lahar

A lahar is a volcanic mudflow composed of water-saturated volcanic ash, pumice, and rock fragments that flows down the slopes of a volcano and into adjacent valleys and rivers. Lahars can form during explosive eruptions when ejecta mix with abundant water from rainfall, melting snow and ice, or breached crater lakes, or when eruptive activity destabilizes added debris.

Lahars behave as dense, fast-moving debris flows or hybrid floods, capable of carrying large boulders and trees. They can travel many kilometers downstream and across flat terrain, destroying bridges, roads, and settlements in their path. Their flow often leaves thick, layered deposits that can persist for decades in valleys and floodplains.

Notable lahar events have caused severe loss of life and property. The 1985 Nevado del Ruiz eruption in Colombia produced deadly lahars that buried the town of Armero, while eruptions of Mount St. Helens in 1980 and Mount Merapi in Indonesia in the 2000s and 2010s also generated destructive lahar flows. Lahars can be triggered by volcanic activity, heavy rainfall, rapid snowmelt, or the failure of crater lakes or volcanic structures.

Hazard mitigation relies on monitoring and forecasting lahars, hazard mapping, and engineering controls such as sediment-retention basins, deflection channels, and reinforced barriers. Early warning systems and evacuation planning are essential for reducing casualties in populated volcanic regions. The term lahar is derived from Malay/Indonesian usage describing flowing volcanic material.