the crater and drifted NW.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during May through August 2023 (figure 140). Although activity was often obscured by weather clouds, a thermal anomaly was visible in an infrared satellite image of the crater on 12 May, accompanied by an eruption plume that drifted SW (figure 141).

Figure 140. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during May through August 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 141. A single thermal anomaly (bright yellow-orange) was visible at Krakatau in this infrared (bands B12, B11, B4) satellite image taken on 12 May 2023. An eruption plume accompanied the thermal anomaly and drifted SW. Courtesy of Copernicus Browser.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/).

Merapi, located just north of the major city of Yogyakarta in central Java, Indonesia, has had activity within the last 20 years characterized by pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome. The current eruption period began in late December 2020 and has more recently consisted of ash plumes, intermittent incandescent avalanches of material, and pyroclastic flows (BGVN 48:04). This report covers activity during April through September 2023, based on information from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

Activity during April through September 2023 primarily consisted of incandescent avalanches of material that mainly affected the SW and W flanks and traveled as far as 2.3 km from the summit (table 25) and white gas-and-steam emissions that rose 10-1,000 m above the crater.

Table 25. Monthly summary of avalanches and avalanche distances recorded at Merapi during April through September 2023. The number of reported avalanches does not include instances where possible avalanches were heard but could not be visually confirmed as a result of inclement weather. Data courtesy of BPPTKG (April-September 2023 daily reports).

BPPTKG reported that during April and May white gas-and-steam emissions rose 10-750 m above the crater, incandescent avalanches descended 500-2,000 m on the SW and W flanks (figure 135). Cloudy weather often prevented clear views of the summit, and sometimes avalanches could not be confirmed. According to a webcam image, a pyroclastic flow was visible on 17 April at 0531. During the week of 28 April and 4 May a pyroclastic flow was reported on the SW flank, traveling up to 2.5 km. According to a drone overflight taken on 17 May the SW lava dome volume was an estimated 2,372,800 cubic meters and the dome in the main crater was an estimated 2,337,300 cubic meters.

Figure 135. Photo showing an incandescent avalanche affecting the flank of Merapi on 8 April 2023. Courtesy of Øystein Lund Andersen.

During June and July similar activity persisted with white gas-and-steam emissions rising 10-350 m above the crater and frequent incandescent avalanches that traveled 300-2,000 m down the SW, W, and S flanks (figure 136). Based on an analysis of aerial photos taken on 24 June the volume of the SW lava dome was approximately 2.5 million cubic meters. A pyroclastic flow was observed on 5 July that traveled 2.7 km on the SW flank. According to the Darwin VAAC multiple minor ash plumes were identified in satellite images on 19 July that rose to 3.7 km altitude and drifted S and SW. During 22, 25, and 26 July a total of 17 avalanches descended as far as 1.8 km on the S flank.

Figure 136. Photo showing an incandescent avalanche descending the flank of Merapi on 23 July 2023. Courtesy of Øystein Lund Andersen.

Frequent white gas-and-steam emissions continued during August and September, rising 10-450 m above the crater. Incandescent avalanches mainly affected the SW and W flanks and traveled 400-2,300 m from the vent (figure 137). An aerial survey conducted on 10 August was analyzed and reported that estimates of the SW dome volume was 2,764,300 cubic meters and the dome in the main crater was 2,369,800 cubic meters.

Figure 137. Photo showing a strong incandescent avalanche descending the flank of Merapi on 23 September 2023. Courtesy of Øystein Lund Andersen.

Frequent and moderate-power thermal activity continued throughout the reporting period, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 138). There was an increase in the number of detected anomalies during mid-May. The MODVOLC thermal algorithm recorded a total of 47 thermal hotspots: six during April, nine during May, eight during June, 15 during July, four during August, and five during September. Some of this activity was captured in infrared satellite imagery on clear weather days, sometimes accompanied by incandescent material on the SW flank (figure 139).

Figure 138. Frequent and moderate-power thermal anomalies were detected at Merapi during April through September 2023, as shown on this MIROVA plot (Log Radiative Power). There was an increase in the number of anomalies recorded during mid-May. Courtesy of MIROVA.

Figure 139. Infrared (bands B12, B11, B4) satellite images showed a consistent thermal anomaly (bright yellow-orange) at the summit crater of Merapi on 8 April 2023 (top left), 18 May 2023 (top right), 17 June 2023 (middle left), 17 July 2023 (middle right), 11 August 2023 (bottom left), and 20 September 2023 (bottom right). Incandescent material was occasionally visible descending the SW flank, as shown in each of these images. Courtesy of Copernicus Browser.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Øystein Lund Andersen (URL: https://www.oysteinlundandersen.com/, https://twitter.com/oysteinvolcano).

Villarrica, in central Chile, consists of a 2-km-wide caldera that formed about 3,500 years ago and is located at the base of the presently active cone at the NW margin of a 6-km-wide caldera. Historical eruptions eruptions date back to 1558 and have been characterized by mild-to-moderate explosive activity with occasional lava effusions. The current eruption period began in December 2014 and has recently consisted of nighttime crater incandescence, ash emissions, and seismicity (BGVN 48:04). This report covers activity during April through September 2023 and describes occasional Strombolian activity, gas-and-ash emissions, and nighttime crater incandescence. Information for this report primarily comes from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN) and satellite data.

Seismicity during April consisted of long period (LP) events and tremor (TRE); a total of 9,413 LP-type events and 759 TR-type events were detected throughout the month. Nighttime crater incandescence persisted and was visible in the degassing column. Sulfur dioxide data was obtained using Differential Absorption Optical Spectroscopy Equipment (DOAS) that showed an average value of 1,450 ± 198 tons per day (t/d) during 1-15 April and 1,129 ± 201 t/d during 16-30 April, with a maximum daily value of 2,784 t/d on 9 April. Gas-and-steam emissions of variable intensities rose above the active crater as high as 1.3 km above the crater on 13 April. Strombolian explosions were not observed and there was a slight decrease in the lava lake level.

There were 14,123 LP-type events and 727 TR-type events detected during May. According to sulfur dioxide measurements taken with DOAS equipment, the active crater emitted an average value of 1,826 ± 482 t/d during 1-15 May and 912 ± 41 t/d during 16-30 May, with a daily maximum value of 5,155 t/d on 13 May. Surveillance cameras showed continuous white gas-and-steam emissions that rose as high as 430 m above the crater on 27 May. Nighttime incandescence illuminated the gas column less than 300 m above the crater rim was and no pyroclastic emissions were reported. A landslide was identified on 13 May on the E flank of the volcano 50 m from the crater rim and extending 300 m away; SERNAGEOMIN noted that this event may have occurred on 12 May. During the morning of 27 and 28 May minor Strombolian explosions characterized by incandescent ejecta were recorded at the crater rim; the last reported Strombolian explosions had occurred at the end of March.

