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Lahars at Lassen Volcanic Center

When pyroclastic flows mix with large quantities of water, they can produce highly mobile flows of mud and debris, called lahars, that may rush down valleys leading away from a volcano.

Lahars resulting from the May 18, 1980 eruption of Mount St. Helens left a mudline on riverside trees. A scientist (middle right) stands on the bank of the Muddy River (southeast of the volcano). (Public domain.)

Hot pyroclastic flows may melt snow and ice to generate these hazards, but they can also occur if large quantities of ash, tephra, and other volcanic material enter a river. Volcanoes, like Lassen, that have a significant snow and ice cover are at greater risk for producing lahars that may cause devastation in downstream communities.