Ash plumes and frequent Strombolian activity during February-July 2021

A brief period of ash emissions in January 2012 was the first documented activity in over 50 years (BGVN 36:12) at Lewotolok (also known as Lewotolo), on the island of Lembata (Lomblen) in the Lesser Sunda Islands of Indonesia. Between then and November 2020, intermittent thermal activity was identified using MODVOLC and Sentinel-2 infrared data (BGVN 41:09, 46:02) and a single episode of increased seismicity was noted by PVMBG in October 2017. The only emissions reported were white steam and gas plumes rising up to 500 m above the summit crater. Increased seismicity at the end of November 2020 was accompanied by explosions with ash plumes and incandescent ejecta (BGVN 46:02) that resulted in ashfall and evacuations from surrounding communities. Explosions with ash emissions rising up to 1,500 m and incandescent ejecta rising 600 m above the crater were reported through the end of January 2021. This report covers similar explosive activity that continued from February-July 2021 with information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, and satellite data.

Multiple daily explosions were reported throughout February-July 2021 except for a brief pause during late April and early May (figure 13). Multi-day periods of incandescent ejecta rising hundreds of meters above the summit and extending a similar distance from the crater continued in February and increased to near-daily during June and July; multiple episodes of ejecta fell 1 km from the crater rim. Dense gray and black ash plumes were infrequent in January and February but increased to multiple times a day in mid-March, rising 1,000-1,500 m above the summit; the highest plume of the period reached over 3 km above the summit on 13 June. Daily explosive activity with ash plumes and incandescent ejecta continued through the end of July. Thermal anomalies in MIROVA data indicated pulses of higher activity during March-early April and late June-July (figure 14). Satellite imagery provided thermal and visual evidence of fresh lava flow activity inside the summit crater on multiple occasions.

Figure 13. The number of daily explosions at Lewotolok during 1 February-31 July 2021 ranged from 0-41. Data compiled from PVMBG daily reports.

Figure 14. Periods of increased thermal activity at Lewotolok were recorded by the MIROVA project during December-2020-early January 2021, March-early April 2021, and late June-July 2021. Courtesy of MIROVA.

Activity during February-April 2021. PVMBG reported daily white and gray emissions that rose 200-700 m above the summit, and occasionally higher, during February 2021. Explosions occurred virtually every day with a maximum of 21 reported on 3 February, but 5-10 per day were typical. During 5-9 February loud noises accompanied bursts of incandescent ejecta that rose 300 m above the summit and extended 300-500 m SE (figure 15). MAGMA Indonesia reported an eruption on 10 February that produced an ash plume that rose 700 m and drifted E (figure 16). Thermal anomalies appeared in satellite images on 7, 12, 17, and 22 February.

Figure 15. A strong thermal anomaly recorded at the summit of Lewotolok on 7 February 2021 was likely due to the incandescent ejecta reported by PVMBG that rose 300 m high and sent material 500 m SE. Sentinel-2 satellite imagery uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 16. An ash plume rose 700 m above the summit of Lewotolok and drifted E on 10 February 2021. Courtesy of MAGMA Indonesia and PVMBG.

The abrupt increase in the number of daily explosions at the beginning of March 2021 was accompanied by loud rumbling noises and observations of incandescent ejecta. During 2-5 March incandescent ejecta rose 200-800 m above the summit towards the SE. On 9 March the ejecta was reported 1,300 m E and SE of the summit. From 11-15 March the incandescent ejecta rose 500 m above the crater rim and was ejected as far as 1,000 m E and SE. The Darwin VAAC reported ash plumes on 13 and 14 March that drifted SW at 1.2 km altitude. MAGMA Indonesia reported two explosions, one on 14 March that rose 1,300 m above the summit and drifted S (figure 17) and one on 15 March that rose 1,000 m and drifted E. During 19-31 March there were 4-20 explosions each day that produced audible booms and rumbling; incandescent ejecta rose as high as 500 m and was thrown the same distance E and SE. Thermal anomalies at the center of the summit crater were apparent in Sentinel-2 satellite imagery on 4 and 19 March.

Figure 17. An explosion at Lewotolok on 14 March 2021 produced an ash plume that rose 1,300 m above the summit and drifted S. Courtesy of MAGMA Indonesia and PVMBG.

