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/).

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/).

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).

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/).

The southern-most cone at Sakura-jima, Minami-dake, manifested increased eruptivity from late October to early November 1999. Following a lull in the second half of November, vigorous activity in December was marked by incandescent columns, large amounts of bomb ejections, and ballistics falling as far as 4 km from the crater.

High eruptive activity occurred in late October and early November 1999. On 31 October the JMA issued a Volcanic Advisory. In early November, 19 eruptions (including 18 explosions) occurred at Minami-dake before activity declined to lower levels later in the month. Activity increased again in early December with a few explosions each day and small numbers of ballistic clasts falling onto the upper slopes. On the afternoon of 10 December JMA issued another Volcanic Advisory. At 0555 this day, Sakura-jima issued a large amount of bombs. Incandescent columns as high as 100 m were accompanied 116 times by volcanic lightning. According to a JMA field inspection, ballistics were scattered 3-4 km away from the Minami-dake crater; the maximum size was 4 cm across. Incandescent columns rose as high as 300 m at 0554 on 24 December and were accompanied by volcanic lightning six times.

Daily numbers of eruptions ranged from 2 to 8 during early- to mid-December; eruptions were mostly explosive. The maximum amplitude of explosion earthquakes recorded at JMA observation point A, 4.6-km WNW of the crater, reached up to 28 µm; the largest value was caused by an explosion at 1301 on 12 December. The plume heights of December explosions ranged from 1,500 m to 2,000 m. Explosions took place on 23 consecutive days between 3 and 25 December. This is the longest record of daily explosions since JMA started observing Sakura-jima in 1955; the previous record was 21 days in 1960. Explosions began again late in the month, with six more on 31 December.

The total of 88 explosions during December 1999 was the second highest monthly count since 1955; the highest was 93 explosions in June 1974. According to the JMA, the total number of eruptions in 1999 was 386, including 237 explosions.

Frequent explosive eruptions continued in early January (figure 21). Explosions on 2 January sent an eruption column to 2,200 m above the crater rim and emitted abundant cinders, as well as bombs that fell midway down the flanks of the volcano. Nine explosive eruptions occurred on 5 January, one of which again ejected cinders and bombs as far as the middle flank of the volcano. The highest plumes in early January reached 2,200 m above the crater rim during explosions at 0821 on 5 January and at 0746 on 14 January. The maximum amplitude of explosion seismic signals at JMA observation point A (4.6 km WNW of the active crater) was 17 µm for the 0513 explosion on 14 January.

Figure 21. Eruption at Sakura-jima at 0900 on 8 January 2000 from 3.5 km SW of the Minami-dake crater. Courtesy of Tatsuro Chiba.

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

Information Contacts: JMA-Fukuoka, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan; Setsuya Nakada, Volcano Research Center, ERI, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Tatsuro Chiba, Nihon University, Japan (URL: http://www.nihon-u.ac.jp/en/).

Eruptive activity continued at Ambrym in late 1999 and through January 2000. A volcanic ash advisory regarding this volcano was issued to aviators on 1 November 1999 reporting "smoke and ash" rising to ~1,500 m altitude. Similar notices were issued on 5 and 6 November. [Aviation reports on 9-10 December] described an ash cloud up to 2,700 m altitude.

John Search and Geoff Mackley investigated Ambrym caldera during a 19-28 January 2000 climb. Lava lakes had disappeared from both Benbow and Mbwelesu craters and a new vent had opened inside the previously inactive 1953 crater. A series of earthquakes were registered around Ambrym Island on 27 November 1999. The largest of these was magnitude 7.1. The earthquakes were followed by a month of reduced activity during which there were no reported observations of lava lakes. Landslides were visible in the caldera and ground cracking visible at Benbow, Mbwelesu, and Niri Mbwelesu craters.

Activity at Benbow Crater. Four vents were active inside Benbow. On 19 January a white plume tinged with blue and yellow rose 1,000 m above the crater rim. Twin plumes were visible the next day rising from the S end of the crater at 15 m/s and from the N end of the crater, where they were tinged with brown. Each time the crater was climbed from the S on 22, 23, and 24 January the pit was full of vapor and no sounds were heard. On 25 January the observers lowered themselves into Benbow using 200 m of rope. The floor of the first level was covered with fine brown ash and a shallow brown pond was present in the SW end of the crater. The inner crater was climbed and observations made from its rim. Below the observers was a ledge 120-140 m down covered with ash and containing a 10-m circular vent emitting white vapor. The main vent was 50 m farther down and 40 m in diameter. This was the vent that contained the lava lake in January 1999 (BGVN 24:02). No lava was observed inside this vent and it made no sound. At 1300 a large roar from the vent was followed by brown ash emission. At the NE end of the inner crater was a plume emission from an unseen vent.

The N end of Benbow crater (on the first level) contained another vent that could not be directly observed but regularly emitted light brown ash. On 26 January a loud continuous 30-second degassing heard from the N vent was followed by brown ash emission and rain of small cinders on observers at the S crater edge. From the central pit the vapor was rising at 5 m/s. During the late afternoon two visible atmospheric perturbations were observed above the main vent. The first followed a loud degassing sound and rose at 40 m/s to a height of 200 m above the vent. Rockfalls were also heard during the afternoon. During the night of 26 January twin skyglows of fluctuating intensity were visible above Benbow followed by a large brown ash emission that rose 1,400 m above the crater in 3 minutes.

Activity at Niri Mbwelesu Taten. On both 19 and 20 January light brown or red/brown ash was emitted from the collapse pit and rose 200-250 m. On 21 January a brown pond of water 150 m NE of the pit was bubbling from both fixed and random locations. Active fumaroles were present on a ledge 60 m down. There were large cracks on the SE side and evidence of wall collapse since August 1999. Ash fell on observers in the area N of the pit. On 23 January larger ash emissions occurred about every hour.

On 24 January the collapse pit was entered using ropes. Fumaroles on the ledge 60 m down averaged 64°C. The pit bottom was 120-140 m below the ledge covered in brown ash. Small clouds of ash were emitted occasionally from two large fissures. Bubbles of hot blue vapors, 6 m in diameter, rose past the observer. Continual degassing sounds were heard in the pit, like the sound of waves crashing on the beach. On 26 January from 0600 to 1100 dark gray ash clouds were continually being emitted from the pit. Plumes rose at 8 m/s to a height of 200 m above the pit, filling the caldera in all directions. During the afternoon the pit returned to a low level of activity. On 27 January a continuous emission of brown ash occurred all day to a height of 800 m above the pit.

Activity at Niri Mbwelesu. On 20 January white vapor tinged with blue was constantly emitted to 600 m above crater. During the evening a very intense pulsating night glow was visible. The glow would brighten (sometimes flicker), then rapidly drop to a lower level of illumination. The bright/dim cycle would repeat every 10-15 seconds. On 21 January in the afternoon degassing was heard from the crater rim and during the evening clouds were illuminated 250 m above the crater. Observers on the crater edge felt hot vapor. When the crater was climbed on the evening of 25 January a clearing of the vapor enabled the bottom to be seen 280 m down. A 40-m-diameter vent was visible emitting bright yellow burning gas, radiant heat was felt on the faces of observers, and moderate degassing was heard.

