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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Shallow earthquakes that began 30 March (table 1) were felt and heard on Anatahan Island, and associated with an apparent increase in thermal activity from the younger E cone's crater lake. Felt seismicity remained frequent through 1 April. Observations limited to early morning and around noon yielded reports of 9 shocks, each lasting 5-7 seconds, 31 March-1 April. No felt events were reported 2-4 April. A helicopter overflight on 1 April revealed that the crater lake had become turbulent and had changed from its usual dirty green color to a bluish gray or whitish blue. Fumarolic activity had increased and a rotten egg smell was noted. A new landslide was visible on the SW wall of the active crater. The 23 residents of the island were evacuated 4 April, and had not returned as of mid-April.

Table 1. Earthquakes near Anatahan recorded by WWSSN stations, 30 March-1 April 1990. All events were shallow, but preliminary data did not allow precise depth determinations. Courtesy of the NEIC.

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

Information Contacts: N. Banks and J. Ewert, CVO; NEIC.

"Seismicity. . . continued throughout March, although at a milder level after the 5th. Following intense February seismicity that involved 83 earthquakes of M_L >=4.0, eight of M_L >=5.0, and one of M_L >=6.0, activity was strong again 3-5 March. More than 720 earthquakes (two of M_L = 5.0-5.1 and 10 of M_L >=4.5) were recorded before seismicity decreased to 20-50 events/day of small-moderate magnitude. The energy released by the February-March seismicity was relatively large, 1.22 x 10²¹ ergs (figure 1).

Figure 1. Daily number of earthquakes (bars) and cumulative energy release (circles) near Bamus, February-March 1990. Magnitudes (ML) of larger events are given over earthquake count bars. Courtesy of RVO.

"An inspection of the Bamus area was carried out on 6 March. Rockfalls had occurred at many places on the volcano and in the limestone ranges to the S. However, no change was observed in the temperatures of the solfataric areas on the summit tholoid (which remained at <=15°C).

"Temporary seismograph networks were operated in the area 13-16 February and 6-8 March. Earthquake locations defined a broad 15-km-long seismic zone trending NNE that extended from the Nakanai Mountains to the S flank of Bamus (figure 2). Within this zone was a concentration of locations trending ENE near the S foot of Bamus. Earthquake focal depths ranged from 0 to 23 km.

Figure 2. Epicenters of seismic events at Bamus, 13-16 February and 6-8 March 1990. Courtesy of RVO.

"Cross-sections . . . (figure 3) suggest that the main cluster of earthquakes defines an ENE-trending near-vertical fault. This orientation is consistent with the structural pattern evident in the Miocene limestone immediately S of, and underlying, Bamus.

Figure 3. Focal depths of seismic events near Bamus during 13-16 February and 6-8 March 1990 projected along lines A-B (top) and A-C (bottom). Horizontal scale (and thus vertical exaggeration) changes from A-B to A-C. Courtesy of RVO.

"The cause of this seismicity remains uncertain. Its ongoing fluctuating character, and the fact that its swarms include but do not occur in response to larger earthquakes, could be consistent with magmatic injection. On the other hand, M_L 5-6 earthquakes are uncommon for magmatic events. Analysis of the magnitude/frequency distribution of the earthquakes shows that the 'b' value is ~1, which is indicative of tectonic earthquake sequences. The seismicity was continuing in early April and was being monitored primarily by the permananent seismograph at Ulawun."

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

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

Steam jets from that rose 300-400 m from fumaroles on the SE flank, 200 m below the summit, were observed during dry weather at about noon on 9 and 16 March.

Geologic Background. The late-Pleistocene to Holocene Callaqui stratovolcano has a profile of an overturned canoe, due to its construction along an 11-km-long, SW-NE fissure above a 1.2-0.3 million year old Pleistocene edifice. The ice-capped, basaltic andesite volcano contains well-preserved cones and lava flows, which have traveled up to 14 km. Small craters 100-500 m in diameter are primarily found along a fissure extending down the SW flank. Intense solfataric activity occurs at the southern portion of the summit; in 1966 and 1978, red glow was observed in fumarolic areas (Moreno 1985, pers. comm.). Periods of intense fumarolic activity have dominated; few historical eruptions are known. An explosive eruption was reported in 1751, there were uncertain accounts of eruptions in 1864 and 1937, and a small phreatic ash emission was noted in 1980.

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

A group from CICBAS (Universidad de Colima) and CONMAR (Oregon State Univ) visited the volcano 15-17 February. Since their last visit, in May 1989, rockfall avalanches have occurred preferentially on the SW flank. Fumarolic activity persisted throughout their visit, forming a dense gray cloud. Poor weather conditions limited additional observations.

The geologists emplaced geoceivers for satellite communication, to determine geodetic positions of sites near the volcano for installation of two new telemetering seismographs. On 15 December 1989, the CICBAS seismology group had installed the 4th telemetric station of the Red Sismológica Telemétrica de Colima, 7 km from the volcano (at la Yerbabuena, site EZV6 on figure 6).

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

Information Contacts: Guillermo Castellanos, Gilberto Ornelas-Arciniega, C. Ariel Ramírez-Vazquez, G.A. Reyes-Dávila, and Hector Tamez, CICBAS, Universidad de Colima.

"Spanish scientists visited Deception Island in December 1989 and January-February 1990. A geophysical station is located on the island and the Spanish oceanographic vessel Las Palmas operated in the area. Geological, tectonic, and geophysical features on and near the island were investigated. A regional, higher precision GPS geodetic network spans the Deception section of the Bransfield Rift.

"During the 1989-90 field season, an array of six digital seismic stations was installed on Deception Island. More than 1,000 events (0.5-2.1 m_b) were digitally recorded. The major shocks were located in de Neptune Bowels (S of the island). The distribution of events shows a good correlation with tectonic features on and near the island (figure 2). A low seismic velocity, high-attenuation body was inferred under the NE sector of the island. A negative magnetic anomaly (-4,900 nT) is located in the same area.

Figure 2. Distribution of seismic events (circles) recorded by the Spanish Antarctic Program seismic array (triangles) on Deception Island, 20 January-20 February 1990.

"Chemical compositions of samples from fumaroles and thermal springs suggest a thermal anomaly related to an underlying magma body. Gas geothermometry shows a formation temperature >250°C, with an outflow temperature of about 100°C. The phreatomagmatic character of the recent episodes is hypothesized as the result of a magma intrusion into shallow and confined water-saturated layers.

"A permanent seismic station monitoring the seismic activity in the area has been established at Spain's Juan Carlos I facility (35 km from Deception)."

Geologic Background. Ring-shaped Deception Island, at the SW end of the South Shetland Islands, NE of Graham Land Peninsula, was constructed along the axis of the Bransfield Rift spreading center. A narrow passageway named Neptunes Bellows provides an entrance to a natural harbor within the 8.5 x 10 km caldera that was utilized as an Antarctic whaling station. Numerous vents along ring fractures circling the low 14-km-wide island have been reported active for more than 200 years. Maars line the shores of 190-m-deep Port Foster caldera bay. Among the largest of these maars is 1-km-wide Whalers Bay, at the entrance to the harbor. Eruptions during the past 8,700 years have been dated from ash layers in lake sediments on the Antarctic Peninsula and neighboring islands.

Information Contacts: R. Ortiz, Museo Nacional de Ciencias Naturales, Spain; Rafael Soto, Real Instituto y Observatorio de la Armada, Spain.

Scientists visited the summit of Mt. Erebus several times from mid-November 1989 through mid-January 1990. Activity was at a low level compared to that of the early 1980s. Anorthoclase phonolite lava in the summit inner crater was mainly confined to two small convecting lakes; one circular and about 20 m in diameter, and the other irregular and ~20 m long. This was the largest area of convecting lava seen at Mt. Erebus since late 1984, when eruptions buried an older, larger, lava lake system. Three hornitos were actively degassing around the lava lakes, and small fumaroles were present within the inner crater.

From mid-November to mid-December, infrequent small Strombolian explosions ejected bombs to a few tens of meters from the lava lakes. A small gas bubble burst was observed in one of the hornitos. In mid-December, an increase in the frequency and size of small Strombolian eruptions was recorded by Victoria University's remote video camera mounted on the crater rim 220 m above the lava lakes. Images transmitted to Scott base, 35 km from the volcano, showed bombs being ejected to more than 100 m height.

SO₂ emission, monitored by COSPEC, has increased substantially over the previous 5 years, commonly exceeding 100 t/d. This increase was consistent with previous observations suggesting that the surface area of the lava lakes correlates with SO₂ emission rates.

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: P. Kyle and W. McIntosh, New Mexico Institute of Mining and Technology; R. Dibble, Victoria Univ.

Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio, and V. Scribano.) February activity was characterized by a single strong eruptive episode at Southeast Crater. Summit-area craters generally remained quiet through the rest of February and March. The 1-2 February eruptive episode was similar to several in January. A gradual increase in Strombolian explosions was followed by lava fountaining, and lava flowed over the crater's E rim for 5 hours beginning at 2200 on 1 February. The flow turned toward the Valle del Bove, advancing to ~ 2,000 m altitude, near the terminus of the mid-January flow. During the morning of 2 February, discontinuous Strombolian activity was followed by ejection of scoria that seldom reached a few tens of meters from the rim. Activity changed at about 1330 to energetic, discontinuous explosions that generated rumbling heard at a considerable distance. Blocks more than a meter across fell within a few hundred meters of the crater; much of the slightly vesicular ash was non-juvenile. Similar activity continued until about midnight. After the eruptive episode, the crater was completely obstructed, without any gas emission, until 27 February, when sporadic ejection of dark tephra began from two vents on the crater floor. February activity at other summit-area craters was limited to vapor emission from floors and walls. Emissions were particularly strong from Northeast Crater, where the active vent's walls were strongly incandescent.

