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

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

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

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

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

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

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

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

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

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

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

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

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

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/).

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

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

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

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

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

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

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

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

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

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

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

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

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

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

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

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

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/).

The eruption from the S flank of Mt. Cameroon that began on 28 March was followed by the opening of a second set of fissures opening on 30 March, sending a voluminous aa flow towards the ocean that continued throughout the first two weeks of April (BGVN 24:03 and 24:04). On 11 April the flow front was 150-200 m wide and 30 m thick and progressing at a rate of several m/hour; lava production ended on 14 April. A notice on 5 June from Henri Hogbe Nlend, the Minister of Scientific and Technical Research (Ministre de la Recherche Scientifique et Technique, MINREST), said abnormal and repeated high-amplitude seismic events were recorded on the night of 30 May by seismographs in Ekona. This was the first time since the end of the eruption that such events have been registered.

On July 11 the head of the Scientific Committee monitoring Mt. Cameroon, Samuel Ayongue, was quoted in The Post, a weekly newspaper, as being "...worried about the tremors going on now because they have increased in intensity and frequency." According to Ayongue, the tremors were being caused by magma refilling spaces created during the eruption. It was difficult to locate the earthquakes because of inadequate seismic equipment.

The Assistant Director of the Institute for Mining and Geological Research (IRGM) at Ekona, Richard Ubangoh, disclosed on 13 October that during 4-6 October, 54 seismic events ("earth tremors") were recorded. A notice to the Minister of Scientific and Technical Research confirmed earlier reports of frequent felt earthquakes by residents living on the foot of Mt. Cameroon. A source at MINREST, quoting the notice for Isaha'a Boh, stated that the events "... were not serious [enough] to cause any damage or immediate threat." The Assistant Director regretted that "... the equipment in use presently, are quite old and cannot provide reliable results." While waiting for 10 new seismographs from Europe in the next six months Ubangoh stated that provisional equipment would be installed at the foot of the mountain in the next three months.

Geologic Background. Mount Cameroon, one of Africa's largest volcanoes, rises above the coast of west Cameroon. The massive steep-sided volcano of dominantly basaltic-to-trachybasaltic composition forms a volcanic horst constructed above a basement of Precambrian metamorphic rocks covered with Cretaceous to Quaternary sediments. More than 100 small cinder cones, often fissure-controlled parallel to the long axis of the 1400 km3 edifice, occur on the flanks and surrounding lowlands. A large satellitic peak, Etinde (also known as Little Cameroon), is located on the S flank near the coast. Historical activity was first observed in the 5th century BCE by the Carthaginian navigator Hannon. During historical time, moderate explosive and effusive eruptions have occurred from both summit and flank vents. A 1922 SW-flank eruption produced a lava flow that reached the Atlantic coast, and a lava flow from a 1999 south-flank eruption stopped only 200 m from the sea. Explosive activity from two vents on the upper SE flank was reported in May 2000.

Information Contacts: Isaha'a Boh Cameroon, Media Research and Strengthening Institute, P.O. Box 731, Yaounde, Cameroon.

The information for this report was compiled by Boris Behncke at the Dipartimento di Scienze Geologiche, University of Catania (DSGUC), and posted on his internet web site. The compilation was based on personal visits to the summit, observations from Catania, and other sources cited in the text. Additional information was provided by Jean-Claude Tanguy (DSGUC), mostly about the activity during September.

Mild eruptive activity resumed at Etna's summit craters (figure 80) in early June, and gradually increased through late August before culminating with a powerful eruptive episode from the Voragine on 4 September. During the same period, lava continued to flow from fissures at the base of Southeast Crater (SEC), and occasional phases of mild lava spattering built hornitos and spatter cones at the eruptive vents.

Figure 80. Sketch map of the summit craters of Etna, based on fieldwork between 7 September and 1 October 1999 by Behncke and others. Courtesy of Boris Behncke.

Activity during June 1999. During early June, lava emission from the 4 February fissure on the SE base of SEC continued at a low rate. Lava issued from ephemeral vents and flowed for a few hundred meters towards the W face of the Valle del Bove (VdB).

The following information regarding activity from 30 May to 2 June was provided by John Guest (University College London, UK) and Angus Duncan (University of Luton, UK). Several explosions were heard from the summit craters on 30 May. On 1 June a brief bright red glow was seen over Bocca Nuova (BN). The active lava pile in the vent area at the foot of SEC on 2 June had increased in thickness since 30 May. Fresh lava now partly buried the 'old' tumulus of altered lava blocks, but a new tumulus had formed a few meters downflow. On 3 June Sandro Privitera (IGGUC) observed three emissions of reddish gray ash to more than 500 m above the crater.

On 4 June the two main sites of activity were generally the same as on 19 May (BGVN 24:05): an effusive vent ~25 m below the hornitos at the upper end of the fissure that became active on 4 February, and a cluster of vents at about 2,600 m elevation on the W slope of the VdB. The upper site had shifted ~30-40 m upslope. During the 16 days between the two visits, the site of lava emission had shifted frequently, sending lava flows in various directions. By 4 June lava flows had covered most traces of the tumulus collapse depression formed on 12 May. It appeared that the effusion rate had remained nearly constant for about 2 months (at ~1 m³/s). About 25-30 x 10⁶ m³ of lava had accumulated since 4 February on the western VdB rim and the slope below.

A brief visit on 10 June by Behncke and Francesca Ghisetti (DSGUC) revealed that the output of lava from the 4 February fissure had increased. The active vents were ~10-20 m below the hornitos at the upper end of the fissure. One vigorous vent was on the fissure, but lava also issued from within and on the margins of recent flows on the SW side of the lava field. A flow down the N side of the lava field appeared to have spilled over the rim of VdB. One vent continued to emit lava on the western VdB slope.

Weak explosive activity at the 4 February fissure resumed in mid-June, accompanied by an increase in the lava output. According to Giuseppe Scarpinati (L'Association Européenne Volcanologique, LAVE), an intermittent glow in the eruption area was visible from Acireale (SE of Etna) on the evening of 16 June. This glow was also clearly visible from Catania on the following evenings, and lava was seen extending from the glow area.

By the afternoon of 19 June one large and several smaller hornitos had grown on a large lava shield, ~50-80 m below the cluster of hornitos built during February-March 1999. Two lava rivers extended a few hundred meters in the direction of the VdB. The effusion rate had increased to 2-3 m³/s (it had been less than or equal to 1 m³/s during the previous month), and the volume of lava emitted since 4 February exceeded 30 x 10⁶ m³.

Between 19 and 23 June there was a notable decrease in activity at the eruptive fissure. After a visit on 26-27 June, Scarpinati reported that variable emission of lava from the 4 February fissure continued. Scarpinati also noted that the Voragine produced explosions, but made no direct observations.

On the evening of 29 June Behncke noted that the 4 February fissure had one eruptive site that produced mild lava spattering and two lava flows. Spattering from three closely spaced vents threw blobs of lava up to 3 m away. A partially drained lava tube containing incandescent but stagnant lava was seen 50 m downslope from the vents. The output was estimated at 1-3 m³/s. The SW ("diaframma") vent in the Voragine produced loud explosions every 2-10 minutes that ejected incandescent bombs above the vent.

Activity during July 1999. Another summit visit by Behncke on 1 July benefitted from perfect viewing conditions and very little wind. The generally flat floor of NEC had changed little since 5 October 1998, but now contained a large pit emitting a high-pressure gas plume charged with SO₂. There were periods lasting a few minutes when the noise level increased notably, and the plume became much denser; one time it contained brownish ash. BN had its usual two large eruptive centers, one in its NW part and the other at the base of its SE rim. While the latter periodically emitted plumes of grayish-brown ash, the former was the site of alternating ash emission and magmatic degassing.

The Voragine, according to a guide, had intensified its activity on 26 May. On the morning of 1 July explosions occurred at the SW vent every 1-10 minutes. Explosions at the SW vent started with a noise followed by large bombs that rose tens of meters above the vent, and sometimes even tens of meters above the crater rim itself, and then by a brownish ash plume. A few fresh vesicular bombs were found on the outer SW slope of the Voragine.

Claude Grandpey (LAVE) visited the eruptive fissure on 2 July and observed vigorous lava emission. The next day, lava emission had decreased. Activity was intense at the SW vent of the Voragine, with explosions ejecting bombs outside the crater on the northern side. Many bombs also fell into BN. The central vent in the Voragine had periodic gas and ash emissions. In the BN, noisy activity occurred in the SE vents (which during the 1 July visit only emitted ash), while the NW vent was relatively quiet.

