lahares

Lahares are volcanic mudflows that form when water mixes with volcanic ash, pumice, and other erupted debris, and then rushes down river valleys and channels around active volcanoes. They can be triggered by heavy rain or rapid snow and ice melt during an eruption, by the sudden drainage of crater lakes, or by the remobilization of ash deposits by floodwaters.

They resemble dense, fast-moving mud and can contain large rock fragments. Depending on water content and terrain, lahars may travel at high speeds and cover long distances, often following existing river valleys but capable of altering flow paths by damming and breaching channels. They can occur hours to days after an eruption and may recur over an extended period as new ash deposits are mobilized or rainfall continues.

Hazards and impacts of lahars are significant due to their speed, persistence, and ability to transport debris. They can bury villages, destroy roads, bridges, and crops, contaminate water supplies, and cause long-term environmental disruption. Because lahars can travel far from their source, areas downstream of volcanoes and along river corridors are routinely designated as at risk, even if the volcano appears dormant.

Prevention and risk reduction involve monitoring and early warning systems, hazard mapping, and engineering measures. Agencies may deploy lahar alert networks, weather-based alerts, and river-channel defenses such as check dams, retention basins, and diversion channels to mitigate impacts and facilitate evacuation when needed.

Notable examples include the 1985 Nevado del Ruiz eruption in Colombia, where lahars killed about 23,000 people in Armero; the 1980 eruption of Mount St. Helens in the United States, which generated damaging lahars downstream; and the widespread lahar activity associated with Mount Pinatubo’s 1991 eruption in the Philippines.