Cloudy weather obscured views of the summit for several days in early April 2021, but during 5-17 April multiple daily explosions produced incandescent ejecta often reaching 500 m high, loud noises, and dense gray and black ash plumes (figure 18). Sentinel-2 imagery indicated thermal anomalies inside the summit crater likely caused by fresh lava flows on 8 April. The thermal anomalies and visual imagery from 13 April revealed a new dark area, still hot around the edges, that also likely represented fresh lava (figure 19). Ejecta reached 300-500 m E and SE of the summit during 8-12 April; on 16 April PVMBG reported incandescent ejecta 1,000 m above the summit. MAGMA Indonesia noted one to five eruptions each day during 12-18 April; dense white, gray, and black ash plumes rose 500-1,500 m above the summit and drifted W or SW most days. The Darwin VAAC reported ash emissions on 15 and 16 April which rose 400-700 m above the summit and drifted WNW, and a plume that rose to 3,000 m altitude (1,500 m above the summit) on 18 April and drifted W (figure 20). Additional explosions were reported on 20-21 and 24-25 April, producing gray and black ash plumes that rose 600-1,000 m above the summit. By 28 April only steam emissions were observed, and no thermal anomalies were present around the fresh dark flow visible inside the summit crater.

Figure 18. Incandescent ejecta rose several hundred meters above the summit of Lewotolok on 13 April 2021. Courtesy of MAGMA Indonesia and PVMBG.

Figure 19. A comparison of visual and thermal imagery of Lewotolok from 4 March (left), 8 (middle) and 13 April 2021 (right) shows the appearance of fresh dark lava (natural color images, top) and thermal anomalies (Atmospheric penetration images, bottom) in the 8 and 13 April images that were not present in the 4 March images. Two plumes of steam and ash rose from the summit crater of Lewotolok on 8 April 2021 (top center) and corresponded with two thermal anomalies inside the crater (bottom center). The NW -trending linear anomaly was likely caused by a fresh lava flow. Small anomalies around the dark area on 13 April (bottom right) suggested cooling lava. Natural color images use bands 4, 3, and 2; Atmospheric penetration rendering uses bands 12, 11, and 8a. Courtesy of Sentinel Hub Playground.

Figure 20. A dense gray and black ash plume rose 1,500 m above the summit of Lewotolok and drifted S and SW on 18 April 2021.

Activity during May-July 2021. A single explosion reported by PVMBG and Magma Indonesia on 2 May caused a VONA and a VAAC report to be issued. The ash plume rose 500-600 m above the summit and drifted W. Lewotolok was quiet with white and gray emissions, and no explosions reported, until 14 May when incandescent ejecta and ash emissions resumed for the rest of the month. Strombolian activity accompanied by loud noises sent incandescent ejecta 300 m high and 400-700 m SE from the summit; satellite imagery from 18 May showed a small thermal anomaly inside the summit crater. The ash plumes from the multiple explosions were dense and gray, and rose 400-800 m high. The Darwin VAAC reported multiple ash emissions during 21-25 and 28-29 May that were clearly seen in satellite imagery extending SE, S, or SW at 2,100 m altitude; the plume on 28 May (figure 21) rose 1,000 m above the summit to about 2,400 m altitude and drifted SW.

Figure 21. An ash plume at Lewotolok on 28 May 2021 rose 1,000 m above the summit and drifted SW. Courtesy of PVMBG and MAGMA Indonesia.

Multiple explosions with either ash plumes or Strombolian activity were reported daily throughout June 2021. During 1-9 June incandescent ejecta fell at least 300 m in all directions from the summit with loud noises reported (figure 22). On 5 June the 300-m-high ejecta reached 1,000 m NW and 200 m SE. During 8-16 June eruptions with ash plumes were reported by MAGMA Indonesia; both VONA’s and VAAC reports were issued each day. The white and gray ash emissions generally rose 500-800 m above the summit. On 10, 14, and 21 June the plumes were reported 2,000 m above the summit drifting W and NW. An explosion on 13 June produced an ash plume to 3 km above the summit, the highest of the period, that was observed drifting W in satellite imagery at 4.9 km altitude. Rumbling reported during 16-20 June accompanied incandescent ejecta sent 300-500 m in all directions from the crater. On 2, 7, 12, and 27 June thermal anomalies appeared in Sentinel-2 imagery inside the summit crater; the strong anomaly on 27 June suggested new flow activity (figure 23).

Figure 22. Incandescent ejecta rose several hundred meters above the summit of Lewotolok on 8 June 2021 and many other days during the month. Courtesy of PVMBG and MAGMA Indonesia.

