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An evacuation Route sign in case of volcanic eruption or lahar.

A lahar is a type of mudflow or landslide composed of pyroclastic material and water that flows down from a volcano, typically along a river valley. The term "lahar" originated in the Javanese language of Indonesiamarker.


Lahars can be best described as volcanic mudflows. They may not necessarily be caused by volcanic activity, but at the very least do originate from some type of volcanism.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 Rainiermarker in Washingtonmarker produced a wall of mud deep in the White River canyon and covered an area of over for a total volume of 2.3 cubic kilometers (0.55 cubic miles).

Lahars can be deadly because of their energy and speed. Large lahars can flow several dozen meters per second and can flow for many kilometres, causing catastrophic destruction in their path. The lahars from the Nevado del Ruizmarker eruption in Colombiamarker in 1985 caused the Armero tragedymarker, which killed an estimated 23,000 when the city of Armero was buried under of mud and debris. New Zealandmarker's Tangiwai disastermarker in 1953, where 151 people died after a Christmas Eve express train fell into the Whangaehu River, was caused by a lahar.


Lahars have several possible causes: In particular, although lahars are typically associated with the effects of volcanic activity, lahars can occur even without any current volcanic activity, as long as the conditions are right to cause the collapse and movement of mud originating from existing volcanic ash deposits.

Places at risk

Several mountains in the world, including Mount Rainiermarker in the USAmarker, Mount Ruapehumarker in New Zealandmarker, and Galunggungmarker in Indonesiamarker, are considered particularly dangerous due to the risk of lahars. Several towns in the Puyallup River valley in Washington state, including Ortingmarker, 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, Sumnermarker, Puyallupmarker, Fifemarker, and the Port of Tacomamarker face considerable risk.The USGS has set up lahar warning sirens in Pierce Countymarker, so that people can flee an approaching debris flow.

A lahar warning system has been set up at Mount Ruapehu by the New Zealand Department of Conservation and hailed as a success after it successfully alerted officials to an impending lahar on 18 March 2007.

The 1991 eruption of Mount Pinatubomarker caused lahars as well, but it was due to the passing of a major typhoon over the Philippinesmarker which resulted in a torrent of volcanic ash and water down to the rivers surrounding the volcano. The lahar was caused by the mixing of settled ash and water from the monsoon which occurred the day after the volcano finished erupting. Although the eruption killed only 6 people, 1500 were killed in the resulting lahar, showing the destructive nature of lahars. In 1985, the volcano Nevado del Ruizmarker erupted, releasing large lahars. These lahar flows reached the town of Armero two hours after the eruption, burying the town and killing 3/4 of the townspeople, an estimated 23,000 people.

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