Karangetang (also known as Api Siau) is located on the island of Siau in the Sitaro Regency, North Sulawesi, Indonesia, and consists of two active summit craters: Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). Its current eruption period began in November 2018 and has more recently been characterized by frequent white gas-and-steam emissions, seismicity, thermal anomalies, and occasional ash plumes (BGVN 46:07). This report updates information during June through December 2021, describing continued white gas-and-steam emissions, thermal anomalies, and intermittent ash plumes, based on reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Center (VAAC), and satellite data.

Activity during June through December dominantly consisted of white-to-gray gas-and-steam emissions that rose 25-500 m above the Main Crater and 25-50 m above the N crater, according to daily reports from PVMBG. Emissions from the N crater rose 150 m above the crater on 23 December and 200 m above the crater on 24 December. Volcanic earthquakes were also detected. A Darwin VAAC notice reported that on 25 October a pilot observed an ash plume that rose to 3 km altitude and drifted SW. Continuous ash emissions were also visible in satellite images, though they had stopped within five hours. On 24 and 31 December PVMBG reported that incandescence from the N crater was periodically detected 10 m above the crater.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data detected intermittent low-power thermal anomalies during the reporting period (figure 53). Sentinel-2 infrared satellite images also showed a thermal anomaly in the N summit crater, a weaker anomaly in the S crater, and intermittent gas-and-steam emissions from both summit craters (figure 54).

Figure 53. Intermittent thermal anomalies (red dots) were detected in the summit area of Karangetang during June through December 2021 as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). These anomalies were slightly higher in power during compared to the previous months. Courtesy of MIROVA.

Figure 54. Small thermal anomalies (yellow-orange) at Karangetang were detected in the N summit crater on 2 June (top left), 27 July (top right), 1 August (middle left), 15 October (middle right), 19 November (bottom left), and 19 December (bottom right) 2021. A weaker anomaly was also detected in the S summit crater during these dates. Gas-and-steam emissions were observed from both summit craters. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geological Summary. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

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); 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/); 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).