Mount St. Helens decides it's time to take a breather

After more than three years of nonstop eruption, the Energizer bunny that is Mount St. Helens appears to have run out of steam — at least for now.

U.S. Geological Survey scientists say the growth of a new lava dome, which began with an explosive outburst in October 2004, has slowed to a halt.

"We're calling it a pause," said Willie Scott, a volcanologist at the USGS Cascades Volcano Observatory in Vancouver, Wash. "We really don't know if it's going to start up again."

The volcano that unleashed a cataclysm on May 18, 1980, proved it has new tricks up its sleeve with the recent bout of prolonged, but relatively subdued, activity.

"It's made liars out of all of us," USGS seismologist Seth Moran said. "Nobody expected it to go on this long."

The first sign Mount St. Helens was waking up after 18 years of dormancy came with a spurt of small, shallow earthquakes in late September 2004. A week later, an outburst of ash and steam rent the crater floor and sent a plume towering into the sky. After several similar throat-clearings, the volcano started pumping out stiff, sluggish lava — and didn't stop until this month.

During the early phase of the eruption, the volcano was disgorging a dump-truck load of lava every second. Columns of ash billowed 36,000 feet into the sky and tourists flocked to southwestern Washington to witness mountain-building in action.

The eruption slowed considerably within a year, subsiding to such a modest level that many Northwesterners weren't even aware it was still going on.

Still, the output has been prodigious.

The new dome that formed in the blasted-out crater is taller than the Empire State Building and half a mile across. It's equal in size to the dome created by a six-year series of eruptions that started after the devastating 1980 blast blew off the mountain top.

"That's about the same volume, in about half the time," Scott said.

By this month, the seismic rattles caused by magma pushing to the surface had largely faded to background levels. Sensors detected virtually no volcanic gas escaping from the crater. Time-lapse photos from cameras mounted on the crater rim showed an unchanging landscape.

Data from instruments that measure tiny up-and-down ground motion revealed the volcano is actually deflating.

"We're seeing what you might expect to see if you have a bunch of hot stuff cooling down," Moran said. "But it's too early to say it's done."

St. Helens showed in the 1980s that it's capable of a series of eruptions spread out over time.

"Because of the nature of Mount St. Helens, I don't think anybody would be surprised if, weeks or months down the line, something starts going on," Scott said.

But Steve Malone, professor emeritus at the University of Washington and former director of the Pacific Northwest Seismic Network, is betting the volcano will be quiet for a while, as fresh infusions of molten rock slowly recharge the magma chamber.

If the volcano starts acting up again, scientists will be able to deploy an array of new instruments and techniques developed and refined over the past three years to take the volcano's pulse from a safe distance. Among them is the Spider, a small device that can be lowered from a helicopter directly onto a growing lava dome. Equipped with GPS, seismometers and gas sensors, Spiders can perch where people can't.

USGS scientists at Mount St. Helens also advanced the use of infrared instruments to identify volcanic hot spots in the crater.

"They came up with some marvelously clever ways of operating in a dangerous environment and getting good data," Malone said.

But scientists still can't explain why St. Helens rumbled back to life in 2004, or why the eruption played out as it did.

One theory is that magma left over from the 1980 eruptions was pushed upward, perhaps by an infusion of fresh magma from deeper in the Earth. Like flat champagne, the old magma had lost much of the gas responsible for explosive eruptions. So by the time it reached the surface, it simply oozed out like thick toothpaste from a tube.

Whatever the mechanism, the lesson is that volcanoes are still unpredictable, Scott said. St. Helens gave only about a week's warning of the recent eruption, which shows why it's crucial to maintain and expand monitoring systems throughout the Northwest, he said.

"What this eruption taught us in spades is that all of our volcanoes can turn around really quickly."

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