Activity at Mbwelesu. Observations were made of Mbwelesu crater on 21 January. The two lava lakes observed in August 1999 had disappeared (BGVN 24:08). A brown pond surrounded by fumaroles was in the Vent B location, with large amounts of ash and rock to the SE. The sill on the SE edge of the crater had large craters and several large sections (over 10 m) that had broken off and fallen into the crater. The fumarole field 40 m SE of the crater rim had a temperature of 72.7°C. Heavy rains caused waterfalls and rockfalls inside the crater. The crater was otherwise quiet with some vapor emissions from many fumaroles on the floor. Fumaroles were also present in the location of the former lava lake at Vent C.

Activity in the 1953 Crater. The 1953 crater contained two levels. The higher (W half) contained a brown pond. The lower (E half) had developed a deep smoking vent. This was in the location of the green pond observed in August 1999.

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; Wellington Volcanic Ash Advisory Center (VAAC), MetService, PO Box 722, Wellington, New Zealand (URL: http://www.metservice.co.nz/).

Starting around dawn on 23 December, INETER registered low-amplitude seismic tremor at the seismic station located at the foot of Concepción. The seismic signal grew gradually and every few minutes small earthquakes were observed. Due to the increasing seismicity, at 1315 on 24 December INETER informed Civil Defense in Managua of the activity and recommended taking precautions for volcanic gases, ashfall, and, in the event of rain, for lahars or mudflows.

On the morning of 27 December INETER received reports from Lacsa and Aviateca airlines that their pilots had observed emission of material from Concepción rising ~300 m above the crater. Residents of Moyogalpa (at the W foot of the volcano) confirmed moderate activity. Minor amounts of gas and volcanic ash blew towards Moyogalpa.

INETER specialists conducted fieldwork around the volcano on 28 December and confirmed the occurrence of low-level eruptive activity based on their own observations and descriptions by local residents. Activity was characterized by sporadic gas explosions from the crater that ejected small amounts W-blown ash. The seismic instrumentation indicated constant tremor with rare volcanic earthquakes related to the crater explosions.

The level of seismicity had decreased by the morning of 29 December, and continued to decline through 1000 on the 30th. Although volcanic activity had also diminished, an explosion at 1600 on 29 December caused ashfall as far as San Jorge (also known as Rivas, a town 25 km SW of Concepción).

Some pilot reports received on 27 December also indicated possible activity from the adjacent Maderas volcano, which has no known historical activity. INETER observers were unable to confirm these reports during fieldwork in the area. However, another seismic station was installed on Ometepe Island in the SW zone of Maderas, which should help to confirm or refute any future reports of Maderas activity.

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

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

On 26 February 2000 the WSW-trending, elongated Hekla volcano erupted. A fissure 6-7 km long opened along the SW flank of the Hekla ridge, from which a discontinuous curtain of lava erupted starting at 1819. Just a few minutes later, at 1825, an ash plume reached a height of 11 km and was carried N by light winds. Based on the tremor amplitude the eruption reached peak intensity in the first hour of activity, then gradually declined.

Seismic networks maintained by the Science Institute at the University of Iceland and the Icelandic Meteorological Office recorded short-term precursors. Small earthquakes were first detected by seismographs at various locations during 1655-1707. These gradually increased and the first well-located earthquakes (M 1-2) started at 1729, centered 1-2 km SE of the summit at depths of a few kilometers. A network of borehole strainmeters operated by the Meteorological Office also detected precursory changes associated with magma movements. A decrease in strain build-up rate, signaling a release of magma pressure, was recorded by a strainmeter in a borehole ~15 km from the summit at 1817.

Notice was given to the National Civil Defense and the Civil Aviation Administration about 40 minutes before the eruption, and the public was alerted about the imminent eruption about 15 minutes before it began through national radio broadcasts. Continuous low-frequency tremor began at 1819, at the same time the eruptive cloud was spotted.

Ashfall was reported on 26 February from Grimsey Island, ~70 km off the N coast of Iceland and 300 km N of Hekla. Although small amounts of ash fell in inhabited areas of N Iceland, most fell in uninhabited areas of the island's interior. Seven hours after the eruption's onset the ash deposit 21 km N had a maximum thickness of 4-5 cm.

Lava flows on 27 February covered a large part of the SE flank. That evening a lava stream flowed N from the erupting fissure at a rate of several meters per hour. Another more active lava stream emanated from three craters near the S end of the fissure; the stream was several kilometers long and advancing at ~1 m/minute.

On 28 February an eruption cloud was deflected towards the S by northerly winds. However weather conditions precluded direct observations. Tremor amplitude continued to slowly decline, and the strength of the eruption was decreasing (figure 1). Two eruption clouds were seen at 0500, confirming that activity was ongoing. Although the craters were not visible in the daylight, the most active crater just S of the summit produced three lava streams down the S flanks. Activity in the N declined during the day on 28 February. At 0630 one lava flow had reached the Vatnafjoll mountains at Lambafell, 5 km S of the summit. Advancing at ~2-3 m/hour, the lava front was estimated to be 8-10 m wide. That evening observers watched Strombolian activity in three craters at the southernmost part of the fissure.

Figure 1. Tremor at Hekla during 26-28 February 2000 recorded at Haukadalur, 10 km W. At the beginning of the eruption, 1819 on 26 February, the tremor increased rapidly and reached a maximum at 1850. Tremor then decreased until about 0700 on 27 February and became steady. The tremor was approximately 10 % of the maximum value, on 28 February, but over 10 times greater than the normal value. Courtesy of Páll Halldórsson, Science Institute at the University of Iceland.

Ashfall was reported on the morning of 29 February 35-40 km S in Fljótshlíð. At 0500 volcanic tremor had started to increase and continued until 1000-1100. By about 0800 all activity in the summit had ceased. During the afternoon of 29 February activity at the southernmost end of the fissure increased again, producing eruption clouds ascending above the summit. In the darkness of the evening, three craters at the southernmost end of the fissure produced lava flowing SW. People watching the lava on the NE flanks reported that they could walk on the stopped flow there.

Vigorous Strombolian eruptions and lava flows on the fissure that cuts the SW slopes were seen during a reconnaissance flight on 1 March during 1100-1230. Four main vents and three smaller vents produced explosions at intervals of 4-5 minutes. At the base of the fissure a large tumuli had developed. The lava streams coming out through the opening of the tumuli joined a stream coming from overflows of the uppermost craters. The S-directed lava flows were fed by the crater closest to the summit. The lava field in the S had only advanced ~100 m since 28 February, but on 1 March it was growing toward the E. By 1 March lava had covered approximately 17 km².

Increased activity was observed in the upper craters on 2 March, although bad weather persisted from 1200 on 1 March until midday on 3 March. There was also constant steaming from the SW craters, and, compared to 1 March, much larger steam clouds rising from the upper craters. At nightfall explosions were observed at ~30-minute intervals. Glowing lava streams were noted on the flank of the mountain on 2 March.

