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/).

This report summarizes observations and monitoring data from Kīlauea during January-December 2013; activity during 2010-12 was covered in BGVN 38:05. The primary reporting source was the U.S. Geological Survey-Hawaiian Volcano Observatory (HVO) which provided monitoring and communication resources for the Hawaiian volcanoes, namely Kīlauea, Mauna Loa, Mauna Kea, Hualalai, and Lo`ihi.

2013 Overview. During 2013, Kīlauea's summit lava lake persisted , and lava flows erupted from Pu'u 'O'o. Two minor ocean entries were visible during the year until mid-July; both were branches of the Peace Day flow while, later in the year, two lava flows (Kahauale'a 1 and Kahauale'a 2) extended N-NE from Pu'u 'O'o. Both lava flows crossed into the nearby forest, causing fires and significant smoke along their margins. Petrology of the summit tephra and East Rift Zone (ERZ) did not show significant changes during the year. SO₂ emissions from both, the summit and the active ERZ were closely monitored by HVO and those observations led to new innovations in quantifying the flux. HVO reported that SO₂ and also CO₂ fluxes were relatively low but still above safe levels as established by the Centers for Disease Control and Prevention. Flank deformation and seismic monitoring determined that, although variable conditions were detected, very little accumulated change had occurred at Kīlauea.

Due to the Federal shutdown during 1-16 October 2013, HVO focused on only the most critical operations. Activities that were not directly related to critical operations were postponed, including research and outreach.

One such outreach opportunity that became curtailed was the first Great ShakeOut for Hawai`i which took place on 17 October 2013 and included almost 16,000 participants across all of the islands. This was a large-scale earthquake drill that followed in the tradition of The Great Southern California ShakeOut, which took place in 2008. HVO partnered with the State and County Civil Defense, Center for the Study of Active Volcanoes (CSAV), National Oceanic and Atmospheric Administration (NOAA), University of Hawai`i (UH-Hilo), American Red Cross, and FEMA. HVO staff generated a significant amount of information for the media including several press releases and web content; they also attended preparedness fairs and gave public talks.

Persistent thermal anomalies during 2013. More than 200 alerts per month were released by the MODVOLC program during 2013 for the Big Island of Hawai`i (figure 211). These alerts came from sites around the island that exhibited elevated radiance and were dominated by Pu'u 'O'o and the Kīlauea summit. One exceptional thermal anomaly was a site along the Mamalahoa Highway (Highway 190) in the NW sector of the island. News sources reported that, during 25-26 November 2013, a significant brushfire burned 300 acres in South Kohala. The burn site was near the highway mile marker 14 and caused segments of the highway to close while emergency crews contained the fire.

Figure 211. More than 200 thermal alerts were posted each month in 2013 by the MODVOLC program for the island of Hawai`i. This image captures the thermal alerts registered during January-December 2013. Note the concentration of red-to-yellow thermal alert pixels at the summit of Kīlauea and at the Pu'u 'O'o vent along the E rift that also reached the sea. The anomalous pixel located N of Hualalai (green box) was attributed to a fire that burned near Highway 190 during 25-26 November 2013. Courtesy of MODVOLC.

Summit lava lake activity. "Now in its sixth year, the current summit eruption harks back to the persistent lava lake in Halema`uma`u during the 1800s and early 1900s, suggesting that it has the potential to last for many years" (Patrick and others, 2013). Based on Hawai`i's written record, one earlier summit lava lake occupied Halema`uma`u during 1823-1924.

During 2013, the summit lava lake within the Overlook crater, a nested crater within Halema`uma`u, fluctuated in height, by tens of meters, resulting in perched lava deposits (bathtub rings) and collapse of the crater walls. The crater englarged slightly as a result.

Also, observers frequently noted nighttime incandescence (figure 212). Local webcameras (infrared and visible-light) captured images of the lava lake from the Halema`uma`u Overlook site as well as from the highest point of the HVO facility.

Figure 212. During 2008-2013, an active lava lake resided within the Overlook crater, a feature within Halema`uma`u crater at Kīlauea's summit. A) This shaded relief map indicates where HVO installed a thermal camera (HT cam) to view the entire surface of the lava lake. HVO and the summit tiltmeter (UWE) are 1.9-2.0 km from the lava lake. B) This oblique view is an aerial photo of the SE crater rim of Halema`uma`u. Modified from Patrick and others (2014).

The HT infrared camera occasionally documented crater rim collapse events in 2013. These events were relatively small-sized and tended to occur more frequently when the lava lake level was relatively deep within the Overlook crater (for example, a small collapse occurred when the lava lake was at a depth of ~75 m during 25-26 July 2013). When the lava lake was high, however, the interior walls were subjected to heating and cracking and HVO scientists concluded that collapse events could be triggered during these conditions as well. One collapse event, on 15 November 2013, was likely triggered by slumping due to heavy rain; several Park Rangers observed the event and the collapse was heard by an HVO scientist standing at the Jaggar Overlook (the same location shared with HVO on the crater rim).

A crust of lava had formed an inner rim within the Overlook crater and, on 25 July at 2033, a portion of that rim collapsed into the lava lake (figure 213). The main event was followed by smaller collapses of the deep inner ledge during the following day. Based on webcamera images, explosive events were not triggered by the collapse. HVO reported that, since the formation of the lava lake (March 2008), the largest gas-and-ash emissions from the summit were triggered by gravitational collapses along the crater rim; when rockfalls hit the convecting lava's surface, violent gas release could occur.

Figure 213. These two thermal images were taken before (25 July 2013) and after (30 July 2013) the collapse of the inner rim of Kīlauea's Overlook crater. The inner rim had been constructed during high lake levels in October 2012 (~22 m below the Halema`uma`u crater floor). The webcam (HT cam) was located on rim of Halema`uma`u crater and it captured a new image every ~15 minutes; the temperature scale is in degrees Celsius up to a maximum of 500°C and it automatically scaled based on the maximum and minimum temperatures within the frame. At the time of these photos, the surface of the lava lake was ~75 m below the Overlook crater rim. Courtesy of HVO.

According to HVO, the lava lake level within the Overlook crater generally fluctuated 30-60 m below the rim during 2013. A laser rangefinder was used to obtain regular measurements during the year. Lava was closest to the rim and flooded part of the inner ledge of the crater in January 2013 (an event that also occurred in October 2012). Lava at the flooded lake's margin chilled and reinforced the bathtub-like ring that persisted above the active lava surface (note the "inner rim" in figure 213). In daily online reports, HVO noted: "The lake level responds to summit tilt changes with the lake generally receding during deflation and rising during inflation."

Starting in 2009, HVO scientists noted rise/fall events and determined that the pattern began with decreasing tremor from the summit at a time when lava rose within the lake, spattering would decrease or completely stop, and summit tilt would also decrease. "After a period of minutes to hours, the lava will abruptly drain back to its previous level amidst resumed vigorous spattering, seismic tremor amplitude will increase for a short time (a seismic tremor burst) before resuming background levels, and summit tilt will return to its previous level. Gas emissions decrease significantly during the high lava stand (the plume gets wispy), and resume during its draining phase. Taken together, the geophysical characteristics suggest that, during the high lava stand, lava is puffed up with gas trapped under the lava lake crust."

During 2013, explosive events at the summit rarely occurred; intermittent spattering and degassing dominated summit activity. The plume from the vent continued to deposit variable amounts of ash, spatter, and Pele's hair onto nearby areas, particularly downwind of the crater (figure 214). The Overlook crater diameter was 35 m in March 2008 and, by the end of 2013, the dimensions had increased to 160 x 215 m. The size increase followed minor explosions and rockfalls from the interior crater walls.

Figure 214. Pele's hair (fine strands of natural glass) continued to accumulate downwind of Kīlauea's active summit crater. A) This photograph from 3 May 2012 was taken looking along the curb of the Halema`uma`u parking lot (closed to the public since the onset of summit activity in 2008), and shows a mat of Pele's hair accumulated on the windward side of the parking curb. Courtesy of Matthew Patrick, USGS. B) On 9 December 2013, a continuous carpet of Pele's hair was observed shining like gold near the Halema`uma`u Overlook trail next to the parking lot. Courtesy of Ben Gaddis, USGS.

Kīlauea's Overlook crater lava lake produced a small explosion during 2148-2149 on 23 August 2013 (figure 215). A portion of the overhanging SE crater rim collapsed and struck the surface of the lava lake. The debris had fallen into an area where nearly persistent spattering had previously been observed. The ensuing explosion generated a plume containing ash, lapilli, bombs (up to 34 cm in diameter), and lithics (ash, lapilli, and blocks up to 10 cm in diameter). The plume deposited material across the Overlook area. The level of the lava lake had been measured as ~38 m below the rim of Halema`uma`u crater earlier that day. Normal conditions prevailed after visibility returned within the camera's field of view at ~2149.

Figure 215. Images captured between 2148 and 2149 on 23 August 2013 by the HT camera (see figure 212 for location) during a small explosion from Kīlauea's lava lake. The thermal images A-D highlight the incandescence that persisted from the lake's surface as well as the hot spatter and debris that exploded after a portion of the inner crater rim fell into the lava lake. Courtesy of HVO.

Pu'u 'O'o and East Rift Zone lava flows. The Pu'u 'O'o eruption consisted of three lava flows during 2013: the Peace Day, Kahauale'a, and Kahauale'a 2 flows (figure 216). The Kahauale'a flows were unique in that they traveled N of the rift zone, unlike the numerous other lava flows that have spread generally toward the ocean (including the Peace Day flow) (figure 217). This activity was considered the continuation of Episode 61, which began on 20 August 2011 and continued through the end of this reporting period (December 2013).

Figure 216. The "spillway"—Pu`u `O`o's eastern flank—has been buried by flows fed mostly from a spatter cone on the NE side of the crater floor. Most of the dark-colored lava in the foreground is new lava that has resurfaced the spillway during the past year. The fume to the left is the trace of the Peace Day tube which carried lava to the coast and had been covered by lava flows from the crater . The tube carrying lava to the NE is inconspicuous, but extends toward the lower right side of the photo. Photo taken on 25 February 2013. Courtesy of HVO.

Figure 217. Geologic map of Kīlauea's East Rift Zone and lava flows from the active vent, Pu`u `O`o. The distribution of lava flows emplaced during 2013 is shaded red (bright red, pink, and red-orange). The yellow lines extending from Pu`u `O`o represent the general path of lava tubes that directed the flows Peace Day, Kahauale'a 1, and Kahauale'a 2. Changes to the surface area of the Kahauale'a 2 and Peace Day flows are shaded bright red, corresponding to activity during 19 September-26 December and 19 September-2 November 2013 respectively. Note that Kahauale'a 1 was active during 19 January-17 April 2013. Courtesy of HVO.