Figure 23. Thermal anomalies appeared inside the summit crater at Lewotolok on multiple days during June 2021 including 12 (left) and 27 June (right). The strong anomaly on 27 June suggested new flow activity with a weaker anomaly just off the N end of the larger one. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Heightened levels of activity continued throughout July 2021. Thermal and visual evidence of fresh lava flows inside the summit crater appeared in satellite images on 2 July (figure 24). Additional thermal anomalies were recorded on 12, 22, and 27 July. Bursts of incandescent ejecta frequently caused loud noises. Vegetation caught fire around the summit from incandescent material on 3 and 28 July (figure 25). The tephra was usually reported 300-500 m from the crater but was sent 1,000 m SW on 3 July, and 1,000 m SE on 12, 18, 28, and 30 July. Five or more explosions were reported daily by PVMBG during 1-20 July, with a maximum of 28 reported on 9 July. The dense gray or gray-and-black ash emissions consistently rose 800-1,000 m above the summit, usually drifting W or NW (figure 26). The tallest plume on 7 July was reported by MAGMA Indonesia at 1,100 m above the summit, drifting E.

Figure 24. Evidence of fresh lava flow activity at Lewotolok appeared in Sentinel-2 satellite imagery on 2 July 2021. A thin line of dark material seen in natural color rendering (left, bands 4,3,2) is the area of strongest thermal anomaly seen in the Atmospheric penetration rendering image (right, bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 25. Vegetation near the summit of Lewotolok caught fire from incandescent ejecta on 28 July 2021. Courtesy of PVMBG and MAGMA Indonesia.

Figure 26. Gray ash and steam plumes rose 800 m from a pyroclastic cone-like feature inside the summit of Lewotolok on 14 July 2021. Courtesy of PVMBG and MAGMA Indonesia.

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

Information is preliminary and subject to change. All times are local (unless otherwise noted)

December 2011 (BGVN 36:12) Cite this Report

December 2011-January 2012 seismicity, incandescence, and evacuations

Plumes and seismic activity at Lewotolo volcano, Indonesia, increased during December 2011 and early January 2012. Lewotolo has erupted potassic calc-alkaline lavas containing as an accessary phase in vessicle fillings, the rare, complex zirconium-titanium-oxide mineral zirconolite (Ca_0.8 Ce_0.2 Zr Ti_1.5 Fe²⁺_0.3 Nb_0.1 Al_0.1 O₇; de Hoog and van Bergen, 2000). Lewotolo last erupted in 1951. All historical eruptions were small (Volcanic Explosivity Index, VEI 2) with the exception of the first recorded eruption, which took place in 1660 and was as large as VEI 3. According to de Hoog and van Bergen (2000), strong fumarolic activity at the summit of Lewotolo indicates the presence and degassing of a shallow magma chamber.

December 2011-January 2012 activity increase. According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), Lewotolo produced thick white plumes reaching 50-250 m above the summit during December 2011. Seismicity increased on 31 December, and intensified on 2 January 2012 with tremor commencing at 1400. Accordingly, CVGHM raised the Alert Level from 1 to 2 (on a scale from 1-4) at 1800 on 2 January. Between 1800 and 2300 the same day, the maximum amplitude of recorded seismicity increased, and at 2000, incandescence was noticed at the summit.

At 2330 on 2 January, CVGHM increased the Alert Level to 3. Under the recommendation of CVGHM, access was prohibited within 2 km of Lewotolo (Hazard Zone III, figure 1), and residents in villages SE of the volcano were advised to keep vigilant and secure a safe place to flee to one of the towns to the N, W, or S in the event of an eruption.

Figure 1. Map of areas around Lewotolo showing Hazards Zones I-III. Hazard Zone I includes areas possibly threatened by ash fall and incandescent bombs (within 7 km of Lewotolo, yellow dashed circle) and areas possibly affected by lahars (shaded yellow). Hazard Zone II includes areas possibly threatened by heavy ash-fall and incandescent bombs (within 4 km of Lewotolo, dark pink dashed circle) and areas possibly affected by pyroclastic flows, lava flows, and lava avalanches (shaded light pink). Hazard Zone III includes areas very likely to be threatened by heavy ash fall and incandescent bombs (within 2 km of Lewotolo, light pink dashed circle) and areas very likely to be affected by pyroclastic flows, lava flows, lava avalanches, and volcanic gases (shaded dark pink). Other symbols are explained in the legend at the right. Authorities prohibited access to Hazard Zone III on 2 January 2012. Modified from CVGHM.

Residents decide to evacuate. According to Antara News, evacuations began on 4 January spurred by increased activity of the previous few days, as well as minor ash falling in the villages. Antara News stated that most of the residents went to Lewoleba, the closest city to the volcano (~15 km to the SW of the summit). Of the evacuees in Lewoleba, all but about 50 people were reported to have found temporary housing with other residents of the city.