The summit area was visited on 6 and 7 July by Behncke, Peter Ippach, and Eduard Harms (German Volcano Museum, Mayen, Germany). During the first of these two visits there was strong gas emission from the central pit of the NEC, and every 10-45 minutes there were explosive ejections of rocks and ash emissions. In the Voragine, explosive activity at the SW vent had decreased, and only one explosion was observed during two hours. However, the central vent was the site of Strombolian eruptions every 1-10 minutes. Incandescent bombs were ejected but only in one case rose as high as the rim of the vent, which was estimated to be at least 35-40 m deep and had a pit about 5 m wide in its floor. Recently ejected bombs up to 1.5 m long littered most of the Voragine floor.

The 7 July visit to the fissure disclosed continuing activity from two major effusive vents, one located in the area of the hornitos that formed in the past few weeks, while the other lay ~100 m downslope at the end of a lava tube. During four hours of observations, explosion sounds coming from the Voragine (and maybe also from BN) were heard every 5 to 45 minutes.

On 9 and 10 July, Behncke, Ippach, and Harms visited the summit area again, and additional information about the activity on 10-11 July was provided by Scarpinati and Charles Rivière (of Tremblay-en-France, France). Observations were restricted to the area of the 4 February fissure, but Rivière visited the summit craters early on 10 July. At the fissure, three vents were active at the tumulus ~150 m downslope from the uppermost February-March hornitos. Several lava flows were active during 9-10 July, and incandescent lava was seen in many places on the lava field. Lava also issued from several vents along the N margin of the flow-field.

Rivière, who visited the summit craters during the forenoon of 10 July, reported continuous pyroclastic activity deep within the pit of the NEC. In the Voragine, Strombolian activity occurred from the central andSW vents, with bombs at times rising high above the crater rim; Rivière noted that explosions occurred about every two minutes.

Scarpinati and Alain Catté (LAVE) observed the activity from the late afternoon of 10 July through the next morning. Shortly after 1800 on the 10th, the tumulus where the main vent had been emitting lava was seen to "inflate rapidly, and then lava came down on all its sides, forming three lava rivers." On the next morning, none of the vents on the tumulus were active, but a new vent had formed 30 m SE, burying the tourist path to the vent area; lava effusion diminished later that morning. Between 13 and 24 July lava continued to flow from the 4 February fissure, but the amount was relatively small, and short-lived flows extended only a few hundred meters downslope.

On 16 July Grandpey noted clouds of brownish ash from NEC. The Voragine was quiet, but Grandpey learned that the SW vent was active earlier during the week (around 12 or 13 July) with explosions, while lava was visible at the bottom. The NW vent inside BN was quiet, and parts of it had collapsed. Strong explosions heard every few minutes in the SE vent had been audible throughout the night.

Activity was particularly intense in the Voragine on 18 July when Rivière filmed the SW vent. Lava had again risen to ~20 m below the rim, and a small, dome-shaped mound of lava produced numerous small explosions. The mound was partly incandescent and was blown to pieces in some of the larger explosions, then rose again. During the days preceding 24 July, however, Rivière observed a diminution of activity in the Voragine, but there was explosive activity within BN.

The summit craters were visited again on 28 July by Behncke, Carmelo Monaco and Angelita Rigano (DSGUC), and others. Deep within the central pit of the NEC there were near-continuous detonations. Within the BN, explosive activity occurred deep within the two main vents. The SE vent produced near-continuous emissions of brownish ash. The Voragine central vent produced powerful explosions and at times prolonged fountains of incandescent bombs, some of them up to 1 m across. Some of the explosions ejected bombs to ~100 m above the crater rim. Many eruptions were accompanied by high-pitched roaring noises indicating high-pressure gas emission from the top of the magma column in the vent, which had risen by tens of meters since last observed directly by Behncke and others on 6 July. At the 4 February fissure, lava emission continued at a low rate. One area of effusive activity lay on the NE side of a large tumulus ~100 m downslope from the upper hornito cluster. The effusion rate was ~1-2 m³/s, and the volume of lava emitted since 4 February was estimated to exceed 35 x 10⁶ m³.

1 August-3 September 1999. Axel Timm from Germany visited on 15 and 16 August and made the following observations. There was little activity in the BN on 15 August, with quiet degassing at the NW vent, while dilute ash clouds were emitted at intervals of several hours from the SE vent. In the Voragine there was only gas emission from the SW vent, but minor eruptions occurred at intervals of 5-60 minutes from the central vent. Rumbling noises and dense gas emissions came from deep within NEC. Several small lava flows issued from the hornito area at the upper end of the 4 February fissure.

On 16 August the SE vent continued to quietly emit ash to 50-100 m above the vent at intervals of about 30 minutes. Voragine eruptions every 10-30 minutes from the central vent varied from noisy gas emissions to explosions that ejected bombs and scoriae far beyond the rim of the vent.

Grandpey reported that lava effusion from the 4 February fissure decreased notably around 20 August. Activity ceased on 25 August, and no effusive activity occurred thereafter for two days. Grandpey noted that the end of the effusive activity corresponded to a increased activity inside the Voragine. On 24 August he saw explosions from two small vents on the N rim of the SW vent. On 26 August Grandpey observed the central part of the Voragine inflate over a surface ~50 m in diameter, followed by an explosion that disrupted about half of that area, ejecting large pyroclasts. A few minutes later a much stronger explosion sent bombs as far as the center of BN and all over the W slope of the Voragine. Similar explosions followed through the next day. When Grandpey returned on 27 August, a new "cavity" had formed at the center of the Voragine and explosions were occurring near the SW vent.

The cessation of activity from the 4 February fissure on 25 August was followed two days later by the opening of a ~50 m long fissure located 40-50 m N of the hornitos. Mild Strombolian activity occurred during the following days and a small lava flow moved along the rim of the lava field.

4 September 1999 eruption from the Voragine and SEC activity. Scarpinati was observing the effusive activity at the new vents at the SE base of the SEC cone at around 1700 on 4 September and noted a hissing sound at around 1745, which gradually increased until it was "like a jumbo jet taking off." Guides at the Torre del Filosofo hut heard a loud detonation at about 1810, and saw intense red glow above the main summit cone ("the BN was incandescent all over"). Strong continuous incandescence between the Voragine and NEC suggested that lava was flowing down the E side of the main summit cone. At about the same time, Scarpinati saw through a gap in the clouds that gas and ash were rising from the summit area. Shortly afterwards he heard the crashing of impacting blocks and bombs, and retreated to the Piccolo Rifugio at about 2,500 m elevation. The climax of the eruption probably occurred between 1900 and 1930, judging from the audible detonations.

Bad weather during most of 4 September precluded observations, but a relatively clear view from Piano Provenzana (on the N flank, ~6 km from the Voragine) revealed the sudden uprise of a dark, ash-laden column that was bent eastwards. Observers at the Piano delle Concazze, about 2,600 m elevation on the N flank and ~2.5 km from the Voragine, enjoyed a splendid view of the eruption. By the time of their arrival, probably between 1830 and 1900, a huge lava fountain was rising hundreds of meters above the Voragine, and a pitch-black, tephra-laden eruption column rose ~2 km high before being blown E by winds. Large bombs fell onto the upper slopes of the NEC, which continuously emitted a dense brown ash plume, and onto the W side of the fountain. At the climax of the activity, the fountain roared to at least 1,500 m above the Voragine, an unprecedented height in the recent history of Etna.

At 1945 the cloud cover lifted, and the group at Piccolo Rifugio saw "an awesome spectacle of gigantic explosions" occurring at intervals of about 2 minutes, one of which was described by Scarpinati as "the biggest I have ever seen" (he has climbed Etna more than 500 times in the past 35 years), and which showered the main summit cone with meter-sized bombs. Some of this late activity may have come from the BN.

By 2045 all activity on the main summit cone had ended, but explosive activity began from the SEC summit vent consisting of dark "smoke" emissions mixed with incandescent pyroclasts. Ten minutes later the activity became purely Strombolian with 20-25 explosions per minute. Observations from the Piccolo Rifugio continued until about 2200 and were curtailed by bad weather; later that evening lava began to spill from the lower part of the fissure on the SE flank of the SEC cone. Lava supply increased at the vents that had become active on 27 August, and on early 5 September, a lava flow ~1 km long was observed by J.-C. Tanguy and local guides.

