A destructive mudflow or debris flow composed of volcanic material mixed with water, generated when volcanic ash, rock and debris on a volcano's slopes are mobilized by melting snow and ice, heavy rainfall, or the sudden release of a crater lake. A lahar can travel tens of miles from its source at speeds rivaling a fast-moving river, burying valleys in dense, cement-like mud long after an eruption itself has ended, as in the 1985 Nevado del Ruiz disaster in Colombia, where a comparatively small eruption's lahars killed more than twenty thousand people.
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Loose pyroclastic-flow deposits on a volcano's flanks are readily remobilized by rainfall into lahars long after the flow itself has stopped, a documented secondary hazard of pyroclastic deposition.
Lahars are volcanic mudflows commonly triggered directly by an eruption, when hot pyroclastic material rapidly melts snow and ice or mixes with a volcano's crater lake and heavy rainfall on its flanks.
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