Seismic activity during June consisted of five volcano-tectonic (VT)-type events, 21,606 LP-type events, and 2,085 TR-type events. The average value of sulfur dioxide flux obtained by DOAS equipment was 1,420 ± 217 t/d during 1-15 June and 2,562 ± 804 t/d, with a maximum daily value of 4,810 t/d on 17 June. White gas-and-steam emissions rose less than 480 m above the crater; frequent nighttime crater incandescence was reflected in the degassing plume. On 12 June an emission rose 100 m above the crater and drifted NNW. On 15 June one or several emissions resulted in ashfall to the NE as far as 5.5 km from the crater, based on a Skysat satellite image. Several Strombolian explosions occurred within the crater; activity on 15 June was higher energy and ejected blocks 200-300 m on the NE slope. Surveillance cameras showed white gas-and-steam emissions rising 480 m above the crater on 16 June. On 19 and 24 June low-intensity Strombolian activity was observed, ejecting material as far as 200 m from the center of the crater to the E.

During July, seismicity included 29,319 LP-type events, 3,736 TR-type events, and two VT-type events. DOAS equipment recorded two days of sulfur dioxide emissions of 4,220 t/d and 1,009 t/d on 1 and 13 July, respectively. Constant nighttime incandescence was also recorded and was particularly noticeable when accompanied by eruptive columns on 12 and 16 July. Minor explosive events were detected in the crater. According to Skysat satellite images taken on 12, 13, and 16 July, ashfall deposits were identified 155 m S of the crater. According to POVI, incandescence was visible from two vents on the crater floor around 0336 on 12 July. Gas-and-ash emissions rose as high as 1.2 km above the crater on 13 July and drifted E and NW. A series of gas-and-steam pulses containing some ash deposited material on the upper E flank around 1551 on 13 July. During 16-31 July, average sulfur dioxide emissions of 1,679 ± 406 t/d were recorded, with a maximum daily value of 2,343 t/d on 28 July. Fine ash emissions were also reported on 16, 17, and 23 July.

Seismicity persisted during August, characterized by 27,011 LP-type events, 3,323 TR-type events, and three VT-type events. The average value of sulfur dioxide measurements taken during 1-15 August was 1,642 ± 270 t/d and 2,207 ± 4,549 t/d during 16-31 August, with a maximum daily value of 3,294 t/d on 27 August. Nighttime crater incandescence remained visible in degassing columns. White gas-and-steam emissions rose 480 m above the crater on 6 August. According to a Skysat satellite image from 6 August, ash accumulation was observed proximal to the crater and was mainly distributed toward the E slope. White gas-and-steam emissions rose 320 m above the crater on 26 August. Nighttime incandescence and Strombolian activity that generated ash emissions were reported on 27 August.

Seismicity during September was characterized by five VT-type events, 12,057 LP-type events, and 2,058 TR-type events. Nighttime incandescence persisted. On 2 September an ash emission rose 180 m above the crater and drifted SE at 1643 (figure 125) and a white gas-and-steam plume rose 320 m above the crater. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. Strombolian activity was observed on 2 September and during the early morning of 3 September, the latter event of which generated an ash emission that rose 60 m above the crater and drifted 100 m from the center of the crater to the NE and SW. Ashfall was reported to the SE and S as far as 750 m from the crater. The lava lake was active during 3-4 September and lava fountaining was visible for the first time since 26 March 2023, according to POVI. Fountains captured in webcam images at 2133 on 3 September and at 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. Sulfur dioxide flux of 1,730 t/d and 1,281 t/d was measured on 3 and 4 September, respectively, according to data obtained by DOAS equipment.

Figure 125. Webcam image of a gray ash emission rising above Villarrica on 2 September 2023 at 1643 (local time) that rose 180 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 02 de septiembre de 2023, 17:05 Hora local).

Strong Strombolian activity and larger gas-and-ash plumes were reported during 18-20 September. On 18 September activity was also associated with energetic LP-type events and notable sulfur dioxide fluxes (as high as 4,277 t/d). On 19 September Strombolian activity and incandescence were observed. On 20 September at 0914 ash emissions rose 50 m above the crater and drifted SSE, accompanied by Strombolian activity that ejected material less than 100 m SSE, causing fall deposits on that respective flank. SERNAGEOMIN reported that a Planet Scope satellite image taken on 20 September showed the lava lake in the crater, measuring 32 m x 35 m and an area of 0.001 km². Several ash emissions were recorded at 0841, 0910, 1251, 1306, 1312, 1315, and 1324 on 23 September and rose less than 150 m above the crater. The sulfur dioxide flux value was 698 t/d on 23 September and 1,097 t/d on 24 September. On 24 September the Volcanic Alert Level (VAL) was raised to Orange (the third level on a four-color scale). SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

During 24-25 September there was an increase in seismic energy (observed at TR-events) and acoustic signals, characterized by 1 VT-type event, 213 LP-type events, and 124 TR-type events. Mainly white gas-and-steam emissions, in addition to occasional fine ash emissions were recorded. During the early morning of 25 September Strombolian explosions were reported and ejected material 250 m in all directions, though dominantly toward the NW. On 25 September the average value of sulfur dioxide flux was 760 t/d. Seismicity during 25-30 September consisted of five VT-type events, 1,937 LP-type events, and 456 TR-type events.

During 25-29 September moderate Strombolian activity was observed and ejected material as far as the crater rim. In addition, ash pulses lasting roughly 50 minutes were observed around 0700 and dispersed ENE. During 26-27 September a TR episode lasted 6.5 hours and was accompanied by discrete acoustic signals. Satellite images from 26 September showed a spatter cone on the crater floor with one vent that measured 10 x 14 m and a smaller vent about 35 m NE of the cone. SERNAGEOMIN reported an abundant number of bomb-sized blocks up to 150 m from the crater, as well as impact marks on the snow, which indicated explosive activity. A low-altitude ash emission was observed drifting NW around 1140 on 28 September, based on webcam images. Between 0620 and 0850 on 29 September an ash emission rose 60 m above the crater and drifted NW. During an overflight taken around 1000 on 29 September scientists observed molten material in the vent, a large accumulation of pyroclasts inside the crater, and energetic degassing, some of which contained a small amount of ash. Block-sized pyroclasts were deposited on the internal walls and near the crater, and a distal ash deposit was also visible. The average sulfur dioxide flux measured on 28 September was 344 t/d. Satellite images taken on 29 September ashfall was deposited roughly 3 km WNW from the crater and nighttime crater incandescence remained visible. The average sulfur dioxide flux value from 29 September was 199 t/d. On 30 September at 0740 a pulsating ash emission rose 1.1 km above the crater and drifted NNW (figure 126). Deposits on the S flank extended as far as 4.5 km from the crater rim, based on satellite images from 30 September.

Figure 126. Webcam image of a gray ash plume rising 1.1 km above the crater of Villarrica at 0740 (local time) on 30 September 2023. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 30 de septiembre de 2023, 09:30 Hora local).