On 5 January, Channel 6 News reported that around 500 residents had evacuated leaving their homes in villages surrounding Lewotolo. They noted that residents who evacuated did so on their own accord, as the government had not yet called for evacuation. The Deputy District Chief of Lembata, Viktor Mado Watun, said "Black smoke columns are coming out of the mountain's crater, the air is filled with the smell of sulfur while rumbling sounds are heard around the mountain."

According to UCA News on 9 January, the health of the evacuees was cause for concern. Father Philipus da Gomez stated that "there are many refugees who have started suffering from acute respiratory infections."

Alert Level lowered. On 25 January 2012, CVGHM lowered the Alert Level of Lewotolo from 3 to 2 following decreased activity after 2 January. The lowered Alert Level restricted access to the summit craters only. CVGHM stated that the observed seismicity (table 1) showed a declining trend, tending towards normal conditions after 23 January. Visual observation revealed thick, white plumes reaching 400 m above the summit during 2-14 January (and a dim crater glow), and thin white plumes reaching no more than 50 m above the summit during 16-24 January (with no accompanying crater glow).

Table 1. Seismicity at Lewotolo during 3-24 January 2012, showing a declining trend in seismicity prior to CVGHM's lowering of the Alert Level from 3-2 on 25 January. Data courtesy of CVGHM.

On 15 January, direct observation of the crater was made, and revealed incandescence in solfataras, a weak sulfur smell, and hissing sounds in both the N and S side of the crater. CVGHM especially noted that the N side of the crater was quite different than when it was last observed in June 2010, when no solfataras were present. Differential Optical Absorption Spectroscopy (DOAS) measurements revealed fluctuating and increasing SO₂ flux between 11-90 tons/day during 8-16 January.

References. de Hoog, J.C.M. and van Bergen, M.J., 2000, Volatile-induced transport of HFSE, REE, Th, and U in arc magmas: evidence from zirconolite-bearing vesicles in potassic lavas of Lewotolo volcano (Indonesia), Contributions to Mineralogy and Petrology, v. 139, no. 4, p. 485-502 (DOI: 10.1007/s004100000146).

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.go.id/); Channel 6 News (URL: http://channel6newsonline.com/); Antara News, Wisma ANTARA 19th Floor, Jalan Merdeka Selatan No. 17, Jakarta Pusat (URL: http://www.antaranews.com/); UCA News, Yayasan UCINDO, Gedung Usayana Holding, Lt.3, Jl. Matraman Raya No.87, Jakarta Timur 13140 (URL: http://www.ucanews.com/).

September 2016 (BGVN 41:09) Cite this Report

Thermal hotspots during 27 September-4 October 2015

During December 2011-January 2012, Lewotolo's seismic activity increased and the volcano produced thick, white plumes that rose as high as 250 m above the summit before subsiding (BGVN 36:12). Since that episode, no further activity was observed through 31 December 2016, except for several thermal anomalies during 27 September 2015-4 October 2015, as recorded by MODIS satellite instruments analyzed using the MODVOLC algorithm (figure 2).

Figure 2. The MODIS/MODVOLC composite image that indicates the location of thermal anomalies at Lewotolo between 27 September and 4 October 2015. One is N of the summit (27 September), the others are somewhat E. One of the two hotspots on 4 October appears on the E summit rim. The daily thermal alert maps indicated that the five hotspots were all weak.

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/, http://modis.higp.hawaii.edu/).

February 2021 (BGVN 46:02) Cite this Report

New eruption in late November 2020 consisting of ash plumes, crater incandescence, and ashfall

Lewotolok (also known as Lewotolo) is located on the eastern end of a peninsula connected to Lembata (formerly Lomblen) that extends north into the Flores Sea. Eruptions date back to 1660, characterized by explosive activity in the summit crater. Typical activity has consisted of seismicity and thermal anomalies near the summit crater (BGVN 36:12 and 41:09). A new eruption that began in late November 2020 was characterized by increased seismicity, dense, gray ash plumes, nighttime crater incandescence, and ashfall. This report covers activity through January 2021 using information primarily from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, and satellite data.

Summary of activity during February 2012-October 2020. Activity from February 2012 to November 2020 was relatively low and consisted primarily of a persistent thermal anomaly in the summit crater since at least March 2016 and occasional white gas-and-steam emissions. During January 2012 intermittent white gas-and-steam plumes rose 15-500 m above the crater, accompanied by crater incandescence; no thermal anomalies were reported during 16-24 January. On 6 January there were 500 people in the Lembata district evacuated due to reports of ash plumes that were observed by local residents, the smell of sulfur, and the sound of rumbling (BGVN 36:12).

