Eruption Assumptions : Science File / An exploration of issues and trends affecting science, medicine and the environment. : Scientists at Mt. St. Helens are studying how to predict volcanic activity.
From the Coldwater Ridge Visitors Center, well inside the devastated zone formed 15 years ago by the volcanic blast on a sunny Sunday morning, the view is one of barren splendor.
A few small trees and an occasional bank of flowers have grown back. But much of the North Fork Toutle River Valley viewed by more than 700,000 visitors a year remains brown dirt terrain resembling a moonscape.
There are countless hillocks of sediments that piled up irregularly from the blast rather than washing downstream. Created that cataclysmic morning of May 18, 1980, two large lakes, one on each side of the valley, have formed behind marginally stable debris dams.
Five miles away looms Mt. St. Helens’ peak. Inside the crater that opened when the top 1,300 feet of the mountain was blown away, there is a lava dome 920 feet high. The dome began growing through small eruptions within weeks of the big one, but just a portion of it is visible from the visitors center.
Far below, road builders are pushing forward a new highway, across another ridge, to bring visitors next year closer to the mountain for a better view of the crater.
Fifteen years after its famous eruption, Mt. St. Helens is quiet. Scientists do not know when the volcano will revive, but they do know that this and other Cascade volcanoes are among the most potentially devastating on the planet, close enough to urban areas to be menacing, yet infrequent enough in their eruptions to lull residents into a false sense of security.
Since blowing its top, St. Helens has become the focal point in the continuing search for precursory events to volcanoes that, it is hoped, will prevent deadly surprise in the future. Researchers are also studying, with great interest, the entire Cascade range, especially Mt. Rainier and nearby Newberry Crater; and outside the Cascade range, Mammoth Mountain in California.
There have been no dome-building eruptions at Mt. St. Helens since 1986, and no steam blasts from water percolating onto hot rocks below the surface since 1991.
Minor renewals of activity in the dome are possible soon, and scientists know from studying deposits that over the last 4,000 years the mountain has erupted in a major way twice as frequently as any other volcano in the Cascade range. It was dormant for 123 years before 1980, but this is only a speck of geologic time. (In the entire Cascade range there is an average of only one or two eruptions every century).
“Researchers have been surprised by the brevity of the dome-building period following 1980,” said Ed Wolfe of the Cascades Volcano Observatory in Vancouver, Wash. “It would not be surprising if dome-building resumed. It would probably start with an explosive eruption, clearing the vent.”
A staff of 65 at the observatory monitors the volcano, but it also studies other Cascade volcanoes and occasionally undertakes missions to monitor eruptions around the world.
Wolfe, who is charged with preparing a hazard assessment of Mt. St. Helens and mapping it geologically, said the next dome-building episodes--like the previous ones--will probably be confined to the crater and hence affect only a restricted area free of human habitation.
But, he said, at some indeterminate time in the future--”we don’t know if the next eruption is a year, a decade or a century from now”--another major event will occur.
For planning purposes, he said, he is assuming it will be as big as the one in 1980, although it will certainly vary in character because the mountaintop has lost its top. The next eruption will have a different focus and may be pointed in a different direction, but it will carry a devastation of its own.
Six Cascade volcanoes have erupted, some on a small scale, in the last 200 years--Rainier, Baker, Hood, Lassen, Shasta and St. Helens. Although the St. Helens eruption was by far the largest, scientists have warned that Mt. Rainier could pose the greatest hazard.
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Not only is it closest to a metropolitan area (Seattle-Tacoma), but at 750,000 years old and 14,410 feet high, it is an older and bigger mountain than the others. Steam vents have sent up acidic gases that may further weaken it.
Volcanologist Carolyn Driedger said collapses of the Rainier superstructure, even without an eruption, could come without warning, sending torrents of mud into populated areas around Puget Sound within hours. Deposits indicate that such events have been happening every 500 years.
Mt. St. Helens is only 50,000 years old, Driedger said, and such unexpected events are less likely. Frequent earthquakes and ground deformation gave warning of the most violent 1980 eruption, although it was larger than most scientists expected.
But there are also hazards at St. Helens that are not directly related to eruptions.
For example, snow and ice have been building up in the crater, which is mostly in shadow, and a glacier appears to be forming at the bottom, at about the 6,000-foot level.
This contributes to what Daniel Dzurisin, the scientist in charge of the Cascades Observatory, calls the greatest concern about Mt. St. Helens.
Dzurisin and the observatory’s chief hydrologist, John Costa, explained that the instability of sediments from the 1980 eruption, the possible catastrophic collapse of the debris dams holding back the lakes, or even the possibility of a much larger than normal rainfall year have ominous potential for downstream areas as far away as the Columbia River.
A 100-year-flood could dislodge sediments from the 1980 eruption, posing serious dangers of mudflows to such cities as Longview and Kelso, Wash., and could again clog the Columbia, which had to be dredged to free mud-bound shipping 15 years ago.
“Mt. St. Helens hasn’t come close to generating a big flood since 1980,” Costa said, “but, still, sediment flow downstream is 10 times what it was before the eruption.”
Since 1980, the Army Corps of Engineers has undertaken major projects to try to reduce the danger.
Spirit Lake, the largest lake close to Mt. St. Helens, began rising after the eruption, its natural outlet to the Toutle River Valley dammed by sediments. The corps first pumped water out and in 1985 drilled a tunnel through rock, allowing the regular flow to resume and stabilizing the level of the lake.
In 1989, 20 miles down the Toutle River from the volcano, the engineers completed a sediment retention structure, or dam, that was designed to keep as much as 258-million cubic yards of sediment upstream, rather than permit it to flow out to the Columbia.
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The empty reservoir created behind the dam will probably be filled by sediment after 45 years, but by that time, Costa said, the loose sediments near the volcano are expected to have stabilized and their downstream flow to have diminished.
The engineers have also built reinforced spillways for Coldwater and Castle lakes, reducing the chance that the outflow from those bodies of water will erode the debris dams and cause, at some point, a sudden emptying of either lake.
Despite such precautions, the hazards are many. “There are big eruptions in the Cascades on the average only once or twice every 100 years,” Dzurisin said. “But that makes them a particularly difficult hazard to mitigate. If the eruptions occurred frequently, as in Hawaii, everyone would be aware of the dangers. But coming here as infrequently as they do, people forget.”