The morphology of Pu'u 'O'o crater was relatively stable through 2013. The crater remained very shallow and at or near the level of the original E spillway rim (figure 216). There were four spatter cones, all consistently active and often exhibiting incandescent openings at their tops (figure 218). These cones also emitted gas-jetting sounds and occasional, effusive spattering. The main center of activity through the year was the NE spatter cone. This cone often hosted a small lava pond and served as the vent for the Kahauale'a and Kahauale'a 2 flows.

Figure 218. A small lava lake, several meters in diameter, had persisted for nearly a year on the NE side of the Pu`u `O`o crater. The lake was perched several meters above the surrounding crater floor (seen behind the topographic high, shrouded in steam). The feature was near the top of a mound of lava composed of spatter cones and lava lake overflows. Flows from the lake and other nearby spatter cones had inundated the E rim of Pu`u `O`o's crater, which would normally be visible in the background just behind the area seen here. Photo taken on 31 January 2013. Courtesy of HVO.

In late 2012, Pu'u 'O'o crater was slowly infilling, and by the beginning of 2013, lava from the NE spatter cone reached the E spillway rim. A dramatic inflation event in mid-January triggered numerous overflows from the NE spatter cone, and the SE cone spread more lava across the crater floor but also sent flows over the E spillway. On 19 January, an overflow from the NE spatter cone sent lava down the E spillway in what would become the Kahauale'a flow. Over the next month, overflows from the cone covered much of the E spillway. Inflation in late April correlated with abundant venting and more overflows from four cones on the crater floor, with some spilling out toward the E, adding to the recent flows mantling the upper E flank of Pu'u 'O'o (figure 219). After the Kahauale'a flow eventually stalled in April, overflows in early May from the NE spatter cone fed a new flow, following the same course; this became the Kahauale'a 2 flow. Small overflows occurred sporadically from the cones through the remainder of the year, with larger events in mid-August and mid-November.

Figure 219. This thermal image of Pu`u `O`o was captured on 27 November 2013. The SE and NE spatter cones had produced small flows that extended out of the crater, shown clearly here by their warm temperatures. The vent for the Kahauale`a 2 flow is at the NE spatter cone, and the lava tube supplying the Kahauale`a 2 flow is obvious as the line of elevated temperatures extending to the lower right corner of the image. The distance between the black scarps is ~ 300 m. Courtesy of HVO.

Coastal plain lava flows and ocean entries. The Peace Day flow (episode 61b) began on 21 September 2011, and it was active for much of 2013 before ceasing in November 2013. This lava flow reached the sea and generated scattered, branching flows (breakouts) on the coastal plain, as well as several isolated breakouts above the pali (fault scarp).

The ocean entry consisted of two main entry points during 2013, with an E entry at Kupapa`u (just E of the Park boundary) and a smaller, weaker entry immediately to its W (within the Park). These entry points were not vigorous; there were little-to-no-observed littoral explosions; a delta formed that extended several meters out from the sea cliff (figure 220 A).The view from the E margin of the Peace Day flow field on the sea cliff was relatively good, and the ocean entry provided a destination for guide services (not all sanctioned) operating out of Kalapana (numerous, possibly over 100 tourists made the hour-long walk out to the site each evening). As activity on the coastal plain declined in the summer, the W entry shut down in mid-July; the E entry ceased on 21 August.

Figure 220. Thermal images have helped HVO geologists map Kīlauea's lava tube system. (A) This thermal image from 27 June 2013 shows Kīlauea's E ocean entry (spanning ~ 1 km along the shore) at Kupapa`u Point. Just inland from the entry point a patch of slightly warmer temperatures indicates an area of recent small breakouts. Inland from this warm patch you can see a narrow line of elevated temperatures that traces the path of the lava tube beneath the surface that is supplying lava to this ocean entry. Two plumes of higher temperature water (~50°C in areas close to the ocean entries) spread out from the entry point. Courtesy of HVO. (B) This image shows the Peace Day lava tube coming down the pali in Royal Gardens subdivision on 24 May 2013. The lava tube parallels Ali`i avenue (see figure 217 for the location of Royal Gardens), shown by the straight line of warm temperatures that represent asphalt heated in the sun. This tube feeds lava to the ocean entry and breakouts on the coastal plain. There is no active lava on the surface in this image - the warm surface temperatures are due to heating by the underlying lava tube. Courtesy of HVO.

Most of the Peace Day flow activity during 2013 was constrained to the coastal plain. From January through August, the coastal plain featured episodic of breakouts near the base of the pali in Royal Gardens. Those branches from the main flow slowly migrated toward the ocean before halting on the coastal plain (figure 220 B). Eventually, another breakout occurred at the base of the pali and sent out another flow that presumably drained supply from the previous flow. The new flow reached the location of the stagnating previous flow, and the flows became an indistinguishable mix of small, scattered breakouts in the middle of the coastal plain (figure 221). Minor, scattered breakouts were common on the coastal plain during January-August. Activity levels declined by August, the ocean entry diminished, and the last coastal plain flows ended around 8 September. With no ocean entry or surface flows, the coastal plain (and Kalapana-based lava tourism) became quiet again.

Figure 221. Lava flows from Pu'u 'O'o consisted of only a few scattered breakouts near the shoreline on 18 January 2013, with most of the activity focused on the coastal plain closer to the base of the pali. This pahoehoe lobe (~1 m wide) was active near the E margin of the Peace Day flow field just a few hundred meters from the coastline. Courtesy of HVO.

Lava flow activity above the pali. From mid-January to the end of May 2013, a large amount of lava escaped the Peace Day tube to create a divergent flow above Royal Gardens. It did not advance very far until April, when it crept slowly downslope into the upper reaches of Royal Gardens. This breakout flow ceased on 30 May. As the coastal plain breakouts progressively decreased during September, two small flows appeared above the pali, presumably resulting from the abandonment of the lower Peace Day lava tube. The smaller of the two breakouts was at the top of Royal Gardens, about 6 km from Pu'u 'O'o, and appeared to start between 7 and 14 September but was inactive by mid-October (timing was determined in large part by satellite images as opposed to direct observation). This small breakout flow was visible from the Kalapana lava-viewing area.

The larger of the two breakouts began around 5 September and was about 3 km SE of Pu'u 'O'o, advancing a little over a kilometer before stalling. This breakout was active until 7 November, when it and the rest of the Peace Day flow stalled. This wasn't the end, however, and the Peace Day flow gasped a final breath when a very small, brief breakout occurred on the upper Peace Day lava tube, near Pu'u Halulu, on 15 November. It was probably active for only minutes or hours and marked the end of the Peace Day flow.

The Kahauale'a flow (episode 61c) began as an overflow from the NE spatter cone on 19 January 2013, occurring simultaneously as Kīlauea inflated. It advanced down the NE flank of Pu'u 'O'o, N of the Peace Day tube, until it hit flat topography N of the cone where it developed a lava tube and covered early Pu'u 'O'o 'a'a flows. The flow consisted of scattered pahoehoe lobes, and these migrated slowly (~50 m/day) E toward Kahauale'a cone, reaching it in mid-February (figure 222). From there, it followed the N margin of an earlier flow emplaced during the episode 58 flow. The path of this new flow abutted the steep northern slope of the 2007-2008 perched lava channel. This confinement led to a narrowing of the advancing flow front, resulting in increased advance rates (>100 m/day) in early March. As the front passed the perched channel, it became less confined, and advance rates dropped to under 50 m/day. By the first week of April, the flow had reached 4.9 km from the vent on Pu'u 'O'o but ceased on 17 April during a deflation-inflation (DI) event (see figure 199 in Bulletin 38:02 where DI events are illustrated). Due to infrequent overflights by HVO scientists during 2013 (resulting from budget cuts), staff relied heavily on satellite images--particularly EO-1 Advanced Land Imager images--to track the advance of the flow.

Figure 222. Kahauale`a Cone, a local topographic high several hundred meters long, has long been a small oasis of vegetation in the midst of Pu`u `O`o lava. This photo from 19 March 2013 shows new lava from the active Kahauale`a flow surrounding the cone, which has also partly burned. Vent structures (such as episode 58, active from 2007 to 2011), are in the background just behind Kahauale`a. Pu`u `O`o is out of sight to the right. Courtesy of HVO.

HVO noted that the Kahauale'a flow was unusual in that the most recent flows from Pu'u 'O'o traveled S toward the ocean, providing minimal threat to residential areas. The Kahauale'a flow, however, was directed N of the rift zone, along a NE trend. This put the flow on a downslope trajectory that could have threatened residential areas of including Ainaloa and Paradise Park. HVO and Hawai'i County Civil Defense increased their communications through that time period but just a few weeks later, in mid-April, an abrupt change in magma supply occurred at Pu'u 'O'o and the flow ceased.

Inflation at Pu'u 'O'o produced another overflow from the NE spatter cone, which started on 6 May. This became the Kahauale'a 2 flow (episode 61d) and was directed slightly more to the N by the original Kahauale'a flow, reaching the forest boundary ~2 km NW of Pu'u 'O'o in early June. These flows invaded the forest a short distance and created steady forest fires. During July, the flow front took a more northeasterly course, following the N margin of the original Kahauale'a flow (figure 223). Its advance slowed during late July to mid-August, but during September the advance increased when the flow entered the previously mentioned narrow channel along the episode 58 perched lava channel.

Figure 223. (top) A photo taken looking W from a helicopter on 19 September 2013 of the burning forest due to Kīlauea's Kahauale`a 2 flow (approximately 7 km long). This lava flow extended from Kīlauea's Pu'u 'O'o NE vent. Active breakouts on the Kahauale`a 2 were scattered over a broad area. Here, a breakout near the edge of the forest engulfed trees and burned dead foliage. Courtesy of HVO. (bottom) The flow front of the Kahauale`a 2 flow cut a narrow swath through forest NE of Pu`u `O`o. The narrow lobe at the front was inactive at the time of this photo on 27 November 2013, with the main area of surface flows about 2 km behind the end of this lobe. Some of these surface flows slowly expanded N into the forest, igniting fires. Pu`u `O`o is in the upper left, ~7 km SW. Courtesy of HVO.

By mid-October, Kahauale'a 2's narrow flow front had reached the distant forest boundary and surpassed the length of the original Kahauale'a flow. A narrow finger of lava forming the flow front advanced into the forest in mid-November, reaching just over 7 km distance from Pu'u 'O'o, before stalling soon after 20 November. Behind the flow front, branching flows began to migrate along a more northerly direction into the forest, triggering more fires. This area of breakouts soon turned NE, paralleling the narrow finger that had stalled in late November. By 26 December, the active flow front was 6.3 km NE of the vent and persisted into the New Year.