On 3 March a group of scientists reached the SW lava flow at 1300 and found that the lava front was ~10 m wide and advancing very slowly, ~1-2 m/day. While tracing the lava to the W the group noted that at some places the flow was spreading much faster, up to ~1 m/hour. Following the lava flow along its W side, the group reached its origin at the foot of the volcano, where it emerged from the end of the erupting fissure. At the origin, the estimated flow rate was 0.06 m/s, producing about 10 m³/s of lava. Due to the continuous degassing along the lava stream a blue mist was formed. The blue mist was also observed farther E along the flank of the volcano, indicating that lava was still flowing from the crater close to the summit area. The craters in this region fed the lava flow that moved S toward the Vatnafjoll glacier 10 km SE from Hekla. Later in the evening observers reported that lava was still flowing slowly towards Vatnafjoll. Explosive activity in the uppermost crater of the SW-fissure was characterized by small explosions at 10-20 minute intervals that produced white steam clouds with only trace amounts of ash.

Due to bad weather conditions on 4 March, no direct observations could be made of the eruption. Decreasing eruption tremor was detected. On 5 March the lava flow to the SW was still ongoing according to observations made in the afternoon. At sunset, a red pulsing glow was observed in the uppermost craters of the SW-fissure from the town of Selsundsfjall, 15 km SW. Small eruption clouds were observed on 6 March penetrating the weather clouds covering the summit of Hekla.

During a reconnaissance flight between 1730 and 1830 on 6 March the whole fissure was steaming vigorously and all of the lava flows appeared to have stopped. The lava stream in the SW had left behind an empty channel. Neither incandescence nor explosive activity were observed from the craters. Minor tremors continued on 6-7 March, but may have been related to lava degassing in the feeder dike.

At 0844 on 8 March the last eruptive tremor was detected on seismometers. Based on the end of detectable tremor, and with no signs of new eruptive products since 5 March, it was determined that the eruption ended on the morning of 8 March. Lava covered approximately 18 km²; the preliminary estimate of lava production was 0.11 km³.

Plume investigation. Sulfur dioxide (SO₂) contained in plumes from Hekla was detected by the Earth Probe TOMS (Total Ozone Mapping Spectrometer) instrument. TOMS imagery at 1154 on 27 February showed that the volcanic cloud was a narrow plume arcing from the volcano in southern Iceland, then N to Greenland, and finally E towards Norway. The plume primarily contained SO₂ because almost all of the ash fell out locally. On 28 February the TOMS imagery indicated that plume stretched out over the Barents Sea and possibly into eastern Russia. By 29 February the SO₂ cloud had drifted E in a band along the Norwegian and Russian coasts of the Barents Sea.

During a transit flight on 28 February a SOLVE (SAGE III Ozone Loss and Validation Experiment) mission with an instrument-laden DC-8 aircraft flew through the plume shortly after the eruption ~11.3 km NNE of Iceland at 76°N and 5°W, just off the Greenland coastline. The plume extended up to ~13 km altitude, well into the lower stratosphere. Instruments also measured many in situ trace gases, SO₂, HNO₃, NO, NOy, O₃, and aerosols (volatile and non-volatile), including their size distribution. From about 0508 until 0518 on 29 February the SOLVE aircraft again entered the volcanic cloud. The scientific team reported large enhancements in CN, NOy, HNO₃, CO, and particle counts, ozone went to nearly zero, H₂O jumped up, and there were strong scattering layers up to 13 km. The plume was a very impressive, orange, airfoil-shaped feature in the pre-dawn sky. The DC-8 engines needed an oil change and new filters after passing through the plume. A flight on 5 March detected enhanced aerosols and SO₂ at 1301, but by that time the plume was so diluted that it represented no danger to the aircraft. During the three weeks following the initial encounter the DC-8 detected remnants of the plume trapped within the polar vortex. The resulting analysis concluded that volatile aerosols increased and the sizes of non-volatile large aerosols decreased.

Fluoride analysis. Ash from previous Hekla eruptions has often been the cause of fluorosis in grazing animals. However, during this time of the year most domestic animals are kept indoors, so fluorosis is not expected to become a problem. Freshly fallen ash was measured for soluble fluoride ions (F-). The result was 800-900 mg F/kg. Snow melted by the ash contained about 2,200 mg/l (ppm) of fluoride.

Geologic Background. One of Iceland's most prominent and active volcanoes, Hekla lies near the southern end of the eastern rift zone. Hekla occupies a rift-transform junction, and has produced basaltic andesites, in contrast to the tholeiitic basalts typical of Icelandic rift zone volcanoes. Vatnafjöll, a 40-km-long, 9-km-wide group of basaltic fissures and crater rows immediately SE of Hekla forms a part of the Hekla-Vatnafjöll volcanic system. A 5.5-km-long fissure, Heklugjá, cuts across the 1491-m-high Hekla volcano and is often active along its full length during major eruptions. Repeated eruptions along this rift, which is oblique to most rifting structures in the eastern volcanic zone, are responsible for Hekla's elongated ENE-WSW profile. Frequent large silicic explosive eruptions during historical time have deposited tephra throughout Iceland, providing valuable time markers used to date eruptions from other Icelandic volcanoes. Hekla tephras are generally rich in fluorine and are consequently very hazardous to grazing animals. Extensive lava flows from historical eruptions, which date back to 1104 CE, cover much of the volcano's flanks.

Information Contacts: Freysteinn Sigmundsson, Nordic Volcanological Institute, Grensásvegur 50, IS-108 Reykjavik, Iceland (URL: http://nordvulk.hi.is); Páll Einarsson, Science Institute, University of Iceland, Hofsvallagata 53, IS-107 Reykjavík, Iceland; Ragnar Stefánsson, Icelandic Meteorological Office, Bustadavegur 9, 150 Reykjavík, Iceland (URL: http://www.vedur.is/); Mark Schoeberl, Code 910, NASA/GSFC, Greenbelt, MD, 20771 USA; Michael Fromm, Computational Physics, Inc., 2750 Prosperity Ave., Suite 600, Fairfax, VA 22031 USA (URL: http://cloud1.arc.nasa.gov/solve/); Arlin Krueger, Code 916, NASA/GSFC, Greenbelt, MD, 20771 USA.

At 1800 on 18 October 1999, the National Coordination Committee for Prediction of Volcanic Eruptions reported that the volcano's fumarolic area had expanded and the amount of steam had increased in the western part of Iwate volcano. New fumaroles have been observed since May on the N slopes of Mts. Ubakura-yama and Kurokura-yama and in the western stream of Ojigokudani (inside the erosion caldera). This fumarolic activity has intermittently increased since July, and ground temperatures between Mts. Kurokura-yama and Ubakura-yama also increased with time. Analyses of fumarolic gas collected between Ojigokudani and Mt. Ubakura-yama in August and October revealed a magmatic component. Although GPS measurements showed the end of the elongation trend observed since July, relatively large volcanic earthquakes occurred during May and June. Deep-seated (~30 km depth) low-frequency earthquakes, relatively deep-seated (6-13 km depth) low-frequency earthquakes, and shallow high-frequency earthquakes occurred under the eastern cone of Iwate. However, the overall level of seismicity has decreased compared to 1998 (figure 5).

