Potential hazards from future eruptions of Mount St. Helens volcano, Washington

Abstract

Mount St. Helens has been more active and more explosive during the last 4,500 years than any other volcano in the conterminous United States. Eruptions of that period repeatedly formed large volumes of pumice, hot pyroclastic flows, domes, and, during the last 2,500 years, lava flows. Some of this activity also resulted in mudflows that moved tens of kilometres along the floors of valleys that head at the volcano. This report describes the nature of these phenomena and their threat to people and property; the accompanying maps show areas likely to be affected by future eruptions of Mount St. Helens. Explosive eruptions which produce large volumes of pumice affect large areas because winds carry the lightweight material hundreds of kilometres from the volcano. Because of prevailing winds, the 180° sector east of the volcano will be affected most often and most severely by future eruptions. However, the pumice from any one eruption probably will fall in only a small part of that sector. Pyroclastic flows and mudflows also can affect areas far from the volcano, but their effects are limited because they follow valleys. Mudflows and possibly pyroclastic flows moving rapidly down Swift and Pine Creeks could displace water in Swift Reservoir, which could cause disastrous floods farther downvalley.