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Scientists haven’t yet come up with a crystal ball that can tell which volcanoes will explode and when. But they are getting better at the forecasts.
In 1975 Dwight R. Crandell, Donal R. Mullineaux, and Meyer Rubin of the U. S. Geological Survey predicted that Mount St. Helens was the Cascade volcano most likely to reawaken from dormancy.
“We had predicted Mount St. Helens would erupt within 100 years,” said Crandell. “But then we went out on a limb and said ‘before the end of the century.' ”
Forecasting from the Past
Predicting a volcano’s future is based largely on its past—what Crandell and his colleagues call the “geologically recent time” of the last 11,000 or 12,000 years.
One reason they expected trouble from Mount St. Helens, for instance, is that-it has been the most active and the most explosive of the Cascade volcanoes in the last 4,500 years. It’s also the youngest—another indicator that there may be more eruptions ahead. Its last eruption before this spring was in 1857.
For many years, USGS scientists have been studying evidence from old eruptions in or near the Cascades. By finding out what came out of the volcano, how often, and how far it traveled, they have been able to map out the areas of the western United States most likely to be affected by lava flows, mudflows, flooding and ashfall.
A mudflow from Mount Rainier about 5,000 years ago covered more than 300 square kilometers of southern Puget Sound. Thousands of people live there now in communities southeast of Tacoma, Wash. And flooding from a large eruption at Rainier could reach as far north as Seattle.
Oregon's Mount Hood has been better behaved than Rainier or St. Helens in the
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