Skip to main content
U.S. flag

An official website of the United States government

Pyroclastic Flows

Pyroclastic flow from the August 7, 1980 eruption stretches from Mo...
During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. (Credit: Lipman, Peter. Public domain.)

Pyroclastic flows are a rare occurrence at Mount Adams, but it is possible for them to occur. If lava flows erupted onto steep or icy terrain the leading edge of the flow may break apart and produces avalanches of hot lava fragments that can form pyroclastic flows. Such lava flow collapses from near the summit of Mount Adams can form pyroclastic flows that may travel downslope as far as 15 km (9 mi). As they move down slope, they can swiftly melt snow and ice to form farther-traveled lahars. Pyroclastic flows formed by avalanches from the fronts of hot lava flows have occurred less frequently at Mount Adams during the past 10,000 years than at Mount Hood or other nearby volcanoes.