MOUNT ST. HELENS NATIONAL MONUMENT, Wash. – Mount St. Helens (search) stewed in volcanic gases and low-level earthquakes Sunday, with crowds of eager tourists hoping to glimpse an eruption that scientists said could happen immediately or take a few weeks.
A second long tremor early Sunday and an increase in volcanic gases strongly suggest magma is moving inside, researchers from the U.S. Geological Survey (search) said. The mountain's alert was raised to Level 3, the highest possible, after a volcanic tremor was detected Saturday for the first time since before the mountain's 1980 eruption.
"I don't think anyone now thinks this will stop with steam explosions," geologist Willie Scott said Sunday at the Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., about 50 miles south.
But Scott said scientists discussed lowering their alert from a Level 3 "volcano advisory," which indicates eruption is imminent, to Level 2 "volcanic unrest," which indicates an eruption is possible. They needed more data before making any change, he said.
"What we haven't gotten back today yet is a lot of field measurements — there's a gas flight going on, a flight to use thermal imaging to look at the [lava] dome, GPS data needs to be downloaded," Scott said Sunday. "There's a lot of work that needs to be done. That will occur overnight and tomorrow morning."
Scientists said they do not expect anything close to the devastation of the May 18, 1980, explosion, which killed 57 people and coated much of the Northwest with ash.
"Of course the volcano reserves the right to change its mind," said monument scientist Peter Frenzen with the U.S. Forest Service (search), which operates the park.
Some experts had said Saturday that an explosion would probably happen within 24 hours. But as the hours passed, others cautioned that the timing is difficult to predict.
"No one is predicting it as a sure thing," said Bill Steele at the University of Washington's seismology lab in Seattle. "This could be going on for weeks."
Crowds gathered along a park road 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 were selling hot dogs and coffee to people camped along the side of the road in lawn chairs and pickup beds.
"It'd be neat if it spews something over and out," said Chris Sawyer, 40, of Dundee, Ore., who had a large camera set up on a tripod at the Coldwater Ridge Visitors Center.
Hundreds of people were cleared from a popular observatory closer to the peak Saturday following a tremor and brief release of steam.
The mountain was outwardly quiet at midday. Clouds of dust rose occasionally, caused by rockfall from the towering canyon walls. But earthquakes were occurring "multiple times per minute," Steele said, peaking every few minutes at magnitudes as high as 3.
Seismic activity became more sporadic over the day, said seismologist Tony Qamar at the University of Washington's seismic lab in Seattle.
Scientists were unsure how explosive the eruption may be; depending on the gas content of the magma and conditions, it could range from a passive emission to an explosion that throws up a column of ash, Scott said.
Besides lava flows, ash and rock-throwing, an eruption could cause melting of the volcano's 600-foot-deep glacier and trigger debris flows to the barren pumice plain at the foot of the mountain.
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, scientists expected populated areas to get little ash if the light west-northwest wind holds. The closest community is Toutle, 30 miles west near the entrance to the park in the Gifford Pinchot National Forest about 100 miles south of Seattle.
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.
Steele said the mountain took scientists on a "rollercoaster ride" early Sunday when instruments detected the second extended volcanic vibration in two days — 25 minutes long compared to Saturday's 50-minute vibration.
"It died off and quickly became a non-issue. But had it been as long as the one following that little steam burst yesterday, we could be moving to an eruption pretty quickly," Steele said.
Scientists also detected elevated levels of carbon dioxide and other volcanic gases, including the rotten egg smell of hydrogen sulfide, that reflect changes in the volume of magma rising within the mountain.
Gas-sampling flights continued Sunday, and acoustic equipment had been placed around the crater. Dozens of global positioning satellite stations — to alert scientists to changes in ground formation — have been placed on the mountain, though Friday's steam blast destroyed equipment on the 1,000-foot lava dome.
Most of the action has occurred beneath the dome, which has been building up on the crater floor and essentially serves as a plug for magma, or molten rock. The dome is filled with lava that came up during 1998 earthquakes but never surfaced. New lava may be coming up as well.
Roberta Miller, 62, said there was an "amazing energy" among those gathered at the Coldwater observatory, where the wraparound veranda was jammed with people in lawn chairs.
She said she was living in Yakima in 1980, when "the mountain came to us" in the form of heavy ashfall. This time, she said, "We came to the mountain."