November 21, 2025 | 09:01 pm
TEMPO.CO, Jakarta - The head of Indonesia’s Geological Agency, Muhammad Wafid, has warned that deposits from Mount Semeru’s recent pyroclastic flows could trigger secondary disasters if not properly managed.
The pyroclastic flow deposits currently filling the mountain’s valley could overflow, potentially spreading to the sides of the river channel.
“As long as the valley remains V-shaped, it can hold the material,” Wafid explained in a virtual press conference on Thursday, November 20, 2025. “But when it becomes full, the lahar flow may expand to the right and left.”
Lahar is a type of volcanic mudflow or debris flow that occurs when pyroclastic deposits mix with water, often from rain or melted snow. These flows can be extremely fast and destructive, carrying rocks, ash, and debris downstream.
To reduce potential risks, the agency is considering constructing buffer zones. “A larger buffer zone may be needed depending on field conditions. If the material deposits in the middle of the river, it could spread outward,” Wafid added.
Officials show the pyroclastic flow from the CCTV monitor at the Mount Semeru Observation Post, Lumajang, East Java, November 19, 2025. Antara/Irfan Sumanjaya
Monitoring and Field Assessment
Field teams are currently evaluating the need for a buffer zone and assessing the full condition of the river affected by the pyroclastic deposits. Visually, the lahar flow appears nearly full.
Wafid emphasized that heavy rain, typical during Indonesia’s rainy season, will gradually wash away the deposits. “The material is not yet compact, making it highly susceptible to erosion. Over time, water will clear the river and return it to its normal flow,” he said.
The Geological Agency is also monitoring additional hazards downstream, where high rainfall combined with thick pyroclastic deposits, sometimes reaching the height of river embankments, could increase the risk of flooding and further lahar flows.
Mount Semeru erupted on November 19, 2025, sending a pyroclastic flow as far as 13.8 kilometers toward Besuk Kobokan. The flow registered a maximum amplitude of 47 mm on the seismograph, with volcanic tremors lasting four hours. As of 05:00 PM WIB on the day of the eruption, Mount Semeru remains at Alert status, Level IV.
Authorities advise people, visitors, and tourists to avoid the southeast sector along Besuk Kobokan within 20 kilometers of the eruption center. Beyond that distance, activities should stay at least 500 meters from the river boundaries along the affected flow.
Officials monitored the pyroclastic flow on CCTV from the Mount Semeru Observation Post in Lumajang, East Java, on November 19, 2025.
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