Mount St. Helens Eruptive Activity, 1980-1984

Pyroclastic Flows

The term "pyroclastic" -- derived from the Greek words "pyro" (fire) and "klastos" (broken) -- describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Pyroclastic flows are composed of hot gas, entrapped air, and different-size particles of fragmented magma and old volcanic rock (ash, blocks, bombs). Pyroclastic flows travel at great speeds in response to gravity (up to 60 to 100 miles per hour).

Pyroclastic flows were first directly observed at 12:17 p.m. and continued intermittently during the next five hours of strong eruptive activity. Smaller pyroclastic flows were erupted during the first few minutes of the avalanche lateral blast sequence. The successive outpourings of pyroclastic material consisted mainly of pumice and ash derived from new magma. Fragments of preexisting rocks were minor components.

The pyroclastic flow deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes that extend five miles north of the crater. Temperature measurements made in these pyroclastic flows were still 780 degrees Fahrenheit two weeks after the eruption. Many "rootless" steam-blast explosions formed small craters on the northern margin of the deposits near Spirit Lake, as encroaching ground water was flashed into steam by the hot material. These steam-blast explosions continued intermittently for several weeks or months after the emplacement of the pyroclastic flows.

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