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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Remote Heard Island in the southern Indian Ocean is home to the snow-covered Big Ben stratovolcano, which has had confirmed intermittent activity since 1910. The nearest continental landmass, Antarctica, lies over 1,000 km S. Visual confirmation of lava flows on Heard are rare; thermal anomalies and hotspots detected by satellite-based instruments provide the most reliable information about eruptive activity. Thermal alerts reappeared in September 2012 after a four-year hiatus (BGVN 38:01), and have been intermittent since that time. Information comes from instruments on the European Space Agency's (ESA) Sentinel-2 satellite and MODVOLC and MIROVA thermal anomaly data from other satellite instruments. This report reviews evidence for eruptive activity from November 2017 through September 2018.

Satellite observations indicated intermittent hot spots at the summit through 12 December 2017. A few observations in January and February 2018 suggested steam plumes at the summit, but no significant thermal activity. An infrared pixel indicative of renewed thermal activity appeared again on 7 March, and similar observations were made at least twice each month in April and May. Activity increased significantly during June and remained elevated through September 2018 with multiple days of hotspot observations in satellite data each of those months, including images that indicated lava flowing in different directions from Mawson Peak. MODVOLC and MIROVA data also indicated increased thermal activity during June-September 2018.

Activity during October-December 2017. MIROVA thermal anomalies recorded during October 2017 indicated ongoing thermal activity at Heard (figure 32). This was confirmed by Sentinel-2 satellite imagery that revealed hotpots at the summit on ten different days in October (3, 6, 8, 13, 16, 21, 23, 26, 28, and 31), and included images suggesting lava flows descending from the summit in different directions on different days (figure 33).

Figure 32. MODVOLC thermal alerts indicated significant thermal activity at Heard during October 2017 that tapered off during November. Intermittent signals appeared in December 2017, March, and April 2018, and a strong signal returned in June 2018 that continued through September. Courtesy of MIROVA.

Figure 33. Sentinel-2 images of Heard Island's Big Ben volcano during October 2017 showed strong evidence of active effusive activity. a) 3 October 2017: at least three hot spots were visible through cloud cover at the summit and W of Mawson Peak, suggesting active lava flows. b) 6 October 2017: a small hot spot is visible at the peak with a small steam plume, and a larger hotspot to the NW suggested a still active lava flow. c) 16 October 2017: a small hotspot at the summit and larger hotspots W of the summit were indicative of ongoing flow activity. d) 23 October 2017: a steam plume drifted SE from a small summit hotspot and a larger hotspot to the W suggested a lava lake or active flow. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

The MODVOLC thermal alert data showed no further alerts for the year after 22 October 2017, and the MIROVA system anomalies tapered off in mid-November 2017. The Sentinel-2 satellite imagery, however, continued to record intermittent hotspots at and around Mawson Peak, the summit of Big Ben volcano, into December 2017 (figure 34). Hotspots were visible during six days in November (7, 15, 20, 25, 27, and 30) and three days during December (5, 7, and 12).

Figure 34. Sentinel-2 images of Heard Island's Big Ben volcano showed reduced but ongoing thermal activity during November and December 2017. a) 7 November 2017: a steam plume drifts NE from a hotspot at Mawson Peak. b and c) 15 November and 12 December 2017: a small hotspot is distinct at the summit. d) 20 December 2017: a steam plume drifts east from the peak, but no clear hotspot is visible. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Activity during January-May 2018. The satellite images during January and February 2018 were indicative of steam plumes at the summit, but distinct thermal signals reappeared on 7 and 12 March 2018 (figure 35). In spite of extensive cloud cover, the Sentinel-2 imagery also captured thermal signals twice each month in April (4 and 14) and May (9 and 14) (figure 36).

Figure 35. Sentinel-2 images of Heard Island's Big Ben volcano showed only steam plumes at the summit during January and February, but hotspots reappeared in March 2018. a) 4 January 2018: a steam plume drifts SE from the summit under clear skies. b) 8 February 2018: a steam plume drifts SE from the summit adjacent to a large cloud on the N side of the volcano. c) 7 March 2018: the first hotspot in about three months is visible at the summit. d) 12 March 2018: a distinct hotspot is visible at Mawson Peak. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Figure 36. Sentinel-2 images of Heard Island's Big Ben volcano showed intermittent low-level thermal activity during April and May 2018. a) 4 April 2018: a small hotspot is visible at the summit through a hazy atmosphere. b) 9 May 2018: a distinct hotspot glows from the summit beneath cloud cover. Sentinel-2 images with Atmospheric Penetration view(bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Activity during June-September 2018. Thermal signals increased significantly in the satellite data during June 2018. The sizes of the thermal anomalies were bigger, and they were visible at least nine days of the month (3, 5, 8, 10, 15, 18, 23, 25, and 30). Five substantial thermal signals appeared during July (3, 10, 15, 18, and 28); images on 23 June and 3 July distinctly show a lava flow trending NE from the summit (figure 37). MODVOLC thermal alerts appeared in June 2018 on three days (2, 26, and 27) and on four days during July (7, 8, 9, 10) indicating increased activity during this time. The MIROVA thermal signals also showed a substantial increase in early June that peaked in mid-July and remained steady through September 2018 (figure 32).

Figure 37. Sentinel-2 images of Heard Island's Big Ben volcano showed significantly increased thermal activity during June and July 2018. a) 8 June 2018: a substantial hotspot is visible through the cloud cover at the summit of Big Ben. b) 10 June 2018: the darker red hotspot at Mawson Peak was significantly larger than it was earlier in the year. c) 23 June 2018: the first multi-point hotspot since 31 October shows a distinct glow trending NE from the summit. d) 3 July 2018: a trail of hotspots defines a lava flow curving NNE from Mawson Peak. e) 18 July 2018: a second significant hotpot is visible a few hundred meters NE of the summit hotspot indicating a still active flow. f) 28 July 2018: the summit hotspot continued to glow brightly at the end of July, but no second hotspot was visible. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Six images in August (2, 7, 9, 22, 27, 29) showed evidence of active lava at the summit, and suggested flows both NE and SE from the summit that were long enough to cause multiple hotspots (figure 38). During September and early October 2018 the satellite images continued to show multiple hotspots that indicated flow activity tens of meters SE from the summit multiple days of each month (figure 39).

Figure 38. Sentinel-2 images of Heard Island's Big Ben volcano showed lava flow activity in two different directions from the summit during August 2018. a) 2 August 2018: lava flows NE from Mawson Peak while a steam plume drifts E from the summit. b) 9 August 2018: a second hotspot NE of the summit hotspot indicates continued flow activity in the same area observed on 2 August. c and d) 27 and 29 August 2018: a different secondary hotspot appeared SSE from the summit indicating a distinct flow event from the one recorded earlier in August. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Figure 39. Sentinel-2 images of Heard Island's Big Ben volcano in September and October 2018 showed hotspots indicating active flows SE of the summit on multiple days. a) 3 September 2018: a small hotspot at the summit and a larger hotspot SE of the summit indicated continued flow activity. b) 3 October 2018: a small steam plume drifted east from a small hotspot at the summit and a larger pair of hotspots to the SE indicated continued effusive activity. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/).

The first confirmed eruption of Kadovar began on 5 January 2018 with dense ash plumes and steam and a lava flow. The eruption continued through February and then slowed during March (BGVN 43:04). This report describes notices of ash plumes from the Darwin Volcanic Ash Advisory Centre (VAAC) and satellite images during April through 1 October 2018.

According to the Darwin VAAC a pilot observed an ash plume rising to an altitude of 1.2 km on 10 June. The ash plume was not identified in satellite data. Another ash plume identified by a pilot and in satellite images rose to an altitude of 1.8 km on 20 June and drifted W. An ash plume was visible in satellite images on 28 September drifting SE at an altitude of 2.1 km. On 1 October an ash plume rose to 2.7 km and drifted W.

Infrared satellite data from Sentinel-2 showed hot spots in the summit crater and at the Coastal Vent along the W shoreline on 10, 15, and 25 April 2018; plumes of brown discolored water were streaming from the western side of the island (figure 18). Similar activity was frequently seen during clear weather in the following months. A steam plume was also often rising from the crater. The Coastal Vent cone was still hot on 8 August, but no infrared anomaly was seen in imagery from 28 August through September.

Figure 18. Sentinel-2 natural color satellite image of Kadovar on 10 April 2018. The island is about 1.5 km in diameter. Steam can be seen rising from the summit and the Coastal Vent just off the western shore; both locations show thermal anomalies in infrared imagery. Discolored water plumes extend NE from the island. Courtesy of Sentinel Hub Playground.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The most recent eruptive period at Karymsky, on the Kamchatka Peninsula of Russia, began on 28 April 2018, with thermal anomalies, gas-and-steam emissions, and ash plumes observed through July 2018. The current report discusses activity through September 2018 (table 11). This report was compiled using information from the Kamchatka Volcanic Eruptions Response Team (KVERT).

KVERT reported ongoing thermal anomalies and intermittent ash plumes over Karymsky during August and September 2018 (table 11). Ash plumes drifted 50 km SE on 7 August, and 40 km S on 25 August. Stronger activity during 10-11 September consisted of continuous dense ash emissions along with explosions that sent plumes 5-6 km high which drifted 860 km NE. Incandescence photographed the next night was attributed to fumarolic activity (figure 41). Ash plumes were identified drifting 365 km E on 22-23 September. The last thermal anomaly was identified in satellite images on 28 September, and an ash plume was last visible on 30 September.

Table 11. Ash plumes and thermal anomalies at Karymsky, 1 August-30 September 2018. Clouds often obscured the volcano. Data compiled from KVERT reports.

Figure 41. Incandescence, attributed to fumarolic activity, was visible above the crater of Karymsky on 12 September 2018. Photo by D. Melnikov; courtesy of Institute of Volcanology and Seismology (IVS FEB RAS, KVERT).

Figure 42. Sentinel-2 satellite imagery of Karymsky on 30 September 2018 showing a diffuse plume and thermal anomaly in the crater. Top: Natural color view (bands 4, 3, 2). Bottom: Short-wave Infrared view (bands 12, 8A, 4). Courtesy of Sentinel Hub Playground.

Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were last observed on 31 July 2018. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected one hotspot in early August (moderate power), and two hotspots in late September (low power).

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

Information Contacts: 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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); 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://hotspot.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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Gas-and-steam emissions were previously reported at Ketoi (figure 1) in January, July, and August 2013 (BGVN 40:09). Intense fumarolic activity originating from the same area, the N slope of Pallas Peak, was reported in 1981, 1987, and 1989. Based on a report from the Sakhalin Volcanic Eruption Response Team (SVERT) using Himawari-8 imagery, the Tokyo VAAC reported an ash plume on 21 September 2018 which drifted to the NE; however, evidence of the plume could not be confirmed by the VAAC from satellite imagery. The original VONA (Volcano Observatory Notice for Aviation) issued by SVERT noted a volcanic cloud without a specific mention of ash, but also remarked that thermal anomalies had been observed on 17 and 20 September.

Figure 1. Natural color Sentinel-2 satellite image of Ketoi on 18 September 2018. A large freshwater lake can be seen SW of the Pallas Peak andesitic cone, which also hosts a crater lake. Lava flows originating from the younger cone extend primarily N to SW, and a white fumarolic area is immediately NE of the crater. The island is approximately 10 km in diameter. Courtesy of Sentinel Hub Playground.

Geologic Background. The circular 10-km-wide Ketoi island, which rises across the 19-km-wide Diana Strait from Simushir Island, hosts of one of the most complex volcanic structures of the Kuril Islands. The rim of a 5-km-wide Pleistocene caldera is exposed only on the NE side. A younger stratovolcano forming the NW part of the island is cut by a horst-and-graben structure containing two solfatara fields. A 1.5-km-wide freshwater lake fills an explosion crater in the center of the island. Pallas Peak, a large andesitic cone in the NE part of the caldera, is truncated by a 550-m-wide crater containing a brilliantly colored turquoise crater lake. Lava flows from Pallas Peak overtop the caldera rim and descend nearly 5 km to the SE coast. The first historical eruption of Pallas Peak, during 1843-46, was its largest.