Petrology of the summit (Halema`uma`u) and rift (Pu`u `O`o) lavas. From 2013 to 2014, the juvenile component of Kīlauea's summit tephra remained essentially as it had during 2008-2013. The overall temporal variation of summit lava mimicked the MgO systematics of ERZ lava for the 2008-2014 interval, with summit glass compositions overlapping those of contemporaneous bulk ERZ lava but erupting 20° to 25°C hotter than at Pu'u 'O'o. There were no changes in trace-element signatures, which matched those of the East Rift Zone (ERZ) lava. Halema'uma'u vent tephra remained sparsely olivine and spinel phyric with ~2 volume percent of 100-300 μm, subhedral to euhedral olivine phenocrysts (typically with melt inclusions). Olivine in summit glasses was consistently complemented by >0.05 volume percent of chromian-spinel microphenocrysts.

2013 Pu'u 'O'o lava also did not show any significant petrologic changes. It contained a five-phase assemblage: olivine(-spinel)-augite-plagioclase-liquid. The assemblage was interpreted as the result of simultaneous growth and dissolution of phenocrysts, reflecting the modeled values for cooling, fractionation, and mixing in the shallow edifice prior to eruption. This multi-phyric condition (see figure 224 for photomicrograph examples from previous years), which had persisted in the steady-state ERZ lava for most of the last ~15 years, attested to a stable shallow magmatic condition perpetuated by near-continuous recharge and eruption.

Figure 224. Two photomicrographs of Pu'u 'O'o thin sections sampled by HVO scientists from vent (a) and lava tube activity (b) during 1996-1998 (100 μm = 0.1 mm). Both show glass containing olivine phenocrysts with melt inclusions and opaque microphenocrysts of spinel. Image B shows an olivine phenocryst and spinel microphenocrysts in glass with round vesicles (one is located behind "b"). Modified from Roeder and others (2003).

SO₂ emission rates. During 2013, HVO reported notable advances in measuring the dense, opaque summit SO₂ plume. It was significant to note that the summit SO₂ emission rates measured since 2008 represented a minimum constraint on emissions, whereas by the end of 2013 it was possible to determine a more accurate estimate of the amount of gas emitted from the Overlook crater. Because traditional gas measuring techniques are subject to multiple scattering effects from incoming radiation that can contribute to significant errors in the calculated SO₂ emission rates, HVO scientists were pursued various approaches to achieve a more accurate emission rate.

HVO scientists addressed the issue of underestimation due to scattered light in two ways: (1) minimize and/or model the effects of scattering on the retrieved results and (2) measure farther away from the emission source where the plume is more dispersed and not as optically thick.

Using HVO's old metric for evaluating SO₂ summit emissions, the total SO₂ released in 2013 was first calculated as 266,000 metric tons. They had long recognized these value as among those that had persistently understated the true mass of SO₂. To account for the summit emission rate underestimation, they used an initial preliminary correction. It was based on early Simulated Radiative Transfer- Differential Optical Absorption Spectroscopy (SRT-DOAS) values. This refinement increased the summit's traditional estimate 3-fold, yielding a summit total SO₂ amount of ~800,000 metric tons for the year 2013.

HVO further reported a preliminary calculation of Kīlauea's 2013 summit emissions using their available Flyspec array data yielded ~1.0 x 10⁶ metric tons for 2013. This was judged more accurate value for the total summit SO₂ release.

East rift zone (ERZ) emissions for 2013 continued at the low level recorded since mid-2012. Early in the year, SO₂ emissions increased coincident with the occurrence of the Kahauale'a lava flow, but emissions stabilized several months later and continued at a low level for the balance of the year. Rift emissions were consistently less than those at the summit for 2013 totaled ~113,000 metric tons (using the refined methods mentioned above). This was ~20% less than reported in 2012, and the lowest amount recorded since the ERZ eruption began in 1983. The low SO₂ emissions from the ERZ were at least partially due to degassing at the summit.

Summit CO₂ emission rates. During 2013, CO₂ emission rates remained at the relatively low level measured since approximately 2009 (figure 225). The continued absence of a strong CO₂ signature in 2013 confirmed that the current summit activity reflects shallow reservoir processes rather than deeper ones. All CO₂ measurements in 2013 were made with the Licor LI-6252 gas analyzer.

Figure 225. Daily average CO2 emissions from the summit of Kīlauea, as measured under trade-wind conditions, during 2003-2013. The vertical bars represent standard deviations of all traverses on a single day. The cyan symbols show CO2, calculated using filtered data to more confidently bracket CO2 emission rates. The black squares are raw CO2 area-count averages; these values provide a measure of CO2 independently of the C/S ratio and SO2 emission rates by accounting for the area traversed through the plume and integrating that area by gas concentration magnitudes (this method also takes into account the plume direction and speed). CO2 values were calculated without any correction to underestimated SO2 emission rates. Courtesy of HVO.

Quantifying summit and rift plume characteristics. In addition to emission-rate studies, HVO continued to monitor the summit and rift plumes using a variety of techniques, including multi-species sensor-based time-series measurements and open-path FTIR. In 2013, FTIR measurements of the summit plume reconfirmed the shallow nature of the degassing source, with plume chemistry characterized by low CO₂, high SO₂, high H2O, and significant HCl and HF (table 10). Measurements of the summit and rift plumes yielded similar chemistry, suggesting a common source for these gases. Also reconfirmed in 2013 were the previously observed short-term changes in gas chemistry correlating with behavior in Overlook crater.

Table 10. The composition of Kīlauea's summit plume for 2013 reported in moles and mole%. Courtesy of HVO.

Gas hazards. In 2013, the maximum ambient concentration of SO₂ measured near the summit along Crater Rim Drive during traverses made with a car was 150 ppm, a value well above the IDLH (Immediately Dangerous to Life and Health) threshold. Concentrations measured inside the vehicle reached a maximum of 12 ppm. The inside-car levels were measured with all air-handling turned off, the operating conditions that minimize SO₂ penetration into the vehicle. Fumarole sampling at the two locations on the rim of Halema'uma'u were subsequently paused during 2013 while shifting to alternative, less-hazardous measurement techniques.

HVO continued to operate the low-resolution SO₂ sensor and rain collector network on Kapapala Ranch in 2013 (within 23 km SW of the summit). In general, maximum SO₂ concentrations on the Ranch in 2013 were lower than in 2012. During the early years of the activity at the Overlook vent, the Ranch's livestock exhibited runny eyes, respiratory issues, weight loss, and tooth mottling and degradation (possibly indicating fluorosis). Additionally, fences and other metal infrastructure on the ranch had been deteriorating more rapidly than before the summit eruption began. New data showed SO₂ one-minute values for 2013 (a single, one-second measurement per minute) up to 4 ppm. Hazard monitoring and communication with the ranch operators, veterinarians, and public health officials remained ongoing.

Ambient SO₂ concentrations measured downwind of Halema'uma'u continued to reach very high levels (~150 ppm) along Crater Rim Drive near the Halema'uma'u parking lot, warranting continued caution along Crater Rim Drive in 2013. HVO scientists maintained communications with community groups and county, state, and federal agencies in order to relay the changing gas-hazard conditions associated with Kīlauea's ongoing eruptions.

In 2013, the National Park Service's (NPS) ambient air quality stations located at HVO and behind the Kīlauea Visitor Center continued to record periods of hazardous air quality resulting from the ongoing eruptions. The National Park continued to close the highly impacted areas of the park during poor air-quality episodes. Closing of park locations, including Kīlauea Visitor Center and Jaggar Museum, were based on the following criteria: a Visitor Center is closed when SO₂ concentrations exceed 1 ppm for 6 consecutive 15-minute periods (1.5 hrs), 3 ppm for 3 consecutive 15-minute averages (45 minutes), or 5 ppm for one 15-minute average. NPS high-resolution SO₂ analyzers located at the visitor centers operated in the extended 0-10 ppm range.

Flank deformation. The variable-rate inflation of Kīlauea that has been ongoing since 2010 continued through 2013. There were periods of slight deflation in March-May, late May, July-August, and September and November. The saw-tooth pattern created by the alternating inflation and deflation is most obvious in the distance change across the Halema'uma'u crater, but can also be seen in the tilt record at summit tiltmeters, such as at station UWE and subtly in the vertical changes at summit GPS sites (figure 226).

Figure 226. (A) Radial tilt measured by borehole instruments at the summit (UWE) and at Pu'u 'O'o (POC) in 2013. Positive change indicating tilt away from the most common magmatic sources, usually indicating inflation, and negative change indicating tilt towards those sources, usually indicating deflation. (B) Changes in distance across Halema'uma'u (UWEV-CRIM) and elevation of GPS stations (HOVL V and OUTL V) from July 2012 through July 2013. Courtesy of HVO.

During 2013, there was a total of almost 10 cm of extension on the approximately 3.5-km baseline between UWEV and CRIM (figure 226 B) and about 10 microradians of inflationary tilt at UWE (figure 226 A). There was very little accumulated vertical change at the summit GPS sites over the year, however. This was also reflected in the lack of appreciable line-of-sight displacement in the interferograms from INSAR spanning 2013. There were 65 deflation-inflation (DI) events in 2013, similar to the rate of occurrence observed since the opening of the summit vent in 2008. Most of these were only weakly detected by the POC tiltmeter at Pu'u 'O'o.

At Pu'u 'O'o, the GPS site on the N rim (PUOC), recorded a fairly steady, slow rate of N-NW motion in 2013, with a slight acceleration in late April-early May. The direction of motion is usually indicative of inflation, but there was no appreciable uplift at the site. There was a net tilt of about 20 microradians to the NW at POC on the N flank, also usually indicative of inflation.

The pattern and velocity of GPS sites on the S flank of Kīlauea in 2013 were similar to the patterns and rates that have been observed in the recent past during times free of slow-slip events and ERZ intrusions.

Deformation monitoring equipment. Two continuous GPS sites (LEIA and SPIL) were lost to lava flows from Pu'u 'O'o in early 2013. After a data outage at the Malama Ki (MKI) tilt site on the lower ERZ in April, HVO discovered that thieves had dismantled the gate to the security enclosure and stolen everything except the actual tiltmeter. This had been part of a string of thefts at this site, forcing HVO to eventually abandon it. This was an unfortunate loss to the monitoring network, especially because the only other tiltmeter station on the lower ERZ, near Heiheiahulu (HEI) had also been stolen late during the previous year. In July, HVO installed a new tiltmeter in a less accessible location a few kilometers NW of Heiheiahulu.