Figure 5. Daily numbers of earthquakes at Iwate (recorded at the Matsukawa station) during 1 January 1998-13 November 1999. Courtesy of JMA.

On the evening of 12 November JMA issued a Volcano Advisory on Iwate after a 4-minute volcanic tremor (M 2.1) saturated local instruments starting at 2054. The event hypocenter was located 2-3 km below the Ubakura-yama and Kurokura-yama areas of western Iwate (figure 6). An earthquake swarm continued for 2 hours after the tremor event at a rate of 16-20 events/hour. Inspection from the air the following day did not show any major change in fumarolic activity or any deposition of new volcanic ash.

Figure 6. Hypocenters of earthquakes under the western section of Iwate during 11-12 November 1999. Courtesy of JMA.

On the evening of 16 November, the extended National Coordination Committee for Prediction of Volcanic Eruptions met in the city of Morioka, Iwate Prefecture, to review the events that occurred on the 12th. They noted that the tremor was similar in shape, amplitude, and duration to one (M 2.4) that occurred on 10 July 1999; hence it was considered likely that the two events occurred in the same place. Changes detected in tilt- and strain-meters located on the flank during the tremor were probably caused by subsurface ground faulting or fluid movement. After the tremor, however, no subsequent changes were observed. Neither the GPS-based, N-S traverse distance across the volcano nor the fumarole temperatures in the Ubakura-yama to Kurokura-yama region changed before or after the event. Fumarolic activity in western Iwate had increased since May as had the number of shallow earthquakes in the Ojigokudani area (erosion caldera). The tremor event on 12 November suggested a continuing possibility of a phreatic explosions in western Iwate.

Geologic Background. Viewed from the east, Iwatesan volcano has a symmetrical profile that invites comparison with Fuji, but on the west an older cone is visible containing an oval-shaped, 1.8 x 3 km caldera. After the growth of Nishi-Iwate volcano beginning about 700,000 years ago, activity migrated eastward to form Higashi-Iwate volcano. Iwate has collapsed seven times during the past 230,000 years, most recently between 739 and 1615 CE. The dominantly basaltic summit cone of Higashi-Iwate volcano, Yakushidake, is truncated by a 500-m-wide crater. It rises well above and buries the eastern rim of the caldera, which is breached by a narrow gorge on the NW. A central cone containing a 500-m-wide crater partially filled by a lake is located in the center of the oval-shaped caldera. A young lava flow from Yakushidake descended into the caldera, and a fresh-looking lava flow from the 1732 eruption traveled down the NE flank.

Information Contacts: Kazuo Sekine, Sendai District Meteorological Observatory, Japan Meteorological Agency, 1-3-15 Gorin, Miyagino-ku, Sendai 983, Japan; Hiroyuki Hamaguchi, Faculty of Science, Tohoku University, Sendai 980-8578 Japan (URL: http://www.sci.tohoku.ac.jp/); Setsuya Nakada, Volcano Research Center, ERI, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Jun-ichi Hirabayashi, Kusatsu-Shirane Volcano Observatory, Tokyo Institute of Technology, Kusatsu, Agatsuma-gun, Gunma 377-17 Japan.

A Volcanic Advisory on Kirishima volcano (figure 4) was issued on 10 November 1999 by the Japan Meteorological Agency (JMA) after seismicity began increasing on 6 November. This is the first advisory at Kirishima since 27 August 1995 (BGVN 20:08 and 20:09). Earthquakes detected at a site 1.7 km SW of Shinmoe-dake totaled 666 during 6-15 November (table 1), peaking at 192 events on the 10th. No volcanic tremor was observed.

Figure 4. Steam from Shinmoe-dake at Kirishima looking towards the SE in 1991. Naka-dake is the adjacent cone with a flat top, and in the background is Ohachi (crater to the right), Takachiho-no-mine (the highest peak in the center), and Futatsuishi (left). Courtesy of T. Kagiyama, ERI.

Table 1. Daily numbers of volcanic earthquake events at Kirishima, 5-15 November 1999. Courtesy of JMA.

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: JMA-Fukuoka, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan; Setsuya Nakada and Tsuneomi Kagiyama, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

Volcanic unrest that began in May 1999, and intermittent explosive eruptions beginning in June 1999, eventually led to growth of a lava dome on 12 February 2000. By 23 February PHIVOLCS had recommended evacuation to 7 km from the summit in the SE and to 6 km for the rest of the volcano. The latter is a permanent danger zone.

At 2206 on 23 February the seismic network detected an explosion-type earthquake that coincided with rumbling and minor ejection of lava fragments from the summit. This earthquake was followed shortly by bright incandescence, indicating that lava emission and ejection had intensified. Low-frequency volcanic earthquakes then occurred beginning at 2217 and lasting until about 2326 when the seismographs began to record harmonic tremor. The tremor became pronounced at about 0034 on 24 February and was accompanied by minor lava fountaining to 50 m above the summit lava dome. The hazard status was raised to Alert Level 4 (hazardous eruption imminent, possible within days) at 0300 on 24 February. No additional evacuation was recommended, but residents within 8 km of the summit were advised to prepare for evacuation.

At 0826 on 24 February another explosion-type earthquake was recorded by the seismographs at Anoling, Sta. Misericordia, and Mayon Resthouse Observatory. The summit was obscured, but at 0829 a pyroclastic flow descended SE towards the Bonga Gully with a run-out distance of ~7.2 km, reaching the distal end of the Bonga fan. The hazard status was then raised to Alert Level 5, (hazardous eruption in progress). Because pyroclastic flows could continue to sweep down along well-incised gullies and channels, especially the Bonga Gully, PHIVOLCS recommended extension of the danger zone to 8 km along the SE sector of Mayon Volcano. Likewise, ashfall was expected mainly W, SW, and NW of the crater.

The SO₂ emission rate increased on 24 February to 4,070-5,700 metric tons/day (t/d). Ground deformation measurements showed that the volcanic edifice swelled significantly in the previous two days, consistent with the growth of the lava dome.

By the morning of 25 February activity was mainly lava extrusion, with a flow channeled along the Bonga Gully. COSPEC readings conducted on 24 February reached 13,500 t/d. The abrupt increase in this value may be attributed to the series of highly gas-charged ash ejections comprising the volcanic plume.

Following a quiet interval that started at 1420 on 26 February, more vigorous activity resumed on the evening of the 27th. Seven ash-and-gas explosions occurred between 1950 and 2237, the most significant of which (at 2144 and 2237) were accompanied by lava fountaining with ejection of volcanic bombs. Large incandescent fragments were ejected to ~500 m above the crater rim. Ground deformation measurements showed that the edifice remained inflated. COSPEC readings of SO₂ flux remained significantly above normal at 4,900 t/d. Explosion earthquakes and harmonic tremor accompanied the lava fountaining and persisted even when the activity had apparently subsided.

