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A lahar is a mudflow or landslide of tephra or pyroclastic material, ash, shattered rock, dust and other material from a volcanic eruption mixed with water. This collection of loose debris, volcanic ash and water, generally has the consistency of flowing concrete and most often flows along a valley. It becomes solid when it stops flowing but until then this volcanic mud slurry is a devastating phenomenon.

While lahars, a word borrowed from the Javanese language of Indonesia, originate in areas of volcanic activity it is not necessary for an eruption to be in progress for a lahar to form. Deposits of ash and other materials from previous eruptions can cause a flow if sufficient water is available to combine with the debris until it can flow. In these cases, a heavy rainfall is usually responsible.

If a lahar occurs during an eruption it is usually the result of the heat of a pyroclastic flow, consisting of gas and debris, which melts snow or ice. This snow melt then mixes with the debris, again causing a mudflow. As the banks of lakes in volcanically active areas are less stable, it is also possible for water to come from a lake breakout.

Lahars can reach a maximum temperature of 100 degrees Celsius, the temperature of boiling water. Flows have a wide range of velocities from one mile per second to 40 miles per second. The rate of flow is determined by the size of the valley along which it flows and the size of the debris. A lahar can be only centimeters wide and deep and flow less than one meter per second but can be hundreds of meters wide, tens of meters deep and flow several tens of meters per second.

Lahars can flow for long distances. Deposits from one lahar traveled for 60km and there is evidence of flow up to 100 km from the volcano. Characteristics of the lahar change the further it travels. Heavier debris drops out and the flow becomes less powerful, finally becoming a hyper-concentrated stream-flow.

The main danger of these deadly mudflows is from burial and suffocation or crushing. Many victims also suffer severe burns. Heavy objects carried by the lahar, boulders, trees, etc., can crush or break structures in its path and the energy of the flow can displace structures, cars and even locomotives.

The final size of a lahar can be huge. The 5600 year old Osceola lahar from Mt. Ranier in Washington left a field of solidified mud over 450 ft. deep and covering 130 square miles.

Lahars are extremely dangerous. In 1953 a lahar in New Zealand, the Tangiwai disaster,

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