Series of pyroclastic flows at Merapi, heavy ashfall blankets nearby communities, Indonesia


At least 7 pyroclastic flows descended down the slopes of Merapi volcano, Indonesia starting at 21:58 UTC on August 7, 2021, with ash rising up to 3.6 km (12 000 feet) above sea level. No casualties were reported. The Aviation Color Code remained at Orange.

According to the Merapi Volcano Observatory, a series of pyroclastic density currents (PDCs) were recorded on a seismogram with a maximum amplitude of 20 mm (0.78 inches) and a duration of 222 seconds (3.7 minutes).

Ash-cloud generated by PDCs was estimated by ground observers reaching a height of 3.6 km (12 000 feet) a.s.l. and moving W.1

The farthest sliding distance was 3 km (1.8 miles) to the southwest (Kali Bebeng).

The rumbling sound of the eruption could be heard several kilometers away from the volcano, said Hanik Humaida, head of Yogyakarta’s Volcanology and Geological Hazard Mitigation Center.2

Humaida said Merapi has seen increased volcanic activity in recent weeks, with the lava dome growing rapidly before partially collapsing during this eruption.

Ash blanketed several villages and nearby towns.


The volume of SW rim lava-dome at the end of July 2021 was estimated at 1 878 million m3 [1.9 km3 / 0.45 mi3] and material continued to collapse down the flank, USGS Volcanologist Sally Sennert noted.3

The volume of the summit lava dome was 2 817 million m3.

Four PDCs were reported from July 23 to 29, with the longest traveling down the SW flank as far as 2.5 km (1.5 miles).

Avalanches of material that descended the W flank originated from lava emplaced in 1992 and 1998, and material that descended the NW flank is from 1948 lava, Sennert said.

The Alert Level remains at 3 (on a scale of 1 – 4), and the public is warned to stay 5 km (3.1 miles) away from the summit.

The current eruptive episode started on December 31, 2021 (VEI 1).

The last major eruption of this volcano (VEI 4) took place in 2010, claiming the lives of 347 people.

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 the 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. 

Subsequently, the growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent eruptive 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 during historical time.4


1 Merapi Volcano Observatory VONA – 23:45 UTC, August 7, 2021

2 Indonesia Java Indonesia’s Mount Merapi erupts with bursts of lava, ash – AP

3 Global Volcanism Program, 2021. Report on Merapi (Indonesia). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 28 July-3 August 2021. Smithsonian Institution and US Geological Survey. – GVP

4 Merapi volcano – Geological summary – GVP

Featured image credit: BPPTKG


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