Mount St. Helens could erupt within days, government scientists said on Wednesday, raising the alert after movement in the volcano's lava crust was detected following a week of small earthquakes.
The volcano last erupted in 1980, killing 57 people and destroying more than 200 homes. Acres of evergreen spruce forest were flattened and ash billowed across North America.
"We think that the likelihood of an eruption has increased," Cynthia Gardner, seismologist for the U.S. Geological Survey told reporters. The USGS raised the alert level for the volcano to its third highest level of a possible explosion. Four indicates an imminent eruption.
"What we're still anticipating is a small to moderate explosion," Gardner said, saying that even a small explosion could send ash into the air that could travel "tens of miles" and obstruct air traffic.
Mount St. Helens is in the southeastern part of Washington state, about 100 miles south of Seattle and 50 miles north of Oregon's largest city, Portland.
Seismologists also said there was a "reasonable probability" that an eruption would not take place, although the U.S. Forest service has closed off the mountain to hikers.
Since last Thursday a swarm of shallow earthquakes, likely caused by hot rock coming into contact with underground water, may have triggered more significant seismic activity involving magma, or molten rock, said Steve Malone, director of the University of Washington's seismology lab.
In the last 24 hours, the lava dome that formed after the previous eruption has moved 1.5 inches, seismologists said, indicating that magma could be moving below the volcano. They added, however, that they had not detected gases indicating heightened magma activity.
A small-scale explosion of the lava dome would most likely spew rocks and muddy debris to the rims of the volcano's crater and down the sides of the mountain.
Seismologists said there was no connection between activity on Mount St. Helens and Tuesday's strong earthquake near Parkfield, California or a smaller series of quakes in Alaska.
The violent eruption of Mount St. Helens on May 18, 1980, blew off the top of the volcano, reducing its summit from 9,677 feet to 8,364 feet.
Since then the lava dome erupted once in October 1986, and strong earthquakes were detected in 1989, when fresh magma entered the volcano's lava system.
Mount St. Helens is considered a stratovolcano, formed by fast-cooling layers of lava that gives it a conical shape. Mt. Rainier in Washington state, Mt. Shasta in California, Mt. Vesuvius in Italy and Mt. Fuji in Japan are all considered typical stratovolcanos