Sangeang Api is a small 13-km-wide island volcano off the NE coast of Sumbawa Island, part of Indonesia's Lesser Sunda Islands. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century. Ash plumes, Strombolian activity and pyroclastic and lava flows during 1985-1987 from the Doro Api summit crater were the first activity in 20 years and led to the evacuation of over 1,200 residents on the island. Explosive activity resumed for about two years during 1997-1999; no further activity was reported until increased seismicity and white emissions were noted in mid-2009. This activity was intermittent from 2009 through early 2014.

On 30 and 31 May 2014, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, or CVGHM) reported large explosions with ash plumes that rose to around 15 km altitude and pyroclastic flows down the flanks (BGVN 39:03, 41:10). The massive ash plume drifted SE, closing airports as far away as Darwin, Australia. The ashfall caused airline disruption and evacuation of thousands in villages on nearby Sumbawa Island. Strong thermal anomalies appeared after the explosions and lasted for the rest of 2014, indicative of flow activity down the E flank; mostly steam plumes, with occasional ash emissions were reported during 2015. Renewed thermal activity appeared in satellite data in mid-January 2017. The thermal signature increased through July 2017 when Strombolian activity and ash emissions were reported (BGVN 42:09). A high level of thermal activity continued through October 2018, with satellite imagery confirming incandescent activity on the E flank, and PVMBG reporting ash emissions that rose a few kilometers above the summit (BGVN 43:11).

During October 2018-April 2019, covered in this report, elevated thermal activity was recorded with satellite data and imagery, and visitors to the island recorded ash plumes, Strombolian activity, lava bombs, and a blocky avalanche flow down the E flank from the pyroclastic cone inside Doro Api cone. Information comes from satellite thermal data and imagery, and reports from visitors to the island. Indonesia's Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) listed Sangeang Api at Alert Level II for the period. There were no reports from the Darwin VAAC.

Thermal activity became more intermittent during December 2018-March 2019 compared with the previous six months, and then increased again in April according to the MIROVA project thermal data (figure 20). The MODVOLC thermal alerts showed a similar pattern, with a decrease in the number of monthly alerts during December through March. In spite of this, a strong thermal signal was still apparent near the summit throughout the period (figure 21). Sentinel-2 satellite images confirmed the presence of hot material at the summit and down the E-flank ravine throughout the period (figure 22).

Figure 20. Thermal activity at Sangeang Api persisted during November 2018-April 2019. Activity was more intermittent from December 2018 through March 2019 but increased again in frequency and intensity in April 2019. Courtesy of MIROVA.

Figure 21. MODVOLC thermal alert images of Sangeang Api for each month from November 2018-April 2019 indicated persistent thermal activity each month, concentrated at the summit and on the upper part of the E flank. Courtesy of Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System.

Figure 22. Sentinel-2 satellite infrared images confirmed thermal activity at the summit crater and down the ravine on the E flank of Sangeang Api during October 2018 through April 2019. The activity migrated and varied in intensity, indicative of ongoing activity throughout the period. Top left: 12 October 2018. Top right: 27 October 2018. Middle left: 16 November 2018. Middle right: 21 December 2018. Bottom left: 14 February 2019. Bottom Right: 20 April 2019. Courtesy of Sentinel Hub Playground ('Atmospheric Penetration' rendering with bands 12, 11, and 8A).

A Google Earth image dated 7 October 2018 shows dark lava in the drainage on the SE flank extending to within 1 km of the shoreline (figure 23). A visit to Sangeang Api by photographer Martin Rietze during 25 September-3 October 2018 provided excellent ground and drone-based photographic and video documentation of the eruptive activity at that time (see information contacts for link to video footage). Regular small ash emissions were recorded multiple times per day from a pyroclastic cone inside the Doro Api cone (figure 24); the explosions produced volcanic bombs that traveled hundreds of meters down the flanks of Doro Api (figure 25) as well as sending lava fragments down all sides of the pyroclastic cone (figures 26 and 27). Steam and ash rose a few hundred meters above the summit of the pyroclastic cone and was incandescent after dark (figures 28 and 29).

Figure 23. A Google Earth image of the SE flank of Sangeang Api taken on 7 October 2018 shows dark channels of lava descending the ravine to within about 1 km of the shore. Data from SIO, NOAA, U.S. Navy, NGA, GEBCO, Copyright 2018, Google, 2019 Maxar Technologies. Courtesy of Google Earth.

Figure 24. A view from the SW of Sangeang Api island with the two cones of the volcano taken during a 25 September-3 October 2018 expedition. In the foreground is inactive Doro Mantai and, in the background, a small ash plume rises from the active Doro Api cone. Copyrighted photo by Martin Rietze (used with permission).

Figure 25. Explosions with ash plumes sent ejecta hundreds of meters down the W flank of Doro Api (left), the active cone at Sangeang Api, photographed during a 25 September-3 October 2018 expedition. Copyrighted photo by Martin Rietze (used with permission).