Explosive eruptions during 28 February-1 March. Mayon had another series of explosive eruptions during 0700 to 2100 on 28 February, with the most significant eruptions occurring at 1641, 1732, and 1940. The first explosion produced a 5-6-km-high eruption column and generated a large pyroclastic flow that descended the W portion of the Bonga Gully on the SE flank and entered the Mabinit channel to the S. This was followed by voluminous eruption clouds beginning at 1732 that rose to ~10 km above the summit and generated multiple pyroclastic flows to the SW, S, and SE. Vigorous explosions sustained the eruption column and discharged large volcanic fragments that splattered the upper portions of the cone. Thick ash clouds hovered around the volcano and created frequent lightning discharges.

Most of the ash clouds were eventually carried to the SW and W, affecting Ligao, Guinobatan, and Camalig. However, the pyroclastic flows did not travel beyond the present danger zones. The ash clouds contained high concentrations of sulfur dioxide, with COSPEC-recorded emission rates of 13,000 t/d, as expected for an eruption cloud. Aircraft were warned to avoid lingering ash clouds to the W of the volcano. The E side, towards the Legaspi airport, remained free from volcanic ash, debris, and SO₂ emissions.

Electronic distance measurements revealed that the volcano's edifice remained inflated. Such inflation was thought to be caused by the ascent of magma as indicated by the near-continuous seismic tremor associated with active magma transport.

The series of major ash ejections and subsequent pyroclastic flows that occurred along Bonga Gully, Mabinit, and Miisi Channels started at 1641 on 28 February 2000. The maximum height estimated for the vertical ash plume was 12 km during the 1732 event. The approximate runout of the pyroclastic flows reached to ~5-6 km downslope. Severe ashfall occurred in the SW sector of the volcano, especially at Barangay Tumpa in Camalig and Barangays Maninila and Masarawag in Guinobatan. Lava fountaining with ballistic bombs was also frequently observed starting at 1732 with maximum heights estimated at 1 km.

After the vigorous activities late in the afternoon to early evening on 28 February, only quiet effusion of lava was noted during times when the summit was not obscured through the morning of 29 February.

Another series of ash ejections began at 1211 on 29 February. The largest event occurred at 1501 and produced a 14-km-high eruption column. This event also generated several pyroclastic flows that descended all sides of the volcano. Pyroclastic flows that were channeled by gullies in the SW, S, and SE reached up to 5-6 km from the summit. Smaller pyroclastic flows that followed gullies in other sectors stopped ~2-3 km from the crater. Ash from the tall eruption column and from pyroclastic flows drifted to the W and SW. The ash ejections were generally accompanied by rumbling sounds. Vigorous lava fountaining began at 1531 and ballistic projectiles fell within 1.5 km of the crater. Lava flows were observed on 1 March to have reached the 1,000 m elevation, or about 2.3 km from the summit.

COSPEC measurements on 29 February were hampered by thick ash cover. Ground deformation measurements made the morning of 29 February along the Buang and Masarawag EDM lines showed that the volcano edifice remain inflated. Significant potential was noted for lahars along major tributaries draining from the NW due to the presence of ash and pyroclastic-flow deposits, which could be eroded and remobilized during heavy rainfall.

Mayon exhibited another series of eruptions that began on 1 March and produced dense and highly convective ash columns that rose up to 7 km above the summit. Part of the eruption column would occasionally collapse to produce pyroclastic flows that traveled along major gullies around the volcano. Pyroclastic flows were observed along the main gullies facing Anoling. The largest of these pyroclastic flows occurred along Bonga Gully and traveled ~6 km from the crater, while smaller flows at other gullies descended some 4 km downslope. Explosive eruptions produced lava fountaining with discrete ballistic volcanic fragments hurled out to ~500 m above the crater rim. Frequent rumbling accompanied the explosions, which lasted until 1609. By the end of this episode of explosive activity, quiet lava extrusion followed and continued to be observed up to the present. Areas SW and W of the volcano were severely affected by ashfall with the most significant deposition in Camalig, Guinobatan, and Ligao. Minor ash and steam were continuously being generated by lava deposits from the summit crater and Bonga Gully and drifted to the SW and W areas by prevailing winds.

Lava emission phase. Mayon was relatively quiet during 2 March as the seismic network recorded short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. This departs from the continuous tremor recorded in the past days during periods of relative quiet. The volcano has apparently entered a phase of lava emission with sporadic episodes of minor ash puffs. Ash and steam emission from both the summit crater and new lava flow deposits produced a haze over the SW sector, particularly in the municipalities of Camalig, Guinobatan, and Ligao. SO₂-flux measurements on 2 March yielded a value of 14,500 t/d. Ash clouds derived from the new lava flow deposits apparently produced a significant portion of this emission rate. Ground deformation measurements indicated that the volcano deflated slightly following the 1 March ash ejections.

Lava emission with sporadic episodes of minor ash puffs dominated the eruptive activity on 3 March. This relatively quiet state was reflected in the low-level but significant seismicity comprised by short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. Thick clouds covered the summit area, but below the cloud line and on the middle and lower slopes of the volcano ash clouds and steam emanated from the new lava flows and pyroclastic-flow deposits. A high emission rate of 8,900 t/d SO₂ was measured by COSPEC. Much of the ash and steam clouds resulting from this degassing drifted to the W and SW sections of the volcano due to prevailing winds. The haze produced by fine ash suspended in the air temporarily precluded ground deformation measurements.

Potential exists for hot lahar flows due to the presence of highly erodible pyroclastic deposits, which may be remobilized during heavy rainfall. Gullies with confirmed pyroclastic-flow deposits in their headwaters, which may therefore be sites for future lahars, are the Mabinit and Matanag river channels in Legaspi City, Miisi channel in Daraga, Basud-Lidong channel in Sto. Domingo, San Vicente and Buang channels in Tabaco, and the Bulawan channel in Malilipot.

Short-duration harmonic tremors and low-frequency volcanic earthquakes continued on 4 March. This type of seismicity indicated that eruptive activity was limited to quiet lava emission. Ground deformation measurements showed that the volcano was still inflated in its lower portion, while the SO₂ emission rate was determined to be at a minimum of 12,100 t/d. Preliminary estimates of the volume of deposits emplaced by the eruptions yielded at least 40 million cubic meters of lava flow and pyroclastic flow deposits. Lava flow deposits account for the major proportion of this estimate.

Activity for the next day was mainly characterized by gentle outpouring of lava. During cloudbreaks the night of 5-6 March, intense glow from the crater and from some portions of the advancing lava flow along the upper and middle Bonga gully were evident. Rockfalls and minor collapses along the length of the flow contributed to some localized ash and steam emission. However, the majority of the thick volcanic plume came from the summit crater which emitted about 8,300 t/d of SO₂. The PHIVOLCS seismic network continued to record short-duration harmonic tremors and low-frequency volcanic earthquakes. Ground deformation measurements showed some slight inflation of the volcano on the lower NW flank. The very high sulfur dioxide emission rate, occurrence of tremor and volcanic earthquakes associated with magma ascent, and slight swelling of the Mayon edifice indicate that some ascent of magma is still ongoing. Due to cessation of explosive eruptions, the sky W and SW of the volcano was generally clear of ash.