Degassing was continuous at the summit craters in March but was not accompanied by Strombolian activity. Degassing occurred from an elliptical vent on the W floor of La Voragine accompanied by sporadic rumbling. Gas was also emitted from two sites on the SE and NW floor of Bocca Nuova. Weak fumarolic activity, from collapse steps that have formed along concentric fractures in Southeast Crater, was strongest from the center of the crater. Degassing also continued in Northeast Crater. On 29 and 30 March, sporadic tephra ejection and incandescence were observed, apparently from a sudden rise in the magma column.

Seismic activity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montaldo, M. Patanè, A. Pellegrino, and S. Spampinato.) Volcanic tremor amplitude began a progressive increase on 1 February at 1239, probably associated with increased Strombolian activity at Southeast Crater. Amplitudes peaked at 1940 that day, and at 0048 the next morning as activity was changing from Strombolian to lava fountaining. Other substantial increases in tremor amplitude occurred at 0600-1100, 1855, and 1935. The first of two sequences of discrete earthquakes on 2 February began at 0352. Eight of the events, centered at ~15 km depth on the volcano's N sector, were larger than M 1, the strongest at M 2.6 between 0424 and 0619. The second series of shocks started at 1321, with the two largest events (M 2.8) at 1322 and 1337. Hypocenters were on the Valle del Bove at <1 km depth. From 3 February until the end of the month, seismic activity was at very low levels, with little variation in tremor amplitude or the number of low-frequency shocks. Nine fracturing events exceeded M 1, with a maximum magnitude of 2.5.

Seismic activity in March was characterized by a significant increase in the number of fracturing events. Swarms on 16 and 18 March totaled 124 shocks (M>=1) and brought the month's recorded earthquakes to 153, ~ 3 times as many as in January and February. The 16 March swarm began at 0530 and continued until 0050 the next day. Of the 107 shocks stronger than M 1, 28 were of M>=2 and three of M>=3. The bulk of the most energetic events originated from the central to W part of the edifice at 10-20 km depth, although one (at 1052) was located just NNW of the central crater at ~5 km depth. The strongest shock of the 18 March sequence, which included 17 events, occurred on the SW flank (a few kilometers S of Monte Nero) at ~10-15 km depth. An M 3.3 earthquake on 22 March at 1159 was ~15 km deep, roughly 6 km SSW of the summit (just S of Monte Vetore). The March seismicity was not accompanied by changes in volcanic tremor amplitude, which remained low throughout the month. The number and amplitude of low-frequency events showed little change after 3 February. A new seismic station (PZF) was installed on the lower NW flank (near Maletto), replacing station RCC, stolen in August 1989. With the new site, IIV's Etna network numbers 8 stations.

Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. Puglisi, and R. Velardita.) Two tilt stations (SPC and CDV) operated during February, both on the S side of the volcano. Data from station SPC generally remained within resolution limits through February and March. A weak anomaly was recorded on the tangential component 18-20 February, then tangential data returned to the normal range. Radial values from recently installed station CDV remained within resolution limits through February, while tangential data began a (negative) excursion on 18 February that totalled 5 µrad by the end of the month. All instruments from this station were stolen on 1 March. Reoccupation of sites that form a triangle along the fracture zone between 1,800 and 1,500 m altitude on the S-SE flank (between benchmarks Bocche 1792, Serra Pizzuta Calvarina, and Mt. Stempato) did not show significant deformation since the previous measurements on 19 January.

Summit SO₂ flux. (T. Caltabiano and R. Romano.) Rates of SO₂ emission during Southeast Crater's eruptive episode on 2 February were three times mean values. Measurements 7, 14, and 21 February showed considerable variation. The five March measurements yielded SO₂ flux of 2,500-14,000 t/d, increasing at the end of the month.

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: R. Santacroce, IIV.

Overflights of Fuego were made on 15 and 16 February by volcanologists from INSIVUMEH and Michigan Tech. The following is from their report.

"Continuous gas emission was observed, with no evidence of any magma at the surface. The geometry of the summit crater and its surroundings (which influences the paths of pyroclastic flows during eruptive activity) was unchanged since 1980. COSPEC measurements of SO₂ emission rates were made from the air, yielding 265 ± 33 t/d on 15 February and 120 ± 30 t/d on 16 February (3 and 8 determinations respectively). These rates are very similar to the 100 t/d measured in February 1980 and much less than the rates measured in February 1978 (660-1,700 t/d) when Fuego was actively erupting (Stoiber et al., 1983; reference under Santiaguito)."

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Otoniel Matías and Rodolfo Morales, Sección de Volcanología, INSIVUMEH; W.I. Rose, Jimmy Diehl, Robert Andres, Michael Conway, and Gordon Keating, Michigan Technological Univ, USA.

Small phreatic ash emissions continued in March, accompanied by spasmodic tremor and long-period seismicity (table 2). Incandescence was mainly observed in the W part of the crater. The number of low-frequency earthquakes increased 47% relative to February values, with an 86% increase in seismic energy release. However, the number of high-frequency events decreased 38% from February and energy release declined 28% (figures 17 and 18). Most earthquakes were centered in two zones under, W of, and S of the summit (figure 19). SO₂ emissions measured on 15 and 22 March by COSPEC were at low-moderate levels, ranging from 630 to 1,380 t/d.

Table 2. Phreatic ash emissions and associated seismicity at Galeras, March 1990. Courtesy of INGEOMINAS.

Figure 17. Number of seismic events at Galeras, February 1989-March 1990. Courtesy of INGEOMINAS.

Figure 18. Daily energy release of high-frequency (dashed line) and low-frequency (solid line) seismicity at Galeras, March 1990. Courtesy of INGEOMINAS.

Figure 19. Epicenters of 67 seismic events at Galeras, March 1990. Courtesy of INGEOMINAS.

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

Information Contacts: INGEOMINAS-OVP.

After 15 months of quiet, phreatic activity began on 16 April at 0221. The activity was confined to the phreatic crater formed in 1981-82, on the NE side of the 600-m-diameter dome that occupies most of the caldera floor. Activity began with spasmodic harmonic tremor of small to intermediate amplitude, accompanied by strong fumarolic emissions generating a vapor column that rose at least 800 m. Several explosions were heard and recorded by seismographs 1.5 km and (very weakly) 9 km from the crater. Seven new fumaroles were observed within the 1981 crater, but by 17 April had joined to form a single fumarole 4 m in diameter. Non-juvenile material, rocks, and mud were thrown outward to 250 m from the vent, forming a layer 4 cm thick. The explosions enlarged the 1981 crater by ~20 m.

Precursory activity began with a M 2.3 earthquake on 5 April and a M 2.2 shock on 13 April. Only a few small events, both A- and B-type, were detected during subsequent days. The tremor had a typical frequency of 1.7 Hz on 15-17 April. Periods of tremor lasted as much as 3 hours, separated by intervals of low-amplitude tremor or quiescence. Intermittent explosions were also recorded, always associated with tremor. Only a few very small B-type events have been recorded since the onset of phreatic activity. Fumarolic waters remained at their normal temperature of 87°C.

Given the shallow character of the activity, geologists believed that it was partly related to the previous week's increased precipitation. Stepped-up monitoring and re-deployment of the Instituto Geofísico's seismic net (dismantled following the 1988 activity) were begun 16-17 April, and tilt stations and EDM lines were being resurveyed. The Instituto's hazard map and previously planned preparedness exercises for a hypothetical eruption of Guagua Pichincha were helping civil defense authorities to prepare for the possibility of increased activity.

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: M. Hall, Instituto Geofísico de la Escuela Politécnica Nacional.

December press reports in Bolivia of an eruption . . .[located 25 km NNW of Olca Volcano] remain unconfirmed, and attempts by Bolivian geologists to fly over the volcano in January were stymied by poor weather. State oil company (ENAP) geologist Patricio Sepulveda reported only normal fumarolic activity at Irruputuncu on 25 March.

Geologic Background. Irruputuncu is a small stratovolcano that straddles the Chile/Bolivia border. It is the youngest and most southerly of a NE-SW-trending chain of volcanoes. It was constructed within the collapse scarp of a Holocene debris avalanche whose deposit extends to the SW. Subsequent eruptions filled much of this scarp and produced thick, viscous lava flows down the W flank. The summit complex contains two craters, the southernmost of which is fumarolically active. The first unambiguous historical eruption took place in November 1995, when phreatic explosions produced dark ash clouds.

Information Contacts: J. Naranjo, SERNAGEOMIN.

A vigorous earthquake swarm occurred off the S flank of Hawaii 11-19 March 1990 (figure 4). More than 300 events were registered, about 15 of M 3-4, and some of M >4. Seismologists associated many of the events, including the larger ones, with processes at Lōʻihi Seamount. No acoustic signals (T-waves) were reported.

Figure 4. Portion of a seismogram recorded during Lōʻihi's 11 March 1990 earthquake swarm, by a station (AHU) 45 km from the epicentral area. Courtesy of R. Koyanagi.