Seismicity. In 2013, HVO's seismic network consisted of 57 real-time continuous stations (25 broadband, 21 strong-motion, 7 three-component short-period, and 25 vertical-component short-period instruments) (figure 227). The network coverage was most dense on and around Kīlauea. In 2013, HVO upgraded of the seismic network which involved installing the digital stations NAHU (to replace the analog station ESR) and TOUO (to replace analog station KII). They also established three arrays of infrasound sensors in order to better track acoustical waves in the air (infrasound) associated with volcanic processes.

Figure 227. (top) Authoritative region of HVO (black line). Red triangles represent permanent, continuous seismic stations and Netquakes instruments, a new type of digital seismograph that transmits data to USGS via the internet after an earthquake. Stations from the National Strong-Motion Program (NSMP) are excluded here because their high triggering threshold means that they produce data for only a handful of earthquakes a year. (bottom) Map showing both HVO stations (red triangles) and Netquakes (blue triangles). Two boxes indicate regions of special interest for seismic monitoring. Netquakes instruments enable the USGS to achieve a "denser and more uniform spacing of seismographs in select urban areas. … The instruments are designed to be installed in private homes, businesses, public buildings and schools" (USGS, 2013a). Courtesy of HVO.

Seismic activity at Kīlauea was generally low in 2013 compared to that of other time periods since the 2008 start of the summit eruption (figure 228). Tremor was a ubiquitous feature of the seismicity near the summit, with discrete very-long-period (VLP) and long-period (LP) events occurring sporadically. Tremor amplitudes appeared to modulate in conjunction with the presence or absence of spattering in the lava lake within Halema'uma'u. In general, increased seismicity in the S caldera and upper ERZ were coincident with rapid increased lava lake level and tilt. None of these swarms were remarkable in number or size compared to previous swarms, especially those in 2011 and 2012.

Figure 228. (A) January-December 2013 earthquake locations, Hawai'i Island, 0-60 km deep, M ≥ 3.0. Earthquake colors are based on depth. The symbol size of the earthquake is based on the preferred magnitude. All plotted earthquakes have been reviewed by an analyst. (B) January-December 2013 earthquake locations, Hawai'i Island, 0-5.0 km deep (shallow), M ≥ 2.0. Earthquake colors are based on time. Symbol sizes are based on the magnitude. Plotted events include both reviewed and automatically determined locations that have horizontal errors < 2 km and vertical errors < 4 km. Courtesy of HVO.

New interactive earthquake webpage launched. In October 2013, HVO launched a new interactive earthquake webpage, informally called Volcweb (USGS, 2013b). The new website used several new technologies that provided a better user-experience and a better compatibility with mobile devices. In addition to providing earthquake location information, the site also creates cross-sections, time-depth plots, cumulative number of earthquake plots, and cumulative magnitude plots for data up to a year old. Webicorders for all stations were available (updated every 10 minutes). The rollout of this website allowed HVO to retire the old "Recent Earthquakes" page.

References. Patrick, M., Orr, T., Sutton, A.J., Elias, T., and Swanson, D., 2013, The first five years of Kīlauea's summit eruption in Halema'uma'u crater, 2008-2013. Hawai`i National Park, HI: U.S. Geological Survey, Hawaiian Volcano Observatory, Fact Sheet 2013-3116.

Patrick, M.R., Orr, T., Antolik, L., Lee, L., and Kamibayashi, K., 2014, Continuous monitoring of Hawaiian volcanoes with thermal cameras, Journal of Applied Volcanology, 3:1.

Roeder, P.L., Thornber, C., Poustovetov, A., and Grant, A., 2003, Morphology and composition of spinel in Pu'u 'O'o lava (1996-1998), Kīlauea volcano, Hawaii. Journal of Volcanology and Geothermal Research, 123, 245-265.

USGS, 2013a (January). Earthquake Hazards Program, Netquakes: Map of Instruments. Retrieved from http://earthquake.usgs.gov/monitoring/netquakes/map/.

USGS, 2013b (December). Hawaiian Volcano Observatory, Recent Earthquakes in Hawaii. Retrieved from http://hvo.wr.usgs.gov/earthquakes/new.

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: https://volcanoes.usgs.gov/observatories/hvo/); Hawai`i Institute of Geophysics and Planetology (HIGP) 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/); Hawaii 24/7 (URL: http://www.hawaii247.com); Great ShakeOut (URL: http://shakeout.org/hawaii/); and West Hawaii Today (URL: http://www.westhawaiitoday.com/).

This report shows satellite thermal alerts from the MODVOLC system showing that they continued for 7 months after the end of coverage in our one report on Nabro's June 2011 eruption (BGVN 36:09), with the last alert occurring on 3 June 2012.

What has emerged regarding the 2011 Nabro eruption since our one previous report is a much more detailed eruptive timeline and some substantially taller plume-height estimates. These new and more carefully assessed details came out in at least eight papers and three technical comments (see References below).

The initial Toulouse Volcanic Ash Advisory Center estimates cited in BGVN 36:09 were made in the time-limited operational setting that identifies volcanic ash for aviation safety. Those altitude estimates, which included maximum plume heights on 13 June 2011 in the range of 9.1-13.7 km altitude, have since been reassessed using an array of satellite and ground-based instruments and processing strategies. The revised heights in the subsequent papers often determined plume altitudes above the 16-18 km tropopause and into the stratosphere. Absent in our earlier report but well documented in the papers was evidence of a 16 June 2011 eruptive pulse.

Overall, Nabro erupted a total SO₂ mass of at ~1.5 Tg (Clarisse and others, 2012), making the eruption the largest SO₂ emitter of the 2002-2012 interval (Bourassa and others, 2013). The various papers and the technical comments have also framed debate on how and when Nabro's plume entered stratosphere.

Thermal alerts. This report does not contain any new in situ observations at Nabro. Table 1 shows MODVOLC thermal alerts during November 2011 and into 2012 on the basis of the number of days with alerts in these months. Those alerts stem from observations made with the MODIS instrument that flies on the Terra and Aqua satellites. Our previous report discussed alerts as late as 5 November 2011, but additional alerts were issued later in the month. For this table, January 2012 was the month with the largest number of days with alerts, 15 days. As of late 2014, the last posted alert was issued on 3 June 2012.

Table 1. MODVOLC thermal alerts recorded for Nabro from November 2012 through September 2014. Courtesy of MODVOLC.

Although the earlier alerts may signify ongoing eruption, some of the later alerts could stem from ongoing post-eruptive thermal radiance from potentially thick lava flows. Absence of alerts could be the result of clouds masking the volcano, although that is unlikely significant in the terminal alert registered in June 2012. It also bears noting that the alerts are at a fairly high threshold.

References. Bourassa, AE, Robock, A, Randel, WJ, Deshler, T, Rieger, LA, Lloyd, ND, Llewellyn, EJ, and Degenstein, DA, 2012, Large Volcanic Aerosol Load in the Stratosphere Linked to Asian Monsoon Transport. Science 337 (6090):78-81. DOI: 10.1126/science.1219371.

Bourassa, AE, Robock, A, Randel, WJ, Deshler, T, Rieger, LA, Lloyd, ND, Llewellyn, EJ, and Degenstein, DA, 2013, Response to Comments on "Large volcanic aerosol load in the stratosphere linked to Asian Monsoon transport. Science, 339 (6120), 647, DOI: 10.1126/science.1227961.

Clarisse, L., P.-F. Coheur, N. Theys, D. Hurtmans, and C. Clerbaux, 2014, The 2011 Nabro eruption, a SO₂ plume height analysis using IASI measurements, Atmos. Chem. Phys., 14, 3095-3111,DOI:10.5194/acp-14-3095-2014.

Clarisse, L., Hurtmans, D., Clerbaux, C., Hadji-Lazaro, J., Ngadi, Y., & Coheur, P. F., 2012, Retrieval of sulphur dioxide from the infrared atmospheric sounding interferometer (IASI). Atmospheric Measurement Techniques Discussions, 4, 7241-7275 [13 March 2012; revised from 2011 version] www.atmos-meas-tech.net/5/581/2012/; DOI:10.5194/amt-5-581-2012.

Fairlie, T. D., Vernier, J.-P., Natarajan, M., and Bedka, K. M., 2014, Dispersion of the Nabro volcanic plume and its relation to the Asian summer monsoon, Atmos. Chem. Phys., 14, 7045-7057, DOI:10.5194/acp-14-7045-2014, 2014.

Fromm, M, Nedoluha, G, and Charvat, Z, 2013, Comment on "Large Volcanic Aerosol Load in the Stratosphere Linked to Asian Monsoon Transport." Science 339 (6120). DOI: 10.1126/science.1228605.

Fromm, M, Kablick, G (III), Nedoluha1, G., Carboni, E., Grainger, R., Campbell, J, and Lewis, J., 2014, Correcting the record of volcanic stratospheric aerosol impact: Nabro and Sarychev Peak, Journal of Geophysical Research. Atmospheres. [Early, online version, accessed August 2014] DOI: 10.1002/2014JD021507

Pan, LL, and Munchak, LA, 2011, Relationship of cloud top to the tropopause and jet structure from CALIPSO data. Journal of Geophysical Research: Atmospheres (1984-2012) 116.D12 (2011).

Penning de Vries, M. J. M., Dörner, S., Pukite, J., Hörmann, C., Fromm, M. D., & Wagner, T. (2014). Characterisation of a stratospheric sulfate plume from the Nabro volcano using a combination of passive satellite measurements in nadir and limb geometry. Atmospheric Chemistry and Physics, 14(15), 8149-8163.

Theys, N., Campion, R., Clarisse, L., Brenot, H., van Gent, J., Dils, B., Corradini, S., Merucci, L., Coheur, P.-F., Van Roozendael, M., Hurtmans, D., Clerbaux, C., Tait, S., and Ferrucci, F.: Volcanic SO₂ fluxes derived from satellite data: a survey using OMI, GOME-2, IASI and MODIS, Atmos. Chem. Phys., 13, 5945-5968, doi:10.5194/acp-13-5945-2013, 2013.

Vernier, JP, Thomason, LW, Fairlie, TD, Minnis, P., Palikonda, R, and Bedka, K M, 2013. Comment on "Large Volcanic Aerosol Load in the Stratosphere Linked to Asian Monsoon Transport." Science 339 (6120). DOI: 10.1126/science.1227817.