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed intermittent thermal activity during April through September, with slightly stronger activity detected during late September (figure 127). Small clusters of thermal activity were detected during mid-June, early July, early August, and late September. According to the MODVOLC thermal alert system, a total of four thermal hotspots were detected on 7 July and 3 and 23 September. This activity was also intermittently captured in infrared satellite imagery on clear weather days (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Villarrica during April through September 2023, according to this MIROVA graph (Log Radiative Power). Activity was relatively low during April through mid-June. Small clusters of activity occurred during mid-June, early July, early August, and late September. Courtesy of MIROVA.

Figure 128. Consistent bright thermal anomalies (bright yellow-orange) were visible at the summit crater of Villarrica in infrared (bands B12, B11, B4) satellite images, as shown on 17 June 2023 (top left), 17 July 2023 (top right), 6 August 2023 (bottom left), and 20 September 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed 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 eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Moderate explosive activity continued during June-November 2023 (figures 50 and 51). According to visual data from Severo-Kurilsk, explosions sent ash 2-3.5 km above the summit (3-4.5 km altitude) during most days during June through mid-September. Activity after mid-September was slightly weaker, with ash usually reaching less than 2 km above the summit. According to KVERT the volcano in October and November was, with a few exceptions, either quiet or obscured by clouds that prevented satellite observations. KVERT issued Volcano Observatory Notices for Aviation (VONA) on 8 and 12 June, 13 and 22 July, 3 and 21 August, and 31 October warning of potential aviation hazards from ash plumes drifting 3-15 km from the volcano. Based on satellite data, KVERT reported a persistent thermal anomaly whenever weather clouds permitted viewing.

Figure 50. Ash explosion from the active summit crater of Ebeko on 18 July 2023; view is approximately towards the W. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Figure 51. Ash explosion from the active summit crater of Ebeko on 23 July 2023 with lightning visible in the lower part of the plume. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

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/).

Ambrym was observed during an aircraft overflight on 25 February 2000. Pilot reports indicated that lava was beginning to reappear in some vents. As previously reported (BGVN 25:02), the long-standing lava lakes in Benbow and Mbwelesu craters had disappeared after the 26 November 1999 earthquake. Activity increased slightly from January indicating the magma column may be rising again.

Brown vapor was being emitted from Mbwelesu crater. The bottom of the crater could be seen but no lava was observed. Four craters, each ~8 m in diameter, were located on a 40 x 10 m section of the sill on the E rim of the crater. This rim had been weakened by the November 1999 earthquake and appeared ready to fall. Both vents inside Benbow crater emitted white vapor, which rose 1.5 km as one plume. The bottom of the vents could not be seen.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: John Seach, PO Box 16, Chatsworth Island, N.S.W. 2469, Australia.

This report covers the period January-April 2000. As of 28 April 2000, KVERT (Kamchatkan Volcanic Eruption Response Team) temporarily suspended operations because of a lack of funding. Except for brief episodes of volcanic and seismic activity in mid-February and mid-March, little to no seismicity was registered beneath the volcano through the remainder of the period. Occasional, weak fumarolic activity was frequently accompanied by fumarolic plumes rising 50-800 m above the volcano and extending up to 50 km away. The volcano frequently was obscured by clouds, which prevented both visual and satellite observations.

During the week of 11-17 February seven shallow earthquakes were registered beneath the volcano. Seismic activity increased above background levels during 10-13 March, and on 14 March the hazard level color code was raised to Red due to continuous volcanic tremor after 0425, which occasionally registered at distances 100 km. At 0430, a satellite image showed a 4-pixel thermal anomaly with a temperature of 50°C and a small ash cloud. At 0742, the ash cloud rose to 5 km altitude and spread to the W; at 0815 ashfall was recorded in Kozirevsk; and at 1100 the ash cloud covered Kozirevsk and spread up to the Sredinny Range, a distance of 225 km. After 1000, the level of volcanic activity decreased and on 15 March the hazard status was reduced to Yellow after seismicity decreased to background levels. Analysis by the Alaska Volcano Observatory (AVO) of a satellite image taken at 0630 on 15 March revealed a 6-pixel thermal anomaly but no ash cloud. According to GMS-5 satellite data (Tokyo VAAC), between 0932 and 2032 on 15 March the ash cloud drifted across Shelekhov Bay and moved NW at an altitude of ~7 km.

At 0500 on the morning of 16 March the volcano erupted again, prompting a hazard status change to Orange. AVO reported that a satellite image taken at 0530 showed an 8-pixel thermal anomaly and a detached ash cloud drifting SW with the leading edge ~140 km from the volcano. The size of the cloud was estimated to be ~100 km E-W by 60 km N-S, but no height estimate was available. KVERT detected increased seismic activity under the volcano with volcanic tremor beginning around 2350 on 15 March and continuing until 0930 the next morning.

Visual reports at 2106 on 18 March from the village of Kozirevsk indicated that a dark dense plume rose to 5,000 m altitude and extended more than 150 km SW. The base of the cloud was not distinct and ashfall may have occurred. Seismicity returned to background levels after 19 March. Rock avalanches continued, although no volcanic tremor was recorded. On 19-22 March, a steam-and-gas plume rose 1-2 km above the volcano and extended up to 10 km NE and E. On 23 March, a plume rose 200 m above the volcano and satellite images analyzed by AVO continued to show a thermal anomaly, but of decreased intensity.

During the last week in March and throughout April seismicity returned to background levels and occasional fumarolic activity and shallow earthquakes were typical of the activity during the earlier part of the year.

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: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

This report covers the period from January-April 2000. As of 28 April 2000, KVERT (Kamchatkan Volcanic Eruption Response Team) temporarily suspended operations because of a lack of funding. Throughout January and early February, seismicity was at background levels with about 1-5 small local earthquakes each day. However, after two months of quiescence, seismic activity beneath the volcano renewed at 0635 on 12 February; seismic data indicated the occurrence of a probable, short-lived gas-and-ash (or gas) explosion as a 5-minute-long seismic signal was recorded by the nearest seismic station (KRY). As a result of this activity, the hazard level was raised to Yellow.

Following this event, an average of 5-10 seismic events/hour occurred, mainly corresponding to explosions along with a few blowouts and pyroclastic flow (?) signals. After 1200 on 15 February, seismicity increased to 40 events/hour. A continuous series of seismic events (pyroclastic flows?) were registered from 1620 to 1810 on 15 February, but satellite images at 1715 and 1855 that day did not show a plume rising above the weather clouds at ~4 km altitude. From 1800 on 15 February to 1340 on 16 February, activity varied from 40-60 events/hour at the end of which a new 80-minute-long series of seismic events was registered. Subsequently, during 1500-1750, 200 events occurred and, at 1750, strong low-frequency, 4-minute-long events were registered. Activity decreased from 40 to 20 events/hour from 1800 on 16 February until 1200 on 17 February. A satellite image from 0630 on 17 February, the first clear image that the Alaska Volcano Observatory had received since the increase of activity on 12 February, showed no plume, but a thermal anomaly covered at least four pixels with a maximum temperature of 40°C.