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Kīlauea's East Rift Zone (ERZ) has been intermittently active for at least two thousand years. Open lava lakes at the summit caldera, and a lava lake and flows from the East Rift Zone, have been almost continuously active since the current eruption began in 1983. A marked increase in seismicity and ground deformation at Pu'u 'O'o Cone on the upper East Rift Zone during the afternoon of 30 April 2018 and the subsequent collapse of its crater floor marked the beginning of the largest lower East Rift Zone eruptive episode in at least 200 years. The daily events of this episode underscored the nature of the interconnected components of the volcanic system. The lava lake level at Halema'uma'u began dropping on 1 May 2018, and fissures first opened on the lower East Rift Zone two days later. The eruptive events of May 2018 (figure 332), the first month of this episode, are described in this report with information provided primarily from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) in the form of daily reports, volcanic activity notices, and abundant photo, map, and video data.

Figure 332. A timeline of events at Kīlauea for 1-28 May 2018. Blue shaded region includes seismic events greater than M 4.0 and activity at Halema'uma'u crater at the summit. Green shaded area lists activity on the lower East Rift Zone.

Summary of events. During 1-11 May, seismicity propagated eastward from Pu'u 'O'o, indicating the intrusion of magma into the middle and lower East Rift Zone (LERZ). The first surface cracks appeared in the LERZ on 2 May, and the first eruptive fissure opened the next afternoon. Fissures were several hundred meters long and formed a NE-SW trending line near the axis of the LERZ that reached about 4 km in length by 8 May. Three large (greater than M 5) earthquakes shook the island on 4 May. New fissures opened daily, with activity consisting of spattering and lava flows; the largest effusion was a flow from fissure 8 that traveled N for a little over 1 km. By 9 May, 15 fissures had opened in the vicinity of the Leilani Estates subdivision; the residents had been evacuated and numerous structures were destroyed by the flows, spatter, and fissures. Active spattering paused on 10 and 11 May, although strong degassing continued, and many cracks within the fissure zone continued to widen.

The lava lake level in the Overlook vent at Halema'uma'u crater at the summit began to drop slowly on 1 May; the rate of deflationary tilt increased by late the next afternoon. The lake level had dropped about 128 m below the vent rim by 5 May, and satellite data indicated a 10 cm subsidence of the Halema'uma'u crater floor during that time. Rockfalls from the crater walls produced ash plumes above Halema'uma'u, resulting in light ashfall in the summit area. By 8 May the lake level was 295 m below the floor of Halema'uma'u crater. Larger rockfalls caused by the dropping lake level generated larger explosions and ash plumes on 9 May.

New fissures began opening again on 12 May in an area about 1.5 km NE of the previous activity on the LERZ, and over the following 11 days flow activity increased substantially, creating multiple flow channels. The fissure 17 flow reached 2.5 km in length on 15 May. A fast-moving flow from fissure 20 headed 4 km SE on 18 May, traveling over 300 m per hour. The two lobes of the flow reached the SE Puna coast overnight on 19-20 May and were joined by another adjacent flow to the west two days later. Fissures 16-23, all located in the NE half of the fissure zone, were active during 12-23 May.

Steam and ash emissions were persistent at Halema'uma'u crater during 12-23 May; they varied in intensity with abrupt increases associated with large rockfalls into the vent, and ashfall reported more than 50 km from the summit. Strong earthquakes at the summit continued in response to the deflation, causing frequent ground shaking and damage to roads and buildings. A large explosion on 17 May generated the highest ash plume for the period; it reached 9.1 km altitude and drifted NE. Intermittent explosive eruptions continued at the summit, and robust plumes of gas, ash, and steam periodically emerged from the Overlook vent.

Beginning on 24 May, activity on the LERZ shifted back towards the SW part of the fissure zone, again impacting the residents of the Leilani Estates subdivision with reactivation of fissures and new flows. While flows continued to reach the ocean on the SE coast, the volume of lava gradually decreased until the supply of lava ceased by 28 May. During 24-26 May fissures 7 and 21 were feeding a perched lava pond and flows that moved E and N within the subdivision. Overnight on 26-27 May activity increased substantially at fissure 7 with a 30-m-tall spatter rampart, and fountains that reached 70 m high feeding a flow moving N. Large cracks opened into fissure 24 adjacent to the reactivated fissure 8; fast-moving flows traveled W then N through the subdivision. By 28 May the eruptive activity was focused on vigorous fountaining at fissure 8, which supplied a voluminous flow that headed rapidly NE.

Intermittent explosions continued from the summit Overlook vent during 24-28 May as a result of the ongoing subsidence at Halema'uma'u. Ash clouds generally rose to about 3.1 km altitude and drifted SW. Earthquakes in the summit region continued as the summit area subsided and adjusted to the withdrawal of magma.

Activity during 1-11 May 2018. An intrusion of magma began overnight on 30 April-1 May into the lower East Rift Zone (LERZ), extending from the vicinity of Pu'u 'O'o eastward at least as far as Highway 130 (15 km E of Pu'u 'O'o). The intrusion began after the collapse of the Pu'u 'O'o crater floor on the afternoon of 30 April; about 250 located earthquakes were reported through the afternoon of 1 May, with the locations migrating eastward during the day. The seismicity consisted primarily of small-magnitude (less than M 3) earthquakes at depths of less than 10 km. During a helicopter overflight to Pu'u 'O'o on 1 May, HVO geologists observed a new fissure and crack extending about 1 km uprift (west) from the W flank of the Pu'u 'O'o cone (figures 333 and 334). A small amount of lava had erupted from the crack, apparently during the collapse of the Pu'u 'O'o crater floor. They also noted a few small, sluggish breakouts of the 61g lava flow, likely from lava still moving through the lava-tube system.

Figure 333. HVO geologists observed a fracture on the W flank of Pu'u 'O'o at Kīlauea during an overflight on the afternoon of 1 May 2018. The 61g flow field as of 13 April 2018 is shown in pink. The crack that formed on the west side of Pu'u 'O'o on 30 April 2018, during or immediately after the crater floor collapse, is shown as a solid red line. Older Pu'u 'O'o lava flows (1983–2016) are shown in gray. The yellow line is the trace of the active lava tubes. The blue lines over the Pu'u 'O'o flow field are steepest-descent paths calculated from a 2013 digital elevation model (DEM), while the blue lines on the rest of the map are steepest-descent paths calculated from a 1983 DEM. Courtesy of HVO.

Figure 334. A new fissure appeared on the W flank of Pu'u 'O'o at Kīlauea on 1 May 2018. Top: In this view to the NE, the fissure was visible as a line of white steam extending roughly 1.5 km W of Pu'u 'O'o Crater. Photo taken 3 May 2018. Bottom: A telephoto view of a small lava flow (lighter in color) and spatter (blue-gray) that were erupted from a section of the crack on the west flank of Pu'u 'O'o. Photo taken during overflight on 1 May 2018, courtesy of HVO.

Overnight on 1-2 May, earthquakes continued at a high rate in the area from Highway 130 eastward towards Kapoho (32 km NE of Pu'u 'O'o). Many events were felt by residents and there were reports of nearly constant ground vibration in some areas; seismicity generally migrated eastward (figure 335). During the morning of 2 May a GPS station located about 1.5 km SW of Nanawale Estates (24 km NE of Pu'u 'O'o ) began moving toward the north by several centimeters, indicating the approaching magma intrusion. A tiltmeter at Pu'u 'O'o recorded steady, deflationary tilt throughout the day, with several sharp inflation offsets. Some of these offsets corresponded to short-lived ashy plumes rising from the crater. New small ground cracks less than a few centimeters wide developed across some roads in and adjacent to Leilani Estates (23 km NE of Pu'u 'O'o).

Figure 335. Starting on the afternoon of 30 April 2018, magma beneath Kīlauea's Pu'u 'O'o Cone drained and triggered the collapse of the crater floor. Within hours, earthquakes began migrating east of Pu'u 'O'o, signaling an intrusion of magma along the middle and lower East Rift Zone (ERZ). As of about noon on 2 May there were many reports of earthquakes felt by residents in nearby subdivisions. The orange oval marks the approximate area within which most of the earthquakes were located based on automatic earthquake locations and analysis by seismologists. A GPS device located in Nanawale Estates began moving towards the N on 2 May, and new, small ground cracks were reported in the Leilani Estates area. Courtesy of HVO.

The summit lake level showed very little change immediately after the collapse of the Pu'u 'O'o crater floor, but tiltmeters at the summit began recording deflationary tilt in the early morning of 1 May. The lava lake level had dropped about 20 m by the afternoon of 2 May when the rate of deflationary tilt increased. By the evening of 3 May, it had dropped an additional 37 m.

At 1030 HST on 3 May 2018 ground shaking from a M 5.0 earthquake south of Pu'u 'O'o caused rockfalls and possibly additional collapse into the Pu'u 'O'o crater (figure 336). A short-lived plume of ash produced by this event rose and dissipated as it drifted SW (figure 337).

Figure 336. Clear weather on 3 May 2018 allowed good airborne observations of the collapse crater in Pu'u 'O'o. This view to the E shows the deep collapse crater that formed on 30 April when magma beneath Pu'u 'O'o drained. For scale, the crater is about 250 m wide. Courtesy of HVO.

Figure 337. At 1030 HST on 3 May 2018 ground shaking from a preliminary M 5.0 earthquake south of Pu'u 'O'o caused rockfalls and possibly additional collapse into the Pu'u 'O'o crater on Kīlauea's East Rift Zone. A short-lived plume of ash produced by this event rose and dissipated as it drifted SW. USGS photo by Kevan Kamibayashi, courtesy of HVO.

New ground cracks were reported in Leilani Estates late in the afternoon of 3 May. Hot white vapor and blue fumes emanated from an area of cracking in the eastern part of the subdivision. Spatter began erupting from the cracks shortly before 1700 local time. The Hawaii County Civil Defense coordinated the evacuation of the subdivision. Lava spatter and gas bursts erupted from the fissure for about two hours; lava spread less than 10 m (figure 338).

Figure 338. An eruption from fissure 1 began in the Leilani Estates subdivision at the lower East Rift Zone of Kīlauea during the afternoon of 3 May 2018. Hot white vapor and blue fumes emanated from an area of cracking in the eastern part of the subdivision. Spatter began erupting shortly before 1700 HST; lava was confirmed at the surface in the eastern end of the subdivision. According to the Hawai'i County Civil Defense update at 1740 all residents in Leilani Estates and Lanipuna Gardens subdivsions were required to evacuate. View is to the NE on 3 May, courtesy of HVO.

By the morning of 4 May 2018 three fissures had opened in the eastern portion of Leilani Estates; activity consisted primarily of vigorous lava spattering and development of short lava flows (figure 339). Additional eruptive fissures or vents opened during the day, each several hundred meters long (figure 340). Spatter and lava accumulated primarily within a few tens of meters of the vents. Fissure 6 opened on the eastern edge of the subdivision by the afternoon. Between 1130 and 1500 three large earthquakes (M 5.4, M 6.9, and M 5.3) shook the island along with numerous lower-magnitude events. The M 6.9 event at 1232 HST (the largest on the island in 43 years) produced a robust ash plume at Pu'u 'O'o (figure 341), and numerous rockfall events were triggered at both Pu'u 'O'o and Halema'uma'u craters.

Figure 339. Fissure 3 was actively erupting at Leilani and Kaupili Streets in the Leilani Estates subdivision of the lower East Rift Zone at Kīlauea at 0807 HST on 4 May 2018. Lava on the road was approximately 2 m thick. Courtesy of HVO.

Figure 340. Fissure 5 in the lower foreground was actively erupting at 1207 HST on 4 May 2018 in the Leilani Estates subdivision at Kīlauea. View is to the SW. Behind fissure 5 are fissures 1, 4, and 3 from front to back. Courtesy of HVO.