Geologic Background. The Nabro stratovolcano is the highest volcano in the Danakil depression of northern Ethiopia and Eritrea, at the SE end of the Danakil Alps. Nabro, along with Mallahle, Asavyo, and Sork Ale volcanoes, collectively comprise the Bidu volcanic complex SW of Dubbi volcano. This complex stratovolcano constructed primarily of trachytic lava flows and pyroclastics, is truncated by nested calderas 8 and 5 km in diameter. The larger caldera is widely breached to the SW. Rhyolitic obsidian domes and basaltic lava flows were erupted inside the caldera and on its flanks. Some very recent lava flows were erupted from NNW-trending fissures transverse to the trend of the volcanic range.

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 (URL: http://modis.higp.hawaii.edu/); and Toulouse Volcanic Ash Advisory Centre (VAAC) (URL: http://www.meteo.fr/vaac/).

Pacaya, which in recent years has consistently erupted olivine-bearing high alumina basaltic lavas, erupted with remarkable violence on both 27 and 28 May 2010 with an explosion on the 27th lasting ~45 minutes. This was followed by a smaller explosion the next day that generated a plume assessed from satellite and meteorological data as reaching 13 km altitude. In this report we describe those events as explosions in order to distinguish them from the ongoing, decades-long, and often effusive eruption generally seen at Pacaya. The terms 'explosion' and 'explosive' appear warranted given such factors as the suddenness of escalation, the ~13 km plume altitude (~10 km over the summit when measured during the weaker explosion on the 28th, the density of projectiles, and the scale of the tephra fall. The term explosion seems consistent with common practice (Sparks, 1986; Fiske and others, 2009).

The following report emphasizes Pacaya's behavior in 2010, including the 27 and 28 May explosions and impacts continuing into early June 2010. Our last report (BGVN 34:12) discussed behavior into mid-January 2010. Some of the reporting came from reports of Guatemalan agencies (eg. INSIVUMEH and CONRED, acronyms spelled out in the Information contacts section at bottom), newspapers (eg. Prense Libra, 2010a, b), videos and photos, and cited manuscripts and papers. It especially benefited from a draft manuscript prepared by Rüdiger Escobar Wolf (REW, 2014) and graciously provided to Bulletin editors. REW also provided reviews, insights, and numerous tailored graphics but bears no responsibility for possible errors induced by Bulletin editors.

The explosions were preceded months to weeks earlier by extra-crater venting of lava flows on the E and SE flanks. The lava flows covered substantial areas after emerging effusively at two widely spaced vents in atypical extra-crater or crater-margin locations.

Subsections address the following topics: (1) the Guatemalan hazard agency CONRED's reports, (2) a sample of available video and photo documentation of Pacaya's behavior, (3) events prior to the 27 May explosion, (4) the explosions and some of the impacts, (5) the seismic record showing the pattern of escalation around the time of the explosions, (6) a brief summary of the critical initial aviation reports, and (7) a geotechnical slope stability study that suggests gravitational instability at Pacaya, particularly owing to the cone's magma pressure and seismic loading.

Pacaya , which has a record of eruptions dating back over 1,600 years, has been erupting the majority of the time since 1961, often emitting rough-surfaced lavas but also occasionally discharging explosions. The centerpiece of the National Park of the same name, it is the most often climbed volcano in Guatemala. There have been 69 prior Smithsonian-published reports describing behavior from 1969 to early January 2010 (CSLP 03-70 to BGVN 34:12). REW (2013) ranked the 27 May explosions as sub-plinean and the associated lava emissions as the largest since similar events in 1961.

Figure 42 shows two simplified regional maps of Guatemala and neighboring countries including Mexico, Belize, Honduras, and El Salvador.

Figure 42. (Top) A map showing Pacaya's location in Central America. (Bottom) A map emphasizing Pacaya's location with respect to the central portion of Guatemala City (red square labeled 'Guatemala'). The larger combined urban area associated with that Capital city stretches well beyond the square symbol and contains ~3.5 million residents. AmatitlÁn was heavily damaged by Pacaya's May 2010 ashfall and the knock-on effects of Tropical Storm Agnes that arrived two days later. Top map taken from Morgan and others (2012); bottom map revised from a base map found online at Ezilon Maps.

The larger tephra blanket spread N, covering an area of more than 1,000 km² including the bulk of the Guatemala City metropolitan area, the largest city in Central America, population ~3.5 million. The City's center lies ~25 km NNE of Pacaya's summit but a 5-km-wide strip of urban and suburban development now stretches from its older core (red square, figure 42)to ~9 km N of the summit. The tephra shut down La Aurora, the county's primary international airport and among the region's busiest, for 5 consecutive days.

The 27 May 2010 explosion destroyed or damaged nearly 800 houses in nearby communities, forcing ~2,000 residents to evacuate and injuring 59 people. A high density of ballistics fell on nearby hamlets and villages, particularly those 2.5-3.5 km N of the MacKenny cone (El Cedro, San Francisco de Sales, and Calderas). The ballistics had sufficient mass and velocity to puncture roofs with a density on the order of one puncture per square meter in some places. Many more smaller ballistics bent but did not penetrate the corrugated sheet metal roofs common in many of the region's dwellings. Some of the ballistics were sufficiently hot to start fires.

Ash caused widespread damage locally, and up to ~8 cm of ash fell on parts of metropolitan Guatemala City, the nation's capital, centered ~35 km NNW of Pacaya. Up to 20 cm of tephra accumulated at and near Pacaya. According to available census data, the population within 10 km of Pacaya was 57,000 (John Ewert, USGS-CVO, personal communication).

Accounts from Guatemalan meteorological stations reported that detectable ash from the 2010 explosions fell as far away as the Caribbean coast. Brianna Hetland was both a graduate student in volcanology and a US Peace Corps Volunteer in Guatemala during 2010-2012. Hetland noted in a message that she had spoken with another Volunteer who said ash had blanketed his neighborhood near Coban (in Samac, Alta Verapaz) ~180 m N of Pacaya (figure 42, bottom). Hetland documented post-eruptive conditions at Pacaya, composed a blog on the impact and clean up, and gave a talk on those aspects as well as multifaceted monitoring conducted by fellow students and faculty at Michigan Technological University (Hetland, 2012a, b; Walikainen, 2010).

Some of the impacts of the freshly fallen ash were amplified and other impacts were diminished by heavy rains and flooding due to Tropical Storm Agatha that struck the region 2 days later, with some areas receiving 0.9 m of rain. The floodwater run carried ash that dislodged debris, clogged drainage systems, left thick deposits on valley floors, and damaged many bridges. The scale of the combined disasters led to more analysis of hardships, mitigation, and economic impact than usual at many eruptions, as exemplified by the detailed assessments by Wardman and others (2012). Those authors visited in the aftermath from New Zealand in order to study impacts that might be analogous to hazards elsewhere. They found that one moderating impact of the rain was to cee crops, which were washed clean of ash and residual acids. The authors also found that that a prompt and efficient cleanup was initiated by the Capital municipality to remove tephra from the 2,100 km of roads in the Capital. An estimated 11,350,000 m³ of tephra was removed from the city's roads and rooftops.

Diminishing strombolian activity and lava flows in the crater area continued into at least late June 2010. By this time the emissions had become more like the generally effusive decades-long eruption, which was still ongoing when this was written in late 2014. In addition to the information here, Pacaya's discharge rates have been summarized for the years 2004-2010 on the basis of infrared satellite images (Morgan and others, 2013). As would be expected, a strong peak in radiance developed in late May 2010.

REW (2013) noted one death attributed to the explosion and tephra fall and 179 deaths attributed to the Tropical Storm. Two people died at Pacaya days prior to the explosion of 27 May 2010. Wardman and others (2013) mentioned two further deaths due to people cleaning tephra from roofs.

Geochemical analysis of material erupted on 27 and 28 May is not yet reported. As background, Matías and others (2012) describe Pacaya's recent lavas as all high-alumina basalts with SiO₂ contents of 50-52.5 weight percent and MgO contents of 3-5 weight percent. Common phynocrysts (visible minerals) included plagioclase, olivine, and opaque minerals (Conway, 1995). There is a slight variation of CaO in this group of lavas, which suggests a phenocryst enrichment or depletion. The lava compositions have remained broadly similar since 1961, and for many previous lavas as well, although some more felsic compositions are represented at older flank eruptions (Eggers, 1971).

CONRED reports. Perspective on the disaster can be gained from the chronology and content of announcements issued by CONRED (the Guatemalan agency for disaster reduction; Coordinadora Nacional para la Reducción de Desastres, table 4). These will be referred to in text by "CONRED" followed by their bulletin number.

Table 4. A summary of key CONRED information bulletins issued relevant to Pacaya's May 2010 eruption (http://conred.gob.gt/). After Escobar Wolf (2012) in addition to a similar table by Wardman and others (2012). Not all bulletins are included in this table.

Events prior to the energetic 27 May explosion. Figure 43 highlights Pacaya's vent locations (1961 to 2009 vents as green dots), including the two new E and SE flank vents that emitted lava flows (red areas). Changes in eruption behavior preceded the 27-28 May explosions by several months.

Figure 43. Simplified geological map of Pacaya, based on cited references, INSIVUMEH mapping, and GOES satellite data. The key at right calls attention to features such as the collapse scarp forming the N and E of margin of the main crater and the lava flows of prehistoric age (Eggers, 1969, 1972; Bonis, 1993) through about mid-2010. Migrating vents mapped during 1961-2012 (Matías, 2010; Rose and others, 2012) appear as dark-green dots (many clustered on or near the MacKenney cone's summit). The red areas on the SE flank and E flank represent lava with the noted age constraints from REW's analysis of satellite data. The SE flank vent had emitted by mid-2010 a field of lava approaching the size of the 1961 Cachiajinas lava flow (purple). The latter flow both vented and advanced within Pacaya's collapse scarp. In contrast, the SE flank flow was the first in historical times to vent and flow outboard of the scarp. The cone residing on Pacaya's NW rim, Cerro Chino, enters discussion frequently in this report. Note the depression (notch or trough) here labeled "New fissure like structure." Map created and provided by REW.

From 2004 to around the end of 2009, Pacaya's eruptive intensity was often low. A clear sign of changes took place in February 2010 when lava flows emerged at vents on the S and SE flanks (table 4). These vents sit well outboard of the usual points of lava emission, which have in recent decades been limited to spots within the central crater, an area bounded by a large engulfing collapse scarp (a Somma rim; Eggers, 1969; figure 43). The two previously mentioned deaths occurred on 18 April when, according to the news, they were hit by a rock avalanched caused by an explosion. By 17 May, SE flank lava flows had reached 1.5 km long and the Park began restricting access (table 1). The scene on the SE flank appears in figure 44.

