Mount St. Helens

            On May 18, 1980, after lying dormant 123 years Mount St. Helens erupted powerfully and had a profound impact on the Pacific Northwest. On that summer day in 1980 Mount St. Helens produced a huge debris avalanche, an explosive lateral blast, lahars and an eruption column. In an instant the countryside and lakes surrounding a great distance around became victims of devastation.
             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 approximately 18,000 years and indicate that Mount St. Helens experienced at least one episode of glaciation. Present day lava content of St. Helens is composed of olivine basalt, dacite, and pyroxene andesite. The ancestral St. Helens, although almost buried beneath the recent core, has left behind much evidence of its previous eruptions. Pumiceous ash blankets the land for hundreds of miles and many valleys are filled with accumulations of explosive rubble. Modern day Mount St. Helens is a result of avalanche debris, explosion rubble and mudflow deposits erupted by its predecessor. .
             Modern day St. Helens also grew "through the extrusion of at least two large dacite domes and the production of glowing avalanches similar to those erupted by the ancestral volcano" (Harris 1980:171).