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/).

Nineteen explosions occurred . . . in August . . . . Ejecta from an explosion on 5 August at 1057 cracked the windshield of an airliner in flight. A car windshield was cracked by tephra from an explosion at 1249 the same day and another was broken on 20 August at 0851, both on Sakura-jima Island, 3 km from the crater. The month's highest ash cloud rose 4,000 m. A total of 583 g/m² of ash was deposited [at KLMO]; a change in the usual wind direction had carried ash away from this site in July. Typical volcanic earthquake swarms were recorded on 3, 15, 16, and 29 August.

Similar activity continued through mid-September, adding 15 explosions as of the 14th . . . . The highest September ash cloud reached 1,800 m height.

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.

An average of 3 explosions/day was recorded in August . . . . Seismicity also decreased, to a daily average of 20 earthquakes (figure 40). Fumarolic activity continued from the active crater, and lava flows continued to travel down the W and SW flanks of the volcano.

Figure 40. Daily number of earthquakes at Arenal, August 1991. Courtesy of the Instituto Costarricense de Electricidad.

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

Information Contacts: R. Barquero and G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Escuela de Geología, Univ de Costa Rica, San José, Costa Rica.

A plume from the summit area of Welirang . . . was photographed by Space Shuttle astronauts on 13 September at [1535] (photo no. S48-151-064) (figure 1). The dense portion of the apparently ash-poor plume extended roughly 50 km N and more diffuse material continued for another 65 km. The summit area was white and apparently de-vegetated. A plume was observed again on direct video downlink from the spacecraft on [17] September at [1306]. No ground reports were available at press time.

Figure 1. Space Shuttle photograph showing a steam plume from Welirang (just east of the central cloud mass). Also, the lack of vegetation at the peak indicates volcanic activity. Volcanoes on Java form an E-W line of peaks the length of the island; five are in this image. NASA Photo ID: STS048-151-064, 13 September 1991.

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

Information Contacts: C. Evans and D. Helms, NASA-SSEOP.

Lava production continued from the subsidiary vent on the NE face of the volcanic cone, 80 m below the main crater, during a visit on 26 June. Incandescent material was ejected in a pulsating fountain, to [80] m height, more intensely than during the previous visit on 16 May. Satellite monitoring had indicated a temperature of 1,100°C around the vent on 6 May. A dark plume rose 300-400 m from the crater of a large spatter cone that had formed at the eruptive vent. The main crater remained quiet. The lava flow observed in May had bifurcated, with one branch extending along the NW and W valleys, and a new branch extending S. By 26 June, lava had reached the sea at the boat landing near the NW corner of the island (~1.2 km from the vent); during the 16 May fieldwork, the lava front was still 200 m from shore. Vigorous boiling and thick jets of steam were observed for 100 m along the shore. Studies of water near the shore indicated a considerable decrease in pH, and visibility dropped to <10 cm (Srinivas, 1991). Nearby coral was destroyed.

The following is from a GSI report on lava chemistry and petrography. "Thirteen chemical analyses on samples of recent lava collected on 16 May indentify the rocks as basaltic andesites (table 1). They are porphyritic with phenocrysts of plagioclase (dominant; some grains show labradorite composition), with minor clinopyroxene (augite) and forsteritic olivine, set in a fluidal [intersertal] groundmass of brown glass, plagioclase microlites, and Fe-Ti oxides. The amount of mafic phenocrysts is relatively low. The average ratio between phenocryst and groundmass components is around 0.44. The volumetric composition of the phenocrysts indicates: 72% plagioclase, 17% clinopyroxene, and 11% olivine; while the groundmass consists of 43% plagioclase microlites, 37% glass, and 20% Fe-Ti oxides. The amount of glass in the groundmass is highly variable, exceeding 70% in some sections. There is a complete lack of amphibole grains, in both the phenocrysts and [in] the groundmass."

Table 1. Range and average compositions from 13 chemical analyses of recent lava erupted from Barren Island, collected 16 May 1991. Courtesy of the GSI.

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

Information Contacts: Director General, GSI; S. Acharya, SANE.

"Five high-temperature fumaroles on the SW rim of the summit lava dome have been monitored continuously since May. These fumaroles are ~75 m W of the site of the March-May lava extrusion and occur along fractures radial to the dome. Temperatures were measured at 20-minute intervals and radio-telemetered to the Science Center in the city of Colima. Temperatures at two of the fumaroles have increased at a steady rate between May and August (figure 16). Mean late-August temperatures were 506 and 386°C, increases of 66 and 43°C, respectively, since May. Mean temperatures in three other fumaroles have changed <10°C during the same period. Throughout the sampling period, all fumaroles exhibited marked diurnal temperature variation, on the order of 30-80°C/day. The rainy season, which began in mid-June and has continued through August, has had little effect on fumarole temperatures other than occasional low readings during rainstorms."

Figure 16. May-August 1991 temperatures at two fumaroles on the SW rim of Colima's summit lava dome, about 75 m W of the site of March-May lava extrusion. Measurements, at 20-minute intervals, were radio-telemetered to the Science Center, city of Colima. Courtesy of C. Connor.

Geologic Background. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: C. Connor, FIU, Miami.

People living near the volcano reported that a new eruption began during the night of 8-9 June, after nine years of relative quiet. At the onset of the eruption, residents were awakened by rumblings and a red glow from the volcano, which has since remained active. Ashfalls have occurred regularly in coastal towns 15 km NNW to 15 km ENE of the summit (Galela, Mamoya, and Tobelo). When visited by a geologist on 23-28 June, small to moderate explosions occurred every 4-5 minutes, sometimes accompanied by noise and night glow. Small lahars occurred in rivers draining the volcano.

Space Shuttle astronauts photographed an apparently ash-rich plume extending ~30 km from the summit to slightly beyond the coast on 15 September at 2156 (photos STS048-110-34 & 35). The entire summit area appeared ash-covered.

Figure 1. Photograph of Dukono taken from the Space Shuttle, 2156 on 15 September 1991. The summit area appears to be covered with ash, and the plume extends ~30 km W from the summit. Courtesy of NASA-SSEOP; photo STS048-110-35.

Geologic Background. The Dukono complex in northern Halmahera is on an edifice with a broad, low profile containing multiple peaks and overlapping craters. Almost continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the Gunung Mamuya cone, 10 km NE. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also had reported eruptions.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland; C. Evans, NASA-SSEOP.

Seismic activity increased significantly in August, reaching the highest number of events (>150/day), the greatest reduced displacement (>800 cm²), and the highest released energy (~5.0 x 10⁸ ergs; see figure 52) by long-period events since monitoring began in February 1989. Explosions and continuous ash emission from the crater were accompanied by periodic ejections of incandescent blocks up to tens of centimeters in diameter. Incandescence was visible within the crater at dispersed sites. Although weather conditions impeded direct observations, it was possible to confirm that many of the long-period earthquakes and tremor episodes had associated surface activity. SO₂ flux was low, ranging from 7 to ~370 t/d.

Substantial deformation changes were measured by the electronic tiltmeter [at Crater Station], with a resultant vector of 231 µrad of inflation (118° azimuth) in the 2 weeks ending 14 August. Lower levels of deformation, 3.7 µrad at 183° azimuth, were measured [at Peladitos Station].

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

Information Contacts: INGEOMINAS-OVP.

Two strong explosions were seen from Ternate, 6 km ESE of the summit, on 15 June, ejecting mainly white clouds. A 20 June climb revealed only white vapor filling the summit crater.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the extensive documentation of activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano; the S-flank Ngade maar formed after about 14,500–13,000 cal. BP (Faral et al., 2022). Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.