Figure 26. A large pyroclastic cone fills the summit crater of Doro Api cone on Sangeang Api, photographed during a 25 September-3 October 2018 expedition. Copyrighted photo by Martin Rietze (used with permission).

Figure 27. Ash explosions sent plumes to a few hundred meters above the summit of Sangeang Api's Doro Api cone, and large bombs and tephra fell around the flanks of the cone. Photographed during a 25 September-3 October 2018 expedition. Copyrighted photo by Martin Rietze (used with permission).

Figure 28. Ash and steam rose from the pyroclastic cone inside Doro Api, the active cone at Sangeang Api. Photographed during a 25 September-3 October 2018 expedition. Avalanche blocks form channels of lava in the ravine on the E flank (right). View is from the SW, taken by drone. Copyrighted photo by Martin Rietze (used with permission).

Figure 29. Incandescence in the ash plumes rising from the pyroclastic cone inside the Doro Api crater was visible at dusk at Sangeang Api when photographed during a 25 September-3 October 2018 expedition. Copyrighted photo by Martin Rietze (used with permission).

Incandescent bombs and blocks from Strombolian activity cascaded down the flanks of the pyroclastic cone and also reached outside of the Doro Api crater, landing on the W flank (figures 30 and 31). Steaming tephra on the flanks of the pyroclastic cone was visible during daylight hours (figure 32); the active Doro Api cone is near the center of the island, the inactive Doro Mantai is on the SE flank. In a nighttime view of Doro Api as seen from the summit of Doro Mantai, the incandescent ejecta is clearly visible hundreds of meters from the erupting pyroclastic cone (figure 33). Video footage showed incandescent lava emerging from a fissure near the base of the pyroclastic cone, and a blocky avalanche on the E flank of the cone; as blocks rolled down the slope, incandescence was visible just below the surface.

Figure 30. Incandescent blocks and bombs were visible at night descending the flank of the pyroclastic cone inside the crater of the Doro Api cone at Sangeang Api when photographed during a 25 September-3 October 2018 expedition. Copyrighted photo by Martin Rietze (used with permission).

Figure 31. Strombolian activity lit up the night sky and scattered bombs around the flanks of the pyroclastic cone inside of the Doro Api cone at Sangeang Api when photographed during a 25 September-3 October 2018 expedition. Fragments also landed on the outer flank of Doro Api (left). Copyrighted photo by Martin Rietze (used with permission).

Figure 32. Steaming tephra surrounded the flanks of the pyroclastic cone inside the Doro Api cone on Sangeang Api when photographed during a 25 September-3 October 2018 expedition. The view, taken by drone, is from the NW, and the quiet Doro Mantai peak lies SE of the active cone closer to the coast. Copyrighted photo by Martin Rietze (used with permission).

Figure 33. This nighttime view of Doro Api cone on Sangeang Api was taken from the summit of adjacent Doro Mantai to the SE during a 25 September-3 October 2018 expedition. Ash rose from the vent at the top of the pyroclastic cone, and incandescent ejecta descended the flanks of the pyroclastic cone, covered the inner walls of the Doro Api summit crater, and also fell outside of the crater onto the W flank of Doro Api (left). In addition, lava emerged from a fissure near the base of the cone (lower right). Copyrighted photo by Martin Rietze (used with permission).

Blocks of lava formed an avalanche down the ravine on the E flank of Doro Api; the avalanche formed channels tens of meters wide on the steep slope (figure 34) when photographed by drone in early October 2018. The PVMBG webcam located on nearby Sumbawa island about 10 km SW captured an image of a small ash emission rising from the Doro Api summit in late October (figure 35). Visitors to the island in November recorded continued activity in the form of ash plumes and incandescent lava emerging from a fissure on the flank of the pyroclastic cone (figure 36).

Figure 34. Fresh blocky lava on the E flank of Doro Api on Sangeang Api was visible in drone images taken by visitors in early October 2019. Bushes in bottom center of the image suggest that the channels of lava blocks are tens of meters across. Courtesy of Doro Adventures.

Figure 35. The PVMBG webcam located about 10 km SW on nearby Sumbawa island captured an ash emission from the Doro Api cone at Sangeang Api on 29 October 2018. Courtesy of PVMBG and Øystein Lund Andersen.

Figure 36. Visitors to Sangeang Api in November 2018 witnessed incandescent bombs and lava emerging from a fissure on the flank of the pyroclastic cone inside Doro Api crater. Courtesy of 80 Jours Voyages.

Geological Summary. Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, Doro Api and Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent eruptions have been recorded since 1512, most of them during in the 20th century.

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.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/); Google Earth (URL: https://www.google.com/earth/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, Videos at https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); 80 Jours Voyages, Lyon, France (Twitter: https://twitter.com/jours_80, URL: https://80joursvoyages.com/); Doro Adventures (Twitter: https://twitter.com/DoroAdventures, URL: http://doroadventures.com/).