Further Reference. Malahoff, A., 1987, Geology of the summit of Lōʻihi submarine volcano, in Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in Hawaii: USGS Professional Paper 1350, p. 133-144.

Geologic Background. The Kama’ehuakanaloa seamount, previously known as Loihi, lies about 35 km off the SE coast of the island of Hawaii. This youngest volcano of the Hawaiian chain has an elongated morphology dominated by two curving rift zones extending north and south of the summit. The summit region contains a caldera about 3 x 4 km and exhibits numerous lava cones, the highest of which is about 975 m below the ocean surface. The summit platform also includes two well-defined pit craters, sediment-free glassy lava, and low-temperature hydrothermal venting. An arcuate chain of small cones on the western edge of the summit extends north and south of the pit craters and merges into the crests prominent rift zones. Seismicity indicates a magmatic system distinct from that of Kilauea. During 1996 a new pit crater formed at the summit, and lava flows were erupted. Continued volcanism is expected to eventually build a new island; time estimates for the summit to reach the ocean surface range from roughly 10,000 to 100,000 years.

Information Contacts: P. Okubo and R. Koyanagi, USGS Hawaiian Volcano Observatory.

The volcano was generally quiet during a 2 February overflight (figure 1). Pre-existing thermal areas were visible in the S and SW parts of the crater, although the vent was snow-covered. Slightly warm zones were also noted on the upper S flank.

Figure 1. Summit crater of Karymsky, looking roughly SW on 2 February 1990. Courtesy of B. Ivanov.

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

Information Contacts: B. Ivanov, IV.

Seismic stations... began to record very strong acoustic (T-phase) signals, probably associated with an eruption of the... Kick-'em-Jenny... on 26 March at 1112. Overflights of the area during the period of vigorous seismicity did not reveal any water discoloration or other surface changes above the volcano, which had a summit depth of about 160 m in 1982.

Thirteen distinct seismic bursts, lasting up to 19 minutes, were recorded 26-27 March on instruments operated by the Seismic Research Unit, Univ of the West Indies. The IPGP's Mt. Pelée seismic network on Martinique, 250 km NNE of Kick-'em-Jenny, recorded strong T-waves on 26 March at 1117:22, 1502:30, 1723, and 2034 (the latter felt by residents of NW Martinique), and on 27 March at 0035:40 and 0424:25. T-waves reached IPGP's Soufrière de Guadeloupe net, 450 km N of Kick-'em-Jenny, on 26 March at 1118. The initial activity saturated the Grenada seismograph and the largest burst of seismicity, at about 1721 on 26 March, was felt on northern Grenada. After a single 14-minute episode that started at 0103 on 28 March, seismicity stopped on all but the Grenada instrument, which continued to record occasional low-frequency (0.5-2 Hz) signals for periods of about 30 seconds to more than 3 hours. The latest reported low-frequency episode occurred on 5 April between about 0500 and 0800.

Geologic Background. Kick 'em Jenny, an active submarine volcano 8 km off the N shore of Grenada, rises 1,300 m from the sea floor. Recent bathymetric surveys have shown evidence for a major arcuate collapse structure, which was the source of a submarine debris avalanche that traveled more than 15 km W. Bathymetry also revealed another submarine cone to the SE, Kick 'em Jack, and submarine lava domes to its S. These and subaerial tuff rings and lava flows at Ile de Caille and other nearby islands may represent a single large volcanic complex. Numerous eruptions have occurred since 1939, mostly documented by acoustic signals. Prior to the 1939 eruption, when an eruption cloud rose 275 m above the ocean and was witnessed by a large number of people in northern Grenada, there had been no written mention of the volcano. Eruptions have involved both explosive activity and the quiet extrusion of lava flows and lava domes in the summit crater; deep rumbling noises have sometimes been heard onshore. Recent eruptions have modified the morphology of the summit crater.

Information Contacts: W. Ambeh, K. Rowley, L. Lynch, and L. Pollard, UWI; A. Redhead, Office of the Prime Minister, Grenada; J.P. Viode and G. Boudon, Observatoire Volcanologique de la Montagne Pelée, Martinique; C. Antenor and M. Feuillard, Observatoire de la Soufrière, Guadeloupe; J.L. Cheminée, N. Girardin, and A. Hirn, IPGP Observatoires Volcanologiques, France.

Lava flows . . . remained active during the first half of March. The main (Quarry) and low-volume (Roberts) flows continued to enter the ocean, while a third (Keone) flow advanced slowly to within 600 m of a highway at 30 m elevation (figure 66). Activity was periodically observed at Pu`u `O`o. Crusted lava in Kupaianaha pond averaged 30 m below the rim and only overturned a few times/day, in contrast to vigorous past activity. On the 19th, the eruption stopped and the lava pond roofed over. Small collapse pits were found in the lava pond's crust the next day. Only residual lava from the Quarry and Roberts lava tubes drained into the ocean on the 21st.

Activity resumed on the night of the 21st, with glow reported from the East rift zone. By the next day, active lava was visible in Pu`u `O`o, had risen to 20 m below the rim at Kupaianaha, and had reoccupied the tube system to 550 m elevation. Surface lava breakouts at 550 and 600 m elevation fed two flows. Lava followed the course of the January 1990 flow between the December 1986 and 1977 aa flows, and by the end of the month had reached 200 m elevation. Lava also followed the course of the Keone flow, to within 500 m of the intersection of highways 130 and 137. Kupaianaha pond remained active through 23 March when it again began to roof over ~30 m below the rim, and by the 26th, only small pahoehoe lobes were periodically active around the pond's margins.

Seismic signals . . . marked the eruption's changes. From early to mid-March, sporadic gas pistoning was recorded, manifested as background volcanic tremor decreasing to an essentially quiet state for several minutes, generally ending with a sharp burst of energy followed by continued background tremor. This activity subsided after 17 March, succeeded by a marked increase in tremor and, on the afternoon of 18 March, brief summit deflation.

At Kīlauea's summit, swarms of long-period tremor events occurred from late 16 March through midday 18 March and from the evening of 19 March through the early morning of the 21st (figure 67). A swarm of short-period microearthquakes began later that morning and continued until early 22 March. Five hours after the onset of the summit swarm, and several hours before eruptive activity resumed, a sudden increase in earthquakes occurred in the upper East rift zone between the summit and the active craters. The hypocenters were in two areas: near Makaopuhi (roughly midway between the summit caldera rim and Kupaianaha) and Pauahi (~5 km uprift from Makaopuhi). The swarm continued until the morning of 25 March.

Figure 67. Preliminary locations of earthquakes in the Hawaii Island region, including Kīlauea and Lōʻihi, 1-26 March 1990. Courtesy of R. Koyanagi.

After lava returned to Kupaianaha on 22 March, variations in seismicity became less obvious. Tremor near Pu`u `O`o increased gradually and was relatively steady from the 24th until the end of the month.

Addendum: Eruptive activity declined on 5 April [see also 15:4], but had resumed by the night of the 6th. Lava entered Kalapana Gardens subdivision on 3 April, and within three weeks had destroyed a dozen houses.

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

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

During an overflight by geologists on 2 February, vigorous ash emission fed a large eruption column that rose to ~5 km height and had a basal diameter of ~400-600 m (figure 3). Individual ash bursts were visible at the base of the column, although ash emission appeared to be continuous. A new vent was noted at 4,500 m elev on the NE slope of the Apakhonchich valley, on the upper SE flank. Vapor jets 200-300 m high were distinctly visible above this vent. A subsidiary vent downslope (at 3,970 m elev) fed basaltic lava flows. An ash plume extended 60-80 km E. The ashfall area on 2 February was ~1,600 km².

Figure 3. Tephra cloud from Kliuchevskoi's summit crater on 2 February 1990, in photograph looking roughly E. Arrow 1 indicates the new vent at 4,600 m elev on the SE flank, arrow 2 the effusive vent at 3,970 m elev. Courtesy of B. Ivanov.

Images from the NOAA 10 and 11 polar orbiting satellites showed several plumes from Kliuchevskoi. On 22 February at 1548, a thin plume extended ~80 km SE. A plume was next visible on 10 March at 0956. Although obscured by weather clouds a short distance ENE of the volcano, it formed a distinct cold area on the infrared image, indicating that it was at relatively high altitude. On 12 March at 0335, a very thin plume stretched 15-20 km NE from the Kliuchevskoi area, and on 15 March at 0942, a small diffuse plume extended S from the volcano. A thin plume extended 250 km NE on 3 April at 0903. Weather clouds . . . may have obscured additional eruptive activity.

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

Information Contacts: B. Ivanov, IV; W. Gould, NOAA/NESDIS.

"Activity consisted of weak to moderate white-grey emissions from Crater 2. Weak, steady, red glow was observed 1-4 and 25-31 March. Rumbling noises were heard on the 28th and 29th. Crater 3 remained quiet throughout the month. Seismicity was at a low level."

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

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

After the 20 February eruption, Lascar returned to its normal fumarolic activity with the generation of mainly white plumes that rise 300-500 m above the rim of the active central crater. Between 20 and 24 March, geologists from the SERNAGEOMIN and several British universities observed the volcano from the ground and from the active crater's rim, reached on the 23rd from the N slope and on the 24th from the S slope. The following is from their report.

