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/).

At the request of the Civil Defense office of the CNMI, a team from the USGS and the Hawaii Institute of Geophysics monitored seismicity, deformation, and thermal activity, and investigated the geologic and eruptive history of the island during fieldwork 19-27 April. The following is from their preliminary report.

Geologic overview. "Anatahan, ~9 km long by 3 km wide and elongate along an E-W axis, is topped by a compound caldera of both collapse and explosive origin. The caldera's shape roughly parallels the outline of the island. It can generally be divided into E and W craters; the floor of the E bay is ~250 m below the floor of the main caldera and contains a lake. The W crater essentially comprises the caldera floor and is made up of at least five phreatomagmatic explosion craters. The two highest points on the volcano are peaks at the E and W ends of the caldera (540 and 790 m above sea level, respectively). Slopes dip steeply to the sea from these high points as well as from the caldera walls, at angles averaging about 25°. A number of extra-caldera explosion craters have been identified, especially at the E end of the island.

"The oldest units are interbedded phreatomagmatic ashes and andesite flows exposed at the coast. Most of the flows are plagioclase-phyric and 5-15 m thick, and a number are glassy or partly glassy with well-developed flow lineations. The interbedded ashes and flows dip 20-25° in a generally radial direction from the center of the volcano. At numerous locations along the coast, oxidized cinder layers are exposed, usually accompanied by co-magmatic flows. Along both the N and S coasts, slopes are significantly steeper near the ocean, and ocean-induced mass wasting is undoubtedly the cause. Additionally, however, an E-W-trending set of faults on the S flank has probably contributed to both the steepness of the slopes and the straightness of the coastline.

"The youngest volcanic unit is a light brown to gray phreatomagmatic surge deposit. It is very young, in places plastered on the wave-cut cliffs just above the coastline, or even banked against large talus boulders. This ash blankets all surfaces on the slopes and within the caldera, and has only been eroded from gullies. Its thickness is extremely variable, and isopachs do not give an indication of source location. This deposit appears to be no more than a few hundred years old. Human cultural remains were found under 4 m of base surge deposits, and will be radiocarbon dated to give the deposit's maximum age.

Present activity. "An acid lake and a number of acid pools were present within the E crater. The water in the pools was boiling, turbid due to very fine suspended sediment, and had a pH ranging between 0.7 and 1.2. Water within the larger lake was clear except for areas where upwelling stirred up sediment. Water collected at the edge of one of the upwelling areas was warm and had a pH ranging between 1.2 and 1.9. A large area of vegetation had been killed near the lake and ponds, most likely due to overflows of the hot acid water. The air around the lake was breathable, smelled slightly of sulfur, and probably contained an elevated amount of CO₂. Gas had accumulated in one of the water samples that had been collected gas-free. A small explosion crater at the W end of the W bay was observed to be steaming.

"Seismicity was monitored from the village and periodically from the center of the W bay during the 9-day investigation. From 19 until 25 April, daily seismicity consisted of 3-4 distant earthquakes, possibly aftershocks of the 6 April M 7.5 earthquake in the Mariana Trench (centered at 15.27°N, 147.53°E, 32 km depth). Additionally, 3-4 local events of M 3-4 were recorded each day, usually in pairs (figure 1 and table 2). Only one was felt.

Figure 1. Portion of seismogram from the temporary station on Anatahan, 22-23 April 1990. Courtesy of Robert Koyanagi, USGS.

Table 2. Earthquakes recorded on Anatahan by the USGS, 19-27 April 1990, using a revolving drum portable seismograph with 1.0 Hz geophone. Magnitude threshold was greater than 0.5 for distances less than 10 km. Events with distances less than 40 km may be volcanically related and located within ~10 km of Anatahan at varying depths beneath the island (the 10 km distance also includes Sarigan Island, N of Anatahan). Seismic recording did not begin until late 19 April. Data courtesy of R. Koyanagi.

"An EDM network and two radial tilt stations were installed and monitored during the investigation. The tilt stations showed no changes. Because of limited helicopter time, every line of the EDM network was not reoccupied every day, but extensions of 6-91 mm were recorded between measurements on 25 and 26 April."

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: F. Sasamoto, Office of Civil Defense, Saipan; J. Lockwood, M. Sako, R. Koyanagi, and G. Kojima, HVO; S. Rowland, Univ of Hawaii.

A significant increase in Strombolian activity during March started with a corresponding increase in volcanic earthquakes at the beginning of the month. Between 20 and 24 March, substantially enhanced tremor accompanied lava production from Crater C. A small flow descended the NW flank toward the Río Tabacón, reaching ~700 m elevation. Lava continued to flow onto the NW flank in April, and a second flow began to advance down the S flank. Strombolian eruptions were larger and more frequent than in previous months and gas emission was continuous. Blocks and bombs fell 800-900 m from Crater C. Ash columns rose as much as 1 km and were carried by the wind toward the NW, W, and SW, the area most affected by acid rain. The activity occasionally produced small nuées ardentes.

Arenal remained highly active 1-22 April during 24 hour/day observations by Smithsonian Research Expeditions. Activity consisted of frequent pyroclastic events as well as production of small block lava flows that advanced down the S and WNW flanks. Flow movement down the NNW slope, the dominant direction during the past 3 years, had ceased. A pronounced leveed ridge had been produced by the most active (WNW) flow. Smaller flows were extruded from a small fissure ~100 m S of the summit crater rim, and a few hundred meters S from a spillover along the SSE part of the crater rim. Frequent explosions continued to eject blocks and bombs around the crater. Some fell more than a kilometer away, creating extremely hazardous conditions for ground observation of the crater area. Because the active flow fronts were all within the impact zone, there were no direct observations of flow rates.

Over the 23-day period, 807 pyroclastic events were recorded (figure 25), including sharp explosions [Type 1 in 14:06]; intense gas, block and bomb fountains [Type 2 in 14:06]; and rhythmic, less intense gas emissions, commonly accompanied by blocks and bombs [Type 3 in 14:06]. A "flashing arc", as described by Perret (1912) from eruptions at Vesuvius and Mont. Pelée, was observed from an explosion on 4 April at 1047 (figure 26). Two blocky pyroclastic flows moved down the S flank, each reaching ~800 m from the crater.

Figure 25. Number of pyroclastic events/day at Arenal, 2-24 April 1990. Poor weather conditions frequently prevented viewing of the summit, so different activity types were identified by sound: type 1 ('Explosions') are sharp explosions; type 2 ('Whooshes') corresponds to intense gas, block, and bomb fountains; type 3 ('Chugs') are rhythmic, less intense gas emissions commonly accompanied by blocks and bombs.