In one of the largest eruptions of the century, Hudson's mid-August paroxysm produced an eruption cloud 18 km high and deposited ash up to 1,000 km SE. Estimates of tephra volume range between 2 and 6 km³; >1 km³ was deposited in Chile, around 2 km³ in Argentina, and 2 km³ may have fallen in the Atlantic Ocean or been lost to the atmosphere. Satellite data showed that the eruption produced a large SO₂-rich cloud, estimated to contain 1.5 megatons of SO₂ on 16 August, that was transported twice around the globe in 2 weeks.

The following is from a report by Norman Banks. "The eruption produced 1 to 2+ km³ (dense rock equivalent) of magma. The initial 8-9 August eruption (beginning about 1820 on 8 August) was from a basaltic (50% SiO₂) dike through a fissure 4 km long, trending through the NW rim of the 10 x 7 km, ice-filled caldera. The basalt erupted both as a lava fountain and phreatomagmatically, producing a tephra column 12 km high, scoria flows that covered 10 km² of the western caldera floor and an unknown area outside of the caldera, a 4-km-long lava flow over the WNW flank's Huemules glacier, long-lived (12 hours) floods down the Río Sorpresa (WSW flank) and Río Huemules valleys, and a rather low-volume tephra-fall deposit N of the volcano. This ash had a moderate level (100-300 ppm dry weight) of soluble fluorine that was quickly reduced to 2-10 ppm by heavy rains during the next 2 weeks. Grass growing through this deposit has a fluorine content of about 30 ppm.

"The andesitic eruption of 12-15 August may have been due to secondary boiling triggered by intrusion of the 8 August basalt, or other basaltic dikes into the andesitic magma body under the caldera; bombs and lapilli of pumiceous andesite (60% SiO₂) mixed with chilled basalt are common in the tephra-fall deposits. The 3-day andesitic eruption produced a strong Plinian column that ejected pyroclastic material into a very strong SE-directed stratospheric wind (185 km/hr) that kept the plume narrow even 700 km from the volcano. Pumiceous ballistic bombs 1 m in diameter were found 10 km from the vent, where tephra-fall deposits were >2.5 m thick. The 10-cm isopach reached just SE of Chile Chico (120 km SE of the vent) and 1-2 cm of ash was deposited at Argentina's coast (figure 5). [As many as 13 distinct layers of ash were deposited in some locations.] Fortunately, pyroclastic flows did not spill onto the outer snow-covered flanks during this episode, and no additional mudflows were reported. Shortly after the 12-15 August eruption, however, secondary water-and-pumice flows formed on the volcano's flanks during daily melting of the snow. Because most of the thick deposits on the steep mountainous terrain SE of the volcano are on and interleaved with snow, downslope movement and associated hydrological problems for the downstream valleys are certain to accelerate as the summer melting and rains begin. The andesitic ash in Chile had low amounts of soluble fluorine (<20 ppm), and grass covered by or growing through the ash deposits has a relatively low fluorine content. Analysis of fine fractions of the Chilean deposits suggest that downwind (Argentinean) fluorine values will not be significantly higher."

Figure 5. Preliminary isopach map of the 12-15 August 1991 tephra-fall deposits from Hudson. Prepared by N. Banks, H. Moreno, H. Corbella, M. Haller, and H. Ostera.

Steam emission, occasionally containing minor quantities of ash, declined rapidly following the eruption's end on 15 August.

Major reworking of ash deposits in Argentina by strong winds led to several false reports of renewed activity at Hudson. Ash was redistributed N to Comodoro Rivadavia (2 mm at 400 km E of Hudson) and was reported S to Río Gallegos (700 km SSE). In early September, GOES satellite images detected ash clouds, probably below 3 km, carried by ground-level winds at 55-65 km/hr; these clouds extended from near the volcano to over the Atlantic ocean. The densest part of the clouds appeared to be ~250 km SE of the volcano, about halfway to the Argentine coast. Poor visibility, down to a few hundred meters, was reported at Puerto Deseado and Puerto San Julián. Argentine officials have expressed concern over the >2 million sheep and 3,000 cattle in the affected region.

Geologic Background. The ice-filled, 10-km-wide caldera of Cerro Hudson volcano was not recognized until its first 20th-century eruption in 1971. It is the southernmost volcano in the Chilean Andes, related to subduction of the Nazca plate beneath the South American plate. The massive volcano covers an area of 300 km2. The compound caldera is drained through a breach on its NW rim, which has been the source of mudflows down the Río de Los Huemeles. Two cinder cones occur N of the volcano and others occupy the SW and SE flanks. This volcano has been the source of several major Holocene explosive eruptions. An eruption about 6,700 years ago was one of the largest known in the southern Andes during the Holocene; another eruption about 3,600 years ago also produced more than 10 km3 of tephra. An eruption in 1991 formed a new 800-m-wide crater in the SW portion of the caldera.

Information Contacts: N. Banks, USGS; H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN; P. Bitschene, Patagonia Volcanism Program, Argentina; P. Maxwell, US Embassy, Buenos Aires; D. Helms, Lockheed, Houston; S. Doiron and G. Bluth, GSFC.

Fumarolic activity continued in August, mainly in a large zone of sulfur and chloride deposition in the N section of the crater, while a new zone of fumarolic activity appeared in the SSE part. The crater lake grew to cover almost the entire floor, >150 m in diameter. Seismicity, abnormally high since late May, continued to decrease in August (figure 4). During the second week in June, a new group of fumaroles appeared in the crater.

Figure 4. Monthly number of earthquakes at Irazú, January-August 1991. Courtesy of the Instituto Costarricense de Electricidad.

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

Information Contacts: R. Barquero and G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Univ de Costa Rica.

Explosions were clearly visible from the coast (at Ulu Siau) during a visit 2-4 July. A diffuse, red, summit-area glow was continuously observed. Some small earthquakes were felt.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.

The bottom of the summit's Choungou-Chahalé crater, obscured by a cloud of white gas and vapor following the 11 July phreatic eruption, became visible in early August. A new explosion crater (~250 m in diameter) was observed in its SE section. Vigorous degassing occurred through the crater lake and from the wall of the new crater. The following, from Patrick Bachélery, supplements the report in 16:6.

Karthala's 11 July explosion followed an increase in seismicity from 3-5 events/month (June 1988 start of monitoring through early April 1991) to 3-10 events/day in May (figure 2). Earthquakes were centered beneath the crater, mostly at 0-2 km below sea level, with a few 10-20 km below sea level. On 4 May, a swarm of 161 earthquakes (M 0.5-2) was recorded during a 1-hour period beginning at 1609. The number of earthquakes increased to 30-50/day by the end of June, and all were at shallow depths. Deformation measurements showed summit inflation of ~20 µrad during this time; only weak changes in deformation had been measured between the network's installation (May 1987) and June 1989.

Figure 2. Daily number of earthquakes at Karthala, March-June (inset) and May-July 1991. Courtesy of P. Bachélery.

A notable change in seismicity occurred on 30 June at 1645. More than 500 earthquakes (long- and short-period) were recorded that day, as many on the next, and >1,500 daily 2-4 July (figure 2). The short-period events (M 0.5-3.1) were centered in a roughly N-S line below the S part of the summit caldera and the S flank of the volcano (figure 3). Felt shocks caused ~1,000 people to leave the lower part of the S flank.

Figure 3. Epicenter map of short-period earthquakes at Karthala during 30 June-4 July (open squares) and 5-10 July 1991 (filled squares). Courtesy of P. Bachélery.

Seismicity continued to increase from 4 July, with 4,000 earthquakes recorded daily by 10 July. A swarm of nearly continuous seismic events was recorded between 0040 and 0110 the next day. The 4-10 July seismicity was characterized by low-magnitude (mostly M <1, sometimes to M 3.4) short-period events located under the summit at 1-4 km depths, and less numerous deeper earthquakes at 4-8 km depth. Some long-period events with cigar-shaped waveform envelopes were also recorded. The center of seismicity shifted N, resulting in fewer felt shocks in the S part of the island, while several M 3 earthquakes were felt in Moroni (13 km NW of the crater).