Figure 341. At 1246 HST on 4 May 2018 a column of reddish-brown ash rose from Pu'u 'O'o crater at Kīlauea after a M 6.9 south flank earthquake shook the island. View is to the S with the steaming fissure on the SW flank of the cone visible behind to the right of the ash plume. Courtesy of HVO.

Fissures 7, 8, and 9 opened in the Leilani Estates subdivision on 5 May. Fissure 7 was only active until mid-afternoon (figure 342). Fissure 8 activity included fountaining and occasional bursts of spatter to 100 m as well as building a spatter cone (figure 343); the flow from fissure 9 migrated W. New ground cracks were reported on Highway 130 along the W edge of the subdivision.

Figure 342. Fissure 7 began erupting around dawn on 5 May 2018 at Kīlauea and was active for several hours. At the peak of its activity, large bubble bursts occurred at one spot (lower left) in the fissure while spattering was present in other locations. A short lava flow was erupted from the fissure around 0800, moving NE and crossing Hookupu Street (left). View is to the S, emissions in upper right are from fissure 2. Image taken on 5 May 2018, courtesy of HVO.

Figure 343. Fissure 8 erupted in the evening on 5 May 2018 at Kīlauea, located near fissures 2 and 7; it began with small amounts of lava spattering at about 2044 HST. By 2100, lava fountains as high as about 70 m (when this photo was taken) were erupting from the fissure. Courtesy of HVO.

Tiltmeters at the summit continued to record a deflationary trend. Satellite InSAR data showed that between 23 April and 5 May 2018 the summit caldera floor subsided about 10 cm. Corresponding to this deflation, the lava lake in the Overlook vent had dropped about 128 m below the crater rim during that same period. Rockfalls from the crater walls into the retreating lake produced ashy plumes above Halema'uma'u crater, resulting in light ashfall in the summit area. During 4 and 5 May about 152 M 2 and M 3 earthquakes occurred at depths less than 5 km beneath the summit area. These earthquakes were related to the ongoing subsidence of the summit area and south flank of the volcano.

Fissure 8 erupted lava fountains until about 1600 on 6 May. By early that afternoon, ten fissures had opened in the Leilani Estates subdivision, but not all were continuing to erupt (figure 344). A lava flow from fissure 8 advanced northward, reaching 1.1 km in length by early evening (figure 345). Deflation continued at the summit with the lava lake dropping at a rate of about 2 m per hour throughout the day, and by evening it had dropped a total of 220 m since 30 April (figure 346).

Figure 344. Between 3 and 6 May 2018 ten fissures opened in the Leilani Estates area of the lower East Rift Zone at Kīlauea. This thermal map shows the ''a'a flow from fissure 8 spreading northward (top) during an overflight of the area on the afternoon of 6 May. The dark area is the extent of the thermal map. Temperature in the thermal image is displayed as gray-scale values, with the brightest pixels indicating the hottest areas (whitish areas show the active lava flow). The gray linear features are the other fissures (numbered in red). The thermal map was constructed by stitching overlapping oblique thermal images collected by a handheld thermal camera during a helicopter overflight of the flow field. The base is a copyrighted color satellite image (used with permission) provided by Digital Globe. Courtesy of HVO.

Figure 345. A lava flow from fissure 8 flowed N on Makamae Street in Leilani Estates at Kīlauea at 0932 HST on 6 May 2018. Courtesy of HVO.

Figure 346. The Kīlauea summit lava lake began dropping on 1 May 2018, and by the evening of 6 May when this image was taken it was roughly 220 m below the crater rim. This very wide-angle camera view captured the entire north portion of the Overlook crater. Courtesy of HVO.

Two new fissures broke ground in the morning on 7 May. The first (fissure 11) opened in a forested area SW of Leilani Estates and was active for only 3 hours. The second (fissure 12) opened around noon between fissures 10 and 11. By 1515 both new fissures were inactive but the west end of fissure 10 was steaming heavily. Cracks on Highway 130 widened from 7 to 8 cm over the course of the day and additional cracks were found just W of the highway on trend with the previous fissures (figure 347).

Figure 347. Cracks several centimeters wide had opened along Highway 130 by 0930 HST on 7 May 2018. The highway runs N-S along the W edge of the Leilani Estates subdivision on Kīlauea's East Rift Zone, about 15 km E of Pu'u 'O'o. Orange paint was used to outline the cracks. Courtesy of HVO.

By the evening of 8 May 2018, the overall fissure zone was about 4 km long (figure 348), stretching SW-NE across most of the now-evacuated Leilani Estates subdivision; 14 distinct fissures had been mapped, and a lava flow starting from fissure 8 had traveled about 1 km NE from its source. Officials noted that 35 structures had been destroyed. Loud jetting and booming noises were heard from fissure 13 that evening. Rockfalls into the Overlook vent at the summit were intermittently producing small ash plumes that rose several hundred meters above the summit and traveled downwind as the lava lake continued to fall. Based on model data collected in the afternoon, the lake level was about 295 m below the floor of Halema'uma'u Crater by the end of the day on 8 May.

Figure 348. The 4-km-long fissure zone that began erupting on 3 May 2018 at Kīlauea's lower East Rift zone had crossed most of the Leilani Estates subdivision by 1900 on 8 May with 14 distinct fissures within the zone. An ''a'a flow from fissure 8 had traveled about 1 km NE since it emerged on the evening of 5 May. Inset shows numbered locations of each fissure in red. The purple areas are lava flows erupted in 1840, 1955, 1960, and 2014-2015. Courtesy of HVO.

At 0832 HST on 9 May 2018, a large rockfall from the steep crater walls into the summit lava lake triggered an explosion that generated an ash column above Halema'uma'u crater; the ash was blown SSW (figure 349). During the day on 9 May fissure 15 broke ground at the NE edge of the LERZ fissure area and generated a pahoehoe flow about 20 m long. Severe ground cracks associated with fissure 14 were steaming vigorously in the morning (figure 350). During an overflight around 1500 in the afternoon HVO geologists also noticed an area of steaming uprift (west) of Highway 130 at the SW edge of the fissure area. Rates of motion increased late in the morning at a GPS station located 1.5 km SE of Nanawale Estates (about 2 km N of Leilani Estates). The direction of motion was consistent with renewed movement of magma in the downrift direction (to the NE).

Figure 349. An ash plume rose from Halema'uma'u crater at the summit of Kīlauea around 0830 on 9 May 2018. HVO's interpretation was that the explosion was triggered by a rockfall from the steep walls of the Overlook vent. The photograph was taken at 0829 HST from the Jaggar Museum overlook. Geologists examining the ash deposits on the rim of Halema'uma'u crater found fresh lava fragments ejected from the lava lake. Courtesy of HVO.

Figure 350. Severe ground cracks associated with fissure 14 in Leilani Estates at Kīlauea were steaming vigorously at 0953 on 9 May 2018. Courtesy of HVO.

Strong degassing continued from existing fissures on 10 and 11 May; although active spatter and lava had paused, several cracks within the fissure zone continued to widen (figure 351). A 3D model constructed of thermal images of Pu'u 'O'o crater taken on 11 May indicated that the deepest part of the crater was 350 m below the rim (figure 352). Hawai'i Volcanoes National Park closed to the public on 11 May due to heightened (daily) earthquake activity at the summit, and concerns about a potentially larger summit explosion.

Figure 351. Ground cracks continued to widen near Leilani Estates subdivision at Kīlauea on 10 May 2018 even though active spatter and lava flows had paused. At 1354 a geologist inspected a crack that widened considerably during the previous day on Old Kalapana Road. In other areas, new cracks appeared along sections of Highway 130, some with visible escaping fumes. Courtesy of HVO.

Figure 352. A clear view into Pu'u 'O'o crater of Kīlauea was possible on 11 May 2018. The upper part of the crater had a flared geometry, which narrowed to a deep circular shaft. The deepest part of the crater was about 350 m below the crater rim according to a 3D model constructed from thermal images. The crater is about 250 m wide, and N is to the left. Courtesy of HVO.

Activity during 12-23 May 2018. Minor spattering resumed from a new fissure (16) that opened about 0645 on 12 May around 1.5 km NE of fissure 15, at the NE end of the existing vent system (figure 353). It produced a lava flow that traveled about 230 m before stalling in the early afternoon. A steady, vigorous plume of steam and variable amounts of ash rose from the Overlook vent and drifted SW. Over the course of the day, rockfalls from the steep enclosing crater walls at the summit crater periodically generated small ash clouds mixed with water vapor. These ash clouds rose only about a hundred meters above the ground, a few generating very localized ashfall downwind.

Figure 353. Fissure 16 opened in the morning on 12 May 2018 at around 0830 HST; it was located about 1.3 km NE of fissure 15, visible at the top left. The fissure is also located 500 m NE of the Puna Geothermal Venture (PGV) site (green piping, top right). View is uprift to the SW. Photograph courtesy of Hawai`i County Fire Department and HVO.

A plume of steam and volcanic gas, occasionally mixed with ash, rose from the Overlook vent within Halema'uma'u for much of the day on 13 May 2018. That morning, a new outbreak was reported about 0.5 km NE of fissure 16. Aerial observations of this new fissure (17) indicated it was at least several hundred meters long and produced spatter rising tens of meters into the air. By late in the day, activity was dominated by lava fountaining, explosions of spatter bombs, and several advancing lava flow lobes moving generally NE at the downrift (NE) end of the new fissure system. As of about 1900 on 13 May, one lobe was 2 m thick and advancing roughly parallel to Highway 132 (figure 354). A smaller fissure 18, a few hundred meters S of 17, was weakly active late in the day. A new fissure (19) was spotted early on 14 May producing a sluggish lava flow. By 1430 on 14 May, fissure 17 was producing a lava flow extending about 1.7 km from the fissure (figure 355).

Figure 354. Fissures 15-19 opened along the fissure zone NE of Leilani Estates between 9 and 14 May 2018 on Kīlauea's East Rift Zone. As of the early morning on 14 May, lava from fissure 17 had traveled about 1.2 km, roughly ESE parallel to the rift zone, and was turning slightly S; at 0830 HST, the flow was about 0.9 km S of Highway 132. Fissure 18, which became active late on 13 May, and fissure 19, which opened early on 14 May, were both weakly active. Updated at 1430 on 14 May, this map shows the location of the front of the fissure 17 flow at that time. The flow is following a path of steepest descent (blue line), immediately south of the 1955 ''a'a flow boundary. Shaded purple areas indicate lava flows erupted in 1840, 1955, 1960, and 2014-2015. Courtesy of HVO.

Figure 355. A thermal map of the NE end of the fissure system on the lower East Rift Zone of Kīlauea as of 1430 on 14 May 2018 shows the active fissure 17 flow extending about 1.7 km from the fissure. The black and white area is the extent of the thermal map. Temperature in the thermal image is displayed as gray-scale values, with the brightest pixels indicating the hottest areas. The thermal map was constructed by stitching many overlapping oblique thermal images collected by a handheld thermal camera during a helicopter overflight of the flow field. The base is a copyrighted color satellite image (used with permission) provided by Digital Globe. Courtesy of HVO.

Activity on the LERZ on 15 May 2018 was concentrated at fissure 17 with intermittent spattering at fissure 18; the fissure 17 flow continued to slowly advance ESE reaching a length of nearly 2.5 km in the early morning (figures 356 and 357). A new fissure (20) opened up SW of fissure 18 and produced two small pads of lava. During the morning, ash emissions from the Overlook vent inside Halema'uma'u varied greatly in intensity with abrupt increases likely associated with large rockfalls deep into the vent (figure 358). Ashfall was reported as far away as Discovery Harbor (55 km SW), Pahala (30 km SW), at locations along Highway 11 from Pahala to Volcano, and in the Ka'u Desert section of Hawaii Volcanoes National Park near the summit. NWS radar and pilot reports indicated the top of the ash cloud ranged from 3.0 to 3.7 km altitude.