Figure 26. Airwave train of an explosion from Arenal that produced a "flashing arc" on 4 April at 1047. The tape recording from which this was derived was made 2.7 km S of the crater, where the event's maximum sound intensity was ~90 dB.

The level of pyroclastic activity (figure 27), sampled at various times over the past 1,000 days at 10-23-day intervals, as well as lava flow directions and rates, continued to show wide variations on both day-to-day and longer time scales. However, the volume of individual lava and pyroclastic flows appears to have gradually declined over the 1,000-day period, with few reaching more than ~800 m from the summit crater. The crater appeared to contain at least two vents, with the S vent the source of much of the pyroclastic activity, while the lava flows emerged from the vent to its N and W.

Figure 27. Comparisons of the number of pyroclastic events heard and (sometimes) seen at the Arenal Observatory, 2.7 km S of the summit crater, by Earthwatch and Smithsonian Research Expeditions volunteers since 27 April 1987. Activity types are as in figure 25. Sampling was over 10-22-day intervals during the periods shown.

Chemical compositions of the highly phyric basaltic andesitic blocks and bombs (table 4) have remained essentially constant over the past 3 years, reflecting both a constant source composition and high magma viscosity. However, petrography has varied, particularly the glass/crystal ratio, high in bombs and near-vent lava flows and much lower in blocks.

Table 4. Compositions of 1987-89 eruption products from Arenal; XRF analyses by Joseph Nelen, SI. The August 1989 lava flow sample is from J. Barquero and E. Fernández. pf = bomb from pyroclastic flow; b = ejected block; B = ejected scoriaceous bomb; L = lava flow.

Reference. Perret, F.A., 1912, The flashing arcs: a volcanic phenomenon: American Journal of Science, ser. 4, v. 34, p. 329-333.

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: W. Melson, SI; E. Fernández and J. Barquero, OVSICORI; Red Sismológica Nacional, ICE; Escuela Centroamericana de Geología, Univ de Costa Rica.

"In the current period of social unrest on Bougainville Island Island, no instrumental data is being recorded, and the only information on Bagana's activity is from visual observations from a site 15 km SSW of the volcano.

"When observations resumed on 3 April, Bagana was in a fairly high level of activity. Thick, white, ash-laden vapour was being forcefully emitted from the summit area. [An explosion in the summit crater] on the 3rd produced a black column, and loud rumbling noises were heard until the 4th.

"Numerous rockfalls (including daytime glowing avalanches) were observed in early April on the SE and E flanks of the cone, where a slowly progressing blocky lava flow has been active since 1987. This activity together with the reportedly stronger vapour and ash emission may suggest that a new pulse of viscous lava extrusion took place in the summit crater in the first few days of April.

"The mountain was often covered by atmospheric clouds or rainstorms, but a weak, night, summit glow was intermittently observed until the 21st, with occasional (night-glowing) rockfalls occurring until the 24th."

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: P. de Saint-Ours and C. McKee, RVO.

"Seismic activity . . . decreased markedly in April. Following a period of intense activity in early March, the frequency of occurrence and magnitude of earthquakes decreased gradually, with only 27 events of M_L >=3 recorded in April (from a total of 200 events picked up by the Ulawun station, 25 km away). Event frequency ranged between 2 and 7/day. Two isolated earthquakes of M_L 5.6 and 4.2 occurred on the 26th."

Geologic Background. Symmetrical Bamus volcano, also referred to locally as the South Son, is located SW of Ulawun volcano, known as the Father. The andesitic stratovolcano is covered in rainforest and contains a breached summit crater filled with a lava dome. There is a cone on the southern flank, and a prominent 1.5-km-wide crater with two small adjacent cones halfway up the SE flank. Young pyroclastic-flow deposits are found on the flanks, and residents describe an eruption that took place during the late 19th century.

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

After ~15 days of increased seismicity, an eruption began on 18 April at 1252. Lava production occurred from the SE part of the Enclos Fouqué caldera, with vigorous fountains (~30-50 m high) that built the "Catherine N" eruptive crater, and extrusion of flows that advanced down the Grand Brûlé area. Poor weather during the eruption severely hampered observations.

Geologic Background. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: J-P. Toutain and P. Taochy, OVPDLF; P. Bachelery, Univ de la Réunion; J-L. Cheminée, IPGP.

Small phreatic ash emissions were frequent through March, but the last episode was recorded on the 29th (table 2), and none were detected in April. Incandescence continued in April, mainly from the SE part of the crater, which showed changes in morphology, fumarolic activity, and temperature.

The number of seismic events increased 21% in April (to 1,343 low-frequency and 154 high-frequency earthquakes) from March values, but energy release declined 17% in April, to 1.48 x 10⁸ and 7.49 x 10⁷ ergs for high- and low-frequency shocks respectively. The month's largest earthquake reached M 2. Long-period events and tremor bursts decreased in both number and maximum reduced displacement. Continuous tremor remained at very low levels and showed no changes in characteristics. Most high-frequency earthquakes were centered in one of two main source regions, W of the active crater at 2-6 km depth, or SSE of the crater at 2-4 km depth. Two peaks in seismicity were noted. On 4 April, 47 high-frequency earthquakes were recorded, and on the 24th there were 115 low-frequency shocks, 83 long-period events, and 16 bursts of spasmodic tremor.

Deformation data showed no significant changes. Twelve COSPEC measurements of SO₂ emission yielded rates of 961-4,078 t/d, with a mean of 2,146 t/d (calculated using measured wind speeds). Maximum and minimum values occurred on 14 and 16 April respectively.

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.

Press reports indicated that an eruption began at 1847 on 25 April, ejecting a thick reddish column ~ 1.5 km high. Authorities inspected areas believed to be at risk from lava flows, but did not immediately order evacuations.

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: Jakarta Domestic Service.

Fumarolic activity continued on the NE flank, with a mean temperature of 89°C. The lake in the main crater had disappeared. Small landslides persisted on the W and N crater walls.

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: J. Barquero, OVSICORI.

. . . lava production continued from Kupaianaha vent, feeding flows that overran dozens of houses in a community near the coast. Lava production halted briefly in early April and early May but promptly resumed each time, and activity remained vigorous in mid-May. Shallow tremor . . . continued at a low level.