The low-level Strombolian eruptive activity that has characterized the volcanism for much of the past four years continued for the remainder of February and throughout March. The number of gas-and-ash explosions decreased from 600/day to 25/day during late February. From 1230 to 1620 on 18 February weak volcanic tremor was registered. According to pilot reports on 20 and 21 February, these explosions sent material up to 1,500 m above the volcano. The number of gas-and-ash explosions varied from 5 to 300/day (although 5-50/day was most typical) through March; suspected small pyroclastic flows were occasionally detected.

At 0625 and 2336 on 3 April and 0743 on 4 April, seismic data indicated short-lived ash-and-gas explosions more intense than those previously recorded. These explosions probably occurred during 3-6-minute-long seismic events recorded by the nearest seismic station (KRY). However, during the following week, no more than three small events/day were recorded and the hazard level was returned to Green. This level of activity continued through April with occasional 2-15-minute-long seismic events, related rock avalanches, and possible short-lived explosions.

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

On 14 May 2000 Australian research vessel Franklin visited Kavachi seamount to discover that... a new eruption was underway (figure 6). The vessel was supporting an Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) research cruise (FR04/00) entitled Project SHAARC (Submarine Hydrothermally Active Arc Volcanoes). Upon arrival at 0600 hours observers saw waves breaking on the peak of the volcano. Every 5-7 minutes the scientists on the CSIRO vessel witnessed violent eruptions that lasted 2-3 minutes each, prevailing for the approximately 20 hours that they observed the volcano. The position of the eruption column was fixed specifically at 8.99°S, 157.97°E, and the volcano's peak was estimated at 2-5 m below the water surface. The eruptions ejected ash and incandescent blocks of lava up to 70 m above the sea surface and sulfurous steam plumes rose to heights of 500 m.

Figure 6. Photograph showing an eruption at Kavachi Seamount, 14 May 2000. Courtesy of CSIRO.

The science team aboard Franklin documented the apparent new phase of island-building eruptive activity. A bathymetric map was produced that differed significantly from the previous map of the volcano made in 1984. Hydrocast/CTD (conductivity-temperature-depth) surveys were undertaken; these detected numerous chemical and particle plumes in the water column; the plumes extended at least 5 km from the eruptive center. Systematic sampling of rocks and ash from the flanks of the volcano returned glassy, vesicular basalts and hydrothermally altered sulfide-rich volcanic breccias. In addition, approximately 500 photographs and 2 hours of digital video captured ~20 eruptions during daylight.

The scientific participants on the CSIRO-led cruise were: Brent McInnes, CSIRO Exploration & Mining Chief Scientist; Richard Arculus, Principal Investigator, and Cromwell Qopoto, Australian National University (ANU); Gary Massoth, Principal Investigator, and Cornel DeRonde, New Zealand Institute of Geological and Nuclear Sciences Ltd (IGNS); Ed Baker, National Oceanic and Atmospheric Administration (NOAA); Pamela Brodie, Dan Conwell, Tim MacConachy, and Ron Plaschke, CSIRO; John Chadwick, University of Florida; and Philip Pousai, University of Papua New Guinea.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is located in the Solomon Islands south of Vangunu Island. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: Brent McInnes, Chieft Scientist (FRO4/00), Division of Exploration & Mining, Australian Commonwealth Scientific and Industrial Organization (CSIRO), PO Box 136, North Ryde, NSW 1670, Australia (URL: http://www.csiro.au/).

The period of activity at Kīlauea between 13 September 1999 and May 2000 was marked by multiple lava breakouts that were caused by damming of the lava-tube system. The lava flows traveled from Pu`u `O`o crater, down Pulama pali, and along the coastal plain towards the sea, covering an area several kilometers wide (figure 145).

On 12 September 1999 magma intruded into the upper rift zone of the volcano (BGVN 25:01), with most of it draining underground afterwards leaving a small lava lake near the center of Pu`u `O`o crater. By 16 September the lake was 50-60 m in diameter and ~92 m below the E rim of the crater. From September to mid-October the level of the lava lake was ~63 m below the E rim of the crater with periods of episodic ascent by 10-15 m. During the intrusion and withdrawal of the lava lake a new collapse pit, termed the West Gap Pit, formed in the W gap of the crater. The pit is 30-50 m wide and 12-15 m deep.

Figure 145. Distribution of lava flows from Kīlauea's E rift zone during January 1983-May 2000. Contours are in meters and the contour interval is ~ 150 m. Courtesy of HVO.

After the intrusion of magma, the 25th eruptive pause of the current eruptive episode (55) occurred when the supply of lava to a well-established lava-tube system, that ended near Kamokuna, stopped. Activity recommenced 23 September when lava erupted from a new spatter cone termed the 'mini-vent,' in Puka Nui collapse pit. Puka Nui is located on the S side of Pu`u `O`o crater. The pit formed in December 1997 and has been growing ever since by the collapse of tephra from the north wall of the pit and by engulfing other cones and smaller pits. Lava had not previously erupted from Puka Nui.

From 12 September through 22 October the lava-tube system was blocked by dams made up of debris that fell from the roof and walls of the tube. After the lava drained from the tube following intrusion, the walls cooled, shrank, and broke, causing the roof to collapse in places and the walls to crumble forming piles of debris. When lava re-entered the system it was blocked by a dam at 530 m elevation and reached the surface through cracks and holes in the tube upstream of the dam. Upslope breakout points were originally thought to be perched ponds (low shield-shaped structures with a pond of lava on top), but because the features were not vents and there was no ponded lava at the sites, they are referred to as breakout points from the lava tube. The lava traveled down Pulama pali towards the sea, but did not reach it. By 16 October activity occurred at the West Gap Pit with spattering from a vent in the floor of the pit, and by 18 or 19 October it filled with lava and overflowed, producing a shelly pahoehoe flow 160 m long.

Over the next several months the lava continued to flow down Pulama pali and just after sunset on 13 December lava reached the sea for the first time since the September intrusion. This took place at Highcastle entry and continued until 17-18 December. On 19 December lava reached the sea at the Lae`apuki lava bench, which had not been active since January 1997. As the lava flowed to the sea, it traveled down the sea cliff with some of it eventually cooling and solidifying to form wonderful examples of lava drapery (structures resembling the drips of wax down a candle). Throughout January lava continued to flow towards the sea causing the Lae`apuki bench to grow. On 3 February a second ocean entry became active at Waha`ula, 4 km E of Lae`apuki. Waha`ula was last active in July 1997 to late 1998.