Deformation measurements (dry-tilt) the morning of 10 July showed >120 µrad of inflation centered on Choungou-Chahalé and Choungou-Chagnoumeni (figure 4) craters since 28 June. That night, the eruption took place, but no eyewitness accounts are available. Seismicity reached its highest intensity during an 11-hour period that night [but see 16:6], dropping abruptly at 0335 on 11 July to ~100 recorded events/hour. About 1.5 hours later, a strong sulfur odor was detected in Moroni for ~2 hours.

Figure 4. Map showing Karthala's summit region and deposits from the 11 July 1991 explosion. Courtesy of P. Bachélery.

Later visits to the summit revealed that a sizeable phreatic explosion had occurred in Choungou-Chahalé crater. The southern 2/3 of the summit caldera were covered by blocks (up to 10 m³) and ash (figure 4), and the summit vegetation was completely removed from within the limits of the caldera. The crater bottom was hidden by gas and vapor clouds, obscuring the source of a "fountaining" sound heard two weeks after the 11 July explosion. Geologists later believed the sound to have been caused by the forceful arrival of water into the new crater, forming the crater lake.

Seismicity rapidly decreased after the explosion, although several earthquakes of M 3.0-3.5 were recorded through the end of July. In August, 20-40 events/day were recorded, the same level as in June.

Geologic Background. The southernmost and largest of the two shield volcanoes forming Grand Comore Island (also known as Ngazidja Island), Karthala contains a 3 x 4 km summit caldera generated by repeated collapse. Elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style basaltic shield, which has an asymmetrical profile that is steeper to the S. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. Historical eruptions have modified the morphology of the compound, irregular summit caldera. More than twenty eruptions have been recorded since the 19th century from the summit caldera and vents on the N and S flanks. Many lava flows have reached the sea on both sides of the island. An 1860 lava flow from the summit caldera traveled ~13 km to the NW, reaching the W coast to the N of the capital city of Moroni.

Information Contacts: P. Bachélery, Univ de la Réunion; D. Ben Ali and J-L. Klein, CNDRS, RFI des Comores; F. Desgrolard, Centre de Recherche Volcanologique, Clermont-Ferrand, France; J-L. Cheminée, J-P. Toutain, and J-C. Delmond, IPGP.

Lava . . . continued to enter the ocean at two main sites through August (figure 79). By the end of the month, numerous breakouts from the tube system had reduced the volume of lava reaching the sea. Flows produced by major breakouts at ~180 and 340 m (600 and 1,100 ft) elevation spread over the W third of the lava field. Most remained on older lava, but a few lobes reached the field's W edge and ignited small brush fires in the remnants of the Royal Gardens subdivision. One flow from the breakout at 180 m reached 20 m elevation in early August.

Since at least January, a small lava pond has been continuously active in the bottom of Pu`u `O`o crater, covering ~20% of the crater floor on its E side. By April, the pond was circular and surrounded by levees. During the evening of 27 August, bright glow was visible over Pu`u `O`o, and a nearby seismometer recorded frequent bursts of higher amplitude tremor lasting 1-3 minutes. Overflights the next morning revealed that the pond had overflowed its levees, covering the entire crater floor with several meters of active lava that had a thin, frequently overturning, crust. Lava periodically drained back to its former level, remaining confined within the original pond until the next overflow. Similar activity continued through the end of the month. Crater depth remained roughly 80 m.

Seismicity in August included the upper East rift zone's third intrusive swarm since December 1990. More than 200 shallow summit microearthquakes were registered between 1100 and 1200 on 21 August. Earthquake counts quickly declined during the next hour, but elevationated levels of seismicity . . . continued through the next day. The largest concentration of events appeared to be centered just SE of the caldera, and very few occurred beyond Hiiaka crater, 4.5 km from the caldera rim. Most of the month's seismicity in the summit/upper east rift area occurred during the swarm.

Earthquake epicenters since December 1990 (figure 80) have been concentrated in several clusters, the largest of which were associated with the period's three intrusive episodes. The three swarms occurred in different portions of what geophysicists infer to be the same shallow (<5 km deep) structure between the summit and the East rift zone, suggesting a significant role for the summit in the current East rift eruption. During the early December swarm earthquakes were located from the summit roughly 6 km downrift (to Pauahi crater). The largest concentration of events was in the SE part of the caldera, perhaps extending a short distance into the rift zone (toward the Chain of Craters). The March activity occurred away from the summit, with the majority of located events between Pauahi and Mauna Ulu, roughly 3 km farther downrift. Following the early December seismicity and a long-period summit swarm late in the month, seismicity increased between the summit and Hiiaka crater. The same segment of the uppermost East rift zone has consistently shown low levels of shallow seismicity throughout Kupaianaha vent's post-1986 eruptive activity. After the March swarm, seismic activity along this rift segment appears to have increased further, and the August swarm was largely confined to this area.

Figure 80. Plot of earthquake epicenters in Kīlauea's summit, upper to middle East rift zone, and south flank areas, December 1990-11 September 1991. Some of the larger craters are labeled. The eruption's two currently active vents, Pu`u `O`o and Kupaianaha, are off the map ~3 and 6 km ENE of Napau Crater. Courtesy of HVO.

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

Information Contacts: T. Mattox and P. Okubo, HVO.

"In August, Crater 3 frequently erupted moderate to strong, pale grey-brown ash and vapour clouds accompanied by weak to loud detonations, roaring or rumbling. The eruptions occurred at intervals of several minutes to a few hours. The emission clouds rose as much as 500 m above the crater. Dull to bright red crater glow was observed on the nights of 7-9, 12, and 13 August.

"During an aerial inspection on the 14th, two active vents were observed in a mound of lava filling Crater 3. The vents were ~5-10 m in diameter, 40 m apart and aligned approximately N-S. The N vent was more active and was filled with incandescent lava. The S vent was clogged with dark lava. Both vents released blue vapour. Lava had flowed eastward to form a short (70 m) lobe in the E part of the crater. A longer (~150 m) lobe of lava was present on the NE flank of Cone 3. This lobe was fresh, having a dark surface, and its source appeared to be a tube within the E lobe. The NE-flank flow was first observed on 13 August, and appeared to be inactive then. However, some activity of this flow had been evident the previous night when prolonged incandescence in this area and some movement of incandescent material were observed. Two other very small lava lobes (both inactive) were observed on the NW flank of Cone 3.

"Throughout the month, Crater 2 (roughly 200 m E of Crater 3) almost continuously emitted moderate amounts of pale grey-brown ash and vapour. This activity was accompanied by nearly continuous low roaring sounds. Occasional stronger explosions took place. Dull glow over the crater was observed on the nights of 7-9, 13, 22, 24, and 27 August. A 30-minute period of strong explosive activity on the night of 13 August resulted in a large volume of incandescent lava fragments being ejected onto the NE flank of Cone 2. Incandescent lava-fragment ejections from Crater 2 were also seen on the night of 20 August. A brief aerial view of the interior of Crater 2 on 14 August indicated that it remains funnel-shaped, with several benches. Detailed observation was prevented, however, by emissions of ash and vapour.

"The ash plume from the combined emissions of the craters was usually directed in a sector between NNE and NW. Fine ashfalls were recorded in coastal areas (9 km distant) on 1, 2, 6, and 12 August.

"Seismicity remained at a moderate to high level throughout the month. It appeared that most of the stronger seismicity was associated with events at Crater 3. The daily number of explosion earthquakes recorded by the summit station fluctuated between 20 and 70, with the largest totals of 40-70 events on 16, 25, and 30-31 August. Meanwhile, the remote station (9 km distant) recorded 0-29 events/day. Numerous low-amplitude, short-duration, tremor-like signals were produced by weaker explosions. Several periods of harmonic tremor were recorded but the source was not determined."