Lava that had advanced to just above [Hwy 130] by the end of March crossed the highway on 2 April (100 m E of its intersection with Highway 137). A small N-facing fault scarp (the Hakuma horst) blocked the flow's direct path to the sea, turning it ESE toward Kalapana Gardens subdivision, in a low-lying area below the scarp (figure 68). Lava entered the subdivision's W edge on 3 April, destroying two houses the next morning, but lava production stopped late that day. Tremor amplitude near Kupaianaha decreased appreciably 4-6 April while lava production was stopped. As in previous instances, USGS geologists believe that the pause in lava production was due to blockage in the upper East rift zone. The summit responded with a sequence of deflation and shallow tremor followed by inflation and an increased number of shallow microearthquakes. Eruptive activity resumed during the night of 6-7 April. By the time of an early morning overflight, lava had reoccupied the tube system along the E side of the flow field, and numerous breakouts were occurring along much of its length, between ~600 and 120 m (1,950 and 400 ft) altitude. The lava formed three flows, the largest advancing along roughly the same path as previous flows toward Hwy 130.

Figure 68. Kalapana Gardens and the E side of Royal Gardens subdivision on Kīlauea's S flank, April 1990.

The primary flow moved down the main flow field to ~120 m (400 ft) elevation, then turned E, following approximately the same path as the previous Kalapana Gardens flow. The lava advanced slowly, but by 13 April two separate lobes had crossed Highway 130, one on top of the early April flow along the Hakuma horst, the other farther E. The W lobe entered Kalapana Gardens on 17 April. The two lobes merged by the 20th. Two homes just outside Kalapana Gardens were destroyed on 13 and 15 April, and destruction of houses within the subdivision began on 18 April. By the end of the month, the April flows had overrun nearly 4 dozen houses, had cut off the main access road into the subdivision (Highway 137) and were moving along both Highway 137 and the fault scarp below it. By 3 May, the flows reached Kalapana village, an older settlement to the E. As of 5 May, the Hawaii County Civil Defense Agency reported that the eruption had destroyed 63 houses since 4 April.

A smaller flow, originating from the breakout at ~600 m elevation, moved diagonally toward the E side of the lava field . . . into the "woodchip area," then onto the December 1986 flow. Lava in this area advanced slowly through April, reaching 30 m (100 ft.) elevation by the end of the month. Another small flow originated from a breakout high in the tube system at slightly above 600 m (2,050 ft) elevation, cutting across lava from Kupaianaha vent and advancing W onto earlier aa lava from Pu`u `O`o . . . . This flow entered the extreme NE corner of Royal Gardens subdivision by mid-April, but moved down the E edge of the subdivision without threatening any homes, reaching ~250 m (800 ft) elevation by the end of the month.

When lava production halted briefly on 7 May [see also 15:3], two lobes had reached the water. One partially filled a spring-fed brackish water inlet behind the fault scarp, and the second (W) lobe reached the open ocean (at Harry K. Brown Park) E of Kalapana Gardens. Lava production resumed during the evening of 9 May. Onset of the renewed activity was gradual, but the eruption was again vigorous by the 11th. On 14 May, lava traveling over roughly the same route as previous flows had reached 120 m (400 ft) elevation and was again turning E toward Kalapana Gardens.

Kupaianaha vent . . . generally remained covered with frozen lava at ~18 m below the rim throughout April, having effectively become part of the tube system. After lava production resumed on 9 May, lava in the pond was at times again actively overturning. Lava also returned to the bottom of Pu`u `O`o after 9 May, but dimensions were difficult to estimate at the base of the 180-m-deep crater.

April seismicity generally conformed to the pattern of crustal earthquakes that has persisted beneath Kīlauea and the SE part of Mauna Loa. Small (M <2.0) shallow (<5 km) events were mainly centered beneath Kīlauea's summit and East rift zone. Of the hundreds of earthquakes processed in April, eight ranged from magnitude 3.0 to 4.4.

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: C. Heliker, P. Okubo, and R. Koyanagi, HVO; AP.

"Activity was limited to weak or moderate emissions of white vapour from Crater 2, with a weak, steady, red glow at night from 25 March until 6 April, and 9-11 and 27-28 April. Occasional weak, deep rumbling noises were heard on 3 consecutive days 12-14 April. Crater 3 remained inactive, apart from thin white vapour released by fumaroles in the crater."

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: P. de Saint-Ours and C. McKee, RVO.

An 8-14 µm infrared thermometer was used on 23 April to measure temperatures of the inner wall of the prominent 20-m-deep chasm formed in the [1952] eruption. Weak fumarolic activity was occurring there, and the maximum recorded temperature was 96°C, probably corresponding closely with the gas temperature.

Geologic Background. Las Pilas-El Hoyo volcanic complex, overlooking Cerro Negro volcano to the NW, includes a diverse cluster of cones within about a 3-km-diameter area. A N-S-trending fracture system cutting across the edifice is marked by numerous flank vents, including maars, that are part of a 30-km-long volcanic massif. The Cerro Negro chain of cinder cones is listed separately in this compilation because of its extensive historical eruptions and possible distinct magmatic system. The lake-filled Asososca maar is located adjacent to the Cerro Asososca cone on the southern side of the fissure system, south of the axis of the Marrabios Range. Two small maars west of Lake Managua are located at the southern end of the fissure. Aside from a possible eruption in the 16th century, eruptions of Las Pilas took place in the 1950s from a fissure that cuts the eastern side of the 700-m-wide crater and extends down the N flank.

Information Contacts: C. Oppenheimer, Open Univ; B. van Wyk de Vries, INETER.

Field observations suggest that the dome extrusion . . . has stopped since at least November and that the dome has continued to collapse above a withdrawing, degassing, magma column, accompanied by small, mainly phreatomagmatic eruptions.

During a summit climb by C. Oppenheimer on 4 April, little activity was seen on the collapsed dome region during daylight. Almost all of the visibly fuming vents were located beyond its margins, particularly on the E side where several powerful fumaroles were active (figure 6). After dark, very few if any of those vents were seen to be incandescent. The collapsed dome, however, showed numerous glowing red patches, presumed to be high-temperature fumarolic vents concentrated along ring fractures (figure 7). Individual vents were probably <0.5-1 m across; the majority appeared to be only a few centimeters across but formed clusters along roughly arcuate trends close to the edge of the collapsed dome. A broad area in the dome's center had no incandescent sites. There were a few groups of incandescent fumarolic vents beyond the collapsed dome, at locations that seemed to correspond spatially to distinct hot pixels on Landsat TM images of October-December 1989 (processed at Open Univ).

