Scientists detected a volcanic tremor at Mount St. Helens early Sunday, just hours after officials raised the volcano's alert level, cleared hundreds of visitors from the area and warned a major eruption was imminent.
Sunday's tremor lasted about 25 minutes and was milder than the 50-minute tremor that followed a steam release Saturday, said Jeff Wynn, chief scientist for volcano hazards at the U.S. Geological Survey's Cascade Volcano Observatory.
"It just means that what's been happening is still happening," Wynn said.
The vibrations told scientists molten rock, or magma, could be moving toward the surface, reports CBS News Correspondent John Blackstone.
"The changes we've seen in the last ten days are the kind of thing that precede an explosive eruption," says seismologist Dan Miller of the U.S. Geological Survey.
Scientists expect the impending blast to be much smaller than the May 18, 1980 explosion that killed 57 people and coated much of the Northwest with ash. But the tremors and steam detected since quake activity began Sept. 23 signaled more seismic energy than at any point since the 1980 explosion.
The volcano alert was raised to Level 3, which "indicates we feel an eruption is imminent, or is in progress," said U.S. Geological Survey geologist Tom Pierson. He said Saturday afternoon that an explosion probably would happen within the next 24 hours.
On Friday, the volcano spewed a plume of steam and ash thousands of feet into the air, but there was a scant release of steam Saturday — a puff of white cloud, followed by a dust-raising landslide in the crater. A volcanic tremor signal that came next was what prompted the heightened alert level, and scientists detected elevated levels of volcanic gases later in the day.
Hundreds of visitors at the building closest to the volcano — Johnston Ridge Observatory five miles away — were asked to leave Saturday. Some relocated several miles north to Coldwater Ridge Visitors Center, which officials said was safe.
People pitched tents alongside park roads and spent the night waiting to see what the rumbling volcano would do. Saturday was the busiest day ever at visitors' centers on the mountain, with thousands of people packing buildings, crowding parking lots and sitting alongside roads in lawn chairs.
Barbara Jardin, 53, of Camas, said she saw the plume at midday and was afraid she'd miss something if she left the area. "I just stare at it and stare at it. It's too awesome to leave," she said.
Interior Secretary Gale Norton, who flew over the mountain Saturday, said the seismic activity has weakened the 1,000-foot lava dome that began forming in the volcano's crater after the 1980 eruption.
"The greatest concern at this point is an ash plume and the spread of ash itself that might come from an explosion," Norton said. "This is a concern for aircraft travel."
Saturday's tremor lasted about an hour before it was drowned out by a series of earthquakes — one or two a minute, with a maximum magnitude of "well over 2," said Tom Yelin, a USGS seismologist at the University of Washington's seismic laboratory in Seattle.
The growing consensus among scientists is that new magma is probably entering the volcano's upper levels, bringing with it volatile gases that could lead to eruptions, said Bill Steele at the University of Washington lab.
Late Saturday, scientists flew past the volcano to measure its emissions. USGS spokeswoman Stephanie Hanna said for the first time in this Volcano Alert the instruments are detecting elevated levels of carbon dioxide, which escapes as magma rises toward the surface from the Earth's interior.
The increasing rate at which a volcano releases gases like CO2 and sulfur dioxide reflects changes in the volume of magma rising within its reservoir. Scientists at the rim of the volcano smelled hydrogen sulfide, similar to rotten eggs, Wynn said.
The USGS said explosions from the crater could occur without warning, possibly throwing rock onto the flanks of the volcano. Still, scientists said the evacuation of the observatory was primarily a precaution in case of heavy ash, which could make it difficult to drive.
"We still feel the risk is confined to this area," Pierson said.
No communities are near Mount St. Helens; the closest, Toutle, is 30 miles west. Few people live near the mountain, the centerpiece of the Mount St. Helens National Volcanic Monument in the Gifford Pinchot National Forest about 100 miles south of Seattle.
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.
Until late Saturday, air sampling had detected only tiny amounts of the volcanic gases that new magma produces, but scientists said the gases could have been sealed inside the system or have been dissolved by water on the mountain. The volcano holds a 600-foot-deep glacier and has received several inches of rain recently.
Melting of the glacier could trigger debris flows down onto the barren pumice plain at the foot of the mountain, the USGS said, noting a "very low probability" that downstream communities would be affected.