Big burst from St. Helens

Mount St. Helens cranked up the drama yesterday, sending a plume of ash and steam 36,000 feet in the air shortly before sunset.

Clearly visible from Portland, the cloud was at least as big as any since the volcano rumbled back to life in late September.

The explosion also appeared to be the most powerful in the current round of volcanic activity, University of Washington seismologist Steve Malone said. It started about 5:25 p.m. and lasted about 30 minutes.

The volcano seemed to quiet down almost immediately afterward, and there's no reason to expect a more significant eruption is imminent, Malone said.

"It looks like it's gone back to roughly the same type of signal that we were seeing before."

However, yesterday's explosion occurred with very little warning, cautioned Carolyn Driedger, spokeswoman for the U.S. Geological Survey's Cascades Volcano Observatory.

"We've had this relatively placid dome-building eruption going on, but we've been saying all along that could change at any time," she said. "It could intensify with little warning and produce more explosions, or it could go back to dome-building."

When university roommates Scott Miller and William Nicoll, both 19, saw the plume explode in front of them at the Mount St. Helens National Volcanic Monument, they feared their lives were in danger.

Miller said: "The first thing that went through my mind was, 'Is this 1980 again?' "

An eruption in 1980 blew off the mountain's top, leveled forests for miles around and killed 57 people.

Against Nicoll's protests yesterday, Miller quickly took two photos, and then the pair jumped back into their car and floored it. They yelled at other motorists to turn back, and drove about a mile before feeling safe.

Nicoll, who is from New Jersey, was on his first visit to Washington and was spending the spring-break week sightseeing with Miller, who grew up in Lake Stevens. Before the explosion, Nicoll joked that he wanted to see a show from Mother Nature.

"It was a pretty big adrenaline rush," Nicoll said.

The plume drifted slowly to the east and northeast, leaving a light layer of dust in nearby towns.

The explosion could have been triggered by a collapse or rock fall on the lava dome that has been building inside the crater for the past six months, Driedger said.

"We're not going to be able to tell until we can get up there in the daytime," she said.

SCOTT MILLER / SPECIAL TO THE SEATTLE TIMES The plume of ash and steam sent up yesterday by Mount St. Helens is seen from the national volcanic monument's Coldwater Ridge Visitor Center.

Scientists hope to fly over the crater today, if the weather allows.

The explosion knocked out at least two sets of scientific instruments and flung rocks around the crater floor, Driedger said.

The only hint that something was up in the hours before the outburst was a subtle increase in earthquake activity, Malone said. Scientists will analyze that data in more detail to see if they hold any clues to what happened.

St. Helens has been rocked by at least four similar explosions since early October. The most recent, on Jan. 16, was obscured by clouds but seems to have been very like what happened yesterday, Malone said.

For the past six months, the volcano has continued to quietly build a new lava dome. The dome had reached about half the size of the dome produced in the mid-1980s, after the cataclysmic 1980 eruption.

Over the past week, the new dome had been crumbling slightly, sending up small clouds of ash and steam. A series of magnitude 2 to 3 earthquakes occurred late last week but had subsided by yesterday, Malone said.

In the town of Packwood, about 40 miles northeast of the volcano, Lili Abella heard about the outburst from her cousin in Seattle, who saw it on TV.

"She told me, 'Pack up your stuff and get out of there,' " said Abella, owner of the Mountain View Lodge. "I told her, 'No, I'm too far away to worry about it.' "

MICHAEL LLOYD / AP The eruption draws attention yesterday in Portland, where the cloud of ash was clearly visible.

But her cars were coated with a light dusting of ash.

In Stevenson, near the Columbia River and southeast of the volcano, Emily Watts said she could see the plume rising in the distance.

"It looked like a big thundercloud coming in," she said.

Federal Aviation Administration spokesman Mike Fergus in Renton said that immediately after the eruption, the agency sent out an alert to air-traffic controllers and industry representatives across the country. Ultimately it was up to individual pilots and airlines to change flight paths or come up with other strategies, he said.

Bob Parker, spokesman at Seattle-Tacoma International Airport, said the airport was monitoring the situation and was not aware of any delays. Because the ash cloud was not moving toward the airport, Sea-Tac runways and facilities would likely not be directly affected, Parker added.

Alaska Airlines spokesman Sam Sperry said the plume was having no impact on its flights last night.

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

Nick Perry: 206-515-5639 or nperry@seattletimes.com.