Fitful Doze Volcanic activity at Mount St. Helens declined
significantly after the May 18 blast. Smaller explosive eruptions continued throughout the
remainder of 1980, with a final one in early 1982. Most of the ash from these eruptions
blew eastward, but on two occasions winds carried some ash westward into the heavily
populated Columbia River valley. A volcanic dome was formed on the floor of the central
crater by a series of small lava eruptions, the last of these occurring in 1986. Minor
steam explosions continued into the early 1990s.
The only activity at present is marked by the rumble
of small earthquakes, but the situation may change at any time. Mount St. Helens sleeps,
but the sleep is fitful. Given its violent past, Mount St. Helens will awake again in the
not too distant future. Photo: University of Colorado. Courtesy of NGDC/NOAA Photo: Dept. of Natural Resources, State of Washington.
Courtesy of NGDC/NOAA.
The Toll To the nation--and especially to those living nearby--the May 18 eruption seemed of apocalyptic proportions. The crown and heart of a whole mountain had been blasted away, and the surrounding countryside devastated. The energy released by the eruption was estimated at ten megatons, an explosion thousands of times stronger than the Hiroshima bomb. Thousands of deer, elk, bear, and smaller animals perished--in addition to 57 humans. Two hundred thirty-two square miles of forest were destroyed, including three billion board feet of timber estimated at $400 million in value (U.S. Senate Hearings, 1980). Numerous buildings, bridges, roads, and machines were destroy ed, and farms and communities up to a thousand miles away were partially buried in ash. One hundred sixty-nine lakes and more than 3,000 miles of streams had either been marginally damaged or destroyed (U.S. Senate Hearings, 1980). Losses to property and crops were set at more than $1.8 billion. Yet, the impact on human life could have been much greater if the main eruption had occurred on a workday or if the blast had been directed southwest toward the Portland/Vancouver metropolitan area, just 45 miles away, or if the wind had been blowing toward the southwest. Photo: Dept. of Natural Resources, State of Washington. Courtesy of NGDC/NOAA. Map: Courtesy of NGDC/NOAA.
As large and destructive as the May 18 eruption appeared, seen in context, it was a relatively small eruption. Thick deposits of older volcanic rock around Mount St. Helens attest to much larger eruptions in its past. Mount St. Helens is also only one of many volcanoes that dot the Cascade Range from northern California to southern Canada (see map). All of these volcanoes grew in the same geologic setting and are of the same explosive type as Mount St. Helens. Some eruptions at other Cascade volcanoes have been truly huge, such as the explosion nearly 7000 years ago--one hundred times larger than the May 18 eruption--that reduced Mount Mazama to Crater lake and spread ash all across the United States. Eruptions ranging in size from the May 18 eruption to the Mazama blast could occur at any time at any of the Cascade volcanoes. The metropolitan centers of Portland, Seattle-Tacoma, and Berkeley-San Francisco have grown up among the Cascade volcanoes. Map: Courtesy of NGDC/NOAA.
From the events in this narrative of the 1980 Mount St. Helens eruption, consider what might happen if Mount Hood or Mount Rainier or Mount Shasta should awaken from its dormant state with a May 18-size eruption, or more incredibly, with a Mazama-scale eruption.
For an extended discussion of Mount St. Helens, see http://vulcan.wr.usgs.gov/ljt_slideset.html
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