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

Information Contacts: B. Talai, C. McKee, and P. de Saint-Ours, RVO.

Photographs taken . . . by D., M., and T. Peterson on 25 January showed few changes since late 1990. Lava flows of varying ages were evident on the crater floor, with the youngest (F25) extending N toward the crater wall from a hornito on the N flank of . . . T5/T9 (figure 22). Its dark brown color and clearly defined margins indicated that it may have been active during the Petersons' visit. Light gray-brown lava had spread from a source near vent T11, across the former saddle (M1M2) to the S wall of the crater, covering more than half of the floor of the former southern depression. Lava of similar age also covered much of the N part of the main crater.

M. Peterson returned . . . 29-30 March, and reported 10-15 minutes of lava production during the evening of the 30th from 2 or 3 vents on the N side of T5/T9, very close to the source of the freshest flow photographed on 25 January. A number of flows moved away from the vents, the longest advancing ~50 m. Flow widths averaged 1-2 m and thicknesses varied from 10 to 20 cm. Steam and sulfur fumes were issuing from several sources on the crater rim, walls, and floor. Older flows in the N part of the crater were dominantly pahoehoe but some aa lava was also observed. Flows entering the S depression were blocky and ~2/3 m thick.

Figure 22. View SE across the crater floor of Ol Doinyo Lengai, 25 January 1991. A recent flow from vent T5/T9 is shown in black. Prepared by C. Nyamweru from a photo taken by the Peterson group.

Little fresh lava was evident on the dominantly pale gray to white crater floor during a visit by Benoit Wangermez on 6 May. A slightly darker flow covered most of the southern depression, showing that lava had advanced S since January from a source slightly NW of T11. Small flows around the base of T5/T9 (active in late March) did not look very young. One new light-colored zone (at M2) appeared to be a vent, currently inactive, that had formed since March.

When T. Peterson arrived at the crater rim on 28 June at about 1000, lava was flowing W from a new vent (T18) W of T5/T9. Activity had subsided 30 minutes later, and the level of lava in the vent had fallen 5 m. Heat was rising from older vents (T5/T9 and T14), while T11 had partially collapsed and looked like a "sulfur cave." Lava flows on the crater floor ranged from dark (fresh) to almost white.

A group led by Luigi Cantamessa climbed to the summit on 12 July. No effusive activity was evident, but black to grayish flows [were] perhaps 1-2 days old . . . . Fumarolic activity occurred from some small hornitos. Many fissures were seen; one extended E-W, parallel to the former saddle dividing the main and southern craters, and cut across the W rim, but was not visible on the volcano's outer flank.

Eruptive activity was very minor . . . on 9 August between 1000 and 1400. Hot, fresh, dark gray natrocarbonatite lava was found near the H6 vent complex (figure 23). Water poured on the lava boiled violently. The extent of other fresh lava flows was similar to that observed 4 days later (see below). A small hornito on the S side of H6 ejected 2-3-mm droplets of spatter. A frozen, but still fresh lava pool ~4 m in diameter was found ~2 m below the average elevation of M1's crater floor (a group of tourists and a local guide reported that vents H6 and M1 had been active 2 days earlier). Vent T5/T9 emitted hot colorless gas, while T11 exhaled SO₂. Radial fissures on the W flank of the crater produced almost pure (>95%) CO₂, with some SO₂. Holes ~0.5-1 m across on the crater's W rim released hot, humid air with no detectable SO₂ or CO₂. These holes contained a variety of water-loving plants such as moss and algae. Gas compositions were measured with Dräger tubes.

Figure 23. Sketch map of the crater floor of Ol Doinyo Lengai, 13 August 1991. Fresh lava is shown in black. Courtesy of Alain Catté.

Lava production from one vent complex was continuing during a summit climb by Alain Catté and others on 13 August. Irregular, weak, but clearly audible explosions occurred from the 4-5-m-high hornito complex H6, ejecting lava fragments horizontally to 10-15 m from two vents (E1 and E2). Weak effusive activity occurred from a site ([E4]) 5 m below the hornito complex. Young, chocolate-brown flows extended from its base in three directions atop older (>48 hours) whitish flows: ~10 m E; ~40 m NE; and > 100 m N, flowing around other small cones. Production of small flows accompanied vent E1's explosions from the initial observations at 0845 until its activity stopped at about 1000.

When clouds cleared at 1030, a very fluid lava flow 40-50 cm wide was emerging from neighboring vent E2. The flow quickly subdivided into many black lobes ~10 cm wide, with a consistency like lubricating oil. Within a few seconds, these formed channeled pahoehoe flows that turned to aa at their distal ends. Lava also formed tubes that carried it >100 m from the source. No lava temperatures were taken, but it was possible to place one's hand a few centimeters from an active flow, and to touch it after ~2 minutes of cooling. A cascade of lava ~10 cm wide began from a third vent (E3) on the hornito complex at about 1145. Vents E2 and E3 erupted simultaneously and showed parallel fluctuations in activity. Later . . . lava outflow from E2 occurred in a jet 2 m long.

At about noon, lava production resumed from the base of the hornito complex (at [E4]) bubbling out in a manner reminiscent of mud pots. It overflowed after ~45 minutes, gradually building a hornito that grew to 1 m height before activity ceased at about 1330. Above [E4], lava effusion from vent E3 stopped at 1230, emerging from a channel 2 m below in a violent, 3-m jet that reached the base of [E4], beginning to fill the area with lava. The outflow rate increased progressively, and lava had advanced 60 m W by the end of observations at about 1400. Lava production from the H6 complex had roughly quadrupled its size since . . . March.

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

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, M. Peterson, and T. Peterson, Arusha, Tanzania; B. Wangermez, Nairobi, Kenya; L. Cantamessa, Geo-decouverte, Switzerland; P. Vetsch, SVG, Switzerland; T. Dunai, R. Ragettli, K. Schenk-Wenger, and U. Ziegler, ETH Zürich, Switzerland; A. Catté, B. DeMarne, and P. Barois, LAVE.

The press reported that renewed activity on 19 September ejected a plume to ~700 m. Incandescent tephra fell 500 m from the crater, starting fires that burned plantations in seven villages. No casualties were reported. As of the next morning, the eruption was continuing and VSI observers were recording accompanying earthquakes. VSI advised local authorities that residents of nearby villages should remain on alert, but an evacuation was not ordered.

Geologic Background. The Lokong-Empung volcanic complex, rising above the plain of Tondano in North Sulawesi, includes four peaks and an active crater. Lokon, the highest peak, has a flat craterless top. The morphologically younger Empung cone 2 km NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century. A ridge extending 3 km WNW from Lokon includes the Tatawiran and Tetempangan peaks. All eruptions since 1829 have originated from Tompaluan, a 150 x 250 m crater in the saddle between Lokon and Empung. These eruptions have primarily produced small-to-moderate ash plumes that sometimes damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred.

Information Contacts: VSI; UPI.

Widely distributed reports of increased activity and up to 20,000 evacuees in mid-September proved false. Heavy cloud cover over the volcano and coincidental tectonic earthquakes prompted claims of an imminent eruption. PHIVOLCS scientists found no signs of activity, although they did locate a previously unknown geothermal area on a remote section of the volcano.

Geologic Background. The Quaternary Malindang volcano is located at the southern end of an eroded massif (Ampiro volcano is at the northern end), W of Iligan Bay in N-central Mindanao. Legends record a large eruption from the dominantly basaltic-to-andesitic volcano in the past, although no confirmed eruptions have been documented (Salise et al., 1991). Reports of increased activity in 1991 at the time of tectonic earthquakes prompted widespread evacuations, and although an eruption did not occur, a previously unknown geothermal area was discovered.