"Examination of photographs taken by J.R. Gerneck (Chile Hunt Oil) during the 20 February eruption revealed three discrete plumes. The first, white in color, consisted mainly of steam, and was overtaken by two smaller, grayish, higher velocity clouds. Geologists interpreted this sequence as an initial steam explosion related to the partial destruction of the dome that fills the bottom of the active crater, followed by phreatomagmatic eruptions. The eruption products, primarily fragments of the dome, occurred as shattered, dark, dense blocks of porphyritic pyroxene andesite, ranging to white, semi-vesicular, largely disaggregated blocks of similar composition, with thin, darker, quenched rims. The blocks were composed of plagioclase, clinopyroxene, and orthopyroxene phenocrysts, small amounts of magnetite, and scarce reacted olivine and hornblende crystals in a glassy groundmass. They are enriched in crystals compared to bombs from the 1986 eruption, with larger phenocrysts (up to 2 mm), and a larger proportion of pyroxene. No olivine or hornblende were found in the 1986 bombs, which included occasional xenoliths of partially molten granite. The 20 February blocks were distributed almost symmetrically in a radius of 4 km around the crater, associated with asymmetrical impact craters, elongate parallel to block trajectories. The number of blocks increased dramatically close to the vent where they covered 70-90% of the surface. No fresh ash was observed close to the volcano.

"Preliminary calculations, based on the volume of ejecta and the size of the plume, indicate that between 10 and 30% of the dome was erupted on 20 February. This estimate is supported by 5 March airphotos of the interior of the crater and by observations made from the crater rim, where a large part of the dome can still be observed in the bottom of the crater. The dome has apparently continued deflating since our last observation in November 1989 (14:11). A hole appeared to be present in its center, produced by collapse into the vent. Fumaroles were located around the dome, along ring fractures as observed in April 1989. Gas was still venting at extremely high velocity, creating the same jet-like noise reported in November. The strongest fumaroles were on the dome's NE and SW edges. A strong smell of HCl and SO₂ was recorded from the N rim. Deposits of yellow sulfur are visible associated with the fumaroles. Temperatures were measured (by Clive Oppenheimer) using an infrared radiometer (after dark, to eliminate the effects of sunlight). The fumaroles were observed to be glowing red hot and bright spots were seen over the dome. Preliminary data show the largest fumarole to have a temperature of 700-800°C, while the surface of the dome had an average temperature of 100-200°."

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

Information Contacts: M. Gardeweg, SERNAGEOMIN, Santiago; S. Matthews, Univ College London; C. Oppenheimer, Open Univ; S. Sparks and M. Stasiuk, Univ of Bristol.

Airphotos taken between 16 and 18 October 1989 by Geoff Price and 7 March 1990 by Lester Eshelman suggest that no large-volume lava flows have been extruded since June 1989. Only minor changes appear to have occurred to cones in the crater since . . . 24 June-1 July and 22-25 November 1988.

During the October 1989 overflight, clouds partially obscured the crater floor, which appeared pale gray, with a slightly darker lava flow (F13), previously seen June-August 1989, near the W wall (figure 14). Cones and vents on the crater floor had changed little since June-August 1989. A vent (T12) seen in September 1989 was no longer visible at the base of the E crater wall. A new vent (T13) had been added to the old complex (T5/T9) which now appeared as several closely spaced cones joined at the base. A possible small hornito (H6) was observed between T5/T9 and T8. The width of the overflow across the former saddle (M2M1) had not changed, but the area of lava S of the saddle may have increased slightly, particularly on the W side of the southern depression.

Figure 14. View of the N crater and southern depression at Ol Doinyo Lengai, looking roughly S between 16 and 18 October, 1989. Traced from a photograph by Geoff Price; courtesy of C. Nyamweru.

On 7 March 1990, bright sunshine and clear visibility revealed small lava flows of varying colors on the crater floor. However, none were dark gray or black, suggesting that they were of different ages and probably more than a few days (but at most a few weeks) old. No new vents were recognized, and the area of lava in the southern depression had not increased. Flow F13 was white, but had been partially covered by younger brown flows from the W side of T5/T9T13 (figure 15). Many flows of different colors were seen on its W and N slopes, including a narrow white tongue of lava (roughly 4-5 m long and 50 cm wide) stretching from the vent down the flank of the cone complex. Similar features were observed forming on T4/T7 in 1988. Several dark grooves extending from the slopes of T5/T9 appear to be narrow channels formed when a lava flow built levees, restricting it to a narrow stream. The formation of similar features was observed . . . in June and November 1988.

Figure 15. View of the N crater and southern depression at Ol Doinyo Lengai, looking roughly S on 7 March 1990. Traced from a photograph by L. Eshelman; courtesy of C. Nyamweru.

Notes on individual vents and cones are as follows: T5/T9/T13: Probable center of activity since October 1989, with emission of small thin flows from very small vents, mostly on its W slopes. The top has merged into a single broad cone with several dark patches indicating cracks or vents near the top. T4/T7: Brown and buff colors dominate. Small black patches at the top of two mounds on the E side indicate vents still open. No sign of new material extruded from these vents. Generally smooth and weathered. Lava production from T4/T7 was last reported in November 1988 (13:12). T8: Brown and buff colors dominate. Top of pinnacle appears slightly less steep. No sign of new material. Lava spattering was seen in November 1988, but only gas emission has been observed since then. T10: Gray; part of ridge that joined this cone to the E crater wall may have collapsed. Bubbling lava was seen near T10 in May 1989 (14:06). T11: Pale gray; center of cone is flat and inactive. Possible collapse at N edge. No recent lava emission was apparent and none has been reported since November 1988.

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

Information Contacts: C. Nyamweru, Kenyatta Univ.

A small explosion on 25 February, followed by the ejection of a glowing column from the main crater, was reported by Conguillio National Park administrator Omar Toledo. He added that small sediment-laden streams of water had flowed down the E flank at times when thawing does not normally occur. Field observations by geologists 5-18 March revealed occasional increases in fumarolic activity from the main crater. On 10 March, vigorous 40-60-second puffs of gas were emitted every minute during the early evening. After a summit climb, Conguillio National Park rangers reported that intense fumarolic activity produced grayish gases and a strong sulfur odor. Rockslides occurred every 1-2 hours on the NE flank.

A portable seismograph was operated 19-22 March at the volcano's W foot (in Los Paraguas National Park) by Jaime Campos and Bertrad Delovis, Dept de Geofísica, Univ de Chile. Intense volcanic earthquakes and tremor were recorded. Another portable seismograph will be installed at the NE foot (near Conguillio Lake) by Univ de la Frontera scientists.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

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

Earthquake swarm activity in the caldera's S moat continued through March. A swarm of >300 events of magnitude greater than or equal to 2.8 occurred 3 March, followed by smaller swarms on 9, 18, 28, and 30 March. The swarm on the 30th included more than 100 events, all of which were smaller than M 2. Only a few isolated events occurred beneath Mammoth Mountain. Two-color geodimeter measurements indicate that extension across the S moat and resurgent dome continued through March at the 5 ppm/year rate that began in late September.

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

Information Contacts: D. Hill, USGS Menlo Park.

The following is a report from José A. Naranjo and Hugo Moreno R. Most field observations were made in collaboration with R.S.J. Sparks and Mark Stasiuk, Bristol Univ, and Clive Oppenheimer, Open Univ.

"Field evidence suggests that the eruption from Navidad Cone ended between 22 and 25 January 1990, after 13 months of activity. Explosions with pyroclastic ejections stopped between 29 December and 10 January. José Córdoba, a teacher from Malalcahuello, observed and photographed one of the last explosions, on 27 December at 1930-2000. Strong explosions ejected bombs, and white clouds consisting mainly of water vapor rose as much as 600 m above the crater. He also observed two small landslides that originated from the cone's flank (above the vent), followed by white steam clouds that rose along the scar left on the N flank (see below). These collapses may represent the early stages of the slumping observed on 20 January.

"Chlorine gases and minor water vapor fumaroles remained along concentric fractures within the main crater 3-17 March. Compared with previous observations on 21 November and 20 January, the innermost annular fractures exhibited clear evidence of collapse, leaving scarps 1.5-2 m high (figure 16). Fumes from the outermost fractures near the crater rim yielded temperatures of 86°C.

Figure 16. View N across the crater of Navidad scoria cone, Lonquimay volcano, from the highest (S) part of the rim. 21 November 1989 (top): Concentric fractures had formed on the W side of the innermost nested crater; intense water vapor fumaroles aligned with them, and a strong steam jet was emitted from a glowing vent on the inner wall. 20 January 1990 (middle): Vapor emission had ceased and collapse had occurred along the eastern inner wall, the southern fractures, and around the N wall-vent. A funnel-shaped crater about 120 m in diameter had clearly widened by collapse since November. 5 March 1990 (bottom): Only dry gases were emitted along the annular fractures, while no fumes were visible at the main crater vents. Fractures had widened on the S part of the cone, and collapse scars appeared on the E part. Sketched from photographs by J.A. Naranjo.

"By March, the source vent was completely covered by talus from the unstable flank material above it. Discontinuous slumping of this debris left a funnel-shaped scar about 90 m high and 30 m deep, with walls that project upward through the crater's inner concentric fractures. The channel was enlarged by successive collapses that were up to 30 m deep and 25 m wide near the vent.