Figure 356. The 2.5-km-long fissure 17 lava flow on Kīlauea's lower East Rift Zone at 0844 on 15 May 2018 was an active ''a'a flow moving ESE from fissure 17, which was visible as low lava fountains in the middle of the photo. Highway 132 appears on the right side of the photograph; the view is toward the W. Photograph courtesy of the Hawai`i County Fire Department and HVO.

Figure 357. Highly viscous (sticky) lava oozed from the edge of the ''a'a flow spreading slowly ESE from fissure 17 at Kīlauea's lower East Rift Zone on 15 May 2018. Courtesy of HVO.

Figure 358. Activity at Halema'uma'u crater at Kīlauea increased in the morning of 15 May 2018 to include the nearly continuous emission of ash with intermittent stronger pulses that formed occasional higher plumes 1-2 km above the summit. At about 0900 HST the plume was drifting SW with the tradewinds toward the Ka`u Desert. The dark area to the right of the ash column rising from the Overlook vent is ash falling from the cloud. Courtesy of HVO.

On the morning of 16 May dense ballistic blocks up to 60 cm across were found in the parking lot a few hundred meters from Halema'uma'u crater, reflecting the most energetic explosions to date. Strong earthquakes within the summit continued in response to ongoing deflation and lava column drop. By the afternoon of 16 May the floor of the larger Kīlauea Caldera had dropped 90 cm from the ongoing deflation, stressing faults around the caldera and causing multiple earthquakes. Employees at the Hawaiian Volcano Observatory, Hawai`i Volcanoes National Park, and nearby residents reported frequent ground shaking and damage to roads and buildings. The decision was made to evacuate HVO's office building on Uekahuna Bluff overlooking Halema'uma'u Crater. Low-level eruption of lava continued from multiple points along the NE end of the active fissure system on the lower East Rift Zone, but spattering generally decreased in vigor throughout the day.

At about 0415 on 17 May an explosion from the Overlook vent produced a volcanic cloud that reached 9.1 km altitude and drifted NE. Traces of ash fell with rain on the Volcano Golf Course, in Volcano Village, and in other areas immediately around the summit (figure 359). Subsequent continued emissions reached 3.7 km altitude; vog or volcanic air pollution produced by volcanic gas was reported in Pahala. After the explosion, seismic levels increased gradually throughout the day.

Figure 359. At 0745 on 17 May 2018 the view of Halema'uma'u crater at Kīlauea from the visitor viewing area in front of the Jaggar Muesum at Hawai'i Volcanoes National Park included a light coating of ash on the Park's interpretative sign caused by ashfall after significant explosive events the previous day. Note the contrast of the mostly-steam plume rising from the Overlook vent in the background with the eruption column that emerged during explosive activity in May 1924 (shown in the middle photograph on the sign). Courtesy of HVO.

At the LERZ, fissures 18, 19, and 20 reactivated during the afternoon of 17 May, and a new fissure opened (21) between fissures 7 and 3 (figure 360). Ground cracks continued to open and widen around Leilani Estates, several with both horizontal and vertical offsets (figures 361 and 362). An area 50-90 m wide, parallel to and N of the line of fissures between Highway 130 and Lanipuna Gardens, dropped slightly, forming a depression that pahoehoe flows from fissures 20 and 21 began filling. Fissure 22 opened just downrift of fissure 19.

Figure 360. Fissure 21 opened between fissures 3 and 7 on 17 May 2018 on the lower East Rift Zone at Kīlauea. Around 1500 an aerial view of the fissure showed fountaining and a lava flow expanding outward from the fissure. View is toward the west. Courtesy of HVO.

Figure 361. SW-trending en-echelon ground cracks dissected and offset NW-trending Pohoiki Road around 0700 on 17 May 2018 as seen during an overflight by HVO of the eruptive fissure area at Kīlauea's East Rift Zone. Cracks continued to open and widen, many with both horizontal and vertical offsets. These cracks were caused by the underlying intrusion of magma into the lower East Rift Zone. Courtesy of HVO.

Figure 362. HVO geologists examined widening cracks on Nohea Street in Leilani Estates at Kīlauea's lower East Rift Zone on 17 May 2018. These cracks had expanded significantly during the previous day. Courtesy of HVO.

Spattering continued on 18 May 2018 from fissures 15, 17, 18, 20, 21, and new fissure 22. Pahoehoe lava flows were also erupted from fissures 17, 18, and 20 (figure 363). During the afternoon, fissure 17 was actively spattering fragments as high as 100 m, and the flow was active but had not covered new ground (figure 364). A flow from fissure 18 had traveled approximately 1 km SE. The graben area N of the fissures was still being filled by pahoehoe flows from fissures 20 and 21; fissure 15 produced a flow that crossed Pohoiki Road. Later in the afternoon a fast-moving pahoehoe flow emerged from fissure 20 and traveled SE, moving over 300 m per hour. By late that evening, the flow had three main lobes; the easternmost was E of Pohoiki Road moving about 200 m per hour while the westernmost was near Malamaki Road and moving about 40 m per hour. At the summit, for much of the day, a steady steam plume rose from the Overlook vent. Several minor emissions of ash were observed in web cameras; no significant explosions and no earthquakes greater than M 3.5 had occurred in the previous 24 hours. At 2358 local time, however, a short-lived explosion from Halema'uma'u created an ash cloud that reached up to 3.1 km altitude and was carried SW by the wind.

Figure 363. By the early afternoon of 18 May 2018, fissures 21, 4, 15, 22, 20, 18, and 17 (SW to NE) were all erupting with either pahoehoe flows, fountaining, or spatter on Kīlauea's lower East Rift Zone. Shaded purple areas indicate lava flows erupted in 1840, 1955, 1960, and 2014-2015. Courtesy of HVO.

Figure 364. The line of fountains on fissure 17 coalesced into a large fountain sending lava fragments 50 m into the air in the morning on 18 May 2018 at the lower East Rift Zone of Kīlauea; small bits of spatter reached 100 m high. Courtesy of HVO.

The rate of lava eruption increased overnight on 18-19 May; fountaining continued at fissure 17, and fissures 16 and 20 merged into a continuous line of spatter and fountaining. Two flows from this consolidated fissure complex were wide, very active, and advancing southward at rates up to 300 m per hour (figures 365 and 366). Flows from fissures 17 and 18 were also still active but advancing slowly, and fissure 18 had stalled by the end of the day. By mid-afternoon on 19 May the two fast-moving flows had joined about a kilometer from the coast and continued to flow southward (figure 367). The GPS instrument located on the NE side of Leilani Estates was no longer moving. While earthquake activity continued, it had not moved farther downrift in the previous few days.

Figure 365. Channelized lava flows originating from a merged, elongated fountaining source between fissures 16 and 20 in Kīlauea's lower East Rift Zone split into two flows that both traveled rapidly S as seen at 0737 on 19 May 2018. This view to the SW also showed the steaming line of fissures on the lower East Rift Zone that continued SW of the fountaining source. Courtesy of HVO.

Figure 366. 'A'a lava flows emerged from the elongated fissure 16-20 at Kīlauea to form several channels. The flow direction is from upper center to the lower left of image. Incandescence from the second flow is visible in the upper left. Image taken around 0818 on 19 May 2018 during a helicopter overflight of Kīlauea's lower East Rift zone by HVO geoscientists. Courtesy of HVO.

Figure 367. Around 1215 on 19 May 2018 the two primary lava-flow fronts originating from the fissure 20-22 area on Kīlauea's lower East Rift Zone were about to merge as they flowed SE. The flow front position based on a later update at 1840 is shown by the red circle. The black and white area is the extent of the thermal map. Temperature in the thermal image is displayed as gray-scale values, with the brightest pixels indicating the hottest areas. The thermal map was constructed by stitching many overlapping oblique thermal images collected by a handheld thermal camera during a helicopter overflight of the flow field. The base is a copyrighted color satellite image (used with permission) provided by Digital Globe. Courtesy of HVO.

The two flows from the fissure 20 complex entered the ocean at two points along the SE Puna coast overnight on 19-20 May (figure 368). Soon after, a crack opened under the E lava channel diverting some of the lava into underground voids (figure 369) and reducing the amount of lava flowing into the ocean. Spattering continued from fissures 6 and 17 during the day on 20 May. Fissure 23 first appeared at 1100 on 20 May, at the NE corner of the Leilani Estates subdivision near fissures 4 and 14, about 2 km SW of fissure 20; it was only active during 19-20 May. Volcanic gas emissions tripled as a result of the voluminous eruptions from the fissure 20 complex; satellite instruments measured a major increase in SO₂ emissions on 19 May (figure 370). Intermittent explosive eruptions continued at the summit, and plumes of gas and steam periodically emerged from the Overlook vent and drifted SW.

Figure 368. The fissure 20 complex flows from Kīlauea's lower East Rift Zone reached the ocean overnight during 19-20 May 2018, as seen in this image from an HVO overflight in the early morning on 20 May. Dense white plumes of "laze" (short for "lava haze") formed as lava entered the ocean. Laze is formed as lava boils seawater. The process leads to a series of chemical reactions that result in the formation of a billowing white cloud composed of a mixture of condensed seawater steam, hydrochloric acid gas, and tiny shards of volcanic glass. This mixture has the stinging and corrosive properties of dilute battery acid and is hazardous to breathe. Because laze can be blown downwind, its corrosive effects can extend far beyond the actual ocean entry area. Courtesy of HVO.

Figure 369. Lava from the eastern channel of the fissure 20 complex flowed into a crack in the ground that opened on the morning of 20 May 2018. The resulting decrease in lava volume caused the easternmost channel of lava and the eastern ocean entry to be less vigorous than the western entry point. Courtesy of HVO.

Figure 370. SO2 emissions increased substantially at Kīlauea when the rate of lava emission increased significantly on 19 May 2018 on the lower East Rift Zone. The OMI instrument on the Aura satellite measured 8.2 Dobson Units (DU) of atmospheric SO2 on 3 May, the day fissure 1 opened, and 18.11 DU on 19 May when the flow rates increased at the fissure 20 complex. Courtesy of NASA Goddard Space Flight Center.

The most vigorous eruptive activity during 21-23 May in the lower East Rift Zone, was concentrated in the middle portion of the system of fissures, primarily between fissure 20 on the NE and fissure 23 on the SW (figure 371). Fountaining 50 m high from fissure 22 was feeding the channelized flow reaching the coast (figure 372). Fissure 6 reactivated with spattering and a short flow on 21 May. Fissure 17, at the NE end of the fissure system was only weakly active. On 22 May lava fountains continued from fissures 6 and 22, with fissures 19 and 5 also active; a new area of fountaining also appeared near fissure 23. Fountaining from fissures 5 and 23 fed flows in the eastern part of Leilani Estates and blue methane was observed burning in road cracks overnight on 22-23 May (figure 373). Observers noted that the height of the perched lava pond forming on the NW side of fissures 5 and 6 had reached 11 m above the ground level.

Figure 371. By 20 May 2018, two lava flows from fissures 20 and 22 in the lower East Rift Zone at Kīlauea had coalesced and reached the ocean. Activity at the fissure 17 flow had diminished significantly. The most vigorous eruptive activity during 21-23 May 2018 was concentrated in the middle portion of the system of fissures, primarily between fissure 20 on the NE and fissure 23 on the SW. Courtesy of HVO.

Figure 372. HVO geologists reported fountaining from fissure 22 as 50 m high on 21 May 2018 at Kīlauea's lower East Rift Zone. Courtesy of HVO.

Figure 373. Blue flames of methane emerged from ground and road cracks on 22 May 2018. This early morning photo was taken on Kahukai Street in the Leilani Estates subdivision at Kīlauea looking SE, with an active lava flow from fissure 13 behind the blue flames. Photo by L. DeSmither, courtesy of HVO.

By 23 May, fountains from fissures 5, 6 (figure 374), 13, and 19 were feeding a flow advancing to the S, roughly parallel to the western flow from fissure 22 (figure 375); it reached the ocean late in the afternoon, creating multiple entry points that produced occasional small explosions. Small ash emissions from the Overlook vent occurred frequently during 21-23 May (figure 376). Fissure 8 reactivated briefly in the morning of 23 May and erupted two small pahoehoe flows over the initial `a`a flow.