Information Contacts: D. Sussman, Philippine Geothermal, Inc., Manila; Philippine Daily Inquirer and Manila Times, Manila; Reuters.

"Main Crater produced weak emissions of white vapour with low ash content on 1, 2, and 3 August. Blue vapour was visible on 8, 11, and 12 August and only white vapour during the last week of the month. There were no audible noises and no night glow was seen.

"The emissions from Southern Crater consisted of tenuous white vapour with occasional grey-brown ash clouds resulting in fine ashfalls on parts of the island. Occasional weak deep roaring and rumbling noises were heard 2-14 August and a weak red glow was observed around the crater mouth on the night of 7 August. An aerial inspection was carried out on 13 August. Southern Crater was partly filled with vapour but Main Crater was clear. The floor of Main Crater was occupied by a solid plug or mound of lava, at a level ~20 m below the lower (NE) part of the crater rim. White mofettes were released by numerous fumaroles around the base and lower walls of the crater. The crater floor was mostly covered by debris from the crater walls, but in the central area, the lava plug was visible over an area ~5 m in diameter, and consisted of steaming lava surrounded by small blocks and scoriae ejected during a stronger degassing phase. During the aerial inspection, emissions from Southern Crater were low-energy, thermally buoyant clouds, released fairly regularly at ~15-minute intervals.

"Seismicity was at a moderate level and tilt measurements showed a deflation of ~1.5 µrad since mid-August."

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

Information Contacts: B. Talai, C. McKee, and P. de Saint-Ours, RVO.

Marchena . . . started erupting on 25 September. The TOMS instrument aboard the Nimbus-7 satellite passed at about 1100 and sensed no SO₂, but the next pass, at the same time on 26 September, mapped a 300-km plume to the SW with an SO₂ content estimated to be close to 100 kt. High SO₂ values immediately over the volcano indicated that the eruption was still vigorous at that time. On the following day the plume was nearly twice as long, but had almost vanished by the same time on 28 September. Weather satellite images during this period showed low cloud cover, but no conclusive indication of the volcanic plume. . . .

Geologic Background. The low shield volcano forming Marchena Island contains one of the largest calderas of the Galápagos Islands. The 6 x 7 km caldera and its outer flanks have been largely buried by a cluster of pyroclastic cones and associated lava flows. Its first historical eruption occurred in 1991. Other young lava flows, some of which may be only a few thousand, or even a few hundred years old, filled the caldera and flowed down its outer forested flanks, in some cases to the sea.

Information Contacts: A. Carrasco, Charles Darwin Research Station; S. Doiron, GSFC; SAB.

Activity continued to decline through 15 September, with only three ash/steam emissions since about 25 August. Heavy monsoon rains triggered numerous mudflows and secondary explosions from the 15-16 June pyroclastic-flow deposits. Two large secondary pyroclastic flows occurred, producing associated ash clouds to 15 km height. The press reported continued fatalities from debris/mudflows and disease in evacuation camps, bringing the number of casualties attributed to the eruption to at least 740 by 20 September. Study of the June deposits has resulted in preliminary estimates of 7-11 km³ of material erupted.

5-11 August. Radar at Clark Air Base detected 13 ash/steam emissions rising to 4.5-13.5 km height; plumes were carried NE by the wind. Most RSAM peaks coincided with these emissions. The majority of seismicity was shallow (<=1 km depth), with magnitudes <1. Seven high-frequency earthquakes were felt at Clark Air Base.

12-18 August. Thirteen ash/steam emissions were detected, three with columns >15 km high (maximum 17.5 km). Wind carried the plumes ENE and NE, and ashfall was reported at Clark Air Base on 13 and 16 August. Ejection velocities ranged from about 300-900 m/min, similar to the ejection velocity on 25 June (estimated at about 450 m/min). A large secondary pyroclastic flow occurred sometime on 12-13 August, in the Marunot drainage on the NW flank. The flow was not observed, but satellite imagery was used to identify the deposits and estimate a deposit volume of 31 x 10⁶ m³ (1.25 km² areal coverage). The flow, ~10 km long, created a headwall scarp about 20 m high along a 240° arc in the primary pyroclastic-flow deposit source region. During aerial observations, the still-steaming secondary deposits could be differentiated from those of earlier pyroclastic flows by the absence of rills and dissected morphology.

Seismic energy release decreased notably from the previous week (figure 19), although the number of earthquakes remained about the same (102 recorded events/day compared to 95/day the week before). Several shocks were felt at Clark Air Base. RSAM peaks reflected high-frequency earthquakes generated by mudflows, and occasional long-period signals associated with ash/steam emissions from the caldera. Geologists suggested that small long-period events may also be related to secondary explosions from pyroclastic-flow deposits.

Figure 19. Accumulated RSAM energy at Pinatubo, 28 July-18 August 1991. Courtesy of PHIVOLCS.

19-25 August. Ash/steam emissions averaging ~9.8 km high (maximum 15 km) were detected eight times during the week. Ash was carried E. Some may have originated from secondary explosions at the E flank (Sacobia valley) pyroclastic-flow deposits. Seismicity consisted mostly of high-frequency earthquakes (M < 1.0) centered below the caldera or ~3 km NW, at 0-18 km depths (figure 20). Four events (M 2-4) were felt at Clark Air Base, with intensities to IV (adapted Rossi-Forel scale). RSAM peaks coincided with the larger high-frequency earthquakes, and long-period events were associated with ash/steam emissions.

Figure 20. Epicenters of 648 earthquakes recorded near Pinatubo, 19-25 August 1991. Courtesy of PHIVOLCS.

26 August-1 September. Only two ash/steam emissions were detected; plume heights ranged from about 10 to 16 km. Light ashfall occurred to 40 km SE (San Fernando) during secondary explosions that produced columns to 16 km. Ash related to these events caused poor visibility (300 m) on the highway between San Fernando and Angeles (25 km E of the volcano). The number of felt shocks (M < 4.2) increased to 17, with intensities to V (adapted Rossi-Forel scale). Multiple peaks in RSAM plots were due to mudflows, while single peaks were caused by long-period events associated with the two ash/steam emissions.

2-8 September. One ash/steam emission was detected (2 September), producing a 9-km plume that was carried W (highest portion) and NE (lower portion). Secondary explosions, three of which were recorded as low-amplitude, low-frequency earthquakes, generated ash clouds 2-4.5 km high. Geologists proposed that the heavy ashfall and 15-km-high ash column observed at 1400 on 4 September (figure 21) were from a secondary pyroclastic flow, whose fresh deposits were discovered two days later. The absence of a long-period earthquake coincident with the ash cloud suggested that it had not been generated by caldera explosions. The secondary pyroclastic-flow deposits about 3 km SSW of the caldera (in the upper Marella drainage) were estimated to be 1-2 km wide, and 4-6 km long, with a headwall scarp 15-25 m high. The deposit appeared very recent and seemed water-saturated. It was not known whether the ash cloud was generated purely by convection, or by phreatic explosions resulting from an encounter with water on the river bed. A helicopter overflight of the caldera on 6 September revealed no evidence of activity during the prior several days. Steaming was observed along the margins of the caldera and a bluish lake was present. No evidence of a lava dome was found.

Figure 21. Visible and infrared image from the NOAA 11 polar-orbiting weather satellite on 4 September at about 1445, showing a large, 15-km-high ash cloud above Pinatubo believed to have been generated by a secondary pyroclastic flow. Courtesy of G. Stephens.