"The lava surface remained almost completely covered by a 1-3-m-thick mantle of debris transported on it. Former arched transverse debris ridges were disturbed and a gash of fresher lava was formed along the debris mantle's front axis. The top parts of most ridges showed higher temperatures (up to 390°C at 30 cm depth) than the almost cool gullies between them. After 20 January, the debris-covered lava advanced 120 m before it stopped flowing. This smooth surface texture conspicuously contrasted with the spiny, jagged surface presented by the blocky/aa lava immediately downstream.

"The fumaroles aligned with the central vent and the flow to the ENE showed decreased activity when compared to April 1989, although their temperatures remained at 190° and 250-300°C, 600 and 300 m from Navidad Cone respectively.

"On 17 March, a 948°C thermocouple measurement was obtained ~7 m below the lava surface, 1.5-2 km downstream from the source vent. The main lobe in the Lolco River valley had not advanced since 20 November 1989, although it showed a front thickness that had increased slightly, from 45-50 m in November to 55-60 m in March."

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

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

"Activity remained at a low level in March. The summit was obscured for long periods (4-9 and 11-23 March), but when weather cleared, emissions of white vapour in weak to moderate amounts were observed from both craters. Seismicity remained low, with daily totals of volcanic earthquakes ranging from 900 to 1,200. No significant changes were noted in seismic amplitudes and ground deformation."

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

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

The following observations, made by scientists from the USSR and New Zealand during a cruise of the RV Vulkanolog, were reported by W.F. Giggenbach and I. Menyailov.

"...Thermal activity manifests itself largely in areas of hydrothermally altered, steaming ground. The major thermal feature is a vigorously boiling pool near sea level in Sulphur Bay (Ramsay and Hayward, 1971). As indicated by the occurrence of bubble zones (Glasby, 1971), submarine thermal activity extends well SW of the island.

"During both the 1988 and 1990 cruises of the RV Vulkanolog, gas and water samples were collected from the main pool. The waters are essentially acid sulfate (4,000 mg/kg; Cl, 20 mg/kg), steam-heated, initially non-saline groundwater. Compositions of 1988 gases are compared in table 1 with those of 1974 samples from Sulphur Bay spring and the seafloor at 34 m depth (Lyon and others, 1977).

Table 1. Chemical composition of gases collected from vents on and near Whale Island (in mmol/mol of dry gas), March 1974 (Lyon and others, 1977) and during the September 1988 cruise of the RV Vulkanolog.

"All gases reflect a hydrothermal origin, and their major component is CO₂. The seafloor gases are contaminated with air, probably after sampling. Their higher CH4 and lower H₂ contents suggest longer residence at lower temperatures compared to the island samples. The composition of the latter has remained essentially unchanged over the last 14 years."

References. Glasby, G.P., 1971, Direct observation of columnar scattering associated with geothermal gas bubbling in the Bay of Plenty, New Zealand: New Zealand Journal of Marine and Freshwater Research, v. 5, p. 483-496.

Lyon, G.L., Giggenbach, W.F., Singleton, R.J., and Glasby, G.P., 1977, Isotopic and Chemical composition of submarine geothermal gases from the Bay of Plenty, New Zealand: New Zealand Department of Scientific and Industrial Research Bulletin, v. 218, p. 65-67.

Ramsay, W.R.H., and Hayward, B.W., 1971, Geology of Whale Island: Tane, v. 17, p. 9-32.

Geologic Background. Moutohora (Whale) Island forms the summit of a largely submerged Pleistocene dacitic-andesitic complex volcano that lies 11 km offshore from Whakatane in the Bay of Plenty. The 2.5-km-long island includes a central dome complex flanked, by East Dome and Pa Hill lava dome, which forms the NW tip of the island. Acid hot springs, steaming ground, and fumaroles are present on the island. The central cone and east dome are both older than the roughly 42,000 before present (BP) Rotoehu Tephra, and Pa Hill dome is overlain by the 9,000 years BP Rotoma Ash, but may be considerably older. It was included in the Catalog of Active Volcanoes of the World (Nairn and Cole, 1975) based on its thermal activity.

Information Contacts: I. Menyailov and A. Ivanenko, IV, Petropavlovsk; W. Giggenbach, DSIR Chemistry, Petone.

Fumarolic activity, accompanied by low-intensity seismicity, was described by policemen from Ujina, 15 km SW of Olca, on 13 November 1989. Minor seismicity associated with Olca was noted in mid-March 1990 by state oil company (ENAP) geologist Patricio Sepulveda.

Geologic Background. A 15-km-long E-W ridge forming the border between Chile and Bolivia is comprised of several stratovolcanoes with Holocene lava flows. Andesitic-dacitic lava flows extend as far as 5 km N from the active crater of Volcán Olca and to the north and west from vents farther to the west. Olca is flanked on the west by Cerro Michincha and on the east by Volcán Paruma, which is immediately west of the higher pre-Holocene Cerro Paruma volcano. Volcán Paruma has been the source of conspicuous fresh lava flows, one of which extends 7 km SE, and has displayed persistent fumarolic activity. The only reported historical activity from the complex was a flank eruption of unspecified character between 1865 and 1867, which SERNAGEOMIN notes is based on unconfirmed records.

Information Contacts: J. Naranjo, SERNAGEOMIN.

Volcanologists from INSIVUMEH and Michigan Tech visited Pacaya on 13, 14, 17, 18, and 28 February and 1, 2, 3, and 4 March, and flew over the volcano on 16 February. The following is from their report.

"Activity at Pacaya continued at a low level, consisting of brief (10-60 second), weak (ejecta typically thrown 2-100 m), Strombolian explosions with reposes of <1 to several minutes. All activity was from a small cone, 6 m high and 8 m wide at its rim, within MacKenney crater. The explosions were accompanied by gas emission (with jet-like noise) and often by fine ash clouds.

"On 17 February, during activity that was typical of the observation period, 78 COSPEC scans were made from a ground observation site 1.25 km from MacKenney crater (at Cerro Chino). Pacaya was emitting SO₂ at an average rate of 30 t/d, with the measured range varying between 3 and 130 t/d. Higher fluxes were directly associated with observed small explosions. The new SO₂ observations at Pacaya were much lower than values measured several times from 1972 until 1980 (Stoiber et al., 1983; reference under Santiaguito), which were generally between 250 and 1,500 t/d."

Geologic Background. Eruptions from Pacaya are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the older Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1,500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate scarp inside which the modern Pacaya volcano (Mackenney cone) grew. The NW-flank Cerro Chino crater was last active in the 19th century. During the past several decades, activity has consisted of frequent Strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and covered the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit.

Information Contacts: Otoniel Matias and Rodolfo Morales, Sección de Volcanología, INSIVUMEH; W.I. Rose, Jimmy Diehl, Robert Andres, Michael Conway, and Gordon Keating, Michigan Technological Univ.

"Activity remained at a low level in March. A total of 265 caldera earthquakes was recorded. Daily earthquake totals ranged from 0 to 24, with the highest daily total recorded in a small Greet Harbour swarm on 18 March that included two felt events (M_L 2.8 and 2.6). During the month, seismicity was broadly distributed within the caldera seismic zone. Levelling measurements on 26 March indicated deflation of 2 mm at the S tip of Matupit Island since previous measurements on 20 February."

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

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

The following observations, made by scientists from the USSR and New Zealand during a cruise of the RV Vulkanolog, were reported by W.F. Giggenbach and I. Menyailov. The island was visited on 30 January 1990.

"A considerable increase in microseismic activity to ~180 events/day, starting at the beginning of January 1990, was recorded by the Raoul Island seismic station. A similar swarm of minor shocks (Adams and Dibble, 1967) and an increase in hydrothermal activity (Healy et al., 1965) preceded the 1964 eruption. There were, however, no significant changes in the appearance and emission rate of thermal fluids from the main area of geothermal discharge along the W shore of Green Lake since the last visit of RV Vulkanolog in March 1988. Water and steam samples were collected in 1988 and 1990. The compositions of the 1988 samples are compared in table 1 with those reported by Weissberg and Sarbutt (1966) for samples collected shortly after the 1964 eruption. Gas compositions point to an essentially hydrothermal origin with insignificant contributions from high-temperature magmatic gases. Heavy seas prevented landing on Curtis Island, the other island in the Kermadecs showing thermal activity."

Table 1. Chemical composition (in mmol/mol of dry gas) of steam samples collected from the main fumarolic vents on Raoul Island in December 1964 (shortly after the 1964 eruption; Weissberg and Sarbutt, 1966) and during the March 1988 cruise of the RV Vulkanolog.

References. Adams, R.D., and Dibble, R.R., 1967, Seismological studies of the Raoul Island eruption, 1964: New Zealand Journal of Geology and Geophysics, v. 10, p. 1,348-1,361.

Weissberg, B.G., and Sarbutt, J., 1966, Chemistry of the hydrothermal waters of the volcanic eruption on Raoul Island, November 1964: New Zealand Journal of Science; v. 9, p. 426-432.

Geologic Background. Anvil-shaped Raoul Island is the largest and northernmost of the Kermadec Islands. During the past several thousand years volcanism has been dominated by dacitic explosive eruptions. Two Holocene calderas exist, the older of which cuts the center the island and is about 2.5 x 3.5 km wide. Denham caldera, formed during a major dacitic explosive eruption about 2200 years ago, truncated the W side of the island and is 6.5 x 4 km wide. Its long axis is parallel to the tectonic fabric of the Havre Trough that lies W of the volcanic arc. Historical eruptions during the 19th and 20th centuries have sometimes occurred simultaneously from both calderas, and have consisted of small-to-moderate phreatic eruptions, some of which formed ephemeral islands in Denham caldera. An unnamed submarine cone, one of several located along a fissure on the lower NNE flank, has also erupted during historical time, and satellitic vents are concentrated along two parallel NNE-trending lineaments.

