A lahar, also known as a volcanic mudflow or debris flow, is an Indonesian term used to describe a fast-flowing mixture of rocks, other debris, and water that starts on the slopes of a volcano. The flow can be either hot or cold and looks like wet concrete. The width, depth, and speed of the lahar also can vary a great deal. Smaller lahars tend to be less than a few meters wide and run several meters a second, while larger lahars can be hundreds of meters wide and run tens of meters per second. Larger lahars have been known to move entire trees and boulders the size of houses.

There are multiple different events that can cause a lahar and all result in a disruption of water, soil, and debris on the volcano. They are most commonly caused by strong rainfall that loosens debris deposited after a recent volcanic eruption. Earthquakes can also loosen volcanic debris that has been saturated with water. The heat from current volcanic eruptions can also cause lahars by quickly melting snow or ice or by heating and overflowing lakes present on the volcano. Finally, lahars can start simply because of gravity.

An initial disturbance that causes a lahar often starts out quite small, but quickly grows in size as the water picks up loose rocks and vegetation on the slope of the volcano, as well as any snow, ice, or running water in its way. These additions can easily increase the size of the initial lahar by ten times. The accompanying debris causes huge amounts of damage to the mountain and any buildings or other structures that happen to be in its way. The debris can also cause downstream reservoirs to overflow, adding their water to the strength of the lahar. As the lahar runs down the volcano into the surrounding valley it loses its deposits, slowing and shrinking it until it stops. Lahars tend to leave large sediment deposits in their wake that can be a few to several hundred meters thick, enough to bury whole towns.

Examples of major lahars in recent years include:

There are several regions around the world today that are at risk of a lahar. The most notable is the region around Mount Rainier in Washington State, which has taken steps to develop an early lahar detection system. The system consists of numerous seismometers placed around the volcano that detect a passing lahar. Any positive signals are relayed to a station that immediately sounds an alarm around the Mount Rainier area, encouraging people to seek higher ground until the lahar passes. This system should hopefully prevent or reduce any causalties that could result from the next lahar.



http://volcanoes.usgs.gov/Hazards/What/Lahars/lahars.html
http://volcanoes.usgs.gov/About/Highlights/RainierPilot/Pilot_highlight.html
http://seattlepi.nwsource.com/mountsthelens/rain12.shtml

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