Mount St. Helens eruption chances put at 70 percent

By Sandi Doughton
Seattle Times staff reporter

STEVE RINGMAN / THE SEATTLE TIMES Roderick Smith, of Portland, paints Mount St. Helens after recently painting Mount Rainier. "It's the energy this mountain is giving off right now that lured me here," he says.

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"We're saying there's a 70 percent chance of an eruption within the next few days to a few months," said volcanologist Cynthia Gardner of the U.S. Geological Survey's (USGS) Cascades Volcano Observatory.

The estimate is based on the mountain's notorious history and expert consensus on how to read the signals it has been giving off for the past week — notably a flurry of small, shallow earthquakes that almost certainly indicate magma pushing against rock.

"From the size of these events and the number of them, it's got to be due to motion, and magma is the only thing down there to move," said University of Washington seismologist Steve Malone.

The quakes started very small but exceeded magnitude 3 for the first time yesterday morning, scientists said. By the end of the day, the rate of earthquakes remained constant at three to four per minute.

But predicting what volcanoes will do is not an exact science, and Gardner stressed there's a nearly 1-in-3 chance the activity will end with a whimper, rather than a bang.

But if a bang is coming, how big will it be?

"Small to moderate" is the official description.

That could range from an explosive eruption powerful enough to fling rocks three miles from the crater to a more sedate outpouring of thick, pasty lava — or possibly a combination of both.

The 1993 outburst at Galeras volcano in Colombia that killed seven volcanologists was a moderate-sized explosive event, Gardner said. That's why the USGS has not allowed any of its researchers back in the crater since Monday, when one person was dropped off on the lava dome by helicopter and spent 15 minutes replacing a sensor that measures ground swelling.

"It's just too dangerous," Gardner said.

Because the area immediately around Mount St. Helens is uninhabited, an explosive eruption would pose little or no hazard to people or property. It could set off a mudflow, which probably wouldn't reach much farther than the edge of the debris fan that spreads out from the volcano's blasted-out north face.

It also would kick up a cloud of volcanic ash that might range from 200 to 20,000 feet high, Gardner said. By contrast, the massive 1980 eruption at Mount St. Helens sent up an ash plume 80,000 feet tall.

That event disgorged more than 600 million cubic yards of ash and rock. The best projections for the eruption that now may be brewing is that it could spew out about 2 million cubic yards of ash, rock and lava.

If magma does reach the surface, it could ooze out rather than burst out.

It depends on how much volatile gas the molten rock contains, Gardner explained. Because there is no evidence that fresh, gas-rich magma is moving into the volcano, the best guess is that the magma setting off the earthquakes probably has been sitting a mile or less under the lava dome for several years, Gardner said. That means it will have lost most of its gas and won't be very explosive if it works its way out of the volcano's geological plumbing and onto the crater floor.

But geologists are not taking the current forecast as the final word. They're watching the seismic signals and other sensors carefully for any hints that deep, fresh magma may be moving undetected into the volcano, as it did before the 1980 eruption.

"We made mistakes in 1980, and I think we will make fewer of those types of mistakes now," Malone said. "But that doesn't mean we've got it figured out 100 percent."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

Copyright © 2004 The Seattle Times Company

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