Figure 374. Fissure 6 in the lower East Rift Zone at Kīlauea built a lava berm across Pohoiki Road as seen on 23 May 2018. Flows from fissure 6 and adjacent fissures formed a flow parallel to an earlier flow that traveled SE reaching the coast that afternoon. Courtesy of HVO.

Figure 375. Multiple channels of lava flowed SE from the fissure zone at Kīlauea's lower East Rift Zone to the sea on 23 May 2018. Overflows from the channels spread out over existing, older flows; note the large agriculture buildings as indicators of the scale of the flows. The visible haze is sulfur dioxide gas from the fissures. Photo taken by J. Ozbolt, Hilo Civil Air Patrol, courtesy of HVO.

Figure 376. A pulse of ash rose from Halema'uma'u on 23 May 2018 as part of semi-continuous emissions at Kīlauea's summit. Ash could be seen falling from the plume as it was blown downwind around 1528 HST on 23 May. USGS photo by I. Johanson, courtesy of HVO.

Activity during 24-28 May 2018. Overnight on 23-24 May field crews observed that fissure areas 2, 7, 8, 3, 14, and 21 had reactivated and were spattering (figure 377). Fissure 22 continued to erupt lava flowing SE to the coast (figure 378). Fissures 7 and 21 were feeding a perched lava pond and pahoehoe flow that advanced eastward later that afternoon. An explosion from the summit Overlook vent just after 1800 on 24 May produced an ash cloud that rose to 3.1 km altitude and had more ash than most recent explosions (figure 379). The National Weather Service Nexrad radar tracked the cloud for 15-20 minutes; moderate trade winds were blowing to the SW.

Figure 377. Reactivation of fissures 2, 7, 8, 3, 14, and 21 was noted on 24 May 2018 at the lower East Rift Zone at Kīlauea. Fissures 7 and 21 were feeding a perched lava pond and pahoehoe flow. Several ocean entries were also active from the channelized flow down the SE flank sourced from the region of fissures 6-20. Courtesy of HVO.

Figure 378. During HVO's overflight in the morning of 24 May 2018, the fissure 22 fountain was not as high as several days earlier, but was still erupting significant lava that was flowing to the SE Puna coast at Kīlauea. USGS photo by M. Patrick, courtesy of HVO.

Figure 379. An explosion was detected from the summit Overlook Crater at Kīlauea just after 1800 on 24 May 2018 that produced an ash cloud that rose to 3.1 km altitude, carrying more ash than most recent explosions. This view to the SW is from the caldera rim near Volcano House where USGS scientists were stationed to track the ongoing summit explosions. Courtesy of HVO.

By 25 May, the two flows from fissure 22 and fissures 6 and 13 were still reaching the ocean with two entry points (figure 380); fissures 7 and 21 were feeding a flow that continued to slowly advance NE, covering several streets in Leilani Estates (figure 381). By the next day, the flow front of fissure 21 had become an 'a'a flow and was continuing to move NE (figure 382), reaching the PGV (Puna Geothermal Venture) property by late afternoon on 26 May. Fissure 7 was feeding a flow that had turned S toward the coast, and at dusk the lava was cascading into the Pawaii crater, adjacent to the western margin of the fissure 6 flow that fed one of the ocean entries. On the W side of fissure 7 a perched pahoehoe flow broke out around 0400 on 26 May, feeding short flows to the W. Multiple small eruptions continued to occur at the summit, most ejecting ash to under 3.1 km altitude. One of the largest occurred about 1617 on 25 May sending ash as high as 3.7 km altitude.

Figure 380. Fissures 6 (left) and 13 (right) at midday on 25 May 2018 on Kīlauea's lower East Rift Zone, with lava flows merging into one channel that flows SE into the ocean. Note plume in distance at the ocean entries (top left). Courtesy of HVO.

Figure 381. Reactivated fissures 7 and 21 within the Leilani Estates subdivision at Kīlauea were feeding new flows moving NE towards the Puna Geothermal Venture (PGV) property during 25 and 26 May 2018. Courtesy of HVO.

Figure 382. An 'a'a flow, erupted from fissure 21 at Kīlauea was approximately 3-4 m high at the flow front and slowly advancing to the NE in the Leilani Estates subdivision around 1030 HST on 26 May 2018. Courtesy of HVO.

Overnight during 26-27 May fissure 17 was the source of multiple booming gas emissions. Fissure 7 activity increased overnight, producing a large spatter rampart over 30 m tall from fountains reaching 50-70 m high (figure 383). The fountains fed two perched channels, the N channel, 8-15 m thick, fed a lava flow that advanced toward pad E of the PGV property; the S channel was a flow advanced SE. Large cracks were observed overnight near fissure 9 which developed into fissure 24. Fissure 8 had three vents active overnight on 26-27 May that were spattering and flaming; they had doubled in size over the previous 24 hours.

Figure 383. Pahoehoe lava advanced rapidly W from fissure 7 on Leilani Avenue in Kīlauea's lower East Rift Zone on 27 May 2018. Activity had increased overnight, with lava fountains reaching 50 to 60 m high. Courtesy of HVO.

The fissure 21 'a'a flow continued to advance NE onto PGV property but at a slower pace on 27 May. By the end of the day, fissures 7 and 8 were the most active, fountaining and feeding lava flows that advanced NE onto PGV property. At about 1900 HST a fast-moving lava flow broke from this area and advanced rapidly to the N and W through the eastern portion of Leilani Estates, causing additional evacuations. Activity had noticeably diminished from fissures 22 and 13, and the supply of lava to the channel flowing to the sea had ceased by the next day, 28 May. Ash continued to erupt intermittently from the Overlook vent at the summit. Observations from the ground and by UAV during the previous week documented retreat of the Overlook vent wall due to collapse of the steep enclosing walls and rim. Trade winds carried the ash clouds primarily SW.

Fissure 8 fed a fast-moving flow early on 28 May that moved N along the margin of the existing fissure 7 flow before turning E and crossing out of Leilani Estates (figure 384). Flow activity from fissure 8 diminished abruptly midday and a few other fissures reactivated briefly with fissure 7/21 having the tallest fountains. Vigorous fountaining resumed at fissure 8 late in the afternoon, spawning a flow that traveled an estimated 20 m per hour to the NE over the flow of the previous day; fountains were 50-60 m tall (figure 385). During the evening, fissures 16, 18, 22, 13, and 20 were also active, with flows moving S from fissures 16/18. Pele's hair from vigorous fountaining of fissure 8 was being transported downwind, and there were reports of some strands falling in Pahoa. Residents were warned to minimize exposure to Pele's hair, which could cause skin and eye irritation similar to exposure to volcanic ash. Ash continued to erupt intermittently from the vent within Halema'uma'u crater. A magnitude 4.1 earthquake occurred at 1739 HST on the Koa'e fault zone south of the caldera. Earthquakes in the summit region continued as the area subsided and adjusted to the withdrawal of magma.

Figure 384. A fast-moving flow from fissure 8 moved N and then E out of Leilani Estates on 28 May 2018 marking a new phase in the 2018 Kīlauea eruption. Courtesy of HVO.

Figure 385. Fissure 8 on Kīlauea's lower East Rift Zone reactivated after a brief pause on the afternoon of 28 May 2018 with lava fountains that reached heights of 60 m and fed a lava flow that advanced rapidly to the NE. Courtesy of HVO.

By 26 May 2018, flows on the lower East Rift Zone had destroyed at least 82 structures including 41 homes since the beginning of May. Twelve more were destroyed on 27 and 28 May as flows continued to move across the region, according to USA Today. By 29 May, activity on the lower East Rift Zone was focused on the vigorous eruption of lava from fissure 8 advancing rapidly downslope towards the NE and Highway 132.

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/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); USA Today (URL: https://www.kiiitv.com/article/news/nation-now/hawaii-lava-flow-destroys-12-more-homes-as-Kilauea -volcano-continues-exploding/465-afd62fc3-91d2-4764-9eb9-c3dee473033d).

Krakatau volcano in the Sunda Strait between Java and Sumatra, Indonesia experienced a major caldera collapse, likely in 535 CE, that formed a 7-km-wide caldera ringed by three islands (see inset figure 23, BGVN 36:08). Remnants of this volcano coalesced to create the pre-1883 Krakatau Island which collapsed during the 1883 eruption. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera has been the site of frequent eruptions since 1927. The most recent event was a brief episode of Strombolian activity, ash plumes, and a lava flow during the second half of February 2017. Activity resumed in late June 2018 and continued through early October, the period covered in this report. Information is provided primarily by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG). Aviation reports are provided by the Darwin Volcanic Ash Advisory Center (VAAC), and photographs came from several social media sources and professional photographers.

After the brief event during February 2017, Anak Krakatau remained quiet for about 15 months. PVMBG kept the Alert Level at II, noting no significant changes until mid-June 2018. Increased seismicity on 18 June was followed by explosions with ash plumes beginning on 21 June. Intermittent ash emissions were accompanied by Strombolian activity with large blocks of incandescent ejecta that traveled down the flanks to the ocean throughout July. Explosions were reported as short bursts of seismic activity, repeating multiple times in a day, and producing dense black ash plumes that rose a few hundred meters from the summit. Similar activity continued throughout August, with the addition of a lava flow visible on the S flank that reached the ocean during 4-5 August. Generally increased activity in September resulted in the highest ash plumes of the period, up to 4.9 km altitude on 8 September; high-intensity explosions were heard tens of kilometers away during 9-10 September. PVMBG reported significantly increased numbers of daily explosions during the second half of the month. The thermal signature recorded in satellite data also increased during September, and a large SO₂ plume was recorded in satellite data on 23 September.

Activity during June-July 2018. PVMBG noted an increase in seismic activity beginning on 18 June 2018. Foggy conditions hampered visual observations during 19-20 June, but on 21 June gray plumes were observed rising 100-200 m above the summit (figure 41). Two ash plumes were reported on 25 June; the first rose to about 1 km altitude and drifted N, and the second rose to 600 m altitude and drifted S (figure 42).

Figure 41. Anak Krakatau began a new eruptive episode on 21 June 2018 with an ash plume that rose 200 m above the summit. Photo by undisclosed source, courtesy of Øystein Lund Andersen‏.

Figure 42. The first of two ash plumes rose to about 1 km altitude and drifted N from Anak Krakatau on 25 June 2018; the first events after about 18 months of no activity were reported on 21 June. Courtesy of PVMBG (Eruption Information on Mt. Anak Krakatau, June 25, 2018).

Incandescence was observed at the summit during 1-2 July 2018, and two ash emissions were reported in VONA's (Volcano Observatory Notice for Aviation) on 3 July. PVMBG reported that during 4-5 July there were four additional ash-producing events, each lasting between 30 and 41 seconds. The last three of these events produced ash plumes that rose 300-500 m above the crater rim and drifted N and NW. The Darwin VAAC reported essentially continuous ash emissions during 3-9 July drifting generally W and SW at about 1.2 km altitude (figure 43). They were intermittently visible in satellite imagery when not obscured by meteoric clouds.

Figure 43. A dense gray ash plume rose several hundred meters above Anak Krakatau on 7 July 2018 (local time) while large volcanic bombs traveled down the flanks. Photo by Sam Hidayat, courtesy of Øystein Lund Andersen‏.

Ash plumes were again observed by the Darwin VAAC in satellite imagery beginning on 13 July 2018 at 1.2 km altitude drifting NW. They were essentially continuous until they gradually decreased and dissipated early on 17 July, rising to 1.2-1.5 km altitude and drifting W, clearly visible in satellite imagery several times during the period. Satellite imagery revealed hotspots several times during July; they ranged from small pixels at the summit (9 July) to clear flow activity down the SE flank on multiple days (12, 19, and 24 July) (figure 44). In the VONA's reported by PVMBG during 15-17 July, they noted intermittent explosions that lasted around 30-90 seconds each. PVMBG reported a black ash plume 500 m above the summit drifting N during the afternoon of 16 July. The Darwin VAAC continued to report ash emission to 1.2-1.5 km altitude during 18-19 July, moving in several different directions; Strombolian activity sent incandescent ejecta in all directions on 19 July (figure 45). During 25-26 July the Darwin VAAC noted continuous minor ash emissions drifting SW at 1.2 km altitude, and a hotspot visible in infrared imagery.