Thermal activity dates back to 13 October 2014 using MODIS data in MODVOLC satellite data (BGVN 41:09; figure 3). According to the MODVOLC algorithm, a total of seven thermal alerts were detected on 13 October 2014 (1), 27 September 2015 (1), 2, 3, and 4 (2) October 2015, and 5 November 2017 (1). The number of thermal alerts in both MODVOLC and Sentinel-2 satellite data had increased slightly in 2020 compared to 2018 and 2019, though cloud cover often prevented visual confirmation for the latter (figure 3). Sentinel-2 thermal satellite imagery captured occasional thermal anomalies in the summit crater during 2016-2019 (figure 4). White gas-and-steam plumes were intermittently reported from September 2017 through 2 March 2018 that rose as high as 500 m above the crater and drifted dominantly E and W, according to PVMBG.

Figure 3. Graph comparing the number of thermal anomalies using MODVOLC alerts and Sentinel-2 satellite data for Lewotolok during January 2014-January 2021 for MODVOLC and 20 March 2016-January 2021 for Sentinel-2 thermal satellite data. Data courtesy of HIGP - MODVOLC Thermal Alerts System and Sentinel Hub Playground.

Brief seismicity, which included shallow and deep volcanic earthquakes was detected during October 2017. On 9 October 2017 PVMBG issued a VONA (Volcano Observatory Notice for Aviation) reporting that white gas-and-steam emissions rose 500 m above the crater. On 10 October BNPB (Badan Nacional Penanggulangan Bencana) reported that five earthquakes 10-30 km below Lewotolok and ranging in magnitude of 3.9-4.9 as recorded by Badan Meteorologi, Klimatologi, dan Geofisika (BMKG). These seismic events were felt by local populations and resulted in an evacuation of 723 people. The only activity reported between January 2018 and October 2020 was white gas-and-steam plumes that rose 5-100 m above the crater drifting primarily E and W and an occasional thermal anomaly in the summit crater (figure 4).

Figure 4. Sentinel-2 thermal satellite imagery shows a thermal anomaly in the summit crater of Lewotolok during 20 March 2016 (top left), 8 July 2017 (top right), 13 July 2018 (bottom left), and 12 August 2019 (bottom right). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

New eruption starting in November 2020. On 26 November 2020 a continuous tremor began at 1943, followed by a series of volcanic earthquakes at 1947 and deep volcanic earthquakes at 1951, 1952, 1953, and 2255; white gas-and-steam emissions rose 20 m above the crater. Deep volcanic earthquakes were again recorded at 0242, 0537, 0556 on 27 November. At 0557 an explosion produced a gray ash plume that rose 500 m above the crater and drifted W; by 0630 the plume turned white, according to PVMBG (figure 5). Seismicity decreased slightly after the explosion, but tremor continued. During 27-28 November dense white gas-and-steam plumes rose as high as 500 m above the crater and nighttime crater incandescence was observed.

Figure 5. Webcam image of a dense gray ash plume rising 500 m above the crater of Lewotolok on 27 November 2020. Courtesy of MAGMA Indonesia.

During the morning of 29 November seismicity increased again and consisted of six deep volcanic earthquakes, continuous tremor occurred around 0930. A second explosion was recorded at 0945 that produced an ash plume 4 km above the crater, accompanied by incandescent material that was ejected above the crater (figure 6). The ash plume consisted of two levels: the lower-level drifted W and NW and the upper-level drifted E and SE. The large, gray ash plume was captured in a satellite image as it spread generally E and W (figure 7). Ashfall and a sulfur odor was reported in several surrounding villages; videos from social media showed tephra falling onto the roofs of residential areas. BPBD evacuated residents in 28 villages in two sub-districts; by 29 November at 1300 about 900 people had been evacuated. At 1900 Strombolian activity was observed and during the night, crater incandescence was visible.

Figure 6. Photos of the eruption at Lewotolok on 29 November 2020 that produced a dense, gray ash plume 4 km above the crater. Courtesy of Devy Kamil Syahbana, PVMBG (left) and MAGMA Indonesia (right).

Figure 7. Satellite image showing a strong gray ash plume above Lewotolok on 29 November 2020, expanding roughly E and W. Courtesy of Sentinel Hub Playground and the European Space Agency, Copernicus.

The eruption continued from 29 November into 1 December, where the white-and-gray ash plumes rose 700-2,000 m above the crater and drifted SE and W, accompanied by incandescent material that was ejected above the crater and the smell of sulfur, according to PVMBG (figure 8). A large sulfur dioxide plume was reported drifting SE and extending over the N half of Australia by 30 November (figure 9). By 1300 that day, 4,628 people had been evacuated. Incandescent lava flows near the summit were visible and incandescent material traveled down the flanks during 30 November and 1 December.

