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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. 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. Because such flows are driven by gravity, those that originate at higher altitudes will generally travel farther. Eruptions on these peaks would also melt sufficient snow and ice to generate lahars that could extend for tens of kilometers.