On 23 February at 0142 tiltmeters at the summit showed a downward tilt toward the caldera (figure 146). The downward tilt indicated that magma was moving out of the summit reservoir; data from two other tiltmeters on the east rift zone indicated that the magma was moving into the rift zone. Concurrently, seismometers recorded a swarm of shallow earthquakes at the upper rift zone of the volcano. The swarm was centered near Pauahi Crater, ~7 km SE of Halema`uma`u Crater along the Chain of Craters Road. Hawaiian Volcano Observatory staff believed that the swarm probably recorded the movement of magma intruding to within 1-2 km of the ground surface. Such swarms commonly precede new fissure eruptions, but they more often record intrusions rather than eruptions. The reversal of the summit tilt a few hours later indicated when the intrusion stopped and magma once again moved into the summit reservoir. No new ground cracks were noted. The intrusion caused minor, but noticeable, changes in the ongoing eruption. Lava continued to enter the tube system at Pu`u `O`o, but blockages in the lava-tube system above the Pulama pali resulted in many breakouts on the active flow field. Lava stopped flowing to Lae`apuki entry on 25 March and intermittent ocean entries began along 2 km of coast centered on Waha`ula, which is currently the main entry.

Figure 146. Kīlauea tiltmeter record for 22-24 February 2000 at Uwekahuna (tilt along an azimuth of N50W). Courtesy of HVO.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

This report covers the period January-April 2000. As of 28 April 2000, KVERT (Kamchatkan Volcanic Eruption Response Team) temporarily suspended operations because of a lack of funding. During most of this four-month period, seismicity at the volcano was at background levels, with shallow earthquakes and weak fumarolic activity accompanied frequently by fumarolic plumes. The plumes rose from 50 to 1,500 m above the volcano and extended in various directions as far as 10 km from the vent. The volcano frequently was obscured by clouds.

Twice during the reporting period, however, the activity level increased sufficiently so that the hazard level was raised from Green to Yellow. The first of these events occurred during 29 January-3 February when seismicity was above background levels with shallow earthquakes and tremor registered. On 29 January, a fumarolic plume rose 1,500 m above the crater extending 30 km to the SE. On 30 January and 1 February, a plume rose 50-500 m above the crater and extended up to 15 km S. According to reports from pilots of Northwest and Reeve Aleutian Airlines, and an observer from Alaska Volcano Observatory on the Reeve flight, a gas-and-steam plume was observed at 1440 on 30 January rising as high as 8-10 km altitude. On the morning of 3 February the volcano was quiet. According to visual reports from from Kliuchi (~30 km NE of the volcano) and pilot reports, a short-lived eruption at 1708 on 3 February sent an ash-poor plume to altitudes of 8-9 km; instruments measured an accompanying increase in seismicity. According to analysis of a satellite image at 1825 on 3 February the plume extended 40 km SSE and contained a large amount of water or ice.

During the subsequent week (4-10 February), seismicity continued above background levels and shallow earthquakes and tremor were registered. At 1815 on 8 February, seismic data indicated that a short-lived explosion probably occurred, because a series of shallow events were recorded. On 4-5 February, a fumarolic plume rose 200-500 m above the crater and extended 3-40 km W and SW.

During the period 11 February to 2 March, seismicity at the volcano returned to near background level accompanied by typical events noted earlier. But during 3-9 March, seismicity at the volcano increased again and the hazard level was raised again to Yellow. During this period, shallow earthquakes were registered. On 3-4 March a steam-and-gas plume rose 1,500 m above the volcano. On 5-7 March, a fumarolic plume rose 100-300 m above the crater. On 8 March, a steam-and-gas plume rose 1,000 m above the volcano extending 5 km to the NW.

Seismicity returned to background levels on 13 March and the hazard level was reduced to Green. This situation prevailed throughout the remainder of March and April. Shallow earthquakes, fumarolic activity, and plumes continued as in the earlier portion of the reporting period.

Geologic Background. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Intermittent activity in October and December 1999 and February-March 2000 was reported by pilots flying near the island. Reports sent to the Wellington Volcanic Ash Advisory Center and the Port Vila control tower resulted in aviation advisories that formed the basis for this summary. Additional information about February activity was provided by John Seach, who visited the volcano (figure 7) during 23-24 February and talked to local residents.

Figure 7. Photograph of Lopevi volcano with Epi Island in the distance, February 2000. Courtesy of John Seach.

During 1963-82, ash emissions, lava flows, lava fountains, and Strombolian explosions occurred intermittently. This twenty-year pattern ended on 24 October 1982. Eruptive activity resumed in July 1998. A series of Strombolian explosions in the main 1963 crater (just NW of the central crater) was observed during November-December 1998 (BGVN 24:02). Lava extrusion in December 1998 produced a small intracrater flow from a cone at the NW end of the 1963 crater (BGVN 24:07). Sporadic eruptive activity observed between the end of December 1998 and March 1999 was confined to the 1963 crater at ~1,000 m elevation on the NW flank.

Ground observers reported on 15 October 1999 that a minor eruption approximately one week earlier consisted of a continuous ash plume rising to an altitude of 300-600 m. An ash cloud up to 2,700 m altitude was reported by pilots on 9-10 December.

Activity during February 2000. At 1422 on 3 February an ash cloud was reported up to 4,500 m over the island. Between 0507 and 0520 on 18 February a Vanair pilot reported an ash cloud initially as high as 9 km and then around 4.8 km altitude. A plume was identified on infrared and early morning visible satellite imagery, but there was considerable cloudiness in the area. Activity was continuing as of 0602 on 19 February, with an ash plume to ~5.5 km altitude. Convective activity through the day rapidly obscured any traces of the ash plume on satellite imagery. Analyses indicated that the plume may have contained significant ash that was expected to move ESE-SE and extend to 370 km. Due to the convective activity pilots were advised to respond as though the plume was rising to ~9 km altitude. However, another pilot report at 0621 indicated that activity had ceased.

According to John Seach, around midday on 19 February villagers on the neighboring island of Paama heard two loud explosions from the volcano; the second was the loudest heard in two years. This was followed by a brown ash cloud rising to ~4.5 km altitude visible from an aircraft en route from Sydney to Hawaii. Ashfalls occurred on Paama. A pyroclastic flow from the NW-flank crater (figure 8) traveled 2.5 km SW and reached the sea along a 300-m-wide front, where it created a delta. Blocks of 4 m in diameter were carried down to sea level. A 3-m-diameter block 50 m from the sea had a surface temperature of 170°C.

Figure 8. View of the 19 February pyroclastic flow path from summit of Lopevi to the sea, 23 February 2000. Courtesy of John Seach.

Trees buried in the ash were only visible as circular, black smoking areas when observed on 23 February. On the edge of the pyroclastic-flow deposit trees 0.5 m in diameter were snapped off 2 m from the ground (figure 9); some were upturned and redeposited in the ash. Standing devastated trees were burnt to a height of 18 m and debris was lodged in the canopy. Some fallen trees were still burning. Within 20 m of the edge of the pyroclastic-flow deposit were fallen devastated trees, and standing burnt trees were present for another 10 m beyond that. Observers were forced back when ground temperatures (over 200°C) melted boots. The air was filled with the smell of burnt trees and sulfur. Coconuts superheated and exploded. Sap in smaller branches boiled and formed bubbles on the outside of the branch.