Figure 8. Webcam image of the continuous eruption at Lewotolok showing a dense gray ash plume rising above the cloud-covered summit on 30 November 2020. Courtesy of MAGMA Indonesia.

Figure 9. SO2 plume from Lewotolok captured by the Sentinel-5P/TROPOMI instrument on 30 November 2020 drifting SE and along the N part of Australia. Courtesy of Simon Carn and the NASA Global Sulfur Dioxide Monitoring Page.

White-and-gray plumes continued frequently through January 2021, rising 100-1,500 m above the crater, drifting in multiple directions, accompanied by nighttime crater incandescence and occasional incandescent ejecta (figure 10). During 1-8 December gray plumes rose 100-1,000 m above the crater and drifted E, W, and SW accompanied by nightly crater incandescence and incandescent material ejected as high as 20 m above the crater. By 5 December at 2200 about 9,028 residents had been evacuated to 11 evacuation centers, according to BNPB. Black, gray, and brown ash plumes were visible daily during 9-15 December, rising 1 km above the crater, accompanied by nightly Strombolian explosions that ejected material above the crater. More Strombolian explosions on most nights over 16-29 December ejected material 100-300 m above the crater; in addition, the sounds of rumbling and banging could be heard. The material was deposited as far as 1 km from the crater E and SE during 24-25 and 27-31 December and 4-7 January 2021. Strombolian activity continued into January, accompanied by frequent gray-and-white ash plumes, rumbling and banging sounds, and incandescent ejecta up to 600 above the crater that extended as far as 500 m E, SE, and W. Crater incandescence was visible up to 600 m above the crater.

Figure 10. Webcam images showing continuing dense gray ash plumes from Lewotolok on 1 December 2020 (top) and 8 January 2021 (bottom). Courtesy of MAGMA Indonesia.

A consistent level of thermal activity was recorded in the Sentinel-2 MODIS Thermal Volcanic Activity from February 2019 through October 2020; in early December 2020 a slight increase in thermal anomalies were detected (figure 11). This data reflects the start of the new eruption in late November 2020. According to the MODVOLC thermal algorithm, five thermal hotspots were detected between January 2020 and January 2021 on 3 September (1), 29 November (2), 24 December (1), and 5 January 2021 (1). Some of this thermal activity was also observed in Sentinel-2 thermal satellite imagery in the summit crater (figure 12).

Figure 11. Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A) shows consistent thermal activity (red dots) at Lewotolok during February 2020 through December 2020. Stronger thermal anomalies in early December is likely due to the new eruption that began in late November 2020. Courtesy of MIROVA.

Figure 12. Sentinel-2 thermal satellite imagery showing a thermal anomaly in the summit crater of Lewotolok on 25 October (top left), 9 November (top right), and 3 January 2021 (bottom right). On 14 December (bottom left) a Natural Color image showed a gray ash emission above the clouds and drifted E. On 3 January 2021 (bottom right) two thermal anomalies were visible in the summit crater accompanied by gas-and-steam emissions drifting NE. Sentinel-2 satellite images with “Natural Color” rendering (bands 4, 3, 2) on 14 December 2020, all other images use “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); European Space Agency (ESA), Copernicus (URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus); 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/); Simon Carn, Dept of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA (URL: https://so2.gsfc.nasa.gov/).

August 2021 (BGVN 46:08) Cite this Report

Ash plumes and frequent Strombolian activity during February-July 2021

A brief period of ash emissions in January 2012 was the first documented activity in over 50 years (BGVN 36:12) at Lewotolok (also known as Lewotolo), on the island of Lembata (Lomblen) in the Lesser Sunda Islands of Indonesia. Between then and November 2020, intermittent thermal activity was identified using MODVOLC and Sentinel-2 infrared data (BGVN 41:09, 46:02) and a single episode of increased seismicity was noted by PVMBG in October 2017. The only emissions reported were white steam and gas plumes rising up to 500 m above the summit crater. Increased seismicity at the end of November 2020 was accompanied by explosions with ash plumes and incandescent ejecta (BGVN 46:02) that resulted in ashfall and evacuations from surrounding communities. Explosions with ash emissions rising up to 1,500 m and incandescent ejecta rising 600 m above the crater were reported through the end of January 2021. This report covers similar explosive activity that continued from February-July 2021 with information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, and satellite data.

