Mount Semeru is currently at Level III (Alert) status following a continuous increase in volcanic activity since late June 2026. Experts are now highlighting the threat of cold lahar floods, which are considered more dangerous to communities surrounding the slopes than the eruption itself.
Universitas Gadjah Mada (UGM) Geography lecturer Dr. Indranova Suhendro explained that the accumulation of volcanic material—such as sand, gravel, and large boulders—at the summit is highly vulnerable to being washed away during heavy rainfall. This phenomenon risks triggering sudden lahar floods, even if the weather in downstream residential areas appears clear.
The Besuksemut River basin area has been designated as a red zone most vulnerable to pyroclastic flows and lahar floods. Consequently, the public has been instructed to refrain from activities within the hazard radius established by the Center for Volcanology and Geological Hazard Mitigation (PVMBG) to avoid fatal risks from volcanic material debris.
Regarding ongoing sand mining activities in high-risk zones, Dr. Nova acknowledged the dilemma between residents' economic needs and life safety. He suggested the necessity of strengthening strict occupational safety standards and ensuring full accountability from relevant parties, although the best preventive measure remains halting all activities in the area entirely.
Scientifically, the high-viscosity characteristic of Semeru's magma forms an unstable lava dome. This dome can collapse at any time due to magma pressure from below or the impact of rainfall, triggering pyroclastic flows. Learning from the December 2021 events, mitigation based on understanding the volcano's characteristics is key to keeping residents living in the area safe from eruptive threats.
