Located in the state of Washington, St. Helens is considered to be the youngest and most active volcano in the Cascade Range. Although only 12th in height among the major Cascade volcanoes, scientists predict a significant increase in size in the future because the mountain is still in its cone building stage. Mount St. Helens stands atop an older volcano which historically, was probably one of the most explosive peaks in the Pacific Northwest. This assumption is evident through the amount of debris that is scattered for miles over the countryside. The oldest recognized products of the ancestral cone are a pumice layer which is dated at 37,600 years and a weathered mudflow deposit dated at approximately 36,000 years (Harris 1980). Evidence of glacial sediments containing fragments of the earlier mountain are dated at

The Cascade Mountains, St. Helen’s being one of them, represent a volcanic arc that is created where the Juan the Fuca tectonic plate is moving eastward and subsiding beneath North America. The plate originated as magma, through rifts in the Juan de Fuca Ridge. The Juan de Fuca Ridge is the remaining northern segment of a larger oceanic feature called the Farallon Ridge. Most of this ridge has been over ridden by the westward drifting North American Plate. The rock that makes up the Juan de Fuca plate is heated to a degree that hot fluids are driven out. These hot fluids, which are mainly composed of water, then penetrate into the overlying wedge of mantle rock and cause chemical interactions that lower melting temperatures in the wedge. This causes blobs of magma to rise through the fractures in the crust and accumulate in chambers beneath the volcanic peaks of the Cascade Range and results in eruptions (Sanders et al. 2002).

In the future, Mount St. Helens probably will erupt violently and intermittently just as it has in the recent geologic past, and these future eruptions will affect human life and healthy, property, agriculture and general economic welfare over a broad area…an eruption is… likely to occur within the next hundred years, and perhaps even before the end of the century” (Sanders et al. 2002:232)

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