Figure 6. Lascar's active crater in daylight, 4 April 1990, showing locations of strong fumaroles. Sketch by C. Oppenheimer.

Figure 7. Lascar's active crater at night, 4 April 1990. Dark spots on the collapsed dome and crater walls represent incandescent areas. Note that north is down, the opposite orientation to figure 6. Sketch by C. Oppenheimer.

The highest brightness temperature, measured over a vent close to the E margin of the collapsed dome (by an 0.8-1.1 mm infrared thermometer) was 787°C. The glowing region filled about 1/6 of the instrument's field of view; the temperature measured around the incandescent vent was ~540°C. Oppenheimer noted that use of the Planck function suggests an actual temperature of the glowing vent, and therefore the gas, of ~940°C.

A seismometer (Portable Kinemetrics MEQ-800) installed 17 km W of the volcano (in the village of Talabre) began recording local seismic activity on 4 April. The seismic station was established by Juan Thomas (Antofagasta Branch, Dept de Geofísica, Univ de Chile) who also trained the village teacher, Manuel Castillo, in its operation. A second seismometer, installed the next day 7.5 km from the volcano (at Tumbre), had to be retired 2 days later because of logistics and operation problems. Installation was supported by Nelson Allendes and data interpreted by Sergio Barrientos (both with the Dept de Geología y Geofísica, Univ de Chile, Santiago).

Seismograms 4-19 April indicated that Lascar's seismicity was limited to tremor every 2-3 minutes, interpreted as magma movements in a chamber of unknown depth. Geologists suggested that the absence of discrete earthquakes could indicate that there was no rupturing of material adjusting to pressure from ascending magma. Installation of the Talabre seismometer is scheduled to end in late May. However, strong recommendations were made to local authorities that permanent monitoring of Lascar be established with telemetrically controlled seismometers, given its distance from any research center or large city (270 km from Antofagasta and 1,200 km from Santiago).

A small eruptive episode was observed on 6 April at 0840 from Talabre and by MINSAL geologists in Toconao. A pale grayish cloud rose to ~1,000 m above the volcano in 1-2 minutes. No sounds were audible during the activity, which appeared to be phreatomagmatic. The seismometers at Talabre and Tumbre recorded no seismicity at the time of the eruptive episode. During the following 20 minutes, the plume was dispersed to the SE, rapidly turning white. Some ash could be seen falling from its base. By 0910, the plume was indistinguishable from weather clouds and the normal vapor plume had reappeared, rising to ~300 m above the crater rim. The vapor plume was weaker than normal 7-8 April, reaching <100 m above the rim, but had strengthened to the usual 900 m height by the 9th.

An ascent of the volcano's S side by Steve Matthews on 12 April showed the dome to be essentially unchanged, with continuing strong fumarolic activity. Fresh tension cracks just outside the N margin of the dome, produced by further collapse, were photographed. Geologists interpreted the eruptive episode as the result of a dome collapse event, given the tension cracking and lack of associated seismicity.

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

Information Contacts: M. Gardeweg, SERNAGEOMIN, Santiago; S. Barrientos, Univ de Chile, Santiago; J. Thomas, Proyecto Sismológico Antofagasta; S. Matthews, Univ College London; C. Oppenheimer, Open Univ.

Earthquake swarms continued in the S moat through early May, with bursts of activity (magnitude greater than or equal to 2.5) on 28 and 30 March, 18-20 April, and 6-7 May. The 6-7 May swarm was the most intense activity within the caldera since 1983-84. It began with an earthquake of about M 2.7 on 6 May at 2,238 and produced more than 300 of M >0.5 over the next 24 hours. The earthquakes included nearly 20 of magnitude greater than or equal to 2.5 and three of magnitude greater than or equal to 3, the largest of which was about M 3.5 at 0241. This swarm, as with most of the others that have occurred since the beginning of the year, was centered in the S moat, ~4 km E of the town of Mammoth Lakes (figure 12). Focal depths ranged from <3 km to as deep as 10 km, with most concentrated between 5 and 8 km (figure 13). Earthquakes in the S moat area as small as about M 2.5 are felt in Mammoth Lakes, and residents reported feeling some 15-20 events during a 5 1/2-hour period starting 6 May at 2238.

Figure 12. Epicenters of earthquakes in the Long Valley Caldera region, 1 January-10 May 1990. Courtesy of D. Hill.

Figure 13. E-W cross-section from U to U' (figure 12), showing focal depths of Long Valley area earthquakes, 1 January-10 May 1990. Courtesy of David Hill.

An approximately 5-fold increase in extensional strain across the S moat and resurgent dome began to be recorded by the caldera's 2-color geodimeter network in September 1989. The extension rate continued to average 4-5 ppm/year through early May, although recent measurements indicated that the rate may be slowing somewhat. Small strain changes, apparently associated with the 6-7 May swarm, were detected by borehole dilatometers at distances of 6 and 10 km. The changes were generally ~0.03-0.05 microstrain.

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

Information Contacts: D. Hill, USGS Menlo Park.

"A final cumulative ashfall isopach map was made after the Navidad cone eruption, which ended by 23-25 January 1990. The 3 February 1989 preliminary map (see figure 6) was based on 67 data points. After the eruption ended, 40 were checked, but only 19 sites were considered valid. All were located in open spaces within the forest, on flat surfaces and with some grass; therefore, wind or water action has been minimized, although some variation could have been caused by minor ash removal from leaves through wind action. Values increased by 3-19 times the 3 February 1989 measurements.

"Ash thicknesses at six sites previously sampled in February had increased 3- to 4-fold when measured again by 25 September 1989. Consequently rejected were all new measurements <3x those of February, and those that were meaningless in the context of the 19 chosen values. Among them, nine had <70% of the February value x 3, and were located in open windy areas (in the Colorado and Lonquimay valleys). The other 12 sites had more than the February value x 3 and were located in the Naranjo River valley and the E slope of the Cordillera Las Raíces (figure 17). Undoubtedly, the main ash removal agent has been the wind. Water action was observed on slopes less than 20°, and big volumes of ash had been carried down the Cautín and Naranjo headwaters, burying trees, bushes, and wire fences."

Figure 17. Final ash isopach map (in centimeters) after the full 13 months of the Lonquimay eruption. Courtesy of H. Moreno and J. Naranjo.

