Government scientists raised the alert level Saturday for Mount St. Helens after its second steam eruption in two days was followed by a powerful tremor. They said the next blast was imminent or in progress, and could threaten life and property in the remote area near the volcano.
Hundreds of visitors at the building closest to the volcano - Johnston Ridge Observatory five miles away - were asked to leave. They went quickly to their cars and drove away, with some relocating several miles north to Coldwater Ridge Visitors Center.
The volcano alert of Mount St. Helens 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 from the observatory.
The growing consensus among scientists is that new magma is probably entering the volcano's upper levels, possibly bringing with it volatile gases that could lead to eruptions, said Bill Steele at the University of Washington's seismic laboratory in Seattle.
"We're in an eruptive period where there's a potential hazard," Steele said.
Scientists said that although the risks were growing larger, they did not expect anything approaching the volcano's devastating May 18, 1980, eruption, which killed 57 people and coated much of the Northwest with ash.
They said the evacuation of the observatory was primarily a precaution in case of heavy ash discharge, which could make it difficult to drive.
"We still feel the risk is confined to this area," Pierson said.
A day after
The signal "was far stronger after today's steam eruption" than the tremor that followed Friday's blast, Steele said. "We were picking it up throughout western Washington and into central Oregon. Yesterday we had a very weak tremor signal."
Also, earthquakes continued Saturday during the tremor.
A tremor - a steady flow - indicates movement of gases or fluid within the volcano," Steele said, while individual earthquakes indicate "a pounding and breaking of rock."
More steam explosions are likely, and possibly an extrusion of lava.
"This is the most intense seismic activity we've seen since the May 18th eruption," said geologist Dan Dzurisin at the U.S. Geological Survey's Cascades Volcanic Observatory in Vancouver, Wash., about 50 miles south.
The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forestland and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep.
The intensity "probably just reflects the fact that more rock needs to be broken for magma to reach the surface," Dzurisin said. The 1980 eruption reamed open the route to the surface, and for six years smaller eruptions piled lava into a dome that is now 1,000 feet tall and marks the main conduit for magma.
Friday's relatively small eruption, which generated a plume of ash and smoke 16,000 feet high, was the first since a 1986 dome-building event at the volcano.
Scientists believe the flurry of shallow earthquakes that began Sept. 23 may reflect movement of magma that came up the volcano's pipe during a 1998 swarm of quakes.
Air sampling has detected only tiny amounts of volcanic gases, which could mean the activity only involves the 1998 magma, which has been "degassed" over time - or that there is fresh magma but the gases are sealed inside the system, Dzurisin said.
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 closest structure is the observatory, five miles away.