Multiple daily explosions were reported throughout February-July 2021 except for a brief pause during late April and early May (figure 13). Multi-day periods of incandescent ejecta rising hundreds of meters above the summit and extending a similar distance from the crater continued in February and increased to near-daily during June and July; multiple episodes of ejecta fell 1 km from the crater rim. Dense gray and black ash plumes were infrequent in January and February but increased to multiple times a day in mid-March, rising 1,000-1,500 m above the summit; the highest plume of the period reached over 3 km above the summit on 13 June. Daily explosive activity with ash plumes and incandescent ejecta continued through the end of July. Thermal anomalies in MIROVA data indicated pulses of higher activity during March-early April and late June-July (figure 14). Satellite imagery provided thermal and visual evidence of fresh lava flow activity inside the summit crater on multiple occasions.

Figure 13. The number of daily explosions at Lewotolok during 1 February-31 July 2021 ranged from 0-41. Data compiled from PVMBG daily reports.

Figure 14. Periods of increased thermal activity at Lewotolok were recorded by the MIROVA project during December-2020-early January 2021, March-early April 2021, and late June-July 2021. Courtesy of MIROVA.

Activity during February-April 2021. PVMBG reported daily white and gray emissions that rose 200-700 m above the summit, and occasionally higher, during February 2021. Explosions occurred virtually every day with a maximum of 21 reported on 3 February, but 5-10 per day were typical. During 5-9 February loud noises accompanied bursts of incandescent ejecta that rose 300 m above the summit and extended 300-500 m SE (figure 15). MAGMA Indonesia reported an eruption on 10 February that produced an ash plume that rose 700 m and drifted E (figure 16). Thermal anomalies appeared in satellite images on 7, 12, 17, and 22 February.

Figure 15. A strong thermal anomaly recorded at the summit of Lewotolok on 7 February 2021 was likely due to the incandescent ejecta reported by PVMBG that rose 300 m high and sent material 500 m SE. Sentinel-2 satellite imagery uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 16. An ash plume rose 700 m above the summit of Lewotolok and drifted E on 10 February 2021. Courtesy of MAGMA Indonesia and PVMBG.

The abrupt increase in the number of daily explosions at the beginning of March 2021 was accompanied by loud rumbling noises and observations of incandescent ejecta. During 2-5 March incandescent ejecta rose 200-800 m above the summit towards the SE. On 9 March the ejecta was reported 1,300 m E and SE of the summit. From 11-15 March the incandescent ejecta rose 500 m above the crater rim and was ejected as far as 1,000 m E and SE. The Darwin VAAC reported ash plumes on 13 and 14 March that drifted SW at 1.2 km altitude. MAGMA Indonesia reported two explosions, one on 14 March that rose 1,300 m above the summit and drifted S (figure 17) and one on 15 March that rose 1,000 m and drifted E. During 19-31 March there were 4-20 explosions each day that produced audible booms and rumbling; incandescent ejecta rose as high as 500 m and was thrown the same distance E and SE. Thermal anomalies at the center of the summit crater were apparent in Sentinel-2 satellite imagery on 4 and 19 March.

Figure 17. An explosion at Lewotolok on 14 March 2021 produced an ash plume that rose 1,300 m above the summit and drifted S. Courtesy of MAGMA Indonesia and PVMBG.

Cloudy weather obscured views of the summit for several days in early April 2021, but during 5-17 April multiple daily explosions produced incandescent ejecta often reaching 500 m high, loud noises, and dense gray and black ash plumes (figure 18). Sentinel-2 imagery indicated thermal anomalies inside the summit crater likely caused by fresh lava flows on 8 April. The thermal anomalies and visual imagery from 13 April revealed a new dark area, still hot around the edges, that also likely represented fresh lava (figure 19). Ejecta reached 300-500 m E and SE of the summit during 8-12 April; on 16 April PVMBG reported incandescent ejecta 1,000 m above the summit. MAGMA Indonesia noted one to five eruptions each day during 12-18 April; dense white, gray, and black ash plumes rose 500-1,500 m above the summit and drifted W or SW most days. The Darwin VAAC reported ash emissions on 15 and 16 April which rose 400-700 m above the summit and drifted WNW, and a plume that rose to 3,000 m altitude (1,500 m above the summit) on 18 April and drifted W (figure 20). Additional explosions were reported on 20-21 and 24-25 April, producing gray and black ash plumes that rose 600-1,000 m above the summit. By 28 April only steam emissions were observed, and no thermal anomalies were present around the fresh dark flow visible inside the summit crater.

Figure 18. Incandescent ejecta rose several hundred meters above the summit of Lewotolok on 13 April 2021. Courtesy of MAGMA Indonesia and PVMBG.