Effects of the 4 September 1999 eruptions. Soon after the beginning of the eruption, loud detonations were audible in villages and towns around the volcano. This was followed by a fall of scoriaceous lapilli on the E flank, extending to the coast near the town of Giarre, more than 15 km from the summit (figure 81). Many of the lapilli were walnut-sized, and some, in the area of Fornazzo, were up to 10 cm long (observation by J.-C. Tanguy). Eyewitnesses reported that some of the larger fragments were still hot when falling near the villages of Milo, Fornazzo, and Sant'Alfio, but not hot enough to set vegetation afire. Larger clasts broke windshields and seriously damaged vineyards and fruit gardens. In a narrow sector from the Milo-Fornazzo area towards the coastal strip near Giarre the pyroclastic deposit was several centimeters thick, and traffic was disrupted due to scoriae on roads. On the beach of the Ionian Sea between Riposto and Fondachello, scoriae 5-6 cm in diameter were not rare. Press reports put the damage to agriculture and infrastructure at several tens of billions of Lire (several tens of millions of US $). According to the Catania-based newspaper "La Sicilia," ~1 x 10⁶ m³ of pyroclasts fell on Giarre alone, while the full volume of pyroclasts was given as 5 x 10⁶ m³, a value that fits well with observations by Behncke and others.

Figure 81. Sketch map showing the distribution of pyroclasts from the 4 September 1999 eruption of the Voragine, based on field work during the week following the eruption. Courtesy of Boris Behncke.

Field investigations were made by Behncke and Werner Keller (Proyecto de Observación Villarrica/Internet) in the area of Milo, Fornazzo, and Giarre on 6-8 September, and during a summit visit on 7 September. Measurements were made of the thickness of the deposit in various locations before heavy rainfall swept part of it away, and when the cleaning of roads was still in an initial stage. During the afternoon of 7 September visibility was hampered by clouds, but the effects of the eruption were striking. The cones of the summit craters were hit by countless bombs up to 5 m in largest dimension and lithic blocks up to 1 m across. Many bombs and some blocks had fragmented upon impact, and others were found up to 10 m outside the craters created by their impact. Projectiles had arrived on both fairly flat and vertical trajectories. Some of the larger bombs were still warm about 60 hours after their emplacement.

On the S flank of the main summit cone the accumulation of juvenile scoriaceous pyroclasts had apparently been so rapid that the deposit began to slide down the steep flank, forming something like a dry debris flow that extended ~500 m down the slope to its base. In its distal portion the flow ended in two distinct lobes ~1 m thick. About 80% of this deposit consisted of juvenile clasts 10-30 cm in diameter whose edges were rounded while sliding down the slope, the other 20% were older, slightly smaller clasts (reddish scoriae and gray lithic blocks).

Brief glimpses through the clouds permitted a view on the Voragine from ~500 m W of the crater rim. The heavy fallout close to the crater almost healed the large scar cut into the S flank of the adjacent NEC cone during the 22 July 1998 Voragine eruption (BGVN 23:11). On the SW crater rim, the rapid accumulation of fluid ejecta formed a lava flow ~300 m wide and 250-300 m long. Two similar fountain-fed flows were emplaced on the E side of the Voragine, the longer of which traveled ~700 m towards the VdB. Guides on the N flank indicated that another fountain-fed lava flow cascaded into the Bocca Nuova.

On the lower E flank the lapilli deposit extended in a narrow strip E towards the coast near Giarre. Five communities (including Milo, Mascali, and Giarre) suffered heavy fallout. Going northwards from Zafferana, on the SE flank, the southern margin of the fall deposit was in the forests between Petrulli (~2 km N of the center of Zafferana) and Milo, where isolated scoriaceous lapilli with 1-3-cm diameters occurred. Closer to Milo (1.5 km farther N) the number of clasts per square meter increased as did their mean diameter, and on the southern margin of the village the deposit became continuous. Most of the deposit consisted of lapilli-size scoriae, with little ash mostly coating leaves and grass. The largest clasts found in the S part of Milo were 7 cm across, and many reached 5 cm. In the N part of Milo, the thickness of the deposit exceeded 5 cm, and many leaves were damaged. In the S part, ~1.5 km from Milo, the deposit was 5-6 cm thick, and the largest clasts were up to 10 cm across. Residents reported that larger clasts fragmented upon impact. Going north, the deposit thinned gradually and ended with a relatively sharp margin ~2 km N of Fornazzo. Downslope, near the town of Giarre, the area of fallout was ~5 km wide in N-S extension, and up to 5 cm thick in its central portion. Most of the deposit here was composed of fragments with diameters of a few millimeters to 3 cm. The N and S margins of the deposit were strikingly sharp, it seemed that only very little fine ash fell beyond the margins of the lapilli deposit.

Comparison with the (relatively poor) descriptions of the fall deposit produced by an eruption from the Voragine on 17 July 1960 allows the conclusion that the 4 September 1999 eruption was less voluminous but similarly violent, and therefore among the largest explosive eruptions at Etna's summit craters during the past 100 years. The 1960 eruption produced ~10 x 10⁶ m³ of pyroclasts, and clasts more than 5 cm in diameter were reported.

The activity at SEC on the evening of 4 September had many minor effects. The most impressive changes since 28 July were the presence of the new lava lobe that had issued from the lower part of the 4 February fissure, and the collapse of part of the E crater rim.

Activity after 4 September 1999. During the week following 4 September activity continued at the summit craters, but observations were hampered by bad weather. Intense explosive activity occurred each day at the BN, and at times bombs were ejected onto the outer slopes of the main summit cone. The Voragine remained active, and vigorous seismicity indicated that the most intense activity occurred between 0100 and 0400 on 9 September. During their summit visit on 7 September, Behncke and Keller reached the area of activity near SEC and saw two small lava flows issuing from vents 15 m below the spatter cone formed after 27 August that extended onto the W slope of the VdB after a few days. Mild Strombolian activity occurred from a new cluster of hornitos near the effusive vents.

During the evening of 11 September Scarpinati observed lava flowing from a vent ~200-250 m farther downslope to the E of the SEC effusive area. The next morning a new double spatter cone ~200-250 m E of the previous cone issued fluid lava, at an estimated rate of at least 1 m³/s, that moved along the margin of the flow-field. The new vents were on terrain not covered by lava during the previous months, and it appeared that this was a true new eruptive fissure.

Mild magmatic explosions were observed by guides every few minutes early on 18 September. On the next day, Rivière observed vigorous lava splashing from the NW cone of BN. Strombolian activity was relatively weak until early the next morning. At 0445, Tanguy observed the eruption from Trecastagni (on the SE flank). Continuous jets of incandescent material illuminated a gas plume rising more than 500 m above the crater rim. A bright glow in the area of the effusive vents at the ESE base of the SEC was noted, and weak incandescence was seen in the area of the Voragine. Tanguy arrived at the Piccolo Rifugio at about 0545, by which time the most energetic phase was over, although some incandescent bombs still rose up to 300 m above the crater rim. The activity had virtually ceased by 0630.

During the early morning hours of 20 September, vigorous lava fountaining occurred at the BN, mostly from the vents in the NW part of the crater where a broad cone had been the site of weak degassing for several weeks; previous reports noted that the area of this cone had remained virtually unchanged even during the 4 September Voragine eruption. The episode covered almost the entire floor of the BN with lava to thicknesses of several meters to tens of meters. A lava tongue invaded the depression that had previously hosted the SE vents, and only an irregularly shaped depression was left at the site of the NW vents. Explosive activity was again observed on the evening of 20 September, and a brief surge of activity occurred on late 21 September, after which BN became silent for about two weeks.

Effusive activity from the vents on the ESE base of the SEC was intense on the morning of 20 September when visited by Tanguy; lava issued from a vent that had opened the previous afternoon near the large spatter cone built after 27 August, and mild spattering occurred from this cone itself. A new vent had also formed at the fissure that had become active on 12 September. Vigorous effusive activity was continuing at the 12 September vents.

During the week following the 20 September eruptive episode at BN, the most persistently active summit crater was NEC, which had Strombolian activity in its central pit. A visit by Behncke on 28 September revealed that NEC cone had received heavy fallout of bombs on 4 September, and the footpath on its W side had vanished under a continuous cover of bombs, some up to 1.5 m in diameter. While the collapse scar on the SW flank of the cone had been largely healed by bomb fallout, a portion of the cone's flank farther to the ESE had collapsed, leaving a similar scar. Activity within the central pit consisted of near-continuous expulsions of dark ash. Good views obtained by Rivière on 25 September showed that the pit continued to a depth of several hundred meters with vertical walls.

On 28 September, good panoramic views of the Voragine from the S rim of NEC revealed that the former SW and central vents had merged into one large ~200-m-wide crater, but it appeared that there were still two eruptive centers. On the SW rim a wide U-shaped gap had formed in the former "diaframma" (septum) through which the floor of BN could be seen. Eruptive activity within the Voragine on 28 September consisted of frequent loud explosions.

