A fast-moving avalanche of hot lava fragments, known as a pyroclastic flow, rushes down the side of Augusine Volcano in Alaska as a cloud of ash billows above. Even a small pyroclastic flow like this moves across the ground at speeds of 80 kilometers per hour. Augustine Volcano erupted dozens of small pyroclastic flows like this during eruptions in 1976 and 1986 (shown here).

What Are Pyroclastic Flows?

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.

Quicktime Movie of this animation sequence (1.79 M)

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URL of this page: http://volcanoes.usgs.gov/Hazards/What/PF/pcflows.html