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Pyroclastic Flow Hazards at Three Sisters

Pyroclastic flows can form in two ways: from the collapse of a tephra plume during an explosive eruption or from the collapse of the front of a steep lava flow or dome on a volcano's slope.

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. 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). (Credit: Lipman, Peter. Public domain.)

On the basis of eruptive activity during the recent geologic past, Middle and South Sister are the most likely locations for explosive eruptions that could generate pyroclastic flows. As in the past, vents may open anywhere on these volcanoes, from the summit to lower flanks. Eruptions from the peaks of any of the Three Sisters would also melt sufficient snow and ice to generate lahars, which could extend for tens of kilometers.