MOUNT ST. HELENS NATIONAL MONUMENT, Wash. - Mount St. Helens belched steam thousands of feet above the volcano's crater Monday, apparently the largest burst yet as days of tremors and low-level earthquakes have raised fears the volcano will blow at any moment.
Scientists said it was unclear whether a larger eruption was still expected.
A thick, white cloud billowed and shrouded the volcano's shattered summit, then wind slowly steered the cloud to the west. Within 30 minutes or so, the cloud had dissipated and the summit was clearly visible again.
Monday's steam release followed a similar blast and 20-minute tremor late Sunday.
"Something is driving - like a piston - something is driving up. We believe it's magma," said Jeff Wynn, chief scientist for volcano hazards at Vancouver, Wash.
Researchers said the emission contained little of the black, minerally ash generated by seismic activity deep within the mountain. Instead, researchers believe the steam was generated when hot gases vented off from the volcano and vaporized ice and snow contained in the glacier that on the mountain's highest reaches.
A drumbeat of earthquakes since a plume of steam was released on Friday indicated that pressure was mounting within the mountain, Geological Survey geologist Tom Pierson said.
Crews also observed a shift in the crater floor and on part of the 1,000-foot lava dome that essentially serves as a plug for magma.
Guy Medema, a seismic analyst at the University of Washington's Seismology Lab in Seattle, said earthquakes of magnitude 2 and 3 continued after the steam burst, unlike a steam eruption Friday, when the earthquakes stopped for several hours.
"It apparently didn't release enough stress to shut the earthquakes down," Medema said.
Scientists have said they do not expect anything close to the devastation of Mount St. Helen's May 18, 1980, explosion, which killed 57 people and coated much of the Northwest with ash.
Still, the recent eruptions and long rumblings deep inside the mountain are unprecedented in the past 24 years.
Among the their biggest concerns is that hot and gassy new magma is filling the volcano's interior reservoir and priming the volcano for a significant blast that could send large boulders flying for several miles.
If the weather cooperates, scientist plan to helicopter into the crater and install more instruments on the growing lava dome inside the crater. The lava dome has risen by 50 feet in recent days, they said.
Crowds have gathered along park roads at what was said to be a safe distance - about 8.5 miles from the mountain - to see what happens next. Barbecues were fired up and entrepreneurs sold hot dogs and coffee to people camped along the side of the road.
The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forest and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep.
This time, the main concern was a significant ash plume carrying gritty pulverized rock and silica that could damage aircraft engines and the surfaces of cars and homes.
Many spectators couldn't wait out the mountain. Sunday's sunset brought a mass exodus off the mountain.
"Our attention span is about like this," said James Wilder, 25, of Aberdeen, holding his forefinger and thumb about one-quarter of an inch apart. "We've been here five hours and we need to leave pretty soon."