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Lahar / Volcanic Mud Flow 

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Mt ST Helens Lahar USGS

A lahar is a volcanic mudflow consisting of fast flowing mixture of volcanic debris and water. The flow has the consistency of cement and once it drys out is very hard. They form where water from a craterlake,melted glacier, river or heavy rain mixes with either new eruption material or old ash deposits. Lahars can either be hot or cold. They can travel great distances along existing valleys far from their original source and travel at speeds of 60mph .

In 1985 the volcano Nevado del Ruiz erupted in Colombia caused a Lahar which killed 23,000 people in neighbouring valleys.