Figure 9. Devastated trees at the edge of a pyroclastic flow from Lopevi, 23 February 2000. Courtesy of John Seach.

The active NW crater (figure 10) was climbed by Seach and Geoff Mackley on 23 and 24 February 2000. A lava dome was present on the SW crater rim from where the pyroclastic flow had originated along a 100-m-wide front. Lighter colored deposits marked its track down the volcano. On the N edge of the pyroclastic-flow deposit and 200 m below the NW crater was the source of a debris avalanche. A debris volume on the order of a million cubic meters had moved, exposing a large hole and producing two branching flows that moved ~250 m SW towards the sea. A new 150-m-high cinder cone formed inside the old crater was smoking with blue and white vapor being emitted. Ground temperature was 81°C at the base of the cinder cone. A two-week-old flank lava flow was observed to have traveled 1.5 km WNW to an elevation 250 m below the NW crater, burning trees and vegetation.

Figure 10. Aerial view of the active NW crater at Lopevi, February 2000. Courtesy of John Seach.

Additional eruptions during March and April 2000. Another minor eruption on 15 March sent ash to 1,500 m, where it mixed with meteorological clouds. A stronger eruption began on 21 March with ash reported to 2,700 m altitude along with steam to 450 m. Activity increased the next day, when the plume was reported to be at levels of 4,200-4,500 m. The eruption continued during 23-24 March with plumes rising to altitudes of about 2,400 m.

A pilot report around 1100 on 24 April indicated a very dark-brown ash cloud rising from the volcano to an altitude of ~4,800 m and drifting NW. The report also noted that a new vent on the W side was emitting lava. A notice to aviators stated that they should expect ash to 5,400 m altitude within 550 km in the NW quadrant of the volcano. Ash warnings to aviators in the late morning and night of 25 April indicated that ash and lava continued to be emitted from the W vent. Pilot reports indicated that the height of the ash cloud was difficult to determine due to cloud cover, but was at least to 600 m altitude. The ash appeared to be thinning out beyond 18 km W of the volcano, but the extent of the plume could not be determined from satellite imagery. A notice at 2155 on 26 April stated that no further visual activity reports had been received since 25 April.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Wellington Volcanic Ash Advisory Center (VAAC), MetService, PO Box 722, Wellington, New Zealand (URL: http://www.metservice.co.nz/); John Seach, PO Box 16, Chatsworth Island, N.S.W. 2469, Australia.

Volcanic unrest that began at Mayon during May 1999 led to growth of the lava dome beginning on 12 February 2000, and continued in the form of explosive eruptions from 23 February through 1 March (BGVN 25:02). Since the 1 March 2000 eruption, observations have indicated that activity is declining.

After 1 March activity was relatively quiet with the largest event being an ash puff that was produced on 12 March when hot lava at the summit came in contact with surface water. The ash puff rose to a height of ~1 km and drifted to the NW. After 1 March there was moderate seismicity, high volcanic gas outputs, no increase in ground deformation, continuing glow of the summit, and new lava effusion. These conditions were associated with very gradual return to repose. Therefore on 16 March PHIVOLCS reduced the alert status of Mayan volcano from Alert Level 4 (hazardous eruption imminent, possible within days) to alert Level 3 (less probability of a hazardous eruption).

On 17 March at 1254, 1350, 1609, and 1619 partial collapse of the new lava flow on the volcano's upper middle slopes produced voluminous secondary pyroclastic flows, with the associated ash clouds blown to the SW. On 19 March at 0138, 0203, and 0300 similar secondary pyroclastic flows occurred with ash clouds that blew NE. The billowing ash clouds did not originate from the crater, but from the side of the lava flow that failed on the volcano's upper slope, and from descending detached lava fragments along Bonga Gully. Scientists expected the series of secondary pyroclastic flows due to the instability of volcanic material deposited on steep ground.

After 25 March the number of low-frequency volcanic earthquakes was relatively high (up to 40 per day), presumably due to strong jetting of hot gases emanating from the crater. In addition, relatively high levels of SO₂ emissions (up to 9,000 metric tons per day) also occurred after 1 March that were associated with degassing of residual magma. Since the volcano showed no signs of an imminent eruption, on 1 April PHIVOLCS reduced the Alert Level to 2 (the probability of hazardous explosive eruption is minimal). The volcanic system is expected to continue producing earthquakes and to vent a large amount of gas because fresh magma still resides along the whole length of the volcanic conduit and near the summit. Due to the possibility of sudden explosions caused by the release of gas from localized pockets within the magmatic system, and the threat of secondary pyroclastic flows, rockfalls, and ash fallout, PHIVOLCS maintains off-limit zones around the volcano up to 7 km in radius.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost. gov.ph/).

Prior to 24 March 2000, KVERT (Kamchatkan Volcanic Eruption Response Team) had not included Mutnovsky in any activity reports this year. As of 28 April 2000, KVERT temporarily suspended operations because of a lack of funding.

After more precise analysis of visual reports from Petropavlovsk-Kamchatsky and a comparison of signals from several seismic stations, KVERT determined that at 0700 on 17 March, a short-lived explosion sent a gas-and-steam plume to heights of ~1,000 m above the volcano; the plume disappeared within 30 minutes. At 1300 on 17 March, another gas-and-steam plume rose to about the same altitude and extended to the SE; activity ended by 1700. A corresponding shallow seismic event was registered at 1856, followed by a low-frequency (1 Hz) volcanic tremor. The hazard level was originally Yellow but was returned to Green later in the month, although episodes of low-level, low-frequency volcanic tremor continued. The volcano occasionally was obscured by clouds.

An increase in volcanic tremor occurred on 1 April from 1300-2100 and again at 0930-1700 on 13 April. The volcano remained quiet through the rest of April.

Geologic Background. Massive Mutnovsky, one of the most active volcanoes of southern Kamchatka, is formed of four coalescing stratovolcanoes of predominantly basaltic composition. Multiple summit craters cap the volcanic complex. Growth of Mutnovsky IV, the youngest cone, began during the early Holocene. An intracrater cone was constructed along the northern wall of the 1.3-km-wide summit crater. Abundant flank cinder cones were concentrated on the SW side. Holocene activity was characterized by mild-to-moderate phreatic and phreatomagmatic eruptions from the summit crater. Explosive eruptions have been common since the 17th century, with lava flows produced during the 1904 eruption.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

This report covers the period January-April 2000. As of 28 April 2000, KVERT (Kamchatkan Volcanic Eruption Response Team) temporarily suspended operations because of a lack of funding. Seismic activity was at or near background level throughout this period. Occasional weak fumarolic activity occurred, accompanied by fumarolic plumes that rose from 50 m to several hundred meters above the volcano and extended to 10 km in various directions. The volcano frequently was obscured by clouds, which prevented both visual and satellite observations. The hazard level was Green throughout most of the period except for a few days in late January and a period in March when seismic events caused the level to be raised to Yellow.