Recorded earthquakes averaged 88/day, similar to 89/day the previous week. The majority were of high-frequency, and geologists believed that they were caused by tectonic readjustments. Most of the few low-frequency signals coincided with observed secondary explosions. Seismicity remained shallow (about 38% at less than 2 km depth), centered beneath or NW of the caldera. Long-duration, high-frequency earthquakes corresponding to mudflows created peaks in RSAM plots. A magnitude 5.1 earthquake at 0627 on 5 September, centered ~17 km NNW (15.53°N, 120.31°E) at 10 km depth, was felt at Clark Air Base (intensity RF V).

On 4 September, due to the continued decrease in caldera activity, the volcanic alert was reduced from Level 5 (eruption in progress) to Level 3 (numerous magma-related earthquakes, fumaroles, and gas emission), and the danger zone radius was reduced from 20 to 10 km. The principal remaining hazards and their probable durations were identified (table 4).

Table 4. Principal hazards associated with Pinatubo following the 15-16 June 1991 eruption (as of 4 September 1991). Courtesy of PHIVOLCS.

9-15 September. Although no ash/steam emissions were detected, ash clouds 2-10 km high were produced by secondary explosions. Vigorous steam emission was noted from the S side of the caldera, and the blue crater lake was still present during observations on 10 September. The average number of earthquakes decreased to 54 recorded daily, most centered ~2 km NW or 2 km S of the caldera, at <2 km and 5-10 km depths. The majority of events were M <2. RSAM and accumulated energy both showed decreases corresponding to the drop in seismicity. Multiple RSAM peaks coincided with mudflows, while single peaks were caused by moderate-sized earthquakes.

Debris flows. All of Pinatubo's major drainage systems experienced debris flows, ranging from mudflows to hyperconcentrated flows and floods. Numerous flows also occurred in more distant drainages in which significant quantities of tephra were deposited. To help alleviate hazards and to aid in studying debris-flow processes, rain gauges were installed, observation posts were set up at strategic locations along rivers, and cross sections were monitored at bridges. Timely warnings and evacuations considerably reduced the number of injuries and casualties. High rainfall (to > 30 cm/day) and still-hot pyroclastic-flow deposits generated numerous hot mudflows that deposited as much as several meters of material.

On the SE flank's Pasig-Potrero River, pyroclastic-flow deposits had formed a dam behind which a 1,000 x 600 m lake had formed. The lake drained on 7 September, causing muddy flash floods that reached 1.2 m high in about 5-10 minutes at Bacolar (35 km SE of the volcano). Press reports indicated that 800 homes were destroyed and seven people were confirmed dead. By 15 September, continued flooding and mudflows resulted in the deaths of 12 more people at Bacolar, where 45,000 of the 68,000 residents had fled.

News reports placed the death toll from the eruption, mud flows, and disease at more than 740 by 20 September [see also 16:9]. Of the fatalities in evacuation camps, an estimated 95% were Aeta tribesmen and 75% were children. The Aeta reportedly refused most medical assistance such as vaccinations.

Fieldwork on June eruptive products. Preliminary estimates have been calculated for pyroclastic-flow deposits and airfall tephra from the paroxysmal June eruptive activity. The bulk of the material erupted was found in pyroclastic flow deposits (5-7 km³); several drainage systems included more than 1 km³. An estimated 0.48 km³ of airfall tephra was deposited within the 15-cm isopach (table 5); the total volume of tephra-fall material erupted, including that deposited in the South China Sea or lost to the atmosphere, was believed to be between 2 and 4 km³. The total volume, therefore, is estimated as 7-11 km³ (roughly 3-5 km³ dense rock equivalent).

Table 5. Preliminary volume calculations (±10% error) of June 1991 eruptive products from Pinatubo. Total tephra deposit volume: 0.48 km³. Total pyroclastic-flow volume: 7.0 km³. Courtesy of PHIVOLCS.

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: R. Punongbayan, PHIVOLCS; K. Rodolfo, Pinatubo Lahar Hazards Taskforce, Univ of Illinois; W. Scott, USGS CVO; G. Stephens, NOAA/NESDIS; NEIC; AP; Reuters; UPI.

In August, the crater lake grew to cover all crater fumaroles, while fumarolic activity continued at levels considered "normal" for the volcano. The yearly total of recorded microearthquakes (almost all of low frequency) exceeded 32,500 by the end of the month (figure 40), a decrease from 1990.

Figure 40. Monthly number of earthquakes at Poás, January-August 1991. Courtesy of ICE.

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

Information Contacts: R. Barquero and G. Soto, ICE; M. Fernández, H. Flores, and S. Paniagua, UCR.

The crew of Qantas flight 41 (Sydney-Jakarta) observed a very dense black plume emerging intermittently from a flank vent on 10 September at 1508. The plume was drifting N at ~6 km altitude, well below the aircraft's altitude of nearly 12 km. A voluminous, dense, mostly white plume with small pulses of ash in its center was observed from a commercial flight two days later.

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

Information Contacts: ICAO; J. Post, SI.

After reports of strong sulfur odors, geologists visited the summit area on 28-30 August. A sulfurous odor was noted at Copelares on the S flank (1,400 m elevation), during the evening of 28 August. An explosion was heard at 0151 the next morning, followed several seconds later by the sound of falling material. Examination of 29 August records from a seismic station 6 km SW of the crater (RIN3) showed that a small earthquake occurred at 0148:47, then a larger earthquake sequence lasting 7.5 minutes began at 0151:40, coinciding with the first audible explosion. As the ascent continued later that morning, traces of fresh ash were observed beginning at about 1,500 m elevation. Large quantities of ash and blocks, ranging from 15 to 75 cm in diameter, were found deposited in the summit area. Impact craters reached 120 cm in diameter and 35 cm deep.

Bad weather obscured the view of the crater floor, but several explosions were heard, and the largest, at 0930, rained very wet ash on the scientists. Near the crater, the smell of sulfur was very strong, making breathing difficult and stinging the eyes. Nearby vegetation was partially or completely dead. Rain collected at Copelares had a pH of 4.1.

On 30 August, scientists visited Ríos Azul and Pénjamo, which flow down the N flank from the crater area. Both rivers were gray-white with suspended sediment, which was also visible, but in lower concentrations, in the Ríos Colorado and Blanco on the S and SE flanks.

[On 6 September, strong fumarolic activity (jet engine noise) was seen in the active crater. During explosive events of May-August 1991 the ejecta was mainly composed of gray mud (sulfide-rich), lithics, and bread-crust bombs (~10% by volume).]

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: J. Barquero and E. Fernández, OVSICORI; R. Barquero and G. Soto, ICE; Mario Fernández, Héctor Flores, and Sergio Paniagua, Univ. de Costa Rica.

A brief period of strong heating in Crater Lake was accompanied by small volcanic earthquakes and possibly by minor eruptions. Continuously recorded lake temperature data showed a gradual decline to 16°C by mid-June, then little change until a sharp increase began about 1 July. Temperatures reached 24.4°C on the 18th before declining again to 13° by late August. A series of small volcanic earthquakes occurred 5-14 July, none exceeding M 1.8.

Severe winter weather limited observations near the time of the increased activity, although the lake appeared normal on 11 July. When briefly observed on 12 August, evidence of 1-2 m of surging was visible under fresh (about 10 August) snow around the lake margin. More detailed observations during fieldwork 27 and 29 August revealed dirty, ash-covered ice under fresh snow 1-2 m above lake level, and widening of the lake's outlet channel by previous strong outflow or surging. No clear patterns were evident in summit-area deformation data.

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

Information Contacts: P. Otway, DSIR Wairakei.

Seismicity remained at low levels in August, with earthquakes mainly W and N of the crater at 0-5 km depths. Tremor episodes were brief and of low energy. Deformation showed no significant changes. The monthly average SO₂ flux was 1,135 t/d, similar to July.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

During a brief visit on 11 September, vertical explosions occurred hourly, producing plumes to about 1200 m height. The block lava flow erupting from the E summit of Caliente continued to flow down to the Río Nima II.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing E towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: W.I. Rose, Michigan Technological Univ.