During 6 March the volcano exhibited quiet lava effusion accompanied by intense crater glow and rolling incandescent materials along the upper and middle reaches of the Bonga Gully. Moderate to strong emission of steam drifted generally to the N from the summit crater. The high steam output also yielded an elevated SO₂ emission rate of at least 8,800 t/d. Seismic activity consisted of 11 low-frequency volcanic earthquakes and 25 episodes of short-duration tremors. Slight inflation of the lower NW flank of the volcano continued.

At 0746 on 7 March, a parallel collapse of the new lava flow deposit in the upper middle slopes produced a voluminous secondary pyroclastic flow. The billowing ash cloud descended the Bonga Gully to the SE.

The seismic network recorded low-frequency volcanic earthquakes and short-duration harmonic tremors on 7 March. The measured SO₂ gas emission rate of 3,900 t/d, although low compared to recent measurements, was still well above the volcano's baseline level. Likewise, ground deformation surveys showed that the edifice was slightly inflated. At night on 7-8 March, when the volcano's summit area was visible, intense crater glow continued.

A PHIVOLCS report on the morning of 9 March noted that since the last eruption of 1 March, a waning trend in Mayon's overall activity has been evident. The number of volcanic earthquakes decreased and remained at unremarkable levels. In addition, tremor associated with emission of lava from the crater ceased. Seismic activity only reflected sporadic surface disturbances such as occasional rockfalls caused by oversteepened slopes. The Electronic Distance Meter (EDM) and precise leveling surveys also showed a return to the baseline levels, indicating a probable deflation of the edifice. Mayon continued to vent a large amount of steam, but the SO₂ component measured by COSPEC had decreased. Although the summit and isolated spots on the new lava flow deposits continued to glow at night, this incandescence was attributed to residual heat.

In view of these recent developments at Mayon, PHIVOLCS lowered the volcano status to Alert Level 4. On 9 March the 8-km-radius extended danger zone in the SE quadrant was reduced to 7 km. PHIVOLCS emphasized that the 6-km radius Permanent Danger Zones should remain evacuated at all times because of instability of new pyroclastic and lava deposits that may be dislodged towards the lower slopes with resultant secondary explosions and life-threatening secondary pyroclastic flows.

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/).

A new eruption began at about 2200 on 20 November 1999 (figure 4) with a series of explosions that caused ashfall near the volcano. More than 100 people were evacuated from the Hacienda Las Rojas. A commercial airline pilot reported that the ash plume reached about 2.4-3 km altitude. As of the start of this increased activity, the Nicaraguan Institute of Territorial Studies (INETER) reported that volcanic tremor and sporadic minor ash emissions had occurred for more than a year.

Figure 4. Photograph showing ash emissions from San Cristóbal on the afternoon of 21 November 1999. The view is from the top of Casita volcano, 4 km away. The dark part in the foreground is the edge of the Casita crater. Courtesy of Wilfried Strauch, INETER.

The last previous significant eruptive activity at San Cristóbal began on the night of 19-20 May 1997 and spread fine ash on Chinandega, 18 km W (BGVN 22:05 and 22:06). On 30 October 1998 earth movements on the S flank of Casita volcano (4 km ESE of San Cristóbal) resulted from heavy hurricane rains, killing an estimated 1,560-1,680 people, with hundreds more displaced and several towns and settlements destroyed (BGVN 23:10). The following is based on INETER reports from November through the end of February 2000, with additional information based on crater visits in January and February and satellite imagery of a large eruption plume on 21 February.

Seismic tremor increased during the night of 19-20 November and quickly reached a maximum that was not matched at least through the end of December (figure 5). Tremor amplitude declined throughout 20 November, and then remained at levels of ~25-60% of the 20 November peak until a volcano-tectonic earthquake on 27 November.

Figure 5. RSAM (real-time seismic amplitude measurement) data showing the relative amplitude of seismic tremor at San Cristóbal recorded at the seismic station near the Hacienda Las Rojas, at the foot of the volcano, 19-28 November 1999. Seismicity began during the night of 19-20 November, although volcanic activity at the surface was not reported until the following night of 20-21 November. Ashfall was reported at dawn on the 21st near the volcano. The high peak at 1520 on 27 November was caused by a volcano-tectonic earthquake. Courtesy of INETER.

Small explosions preceded emissions on 22 November. Activity decreased after the initial eruptions, but a lack of winds caused the ash and gases to remain near the volcano. Concentrations of CO₂ and SO₂ measured on 22 November at three different sites exceeded the permissible limits established by the World Health Organization (WHO). INETER inspections showed that expelled gas and ash have been accumulating in local communities. However, on 23 November a slight increase in the wind speed facilitated the transport of volcanic material as far as the city of Chinandega 10 km to the SW. That day an observation post was established near the summit of Casita, where INETER staff could make visual observations and measurements of eruption column heights.

Ash emanations during 24-27 November varied in frequency, and the observer at Casita saw ash columns to heights of 100-300 m above the crater during the same period. At 0920 on 27 November a small earthquake (M 2.3) occurred ~2 km underneath the volcano. Volcanic gas monitoring on 24 November showed a significant increase in the concentrations of SO₂ in the San Rafael and Las Rojas areas. However, the concentration of CO₂ showed a diminution compared to 23 November. Concentrations of SO₂ and CO₂ decreased again on the 25th, although in some communities the level of SO₂ remained unchanged. A correlation spectrometer (COSPEC) for measuring SO₂ flux was brought from the INSIVUMEH of Guatemala on 26 November, with the support of the Coordination Center for the Prevention of Natural Disasters in Central America (CEPREDENAC).

Seismic tremor amplitude remained fairly stable, around 30-50% of the 20 November peak, during 26 November through 13 December. Seismicity fluctuated, with periods of higher and lower amplitude; a corresponding fluctuation in the ash-and-gas emissions was noted. The amounts of gases emitted by the volcano also showed this correlation, with SO₂-flux values that oscillated between 100 and 1,000 tons/day.

By 2 December ash emissions had decreased considerably, although low-level gas emissions remained continuous. There was a slight increase in emissions on 4 December, but activity remained low through the 13th. Higher wind speeds during this period helped keep gas concentrations low in local towns. Seismic tremor began to rise the night of 12-13 December to a high of 70% of the previous peak. On 14 December small amounts of volcanic ash fell in Chinandega. Tremor amplitude returned to ~40% of the peak on 15 December, then decreased to 20-30% of the peak by dawn on the 16th.

Lahars on 16 December 1999. The Civil defense and local residents reported that a mass-flow on the NW side of the volcano on 16 December stopped ~2 km from populated areas. On 21 December, two specialists visited the affected Rancherías region NW of San Cristóbal in the Municipality of Chinandega (Dpto. Chinandega) to investigate the event. According to the meteorological station of Chinandega, 7.9 mm of rain fell from 1824 until 2012; in the first hour strong rains were reported, and from 1920 there were light to moderate rains. Previous strong rains have produced erosion gullies on the slopes of the volcano.

