Lahar
U.S. Geological Survey photo by Tom Casadevall (fair use policy)
A lahar (la-HAR) is a mudflow that occurs in the thick deposits of volcanic sediment that surround volcanoes. Sometimes lahars are hot, sometimes they're cold. This is a picture of a cold lahar, which happened after an earthquake near the Colombian volcano Nevado del Huila in 1994. The Rio Paez, flowing across the bottom of the image, filled with black muck and overran several towns along the river. Smaller slides like those in the image joined it, turning it into what survivors called a black wave of death.
Other lahars can occur during volcanic eruptions as snowfields or glaciers high on a volcano melt and carry down hot ashfall with them. They can be as destructive and as long-lasting as the eruption itself. See what kind of rock layers are built by lahars in an example from California's Sutter Buttes.
Learn more from Landslides in a Nutshell. See other types of landslides in the Landslide Picture Gallery.
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