Before and after images from Indonesia's Center for Volcanology and Geological Hazard Mitigation show an enormous chunk missing from the peak, which it called "completely annihilated."
Volcanologist Devy Kamil Syahbana said Tuesday that the chunk, known as the "lava dome," had a volume of at least 1.6 million cubic meters (56.5 million cubic feet).
The volcano in North Sumatra, which has been active since 2010 after centuries of dormancy, erupted explosively on Monday morning.
Hot ash clouds rolled down its slopes, traveling as far as 4.9 kilometers from the crater, and ash reached Lhokseumawe, a city more than 260 kilometers (162 miles) to the northwest.
No-one was injured. Video showed screaming children fleeing a school outside the volcano's exclusion zone as a billowing column of ash rose in the background.
Mount Sinabung is among more than 120 active volcanoes in Indonesia, which is prone to seismic upheaval due to its location on the Pacific "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin.
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