LaHar



A lahar from the 1982 eruption of Galunggung

A lahar is a type of mudflow or debris flow composed of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of Indonesia.

"Lahar" is an Indonesian word that describes volcanic mudflows or debris flows. Lahars have the consistency, viscosity and approximately the same density of concrete: fluid when moving, then solid when stopped. Lahars can be huge: the Osceola lahar produced 5,600 years ago by Mount Rainier in Washington produced a wall of mud 140 metres (460 ft) deep in the White River canyon and covered an area of over 330 square kilometres (130 sq mi) for a total volume of 2.3 cubic kilometres (0.55 cu mi). A lahar can bulldoze through virtually any structure in its path, but quickly loses force when it leaves the channel of its flow: even frail huts may remain standing while being buried up to the roof with mud. The viscosity of a lahar decreases with time and amount of rain, although the mud solidifies quickly when it stops moving

Lahar flows are deadly because of their energy and speed. Large lahar flows move at approximately 100 kilometres per hour (60 mph), can flow for more than 300 kilometres (190 mi), and can cause catastrophic destruction in their path. The lahars from the Nevado del Ruiz eruption in Colombia in 1985 caused the Armero tragedy, which killed an estimated 23,000 when the city of Armero was buried under 5 metres (16 ft) of mud and debris. New Zealand's Tangiwai disaster in 1953, where 151 people died after a Christmas Eve express train fell into the Whangaehu River, was caused by a lahar. Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997. Lahars can cause fatalities years after an actual eruption: for example, the Cabalantian tragedy four years after the 1991 eruption of Mount Pinatubo.