Information Contacts: I. Nairn, P. Otway, B. Scott, and C. Wood, NZGS Rotorua; W. Giggenbach, DSIR Chemistry, Petone.

Quoted material is from the AVO staff. Information about the 4, 9, and 14 March explosive episodes supplements the initial reports in 15:02.

"Lava dome growth disrupted by moderate explosions and gravitational collapse continued. Since 15 February, explosive episodes have occurred at average intervals of 3-9 days (table 1). Explosive episodes were associated with pyroclastic flows and surges that triggered floods and lahars in the Drift River valley, which drains the volcano's N flank (figure 8). Seismicity remained centered on Redoubt from the surface to a depth of about 10 km, but earthquakes of M >= 2.0 have not occurred since 9 March. The summit seismometer that was damaged during the 15 February event was removed in March and three new seismometers were placed on the volcano's summit and flanks. COSPEC measurements began on 20 March; data are collected as weather permits. SO₂ emission rates have ranged from 1,600 to 6,000 t/d."

Figure 8. Sketch map of the Drift River valley and related drainages on the NE flank of Redoubt. The Drift River oil facility is between the mouth of the Drift River and Rust Slough. Courtesy of AVO.

Since early January, deposition in the Drift River's main channel has diverted significant amounts of flood water and debris into Rust Slough, S of the Drift River oil facility. An L-shaped 4-m-high levee upstream from the oil facility was designed to protect it from Drift River floods, but neither levees nor topography protect its S side. Beginning on 4 March, deposition in Rust Slough has diverted floodwater farther southward into Cannery Creek, just upstream of the Drift River facility. None of the subsequent floods associated with March-mid April explosive episodes have affected the oil facility.

Explosive episode, 4 March. "An explosive event that occurred at 2039 was recorded for 8 minutes at the Spurr station (a regional seismometer about 100 km NNE of Redoubt that has been operating since the onset of the eruption). By 2110, an ash plume was reported to an altitude of 12 km; the plume moved N20°E and ashfall occurred 225 km away. Moderate flooding occurred in the Drift River. A new diversion upstream of the Drift River oil facility caused much of the flow to be diverted S of the facility (from Rust Slough into Cannery Creek).

Explosive episode, 9 March. "An explosive event occurred at 0951 and was recorded for 10 minutes at the Spurr station. Tephra fell primarily W of the volcano; Port Alsworth, 95 km SW of the volcano, received a light dusting from the southern margin of the plume. Floodwater reached the Drift River oil facility about 2 3/4 hours after the onset of the event.

Explosive episode, 14 March. "Explosive activity that began at 0947 was recorded for 14 minutes at the Spurr station. Tephra fell E of the volcano; the Drift River oil facility reported heavy ashfall from 1057 to 1247. Oil facility crews were evacuated because of the heavy ashfall. Traces of ash were reported on the Kenai Peninsula and in the Anchorage area." Satellite images (figure 9) showed the plume moving ENE. The temperature at the top of the dense portion of the plume was -40°C at 1030, corresponding to an altitude of about 7 km. Winds were relatively light, and by 1230, the plume extended less than 150 km N and about 100 km E of the volcano.

Figure 9. Image from the NOAA 10 polar orbiting satellite, 14 March at 1054, about an hour after the onset of the eruptive episode. An elongate plume extends ENE of Redoubt. Courtesy of G. Stephens.

"Moderate flooding occurred in the lower Drift River valley. Peak flow velocity was about 6 m/sec. The flood reached the oil facility about 2 1/4 hours after the onset of the explosive episode. The flood carried numerous ice blocks and hot angular dome rocks 16 km from the glacier, where peak discharge was estimated at 1200 m³/sec.

"On 15 March, after a vigorous 2.5-minute seismic event was recorded at all seismic stations, an AVO field crew was warned about a possible explosion. They reported no changes in steam plume activity and did not hear any noises. However, 20 minutes later, they noted an approximate doubling of the Drift River's discharge 4 km downstream from the glacier. The increased discharge was accompanied by large quantities of cobble-sized ice.

"A small dome in the summit area was observed by field crews on 16, 18, 20, and 21 March. The dome appeared to be growing slowly between observations.

Explosive episode, 23 March. "Seismicity indicating the onset of explosive activity began at 0404 and was recorded for 8 minutes at the Spurr station. Seismic activity at the summit stations had increased around 0000 on 22 March and had stayed at elevated levels for most of the day. Seismic activity then decreased several hours before the 23 March explosive episode. A plume was reported to 10.5 km but appeared to be mostly steam. Light ashfall was observed W of the mountain, but ash did not fall on any community. Discharge increased in the Drift River."

An image from the NOAA 11 polar orbiting satellite at 0430 (figure 10), 26 minutes after the onset of the explosive episode, showed a plume extending WNW from the volcano. The top of the dense portion of the plume had a temperature of -39°C, yielding an altitude estimate of slightly less than 9 km based on the radiosonde temperature/altitude profile over Anchorage 1.5 hours earlier. The plume continued to move rapidly WNW, and by 1430, 10.5 hours after the explosion, its center was about 850 km from the volcano.

Figure 10. Image from the NOAA 11 polar orbiting satellite, 23 March at 0430, about 30 minutes after the start of the eruptive episode. The nearly circular plume is just WNW of Redoubt. Courtesy of G. Stephens.

"Pyroclastic flow deposits covered the lower Canyon (below 825 m) and the upper piedmont area (above 500 m) of the Drift glacier. The deposits were generally hot, dry, and friable; where they rested on snow, the basal part of thick deposits, and those less than 50 cm thick, were wet and warm to the touch. Pyroclastic deposits were still hot (325°C) when measured on 26 March.

"Views into the crater on 23 March were largely obscured by steam but much of the dome appeared missing from the summit area. Poor weather obscured observations of the summit area from 26 March until 6 April.

Explosive episode, 29 March. "Seismic activity indicated that an explosive event began at 1033 and was recorded for 7 minutes at the Spurr station. An increase in discharge of the Drift River was reported, reaching the oil facility by 1307. Pilots reported a plume, consisting chiefly of steam, to 15 km. Tephra fallout appears to have been similar to that of 4 March; light ashfall was reported to 225 km N-NE of the volcano.

"Poor weather prevented ground observations or views of the glacier. Deposits from a debris flow or hyperconcentrated flow were observed in the upper valley and flooding appeared similar to 23 March. No hot debris or ice blocks were observed in the upper valley.

Explosive episode, 6 April. "Seismicity increased throughout the morning of 6 April. An explosive event began at 1723 and was recorded for 7-8 minutes at the Spurr station. Seismicity declined after the explosive event. An ash plume was reported to 9 km; wind shear caused the lower part of the plume to drift NW and the upper part to drift E. The ash plume reached the W coast of the Kenai Peninsula by 1808, but only light ashfall was reported in Kenai during the evening.

"Pyroclastic flow deposits overlay the glacier down to about the 610 m level. A debris flow of dome-rock material and ice boulders flowed onto the Drift River valley, and peak flow velocity was estimated at 22 m/s. Peak discharge attenuated quickly downvalley.

Dome growth and hydrologic events 7-13 April. "A dome was first observed in the summit area on 7 April. This dome appeared to be larger when observed on 10 and 13 April and was greatly oversteepened on the N face.

"On 7 April, discharge near the E canyon mouth of the Drift River glacier fluctuated by 30-50% several times during a 1/2-hour observation period. A flood of ice blocks up to 1 m across caused a 4-fold discharge increase in one of the large glacier canyons. Repeated increases in discharge were noted over a 15-minute observation period. An iceslide blocked the entire width of the canyon bottom upstream of the increased discharge area. Episodic release through a tunnel at the base of the ice jam may explain the surges observed at the canyon mouth.

"On 10 April a rootless phreatic eruption was noted on the Drift Glacier at the 890 m level, causing a vigorous ash and steam plume to rise 1,000 m. A series of explosions migrated N and S of this area along a glacier bed stream, producing an elongate crater perhaps 300 m long. Numerous small pyroclastic flows emanated from the explosion area and formed a small pyroclastic flow fan that dammed the main water flow from the dome area for about an hour. Failure of the dam caused a flood with an estimated discharge of 10 m³/s.

Explosive event, 15 April. "A moderate explosive event occurred at 1440 and lasted about 8 minutes at the Spurr station. The ash plume reached elevations between 9 and 12 km and the plume moved N-NW. There were no clearly identifiable seismic precursors. Seismic activity before and after the event appeared unchanged." [See also 15:04].

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

Information Contacts: AVO Staff; SAB.

Phreatic eruptions had apparently stopped by 1 February. A possible eruption cloud was reported on 19 March, but a field inspection that day revealed only steam rising from the lake surface. There was no evidence of recent surging associated with small eruptions. Crater Lake was battleship gray with yellow and gray sulfur slicks. No convection was observed over the main vent, and only faint upwelling could be detected over the N vents. The lake temperature had cooled to 34.1°C from 46.5°C on 6 February. A sizeable lake had formed in an area of ice collapse in the valley draining Crater Lake to the S. Since 1 February, the lake had grown from ~60 ± 15 m to 100 ± 30 m. Sudden release of the lake could cause flooding in the Whangaehu River.

