Saturday, March 01, 2008

Is Mount St.Helens' eruption near the end?

The eruption of Mount St. Helens may have finally petered out.

More than three years after the volcano reawakened spectacularly in the fall of 2004, scientists say the eruption appears to have stopped ... for the time being.

"Volcanoes do this sort of thing," said Steve Malone, a University of Washington research geophysicist who has studied Mount St. Helens since it erupted catastrophically on May 18, 1980. "They erupt for a while, then it sort of dies out, and something changes in the future, and then they’ll erupt again."

Yet scientists acknowledged that almost 40 months is a long time for a volcano to steadily churn up solid rock. The mountain has built a lava dome of at least 122 million cubic yards, all of it pushed out as blocky chunks similar in chemical composition to the lava dome that emerged in the six years after the 1980 eruption.

The new lava dome radically uplifted, severed and then shoved away a 600-foot-thick glacier that accumulated in the 18 years between the 1980s dome-building and the latest period of dome growth that began with a flurry of earthquakes on Sept. 23, 2004.

By the fall of 2006, St. Helens was continuing to pump out the equivalent of 25 Olympic-sized swimming pools every day.

That rate had dwindled significantly by this past autumn.

Since the end of January, scientists with the U.S. Geological Survey have detected no discernible growth of the lava dome, negligible gas emissions, hardly any earthquakes and no wider ground deformation.

Officially, scientists are terming this a "pause" in the eruption.

"Whether that means this period has ended or has just paused, we won’t know until there are further developments," said Dan Dzurisin, a USGS geologist in Vancouver. "Dome growth has stopped. Whether or not this period of eruptive activity is over, is a different question."

The USGS lowered the alert level from a Watch to an Advisory, but pilots still should keep their heads on a swivel when sightseeing around Southwest Washington’s dyspeptic peak.

"It’s certainly not as big an issue as it has been," said Cynthia Gardner, scientist in charge of the Cascades Volcano Observatory. "But it’s a little ‘heads-up,’ not to ignore Mount St. Helens altogether."

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