"AMERICA'S VOLCANO"

Remembering Mount St. Helens

- BY STEVE VOYNICK

Forty-five years ago this month, the deadliest and most destructive volcanic event in America’s history took place in the state of Washington. On May 18, 1980, the cataclysmic eruption of Mount St. Helens literally removed the entire top of the mountain. It also killed 57 people, devastated forests and wildlife, and destroyed homes and highways.

Today, Mount St. Helens is known as “America’s volcano” and is the namesake attraction at Mount St. Helens National Volcanic Monument. Although the volcano is now quiet, geologists warn that future eruptions are likely.

GEOLOGY

Located 100 miles south of Seattle and 50 miles north of Portland, Oregon, Mount St. Helens is part of the Cascades Volcanic Province, a volcanic arc extending from southwestern British Columbia to northern California. Within this geological province are 20 major volcanoes along with hundreds of lesser volcanic features such as vents, domes, lava flows and cinder cones.

The Cascades Volcanic Province was created by tectonic-plate movement as the North American Plate subducts the oceanic Juan de Fuca Plate, increasing temperatures of free water molecules from subducted rock. This superheated water then moves upward into the overlying North American Plate where it melts mantle rock into magma which, in turn, rises to the surface to explain the region's volcanic activity. Beneath Mount St. Helens, magma accumulates in two chambers, one 3 to 7 miles beneath the surface, the other 7 to 25 miles deep. Mount St. Helens, magma accumulates in two chambers, one 3 to 7 miles beneath the surface, the other 7 to 25 miles deep.

ERUPTION HISTORY

Mount St. Helens began forming only about 40,000 years ago. Studies of volcanic sedimentation reveal eight ancestral eruptive phases of widely varying intensities and duration.