Figure 44. Sentinel-2 satellite imagery clearly documented the repeated thermal activity at Anak Krakatau throughout July 2018. a) 9 July 2018: a small hotspot was visible at the summit and an ash plume drifted NW. b) 12 July 2018: a much larger hotspot showed a distinct flow down the SE flank. c) 19 July 2018: even under partly cloudy skies, incandescent ejecta is visible on the S flank. d) 24 July 2018: incandescent lava had almost reached the SE coast. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Figure 45. Strombolian activity sent incandescent ejecta down all the flanks and into the sea at Anak Krakatau on 19 July 2018, as seen from the island of Rakata (5 km SE). Courtesy of Reuters / Stringer.

Activity during August-early October 2018. A series of at least nine explosions took place on 2 August 2018 between 1333 and 1757 local time. They ranged from 13 to 64 seconds long, and produced ash plumes that drifted N. The Darwin VAAC reported minor ash observed in imagery at around 2 km altitude for much of the day. In a special report, PVMBG noted a black ash plume 500 m above sea level drifting N at 1757 local time. Continued explosive activity was reported by local observers during the early nighttime hours of 3 August (figure 46).

Figure 46. A dark ash plume rose 100-200 m from Anak Krakatau during the early morning hours of 3 August 2018, and incandescent ejecta rolled down the flanks. Tens of explosions were heard in Serang (80 km E) and Lampung (80 km N). Courtesy of Sutopo Purwo Nugroho.

The Darwin VAAC reported continuous ash emissions rising to 1.8 km altitude and drifting E on 5 August, clearly visible in satellite imagery, along with a strong hotspot. The ash plume drifted SE then S the next day before dissipating. PVMBG reported incandescence visible during the nights of 5-15 August. Photographer Øystein Lund Andersen visited Krakatau during 4-6 August 2018 and recorded Strombolian activity, lava bomb ejecta, and a lava flow entering the ocean (figures 47-50).

Figure 47. Strombolian explosions sent incandescent ejecta skyward, and blocks of debris down the flanks of Anak Krakatau on 5 August 2018 as captured in this drone photograph. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 48. Large volcanic bombs flew out from the summit vent of Anak Krakatau while a dark gray plume of ash rose a few hundred meters on 5 August 2018 in this drone photograph. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 49. A blocky lava flow traveled down the S flank of Anak Krakatau on 5 August 2018 in this closeup image taken by a drone. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 50. Views of Anak Krakatau from the SE showed Strombolian activity and incandescent lava (upper photo) and steam from the lava flowing into the ocean and dark ash emissions from the summit (lower photo) on 5 August 2018. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Emissions were reported intermittently drifting W on 11, 14, and 16 August at 1.2-1.5 km altitude. Video of explosions on 12 August with large bombs and dark ash plumes were captured by photographer James Reynolds (Earth Uncut TV). PVMBG reported black ash plumes drifting N at 500 m above the summit on 17 and 18 August after explosions that lasted 1-2 minutes each. The Darwin VAAC also reported ash plumes rising to 1.2 km altitude on 17-18 drifting NE. VONA's were issued during 22-23 August reporting at least three explosions that lasted 30-40 seconds and produced ash plumes that drifted N and NE. The Darwin VAAC reported the plume on 22 August as originating from a vent below the summit. PVMBG noted that a dark plume on 23 August drifted NE at about 700 m above the summit. During 27-30 August, the Darwin VAAC reported ash plumes intermittently visible in satellite imagery extending SW at 1.2-1.5 km altitude.

Ash plumes drifting N and NW were visible in satellite imagery during 3-4 September at 1.2-1.5 km altitude. The Darwin VAAC reported an ash plume moving NW and W at 4.9 km altitude on 8 September, the highest plume noted for the report period. The following day, the plume height had dropped to 1.5 km altitude, and was clearly observed drifting W in satellite imagery. A hotspot was reported on 12 September. During the night of 9-10 September PVMBG reported bursts of incandescent material rising 100-200 m above the peak, with explosions that rattled windows at the Anak Krakatau PGA Post, located 42 km from the volcano. Ash plumes continued to be observed through 13 September. The Darwin VAAC reported continuous ash emissions to 1.8 km altitude drifting W and NW on 16-17 September (figure 51). The ash plume was no longer visible on 18 September, but a hotspot remained discernable in satellite data through 20 September.

Figure 51. On 16 September 2018 a dark ash plume rose several hundred meters above Anak Krakatau as incandescent lava flowing down the SE flank to the sea created steam plumes. Courtesy of Thibaud Plaquet.

PVMBG reported incandescence at the summit and gray and black ash plumes on 20 September that rose 500 m above the summit. A low-level ash emission was reported drifting S on 21 September and confirmed in the webcam. Four VONA's were issued that day, reporting explosions at 0221, 0827, 2241, and 2248, lasting from 72-115 seconds each. PVMBG subsequently reported observing 44 explosions with black ash plumes rising 100-600 m above the summit, and incandescence at night on 21 September. Ash emissions continued on 22 September at 1.5 km altitude, with a secondary explosion rising to 2.4 km altitude drifting W. The plume height was based on and infrared temperature measurement of 12 degrees C. Later in the day, an additional plume was observed in satellite imagery at 3.7 km altitude drifting N. PVMBG reported observations of 56 explosions, with 200-300 m high (above the summit) black ash plumes and incandescence at night on 22 September. Observations from nearby Rakata Island on 22 September indicated that tephra from incandescent explosions of the previous night mostly fell on the flanks, but some reached the sea. A lava flow on the SSE flank had also reached the ocean (figure 52).

Figure 52. Activity at Krakatau during 22-23 September 2018 included substantial Strombolian explosions, a dark ash plume, lava flows, and large volcanic bombs traveling nearly to the ocean. Photo courtesy of Malmo Travel.

By 23 September 2018, a single plume was observed at 2.1 km altitude drifting WNW. A glow at the summit was visible in the webcam that day, and a hotspot was seen in satellite imagery the next day as observations of an ash plume drifting W at 2.1 km continued. A significant SO₂ plume was captured in satellite data on 23 September (figure 53).

Figure 53. A significant SO2 plume dispersed NW of Krakatau (lower right corner) on 23 September 2018 after a surge in activity was observed the previous two days. Courtesy of NASA Goddard Space Flight Center.

On 24 September, PVMBG reported black ash plumes rising 1,000 m above the summit, incandescence at the summit, and lava flowing 300 m down the S flank observed in the webcam during the night. An ash plume was observed by the Darwin VAAC drifting WSW and then W on 25-26 September at 2.1 km altitude, lowering slightly to 1.8 km the following day, and to 1.2 km on 28 September. Continuous ash emissions were observed through 29 September. A new emission was reported on 30 September drifting SW at 1.8 km altitude. Ash emissions were observed daily by the Darwin VAAC from the 1st to at least 5 October at 2.1 km altitude drifting W. A large hotspot near the summit was noted on 3 October. The thermal activity at Anak Krakatau from late June into early October 2018, as recorded in infrared satellite data by the MIROVA project, confirmed the visual observations of increased activity that included Strombolian explosions, lava flows, ash plumes, and incandescent ejecta witnessed by ground observers during the period (figure 54).

Figure 54. The MIROVA project graph of thermal activity at Krakatau from 12 February through early October 2018 showed the increasing thermal signature that appeared in late June at the onset of renewed explosive activity, the first since February 2017. Courtesy of MIROVA.

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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Reuters Latam (Twitter: @ReutersLatam, URL: http://www.reuters.com/); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, Video: https://www.youtube.com/watch?v=UD3SLWtuPZs); Thibaud Plaquet (Instagram: tibomvm, URL: https://www.instagram.com/tibomvm/); Malmo Travel (Instagram: malmo.travel, URL: https://www.instagram.com/malmo.travel/).

Ol Doinyo Lengai is the only volcano on Earth currently erupting carbonatite lavas. Activity is based in the crater offset to the N about 100 m below the summit, where hornitos (small cones) and pit craters produce lava flows and spattering. After displaying effusive activity in the north crater since at least 1983, it became filled and lava began overflowing in 1998. The eruption transitioned to significant explosive activity in September 2007 through March 2008 that cleared out and recreated the crater (figure 175). Since then, intermittent effusive carbonatite eruptions have continued. This report summarizes observed activity from August 2014 through August 2018, including observations by visitors and satellite data (figure 176).

Figure 175. The northern summit crater of Ol Doinyo Lengai on 29-30 November 2017. The crater is ~100-125 m deep and ~175-190 m in diameter at the vertical crater wall, and 260-300 m wide at the crater rim. Top: orthorectified photography showing the light-colored crater floor with dark spots indicating the locations of recently active vents. Bottom: Shaded relief of the crater. Courtesy of M. Kervyn and Antoine Dille at Vrije Universiteit Brussel.

Figure 176. Selected satellite imagery showing typical activity at Ol Doinyo Lengai during 2017-18. These Sentinel-2 thermal (left) and true color (right) satellite images show the active areas indicated by elevated thermal activity (bright orange) and darker gray-black areas on the crater floor. The dark fresh lavas rapidly cool to a light brown-white color. Courtesy of Sentinel Hub Playground.

Lava fountaining was seen by geologists on 5 July 2014 (BGVN 39:07). A tourist report on Trip Advisor for an unknown date in July 2014 described potential fumarolic activity, while another report that month did not note any activity. No clear reports are known describing activity between August 2014 and May 2015. On 20 June 2015 an Earth Sciences group from the University of Glasgow and University of Dodoma, including volcanologist David Brown, visited the crater (figure 177). They observed minor eruptive activity consisting of gentle spattering at one of the mounds. Evidence of this activity continuing through August is seen in Landsat satellite images (figure 178).

Figure 177. The active Ol Doinyo Lengai crater on 20 June 2015 showing the cone along the western wall (top) and the northern wall (bottom). Photos courtesy of David Brown, University of Glasgow.

Figure 178. Landsat-8 satellite images show color variations on the Ol Doinyo Lengai active crater floor. Darker areas may indicate activity and changing morphology during July through August 2015. Landsat-8 true-color pansharpened images courtesy of Sentinel Hub Playground.

Only one Sentinel-2 thermal image (out of 22 cloud-free images) contained elevated temperatures during 2016. The image showed activity in the northern part of the crater. Landsat-8 true color images show color variations on the crater floor in October 2016 indicating activity at that time (figure 179).

Figure 179. Landsat-8 images showing color variations on the Ol Doinyo Lengai active crater floor indicating activity in October 2016. Landsat-8 true-color pansharpened images courtesy of Sentinel Hub Playground.

On 29-30 November 2017, a French-Belgium team including M. Kervyn conducted a summit morphology study. Accounts from previous visitors in September-October 2017 reported significant activity in the large half-cone with regular emission of spatter from the summit vent. They observed significant fumarolic activity and the remnants of rockfalls in the crater. Several secondary vents were visible on the side of the large half-cone along the western wall, but no activity was witnessed at this time (figure 180). Active spattering was occurring from a lava pool within a vent in the north-central part of the crater where spattering up to 10 m above the vent continued for several hours (figure 181). A circular cavity in the north-central part of the crater contained a lava pool that had partially crusted over. Darker surfaces suggested recent activity from several vents in the crater.

Figure 180. The ~50-m-high half-cone that formed by a vent along the western wall of the Ol Doinyo Lengai north crater as seen on 29-30 November 2017. Several secondary vents were observed at the foot of the cone. In the lower right of the image, several pit structures are visible along the northern part of the crater. Photo courtesy of M. Kervyn, Vrije Universiteit Brussel.

