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Lahars move rapidly down valleys like rivers of concrete. If you are on the floor of a valley that leads from a volcano and become aware of an oncoming lahar, seek high ground. Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley.
A variation is the hot lahar ordinarily produced by the heating of the crater lake water by the quiet upwelling of lava or an explosion. Lahars move downslope at very high speeds and may extend for tens of miles. A lahar deposit usually has a hummocky or hilly surface. They often cause much death and destruction, as at Herculaneum during the eruption of Vesuvius in ad 79.