Due to the recent eruptive activity, large amounts of ash and lapilli had accumulated in these gullies; these were mobilized by the rains into lahars. Several sources were identified between 1,400 and 1,500 m elevation. The mobilized material formed a debris flow containing ash and lapilli that carried blocks varying from centimeters to meters in size. The flow quickly cemented into an extremely hard deposit. The material eroded from the highest slopes of San Cristóbal moved along one main gully, leaving a very deep channel and, below 500 m elevation, an extended lobate deposit that reached within ~2 km of the community of Ranchería. The main flow had a length of ~7 km and variable widths between 10 and 150 m; deposit thickness varied from less than 10 cm up to 2 m at the terminus.

Similar events happened on the S slope of the volcano that same day. At least five lahars were visible from the Leon-Chinandega highway.

Minor ash emissions continue. Activity remained consistently low and unchanged until noon on 28 December when earthquakes began. These were centered SE of Casita with magnitudes between 2 and 3.5 and depths of a few kilometers. The occurrence of earthquakes near San Cristóbal is a new phenomenon in the current eruption. Ash emission also increased and small amounts fell in Chinandega.

Seismic tremor on 29 December increased to as high as 40% of the 20 November peak. Small amounts of ash fell in Chinandega and El Viejo. No significant changes in activity were noted on 30 December. Moderate ashfalls were reported near the volcano and in Chinandega. Depending on the wind direction, ashfall sporadically reached the communities of Higueral, 10.5 km NE, and Pelona, 9 km E.

Activity during January-February 2000. Small explosions continued during January with ash and gas emissions (figure 6) and 4,444 registered volcanic earthquakes. Seismicity was higher in the first 17 days of January, but the seismic tremor (RSAM) stayed constant. Between 17 and 23 February activity increased, causing significant ashfall in Chinandega. The number of registered volcanic earthquakes in February was of 1,784.

Figure 6. Ash emissions from San Cristóbal on 13 January 2000. The view is from the summit of Telica volcano. Courtesy of INETER.

Alain Creusot visited the crater on 10 January and observed rhythmic, phreatic explosions, which included rock ejections and ash columns from three vents. At 0600 that day, a violent explosion threw bombs high above the crater rim. Creusot visited the crater again on 4 February and observed a 50-cm-deep ash layer over the crater area and 30-cm depths over the entire summit. Rhythmic phreatic explosions continued and new bombs were observed and sampled on the E crater rim (figure 7). These bombs apparently originated from a Strombolian explosion on 30 January. A third visit on 20 February showed a 1-m-deep ash layer in the crater area and 50-cm depths elsewhere in the summit area. Rocks 50 cm in size had been ejected.

Figure 7. Sketch map of the San Cristóbal summit area showing the new vents within the crater and locations of bombs deposited following an explosion on 30 January 2000. Courtesy of Alain Creusot.

Benjamin van Wyk de Vries noted that GOES images on 21 February showed a long plume extending from San Cristóbal to ~100 km over the Pacific Ocean. He also reported that this was the strongest ash eruption at San Cristóbal since the 1997 eruption. At 1100 on 24 February, a violent explosion threw bombs over the entire summit. A similar explosion and effects occurred at 0900 on the 25th.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua (URL: http://www.ine.gob.ni/); Benjamin van Wyk de Vries, Departement des Sciences de la Terre, Universite Blaise Pascal, 63038 Clermont-Ferrand, France.

The Alaska Volcano Observatory (AVO) reported on 21 January 2000 that investigations of recent seismic data had revealed evidence for small explosions at Shishaldin. Later detailed study of the seismic records showed that the activity may have begun in as early as late September. The numbers of explosions varied from several to over 200/day, but no steam or ash plumes were observed by airborne or ground observers. Also, no thermal anomaly was observed in satellite imagery, indicating that lava had not reached the surface. It was thought that the explosions were phreatic, caused by the flashing of water to steam; these events may represent a local hazard within a few hundred meters of the vent but do not pose a hazard to aircraft. Small explosions continued at a similar rate through 28 January.

Small low-frequency seismic events, present at Shishaldin since June 1999, gradually increased in amplitude after 28 January, with a noticeable increase during 2-3 February. Seismic data continued to show the presence of small phreatic explosions. Reports of steam plumes were received during the week ending on 2 February, with heights reaching as high as ~900 m above the summit. However, no thermal anomaly was observed in satellite imagery and no seismic tremor was identified; both were seen prior to the last eruptive episode in April and May 1999 (BGVN 24:03, 24:04, 24:08). Due to the increased activity, AVO raised the Level of Concern Color Code to Yellow on 3 February, indicating that the volcano is restless and an eruption may occur.

No appreciable number of seismic events were detected after 4 February; that was also the last day that small explosions were observed. Small low-frequency seismic events continued through 11 February, but at a slower rate and slightly lower amplitude. By 18 February seismic activity had declined significantly with no thermal anomalies or observations of unusual activity, so the hazard status was changed back to Green, indicating normal seismicity and surface activity.

Small low-frequency seismic events and very low-level tremor was recorded through 3 March, although at or below the levels observed in the months prior to the 19 April 1999 eruption. Low-level seismicity continued through the end of March. Vigorous steaming was reported in the second half of March, but no thermal anomaly observed in satellite imagery.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: 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.

As of late November 1999, microseismic activity had been occurring at Telica for more than a year. There were phreatic explosions in May and June 1999 (BGVN 24:06). An eruptive phase began in August 1999, generally producing only sporadic small and local ash falls. Intermittent gas-and-ash emissions continued to be reported through December 1999. One of the more vigorous events took place on 29 December, sending ash to several kilometers altitude and inducing falls detected 45 km away.

A noteworthy event began around 0200 on 10 August. Tremor and earthquakes increased abruptly. Small explosions took place in the crater, expelling gas and volcanic ash. Ash fell ~20 km WSW of Telica in the city of Chichigalpa. An interval of relative calm on 12 August lasted approximately one hour. It ended with the gas explosions and ash outbursts starting again at 1315 and continuing until 1515 with ongoing degassing afterwards. According to the summed seismic amplitudes (RSAM values), the greatest activity was between 2000 on 10 August until the morning of 11 August.

Observers saw a lava lake in the crater on 18 August. On that day, INETER's Wilfried Strauch and Armando Saballos, along with visiting North American specialists, climbed Telica to install GPS equipment. Taking advantage of periods of low degassing, they managed to observe the bottom of the new inner crater that had formed in the last few months (figure 11). To their surprise, they saw a lava lake there. In addition they listened to forceful jetting noises probably generated by the water contact with heated material.

Figure 11.Photograph from the crater rim at Telica showing the new inner crater, 18 August 1999. Courtesy of Wilfried Strauch, INETER.

On 21 August INETER's Virginia Tenorio and Julio Alvarez climbed the volcano and saw that the inner crater had enlarged; and, in addition they again heard jet-engine-like noises. Abundant escaping gases thwarted views into the inner crater so the visitors could not assess whether a lava lake remained. The same day between 0800 and 0900, residents who live on the SE flank of the volcano felt two rumblings from the volcano. Possibly, this caused the inner crater to enlarge even more.

