Mount Sinabung in Indonesia's North Sumatra province erupted on Monday morning, 31 August, sending a thick column of volcanic ash over the nearby city of Berastagi and prompting authorities to warn residents to wear masks and stay alert. The eruption is the latest in a series of volcanic events to hit the world's largest archipelago in 2025.

Key Details of the Eruption

Armen Putra, head of the Sinabung Volcano Observation Post, confirmed that a dark ash column rose approximately 3,500 metres above the summit before drifting east and southeast. The eruption registered on seismographs with a maximum amplitude of 120 millimetres and lasted roughly one hour.

Volcanic ash blanketed roads and vehicles across Berastagi, significantly reducing visibility in the area. Residents were urged to wear masks, and schools in affected zones advised students and teachers to do the same when outdoors.

Exclusion Zones and Safety Warnings

Mount Sinabung remains at Level II (Alert status). All activities are prohibited within a three-kilometre radius of the summit and within 4.5 kilometres in the southeast sector. Authorities have also warned communities living near rivers that originate from the volcano to remain on guard against potential lahar flows, which can occur when volcanic debris mixes with water during rainfall.

Indonesia's Broader Volcanic Activity in 2025

The Sinabung eruption follows a string of volcanic incidents across the archipelago this year. In May, three people were killed after Mount Dukono erupted in Indonesia's North Maluku province. That eruption, recorded at around 7:40 am local time on 8 May, sent an ash column up to 10 kilometres into the sky, and local rescue authorities warned residents to stay clear of a 4-kilometre radius around the crater.

Mount Dukono, situated on Halmahera Island, had also erupted in February earlier this year, prompting the country's Volcanology and Geological Disaster Mitigation Centre to issue an aviation warning. A Volcano Observatory Notice for Aviation (VONA) was issued at the orange level — the second-highest warning — prohibiting aircraft from flying below 5 kilometres around the volcano. Ash clouds from volcanic eruptions are known to pose serious risks to aircraft engines and flight systems.

What to Watch

With Mount Sinabung holding at Alert Level II, monitoring teams are continuing round-the-clock observation. The risk of lahar flows during rainfall and further ashfall events remains elevated. Indonesia, which sits on the Pacific Ring of Fire, is home to more than 130 active volcanoes, making it one of the most volcanically active countries in the world.