On Mount St. Helens

by Gabe Meier

mount st helens

Being from the Pacific Northwest, there were plenty of notable geological features from which to choose from. Mount Rainier is the largest mountain in the region, but Mount St. Helens seemed more applicable to this assignment because of its quite recent volcanic activity. Mount St. Helens is an active stratovolcano (also known as a composite volcano) and is located 96 miles from Seattle,WA. The name Mount St. Helens is derived from 18th century British diplomat Lord St Helens, a friend of George Vancouver who took an important survey of the region in the late 18th century. Part of the Cascade Volcanic Arch, Mount St. Helens is one of over 160 active volcanos in the Pacific Ring of Fire. The mountain most recently erupted on May 18, 1980, the most destructive (both in terms of loss of life and economics) volcanic event in the history of the United States. See below for a video documenting the eruption.

Mount St. Helens’ eruptive history dates back 40,000-35,000 years ago, an era referred to as the “Ape Canyon Stage”. The “Cougar Stage” (20,000-18,000 years ago) followed, and subsequently the “Swift Creek Stage” (13,000-8,000 years ago). The mountain itself began to form in the Pleistocene 37,600 years ago with original eruptions of dacite and andecite. 36,000 years ago, a large mudflow occurred, a consistent feature in all of Mount St. Helens’ eruptions. Following the Ape Canyon Stage, the mountain faced around 17,000 years of peace. The Cougar Stage was highlighted by Pyroclastic flows of hot pumice and ash, as well as significant dome growth. 5,000 years of dormancy followed, broken by the Swift Creek stage, highlighted by more dome growth and eruptions of tephra, which blanketed the countryside. The modern period of the mountain (since around 2500 BCE) has been highlighted by a wide variety in the composition of erupted materials, ranging from olivine basalt to andesite and dacite). The mountain’s volcanic activity comes as a result of the Juan de Fuca plate subducting the North American plate.

Mount St. Helens is significant for the tangible damage it caused in the past 50 years and for being one of the few active volcanos within the United States.