Sub-concentric fractures were present on the outer ENE and E rim of the Voragine and on a ridge which now constitutes both the SE flank of the NEC cone and the NE rim of the Voragine. A fountain-fed lava flow that had formed during the 4 September eruption on the W side of the Voragine was up to 150 m wide in its upper part but narrowed to ~30 m in its distal portion where it formed a lobe along the N side of the 22 July 1998 flow; the new lobe, however, was shorter than its predecessor. Two fountain-fed lava flows also formed on the E side of the Voragine. The longer of these flows extended about halfway to the W rim of the Valle del Bove.

Rivière visited SEC on 24 September and reported that discontinuous effusive activity continued from the new vents (first seen by Scarpinati on the morning of 11 September) near the 4 February fissure. Lava flows extended ~1 km and spilled down the W face of the VdB.

Vigorous eruptive activity resumed in the BN on 30 September, ejecting large bombs hundreds of meters beyond the crater rim. At the same time, activity increased at the NEC. On 29-30 September, near-continuous Strombolian activity ejected bombs tens of meters above the crater rim, and larger bursts reached heights of up to 150 m, dropping bombs all over the crater floor and onto the flanks of the NEC cone.

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Boris Behncke and Jean-Claude Tanguy, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy.

Less than 2 months after the end of the eruption of July (BGVN 24:09), a new seismic crisis started at 1037 on 28 September. Most of the observed 189 seismic events had magnitudes of less than 1. All were situated above sea level. Only two of them had significantly larger magnitudes of 1.8 and 2.2, at 1042 and 1053, respectively.

An eruption started at 1158 in the W part of Dolomieu crater with a strong whistling noise. Seconds later, a 10-m-diameter, ~50-m-high lava fountain rose from the SW corner of Dolomieu crater. Immediately after that, a fissure formed going NW, followed by the development of small lava fountains and a lava flow. Less than 5 minutes later the fissure measured ~200 m long and was terminated by another lava fountain 20-30 m high. At 1210, the fissure opened on the S flank "en echelon," ~100 m below the crater rim. The two upper fissures measured ~50 m long, followed by a third one ~250 m. The lava flow down the steep S flank extended ~1 km in less than 15 minutes. It continued to the SE on a more gentle slope and reached "Château Fort" crater, 2 km away, within two hours.

Less than 8 hours after the eruption started, activity was limited to some individual points on the upper S flank, while the main lava flow had stagnated. No further activity was observed in the Dolomieu crater. In the night, small fissures on the S flank at 2,150 m elevation produced some small pahoehoe lava flows.

On 8 October, after a significant increase of tremor, steam release was observed in the south "enclos," at 1,900 m altitude, ~4 km away from Dolomieu crater and on the morning of 11 October a new 600-m-long lava flow was observed 500 m to the SE, on the base of crater "Villèlle," close to southern border of the caldera. On 18 October this lava flow measured ~1.5 km. No further activity was observed at this site on 21 October. As of 22 October tremor was still visible, mainly in form of small "gas piston events," centered on the upper fissures on the S flank of Fournaise, where a small cone was formed. The eruption ended following small "gas piston events" on at about 1800 on 23 October. Residual fumarolic plumes, consisting primarily of water vapor, were visible the following week.

Mapping of the lava flow was performed in the first days by use of small hand-held GPS. Early lava flows, in Dolomieu crater and on the S flank are mainly aa lava flows. In the Dolomieu crater, it represents a surface of ~40,000 m² (?) and a volume of <100,000 m³. It partly covered the July lava flow. On the border of the lava flow we could observe fissuring of the ground, up to 3 m deep, due to the weight of the new up to 3-m-high lava flow.

The main lava flow on the S flank represents about 300,000 m² and <1 x 10⁶ m³. Taking into account an emplacement within less than 5 hours, the eruption rate was estimated to be >50 m³/s. The small pahoehoe flow from the fissures at 2,150 m altitude covered less than 5,000 m².

The southern-most lava flow starting at crater Villèlle also was mainly pahoehoe. There were no projections at its point of emission, indicating a highly degassed magma. On 11 October a ~1 m lava flow emerged from a small "well" on the SW base of "Villèlle." The volume of this lava flow is estimated to be under 50,000 m³. All recovered samples were aphyric basalt.

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: Thomas Staudacher, Nicolas Villeneuve, and Jean Louis Cheminée, Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Institut National des Sciences de l'Univers, 14 RN3 - Km 27, 97418 La Plaine des Cafres, Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

During July and August 1999, low-intensity seismic activity continued, similar to that of previous months (BGVN 24:07). Fifty-six volcano-tectonic (VT) earthquakes were registered during this period compared to 90 during the previous two months. The depths of these VT events were between 0.35 and 19 km below the summit, and the total energy released was estimated as 4.82 x 1013 ergs. The largest magnitude event, on the morning of 16 July, had a coda magnitude 1.7 and depth of 8 km.

Additionally, 20 long-period events and 10 tremor episodes were recorded with an energy release of 5.38 x 1012 ergs. Dominant frequencies during the tremor episodes were 2.0-4.0 Hz. The tremor event on 23 July had a small amplitude with respect to the long coda, a quasi-monocromatic frequency of ~2.01 Hz, and an energy release of 2.09 x 1012 ergs. Periodic fumarole temperature measurements taken during the two-month period in the active crater registered a range of 130-394°C.

Radon-222 emissions measured in the soil at six stations were not significantly different from values in previous months. As in the May-June period, the greatest emissions occurred at the Sismo2 station (~5 km NE of the summit) attaining a maximum value of 2,297 pCi/l.

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: Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

During a 6-10 September visit by John Seach to the Gaua caldera and the cone of Mt. Garat, of the five craters only Crater A was solfatarically active (figure 1). The W side of the caldera lake was stained yellow by sulfurous mud and emitted a strong SO₂ smell. The water temperature was measured at 30.1°C while that of the lake shore mud was 35.1°C. The Mt. Garat cone was largely denuded of vegetation around the craters. Only on the NE side near Crater E was there any regrowth on top. Moss and grasses had regrown on the flanks of the cone, to within 20 m of the E side of Crater E. Fallen trees were scattered around the rims of all five craters.

Figure 1. Sketch map showing the positions of the five summit craters (A-E) on Mt. Garat within the Gaua caldera (larger solid line), 6-10 September 1999. Courtesy of John Seach.

Hot, whistling ground with a temperature of 97.6°C was located 20 m S of the Mt. Garat summit, located along the W summit crater rim. Fumarole fields were found both inside and outside of the summit crater rim. Another fumarole field with a temperature of 45.3°C was located ~25 m SW of Crater B. Steam was observed venting ~2 km SW of Crater E but was not approached due to its remote location.

The active Crater A is located on the SE side of the Mt. Garat cone. The E crater wall contained solfataras emitting white vapor with a strong SO₂ smell and a temperature of 95.0°C. The solfataras were surrounded by bright yellow deposits, and were active up to the rim of the crater. Solfatara plumes were easily visible from a distance of 5 km on the E shore of the lake. On the SE crater floor, a solfatara constantly vented 102.7°C vapor. Continuous loud high-pressure venting noises originated from along the N and W walls and the W floor of the crater. A pile of blocks coated in yellow deposits rested on the SW floor; mild degassing with a temperature of 99.7°C occurred here. Large blocks 1-2 m in diameter littered the SW wall and floor of the crater. The floor of the crater was split into two levels with the N level being ~5 m below the S level. Large cracks ~3 m deep were present on the S floor. Two 3-m-diameter blocks sat near a brown pond on the low, N-level floor. Rockfalls were heard coming from the E wall. Mild acid rain fell inside the crater, which was mostly filled with white vapor. At times, twin plumes emitting from the crater were visible, rising to a height of 100 m.

Craters B and C are similar in size and depth (figure 2), are denuded of vegetation, contain standing, devastated trees (figure 3), have and flat silty floors with brown ponds. The crater walls contain tuffs, cinders, and scattered blocks. Crater D is the shallowest of the five and has a flat and silty floor containing a shallow brown pond and standing, devastated trees. A 3 m-diameter block was observed on the E wall. Crater E is the smallest crater, ~20 m wide and 20 m deep. It is cone-shaped with blocks and a full cover of vegetation inside.

Figure 2. View towards the NNW of two inactive craters (B in the foreground, C in the background) in the summit area of Mt. Garat within the Gaua caldera, September 1999. The peak on the right is at 682 m elevation. Courtesy of John Seach.

Figure 3. Devastated tree at the NW edge of the Mt. Garat cone within the Gaua caldera, 8 September 1999. Courtesy of John Seach.

Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with a 6 x 9 km summit caldera. Small parasitic vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; several littoral cones were formed where these lava flows reached the sea. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. Construction of the historically active cone of Mount Garat (Gharat) and other small cinder cones in the SW part of the caldera has left a crescent-shaped caldera lake named Letas (figure 4). The symmetrical, flat-topped Mount Garat cone is topped by three pit craters.

Figure 4. View of a fumarolic plume rising from a cone on the SE flank of Mt. Garat in the Gaua caldera, September 1999. Lake Letas is in the foreground. Courtesy of John Seach.

Only solfataric activity was recorded from 1868 to 1962. Beginning in 1962, central crater explosions with frequent associated ash columns were reported nearly every year until 1977. Information after 1977 is scarce, but steam was reported in mid-1980 and ash plumes were reported in July 1981 and April 1982. Increased fumarolic activity was noted and the NW slopes of the cone were denuded of vegetation in July 1991 (BGVN 16:07). Strong fumarolic activity was continuing in July 1996 (BGVN 21:09).

Geologic Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with an 6 x 9 km summit caldera. Small vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; littoral cones were formed where these lava flows reached the ocean. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. The active Mount Garet (or Garat) cone in the SW part of the caldera has three pit craters across the summit area. Construction of Garet and other small cinder cones has left a crescent-shaped lake. The onset of eruptive activity from a vent high on the SE flank in 1962 ended a long period of dormancy.

Information Contacts: John Seach, P.O. Box 16, Chatsworth Island, NSW 2469, Australia.

This report chiefly covers the turbulent period of 1 September through 19 October 1999. Histograms available on the Instituto Geofísico's website for the crisis interval through 31 October illustrated that September and October had a striking abundance of both phreatic explosions and earthquakes. The monthly explosion count for October 1999 (53 phreatic explosions) was almost double any other month during the crisis.

Despite the steep increases in explosions and earthquakes during September and October, non-explosive episodes were common during the reporting interval. They were marked with fumarolic emissions rising from a few meters to a few kilometers above the summit vent.

Microscopic inspection of tephra erupted on 30 August led researchers to conclude that the explosions to that point had continued to eject older, non-juvenile material. But John Ewert of the USGS noted that juvenile pumice of dacitic composition began to appear in deposits starting on 26 September. And, the comparatively large 5 and 7 October eruptions both contained similar juvenile pumice.

The intracrater dome, cold at the start of the crisis, began to grow by lava extrusion around 28 September. The volume of material extruded was small,3. The comparatively large eruptions on 5 and 7 October excavated part of the dome and sent pyroclastic flows 4-5 km down the W-flank into the Rio Cristal. Shortly after both events Ewert shot videos of their still-steaming deposits.

Earthquakes. Compared to the earlier stages of the crisis, the number of multiphase, volcano-tectonic, and long-period earthquakes grew sharply during September and October. For all three types, the highest numbers seen during the entire crisis interval (July 1998-October 1999) took place during October when multiphase earthquakes occurred 15,024 times, volcano-tectonic, 1,701 times, and long-period, 15,075 times. Omitting September 1999 and comparing the October 1999 earthquakes to the previous monthly highs during the crisis, one obtains the following: multiphase earthquakes underwent a 7-fold increase; volcano-tectonic earthquakes, a ~10-fold increase; and long-period earthquakes, an impressive ~70-fold increase. It was not just the numbers of events that rose. Seismic amplitudes at stations 7 to 9 km from the summit increased notably during September and October. Many of the earthquakes had depths between the surface and 6 km.

Eruptions. Table 5 provides an overview of some of the interval's larger outbursts. The one on 3 September yielded a reduced displacement (RD) of over 25 cm². The event generated a plume to ~5.5 km altitude, which could be seen from Quito and included four distinct explosions (at 0723, 0726, 0743, and 0751). The plume dispersed after 30 minutes. Ash fall concentrated over the N flank. The next day, aerial observers noted that the 1981 crater had merged with another recent one, leaving a larger, roughly E-W trending crater in the vent area.

Table 5. Noteworthy explosions at Guagua Pichincha during 3 September through 19 October 1999. Cases shown are those where reduced displacements were stated in daily reports, with the exception of 7 October, for which the explosion's RD remained undisclosed. Plume heights were frequently undetermined due to restricted visibility (eg. darkness and clouds). Courtesy of the Instituto de Geofísico.

The 26 September explosion was described as "important." The loud noise accompanying the early morning outburst (at 0315) awakened residents on the SSW flank in Lloa (see maps in BGVN 23:09). Ash fell over some areas; a lahar moved down the W flank Rio Cristal.

Eruption on 5 October. The explosion with the largest reduced displacement disclosed during the reporting interval (36 cm²) happened on 5 October; it was associated with an ash column to over 8 km altitude. The explosion vented on the caldera's W side; observers on the scene saw airborne material move SW and SE. In addition, the next day it was reported that ash thicknesses of 2 and 3 mm were found in central and N Quito as well as the settlement of Nono. Accumulated ash in other sectors (Mindo, Cumbayá, Tumbaco, Conocoto, El Tingo, Pomasqui, and Guayllabamba) reached only minor thicknesses. On the morning of 6 October, technicians visiting monitoring stations found ash-covered solar panels.

The 6 October issue of the newspaper Diario Hoy reported that the 5 October eruption took place at 1409, and that residents in S Quito heard the explosion. They also said that the resulting plume attained a height of 20 km. Diario Hoy further wrote that one hour after the audible sound, the first ash particles descended on N Quito, which became darkened by an enormous gray cloud. In four hours the cloud covered the city in a thick fog-like mantle; Marshal Sucre airport closed at 1730. The paper noted that Quito citizens would find their normal potable water supply intact. The news report added a comment by the mayor that this behavior could persist for months or even years. Although the news report, and other information around this time described the eruption as phreatic, tephra samples indicated the presence of juvenile pumice (mentioned above), indicating that the eruption was at least partly magmatic.

Observers on a flight at around 0800 on 6 October over the S part of the volcano confirmed extensive coverage of ash, but they saw vigorous, 3-km-tall fumarolic plumes-not ash plumes-being emitted. Ash hanging over Quito at that time was therefore assumed to mainly have resulted from earlier deposited ash remobilized by traffic and wind. The 5 October eruption column was captured on NOAA GOES-8 imagery, which can be viewed as a time-lapse animation, revealing some of the dynamics of the ash column (for URL, see discussion below). Portions of the rising column split into components directed E and W, forming what appeared as a dumbbell-shaped bifurcating plume. A plume on 7 October behaved in much the same way. In both cases, analysts attributed the bifurcation to wind shear.

Eruption on 7 October. Another comparatively large explosion took place the morning of 7 October (figure 16). Hugo Yepes, John Ewert, and Dan Miller of the Instituto and USGS accompanied Ecuador's president and members of the media on a flight just after the explosion. The pilot tried to approach the S flank but a curtain of falling ash prevented the occupants from seeing into the caldera. Ash fell over Quito, the Capital. The U.S. National Oceanographic and Atmospheric Administration (NOAA) reported that the plume rose to 16.5 km altitude.

Figure 16. Guagua Pichincha's ascending ash plume at 0730 on 7 October 1999 as shot with a digital camera from the uplands of Quito. Pichincha's summit vent was ~ 11 km W, lying well behind the peak in the foreground. Courtesy of Arden and Debra Burgess.

Regarding the 7 October explosion, the Diario Hoy's headline read "Guagua: A million tons of ash." The article went on to note that the Instituto estimated 1.1 x 10⁶ metric tons of ash lay within 15 km of the summit. Thicknesses of 1-3 mm accumulated in the northern parts of Quito. Ash clean-up proceeded within the city and at the airport. The article went on to caution that in a stronger eruption 5-10 cm of ash might fall on the city.

In similar manner to satellite images of the 5 October plume, those of the 7 October plume showed that it also bifurcated. Figure 17 shows GOES-8 visible imagery available on websites operated by both NOAA and the Cooperative Institute for Meteorological Satellite Studies (CIMSS) at the University of Wisconsin. An initial pre-eruption image was made at 0645 (1145 GMT) (not shown on figure 5); the image a half hour later showed the plume at an early stage. Due to variable wind shear with height, the advecting 7 October plume moved in two directions: the highest portion (~15 km in altitude) drifted W, away from Quito, while a lower portion (~12 km in altitude) drifted E over Quito.

