During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour (100 kilometers/hour) and reach temperatures of over 800 Degrees Fahrenheit (400 degrees Celsius). Photographed here, a pyroclastic flow from the August 7, 1980 eruption stretches from Mount St. Helens' crater to the valley floor below.

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During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour (100 kilometers/hour) and reach temperatures of over 800 Degrees Fahrenheit (400 degrees Celsius). Photographed here, a pyroclastic flow from the August 7, 1980 eruption stretches from Mount St. Helens' crater to the valley floor below.

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Photo credit: © CPC Collection / Alamy / Afripics
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