Explosive activity was restricted to crater C1 (NE part of the summit area; figure 17) during 9 August fieldwork (by F. Iacop, Institute of Earth Sciences, Univ of Udine). C1's central cone ejected hot tephra at ~20-minute intervals, and as a result, it had grown more rapidly than the crater's other two active cones. Glow from two small radial fissures in crater C2 was clearly visible at night. Sustained noisy gas emissions occurred about once an hour. Volcano guides had reported that activity was concentrated in crater C3 (SW part of the summit area), but at its cone 1 only hot vapor emission was occurring, from two vents, on 9 August. Rare explosions, mostly ejecting tephra, took place at bocca 4. The average number of recorded earthquakes remained near the normal value of 6/hour in July, declining below that level in the month's last week (figure 18). Average tremor amplitude also remained relatively constant through the end of July, while large shocks nearly disappeared after a peak on 29 June (figure 19). [see 16:09 for 28-29 August observations].

Figure 17. Active craters at Stromboli as seen from the somma, 6 September 1991. Crosses mark small vents active during the 6 September fieldwork. Courtesy of the Société Volcanologique Européenne.

Figure 18. Average number of explosion events/hour at Stromboli, 22 June-31 July 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.

Figure 19. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude (upper curve) at Stromboli, 22 June-31 July 1991. Courtesy of M. Riuscetti.

Moderate activity was observed in early September, with explosive episodes about every 15 minutes at crater C3 and roughly hourly at C1. Activity increased in the 3 hours of observations after 2300 on 6 September, with many moderate to strong explosions from the SW part of C3. Ejections of incandescent bombs and scoria sometimes lasted several minutes. Thick white vapor plumes rose from C2 and a small cone in its center, while blue SO₂-rich plumes emerged from several other vents. Explosions from C1 were vigorous, ejecting glowing fragments and dark brown columns that rose 200 m above the crater. C3's smaller explosive bursts, consisting of tephra-poor incandescent gas jets, were usually preceded by comparatively brief periods of increasing, noisy gas puffs; larger explosions that ejected a higher proportion of tephra followed longer intervals, with fewer or no precursory gas puffs. Geologists attributed this pattern to intermittent closure (by cooling) of the lava-filled conduits to gas-bubble rise from the underlying magma body, allowing higher pressure to build at depth.

Airborne COSPEC measurements by an Italian-French cooperative program in May-July indicated a total SO₂ flux somewhat lower than that measured by the same means in 1980 and 1984 (1,000 ± 200 t/d average; Allard and others, in press), consistent with the current moderate activity. Geologists concluded that combined with microprobe determination of the initial and residual sulfur content of Stromboli's lava, the SO₂ flux data require the degassing of 0.1 km³/year (average) of magma, three orders of magnitude more than the co-erupted volume. Thus, gas output is essentially derived from magma stored within the volcano. To assess the amount of diffuse magmatic degassing through the volcanic pile, other than from the craters, infrared mass spectrometric profiling of CO₂ concentrations in the ground began on 11 September. High CO₂ levels (80-90%), associated with subsurface thermal anomalies, were found to characterize the Pizzo sopra La Fossa crater terrace (at the summit rim, SE of the active craters). Concentrations gradually decreased toward the rim of this former crater, and no CO₂ anomaly was detected in outer areas to the S (down to the Vancori rampart).

Reference. Allard, P., Carbonelle, J., Le Bronec, J., Metrich, N., and Zetwoog, P., Volatile flux and magma degassing budget at Stromboli volcano: Geophysical Research Letters, in review.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: M. Riuscetti, Univ di Udine; Patrick Allard, CNRS-CEA, France; J.C. Baubron, BRGM, France; H. Gaudru and Rolf Haubrichs, SVE, Switzerland; Yvonne Miller, Univ de Genève, Switzerland.

Lava extrusion continued at Jigoku-ato crater through mid-September, generating destructive pyroclastic flows that advanced down two valleys. More than 12,000 people remained evacuated and no new casualties were reported.

A summit seismic swarm that began 11 August peaked 12-13 August (figure 29), then gradually declined through the 19th. Incandescent block ejection was seen between 0000 and 0200 on 12 August, followed by continuous ash emission through the day. The number of seismically detected pyroclastic flows from the lava dome decreased suddenly to a few events daily on 12 August. A new lava dome, first recognized from the air on 13 August, emerged W of the former dome, and began to produce pyroclastic flows on 25 August. Pyroclastic flows had previously traveled down the Mizunashi River valley but those from the new dome (C dome; see below) moved ENE down the Oshigatani Valley, which extends N of and parallel to the Mizunashi, then joins it several kilometers downstream. Some of the larger pyroclastic flows from the new dome advanced 3 km down the Oshigatani valley from late August through mid-September, and pyroclastic surges burned vegetation. The mayor of Shimabara city ordered the evacuation of about 500 people from an area (Senbongi) 3.5 km NE of the dome on 31 August. Frequent pyroclastic flows during the afternoon of 3 September included one of about 1 x 10⁵ m³ volume that advanced down the Oshigatani Valley at 1611. The accompanying cloud rose about 1,500 m and ash fell to the N part of Shimabara city. Ashfalls from pyroclastic flow elutriation clouds disrupted traffic around Shimabara city throughout the following day; the cloud from a flow at 1311 was 2,500 m high.

Figure 29. Daily numbers of earthquakes (top), tremor episodes (middle), and pyroclastic-flow events (bottom) recorded at Unzen, 1 May-20 September 1991. Courtesy of JMA.

Another seismic swarm began beneath the crater on 6 September, and a pyroclastic flow that evening at 2121 advanced about 3.5 km down the Oshigatani Valley. Hypocenters and seismic wave characteristics were similar to those of mid-August, although the September swarm was more vigorous.

By 12 September, the lava dome had broken into numerous small blocks. Seismic activity declined through 14 September but increased again on the 15th. Seismometers near the summit began to record larger pyroclastic flows, with longer durations than any since 8 June, on 15 September at 1644 (150 seconds) followed by others at 1759 (120 seconds), 1842 (360 seconds), and the largest at 1854 (670 seconds). The latter moved down the Oshigatani valley, entered the Mizunashi valley, and continued to within 500 m of highway 57, a total of 5.5 km. The main body of the pyroclastic flow turned east into the Mizunashi valley, where it damaged 50 houses in Shimabara city, but the pyroclastic surge continued about 800 m southward, destroying 26 houses and 74 other buildings including those of a primary school (in Onokoba district, Fukae town). All of the affected area had previously been evacuated, so there were no casualties. The largest pyroclastic flow was associated with the collapse of a section of the new lava dome about 250 m wide, 300 m long, and 50 m thick, a volume exceeding 3 x 10⁶ m³. This is about 20% of the total volume of lava domes erupted to date, and 3 times the volume of material removed by the 8 June pyroclastic flow. Two days later, a new lobe had grown to 100 x 200 m and 30 m high (0.3 x 10⁶ m³/day), about twice the June-August extrusion rate (see below).

A total of 292 pyroclastic-flow events was recorded in August, down from 326 in July, but the more frequent episodes toward mid-September raised that month's total to 310 as of the 17th. September earthquake counts had reached 2075 through the 17th, up from 559 in August and 133 in July.

The following, from Setsuya Nakada, describes eruption products through early September.