Geologic Background. Lonquimay is a small, flat-topped, symmetrical stratovolcano of late-Pleistocene to dominantly Holocene age immediately SE of Tolguaca volcano. A glacier fills its summit crater and flows down the S flank. It is dominantly andesitic, but basalt and dacite are also found. The prominent NE-SW Cordón Fissural Oriental fissure zone cuts across the entire volcano. A series of NE-flank vents and scoria cones were built along an E-W fissure, some of which have been the source of voluminous lava flows, including those during 1887-90 and 1988-90, that extended out to 10 km.

Information Contacts: H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN, Santiago.

"Activity remained at a low level. The summit craters were often obscured but when clear were seen to release white vapour in weak amounts. Occasionally, Southern Crater emissions were greyish, containing a little ash. These emissions were accompanied by weak, deep, rumbling sounds. Seismicity was low throughout the month, while the water tube tiltmeter accumulated an unusual radial deflation of 4.5 µrad."

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: P. de Saint-Ours and C. McKee, RVO.

During fieldwork on 17 and 25 April, gas emission in Santiago Crater was limited to a few patches of weakly fuming ground within the inner crater, below the level of the frozen 1965 lava lake. The highest temperature measured on the fuming ground (using an 8-14 µm infrared thermometer from the crater rim) was 50.7°C. Small rockfalls from the inner crater walls were frequently audible. Much of the floor of the innermost crater was covered by debris and the "cannon" vent (first reported in February 1989; 14:02) was no longer visible. However, an opening had formed at the site of a former incandescent vent N of the February-March 1989 lava lake. No incandescence was evident in the crater after dusk on 25 April. Tangential fissures crossing the S rim parking area and nearby had widened in recent weeks.

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

Information Contacts: C. Oppenheimer, Open Univ; B. van Wyk de Vries, INETER.

When visited 21 and 27 April 1990, locations of fumarolic vents had changed little since a year earlier but gas temperatures had generally declined by 50-150°C. The highest recorded at vent F9 was 772°C (down from 880° on 15 April 1989) and temperatures at neighboring vents were 634° (779° in April 1989), 662° (716° in 1989), and 742° (844° in 1989). Other temperatures measured on 27 April 1990 were 575°C (F7), 702° (F8), and 439° (F12). Small dribbles, drops, or pools of amber-colored liquid sulfur were common at most fumaroles. Some very small multiple flows had smooth crusts of solid orange-brown sulfur, which turned bright yellow within minutes of removal from the hot surface. At F8, a tiny pool of liquid sulfur, a few mm deep, had a temperature of 137°C. Evidence for older, larger, sulfur flows included eroded remnants as much as 15 cm thick near F12, where up to 5 m of a pahoehoe type flow was preserved, including its front. The upper portion had apparently been eroded by water.

Geologic Background. Momotombo is a young stratovolcano that rises prominently above the NW shore of Lake Managua, forming one of Nicaragua's most familiar landmarks. Momotombo began growing about 4500 years ago at the SE end of the Marrabios Range and consists of a somma from an older edifice that is surmounted by a symmetrical younger cone with a 150 x 250 m wide summit crater. Young lava flows extend down the NW flank into the 4-km-wide Monte Galán caldera. The youthful cone of Momotombito forms an island offshore in Lake Managua. Momotombo has a long record of Strombolian eruptions, punctuated by occasional stronger explosive activity. The latest eruption, in 1905, produced a lava flow that traveled from the summit to the lower NE base. A small black plume was seen above the crater after a 10 April 1996 earthquake, but later observations noted no significant changes in the crater. A major geothermal field is located on the south flank.

Information Contacts: C. Oppenheimer, Open Univ; Benjamin van Wyk de Vries, INETER.

March activity. Continuous gas emission persisted in March. Gases were carried W and SW by prevailing winds, with major impact on the vegetation, infrastructure, and health of the inhabitants of that area. Winds sometimes changed, carrying gas toward the S and SE where other residents were affected.

Water level in the crater lake had dropped, leaving isolated pools around a central remnant lake from which most of the activity occurred. Activity included continuous bubbling, small geyser-like eruptions, and intense gas emission (primarily water vapor). Two shallow ponds to the SE were 5-10 m in diameter with a mean temperature of 80°C. On the NE side there were three shallow molten sulfur ponds with a mean temperature of 160°C. In the N part of the crater the most vigorous fumarole emitted orange gas, probably produced by combustion of sulfur, and fine sulfur particles. The gas temperature was about 400°C and a reddish flame was sometimes observed at the base of the gas column.

A permanent seismic station (POA2) continued to record B-type events, with a mean of 337/day in March (figure 26). An A-type earthquake was felt at MM II, 7 km WNW of the summit (in Bajos del Toro) on 8 March at 0239. A series of A-type shocks began to be recorded on 25 March (figure 27). Inhabitants of flank towns, including Poásito (5.5 km SE of the summit), Fraijanes (7 km SSE), and San Pedro de Poás (13.5 km S) felt a M 2.5 earthquake on 1 April at 0204 that was centered 5 km NE of the active crater at 5 km depth. A second felt event, on 2 April at 0215, was centered 6 km SW of the crater at 15 km depth, and had a magnitude of 3.1.

Figure 26. Number of low-frequency earthquakes recorded at Poás, March-April 1990. March data courtesy of J. Barquero; April data courtesy of G. Soto.

Figure 27. Number of A-type earthquakes recorded at Poás, 25 March-24 April 1990. Courtesy of G. Soto.

Hazel Rymer noted that the March 1990 crater lake was comparable to the lake in April 1989, having apparently evaporated to a low level more rapidly during the 1990 dry season. The base of the mud volcanoes, <1 m above lake level, was 2,282.650 m above sea level on 18 March 1989, while the 30 March 1990 lake level was at 2,278.853 m (elevations tied to a flank benchmark). The location of the hot sulfur lakes, observed last year prior to the ash eruption, was occupied by a vigorous vent, continuously jetting a mixture of sulfurous gases. In early April 1990, boiling mud pools occupied the remainder of the former lake area.