Figure 181. Sporadic activity from an active vent in the northern-central part of the Ol Doinyo Lengai active crater was observed for two hours on 30 November 2017. Explosions were regularly heard emanating from the laval pool and jets of spatter were observed reaching up to 10 m above the crater and depositing on the wall and edges of the pit crater. Courtesy of M. Kervyn, Vrije Universiteit Brussel.

Sentinel-2 thermal satellite images acquired during 2017 show intermittent activity in the crater (figure 182). Out of 21 cloud-free images, 13 contained elevated thermal signatures between April through December. The locations of the activity move around the crater, indicating that the center of activity was variable through time. Lava pond activity was also noted in early December 2017 by Gian Schachenmann, documented with photos taken during an overflight and posted at Volcano Discovery.

Figure 182. Sentinel-2 thermal satellite images showing areas of high temperatures (bright orange to red) in the summit crater of Ol Doinyo Lengai through 2017. The hotpots show where current or very recent activity has occurred at the time of the satellite image acquisition. The active area moves around the crater throughout the year. False color (Urban) images (bands 14, 11, 4) courtesy of Sentinel Hub Playground.

On 1-2 July 2018, K. A. Laxton and F. Boschetty from University College London visited the summit, accompanied by local guides Papakinye Lemolo Ngayeni, Amadeus Mtui, and Ignas Mtui. Vigorous fumarolic activity was observed near the summit, with sulfur deposits and acrid-smelling gases. A small lava flow was observed that had cooled and turned from black to white by later that day. A pool of lava was observed inside a small hornito in the southern area of the crater floor (figure 183). A small cluster of hornitos were developing in the southern area of the crater and one produced a lava flow on 2 July (figure 184).

Figure 183. View of the crater floor at Ol Doinyo Lengai on 1 July 2018. Small inactive carbonatite flows that emanated from the collapse scar and flank vent on the NW hornito. An active hornito with a lava pool is visible in the center-bottom of the image and a semi-collapsed hornito is visible in the bottom-right. Courtesy of K. A. Laxton, University College London.

Figure 184. A view inside the active Ol Doinyo Lengai crater on 2 July 2018. New natrocarbonate flows are visible in the S and SE of the crater floor and one degassing vent is visible and one active vent is visible in the lower part of the image. A second lava flow from a vent just out of this view below the rim produced a lava flow that covered one third of the crater floor. Annotated image courtesy of K. A. Laxton, University College London.

A video taken by Patrick Marcel in August 2018 showed a recent lava flow that had occurred from a vent at the base of the crater wall and an active flow over-spilling from an active lava pond (figure 185). Throughout 2018, there were 18 out of 24 Sentinel-2 thermal cloud-free images which contained areas of elevated thermal activity. Like 2017, the 2018 activity was located in different areas around the crater (figure 186).

Figure 185. Scenes captured from a video taken in August 2018 show activity on the Ol Doinyo Lengai crater floor. A recent faded flow along the crater floor edge can be seen in the upper images and the active black lava lake with an active lava flow is seen in all images. Courtesy of Patrick Marcel.

Figure 186. Sentinel-2 thermal satellite images showing areas of high temperatures (bright orange to red) in the summit crater of Ol Doinyo Lengai through 2018. The hotpots show where current or very recent activity has occurred at the time of the image acquisition. Similar to activity in 2017, the active area moves around the crater throughout the year. False color (Urban) images (bands 14, 11, 4) courtesy of Sentinel Hub Playground.

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

Information Contacts: Matthieu Kervyn, Vrije Universiteit Brussel, Department of Geography, Pleinlaan 2, 1050 Brussels, Belgium (URL: http://we.vub.ac.be/en/matthieu-kervyn-de-meerendre); Kate Laxton and Felix Boschetty, University College London, Gower Street, London, WC1E 6BT, United Kingdom (URL: https://www.ucl.ac.uk/earth-sciences/people/research-students/kate-laxton); Patrick Marcel (URL: https://www.youtube.com/watch?v=ZqxuYOEFNLk); David Brown, School of Geographical and Earth Sciences, University of Glasgow, University Avenue, Glasgow G12 8QQ, United Kingdom (URL: https://www.gla.ac.uk/schools/ges/staff/davidbrown/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Gian Schachenmann, Volcano Discovery (URL: https://www.volcanodiscovery.com/nl/photos/ol-doinyo-lengai/dec2017/crater.html); Trip Advisor (URL: https://www.tripadvisor.com with initial search term 'Ol Doinyo Lengai').

Mayon is a frequently active volcano in the Philippines that produces ash plumes, lava flows, pyroclastic flows, and lahars. In early 2018, eruptive activity included lava fountaining that reached 700 m above the summit, and lava flows that traveled down the flanks and collapsed to produce pyroclastic flows (figure 39). Lava fountaining and lava flows decreased then ceased towards late March. Lava effusion was last detected on 18 March 2018, and the last pyroclastic flow for this eruptive episode occurred on 27 March 2018 (see BVGN 43:04). The hazard status for was lowered from alert level 4 to 3 (on a scale of 0 to 5) on 6 March 2018 due to decreased seismicity and degassing; the level was lowered again to 2 on 29 March. This report summarizes the activity during April through September 2018 and is based on daily bulletins issued by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and satellite data.

Figure 39. Sentinel-2 thermal satellite images showing the lava flow activity at Mayon during January through March 2018. Three lava flow lobes flowed down the Mi-isi, Bonga-Buyuan, and Basud channels, and are shown in bright orange/red in these images. False color images created using bands 12, 11, 4. Courtesy of Sentinel Hub Playground.

The hazard status remained on Alert level 2 (increasing unrest) throughout the reporting period. Activity was minimal with low seismicity (zero to four per day) and a total of 19 rockfall events throughout the entire period. White to light-brown plumes that reached a maximum of 1 km above the crater were observed almost every day from April through September (figure 40). Two short-lived light brown plumes were noted on 27 and 28 August and both reached 200 m above the crater.

Figure 40. An emission of white steam-and-gas at Mayon and a dilute brown plume that reached 200 m above the crater was seen on 24 May 2018. Courtesy of PHIVOLCS.

On the days that sulfur dioxide was measured, the amount ranged from 436 to 2,800 tons per day (figure 41). Mayon remains inflated relative to 2010 baselines but the edifice has experienced deflation since 20 February, a period of inflation from 2-14 April, and slight inflation of the mid-slopes beginning 5 May, which then became more pronounced beginning 25 June. No other notable inflation or deflation was described throughout the reporting period.

Figure 41. Measurements of sulfur dioxide output at Mayon during 1 April-30 September 2018. Data courtesy of PHIVOLCS.

Incandescence at the summit was observed almost every night (when weather permitted) from April through to the end of September 2018, and this elevated crater temperature is also seen in satellite thermal imagery (figure 42). Thermal satellite data showed a slight increase in output during April through to June, although not as high as the earlier 2018 activity, with a decline in thermal output starting in July (figure 43).

Figure 42. Sentinel-2 thermal satellite image showing an elevated thermal signature in the crater of Mayon and a steam-and-gas plume on 15 May 2018. Similar indications of activity in the crater were frequently imaged on cloud-free days from April through September. False color image created using bands 12, 11, 4. Courtesy of Sentinel Hub Playground.

Figure 43. Log radiative power MIROVA plot of MODIS thermal data for the year ending 11 October 2018 at Mayon. An elevated period of activity reflecting the lava flows in January through March is notable, followed by a second period of lower intensity activity during May into June, then a prolonged period of reduced activity through to the end of the reporting period; the August anomaly was not at the volcano. Courtesy of MIROVA.

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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Historical observations of eruptive activity on ice-covered Mount Michael stratovolcano on Saunders Island in the South Sandwich Islands were not recorded until the early 19th century at this remote site in the southernmost Atlantic Ocean. With the advent of satellite observation technology, indications of more frequent eruptive activity have become apparent. The last confirmed eruption evidenced by MODVOLC thermal alerts was during August-October 2015 (BGVN 41:02). Limited thermal anomaly data and satellite imagery since then have indicated intermittent activity through September 2018. Information for this report comes from MODVOLC and MIROVA thermal anomaly data and Sentinel-2, Landsat, and NASA Terra satellite imagery.

Evidence for thermal activity at Mount Michael tapered off in MIROVA data from October 2015 through January 2016. MODVOLC thermal alerts reappeared on 28 September 2016 and recurred intermittently through 6 January 2017. Low-level MIROVA thermal signals appeared in June and September-November 2017. During January-September 2018, evidence for some type of thermal or eruptive activity was recorded from either MODVOLC, MIROVA, or satellite imagery each month except for May and June.

Although MODVOLC thermal alerts at Mount Michael ended on 8 October 2015, the MIROVA radiative power data showed intermittent pulses of decreasing energy into early January 2016 (figure 10, BGVN 41:02). At a high-latitude, frequently cloud-covered site such as Saunders Island, this could be indicative of continued eruptive activity. A white plume in low resolution NASA's Terra satellite data was spotted drifting away from Saunders in April 2016, but no thermal activity was reported. The only high-confidence data available from April 2016 through May 2017 is from the MODVOLC thermal alert system, which recorded two thermal alerts on 28 September 2016, one the next day, one on 30 October, and eight alerts on four days in November. Activity continued into January 2017 with one alert on 17 December 2016, and six alerts on 2 and 6 January 2017 (figure 11).

Figure 11. Seventeen MODVOLC thermal alerts between 28 September 2016 and 6 January 2017 were the best evidence available for eruptive activity on Saunders Island from April 2016 through May 2017. Courtesy of MODVOLC.

A low-level log radiative power MIROVA signal appeared in early June 2017; two more signals appeared in September 2017, one in early October and one in late November (figure 12). Additional signals plotted as more than 5 km from the source may or may not reflect activity from the volcano. Steam plumes were visible in NASA Terra satellite images drifting away from the island in August, October, and December 2017, but no thermal signatures were captured.

Figure 12. The MIROVA log radiative power graph for Mount Michael on Saunders Island from 25 May-30 December 2017 showed intermittent heat sources that indicated possible eruptive activity each month except July and December. Location uncertainty makes the distinction between greater and less than 5 km summit distance unclear.

More sources of evidence for activity became available in 2018 with the addition of the Sentinel-2 satellite data during the months of February-April and September. Multiple thermal signals appeared from MIROVA in January 2018 (figure 13), and the first Sentinel-2 satellite image showed a distinct hotspot at the summit on 10 February (figure 14).

Figure 13. MIROVA thermal data for January-September 2018 indicated intermittent thermal anomaly signals in January, March, April, and July-September (top). A Sentinel-2 image with a hotspot was captured on 23 September, the same day as the MIROVA thermal signal (bottom). Courtesy of MIROVA.

Figure 14. A Sentinel-2 image of Saunders Island on 10 February 2018 revealed a distinct hotspot and small steam plume rising from the summit crater of Mount Michael. Sentinel-2 image with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

A MODVOLC thermal alert appeared on 26 March 2019 followed by a significant hotspot signal in Sentinel-2 imagery on 29 March (figure 15). The hotspot was still present along with a substantial steam plume on 3 April 2018. Sentinel-2 imagery on 11 April revealed a large steam plume and cloud cover, but no hotspot.

Figure 15. Hotspots in Sentinel-2 imagery on 29 March and 3 April 2018 indicated eruptive activity at Mount Michael on Saunders Island. Sentinel-2 image with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

MIROVA thermal signals appeared in mid-July and mid-August 2018 (figure 13) but little satellite imagery was available to confirm any thermal activity. The next clear signal of eruptive activity was evident in a Sentinel-2 image as a hotspot at the summit on 23 September. A small MIROVA signal was recorded the same day (figure 13, bottom). A few days later, on 28 September 2018, a Landsat 8 image showed a clear streak of dark-gray ash trending NW from the summit of Mount Michael (figure 16).