Figure 19. A comparison of visual and thermal imagery of Lewotolok from 4 March (left), 8 (middle) and 13 April 2021 (right) shows the appearance of fresh dark lava (natural color images, top) and thermal anomalies (Atmospheric penetration images, bottom) in the 8 and 13 April images that were not present in the 4 March images. Two plumes of steam and ash rose from the summit crater of Lewotolok on 8 April 2021 (top center) and corresponded with two thermal anomalies inside the crater (bottom center). The NW -trending linear anomaly was likely caused by a fresh lava flow. Small anomalies around the dark area on 13 April (bottom right) suggested cooling lava. Natural color images use bands 4, 3, and 2; Atmospheric penetration rendering uses bands 12, 11, and 8a. Courtesy of Sentinel Hub Playground.

Figure 20. A dense gray and black ash plume rose 1,500 m above the summit of Lewotolok and drifted S and SW on 18 April 2021.

Activity during May-July 2021. A single explosion reported by PVMBG and Magma Indonesia on 2 May caused a VONA and a VAAC report to be issued. The ash plume rose 500-600 m above the summit and drifted W. Lewotolok was quiet with white and gray emissions, and no explosions reported, until 14 May when incandescent ejecta and ash emissions resumed for the rest of the month. Strombolian activity accompanied by loud noises sent incandescent ejecta 300 m high and 400-700 m SE from the summit; satellite imagery from 18 May showed a small thermal anomaly inside the summit crater. The ash plumes from the multiple explosions were dense and gray, and rose 400-800 m high. The Darwin VAAC reported multiple ash emissions during 21-25 and 28-29 May that were clearly seen in satellite imagery extending SE, S, or SW at 2,100 m altitude; the plume on 28 May (figure 21) rose 1,000 m above the summit to about 2,400 m altitude and drifted SW.

Figure 21. An ash plume at Lewotolok on 28 May 2021 rose 1,000 m above the summit and drifted SW. Courtesy of PVMBG and MAGMA Indonesia.

Multiple explosions with either ash plumes or Strombolian activity were reported daily throughout June 2021. During 1-9 June incandescent ejecta fell at least 300 m in all directions from the summit with loud noises reported (figure 22). On 5 June the 300-m-high ejecta reached 1,000 m NW and 200 m SE. During 8-16 June eruptions with ash plumes were reported by MAGMA Indonesia; both VONA’s and VAAC reports were issued each day. The white and gray ash emissions generally rose 500-800 m above the summit. On 10, 14, and 21 June the plumes were reported 2,000 m above the summit drifting W and NW. An explosion on 13 June produced an ash plume to 3 km above the summit, the highest of the period, that was observed drifting W in satellite imagery at 4.9 km altitude. Rumbling reported during 16-20 June accompanied incandescent ejecta sent 300-500 m in all directions from the crater. On 2, 7, 12, and 27 June thermal anomalies appeared in Sentinel-2 imagery inside the summit crater; the strong anomaly on 27 June suggested new flow activity (figure 23).

Figure 22. Incandescent ejecta rose several hundred meters above the summit of Lewotolok on 8 June 2021 and many other days during the month. Courtesy of PVMBG and MAGMA Indonesia.

Figure 23. Thermal anomalies appeared inside the summit crater at Lewotolok on multiple days during June 2021 including 12 (left) and 27 June (right). The strong anomaly on 27 June suggested new flow activity with a weaker anomaly just off the N end of the larger one. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Heightened levels of activity continued throughout July 2021. Thermal and visual evidence of fresh lava flows inside the summit crater appeared in satellite images on 2 July (figure 24). Additional thermal anomalies were recorded on 12, 22, and 27 July. Bursts of incandescent ejecta frequently caused loud noises. Vegetation caught fire around the summit from incandescent material on 3 and 28 July (figure 25). The tephra was usually reported 300-500 m from the crater but was sent 1,000 m SW on 3 July, and 1,000 m SE on 12, 18, 28, and 30 July. Five or more explosions were reported daily by PVMBG during 1-20 July, with a maximum of 28 reported on 9 July. The dense gray or gray-and-black ash emissions consistently rose 800-1,000 m above the summit, usually drifting W or NW (figure 26). The tallest plume on 7 July was reported by MAGMA Indonesia at 1,100 m above the summit, drifting E.

Figure 24. Evidence of fresh lava flow activity at Lewotolok appeared in Sentinel-2 satellite imagery on 2 July 2021. A thin line of dark material seen in natural color rendering (left, bands 4,3,2) is the area of strongest thermal anomaly seen in the Atmospheric penetration rendering image (right, bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 25. Vegetation near the summit of Lewotolok caught fire from incandescent ejecta on 28 July 2021. Courtesy of PVMBG and MAGMA Indonesia.

Figure 26. Gray ash and steam plumes rose 800 m from a pyroclastic cone-like feature inside the summit of Lewotolok on 14 July 2021. Courtesy of PVMBG and MAGMA Indonesia.

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