Figure 17. GOES-8 satellite images showing the dynamics of Guagua Pichincha's ascending 7 October plume. All images came from visible wavelengths; they are all similarly oriented and at identical scales although the upper image covers a larger area. The upper image shows the location of Guagua Pichincha (gp) and Quito (Q). Clouds on that image lay over parts of the Pacific Ocean and Ecuador (E), but N-central Ecuador and much of Colombia (C) remained unobscured. The first image (a) was taken directly from the larger one; both were captured at 0715 (1215 GMT). In the first image (a) the plume was compact and circular. The image at 0745 (b) shows the plume beginning to split into components directed W towards the Pacific, and E over Quito. The two components may have differed slightly in reflectivity. By 0815 (c), the plume had become decidedly dumb-bell shaped with the area above the volcano becoming relatively diffuse. The plume continued to spread at 0845 (d), where now the W component had also become diffuse. The eastern plume still sustained a relatively dense white color. These and associated images are displayed as time-lapse animations on NOAA and CIMSS websites (see URLS below). Courtesy of NOAA and CIMSS.

Other processed views and animations of the 7 October plume dynamics were also available on the web. Scott Bachmeier at CIMSS posted an image prepared from GOES infrared (IR) data. He used a difference or "split window" technique that enhances the ash plume. Radiation escaping from a body can be described in terms of emissivity (emissive power). The emissivity of silicate particles within an ash plume varies with wavelength. This image processed the wavelengths 10.7 and 12.0 micrometers, which led to brightness temperature differences of 1-5 Kelvin. The IR difference product shows the ash plume very well initially; but later, the plume became thinner, losing its identity on the IR difference product images.

The eastern portion of the 7 October ash plume was tracked for a longer time on the GOES 6.7 micrometer IR ("water vapor") channel. Due to the generally dry middle and upper troposphere over northern South America that day, the water vapor content in the higher plume created a discernible contrast that drifted eastward across Ecuador toward Colombia and Perú.

John Ewert took videos of the plume's dynamics, as seen from the ground. From that perspective the ascending plume appeared to have a strong rotational component. He also noted that these plumes' behaviors were hard to forecast from available wind data.

Background. On September 27, the Mayor of Quito closed schools and raised the alert from yellow to orange signifying a possible eruption within days (BGVN 24:08). About a week later the character of the alerts was revised to become more local in scope. For example, on the W flank, small settlements incorporating about 60 families along the Rio Cristal were evacuated and the status there stood at the highest level, red. The SSW flank city of Lloa remained at orange alert; and in Quito, it returned to yellow where it remained throughout the reporting period, including during times of ashfall.

During early October, the U.S. State Department issued these statements: "Geological experts conclude that the city of Quito is protected from possible lava flows, avalanches, and lateral explosions by the bulk of Pichincha Mountain, which stands between the city and the volcano crater. Parts of Quito could be affected by secondary mud flows caused by heavy rains that usually accompany an eruption. The entire city could also be affected by slight to significant ash falls and resulting disruptions of water, power, communications, and transportation. According to geological experts, lava flows, ash falls, avalanches, and lateral explosions would almost certainly head W and SW from the volcano, in the direction of three small communities, Lloa, Mindo, and Nono, popular destinations for birdwatchers. Travelers should avoid these towns."

In addition to Guagua Pichincha, a second volcanic crisis has developed at Tungurahua. Volcanological and geophysical colleagues from multiple countries have participated, or continue to collaborate in instrumenting and monitoring these crises. In the midst of these events Ecuador's economy has undergone a serious downturn with the currency recently declining in value by more than 50%.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Information Contacts: Instituto Geofísico (URL: http://www.igepn.edu.ec/); John Ewert, Volcano Disaster Assistance Team (VDAP), United States Geologic Survey (USGS), Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Diario Hoy ("Hoy Digital,", URL: http://www.hoy.com.ec/); Arden and Debra Burgess, Centro Aereo 1Q1702, P.O. Box 02-5268, Miami, FL 33102-5268 USA; NOAA/NESDIS Operational Significant Event Imagery Support Team, E/SP22, 5200 Auth Road, Camp Springs, MD 20746-4304 USA (URL: https://www.nnvl.noaa.gov/); Scott Bachmeier, Cooperative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin, 1225 West Dayton St., Madison, WI 53706 USA (URL: http://cimss.ssec.wisc.edu/).

Seismicity at Ijen increased starting in early April, when volcanic B-type events rose from 15 during the week ending on 5 April to 41 events during 6-12 April. Tremor during April and May had amplitudes of 0.5-2 mm. The number of B-type events remained high (more than 34/week) for most of the period through mid-June. Seismicity then gradually declined through mid-July, after which the weekly number of B-type events remained stable at an average of 9/week. During the period of 18 May through the week ending on 21 June a "white ash plume" rose 50-100 m. Recorded tremor had an amplitude of 0.5-3 mm.

Two phreatic eruptions occurred at the Sibanteng location inside the active crater lake at 0510 on 28 June. An accompanying detonation was heard at the sulfur mining site 2 km from the summit and volcanic tremor was recorded with an amplitude of 0.5-1 mm. The following week, 6-12 July, yellow-gray sulfur emissions were observed from the crater and a loud "whizz" noise was heard. The crater lake's water was brownish-white and had sulfur agglutinate floating on the surface. Measurements on 8 July showed that the hotspring temperature was 48°C, air temperature at the crater lake was 15°C, the lakewater temperature was 40°C, and the sulfur gas temperature was 207-221°C. Thick haze prevented observations from 13 July through 23 August, but B-type events and continuous tremor was recorded. When J.M. Bardintzeff visited, on 17 August 1999, the solfatara was strongly active and the crater filled with gas. The acid lake was a pale-green color.

Conductivity determinations were made of acid lake waters sampled on 7 December 1998 (BGVN 23:11) by Bardintzeff, Marlin, and Barsuglia. Conductivity in the middle of the lake was 146 mS/cm. Near the S side it was 140 and only 98-120 mS/cm near the hot sub-lacustrine spring. A small affluent in the S side, was (from its source to the lake) 39-27°C, with a pH of 1.6, and conductivity of 17 mS/cm. In the Banyupahit River, 3 km from the dam, conductivity was 138 mS/cm. On 10 December 1998 conductivity in the middle of the lake was 181 mS/cm.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the rim was buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Kawah Ijen is the site of a labor-intensive mining operation in which baskets of sulfur are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor; nearby waterfalls and hot springs are tourist destinations.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); J.M. Bardintzeff; C. Marlin, and F. Barsuglia, Sciences de la Terre, bat 504, Universite Paris-Sud, 91405 Orsay cedex, France.

In the early morning of 18 July, a small jökulhlaup (sudden glacier-outburst flood) lasting less than 24 hours, occurred in "Jökulsá á Sólheimasandi," one of the rivers draining from the Mýrdalsjökull icecap (figure 2) towards the S. Inspection of the icecap revealed that a new ice cauldron, ~2 km wide, and 50 m deep, had formed just above the origin of the Sólheimajökull outlet glacier. The jökulhlaup was preceded on 17 July by a 20-minute-long burst of modest volcanic tremor (reported by P. Einarsson). Intrusion of magma at a low level within the subglacial Katla volcano or even a small subglacial eruption may have occurred, possibly associated with pulse of CO₂ which could have caused boiling in geothermal areas under the icecap.

Figure 2. Topographic map of the Mýrdalsjökull icecap over Katla volcano showing tilt stations. Courtesy of the Nordisk Vulkvanologisk Institut.

From 18 July until mid-August, ten new ice cauldrons formed along the W, S, and E borders of the Mýrdalsjökull caldera (figure 3), signifying increased geothermal activity along a large part of the caldera rim. Changes on the icecap surface have been reported for some of the earlier eruptions of Katla, and the current activity could be a possible long-term precursor to a new eruption. A flight over the area on 9 September by Reynir Ragnarsson at Vík, revealed that the ice cauldrons did not develop much after mid-August.

Figure 3. One of the new ice cauldrons on Mýrdalsjökull, July-August 1999. Photo by Freysteinn Sigmundsson.

Geologic Background. Katla volcano, located near the southern end of Iceland's eastern volcanic zone, is hidden beneath the Myrdalsjökull icecap. The subglacial basaltic-to-rhyolitic volcano is one of Iceland's most active and is a frequent producer of damaging jökulhlaups, or glacier-outburst floods. A large 10 x 14 km subglacial caldera with a long axis in a NW-SE direction is up to 750 m deep. Its high point reaches 1380 m, and three major outlet glaciers have breached its rim. Although most recorded eruptions have taken place from fissures inside the caldera, the Eldgjá fissure system, which extends about 60 km to the NE from the current ice margin towards Grímsvötn volcano, has been the source of major Holocene eruptions. An eruption from the Eldgjá fissure system about 934 CE produced a voluminous lava flow of about 18 km3, one of the world's largest known Holocene lava flows. Katla has been the source of frequent subglacial basaltic explosive eruptions that have been among the largest tephra-producers in Iceland during historical time and has also produced numerous dacitic explosive eruptions during the Holocene.