Figure 16. Satellite imagery confirmed eruptive activity at Mount Michael on Saunders Island in late September 2018. Top: a hotpot in a Sentinel-2 image on 23 September coincided with a MIROVA thermal signal (see figure 13); Bottom: A Landsat 8 image on 28 September has a distinct dark gray streak trending NW from the summit indicating a fresh ash deposit. The lighter gray area SW of the summit is likely a shadow. Sentinel-2 image with Atmospheric Penetration view, (bands 12, 11, and 8A), Landsat 8 image with pansharpened image processing, both courtesy of Sentinel Hub Playground.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); 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/); Sentinel Hub Playground, (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Historical eruptions at Chile's Villarrica, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. An intermittently active lava lake at the summit has been the source of explosive activity, incandescence, and thermal anomalies for several decades. A large explosion on 3 March 2015 included a 9-km-altitude ash plume; significant thermal anomalies from intermittent Strombolian activity at the lava lake and small ash emissions have continued since that time. Sporadic but reduced activity during November 2017-August 2018 is covered in this report, with information provided primarily by 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 Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile, a research group that studies volcanoes across Chile.

Seismicity increased during the second half of November 2017, along with observations of increased incandescence at night from occasional explosions inside the summit crater. Satellite instruments measured a brief surge of thermal activity from late November through early December. The next episode of increased activity occurred in the second half of February 2018 with minor satellite thermal data and webcam views of incandescence. A slow but sustained increase in energy was recorded during March 2018; sporadic incandescence was reported a few times each month between March and May, but observations indicated that the lava lake level was over 100 m below the crater rim. Satellite and webcam observations of incandescence increased in frequency and intensity during June; sporadic ash emissions were noted during mid- and late July. Continuous incandescence was observed in webcams during August 2018; satellite thermal data identified an abrupt rise in thermal energy in late July that remained at a low level into early September 2018.

Activity during November 2017-January 2018. OVDAS reported that during November 2017, the webcams near the summit showed evidence of low-intensity, predominantly white degassing to low altitudes (100 m above the summit). Nighttime incandescence associated with occasional explosions inside the crater were typical. They also noted that long-period (LP) seismicity increased in both energy amplitude and frequency during the last few days of the month. A gradual increase in RSAM values began on 15 November with a continuous tremor signal. A magnitude 4.1 event occurred on 24 November located 2.6 km ESE of the summit at a depth of 1.8 km. A single MODVOLC thermal alert was reported on 28 November. According to POVI the lava lake on the crater floor subsided 8 m between 10 and 20 November (figure 54); during the second half of the month they documented 50-m-high lava fountains, spatter on the crater rim, incandescent jets, and fresh ashfall on the snow cover around the crater rim (figures 55 and 56).

Figure 54. The SW part of the crater floor at Villarrica subsided about 8 m between 10 and 20 November 2017. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Figure 55. During the second half of November 2017, POVI documented 50 m high lava fountains, spatter on the crater rim, incandescent jets, and fresh ashfall on the snow cover around the crater rim at Villarrica. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Figure 56. A sample of reticulite or basaltic pumice collected on 28 November 2017 from the summit of Villarrica. It is a highly vesiculated scoria, with greater than 98% porosity. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

On 5 December 2017, SERNAGEOMIN raised the Alert Level at Villarrica from Green to Yellow (on a 4-level scale), noting a progressive increase in seismic and thermal energy since 15 November. They increased the restricted radius from 500 to 1,000 m from the summit crater. SERNAGEOMIN reported low-intensity degassing during the first half of December 2017, mostly white, and rising not more than 650 m above the crater. Incandescence was visible on clear nights, with occasional explosions that remained below the crater rim. They reported that increased surficial activity was visible during the first few days of December, followed by a decrease in activity (figure 57). POVI images at the end of December (figure 58) showed that the lake level had dropped more than 45 m between 5 and 27 December 2017. Seismicity also decreased throughout the month, reaching its lowest level of the year at the end of December.

Figure 57. POVI reported that on 9 December 2017 at Villarrica the level of the lake at the bottom of the crater was stable at about 70 m below the rim, and five days had passed with no observations of lava ejecta in the webcams. Images by Víctor Marfull, courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Figure 58. The lava lake at Villarrica subsided more than 45 m between 5 and 27 December 2017 when this image was taken. Seismic activity also decreased significantly throughout December, reaching its lowest level of the year. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

On 6 January 2018 SERNAGEOMIN lowered the Alert Level to Green, noting a reduced thermal signal, low-level white degassing rising less than 300 m above the crater, and only occasional nighttime incandescence associated with explosions below the crater rim during the second half of December. POVI noted that the drop in seismicity at the end of December corresponded to the end of a 17-month-long period of increased seismicity (figure 59).

Figure 59. The drop in seismicity at the end of December 2017 suggested the end to a 17-month-long period of increased seismicity that began in July 2016 after a similar decrease in activity at the end of June 2016. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Activity during February-August 2018. Activity remained low at Villarrica during January 2018. Steam plumes rose less than 550 m above the crater and no thermal activity was apparent. After about six weeks of low activity, Sentinel-2 images indicated an increase in thermal activity between 5 and 18 February 2018 (figure 60). The Villarrica webcam also recorded incandescence at the summit for the first time in two months on 25 February 2018.

Figure 60. After about six weeks of low activity at Villarrica, Sentinel-2 images indicated an increase in thermal activity between 5 and 18 February 2018. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

While SERNAGEOMIN reported only white degassing to less than 50 m above the summit in March 2018, POVI noted that seismic instruments recorded a slow but sustained increase in released energy. The lava lake was not visible and remained more than 110 m below the crater rim; a small spatter event was detected by a webcam on 7 March 2018 (figure 61). Sporadic incandescence, including on 13 and 20 March, was captured with a webcam located in Pucón, about 16 km N of the summit.

Figure 61. The surface of the lava lake at the summit of Villarrica remained more than 110 m below the crater rim on 6 March 2018. A small spatter of lava was detected by one of the POVI cameras on 7 March 2018, but little other activity was recorded. A slow but sustained increase in seismic energy was evident in the seismic amplitude data (inset). Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

A research effort in mid-March 2018 by Liu et al. (2019) to capture gas emissions close to the vent using Unmanned Aerial Systems (UAS) demonstrated good agreement between gas ratios obtained from simultaneous UAS- and ground-based multi-GAS acquisitions. The UAS measurements, however, taken from the young, less diluted gas plume revealed additional short-term patterns that reflected active degassing through discrete, audible gas exhalations (figures 62 and 63).

Figure 62. A research expedition to Villarrica on 20 and 21 March 2018 demonstrated the effectiveness of Unmanned Aerial Systems (UAS) in measuring gas emissions close to an active vent. a) This view to the SE shows Lanin and Quetrapillan volcanoes in the distance behind the summit of Villarrica. (b, c) The lava level was extremely low in the conduit during the measurement campaign, with the lake surface only visible as several pixels in aerial imagery. (d) Unmanned Aerial Systems (UAS) were launched from a sheltered plateau on the N rim of the crater, with the multi-GAS station visible on the eastern rim. (e, top right) Location map of the region, showing the position of UV camera. The green shaded region delimits the extent of the national park. Inset: Aerial map of the summit region shown in (d); the summit crater is ~200 m in diameter. (e, bottom left) Two instrumented multi-rotor vehicles were used in the campaign, the Vulcan octocopter with multi-GAS (left) and DJI Phantom 3 Pro with Aeris gas sensor (right). (f) Vulcan UAS in flight on 20 March 2018. UAV = Unmanned Aerial Vehicle. Taken from Figure 1 of Liu et al. (2019).

Figure 63. A comparison between contemporaneous proximal UAV and crater rim SO2 measurements at Villarrica. (a) The same-scaled axis highlights the magnitude of plume dilution between the proximal measurements from the UAV made directly above the conduit and those made at the crater rim only 100 m downwind. (b) When the time series are displayed on individually scaled axes it is apparent that even considering the temporal offset imposed by the downwind travel time, the periodic component of the proximal UAV trace is indistinguishable in the crater rim data. Taken from Figure 6 of Liu et al. (2019).

A minor collapse of the crater wall caused a small plume of ash that rose a short distance above the summit on 29 March 2018. POVI's time-lapse webcams located in Pucón captured the event. Overnight on 1-2 April, sporadic incandescence was observed in the webcams and in Sentinel-2 satellite imagery. SERNAGEOMIN reported a single MIROVA alert signal on 13 April and an abrupt fall of the seismic signal on 27 April. The POVI webcam captured the brightest incandescence since mid-December 2017 on 3 May 2018. SERNAGEOMIN reported incandescence at the summit again on 23 May, and two thermal alerts on 22 and 25 May 2018.

While gas emissions remained less than 150 m above the summit during June 2018, observations of incandescence at night increased and were reported on 14, 18, 24, and 28 June, and were accompanied by satellite thermal signals on 14 and 24 June. Sporadic ash emissions that reached 400 m above the summit were reported by SERNAGEOMIN during July. The POVI webcam in Pucón captured an ash emission on 16 July 2018 that left ash and pyroclastic debris around the crater rim (figures 64 and 65). A second emission was recorded on 18 July;the Sentinel-2 satellite recorded the largest summit thermal signature since 10 December 2017 the same day.

Figure 64. A steam and ash emission at Villarrica on 16 July 2018 was captured by the POVI webcam. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Figure 65. Ash and pyroclastic debris were deposited around the inside rim of the crater at Villarrica on 16 July 2018. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Activity continued to increase during July 2018; POVI photographed significant incandescence at the summit on 19 July and again on 25, 29, and 30 July after a period of cloudy weather. ESA's Sentinel-2 camera measured the largest heat area on the summit since August 2015 on 30 July (figure 66). As a result, the interior of the crater lost much of its snow cover and ice (figure 67). Ash and lapilli were visible in satellite imagery on the eastern edges of the crater.

Figure 66. On July 30 2018 ESA's Copernicus program satellite, Sentinel-2, measured the largest heat area on the summit of Villarrica since August 2015. Due to the heat, the interior of the crater had lost much of its snow cover and ice. Ash and lapilli stand out on the eastern edges of the crater. Left: terrestrial images (objective 120 mm, 0.0001 lux), center: Sentinel-2, filters bands 8, 4 and 3; Right: Sentinel-2, filters bands 12, 11 and 4. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Figure 67. Sequential images of the Sentinel-2 satellite (ESA), with filters of bands 8, 4, and 3, illustrate the evolution of the heat surface emitted by the lava pit, and the decrease in snow and ice within and around the crater rim between 8 July and 2 August 2018. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

SERNAGEOMIN reported continuous incandescence at the summit during August nights when the weather was clear. POVI noted on 31 August 2018 that the lake level had not changed during the month and was about 75 m below the inner W rim of the crater. The lake level remained unchanged during the first 10 days of September 2018 as well (figure 68).

Figure 68. The lava lake level at the bottom of the summit crater of Villarrica was unchanged during the first 10 days of September 2018. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

The thermal signature in the MIROVA graph for the period from October 2017 through August 2018 showed two clear increases in thermal energy between late November and mid-December 2017, and again from mid-June through August 2018 (figure 69). These corresponded well with MODVOLC thermal alert data which recorded one alert on 28 November 2017, 10 alerts during 2-11 December 2017, and five alerts between 30 July and 2 August 2018.

Figure 69. MIROVA thermal anomaly graph of log radiative power at Villarrica from 28 September 2017 through August 2018 shows two clear increases in activity, one in mid-November through mid-December 2017 and a second longer-lived phase that began in June 2018, peaked in late July-early August, and remained steady throughout the month of August. Courtesy of MIROVA.

Reference: Liu, E. J., Wood, K., Mason, E., Edmonds, M., Aiuppa, A., Giudice, G., Bitetto, M., Francofonte, V., Burrow, S., Richardson, T., Watson, M., Pering, T.D., Wilkes, T.C., McGonigle, A.J.S., Velasquez, G., Melgarejo, C., and Bucarey, C., 2019. Dynamics of outgassing and plume transport revealed by proximal unmanned aerial system (UAS) measurements at Volcán Villarrica, Chile. Geochemistry, Geophysics, Geosystems, 20. https://doi.org/10.1029/2018GC007692

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/); 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/).