Volcanic tremor gradually declined in February, nearing background levels by 8 March. Continuous tremor with fairly uniform amplitude changed to bursts of tremor alternating with periods of quiet, similar to small volcanic earthquakes. On 8 March, tremor increased to high levels and broadened its frequency range, with 1 and 1.5 Hz tremor in addition to the usual 2 Hz signal. Tremor remained strong for 2-3 days before declining to more moderate amplitude. During the period of strongest activity, 6-hour energy release reached 400-1,400 x 10⁴ joules, exceeding levels that accompanied the January 1982 eruptions, but less than in September 1982, when there were no eruptions and declining lake temperature. Tremor increased again on 16 March, almost to the level of 8 March, but by the 22nd had decreased to moderate-strong amplitude. EDM measurements on four lines across the N portion of the crater detected only small (<7mm) changes since the 1 February survey.

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

Information Contacts: P. Otway, DSIR Wairakei.

The number of earthquakes and seismic energy release remained low in March. Located events were centered W and SW of the crater. The strongest recorded earthquake (M 2.1) occurred 21 March. Only a few short pulses of low-energy tremor were recorded, except for a high-energy episode on 12 March at 2301, associated with a small ash emission. Five COSPEC measurements yielded an average SO₂ flux of 1,540 t/d, similar to the previous month. Deformation measurements showed no significant changes.

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

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

The following observations, made by scientists from the USSR and New Zealand during a cruise of the RV Vulkanolog, were reported by W.F. Giggenbach and I. Menyailov.

"Considerable uncertainty remains about the minimum depth to the summit of Rumble III seamount. Early bathymetric measurements place it at 117 m depth (Kibblewhite and Denham, 1967), while later data and surveys by the RV Vulkanolog in March 1988 suggest a depth of 200 m. A special effort was therefore made to locate its highest point and to determine its depth.

"From echograms, it appears that the uncertainty may largely be due to the production of gas-rich, probably volcanic fluids from the summit area (Kibblewhite, 1966). Close inspection of the echograms shows that reflections above 200 m are probably caused by a plume of expanding bubbles, as they are invariably Separated from the solid reflector (the true summit) by a non-reflecting zone. There, the bubbles are either too small or the prevailing pressures keep the gases in solution.

"In contrast to March 1988, when echograms suggested that some of the bubble swarms reached the surface and gas bubbles were observed from the RV Vulkanolog, in January 1990 the plumes terminated at 150-120 m depth and no bubbles were observed at the surface. The disappearance of bubbles at depths <120 m is likely to be due to re-dissolution of soluble, probably volcanic gases (CO₂ and SO₂). The decrease in extent of the bubble zones may reflect a decrease in the production rate of thermal fluids and, therefore, of volcanic activity. There were no obvious signs of volcanic activity in either March 1988 or January 1990.

"Several large samples of ferro-magnesian, basaltic pillow lavas were dredged from the slopes of the seamount at depths of 400-1,200 m."

References. Kibblewhite, A.C., 1966, The acoustic detection and location of an underwater volcano: New Zealand Journal of Science, v. 9, p. 178-199.

Kibblewhite, A.C. and Denham, R.N., 1967, The Bathymetry and total magnetic field of the south Kermadec Ridge seamounts: New Zealand Journal of Science, v. 10, p. 52-69.

Geologic Background. Rumble III seamount, the largest of the Rumbles group of submarine volcanoes along the South Kermadec Ridge, rises 2,300 m from the seafloor to within about 200 m of the surface. Collapse of the edifice produced a scarp open to the west and a large debris-avalanche deposit. Fresh-looking andesitic rocks have been dredged from the summit and basaltic lava from its flanks. It has been the source of several submarine eruptions detected by hydrophone signals.

Information Contacts: I. Menyailov and A. Ivanenko, IV, Petropavlovsk; W. Giggenbach, DSIR Chemistry, Petone.

Santiaguito was visited by volcanologists from INSIVUMEH, Michigan Tech, and Arizona State 20-26 February. The following is from their report.

"Eruptive activity was still focused on Caliente vent, capped by a cone-shaped exogenous domal mass of lava that feeds a viscous flow directed toward the SSW. The flow extended about 500 m, dropping about 250 m in elevation below the top of the vent (about 2,500 m above sea level) and terminating on a talus slope at the angle of repose. Rockfalls were frequent, resulting in ash clouds. The frequency of vertical ash eruptions from Caliente vent was only a few/day. The rate of SO₂ emission was measured on 22 February at 48 ± 15 t/d, with a range of 21-76 t/d (24 determinations). This emission rate was slightly less than the average of about 100 t/d (range 40-1,600 t/d) determined in July 1976, when there were many more vertical ash eruptions that had higher values, but was identical to the emission rates measured then between eruptions (Stoiber and others, 1983; especially Table 29.4).

"Figure 12 shows the pattern of Santiaguito's activity from June 1988 until 10 January 1990, five weeks before the dates of the most recent field surveys, as revealed from interpretation of telemetered seismic data by INSIVUMEH. The data demonstrate a good correlation between the frequency of avalanche events and vertical explosions. They also demonstrate that the February field observation dates represented a time of very few vertical explosions compared to the past year's record.

Figure 12. Mean daily number of explosions (crosses) and avalanches (squares) during 2-week periods at Santiaguito, as interpreted from telemetered data by INSIVUMEH, June 1988-January 1990. The 19 June 1989 eruption is marked by an arrow.

"Significant changes have occurred on the N side of Santiaguito since July 1989 (figure 13). The El Monje dome, mostly extruded between 1947 and 1952, had developed a talus slope on its N side that was stabilized and had developed a strong moss coating that prevented rockfalls. This slope allowed access to the summit of Santiaguito throughout a long period (1964-88) and also to the 1902 crater of Santa María. Deep barrancas (canyons) have formed on the N side of the El Monje dome, cutting steep barriers into the talus slopes. These have coalesced at the edge of the talus slope, forming a large barranca between Santiaguito and Santa María that feeds an enormous amount of material into the (Isla) area farther W, and caused another deep barranca to form at the end of the Loma trail. The barrancas on the El Monje dome have deepened and migrated headward until they intersect the top of the dome. They could reflect fracturing of the El Monje dome, perhaps the weakest of three dome units that buttress the N side of the Caliente Vent. If viewed in this way the new barrancas could forecast the site of new dome extrusion from a lateral vent. The increased sediment load from this barranca system is likely to affect the Río Concepción and the Río Tambor to the south when the next rainy season arrives in April or May.

Figure 13. Simplified geologic map of Santiaguito Dome, 1922-February 1990. Streams near Santiaguito are approximately located. Unit dates, such as Rc (1922-90), represent periods of discontinuous activity at each vent. Patterned areas represent very recent activity: Rl - area of active laharic and stream deposition, and very high aggradation rates; Rd - area of recently initiated extensive mass wasting indicating inflation of the El Monje vent area and potential reactivation of the vent; Rc (v pattern) - active block lava flows on Caliente's summit, with very common (hourly) collapse of the broad toe resulting in hot rock avalanches; Rc (dotted pattern) - extent of the 1986-88 block lava flow from Caliente.

"Fieldwork was also directed at examination of the areas affected by the 19 July 1989 eruption (figure 14). The outline of a distinct blast zone, marked by tree blowdown, was mapped. A collapse scarp facing the blast zone was observed. This shows conclusively that partial domal collapse accompanied the 19 July 1989 eruption (14:07)."

Figure 14. Map of Santiaguito and vicinity, showing the zones affected by the 1929, 1973, and 1989 pyroclastic flows. The 1989 and April 1973 deposits have similar areas but different sources. Modified from Rose, 1987.

Reference. Stoiber, R.E., Malinconico, L.L. Jr., and Williams, S.N., 1983, Use of the correlation spectrometer at volcanoes, in Tazieff, H. and Sabroux, J.C., eds., Forecasting Volcanic Events; Elsevier, Amsterdam, p. 425-444.

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

Information Contacts: O. Matías and R. Morales, INSIVUMEH; W.I. Rose, J. Diehl, R. Andres, F.M. Conway, and G. Keating, Michigan Technological Univ; J. Fink and S. Anderson, Arizona State Univ.

During a 2 February overflight, an explosion vent more than 100 m in diameter was observed in the center of the [extrusive] hornblende andesite lava dome (figure 1). Minor fumarolic activity was occurring.

Figure 1. Crater and lava dome at Shiveluch, looking roughly N on 2 February 1990, showing explosion vents. Courtesy of B.V. Ivanov.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: B. Ivanov, IV.

"Activity remained at a low level in March. Summit crater emissions consisted of thick white vapour. Seismicity was low throughout the month."

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

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

Fumarolic activity at Vulcano remained at a very high level in 1989. The temperature of a fumarole (F5) on the crater rim (figure 6) has remained stable at 310 ± 5°C; more than 90 samples have been collected since July 1987. In contrast, a fumarole (FF) inside the crater showed very high temperatures, reaching a maximum of 550°C in August-September 1989, 100° hotter than in 1988. February 1990 temperatures were 515° and 312° at FF and F5 respectively.

Figure 6. Map of Vulcano, showing locations of F5 and FF fumaroles.

