Lahar

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For the town in India, see Lahar (India).

Image:MSH82 lahar from march 82 eruption 03-21-82.jpg Image:Vol.escape.jpg A lahar is a type of mudflow composed of pyroclastic material and water that flows down from a volcano, typically along a river valley. The term 'lahar' originated in Indonesia.

Lahars have the consistency 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 180 m (600 feet) deep in the White River canyon and extends over an area of over 320 km².

Lahars can be extremely dangerous, because of their energy and speed: large lahars can flow several dozen meters per second, and can flow for many kilometres, causing catastrophic destruction along the way. The lahars from the Nevado del Ruiz eruption in Colombia in 1985 killed an estimated 25,000 people in the city of Armero who were buried under 8 m (26 ft) of mud and debris. The 1953 Tangiwai disaster in New Zealand was caused by a lahar.

Lahars have several possible causes:

Snow and glaciers can be melted by a pyroclastic flow during an eruption
A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar, also called glacier run or jökulhlaup.

Several mountains in the world, including Mount Rainier, Mount Ruapehu in New Zealand, and Galunggung in Indonesia, are considered particularly dangerous due to the risk of lahars. Several towns in the Puyallup River valley in Washington state, including Orting, the closest to Mount Rainier, are built on top of lahar deposits that are only about 500 years old. Lahars are predicted to flow through the valley every 500-1,000 years, so Orting, Sumner, Puyallup, Fife, and the Port of Tacoma face considerable risk. The USGS has set up lahar warning sirens in Pierce County, so that people can flee the approaching debris flow. A lahar warning system is also being set up at Mount Ruapehu by the New Zealand Department of Conservation.