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 Lahar

 
A lahar is a fast-moving flood of water which is full of rocks and ash. They travel very quickly and you can't run faster than they travel. Because they carry so much in them, they leave thick deposits of mud, sand, boulders, and other debris behind, often many meters thick. Sometimes they happen during an eruption, but they can happen years later, when heavy rain or an earthquake causes a lahar to start moving.