April activity. By the beginning of April, lake level had dropped 4 m since December 1989, shrinking to a small pool in the center of the former lake, where minor phreatic eruptions occurred. Numerous fissures continually emitted gases; at some sites, combustion of sulfur produced flames. By the 18th, the lake's cumulative descent had reached 7 m and it was nearly dry, leaving areas of mud with occasional bubbling caused by the discharge of fumaroles in the bottom of the crater. In addition to activity on the SE part of the crater floor, decsribed in detail below, hot fumaroles were also found in the NE part of the former lake bed (figure 28). The principal fumarole in this area had a vent 2-3 m in diameter. Sprays of very pure yellow sulfur were emitted, as were bluish SO₂ and water vapor. The gases were expelled under high pressure with a jet aircraft sound, and burned with yellow-orange flames. At the beginning of the month, temperature (190°C with an infrared thermometer), sound level, and pressure were less than on the 18th, when activity was stronger with temperatures to 793°C. Since 17 April, the vigorous discharge has caused vibrations felt inside the crater and registered by the summit seismic station (VPS-2). The fumarole ejected evaporitic sediments to 75 m height, and similar activity continued through the end of the month. On the 1953-55 dome, fumaroles emitted gas dominated by water, and precipitated sulfur and sulfates. Maximum fumarole temperature (measured by thermocouple) was 90.3°C.

Figure 28. Sketch map of the active crater at Poás, 18 April 1990. Courtesy of G. Soto.

Gases, carrying sediments because of the dryer lake bottom, were carried mainly W and SW, affecting coffee farms, pastures, and especially forests. The inhabitants of various towns, including San Luis (about 13 km SSE of the summit), Cajón, and San Miguel de Grecia (~11 km SW of the summit), suffered health problems, principally with the respiratory tract, vision, and skin allergies. From the night of 26 April through the following day, a wind change concentrated gases strongly in the Parque Nacional del Poás area and toward the SE flank, affecting strawberry crops.

Seismic activity changed substantially from the previous month. Recorded events dropped from 9,460 in March to 9,190 in April. Of these, 9,026 were of low frequency and 159 were volcano-tectonic or A-type. The latter averaged 5/day, a substantial increase. Volcanic tremor also occurred during April, manifested as prolonged vibration of the edifice caused by gases emerging from fumaroles at high pressure.

12 April fieldwork. G. Soto and Clive Oppenheimer climbed into the crater on 12 April at about noon. Bubbling mud pools occupied positions very similar to those of sulfur ponds seen on the SE part of the crater floor in April 1989. Nearly continuous geysering of mud and clumps of solid sulfur was depositing a ring around one of the mud pools. Columns of material ejected in the SE part of the crater reached 10-15 m height, and temperatures measured by an infrared thermometer were between 70 and 100°C. A vent at the NE fumarole site produced a gentle roar and a blue-tinged turbulent plume of gases with pink-orange flames, just visible in daylight, at its base. The plume intermittently stopped burning, turned a thick bright yellow color for a few minutes, then re-ignited and returned to its previous color.

By 0900 the next day, a small dull-yellow sulfur cone had grown at the site of the previous day's geyser-like eruptions. The cone was 2.2 m high with a basal diameter of about 6 m and a crater 2 m across. Small bursts of coagulated, rather plastic sulfur mixed with some silicate occurred at about 1-second intervals, sending ejecta to about 3 m above the rim. A warm, white, acid, vapor cloud was continuously emitted from the crater. The maximum temperature measured by a thermocouple pushed into the substrate at the summit was 97.6°C; when thrown over the rim, the highest measured temperature was 96°C. The crater was filled to within about 0.75 m of the rim with various-sized pellets of somewhat malleable sulfur mixed with silicate, appearing fluidized from agitation in the upward gas stream. Some of the crudely spherical pellets, collected from the sides and base of the cone, were up to 4 cm in diameter. In cross section, they revealed accretionary shells of aggregated sulfur crystals and minor clay alternating with gray clay-rich layers. Shortly before 1130, the vent appeared to become more confined, and the crater was quickly filled by a growing sub-cone. Within minutes, it grew above the old crater rim, forming a perfect cone 3.5-4 m high. A constant spray of sulfur up to 10 m high issued from a narrow vent at the cone's apex for about 30 minutes, dispersing a fine layer to about 15 m downwind. The conduit was often momentarily blocked before being cleared by a slightly stronger gas burst. The continuous ejection of material also built other cones, a few meters high and rich in pyroclastic sulfur, which periodically collapsed and recycled their contents.

29 April fieldwork. When geologists returned to the volcano on 29 April at about 1245, a convecting grayish-white plume, combined from numerous individual vents, was rising to more than 300 m, accompanied by a continuous roar, like a distant jet aircraft, from the center of the crater floor. Moist drops of acid mud fell from the plume, and appeared to form a thin veneer on the inner crater. Three recent cones, presumably of sulfur, on the SE part of the crater floor were also coated with mud and only weakly fuming, with bright yellow sulfur deposited around their conduits. One was irregularly erupting yellowish sulfur. To the NW, small gas eruptions ejected dark clouds of lake sediment, forming a line or cluster of several wide cones. Fumarolic vents varied in color from bright yellow, to different shades of gray, to white. Some had a pronounced tinge of bluish haze. The NE vent, which had been burning on 12 April, was considerably quieter, but a nearby vent was producing a strong plume with pink-orange flames clearly visible at its base. The plume changed color to yellow when combustion ceased; re-ignition was accompanied by a roaring sound. Bright flames also licked the steep inner walls and rim of the remnants of a small cone nearby. A peak thermocouple temperature of 662°C was recorded with the probe in the flames. The origin of the cone was uncertain, although samples of pink-gray ash were collected nearby. At about 1445, a brief, more powerful eruption occurred from the center of the former lake floor. The plume, presumably of non-juvenile material (lake sediments) rose roughly 50 m and produced a hail of blocks that fell noisily to the muddy crater floor.

The next day at 0700, nearly constant eruptions of gas, yellow-tinged apparently dry ash, and blocks, continued from several vents around the center of the crater. Some fresh sulfur had been erupted over one of the old sulfur cones at the SE site. The activity was similar to that observed in the same region in April 1989. At roughly hourly intervals, considerably more powerful activity ejected thick cauliflowering columns of dark ash, accompanied by blocks trailing white vapor above the level of the 1953-55 dome. The episodes lasted 30-50 seconds. There was no evidence that the ash was juvenile, although no samples were obtained. Two gas eruptions were observed at previously inactive sites. The first, at about 0800, occurred very near one of the active phreatic cones. Gas bubbles burst noisily through mud, hurling expanding shells of mud spatter. A similar but much briefer episode occurred 40 m away at about 0810, near the first set of lake sediment terraces. It left a crudely horseshoe-shaped scar in the mud, and a gray fluidized mudflow moved toward the center of the crater. Another strong eruption was seen at 0944, shortly before geologists left the crater, producing a plume that rose to an estimated height of 100 m.

