Moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope. As the moving debris rushes down a volcano and into river valleys, it incorporates water, snow, trees, bridges, buildings, and anything else in the way. Debris avalanches may travel several kilometers before coming to rest, or they may transform into more water-rich lahars, which travel many tens of kilometers downstream.

A debris avalanche rushes down the side of a volcano to the valley floor. Many such debris avalanches transform into lahars and travel tens of kilometers from the volcano. Typically, the scar created by the avalanche leaves a horseshoe shaped crater on volcano's side.

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