Figure 44. Pacaya's SE flank eruption as seen during the day on 27 May 2010. The ultimate distribution of lavas appears on the preliminary map by REW (2014). Image courtesy of Gustavo Chigna (INSIVUMEH).

Earlier on the 27th (prior to the explosion), INSIVUMEH volcanologist Gustavo Chigna looked out over the crater area and counted at least 16 distinct vents emitting lava. Chigna was surprised, and his comment was something like 'It looked like water gushing out of a sieve.' That scale of new extrusive sites helped alert authorities that the volcano's behavior had escalated well beyond the norm and led to restricting public access to Pacaya.

During the 5 years prior to the 27 May 2010 explosion, sporadic vent openings limited to the MacKenney cone and adjacent areas (particularly the N crater) extruded lava flows (green dots, figure 43). Many of the resulting lava flows were each only active for periods of days to months. INSIVUMEH sometimes reported multiple simultaneous lava flows from distinct vents on the cone, which occurred, for example, during April 2009. Most of the lava was confined to the main crater or portions downslope and W of the E-bounding collapse scarp. The case in 2005 illustrated that the topographic boundary associated with the NE segment of the collapse scarp had diminished in places to the point where lava flows could cross the scarp (BGVN 33:08).

Around January 2010, Gustavo Chigna (INSIVUMEH) indicted the end of mainly lower effusive activity ongoing since 2004. The new upsurge fed several lava flows from vents on Pacaya's main cone. In harmony with this comment, the video by Crossman (2009) indicates that on 24 December 2009 the volcano emitted considerable lava. Venting was effusive and at both the MacKenney cone's summit and base. Visible plumes were nearly absent.

Table 5 lists a small sample of available videos taken at Pacaya that aid in documenting its behavior. The table includes videos taken before, during, or shortly after the 27 May explosion, with the two pre-explosion videos capturing behavior relevant to this subsection. The videos from other parts of the table are discussed in appropriate sections below.

Table 5. Some photos and videos that advance understanding of Pacaya behavior during December 2009 to about 2 June 2010 (a week after the explosion). The cases presented are a sample, not an exhaustive list. Compiled by Bulletin editors.

Figure 45 shows one of several Landsat views of the E flank in an infrared image acquired on 23 March 2010. It showed high thermal radiance in a narrow linear thermal anomaly headed E outboard of the usual eruptions confined to the crater. The E-flank area is devoid of vegetation, which rules out a local fire there, meaning that the anomaly was due to a lava flow. The number of clear (cloud-free) views of Pacaya available during March through June was limited. REW plotted this anomaly in a KMZ file format (red line, figure 46).

Figure 45. A Landsat 7 thermal image of Pacaya on 23 March 2010 showing high heat flux as red. The small red area is on the MacKenney cone. The larger red area is a lava flow that had extended E. A site visit and video by Moon (2010) on 1 April (8 days later) confirmed lava flows on the order of 2-4 m wide. Black and marginal gray areas are older lava flows; green areas are vegetated with some cultivated or pasture land in shades of brown. This image contains artifacts in the form of gray diagonal stripes. The stripes are due to the failure of the Scan Line Corrector (SLC), which compensates for the satellite's forward motion. Courtesy of REW.

Figure 46. A Google Earth view of the land surface looking radially outward (E) down Pacaya's E flank (N is to the left). The red line indicates the location of the lava flow axis from heat flux in Landsat images. The flow's source was at or very near the collapse scarp. The yellow line indicates the film crew's 1 April 2010 excursion route recorded with GPS as they approached the lava flow, filmed it at close range, and then headed back towards the trailhead (Moon, 2010). For scale, the lava flow is ~0.3 km long. Graphic files, analysis, and compilation created and provided by REW.

The new E flank (extra-crater) lava flow documented by Landsat on 23 March was the subject of a video by Moon (2010) taken on 1 April (table 4; see their excursion route on figure 46). The footage was shot during daylight hours at high resolution [1080p HD] and later processed to obtain vibrant red, orange, and yellow colors. The discharges were effusive and few visible emission clouds accompanied the lava flows seen in the video. A dark plume remained above the MacKenney cone's summit.

As seen in figures 47-50, the lava documented by Moon (2010) in photo and video was several meters wide and passing over irregular terrain. As seen from a distance (e.g. figure 47), some sectors of the flows channel stood well above the surrounding landscape. In the area visited, the lava remained confined behind jumbled but effective levees as it passed through and over the a'a' (rough textured) flow field.

Figure 47. A 1 April 2010 photo of Pacaya's E flank lava flow seen in the distance as it descends across an a'a flow field. Courtesy of H. Paul Moon (see table 5).

Figure 48. Pacaya's E-flank lava flow on 1 April 2010 upon closer approach than previous f. After watching the video Moon (2010), REW commented that the flow looked like "a typical channel-levee aa flow developed on a steep slope." Courtesy of H. Paul Moon (see table 5).

Figure 49.. Pacaya's E-flank lava flow on 1 April 2010 upon closer approach than previous f. For scale, note exposed portion of ~1.4 m long hiking stick in right foreground. Courtesy of H. Paul Moon (see table 5).

Figure 50. A still closer view of Pacaya's E-flank lava (taken from just a few meters away), which was moving swiftly. In his YouTube notes on his teams 1 April 2010 visit Moon commented that "the heat was so intense that I could only hold out for brief shots, needing to turn away regularly to avoid getting scorched." Courtesy of H. Paul Moon (see table 5).

Figure 51 maps the inferred E flank lava flow axis and SE flank fissures on an oblique Google Earth view.

Figure 51. An oblique Google Earth view of Pacaya looking roughly WNW. At left in orange appear the upslope areas of the fissures that fed the SE flank lava flows. Farther NE (to the right) appear another set of fresh black lavas that reside on the upper E flank. The green line traces high heat emissions REW found in Landsat imagery from 23 March 2010, the same lava flow that had been the subject of Moon's video ~8 days later. Both sets of flows and vents were the first clearly documented to extend E of the collapse scarp in historical times. Analysis, compilation, and topographic files all provided by REW.

On 18 April 2010, according to a news report in the newspaper Prensa Libre, a Venezuelan tourist and her Guatemalan guide died on Pacaya. The news report stated the deceased were in the area of high risk when struck by material released from an explosion. Some of the other 14 people on the scene sustained injuries.

On 17 May 2010, observers saw abundant lava escaping from a new SE-flank vent (CONRED 708). A mound had formed at the vent area. The lava from this vent had by 17 May extended as far as 1.5 km. As seen on figure 43, the SE flank lava flows and their fissures ultimately fed lava flows trending roughly S for ~2.5 km then turning sharply (~90 degrees) to the W and extending in that direction another ~2.5 km.

CONRED 708 made a recommendation to the Pacaya National Park authority to restrict visitor access to the lava flows. The 17 May report noted that Pacaya's activity was considered to be relatively high, but it left out language suggesting a crisis at this point. According to the press, access to the volcano was restricted following the recommendation.

On 17 May, the newspaper Prensa Libre featured an undated night photo of the MacKenney cone taken from the N, presumably of this stage of Pacaya's eruption. It showed a dense spray of glowing material thrown from the MacKenney cone's summit and rising hundreds of meters. The cone's N rim contained a recently formed V-shaped notch (or trough). Out of that notch poured a broad lava flow. Several hundred meters down the MacKenney cone's N face, the broad flow split into two flows descending the cone's steep face on diverging paths. The notch in the cone stands out as a clear morphologic change associated with this time interval (~10 days prior to the 27 May explosion), and as will be seen below, it served as a conspicuous vent site for the fissure emissions documented during the explosions.

The day before the explosion, on 26 May, eruptive and seismic intensity both increased markedly. An eruptive plume reached 1 km above the vent and fine tephra fell on villages around the volcano (CONRED 726 on 26 May, table 4). CONRED recommended fully closing Pacaya National Park, and they warned aviation authorities of airborne ash near Pacaya. No call was yet made to evacuate residents living adjacent Pacaya.

Vigorous explosions starting 27 May 2010. Pacaya's eruptive vigor increased to the point of strong strombolian eruption, with the initial increase noted on the 27th in a morning report in Prensa Libre. More intense explosions occurred at around 1500 when observers noted explosions discharging about once per second and saw glowing material thrown ~1.5 km above the crater, and taller rising dark clouds carrying finer tephra that dispersed over nearby villages.

The exact start time of the intense 27 May explosion is variously reported, but available visual observations suggested to REW (2014) that it was during the interval 1800-1900. CONRED 729 indicated the climax (the explosion)began at 1900. Seismic data, discussed in a subsection below underwent the highest (RSAM) amplitudes during 1730-1830 local time on the 27th. Aviation reporting of satellite data on eruptive plumes, discussed in a subsection below, was initially ineffectual for the 27th owing to above-lying weather clouds.

What is clear is that the explosion late in the day on the 27th drove forth intense fire fountaining and vigorous ejection of tephra and ballistics.

Figure 52 shows a broad fire fountain frame taken from a Youtube video posted on 28 May—but it lacked an acquisition date (RT news channel, 2010a). REW interprets this video as taken during the major climax (explosion) during the night on the 27th. The eruption was clearly of fissure style at this point but the upper extent of the glowing material was possibly masked by ash clouds. Some of the textures within the glowing region are explained in the f caption and in the text below.

Figure 52. A frame captured from a news video taken at night from Pacaya's NNW side documenting powerful curtain-style emissions (fire fountains) from the main crater area (which includes the MacKenney cone). The foreground consists of the dark silhouette of Cerro Chino (indicated on figure 43). Some of the tall antenna towers there appear as narrow vertical dark streaks backlit by the brighter orange fire fountains. Many of the towers and radio shacks on the ground near their bases were destroyed. Taken from RT news (see table 5 (RT News, 2010 (a)).

REW described the video source for figure 52 as taken looking at Cerro Chino (indicated on figure 43) from at or near the town of El Cedro, ~3 km to the NNW of the vent. The diffuse zones of near darkness in the midst of the fountains are rising ash clouds locally diminishing the glow. Thus it is clear from the dynamics seen on the video, that the glow of higher reaching clasts in the upper portions of this image could possibly be masked by dense ash plumes.

On the video, the orange streaks from glowing airborne pyroclasts track to points below that suggest emission from multiple vents or an elongate vent with continuous extent, rather than a single point source, a topic returned to below in the context of an elongate trough developed on the MacKenney cone. That said, REW points out that it is hard to get a good idea of the scale from this video and that videos taken from other locations seem to show a wider, and at times two different fountain jets. Available video and photographic data has thus far prohibited estimating the width of the fountain at this stage of the eruption. REW (2013) citing Hetland (personal communication) and CONRED 856 noted that associated with these emissions the major tephra fall began, and it soon spread tens of kilometers to the N.