Several days later seismic tremor increased, but the number of microearthquakes fluctuated, first dropping, then increasing again on the 25th. On 29 August seismic tremor began to drop substantially. Then, however, the number of microearthquakes increased. Telica's eruptive activity is typically associated with slightly increased tremor and over 200 to 300 microearthquakes per day.

During September, a month with 2,116 microearthquakes, gas emanations prevailed until the 10th. A seismic swarm at the beginning of October was followed by a series of explosions with tephra expulsions during 3-15 October. On the 5th, INETER staff on the crater's edge witnessed the discharge of both ash and lava (presumably in the form of bombs). The last similar lava-bearing explosion of this type was in 1988 (SEAN 13:01). On the 12th, W-flank residents reported that on the previous day (at about 1400 on the 11th) they had felt an unusually strong explosion shaking their houses. Later, they witnessed the fall of very fine gray ash. Observers also saw that the inner crater had grown wider than when seen in September. By 12 October the seismic amplitude had decreased to background levels. The number of earthquakes registered for October was 888.

During November the earthquake sum was comparatively low, 144, but that did not signify volcanic quiet. On 19 November, INETER's Julio Alvarez and Virginia Tenorio skirted the volcano along the León-Chinandega highway where they saw an ash column. Erminio Rojas, a farmer on Telica's S flank, told them that in the past few weeks the volcano had almost constantly been expelling gray ash. On 17 November he witnessed a very large explosion that caused an ashfall deposit reaching 2.5 cm thickness near his house, damaging his apples and beans. The observers further noticed that on the crater's SW a possible collapse feature had developed. Burned ash-covered plants lay in the area near the edge of the crater. Ash discharges on 17 November occasionally emitted a noise similar to a gunshot.

On 24 November, Civil defense of León reported a black cloud above Telica. An unusual seismic signal on 28 November prompted a visit to Telica by Tenorio and Strauch, along with Rafael Abelia of the Institute of Geomineras Investigations of Madrid, Spain. When they arrived at the volcano, the group found that a zone of disruption had spread over a great part of the N crater wall, and the edge of the crater was covered with a thick layer of fine dust. This indicated to them that there was no explosion and the cloud that the Civil Defense observed was due to the collapse of the N crater wall. COSPEC measurements conducted on 29 November indicated that the volcano was producing between 50 and 500 metric tons/day of SO₂ per day.

During December 1999 there were 1,085 volcanic earthquakes, of which, four were located. INETER's seismic network located several earthquakes that took place underneath the volcano on 14 December with magnitudes between 2 and 2.5. During December, tremor stayed low until the 24th, when it was punctuated by sporadic degassing and smaller ash-bearing discharges. On the 25th, tremor began to rise slowly; on the 28th there occurred an abrupt increase in the seismic signal, four-fold larger than seen during previous days. The morning of 29 December seismicity was high. The same day reports were received describing almost continuous ash-bearing explosions, with WSW-directed tephra falls.

Two large explosions at 0900 on 29 December sent ash to heights of more than 1,000 m above the crater. Besides affecting cities adjacent the volcano, ash was later known to have affected the cities of Posoltega (~16 km SW), Chichigalpa (20 km WSW), Quezalguaque (20 km SSW), Chinandega (35 km WSW), and Corinto (~45 km SW). INETER noted that civil-aviation pilots reported that ash rose up to 5 km, although whether this was an altitude or the height over the 1-km-tall volcano remained undisclosed. Tremor initially stayed high on 30 December but dropped on 31 December. Activity continued into January 2000.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

The submarine eruption that started on December 1998 (BGVN 24:01 and 24:03) from multiple vents along the Serreta Volcanic Ridge, about 10 km W of Terceira Island, Azores, continued through March 2000. Vents along the ridge were very active between December 1998 and September 1999. Activity then declined to very low levels with rare surface manifestations through December 1999. Activity increased again in late January 2000.

Several times during 1999 basaltic lava balloons were observed floating in the eruptive area. These "balloons" are very hot, gas-rich, lava fragments produced from small submarine lava lakes/fountains. During ascent to the surface, magmatic gas exsolves from the hot fragments, increasing the volume of the balloon while the crust is glassy and expansible. Once at the surface, interaction between the hot blocks and seawater produce white steam columns that can be seen from land when meteorological conditions are favorable (figure 5). The blocks eventually sink after the gas escapes.

Figure 5. Lava balloon from the Serreta Ridge off Terciera floating on the sea surface and producing white steam column. Courtesy of CVUA.

An oceanographic mission supported by the national Foundation for Science and Technology was carried out in April 1999 to study the geological/geophysical characteristics of the eruption and its impact on local ecosystems. Scientists from the University of Azores, University of Lisbon, University of Algarve, Instituto do Mar, and Instituito Hidrográfico used a remotely operated vehicle that crossed an impressive submarine volcanic plume just above an active eruptive center at about 380 m depth. This plume was formed by volcanic particles of ash and lapilli size along with gas bubbles and lava balloons up to 2 m in diameter.

On 28 January 2000 a yellowish spot was observed at the sea surface above the eruptive area due to the dispersion of a volcanic plume that rose from a new vent located at about 250 m depth (figure 6). The area of water discoloration caused by the plume was visible almost continuously for about a month, reaching a maximum diameter of 8 km on 24 February. The plume was generated by multiple eruptive pulses from different eruptive centers located within a few hundred meters of each other.

Figure 6. Aerial view of the edge of a submarine volcanic ash plume spreading at the sea surface. Courtesy of CVUA.

Seismicity along the ridge related to the eruption continued through the end of March, but at low levels. Since the beginning of this volcanic crisis the physical and chemical parameters of waters and fumarolic gases from Terceira Island have been monitored, with no changes detected. Another submarine eruption took place in this general location in June 1867. At that time five months of strong seismicity destroyed about 200 houses at Serreta.

Geologic Background. Terceira Island contains multiple stratovolcanoes constructed along a prominent ESE-WNW fissure zone that cuts across the island. Historically active Santa Barbara volcano at the western end of the island is truncated by two calderas, the youngest of which formed about 15,000 years ago. Comenditic lava domes fill and surround the caldera. Pico Alto lies north of the fissure zone in the north-central part of the island and contains a Pleistocene caldera largely filled by lava domes and lava flows. Guilherme Moniz caldera lies along the fissure zone immediately to the south, and 7-km-wide Cinquio Picos caldera is at the SE end of the island. Historical eruptions have occurred from Pico Alto, the fissure zone between Pico Alto and Santa Barbara, and from submarine vents west of Santa Barbara. Most Holocene eruptions have produced basaltic-to-rhyolitic lava flows from the fissure zone.

Information Contacts: J.L. Gaspar, G. Queiroz, J.M. Pacheco, T. Ferreira, R. Coutinho, M.H. Almeida, and N. Wallenstein, Centre of Volcanology of the Azores University (CVUA), Departamento de Geociencias, Rua da Mae de Deus, 9502 Ponta Delgada, Azores, Portugal (URL: http://www.uac.pt/).