Major chemical species (H₂O, CO₂, H₂S, and SO₂) showed large variations in concentration (figure 7). ³He/4He ratios were very high for all crater fumaroles (~60% mantle-derived He), remaining stable during 1989 at ~ 7.5-8.0 x 10^-6. The 13C/¹²C ratio followed a similar trend to that of CO₂, with very wide oscillations from about d13C 0.00 to -2.20+. Geologists noted that the chemical and isotopic trends suggest mixing of different sources.

Figure 7. Variations in concentrations of H2O (top), CO2, (center) and SO2 and H2S (bottom) at Vulcano's fumarole F5, 1987-90. Courtesy of OV.

Seismic activity was monitored by a permanent network installed by IIV, and a digital mobile seismic network operated by OV since 1987. Seismicity was at a low level and characterized by low-energy earthquakes occurring in swarm sequences. On the basis of their wave shapes and spectral characteristics, the earthquakes were divided into "Volcano-tectonic" and "Volcanic" events (figure 8) using the classification of Latter (1981). Volcano-tectonic earthquakes outside the Fossa cone and around the island showed clear P and S phases, high frequency contents, and represented the most energetic events (M < 1.6). Volcanic-type events showed very regular wave trains that were sometimes sharply monochromatic, and were characterized by low dominant frequencies and an absence of clearly identifiable phases. Their energy reached 1011-1012 ergs and their magnitudes were negative. Particle motion analysis revealed the presence of Rayleigh and Rayleigh-like waves with a prograde rotation; the arrivals of these two phases followed one another during such earthquakes. Geologists interpreted these events, centered in the Fossa crater, as being related to fumarolic gas flow at shallow depth.

Figure 8. Seismograms showing events classified as "Volcano-tectonic" (top) and "Volcanic" (bottom) at Vulcano.

Reference. Latter, J.H., 1981, Volcanic earthquakes and their relationship to eruptions at Ruapehu and Ngāuruhoe volcanoes: JVGR, v. 9, p. 293-310.

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

Information Contacts: D. Tedesco, S. Vulcano, and G. Luongo, OV.

January 1990 fieldwork revealed no fumarolic ice towers or other signs of recent activity. A thick (<=4 m) sequence of tephra was found in blue ice at the foot of the volcano, but its vertical attitude suggested eruptions thousands of years ago.

Geologic Background. Mount Waesche is the southernmost of a N-S chain of volcanoes in central Marie Byrd Land, 20 km SW of Pliocene Mount Sidley. The Waesche shield was constructed around 1.0 Ma on the SE rim of the 10-km-wide Chang Peak caldera; pre-caldera Chang Peak lavas were erupted about 1.6 Ma. Satellitic cinder cones, some aligned along radial fissures, are located on the SW flank. The youngest dated products suggest a pulse of effusive activity between about 200,000 and 100,000 years ago.

Information Contacts: P. Kyle and W. McIntosh, New Mexico Institute of Mining and Technology; R. Dibble, Victoria Univ.

Little eruptive activity has occurred since 29 November fieldwork revealed a new vent and fresh tephra on the main crater floor. Seismic activity has been at low levels, fumarole temperatures have decreased, and deflation on the main crater floor (centered in the Donald Duck area) suggests that heatflow has been redirected from Noisy Nellie fumarole westward to 1978 Crater. R. Fleming reported a small eruption of lithic accessory ejecta from Noisy Nellie in late January 1990, and further collapse of Corporate and Congress Craters.

Geologists from the RV Vulkanolog visited White Island 2-3 March. Only blue "flames" associated with fumarolic discharge were seen over fumaroles E of 1978 Crater (Donald Mound, Blue Duck, and Noisy Nellie) during the night of 2 March. The three most vigorous vents along a small cone on R.F. crater's floor glowed pale red (500-550°C) and a small eruptive episode on 3 March added pebble-sized material to the cone. A shallow green pond that occupied the rest of the crater floor was surrounded by yellow to orange precipitates.

On 6 March geologists found only 4 mm of fine green ash that had fallen since 29 November at a site 35 m E of 1978 Crater. No new ash was found on the 1978 Crater rim or to the SE (S of Donald Mound). Donald Duck emitted white gas/steam clouds, and low-pressure gas emerged from Noisy Nellie. Accessory blocks and smaller ejecta, first seen about a month earlier, extended 30 m SE from Noisy Nellie. Emissions from 1978 Crater obscured R.F. and Corporate craters, but small detonations from R.F. Crater were frequently heard.

Only ~10 small B-type events/day and an average of ~3 A-types/day were recorded in December, with small E-types recorded on the 7th and 21st. About 3-6 B-type events/day plus rare A-types were recorded during January and February, with tremor nearly absent.

A March deformation survey showed strong subsidence of the Donald Mound area following a period of brief uplift measured 29 November. Subsidence since then was centered E of 1978 Crater (between Noisy Nellie and Donald Mound), reaching 30 mm near Donald Duck vent, with a trough extending NW along the line of fumaroles. Noisy Nellie, near the apparent center of the 15+ mm uplift prior to 29 November, lies on the edge of this trough. The recent subsidence of 9 mm/month is similar to the rate observed since mid-1987.

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

Information Contacts: I. Nairn, P. Otway, B. Scott, and C. Wood, NZGS Rotorua; W. Giggenbach, DSIR Chemistry, Petone.

The following observations, made by scientists from the USSR and New Zealand during a cruise of the RV Vulkanolog, are reported by W.F. Giggenbach and I. Menyailov.

"Calypso Mound is a white anhydrite cone some 6-8 m high, formed at 167 m depth by discharge of thermal waters at the ocean floor. It was discovered in February 1987 using the diving vessel Soucoup carried on the RV Calypso (Sarano and others, 1989). It lies within one of the 'bubble zones' extending in a line from White Island to Whale Island in the Bay of Plenty (Duncan and Pantin, 1969) [around 37.64°S, 177.10°E].

"The echograms indicated strong hydrothermal activity with a number of vents producing bubble curtains. However, an extended visual search under calm conditions from both the RV Vulkanolog and a rubber dinghy detected no bubbles at the surface. A possible explanation is re-dissolution of the gas in seawater. Similar gases, collected from more shallow submarine springs in the Bay of Plenty, S of Whale Island, and from Whale Island itself (see below), consisted predominantly of CO₂, which has a comparatively high solubility in water. Re-dissolution is also supported by the distribution of reflections recorded during a slow pass over the area. Most of the individual bubble swarms, now clearly separated, appeared to terminate at ~20 m depth.

"Close inspection of a video recording shows that the fluid discharged from two vents on Calypso Mound is very likely to contain a considerable free vapor phase, indicated by flame-like tongues of free vapor, rapidly quenched on contact with cold seawater. Water leaving the vapor-seawater interaction zone appeared clear and colorless except for schlieren indicating a density difference from seawater.

"The existence of free vapor at 167 m depth and about 18 bars pressure suggests that the temperature of the fluid discharged from Calypso Mound is close to 207°C. The high proportion of vapor, apparently present in the fluid mixture leaving the vents, would indicate high corresponding enthalpies of the fluid feeding Calypso Mound. The temperature of any initial single phase liquid, before flashing and possibly present at greater depth, may therefore be considerably higher. However, Sarano et al. (1989) consider it unlikely that the waters emitted from Calypso Mound were as hot as 160°C. The 'hydrothermal' nature indicated for the Calypso Mound system may also explain the enrichment in typically 'epithermal' elements such as As, Sb, Hg, and Tl, and the absence of a 'volcanic' trace metal signature (Giggenbach and Glasby, 1977) in clays recovered from near the main cone."

References. Duncan, A.R., and Pantin, H.M., 1969, Evidence for submarine geothermal activity in the Bay of Plenty: New Zealand Journal of Marine and Freshwater Research, v. 3, p. 602-606.

Giggenbach, W.F., and Glasby, G.P., 1977, The influence of thermal activity on the trace metal distribution in marine sediments around White Island, New Zealand: New Zealand Department of Scientific and Industrial Research Bulletin, v. 218, p. 121-126.

Sarano, F., Murphy, R.C., Houghton, B.F., and Hedenquist, J.W., 1989, Preliminary observations of submarine geothermal activity in the vicinity of White Island, Taupo Volcanic Zone, New Zealand: Journal of the Royal Society of New Zealand, v. 19, p. 449-459.

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

Information Contacts: I. Menyailov and A. Ivanenko, IV, Petropavlovsk; W. Giggenbach, DSIR Chemistry, Petone.

On 2 February, fumarolic activity was noted in two vents inside the active crater and two vents to the W (figure 1).

Figure 1. Active fumarolic vents at Zhupanovsky, looking roughly E on 2 February 1990. Courtesy of B. Ivanov.

Geologic Background. The Zhupanovsky volcanic massif consists of four overlapping stratovolcanoes along a WNW-trending ridge. The elongated complex was constructed within a Pliocene-early Pleistocene caldera whose rim is exposed only on the eastern side. Three of the stratovolcanoes were built during the Pleistocene. An early Holocene stage of frequent moderate and weak eruptions from 7,000 to 5,000 years before present (BP) was followed by a period of infrequent larger eruptions that produced pyroclastic flows. The last major eruption took place about 800-900 BP. Recorded eruptions have consisted of relatively minor explosions from Priemysh, the third cone from the E about 2.5 km from the summit peak.

Information Contacts: B. Ivanov, IV.