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Geology and History Summary for Mount St. Helens

Mount St. Helens, located in Washington State, is the most active volcano in the Cascade Range, and it is the most likely of the contiguous U.S. volcanoes to erupt in the future. The volcano is almost 53 km (33 mi) due west of Mount Adams and approximately 80 km (50 mi) northwest of the Vancouver, Washington—Portland, Oregon metropolitan area. Volcanism occurs at Mount St. Helens and other volcanoes in the Cascades arc due to subduction of the Juan de Fuca plate off the western coast of North America.

Over its rich and complex 275,000-year history, Mount St. Helens has produced both violent explosive eruptions of volcanic tephra and relatively quiet outpourings of lava. In the beginning stages of eruptive activity, the volcano mostly consisted of a cluster of domes that was surrounded by an apron of tephra and debris fans of fragmented volcanic rocks.

It was only during the past few thousand years that the volcano grew to its pre-1980 elevation of 2,950 m (9,677 ft), making it the, then, fifth highest peak in Washington. Starting about 3,000 years ago, substantial amounts of basalt and andesite began to erupt as lava flows between phases of dacite eruptive activity. These lava flows buried large parts of a central cluster of dacite domes and flanking fans, which started the cone building in earnest.

Based upon detailed chemical analysis of the eruptive products from each stage of Mount St. Helens volcanism, scientists deduce that the volcano's magmatic system has evolved from relatively simple to more complex as the volcano matured. The evidence indicates that interaction between more silicic (dacite) magma batches with more mafic (basalt to andesite) magma increased from the earliest to most recent stages of Mount St. Helens volcanism.