Mount St. Helens likely to have small eruption soon, experts warn

By Hal Bernton
Seattle Times staff reporter

GREG GILBERT / THE SEATTLE TIMES This aerial photo, taken yesterday, looks east at Mount St. Helens and its lava dome.

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Other links Mount St. Helens Seismicity Information Mount St. Helens National Volcanic Monument Forest Service's Mount St. Helens VolcanoCam

Such an event could fling ash and rocks thousands of feet into the air but would not be expected to pose hazards beyond the volcano's crater and flanks.

Scientists monitoring Mount St. Helens hope to gain a better understanding of the situation from a helicopter flight yesterday that searched for the presence of magma gases that might offer hints of a future eruption. Preliminary results did not indicate the presence of those gases, according to a joint statement released yesterday evening from the Cascade Volcano Observatory here and the University of Washington.

But shallow earthquake activity continued yesterday to slowly increase. And scientists caution that predicting any future eruption is a very difficult thing to do.

"It is not an exact science," said Thomas Pierson, a research hydrologist at the Cascade Volcano Observatory here. "Nowhere in the world can anyone accurately forecast an eruption. There is a lot of art and subjectivity in this."

	GREG GILBERT / THE SEATTLE TIMES Scientists searched for magma gases coming from Mount St. Helens, shown looking southeast in this aerial photo taken yesterday.

The key question is the source of an unusual pattern of the shallow earthquakes that grabbed the scientists attention over the weekend. Those quakes continued yesterday with equal frequency but less intensity.

They could be triggered by recent rains transformed to steam as water percolates down into the crater's lava dome, according to Pierson. That's a frequent end-of-summer event.

Or, the quakes could result from hot magma entering an area beneath the 925-foot high lava dome that sits inside the crater and serves as a kind of volcanic plug. From beneath the dome, the magma could combine with pressurized gases and steam to trigger an eruption, Pierson said.

Scientists say that any eruption could throw up a modest ash plume and also be accompanied by mud flows that would ooze out of the crater's open north end. U.S. Forest Service officials, as a precautionary measure, have closed the popular climbing route to the edge of the crater as well as three other sections of hiking trails.

But the ash and mud flows — known as lahars — would not threaten the Johnston Observatory on State Highway 504. It is located some 5.5 miles north of the lava dome and is separated from the dome by the Toutle River Valley.

"We're up high and a good distance away, so any mud flow out of the crater would not reach us," said Todd Cullings, assistant director of the Johnston visitor center.

Quake gear

Scientists have made a lot of progress in understanding Mount St. Helens since the cataclysmic eruption of May 18, 1980.

The crater is now outfitted with sophisticated seismograph equipment. And so far, that equipment does not indicate any movement of magma from deep within the mountain that might presage a large eruption. Instead, any magma now moving into the dome would most likely come from a much smaller, and shallower magma pool that would pose much less of a threat, Pierson said.

That 1980 eruption, which blew off the mountaintop, included both a upward explosion and a huge lateral blast. It unleashed more than 1.42 billion cubic yards of ash, streams of lava and lahar mud flows. The eruption killed 57 people, destroyed nearly 200 homes and claimed thousands of animals. It also felled enough timber to make 300,000 homes, clogged the Columbia River shipping channel and left as much as a half inch of ash at downwind distances of 300 miles.

In the six years after the blast, scientists recorded 21 separate incidents of fresh magma working its way into the dome, with seven of those events triggering at least weak explosive activity, according to the U.S. Geological Survey.

But no such magma events have happened since 1986, although steam explosions sometimes have occurred within the dome. And scientists did not think such an event likely last week. Then Saturday, the quake swarms continued and grew considerably more powerful.

"This is a pattern we have never really seen before," Pierson said. "It's a new chapter for us."

Yesterday, the stylus of an observatory seismograph continued to jiggle every few seconds like the end of a fishing rod reacting to nibbles. It was transmitting from equipment perched on top of the lava dome. And instead of the flat line sketched on a normal day, it recorded the quake swarms in tracings shaped like Christmas trees.

The monitoring equipment indicates the quakes are relatively shallow, anywhere from 500 feet to several thousand feet below the top of the lava dome, Pierson said. And that strengthens the scientists' belief that any hot magma entering the area underneath the dome is from a relatively small, shallow pool.

Measuring gases

Yesterday, on the helicopter flight over the crater, scientists took the atmospheric measurements of gases. The helicopter also lowered geophysicist Michael Lisowski onto the lava dome to replace a failed instrument used to measure tiny movements that indicate whether the dome is swelling, chief observatory scientist Jeff Wynn said.

While the chopper was near the dome, the pilot was in constant radio contact with geophysicist Bobbie Myers, the observatory's aircraft manager, who during the 1980 blast learned to detect subtle changes in seismic monitors.

"She's known to be able to predict explosive events up to a couple of minutes ahead of time," said Wynn, who confessed to being a nervous wreck while the device was being swapped out.

Information from The Associated Press was used in this report.

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com

Copyright © 2004 The Seattle Times Company

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