Mount St. Helens Volcanic Eruption – Washington – May 18, 1980

This was the most recent in the long history of volcanic eruptions in the Cascades. It had a VEI of 5, making it the greatest eruption of the twentieth century in the contiguous United States.

Mount St. Helens is the youngest of the Cascades’ volcanic peaks and the explosion of 1980 is just the most recent of the many intermittent eruptions that took place over the past 40,000 years. Pumice and ash from these past events now cover large areas of the Pacific Northwest. From the 1950s onward, the mountain was intensively monitored, perhaps to a greater degree than any other. Days before the fateful event of Sunday May 18, 1980, there were many signs of impending danger but no one was quite prepared for what finally happened—the largest eruption in the history of the contiguous United States.

It all seemed to take place in seconds. Seattle’s air traffic control tower tracked the mass of ash and rocks hurtling out of the mountain and concluded it was traveling at close to three hundred mph. The earthquake that triggered the explosion measured 5.2 on the Richter Scale but the energy released might be more accurately defined as the equivalent of thousands of Hiroshima-size bombs. An avalanche of mud, rock, and ice roared down the mountainside while the ash cloud rose as high as 54,000 feet.

What had moments before been a beautiful 9,000-foot-high peak was reduced to a 7,500 foot decapitated mountain. Ash, high in the atmosphere, drifted eastward right across the country, covering the ground everywhere it went with a layer of ash and blocking out sunlight in several communities near the mountain. Two hundred square miles of forest was flattened. Mudflows rushed westward down river valleys toward the Columbia River, blocking the navigation channel for ships with logs and mud for a distance of ten miles. It was estimated that fifty-seven people lost their lives on that first day.

A Boeing 737 jet flying from Reno, Nevada, to Vancouver, Canada, at 6,000 feet, was forty miles south of Mount St. Helens when the mountain exploded. The pilot saw the explosion and swung away from his course, a path that would have taken him directly over the eruption, escaping in so doing a dirty gray cloud that was rising quickly to meet him. His 737 rocked in the air from the shock of the explosion as if it were a ship at sea. Fortunately, his flight had been delayed for thirty minutes at Reno or all 122 passengers plus crew would have been added to the list of deaths for May 18.

The explosion that triggered the eruption was an earthquake of magnitude 5.2, not an unusual event in a region that experiences frequent earthquakes of this strength and stronger ones, but the scale of the eruption was a very different matter. It was a rare event even in historical time. Volcanic eruptions are identified by a volcanic eruption index number (VEI) based on the volume and speed of the rock and ash that is ejected. The index is numbered similarly to the Richter Scale, each number representing ten times the volume of the one immediately below it and one tenth that of the next number above it.

For Mount St. Helens the VEI is 5, a volume of eruption greater than that from any other within the contiguous U.S. since 1900. One has to go back into earlier times to find a meaningful comparison. Vesuvius in the year 79 or Krakatau in 1883 each had VEIs of 6; that is, they were ten times more powerful than Mount St. Helens. Volcanoes in the Cascade Range are fairly new in geological time. A few date back several million years. Mount St. Helens is one of the youngest. Much of its visible cone was formed within the last thousand years.

It was frequently active during that time and because of that geologists were convinced that a major eruption would occur sometime in the twentieth century. Further evidence in support of that expectation came more recently when it was found that the mountain had been more active and more violent over the past few thousand years than any other volcanic mountain in the contiguous United States. From the evidence in ash deposits across the western Cordillera it is clear that some of these older eruptions must have had VEIs of 6.

The most recent well-documented eruption, prior to 1980, occurred in 1842. Eyewitness accounts from that time described seeing vast columns of lurid smoke and fire. Ash from that time was subsequently located at The Dalles in Oregon, about sixty-five miles away. Fireworks continued intermittently over a fifteen-year period. Then came a three-year lull followed by a lot of activity in the year of 1857. After that date the volcano seemed to have slept for the 123 years before 1980. The moment of eruption on May 18 of that year was indelibly etched on news reporters’ memories.

David Johnston, the expert geologist from the United States Geological Service, who was monitoring the mountain on the morning of the explosion, talked to news reporters early on that day. He described Mount St. Helens as a keg of dynamite with a lit fuse whose length you do not know. He was well aware of the risks of being so close to the summit but he stayed on there right up to the moment of the eruption. He told the reporters that it was extremely dangerous to stand where they were at that time. “If the mountain explodes,” he told them before they left, “we would all die.” Soon afterward they heard his final words as he yelled into his radio, “Vancouver, Vancouver, this is it!”.

A family watched their $100,000 dream home smashed and washed down the chocolate-brown Toutle River. A couple was on a camping and fishing trip on the same river when they heard the explosion. They tried to grab their camping gear but quickly saw there was no time to escape in their car. They were thrown into the water and carried along in a mass of mud, logs, and rocks, grimly clinging to one log. They were lucky. The log was shunted sideways out of the main stream and sometime later, a helicopter picked them up. A television cameraman was approximately one mile from the base of the mountain filming the event. He saw the mass of mud and debris heading for him so he dropped everything, got into his car and drove madly to keep ahead of destruction. He was able to stay ahead.

Farther east, travelers were stranded in numerous small communities, altogether 10,000 of them in three states. One couple driving west from Spokane saw the black oncoming cloud. Soon they could only inch along the highway at a speed of about 5 mph. They abandoned their car and joined the other stranded ones. Everywhere around, trains, buses, airplanes, and cars came to a stop. Walking was the only mode of transportation that worked. Digging out from under the ash was yet another hazard. It proved to be as hard as getting around it. For some time, people wore masks of whatever material they could find for fear of toxic fumes.

As is common in volcanic eruptions, the magma that had risen and caused the explosion of May 18 left the inner chamber empty for a time until new magma moved up from below. The interior dome then began to grow until pressure rose to a level that caused another eruption. Several of these subsequent eruptions came in May, June, July, August, and October of 1980. By 1983, the dome had grown to six hundred feet and the crater in which it sat was two miles in diameter, waiting for the moment of the next event and meanwhile continuing to grow.

How can volcanic eruptions and earthquakes be predicted? The answer remains elusive but experience at Mount St. Helens shows us some of the things that can be done. As mentioned earlier in this chapter, this mountain received more monitoring since the 1950s than almost any other and the small number of people who were killed is largely due to this as well as the actions that were taken in the months before May 18, 1980. The first earthquake in Mount St. Helens struck on March 20, 1980, and immediately seismologists met with local authorities to warn of the danger ahead and make some preliminary plans.

One week later, steam and ash exploded from the summit of the volcano and this was followed by several minor eruptions over the following weeks. As these eruptions became more frequent public authorities closed off the area around the mountain, causing heated opposition from the general public. Later they lowered the water level in the Swift Dam reservoir to minimize damage from mudflows. Still closer to the eruption, the state governor issued a state of emergency in order to use the National Guard to staff the roadways. So angry were some over the closure that they found ways of circumventing the law by using little known roads and footpaths to gain access. Many of these people were too close to the volcano when it exploded and died. Some like Harry Truman, a veteran resident of the mountain, refused to leave his home on the north side and died.

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