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Report on Merapi (Indonesia) — 18 August-24 August 2021


Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
18 August-24 August 2021
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2021. Report on Merapi (Indonesia). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 18 August-24 August 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (18 August-24 August 2021)



7.54°S, 110.446°E; summit elev. 2910 m

All times are local (unless otherwise noted)

BPPTKG reported that both the lava dome just below Merapi’s SW rim and the lava dome in the summit crater remained active during 13-19 August. Webcam images showed some changes in the SW dome due to lava avalanches and pyroclastic flows; there were no significant changes in the central dome. The volume of the SW lava dome was 1.35 million cubic meters. During 13-19 August a total of 20 pyroclastic flows were observed descending the SW flank as far as 3.5 km. Lava avalanches were observed 172 times to the SW, traveling up to 2 km. BNPB noted that ashfall was reported in several areas on 16 August, including Dukun, Sawangan, Tegalrejo, Secang, Gowok, Mertoyudan, Selo, Mojotengah, Temanggung, Kedu, Pringsurat, Bulu, Tlogomulyo, Kranggan, and Parakan.

PVMBG reported that during 18-19 and 23-24 August white plumes rose 20-200 m above the crater and drifted in different directions. As many as 331 lava avalanches traveled a maximum distance of 1.5 km SW. Two pyroclastic flows moved as far as 2 km, though the direction was not observed. The Alert Level remained at a 3 (on a scale of 1-4), and the public was warned to stay 5 km away from the summit.

Geological Summary. 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.

Sources: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Badan Nacional Penanggulangan Bencana (BNPB), Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)