Gravity data. The following is from Hazel Rymer. "Five years of continuous gravity increases at crater-bottom stations, from March 1985 to March 1989, have been recorded by Open University geophysicists. Since 1987, we have also had good elevation control at these stations and have recorded minor deflation. The maximum changes, >200 microGal increases at stations on the 1953 dome, were accompanied by 30 cm deflation (March 1987-March 1989). Gravity variations were similar elsewhere on the crater floor, but elevation changes were less than 6 cm. These data are interpreted in terms of small dendritic magma intrusions and loss of magmatic gas from beneath the lake area (Rymer and Brown, 1987). Evidence to support this model comes from detailed analysis of the energy budget of the crater lake. While gravity increased gradually from 1985 to 1989, the power output through the lake area jumped in 1986 from a fairly steady 190 MW to about 300 MW, maintained to the present. Thus, steady gravity increase was associated with a sustained power output since the stepped increase in 1986.

"Microgravity and elevation data collected by Open Univ geophysicists and Earthwatch volunteers between 19 and 30 March 1990 revealed gravity decreases of about 50 microGal at crater bottom stations with elevations unchanged from 1989 to within 2 cm."

Reference. Rymer, H. and Brown, G., 1987, Gravity changes as a precursor to volcanic eruption of Poás volcano: Nature, v. 342, p. 902-905.

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: J. Barquero, E. Fernández, and V. Barboza, OVSICORI; Hazel Rymer and C.M.M. Oppenheimer, Open Univ; G. Soto and R. Barquero, ICE; Mario Fernández, UCR.

"Activity was at very low level throughout April, with only 69 recorded earthquakes. There were several days without any recorded events, and the highest daily total was 9 events. Only three earthquakes could be located - one in each of the E, S, and NW parts of the caldera seismic zone. Levelling measurements carried out on 25 April indicated no significant changes from the previous measurements (26 March)."

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

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

This report, from the AVO staff, covers the period 15 April-15 May, 1990. Information about the 15 April explosive episode supplements the initial material in 15:3.

"Lava dome growth punctuated by small explosive events and partial dome collapse continued to occur. In general, seismic and explosive activity were at relatively low levels (figure 11). Intense steaming in the crater area coupled with poor weather have limited viewing of the dome and crater area to 5 observations during the report period. The pattern of explosive eruptions every 3-9 days that had characterized Redoubt's activity from 15 February to 21 April has changed; as of mid-May, the last significant explosive event had occurred on 21 April, 3.5 weeks earlier.

Figure 11. Epicenter map (top) and depth vs. time plot (bottom) of earthquakes recorded near Redoubt by AVO, 15 April-15 May 1990. Squares on the epicenter map mark the positions of seismic stations. Near-summit stations are obscured by numerous seismic signals.

Explosive episode, 15 April. "A moderate explosive event occurred at 1440 and was recorded for 8 minutes at the Spurr station. The explosive event triggered a pyroclastic flow down the N face of the volcano, and a mudflow that carried hot blocks of dense dome rock 1-2 m in diameter 4 km downvalley (to the E end of the Dumbbell Hills; figure 12). A small flood reached the Drift River oil facility about 3 hours after the onset of the episode (about 40% occupied the Drift River channel, the rest the Rust Slough and Cannery Creek channels). Winds carried the tephra N-NW from Redoubt; flat discs of pumice up to 4 cm in diameter were noted about 10 km NW of the vent area. Three cloud-to-ground lightning strikes were detected NW of the volcano.

Figure 12. Sketch map of the Drift River valley and related drainages on the NE flank of Redoubt. The Drift River oil facility is between the mouth of the Drift River and Rust Slough. Courtesy of AVO.

Explosive episode, 21 April. "A small to moderate explosive event occurred at 0611 and was recorded for 4 minutes at the Spurr station. An ash-laden plume was reported to 7.5-9 km and a disc-shaped 'collar' was observed at about half that altitude by personnel at the Drift River oil facility and residents of the Kenai Peninsula. Light ashfalls occurred N-NW of the volcano to about 75 km distance. Heavy steaming prevented good observations of the crater area, but it was apparent that at least some of the lava dome remained in the crater. The episode triggered a small pyroclastic flow and surge. No significant flooding was associated with this episode.

Minor explosive events. "A small seismic event that occurred on 26 April at 1017 was recorded for 2 minutes at Spurr station. An AVO field crew reported an ash plume rising above the volcano and heading SE. Light ashfall reached 100 km SE of the mountain. The episode included a small pyroclastic flow that terminated at about 1,000 m on the N flank. The summit area was partially obscured by clouds, but it appeared that about 80% of the dome was intact and was still oversteepened to the N. An overhanging slab extended from the dome's NE rim; collapse of a similar slab may have triggered the pyroclastic flow. No flooding in the Drift River valley accompanied the event.

"A seismic event on 7 May at 1846 was too small to be recorded at Spurr, but pilots reported a plume to 9.5 km, consisting mostly of steam with a little ash. No pyroclastic flows were reported to have been associated with the event, and no hot blocks were noted by an AVO crew in the field the next day; however, poor weather prevented observations above 750 m altitude. A dusting of ash (presumably from the 7 May event) was seen on the upper flanks on 10 May.

Dome observations. "The dome was only observed five times during the report period, on 21, 25, 26, and 28 April, and 5 May. Thus, no views of the dome were obtained between the explosive episodes on 15 and 21 April, nor since the small explosive event on 7 May. The crater area was exceptionally clear on 25 and 28 April, and in both viewings the dome had a rough, blocky surface with transverse tension cracks exhibiting a smooth, massive interior. On 25 April, the dome was oval in plan, but by the 28th it was elongate N-S and oversteepened on its N side. Blocky talus that covered the canyon floor for a couple of hundred meters N of the dome on 28 April was not observed on the 25th. When the dome was last viewed on 5 May, it was partially obscured by steam and low clouds, but the surface was definitely smooth and massive, not rough and blocky."

Geologic Background. Redoubt is a glacier-covered stratovolcano with a breached summit crater in Lake Clark National Park about 170 km SW of Anchorage. Next to Mount Spurr, Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano was constructed beginning about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit 13,000-10,500 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3,500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Information Contacts: AVO Staff.

Fieldwork on 12 April revealed that the temperature of Crater Lake had dropped to 29.5°C, continuing a decline from 34°C on 19 March and an 8-year peak of 46.5° on 6 February. Upwelling occurred from three vents in the lake's N vent area, from which yellow sulfur slicks were drifting SE. Chemistry of lake water indicated that HCl-bearing steam was the lake's dominant thermal input. Deformation measurements revealed only minor changes.