At 0329 on 9 January, seismic data indicated that a short-lived ash explosion may have occurred. An 81-minute-long series of shallow events was recorded. However, no ash plume was seen on the satellite images at 0513. At 0100 on 23 January, and again at 0428 on 26 January, 20-minute episodes of shallow earthquakes and tremor indicated that brief gas-and-ash explosions may have occurred. The hazard level was raised to Yellow. The volcano was obscured by clouds on 23-26 January; however, on the morning of the 27th the volcano was quiet but its W flanks were covered with gray ash, perhaps from dome explosions. The hazard level was returned to Green.

During the first week of February, seismicity under the volcano was mainly at background levels with occasional fumarolic plumes. At 1653 on 7 February, visual sightings from Klyuchi town reported short-lived, explosive eruptions that sent an ash-poor plume to heights of 1,500 m above the dome. An accompanying increase in seismic activity occurred. At 1800 a plume rose 700 m above the dome and extended 5 km to the NW.

On 9 March, there was a possible ash-gas plume and the nature of the seismicity suggested that this plume may have risen 3-4 km above the crater, but the volcano was obscured by clouds. Again during the week ending on 17 March, seismic data suggested a short-lived explosive eruption sending a plume to 3-4 km above the dome. On 11-13 March, shallow earthquakes and volcanic tremor were registered, but the volcano was obscured by clouds the entire week. During 17-23 March, a steam-and-gas plume rose 500-1,000 m above the volcano, spreading up to 10 km to the NE and E. Visual sightings from Klyuchi at 1752 on 17 March revealed that a short-lived explosive eruption sent an ash-poor plume to about 1,000 m above the dome; it drifted 7 km to the W. This event was accompanied by increased seismicity. At 1345 on 18 March, seismic data indicated another short-lived weak explosive eruption. At 0249 on 24 March, in contrast, seismic data indicated a short-lived but vigorous explosive eruption. The hazard level was raised to Yellow during most of this two-week period but was returned to Green after the seismicity returned to background levels, where it remained throughout April.

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: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Volcanism at Soufriere Hills volcano between 5 February and 26 May 2000 consisted of tremor, continuous dome growth, dome collapses, several pyroclastic flows, and a Vulcanian eruption.

Dome growth that began in November 1999 (BGVN 25:01) continued this report period with growth concentrated in the NE and E portion of the dome through February 2000. On 7 March a switch in the focus of dome growth was marked by a small swarm of earthquakes recorded from 1530 to 1730. The swarm consisted of a mixture of volcano-tectonic, long-period, and hybrid earthquakes with a total of 25 recorded events. Between 2342 on 7 March and 0349 on 8 March a second swarm of different types of earthquakes occurred. The appearance of spiny material in the summit area on 7 March suggested that some growth occurred to the W of the new dome towards Gages wall. Further observations on 9 and 10 March showed a large spine, ~30 m high in the S part of the new dome.

A magmatic explosion occurred at 1530 on 20 March, which was the largest event of this report period. According to MVO, a series of pyroclastic-flow signals started shortly after 1530. These were probably triggered by heavy rainfall and gradually built up in size. Between 1800 and 1900, there were several large pulses of activity with each pulse successively larger. At about 1915, a very large pyroclastic flow traveled out across the sea. This was followed by at least one, and probably more, vulcanian explosions from the volcano's summit. Lightning was seen, and near-continuous thunder was heard during this eruption. Satellite imagery suggested that the ash cloud reached over 9 km and mostly traveled E and SE. However, all of Montserrat received some light ashfall as part of the ash cloud spread to the N. Reports of ashfall in Guadeloupe and Antigua were also received. The following night, heavy rain washed away most of the ash in Montserrat. Observations of the lava dome after the explosion showed that virtually all of the new dome, growing since November 1999, had collapsed and some of the interior of the old dome had been eroded.

On 24 March observations of the scar confirmed that a new spine of lava had been extruded over the vent area. In addition, excellent views of the lava dome on 31 March showed blocky growth in the base of the scar with abundant steaming from around the dome. The new dome was estimated to be 150 m wide and about 100 m high; three small spiny lobes could be seen on top of the new growth. By 20 April the dome's volume was estimated to be 12 to 15 million m³. The average growth rate was 5-6 m³/s, which was higher than the rate in December and July (above ~3 m³/s: BGVN 25:01). Two areas of active growth were noted; one on the dome's S side, and another on its E side. The highest point on the new lava dome was measured at 848 m above sea level, which means the new lava dome had a total height of ~120 m.

Between 28 April and 5 May there was a significant increase in both hybrid and long-period earthquakes. About half of the long-period earthquakes recorded immediately preceded rockfalls, and visual observations confirmed that these events comprised powerful, vertical ash emissions immediately prior to the start of the rockfall. The nature of the seismicity indicated increased levels of pressure within the upper conduit and lava dome. The main dome growth area was on the dome's NE flank. The increased activity culminated on 6 May when there was a moderate collapse of several million m³ of rock from the dome's NE flank. The pyroclastic flows generated from the collapse traveled down Tar River Valley and the longest flows just reached the sea. Observations on 12 May revealed refilling of the small scar created by the 6 May collapse. As of 26 May growth was concentrated in the dome's NE side, with indications that it may move towards the SE. Also, between 19 and 26 May a continuous, low-level ash plume visible in GOES 8 imagery emanated from the volcano.

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

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

This report covers the interval from 1 January to 18 February 2000. What follows are excerpts of the Geophysical Institute's daily reports. Basically, they disclose a pattern of noisy ash-bearing eruptions, tremor, and elevated seismicity. The alert status remained at Orange. As noted on table 3, some ash columns rose as high as 5-10 km; the sounds from one explosion were heard 75 km N in Quito. Tephra frequently blew towards the W (the major population center, Baños, lies N of the summit).

Table 3. Tungurahua plumes and other activity during 1 January-18 February 2000. Plume composition was further described as follows: dominantly ash plumes (a), dominantly steam plumes (s); and direction blown (e.g., N, S, E, W). Where available, selected comments on active processes also follow. Courtesy of the Geophysical Institute.

Histograms and plots on the Institute's web site tell a story of increased activity during late 1999, but the data for 2000 were generally absent. For example, BGVN 24:11 included their plot of daily explosions during 24 October-3 December 1999. This plot showed 1 event on 24 October 1999 and 1-39 events during the first half of November. A prominent peak in the latter half of November reached 105 events. The number of daily events subsequently declined for the last data shown, ending with 39 events on 3 December 1999.

Based on a reading of the daily reports for early 2000 this decline continued. An approximate tally of daily explosions suggests 300 during the month of January, an average of about 10/day. Similarly, for the first eighteen days of February, explosions again averaged about 10/day. In addition, a regional earthquake of M 3.3 took place on 12 January centered below the volcano's S flanks.