Early in the explosion on the 27th (exact timing unknown), a news team from a national television station (Notisiete) endured a shower of ballistics. REW (2013) noted that they were in the vicinity of Cerro Chino at probably less than 1 km from the vent, the zone with critical infrastructure most impacted (figure 53). Although most of the news team survived, reporter Anibal Archila's death was apparently the result of direct impact from a large ballistic. His was the only icially confirmed death caused by the strong explosive phase. During a subsequent eruptive lull, a rescue team spent several dangerous hours in very close proximity to the vent, finding and rescuing missing people, and carrying out Archila's body.

Figure 53. A truck parked directly N of the Pacaya's active crater at Cerro Chino as seen in the aftermath of the 27-28 May explosions. Courtesy of Gustavo Chigna (INSIVUMEH).

Ballistics in excess of 0.5 m on their long axis fell at Cerro Chino and elsewhere within ~1 km of the vent area (figure 54). Some bombs on the ground reached sizes of 80 x 50 cm (Hetland personal communication) but part of that extent may have been due to splattering on impact. Farther away, the sizes of ballistics generally diminished with distance from the source. At Cerro Chino ballistic impacts broke concrete roofs, started fires in the radio shacks, and toppled antenna towers (REW, 2014; Wardman and others, 2010).

Figure 54. An example of a large bomb found in the near-source region. Courtesy of Gustavo Chigna (INSIVMEH).

When the intense phase started on the 27th, the evacuation of villages to the W (El Rodeo and El Patrocinio) was already underway. During the hours after the explosion's onset on the 27th, more than 2,100 people were evacuated from the proximal villages to the town of San Vicente Pacaya (5 km NNW)(see related scenes in RT news, 2010 (b), table 4).

The settlements El Cedro, San Francisco de Sales, and Calderas, towns 2.5-3.5 km to the N, endured both ash as well as a dense barrage of hot ballistic bombs (figure 55). Many of the bombs were below 20 cm in diameter. Some of the ballistics pierced the corrugated (sheet metal or fiber cement) roofing common in Latin America. In some cases the ballistics also ignited fires that consumed most of the combustible contents of the buildings. Some roofs collapsed or buckled due to the load of deposited tephra.

Figure 55. Two photos taken soon after Pacaya's 27-28 May eruptions illustrate the density of projectile penetrations through roofs of two large buildings in San Francisco de Sales (~3 km N of the MacKenney cone). Taken from REW (2013) with photo credit to Hetland.

The ballistics examined were of low density owing to vesicles larger than 1 mm in diameter. They contained sparse phenocrysts (often larger than 1 mm), most likely plagioclase (Hetland personal communication to REW and Hetland (2010).

REW (2013) noted that, from the observed damage to roofs in these villages, the density per unit area of impacts that pierced through the corrugated roofs averaged as high as on the order of 1 per square meter. Portions of the roofs in near-vent settlements also sustained many dents from bombs that delivered impacts with lower force. Although some communities were partially evacuated when many of the ballistics arrived, REW (2013) concluded that some residents remained within the communities and regions mostly affected.

Reports in Prensa Libre give insights into the scene of the evacuation and the barrage. Many of the residents evacuated on foot following narrow paths across the rugged rural terrain. Other residents remained behind in order to protect their belongings from theft. When the barrage came, those too close used whatever hard and resistant objects they could find to protect themselves, including hiding under furniture and using pots and pans to protect their heads. Some corroded metal roofs were weak prior to the eruption. Some people found refuge in buildings with heavier, concrete-slab roofs, which generally fared better.

Figure 56 shows an individual who clearly received medical attention, stitches, for a laceration on his forehead. According to REW (2013), Pacaya's 2010 ballistic barrage caused more injuries than any recent eruptions. That said, data remain scanty on injuries rates and kinds, resultant disabilities, accident location, etc., although Wardman and others (2012) compiled some statistics.

Figure 56. Ballistic projectiles presented the most direct hazard from the 2010 explosions at Pacaya. This photo was found on the Boston Globe photo news site Boston.com (table 5). Their caption read, "A man shows the stitches he received after being injured by volcanic rock on the slopes of the Pacaya Volcano on May 28, 2010. (REUTERS/Daniel LeClair)." Courtesy of Boston.com.

The AmatitlÁn geothermal plant, located ~3 km N of the MacKenney cone to the N of San Francisco de Sales received ~20 cm of mostly lapilli-sized tephra. As Wardman and others (2012) noted, "Ballistic bombs and blocks also bombarded the plant, causing extensive damage to the plant's roof and condenser fans. Fan blades were dented, bent and also suffered damage from abrasion. Minor denting of the intake and outlet pipe cladding was also reported however these impacts were superficial and did not require repair." A photo showed cladding bearing multiple closely spaced dents on the side of a large pipe; the largest dent, 20 cm across, had ruptured through the sheet metal.

Post-explosion assessment of the MacKenney cone shed new light on the form and significance of the previously mentioned notch across it (a linear NW-trending trough passing through the summit, figure 57).

Figure 57. An annotated photo viewing the N side of the MacKenney cone in calm conditions at an unstated date following the May 2010 explosions. The prominent trough included a deep segment that had developed on the cone's lower slopes (labeled 'Possible crater'). During the 27-28 May 2010 eruption the trough appears to have served as an active fissure or series of vents emitting fountains (see figure 52 and related discussion). Courtesy of REW with photo credit to Gustavo Chigna.

The notch formed a prominent depression aligned both with the new SE-flank fissures and Cerro Chino cone on the outer NW crater rim. Portions of the RT video footage taken during vigorous stages of explosion suggests that at a paroxysmal stage of the explosion the trough served as an eruptive fissure emitting a vertically directed fountain as a curtain (table 5). REW (2013) also suggested that the eruptive fissure along the trough may have served as the vent for the ballistics that fell in previously mentioned settlements to the N.

The explosions broadest areal impact came from tephra fall. Figure 58 shows a close up of ash from a sample collected 22 km from the vent. Overall, the grain sizes ranged from sub-millimeter to centimeter size. An abundance of fine suspended particles in the air were not reported during or following the tephra fall.

Figure 58. Close up view looking at Pacaya tephra clasts collected in Guatemala City ~22 km NNE of the source. The smallest increments on ruler are in millimeters; the size range of grains here were mostly below ~ 3 mm diameter but grains under 0.2 mm were scarce to absent. The clasts consisted of black to dark brown vitric (crystal poor) scoria. Taken from REW (2013), who cited R. Cabria (personal communication).

As noted in table 4, in the afternoon on the 28th, high eruptive vigor resumed and tephra again fell on Guatemala City (CONRED 735). The ash fall on this day was lighter than on the 27th. Here aviation data (discussed below) did record the plume via satellite. The Washington Volcanic Ash Advisory Center (VAAC) noted (in their 6th advisory) an eruption in the afternoon on the 28th reaching (based on comparison of plume movement to modeling of winds aloft) ~13 km altitude.

During 29 May and onwards the intensity of volcanic activity decreased, with only relatively small eruptive plumes that occasionally produced minor tephra fall in the communities surrounding the volcano (CONRED 742). CONRED 748 noted that by the 29th, a total of 2,635 people were in shelters due to the eruption, with close to 800 home either damaged or destroyed. In the following days the attention of the emergency managers shifted from the eruption to the Tropical Storm Agatha, which had much broader extent and impact.

In the Pacaya and Guatamala City region, and along drainages carrying ash-charged run, both disasters combined. Lake AmatitlÁn rose, inundating low lying parts of the town with a water-and-ash mix (see photo documentation of impacts at Boston.com). Figure 59 is a photo taken ~12 km downstream of the Lake's outlet.

Figure 59. The Pacaya tephra fall combined with storm run from Agnes led to swollen rivers in a 'dual disaster.' Those rivers formed new deposits along their beds from large amounts of in-swept debris, in this case including large boulders, trees, and a badly battered vehicle in the foreground. This press photograph was taken on 30 May 2010 as the flood water dropped. The location was the municipality of Palin, which sits along the Michatoya river downstream of Lake AmatitlÁn and ~10 km W of Pacaya. Taken from Boston.com with credit to Johan Ordonez/AFP/Getty Images.

Seismic record. INSIVUMEH and REW (2013) suggested a climax on the 27th starting shortly before 1800 local time and lasting ~40 minutes.

The seismic signal (figure 60, upper panel) contained a few scattered high amplitude events during the morning of 27 May 2010. Seismicity rose significantly about 1200 on the 27th, about doubling the RSAM values recorded during the previous 13 hours.

Figure 60. Seismicity recorded at Pacaya during the 2300 of 26 May through 1700 on 28 May (local times). The upper panel shows the seismic record and the lower panel shows the computed RSAM. Station PCG is a short-period seismometer located on Cerro Chino, ~1 km NW of Pacaya's summit on the MacKenney cone. Courtesy of INSIVUMEH.

The first of about 10 strong peaks (seen on both the upper and lower panels of figure 60) took place around 1230 on the 27th. Those peaks represented a large escalation in seismicity an approximate doubling of the RSAM values. The highest peak on the record took place during 1730 to 1830 on the 27th, a ~6-fold increase in RSAM over the background values acquired earlier on the 27th. During the middle part of the 1800-1900 interval there was a peculiar several-minute-long period with low seismicity conspicuous on the seismic record (upper panel). After that, a series of closely spaced peaks of generally decreasing amplitude followed and then seismicity decreased substantially, particularly around 2300-2400 on the 27th. A second escalation of broadly similar size to the earlier one came on the 28th peaking at 1100 and then dropping.

In a later analysis of seismicity, Mercado and others (2012 correlated waveforms for 5 months before and 9 months after the May 2010 eruption. They noted that "No correlation was found between the events of each day during the five-month period before the eruption, thus, establishing no relationship with the periods of correlation found after the eruption. The post-eruptive sources of seismicity discovered were not active before the eruptive event of May 27, 2010, and therefore these sources must be strictly post-eruptive in nature."

Aviation. Although there were 48 reports (Volcanic Ash Advisories, VAA's or simply 'advisories') issued by the Washington Volcanic Ash Advisory Center (VAAC) on Pacaya behavior during the interval 27 May to 26 June 2010, weather clouds frequently masked the plume from the key satellite observation platform, the GOES-13 satellite. Where satellite observations of the plume were scarce or lacking, most of the VAA's conveyed ground-based observations including media reports.