Tremor amplitude declined in late March, was increasing again by the beginning of April, then declined toward mid-month. Low-frequency tremor remained uncommon, with 1-Hz signals recorded only on 7 and 8 April.

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.

Seismic energy release and the number of earthquakes were at low to moderate levels in April. Seismicity peaked on 11 April with 162 low-frequency events. Earthquakes were dispersed around the crater, with focal depths of 0.5-6.5 km. Pulses of low-energy tremor began 26 April and persisted until the 29th, when an episode of continuous low-energy tremor was associated with a small ash emission. Dry and electronic tilt showed no substantial changes. Measurements of glacial behavior showed significant ablation, reaching a rate of the order of 240 m³/day. The average rate of SO₂ emission, measured by COSPEC, was 1,467 t/d.

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.

The Mt. St. Helens seismic network recorded an explosion-type seismic signal on 25 April at 0126. The seismicity appeared similar to that associated with minor tephra emissions on [6] December and 6 January (SEAN 14:11 and 14:12), but both amplitude and duration were much smaller on 25 April. No eruption plume was reported and fresh snowfall shortly after the episode obscured any material that may have been deposited. The number of tiny earthquakes (detected only by crater stations) remained elevated for ~18 hours after the activity, then returned to background levels. Periods of increased local seismicity have continued since late 1987 (figure 43 and SEAN 14:08 and 14:10).

Figure 43. Time vs. depth plot of seismicity at Mt. St. Helens, 1 January 1989-17 May 1990. Arrows mark episodes of explosion-type seismicity. Courtesy of the Geophysics Program, University of Washington.

Geologic Background. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: C. Jonientz-Trisler, University of Washington.

Stromboli was visited on 29 March and 1-2 April. Poor weather on the 29th obscured the active summit vents until late afternoon, when six eruptive episodes were observed between 1745 and 1830. Five were accompanied by emission of dark gray to black ash plumes that rose 150-200 m above the two active vents. The activity produced lava fountains ~100 m high, often falling onto the Sciara del Fuoco. On one occasion, a large glowing block rolled down the NNW side of the Sciara to ~100 m below the vents.

Observations from the summit between 1730 on 1 April and 0200 the next morning revealed morphologic changes that had occurred in the vent area since September 1989 (14:09). Crater A [termed Crater 1 in previous reports; compare figure 2 and figure 3 below] included at least five vents, four of which were active. The vent that had been most active in September had collapsed, but a new deep vent that had formed 10-20 m to the NW produced an average of 2-3 eruptions/hour before midnight. Glowing spatter from the eruptions rarely escaped the vent, but black ash plumes rose ~100 m above the rim. Eruptions became more frequent and intense after midnight, with lava fountains rising to ~100 m above the rim, larger ash plumes, and heavy falls of bombs and spatter onto Crater A's NE rim (figure 4). Each of the eruptions was accompanied by a few seconds of deep but not very loud rumbling. In the NE part of Crater A, a cluster of open vents (several very small and three larger) contained active magma and glowed intensely at night. They emitted burning gases but no spatter.

Figure 3. Sketch map of the summit area of Stromboli showing vent configurations observed 1-2 April 1990. Courtesy of B. Behncke.

Figure 4. Oblique sketch of the summit area of Stromboli, looking roughly W. Courtesy of B. Behncke.

Crater B [coalesced from craters formerly termed 2, 3, and 4; compare figures 2 and 3 below] included at least 4 vents, and others were probably hidden from view by intense gas emission and topographic obstacles. In its center was a symmetrical spatter cone 10-15 m high with a glowing summit vent and a small steaming hornito near its SW base. The cone was covered with yellowish green sublimates and was not ejecting tephra. Vent 5, frequently active in September, erupted only once every 1-2 hours. Its eruptions lasted up to 30 seconds, sometimes consisting of several pulses of lava fountains that reached 100 m above the rim, accompanied by loud rumbling. Vigorous emission of gas (smelling strongly of H₂S and [SO₂]) from the E wall of Crater B obscured vent 5 from direct observation. Vent 4a, at most 2 m in diameter, was near the base of a steep pinnacle, possibly a hornito, ~10 m high. The vent produced very loud high-pressure gas emissions, sometimes lasting 20 seconds, every 20-30 minutes, feeding a very faint bluish gas column ~20-30 m high. A few were accompanied by ejection of several glowing blocks, probably from the conduit walls. A slight continuous tremor was felt from ~50 m away during one of the gas emission episodes. Vent 3, between craters A and B, remained inactive during the observation period.

During the afternoon of 3 April, ash emission reportedly became stronger and could be seen from villages on the SE and E sides of the island.

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: B. Behncke, Ruhr Univ, Germany.

Fumarolic activity continued from the main crater, with temperatures of 90°C, and from the N wall of the central crater.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: J. Barquero, OVSICORI.

"Activity remained at a very low level, with the summit crater releasing white vapour in small to moderate amounts. Seismicity was limited to a few (<=10) very small low-frequency events/day."

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

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

A summit climb on 31 March revealed only minor changes since September 1989 (14:10). Gas emission continued from the fissure on the N rim, at high pressure from its 10-15-cm-wide central portion. Rocks up to 5 mm in diameter were re-ejected when thrown into the fissure's central section. The resulting gas plume rose 300-400 m during rainy weather on 3 April, but was considerably smaller at other times. Weak fumarolic activity was also occurring on the outer SE crater wall, and a new fumarole had formed on the NW flank.

Geologic Background. The word volcano is derived from Vulcano stratovolcano in Italy's Aeolian Islands. Vulcano was constructed during six stages over the past 136,000 years. Two overlapping calderas, the 2.5-km-wide Caldera del Piano on the SE and the 4-km-wide Caldera della Fossa on the NW, were formed at about 100,000 and 24,000-15,000 years ago, respectively, and volcanism has migrated north over time. La Fossa cone, active throughout the Holocene and the location of most historical eruptions, occupies the 3-km-wide Caldera della Fossa at the NW end of the elongated 3 x 7 km island. The Vulcanello lava platform is a low, roughly circular peninsula on the northern tip of Vulcano that was formed as an island beginning more than 2,000 years ago and was connected to the main island in about 1550 CE. Vulcanello is capped by three pyroclastic cones and was active intermittently until the 16th century. Explosive activity took place at the Fossa cone from 1898 to 1900.

Information Contacts: B. Behncke, Ruhr Univ.