lahars

A lahar is a volcanic mudflow or debris flow composed of water-saturated volcanic ash, pumice, and rock fragments that moves rapidly downslope through river valleys. Lahars can behave like a flood and a landslide at the same time, carrying large boulders and sediment, and they often travel far from their source.

They form when loose volcanic materials become heavily saturated with water. Triggers include heavy rainfall on ash deposits, rapid melting of snow and ice due to eruptive heat, the sudden release of crater lake water, or the mixing of hot pyroclastic flows with surface water. They can be hot or cold, depending on the temperature of the material involved.

Lahars vary in density from cohesive debris flows to more fluid hyperconcentrated flows. They can be hot or cold, and they commonly follow river channels but can surge across plains. They may dam valleys, creating lakes that later breach and cause secondary flooding.

Hazards include burial of settlements, destruction of infrastructure, disruption of water supplies, and long-term sediment deposition that raises river beds and alters flood regimes. Because lahars can travel tens to hundreds of kilometers, communities distant from volcanic vents can be affected.

Monitoring and mitigation rely on hazard assessments, early warning systems, and land-use planning. Sensors along rivers, rain gauges, and satellite observations help detect lahar activity. Protective measures include channel stabilization, diversion barriers, retention basins, and clear evacuation protocols.

Notable eruptions associated with deadly lahars include the 1985 eruption of Nevado del Ruiz in Colombia and the 1991 eruption of Mount Pinatubo in the Philippines. Lahars are most common around volcanoes with loose ash deposits and substantial water in valleys.