By the 3rd advisory, which was issued on the 28th, considerable ash had fallen at the International airport Aurora. There is some confusion as to the quantity of ash at the airport and over the region in general, but a photo on the 28th shows ash at the airport. Judging from ash load on the aircraft, the f walking just to the right of the aircraft, and adjacent tire tracks, the ash was on the order of ~1-cm thick (figure 61). This is in accord with INSIVUMEH's summary report that said 5-7 mm of ash had fallen during the entire explosive 27-28 May eruption at the airport. This is also in accord with REW (2014), which discusses the complexities of assessing tephra thicknesses in more detail, and presents a preliminary isopach map that shows the S fringes of the Guatemala City urban area with 10 cm of ash and many parts of the urban area farther N, including the airport, with on the order of 1 cm of ash.

Figure 61. An American Airlines jet sits covered with ash from the Pacaya explosions at the International airport in Guatemala City on 28 May 2010. Runway cleanup took five days. The cleaning of the abrasive ash both destroyed the bituminous runway surface and all markings on it (Wardman and others, 2012). This photo was posted on the Boston Globe news website (Boston.com, see reference in table 5) with the credit to REUTERS/Daniel LeClair.

What follows is a summary of the advisories issued during 27 through 28 May (UTC).

The VAA's frequently refer to the NAM (North American Mesoscale Model), a numerical model for short-term weather forecasting and in this case wind-velocity estimation. The model is run 4 times a day with 12 km horizontal resolution and with 1 hour temporal resolution, providing finer detail than other operational forecast models. An example of a model with less detail is the model called GFS (Global Forecast System), which predicts weather for many regions of the world, and was sometimes also used by the VAAC analysts.

The VAAC issued their 1st VAA for Pacaya during 2010 on May 27 at 1140 UTC, citing as key information sources GFS winds and INSIVUMEH. Eruption details noted small brief ash emissions near the summit at 1115 UTC. The ash cloud was not identifiable from the GOES-13 satellite owing to rain. The ash cloud was inferred to have remained low and near the volcano. GFS wind data suggested that for such a low ash cloud at that time, wind-directed transport would carry a plume S-SW and would only be significant for ~20 km. The analyst noted that eruption as then dominantly lava emission.

The 2nd advisory came out 7 hours later at 1845 UTC on the 27th indicating volcanic ash and gases to ~3.5 km altitude (noting ICAO as an information source). Ash was again not identifiable from the GOES-13 satellite owing to clouds.

The 3rd advisory, noting 'ongoing emission of volcanic ash and gases,' came out at 1257 UTC on the 28th, again lacking clear satellite identification of ash owing to clouds, in this case citing a thick tropical depression. This advisory relied on both a wind model (NAM winds) and an aviation meteorological report (a METAR). The advisory further noted media reports of ash on runways as discussed in the context of figure 61.

The 4th advisory was issued at 1554 UTC on the 28th, noting "increasing emissions" at 1515 UTC with INSIVUMEH reporting ash rising to 3.7 km altitude (FL 120) and spreading up to 27 km NW. Again, owing to extensive weather clouds, ash was again not visible from GOES-13 satellite.

The 5th advisory was issued at 1710 UTC on the 28th, noting "ongoing emissions" recorded at 1645 UTC. Plume has now become visible in [GOES-13] imagery and extends about 15 NMI [Nautical miles, 27 km] to the NNE of the summit. Plume top was at 3.7 km altitude (FL 120).

The 6th advisory was issued at 1915 UTC on the 28th, noting a large eruption recorded at 1815 UTC: "Large eruption seen to FL420 [42,000 feet, ~13 km altitude] based on NAM sounding for the area. Forecast winds remain mostly westerly to northwesterly. Winds at the time of observation blew the plume E at ~18 km/hr.

The 7th advisory was issued at 1930 UTC on the 28th (the last one that day); it repeated information about the eruption seen in imagery around 1815. In this advisory the wind was moving NW at 27 km/hr.

Slope stability study. Schaefer and others (2013) evaluated slope stability at Pacaya and commented on the possible implications of the trough across the MacKenney cone (figure 57). They consider the trough noted above as an example of a recent, smaller-volume collapse.

Specifically, they studied the SW flank of the edifice and developed a geomechanical model based upon field observations and laboratory tests of intact rocks from Pacaya. Their study included analysis of slope stability using numerical techniques and consideration of forces from gravity, magmatic pressure, and seismic loading as triggering mechanisms for slope failure.

Given the cone's structural and seismo-tectonic setting, the likely magma pressures, and the history of past behavior, they suggested Pacaya lacked substantial gravitational stability.

References. Bonis, S., 1993, Mapa Geologico de Guatemala Escala 1:250,000. Hoja ND 15 - 8 - G, "Guatemala". First edition (map). IGN, Guatemala.

Conway, M., 1995, Construction patterns and timing of volcanism at the Cerro Quemado, Santa María, and Pacaya volcanoes, Guatemala. Ph. D. Dissertation. Michigan Technological University, Houghton, Michigan. 152 pp.

Escobar Wolf, R, 2013 (Report in preparation, July 2013), The eruption of VolcÁn de Pacaya on May -June, 2010, Michigan Technological University, 31 pp.

Escobar-Wolf, R., & Tubman, S., in preparation, Compilation of historical and recent accounts of eruptions from volcan de Pacaya (XVI-IXX centuries).

Eggers, A., 1972, The geology and petrology of the AmatitlÁn quadrangle, Guatemala. Ph. D. Dissertation. Dartmouth College, Hanover, New Hampshire. 221 pp.

Eggers, A., 1969, Mapa Geologico de Guatemala Escala 1:50,000. Hoja 2059 II G, " AmatitlÁn ". First edition (map). IGN, Guatemala.

Fiske, R. S., Rose, T. R., Swanson, D. A., Champion, D. E., & McGeehin, J. P., 2009, Kulanaokuaiki Tephra (ca. AD 400-1000): Newly recognized evidence for highly explosive eruptions at Kīlauea Volcano, Hawai'i. Geological Society of America Bulletin, vol. 121, no. 5-6), pp. 712-728

Hetland, B., 2010a, Volcano Pacaya, Cleaning up the eruption of Pacaya, One roof at a time…; Online manuscript by Brianna "Adriana" Hetland, US Peace Corps Volunteer 2010-2012, (URL: http://www.geo.mtu.edu/~raman/Pacaya.Bri.pdf )

Hetland, B., 2010b, Erupción del VolcÁn de Pacaya 27 Mayo 2010; X Congreso Geológico de América Central, Antigua, Guatemala, November 10, 2010. [Power Point from talk at Geologic Conference; in Spanish] (URL: http://www.geo.mtu.edu/rs4hazards/conferencepresentations_files/conferenceshz.htm )

Kitamura S., and Matías O., 1995. Tephra stratigraphic approach to the eruptive history of Pacaya volcano, Guatemala. Science Reports-Tohoku University, Seventh Series: Geography. 45 (1): 1-41.

Matías Gómez, RO, Rose, WI, Palma, JL, Escobar-Wolf, R, 2012, Notes on a map of the 1961-2010 eruptions of VolcÁn de Pacaya, Guatemala. Geol Soc Am Digital Map Chart Series 10: 10 pp.

Matías , O., 2010, Volcanological map of the 1961 - 2009 eruption of Volcan de Pacaya, Guatemala. MS. Thesis. Michigan Technological University, Houghton, Michigan. 57 pp.

Mercado, D, Waite, G, and Rodriguez, L, 2012, Analysis of seismic patterns before and after the May 27, 2010 eruption of Pacaya volcano, Guatemala, Cities on Volcanoes 7 (COV7), Abstract volume, IAVCEI meeting (19-23 November 2012, Colima, Mexico) (URL: http://www.citiesonvolcanoes7.com/vistaprevia2.php?idab=510)

Morgan, HA, Harris, AJL, and L. Gurioli, 2013, Lava discharge rate estimates from thermal infrared satellite data for Pacaya Volcano during 2004-2010, Jour.of Volcanology and Geoth. Res., Vol. 264, pp. 1-11, ISSN 0377-0273, http://dx.doi.org/10.1016/j.jvolgeores.2013.07.008.

Prensa Libre, 2010 [19 April at 01:17 Nacionales]; Mueren tras explosion (URL: http://www.prensalibre.com/noticias/Mueren-explosion_0_246575377.html; http://tinyurl.com/2vvaj4j ) [in Spanish]

Prense Libre, 17 May 2010 [21:38 Nacionales], Restringen acceso al volcÁn de Pacaya (URL: http://www.prensalibre.com/noticias/Restringen-acceso-volcan-Pacaya_0_263373950.html; http://tinyurl.com/2wb8p9t) [in Spanish]

Prensa Libre, 27 May 2010 Aumenta actividad volcÁnica en el Pacaya (URL: http://www.prensalibre.com/noticias/volcan-pacaya-actividad-erupcion_0_269373197.html; http://tinyurl.com/246hohe)

Rose, W. I., Palma, J. L., Wolf, R. E., & Gomez, R. O. M., 2013, A 50 yr eruption of a basaltic composite cone: Pacaya, Guatemala. Geological Society of America Special Papers, 498, 1-21.

Schaefer, L. N., Oommen, T., Corazzato, C., Tibaldi, A., Escobar-Wolf, R., & Rose, W. I., 2013, An integrated field-numerical approach to assess slope stability hazards at volcanoes: the example of Pacaya, Guatemala. Bull Volcanol, 75, 720.

Sparks, R. S. J., 1986, The dimensions and dynamics of volcanic eruption columns. Bulletin of Volcanology, vol. 48, no. 1, pp. 3-15.

Wardman, J., Sword-Daniels, V., Stewart, C., & Wilson, T. M., 2012, Impact assessment of the May 2010 eruption of Pacaya volcano, Guatemala (No. 2012/09). GNS Science (New Zealand)

Walikainen, Dennis, 2010 [June 4], Eyewitness to Disasters: Graduate Student Reports from Guatemala (URL: http://www.mtu.edu/news/stories/2010/june/eyewitness-disasters-graduate-student-reports-guatemala.html)

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: Rüdiger Escobar Wolf, Department of Geological Engineering and Sciences, Michigan Tech University, Houghton, MI 49931; INSIVUMEH Seccion Vulcanologia (Institute National de Sismologia, Vulcanologia, Meteorolgia, e Hidrologia) 7a Avenida, Zona 13, Guatemala City, Guatemala; Gustavo Chigna,.INSIVUMEH; CONRED (Coordinadora Nacional para la Reducción de Desastres) Avenida Hincapié 21-72, Zona 13 Guatemala, Ciudad de Guatemala; and Washington Volcanic Ash Advisory Center (VAAC), NOAA Satellite Analysis Branch, NOAA NESDIS OSPO, E/SP, NCWCP, 5830 University Research Court, College Park, MD 20740 (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).