One of the Institute's plots disclosed a positive correlation between near-term rainfall and tremor during 1993-1999. Notwithstanding that longer-term observation, rainfall decreased during the course of 1999 while tremor escalated strongly, ultimately undergoing more than a 15-fold increase over background by the latter part of 1999.

Epicenters and foci mapped for August 1999 clustered near the summit and also occurred with a much lesser density along a radial band trending NW-SE extending out on either side of the summit. Thus, the main cluster of the August 1999 earthquake swarm lay directly below the summit at less than 10 km depth; with close approach to the surface the cluster appears directed toward the volcano's SE side. A plot of measured SO₂ fluxes for the interval July-8 December 1999 peaked in September-October at ~9,000-10,000 tons/day (BGVN 24:11).

On 21 and 29 January, and 6 February, rainy weather caused mudflows to descend drainages blanketed by ash deposits. On many days, explosions caused unusually strong acoustical signals; the most powerful signal was generated on 8 February when the sound of an explosion was heard 75 km N in Quito.

Some of the other noteworthy events and ideas discussed in the daily reports follow. At 1544 on 3 January significant explosions occurred, followed by high-amplitude tremor lasting about 30 minutes. The emissions continued and the columns again blew W. On the 4th, undisclosed satellite images showed a column of ash 5 km tall, again blown W. The daily report for 7 January mentioned two possible eruption scenarios involving escalating eruptive vigor and increased hazards.

At 0614 on 12 January ground observers noted a dark gray column rose 3-5 km over the summit. Thereafter, NOAA satellite technicians noted two ash clouds. One cloud extended from the volcano toward the W reaching a length of 16 km and a width of 15 km; it rose to 3 km over the summit (i.e., ~8 km altitude). A larger ash cloud extended toward the S reaching a length of 75 km; it also reached ~8 km altitude (table 1).

Ash clouds remained obscure from the ground on 13 January but in the morning roars were heard associated with the continual emission of gases and ash. A light rain of fine ash fell in the NW sector of the volcano. Peculiar, very dense clouds vented on 14 January, rising only 200 m before rapidly collapsing.

At 0743 on 13 February, an Avianca flight in route to Lima-Bogotá reported an ash cloud over the volcano at 8 km altitude (table 1). In the morning on 14 February some loud explosions foretold of ash columns; they rose 2-3 km above the summit and blew SW. In the morning the next day satellite images showed an ash cloud that had reached a length of 100-150 km and width of 15 km. This cloud was directed from Tungurahua toward the NNW, passing over the towns of Píllaro, Latacunga, and Salcedo. At 10 km distance, falling ash reached a thickness of 1 mm.

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

Starting on 7 March, White Island displayed minor eruptive activity, elevated seismicity, and higher than normal SO₂ gas flux (BGVN 25:03). Staff from the Institute of Geological & Nuclear Sciences (GNS) who visited on 18 April observed a gas plume, comprised of steam and small amounts of ash, being erupted to 1,300-1,700 m before being blown S. At distances of more than a few kilometers from the island no ash was visible in the plume, although reports from Whakatane indicated that trace amounts of ash fell during the night of 17-18 April. The erupting vent, named MH vent, is ~10 m across. The seismograph at White Island recorded volcanic tremor starting on 6 April 2000, the first significant tremor since April 1999. In addition, recent measurements of the SO₂ flux were well above typical background levels at White Island. This eruptive activity was sufficient to increase the Alert Level from 1 to 2.

Ash was erupted almost continuously during the weekend of 22-23 April, although there was no evidence that any larger blocks were erupted. During another visit by scientists on 26 April the activity level was much reduced from that of the previous two weeks, but minor eruptive activity continued. A weak gas plume, comprised almost totally of steam, was being erupted to 1,500 m before being blown NE. The plume was not visible more than a few kilometers from the island. The seismograph continued to record moderate to strong volcanic tremor, although the level fluctuated with time. It is unclear if there was a direct relationship between the intensity of the eruptive activity and the level of volcanic tremor. COSPEC measurements on 28 April recorded a SO₂ flux of 927 metric tons/day, down from previous measurements, but consistent with the activity.

Activity was similar during a monitoring visit on 2 May, with a weak gas and ash plume from MH vent. The ash plume was rising to 500 m, while the steam-and-gas plume rose to 760-900 m before being blown NNE. The plume was visible more than 10-15 km from the island. Ash emission was occurring continuously. The seismograph continued to record fluctuating moderate to strong volcanic tremor.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: Brad Scott, Brent Alloway, and Steven Sherburn, Wairakei Research Center, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).

Yasur volcano was visited during 18-20 February by John Seach, Geoff Mackley, and a film crew from the United Kingdom. There was a lower level of activity than in June and July 1999 (BGVN 24:07). "Level 1" activity predominated, defined as normal activity where bombs generally fall within the cone area. The three main craters within the cone, A, B, and C, are aligned S to N.

On 18 February Crater A silently emitted small amounts of white vapor tinged with blue. The following day observers saw emission of brown ash but again heard no noise from the crater. Crater B was also silent and only emitted a small amount of white vapor. Two smaller craters E of Craters A and B were continually full of vapor. Crater C exhibited the main activity. There were two active vents at the N and S ends of the crater. The inside crater wall was strongly fumarolic. Over a 2-hour period Crater C produced four explosions from the N vent and three from the S vent. Incandescent bombs were thrown to a height of 120 m. Some bombs glowed for 30 minutes after dark.

On 19 February observers lowered themselves 50 m to the floor of the Yasur cone from the SE using ropes. Ground temperature 15 m S of Crater C was 96°C. The floor was littered with black bombs up to 1.5 m in diameter and covered with ash. Explosions threw lava to a height of 250 m. Bombs were either thrown straight up or directed towards the NW edge of the cone. Explosions occurred ten times per hour and were accompanied by ground shaking and sometimes shockwaves. Degassing was heard every few seconds between explosions. Inside the cone a number of earthquakes were felt which were not accompanied by explosions or degassing; these earthquakes were not felt by observers on the crater rim, 100 m away. Inside the cone there was a strong sulfur smell and fumes irritated eyes.

On 20 February activity increased. Incandescent lava was ejected every few seconds. Larger explosions about 8 times per hour sent lava to 300 m elevation and 100 m away from the vent. The active vents glowed orange after dark.

The NW flanks of Yasur cone down to Lake Siwi were littered with large bombs from the 1999 activity. Black bombs to 3 m in diameter were observed at the base of the cone. Approaching Yasur remains dangerous at any time for visitors. Vehicles can be driven to within 20 m of the cone and this encourages tourists to visit the crater edge. Unpredictable trajectories of bombs and variable eruptive activity pose threats to visitors. It should be noted that three deaths occurred on Yasur in 1994-95 during a period of low activity (BGVN 20:08).

Geologic Background. Yasur has exhibited essentially continuous Strombolian and Vulcanian activity at least since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island in Vanuatu, this pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide open feature associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: John Seach, PO Box 16, Chatsworth Island, N.S.W. 2469, Australia.