The size and frequency of pyroclastic flows had decreased until July, and travel distances were almost always <2 km. However, collapse episodes from the E lava dome remained frequent and lava blocks had filled the narrow headwaters of the Mizunashi River, along which the 3 and 8 June pyroclastic flows had descended. As a result, cliffs along the valley disappeared, and valley-fill deposits (talus) became thick enough to act as a cushion to soften the shock of falling blocks. The E dome flowed southeastward on the valley-fill deposits. After the end of June, the horseshoe-shaped depression had filled with dome materials, and lava blocks began to fall northeastward onto the floor of Myoken caldera (figure 30). They filled the E end of the floor with talus, which overflowed the caldera rim at the end of July. Lava blocks then fell down the E and NE flanks as pyroclastic flows and their paths widened northeastward. Some reached the N bank of the Mizunashi River. The E margin of the E dome widened; because the NE slope under the dome was steeper than the SE slope, the northern half of the E dome migrated northeastward, while the southern half did not move and solidified. By the middle of August, the caldera rim NE of the dome had been eroded away by the falling lava blocks.

Figure 30. Sketch map of Unzen's lava dome, 8 September 1991. Courtesy of Setsuya Nakada.

At the beginning of August, the ash-laden plume from the small vent at the northern base of the remnant W dome became stronger, and new lava was extruded on the western part of the E dome. On 5 August, many bubbles were observed coming from an old water-filled crater near the W dome. The small explosions that took place from the W dome on 12 August (see above) enlarged the vent to 20 m across and built a tuff cone around it. The E dome temporarily thickened for a few days prior to the new lava extrusion; the western part of the E dome, just above the former Jigoku-ato Crater, had swelled vertically. By the time new lava appeared 13 August, magma supply into the E dome had stopped, since the E dome did not lengthen and the surface of the dome did not move eastward (figure 31). It was difficult to accurately estimate the change in magma supply rate; talus and pyroclastic flows were deposited over an extensive area with irregular topography, which causes difficulties in calculating volumes of talus plus pyroclastic deposits.

Figure 31. Tracings of photographs from a fixed point about 4.4 km from Unzen's E lava dome, illustrating its growth 10 July-20 August 1991. Courtesy of Setsuya Nakada.

At the end of August, the new dome (central, or C dome) was 375 m long, 275 m wide, and 60-80 m high. The C dome grew eastward and northeastward, keeping a constant thickness. It covered the E dome and talus, plus a part of the old volcanic edifice, which was bulldozed by the growing dome from the former crater wall to the caldera rim. Talus also formed on the E dome. At the end of August, the volume of C dome was about 4 x 10⁶ m³ and the total volume of the domes was about 12 x 10⁶ m³. The resulting dome growth rate is about 0.15 x 10⁶ m³/day for 8 June-28 August.

Lava blocks fell down the E and NE margins of C dome into the Oshigatani Valley, forming pyroclastic flows beginning 25 August. The upstream area of the valley was the source area for lahars on 30 June. The pyroclastic flows traveled a maximum distance of 3 km from the dome, and had associated ash-cloud surge and seared zones like those of 3 and 8 June (figure 32). Flows moving down the Oshigatani Valley changed course southeastward when they encountered a high point dividing the valley and a residential area. Ash-cloud surges climbed the barrier, burning or searing trees, but block-and-ash flows did not. The devastated area was widest for pyroclastic flows that took place within the first week. By mid-September, Oshigatani Valley had been almost filled by pyroclastic-flow deposits.

Figure 32. Map showing the distribution of pyroclastic flows from Unzen as of 9 September 1991. Deposits from lahars, which occurred mainly on 30 June, are omitted. Courtesy of Setsuya Nakada.

Average speeds of pyroclastic flows were estimated using travel distances observed by Ground Self-Defense Force radar and durations of tremor signals. The higher the average speed of a pyroclastic flow, the longer its travel distance: about 100 km/hour for flows reaching 3 km distance and 50 km/hour for flows 1 km long. The average speed of a pyroclastic flow at the end of August was estimated at 93 km/hour using the time lag between the start of the tremor signal and the time when the seismometer was broken by the flow.

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

Information Contacts: JMA; S. Nakada, Kyushu Univ; M. Takahashi, SI; Yomiuri Shinbun, Tokyo.

An increase in fumarolic activity and weak explosions were observed in the crater during August-September. On 26 August, water in a nearby river (Río Carmelito) was cloudy and the river level abnormally high. Four days later, on 30 August, small ash emissions and continuous explosions were observed from 1430 to 1500, followed by a strong explosion at 1506. A weak emission of gray ash and a white gas plume 1 km high were observed on 17 September. Seismicity was at normal levels for the volcano.

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: G. Fuentealba and P. Riffo, Univ de la Frontera.

Emission of gas/tephra columns from May 91 vent continued through August. During early-August helicopter overflights, R. Fleming noted flashes and strong low-frequency detonations as a hot, dilute eruption column rose from the vent. Crumbly white lithic blocks and lapilli with rare juvenile scoriae had been deposited nearby. Larger-than-normal plumes were often visible from the North Island coast, roughly 50 km away.

During fieldwork 28-29 August, a convoluting pink-brown column was emitted from May 91 vent. It contained very little ash and no evident incandescent material. Visible shock waves emerged from the vent every few seconds as "flashing arcs," lighting clouds above with a flickering glow like that from a poorly-functioning fluorescent tube. The strongest shock waves were manifested as an instantaneous displacement of the plume at the vent, and could be felt 150 m away. Some could be seen to bounce off the crater walls and travel back through the clouds. The shock waves did not seem to affect the rate of plume emission. The activity was accompanied by dull booming and sloshing noises, and occasional sharp detonations. The sloshing sounds were much like those heard in 1988 at Yasur (Vanuatu), where large gas bubbles were bursting through the surface of an active lava lake. Geologists noted that the activity at May 91 vent was consistent with similar gas-bubble discharge through a liquid magma column.

About 200 mm of coarse and fine ash had been deposited just N of May 91 vent since the previous fieldwork on 27 May. Little new ash was evident elsewhere on the main crater floor, but small (< 0.3 m) lithic blocks and their impact craters were found >200 m SE of the vent and to its W. Scarce, widely scattered scoria bombs, most 0.1-0.2 m across but some reaching 0.3 m, were found on top of the May ash, with only a light ash coating. The bombs seemed most abundant a few hundred meters SE-NE of the vent. They had highly vesiculated interiors of black glass with large pyroxene and plagioclase phenocrysts. Internal vesicles were up to 30 mm across, but decreased rapidly to sub-millimeter size toward the surface.

The pattern of deformation between late May and late August was similar to that of the previous 3 months. Strong subsidence at roughly double the previous rate continued to be centered SE of May 91 vent, while relative inflation persisted ~200 m farther SE. A new zone of inflation was measured E of Noisy Nellie fumarole (NE of May 91 vent). Minor deformation associated with activity at May 91 vent is unlikely to be detected, as the nearest part of the levelling network is 100 m away. Most fumarole temperatures had changed little since May, although values at Noisy Nellie had increased from 240 to 411°C.

The volcano had remained seismically quiet until mid-June, when B-type events became more common, continuing at rates of 2-7/day through the end of the month. Very weak volcanic tremor was sometimes visible on seismic records. A sequence of >45 tectonic earthquakes (to ML 3.7) occurred near White Island 1-2 July. A- and B-type events increased markedly on 7 July, accompanied by a small increase in background volcanic tremor amplitude. E-type eruption earthquakes were recorded on 1, 7, and 11 July. Seismicity had declined by 15 July, but a 3-day swarm of >200 A-type events began on 20 July. Significant volcanic tremor also resumed and continued through mid-August, increasing again 21-28 August. Tremor varied from a nearly pure 1.8 Hz signal to a complex pattern with spectral peaks to 8 Hz. A-type events did not occur daily in August, but often numbered 8-10/day. B-type events were very rare after 24 July. E-type eruption shocks were recorded on 14, 15, 19, 20, 23, 27, 29, and 30 August.

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

Information Contacts: B. Houghton, I. Nairn, and B. Scott, DSIR Geology & Geophysics, Rotorua.