Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly by the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits or breakout of a lake dammed by volcanic deposits.

Facts and figures: A lahar from Nevado del Ruiz volcano destroyed the town of Armero, Colombia, 72 km from the volcano, on 13.11.1985. It reached Armero about 2.5 hours after an explosive eruption sent hot pyroclastic flows across the volcano's broad ice- and snow-covered summit area. Although flow depths in Armero ranged only from 2 to 5 m, three quarters of its 28'700 inhabitants perished. This was the second highest death toll of any volcanic catastrophy during the 20th century (the worst happened on the island of Martinique in 1902 and was caused by a pyroclastic flow). Photo 1 and photo 2 N. Banks 18.12.1985. An extensive report, with numerous illustrations, about this catastrophy can be found on the USGS Volcanic Hazards Program (external link).

An impressive lahar carrying glacial sediments continues to flow in Drift River Valley six hours after a Plinian eruption of Redoubt Volcano (Alaska) has triggered it. Photo Marco Fulle. More images of this lahar.