Volcanic Eruption, Mount Saint Helens, WA, May 18, 1980

Summary

On May 18, 1980, shortly after 8:30 a.m., Pacific Standard Time, Mount Saint Helens exploded with the estimated force of a twenty megaton nuclear bomb. The explosion sent steam and ash from burnt rock over eleven miles into the air. Eighty-five deaths were directly attributed to the blast. Ash fallout from the eruption, up to several inches in some locations, made travel by motor vehicle impossible for days. Roads between major cities, including an interstate highway, were closed, stranding thousands of travelers. Many thousands more were confined to their homes for nearly a week as places of work were shut down and authorities urged people to stay indoors until health risks associated with the ash could be accurately assessed. The event was termed a disaster. The plume of ash was carried east by northeast winds and deposited in varying mounts across the state. The heaviest amounts did not necessarily fall in areas closest to the mountain. In fact, Ritzville, near Spokane, one of the towns with one of the highest levels of ashfall, is located nearly three hundred miles from the mountain. The features of the event which set it apart from other natural disasters were: 1) the direct cause of the disaster was located far (up to three hundred miles) from its consequences; 2) some of those in the affected area did not know the mountain had erupted until they were exposed to the fallout; 3) the sequence of events following and the duration of effects from the eruption were uncertain and could not be predicted based on past experience; 4) this uncertainty contributed to the production of a great deal of contradictory information about what people should do to protect themselves; 5) the event occurred on a Sunday when most people were involved in non-work routines; 6) the event affected an entire region, not just a few isolated communities; and 7) a clear measure of degree of impact could be derived from ashfall depth.

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