Information Contacts: Rósa Ólafsdóttir, Guðrún Sverrisdóttir, Freysteinn Sigmundsson, Erik Sturkell, and Níels Óskarsson, Nordisk Vulkvanologisk Institut, Grenásvegur 50, 108 Reyjavík, Iceland (URL: http://nordvulk.hi.is); Helgi Björnsson, Páll Einarsson, and Magnús Tumi Guðmundsson, Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland (URL: http://www.raunvis.hi.is/); Ármann Höskuldsson, South Iceland Institute of Natural History, Strandvegur 50, 900 Vestmannaeyjar, Iceland (URL: https://www.nattsud.is/).

Early on the morning of 12 September monitoring instruments detected a swarm of small earthquakes and volcanic tremor on the east rift zone, and a sharp deflation (tilt) of the summit area and parts of the east rift zone. A pause in on-going eruptive activity also occurred. These effects were interpreted as due to a new intrusion of magma. Apparently, magma moved from both the summit area and from near Pu`u `O`o into the upper rift zone, forming a dike in the area between Pauahi Crater and Mauna Ulu.

Figure 142 shows the seismic record for part of 11-12 September. After tremor associated with the seismic swarm ceased, another pause in episode 55 of the Pu`u `O`o-Kupaianaha eruption began at 0131 on 12 September. This change was thought to be due to the above-mentioned intrusion.

Figure 142. Part of the 11-12 September seismogram for station STC near Pu`u `O`o at Kīlauea. There is a time difference of 15 minutes between each horizontal line and 1 minute between each small tic. Volcanic tremor was normal before the seismic swarm of 12 September but absent afterward. This absence of tremor was due to a pause in eruptive activity during the time of the swarm. Courtesy of the Hawaiian Volcano Observatory.

The onset of seismic activity and tilting on 12 September was abrupt and simultaneous to within the one-minute resolution of the tilt data. Strong tilt commenced early on 12 September, as indicated by the vertical line on figure 143, where tilt for a station was toward the caldera. A swarm of small earthquakes along the upper rift zone accompanied the ground deformation. The downward tilt (figure 143) suggested that magma was moving away from and out of the summit reservoir. Data from two other tiltmeters on the E rift zone (E of Pauahi Crater and just uprift from Pu`u `O`o) indicated that the magma was moving into the rift zone. The reversal of summit tilt about 4-6 hours later suggests that when the intrusion stopped, magma once again moved into the summit reservoir. An inspection of the ground above the intrusion on 12 September did not reveal new ground cracks, which indicated that the intrusion remained 1-2 km below the surface. On the other hand, leveling across the zone of intrusion on 14 September showed elevation changes indicative of a dike, but its size and depth remained to be calculated. It was estimated that 3-5 million cubic meters intruded into the rift zone.

Figure 143. Kīlauea tiltmeter record for early September 1999 at Uwekahuna (tilt along an azimuth of N50W). Courtesy of Hawaiian Volcano Observatory.

About eight hours after the start of the intrusion, the active lava bench on the S coast of Kīlauea began collapsing into the sea. Several small collapses were observed by scientists on 12 September. The lava bench began to collapse during 0800-0915 on 12 September and this process continued for most of the day (figure 144). By the evening of 13 September, about 2 x 10⁴ m² of the S coast had been removed. The discharge of lava into the sea stopped completely in the afternoon of 13 September.

Figure 144. W-looking view of the lava bench on the S coast of Kīlauea as it appeared on 9 September (left) and at about noon on 12 September 1999 (right). Photo courtesy of J. Kauahikaua.

Background. Kīlauea is one of five coalescing volcanoes that comprise the island of Hawaii. Historically its eruptions have originated primarily from the summit caldera or along one of the lengthy E and SW rift zones that extend from the caldera to the sea. The latest Kīlauea eruption began in January 1983 along the east rift zone. The eruption's early phases, or episodes, occurred along a portion of the rift zone that extends from Napau Crater on the uprift (toward the summit) end to ~8 km E on the downrift end (toward the sea). Mike Garcia has compiled a tabular summary of the episodes, now available on the web.

Activity eventually centered on the area and crater that were later named Pu`u `O`o. Between July 1986 and January 1992, the Kupaianaha lava lake was active ~3 km NE (downrift) of Pu`u `O`o. It was during this period that the town of Kalapana and most of the 181 homes lost were destroyed. In December 1991, one month before the shutdown of Kupaianaha, eruptive activity returned to Pu`u `O`o. More than 1 km³ of lava was erupted from January 1983 through January 1997.

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

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); Mike Garcia, Hawaii Center for Volcanology, University of Hawaii, Dept. of Geology & Geophysics, 2525 Correa Rd., Honolulu, HI 96822 USA (URL: http://www.soest.hawaii.edu/GG/HCV/puuoo-episodes.html)

Low-level activity continued throughout most of July, August, September, and into the first week of October, with only small-to-moderate exhalations and some light gas and steam emissions. Generally, fumarolic activity was low, but clouds frequently obstructed visibility. The hazard status remained Yellow and the radius of restricted access remained at 5 km. A M 7.4 earthquake in the state of Oaxaca on 30 September did not affect the volcano.

Low-magnitude microseismic and/or tectonic events occurred occasionally. Type-A earthquake events were recorded at the following times: M 2.2 at 0141 on 14 July (preceded by a type-A microseism); M 3.3 at 2053 on 15 July; M 2.1 at 2336 on 23 July (followed by a small tectonic type-A event on 25 July); a small-magnitude event at 1638 on 29 August; and two events at 2008 and 2148 on 1 September of M 2.2 and 2.5, respectively.

Several low-magnitude tectono-volcanic earthquakes were also detected as follows: M 2.7 at 0654 on 28 July; two events at 2029 on 29 July with M 2.0 and 2.6, respectively; a M 2.5 event at 1431 on 6 September at a depth of 7.9 km from the summit and 5 km S of the crater; M 3.2 at 2047 on 8 September with its hypocenter at 7.1 km below the summit and 6 km S of the crater; and another M 3.2 event at 0834 on 27 September at a depth of 5.3 km under the summit and 6 km SSE of the crater.

Moderate exhalations starting in late August continued through September and into the first week of October. At 0920 on 27 August two small ash emissions caused light ashfall over several towns on the W flank. Another emission on 1 September caused minor ashfall. A larger event with a duration of two minutes occurred at 2205 on 5 September, causing light ashfall over several towns. At 0757 on 20 September a small exhalation ejected a plume 1 km above the summit before dispersing to the W. Two moderate exhalations occurred at 0916 and 0949 on 29 September, both lasting about 2 minutes, with ash falling W of the volcano about an hour later.

Volcanic activity during the first week of October, subsequent to a M 7.4 earthquake in the state of Oaxaca on 30 September and a number of aftershocks, remained similar to recent months. At 1101 on 3 October, a moderately large exhalation lasted for more than 15 minutes; the ash column rose to 4 km above the crater and ash fell on several towns to the SW.

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

Information Contacts: Servando De la Cruz-Reyna1, 2, Roberto Quaas1, 2; Carlos Valdés G.², and Alicia Martinez Bringas1.1-Centro Nacional de Prevencion de Desastres (CENAPRED), Delfin Madrigal 665, Col. Pedregal de Santo Domingo, Coyoacán, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); 2-Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.

The activity at Jonggring Seloko Crater in mid-September 1999 was very similar to that observed in the last recent years at the volcano. It consisted of short-lived non-sustained Vulcanian explosions producing 300-1,000-m-high ash plumes.

On 17 September there were 17 explosions witnessed during day-time hours. The time interval between two successive explosions ranged from 1 to 71 minutes, with an average of one explosion every 36 minutes. The next day 25 explosions were witnessed with 1 to 75 minutes between explosions and an average of one explosion every 32 minutes. These consistent statistics suggest that the present level of activity is lower than that observed in July 1996 and 1997 (BGVN 22:08). Of the 18 explosions closely witnessed, only two were capable of sending ballistic blocks higher than the N crater rim. All ballistic material felt back into the crater. However, the presence of fresh impact structures on the northern pyroclastic rampart of Jonggring Seloko Crater indicated that it is still occasionally showered by pyroclastic blocks.

The morphology of the crater floor changed considerably after the 1994 and 1995 eruptions. In mid-1996 and 1997 the bottom of Jonggring Seloko Crater was too deep to be visible from the NE crater rim. Observations on 18 September 1999 showed that the floor of the crater had risen several tens of meters and about 2/3 of the crater floor could be clearly seen. No evidence of lava or dome extrusion could be observed because of a thick carapace of pyroclastic ejecta and scree. The floor consists of an irregular platform. The southern part of the platform showed evidence of a recent subsidence event (scalloped normal faulting of ~10 m). The platform contained three principal active vents covered by their own ejecta. The central vent was partly surrounded by a small pyroclastic crescent.

Unsteady noisy steam emissions occurred sporadically either from the major vents or from other smaller vents on the crater floor. Larger explosions occurred only from the three principal vents and frequently progressed from the western to the eastern vent during the same explosion event. A moderate explosion at the central vent, observed from the NE crater rim, started with a booming sound followed by the noisy fallback of ballistic material into the crater. Convective uplift of the ash cloud allowed clear observation of the vent area which showed ash geysering silently ~20-40 m above the vent (with "cocktail" projections) for a few tens of seconds. The floor of the crater showed several dark areas, probably corresponding to wet zones, suggesting that water plays an important role in the explosive activity of Jonggring Seloko Crater.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Jean-Luc le Pennec, Institut de Recherche pour le Developpement and Institut de Physique du Globe de Paris. Tour 26, case 109, 4 place Jussieu, 75 252 Paris cedex 05, France; Sandrine Poteaux, 6 Villa Daviel, 75013 Paris, France; Isya N. Dana, Volcanological Survey of Indonesia, Jalan Diponegoro No 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

In mid-September, increasing seismic activity was recorded at the volcano, continuing into the first week of October. As a result of this increased activity, instrumentation for a new deformation network was installed on the W-side of the volcano and 10 new seismic stations were installed on the N-side and at other locations on the volcano. In late September, an inclinometer was installed adjacent to the seismically active area and a Yellow alert was declared, which continued as of 5 October.

Increased seismicity started on 14 September in conjunction with increased gas emissions, with plumes rising up to 3 km above the volcano. On 1 October, a column of vapor and gas rose to a height of 1 km. COSPEC measurements on 2 and 4 October indicated elevated SO₂ fluxes of ~4,300 and ~9,500 tons/day, respectively. Then on the morning of 5 October three explosions at 0721, 0738, and 0743 threw blocks of rock and ash around the crater. The largest in this sequence, at 0738, yielded a reduced displacement of 25 cm² and explosion hypocenters 4-5 km under the crater. During the night of the 4th, seismicity had reduced considerably and the activity that followed appeared to have produced a seal, leading to the subsequent explosions.

One particularly vulnerable town, Baños, was evacuated during the current crisis.

Reference. Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards. Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.

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

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

The following report, from the scientific team at the Observatorio Volcanologico de Los Andes del Sur (OVDAS), is for the period 20 August through 11 October 1999.

Since 22 August, seismic activity at Villarrica has increased from background levels, shown by an increase in the amplitude of harmonic tremor signals registered at station CVVI, located 19 km from the crater. Periods of high-amplitude tremor lasting 2-30 hours occurred, alternating with background-level tremor (banded tremor). Elevated levels of harmonic tremor lasting for hours-days preceded the last eruption in 1984. OVDAS has therefore recommended to local authorities a move to Level 2 (Green) in the "Semaforo" (traffic light) alert scheme adapted for Villarrica. If the harmonic tremor increases further in amplitude or high levels are maintained for longer periods, recommendations will be made to move to Level 3 (Amber). An energetic long-period event on 15 September, the culmination of this period of high-amplitude tremor, is considered to have been associated with a small explosive event in the crater and ash emission.

The level of seismicity rapidly decreased after 15 September to unusually low levels. Magma level in the crater lake however, is inferred to have been high on 25 September from nighttime observations of glow. Observations by local residents suggest that during the early morning of 26 September a second explosion occurred, depositing new ash. This event was not registered by CVVI so is considered to have been less energetic than the first.

On 1 October, OVDAS scientists on a helicopter flight observed that the level of the magma lake was unusually low (~200 m below the crater rim). The incandescent lava was only visible through a small opening (20-30 m) in a solid crust. Ashfall deposits extended ~5 km ESE from the crater. The deposits clearly exhibited two components, that of the Strombolian fountain (proximally) and that of the upper ash plume. A further increase in tremor amplitude and frequency was observed on 3 October. Observations of new ash and projectiles on the crater rim on the 4th suggested that this tremor episode also culminated in a small explosive event.

A new type of seismic signal, apparently strong hybrid earthquakes, was also registered at the VNVI seismic station (4 km from the crater). They have been increasing in number since 1 October (typically 2-3/day) and are not associated with any visible activity. These events do not comprise the normal background activity.

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: Gustavo Fuentealba¹, Paola Peña S., and Eliza Calder, Observatorio Volcanologico de Los Andes del Sur (OVDAS), Casilla 23D, Temuco, Chile (URL: http://www.sernageomin.cl/); 1-also at Universidad de La Frontera (UFRO), Departamento Ciencias Fisicas, Universidad de la Frontera, Instituto del Medioambiente, Avda. Francisco Salazar 01145, Casilla 54-D, Temuco, Chile.

A series of earthquake swarms began along the NW edge of Yellowstone National Park on the evening of 13 June 1999. Between 13 and 22 June over 630 earthquakes were recorded in a region ~13 km NE of the town of West Yellowstone, Montana and ~5 km SE of Grayling Creek Junction, Montana. The largest of the earthquakes, M 3.5, occurred at 1038 on 16 June. No residents reported noticing the earthquakes. The activity was located along mapped faults that extend eastward from the S end of 1959 Hebgen Lake rupture (the 7.5 magnitude Hebgen Lake earthquake was the largest in the history of the Intermountain region). Earthquake swarms are common in Yellowstone, but this was the largest since June 1997. That swarm also occurred along the NW edge of the park, the area that historically records the most persistent swarms. The most extensive recorded earthquake swarm occurred ~10 km SE of the June activity over a period of several months in 1985 and 1986.

Seismicity in the Yellowstone region is recorded by 22 University of Utah Seismograph Stations and two Global Positioning System stations. The telemetered surveillance system provides coverage for both earthquakes and ground movement related to volcanic or earthquake activity. The project is conducted cooperatively with the U.S. Geological Survey Volcano Hazards Program and the National Park Service.

As discussed by Robert B. Smith on his web pages at the University of Utah, Yellowstone National Park is located on a hotspot within the North American Plate; its three calderas are the most recent in a string that extends to the SW across Idaho. Dubbed "The Restless Giant" for its geological instability, Yellowstone could one day have another major eruption like the one that formed its youngest caldera 600,000 years ago. Symptoms include numerous earthquakes (most too small to be felt), uplift and subsidence of the ground surface, and persistent hydrothermal activity. The current rates of seismicity, ground deformation, and hydrothermal activity at Yellowstone, although high by most geologic standards, are probably typical of long time periods between eruptions and therefore not a reason for immediate concern. Scientists from the U.S. Geological Survey and the University of Utah are studying the Yellowstone region to assess the potential hazards from future earthquakes and eruptions and to provide warning if the current level of unrest should intensify.

Geologic Background. The Yellowstone Plateau volcanic field developed through three volcanic cycles spanning two million years that included some of the world's largest known eruptions. Eruption of the over 2,450 km3 Huckleberry Ridge Tuff about 2.1 million years ago created the more than 75-km-long Island Park caldera. The second cycle concluded with the eruption of the Mesa Falls Tuff around 1.3 million years ago, forming the 16-km-wide Henrys Fork caldera at the western end of the first caldera. Activity subsequently shifted to the present Yellowstone Plateau and culminated 640,000 years ago with the eruption of the over 1,000 km3 Lava Creek Tuff and the formation of the present 45 x 85 km caldera. Resurgent doming subsequently occurred at both the NE and SW sides of the caldera and voluminous (1000 km3) intracaldera rhyolitic lava flows were erupted between 150,000 and 70,000 years ago. No magmatic eruptions have occurred since the late Pleistocene, but large hydrothermal events took place near Yellowstone Lake during the Holocene. Yellowstone is presently the site of one of the world's largest hydrothermal systems, including Earth's largest concentration of geysers.

Information Contacts: U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Michael Finley, Tom Deutch, and Anne Deutch, National Park Service, P.O. Box 168, Yellowstone, WY 82190 USA (URL: https://www.nps.gov/yell/); Robert B. Smith, Department of Geology and Geophysics, 135 S. 1460 East, Room 702, University of Utah, Salt Lake City, UT 84112 USA.