'Volcanic advisory' issued for Mount St. Helens
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 Signs Mount St. Helens will erupt soon.
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| MOUNT ST. HELENS |  Mount St. Helens erupted explosively on May 18,1980, blowing off more than 1,000 feet of the mountain's peak.
The eruption triggered the largest landslide in recorded history, as the blast melted snow on the mountain's peak and ash choked rivers.
The eruption launched ash 15 miles into the sky and flattened more than 150 square miles of forest.
Source: U.S. Geological Survey
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VANCOUVER, Washington (CNN) -- The U.S. Geological Survey issued a volcanic advisory Wednesday for Mount St. Helens with officials warning of a heightened possibility of a small to moderate eruption.
The Associated Press reported the flurry of earthquakes at the volcano intensified further Thursday.
There was no immediate timetable for when an eruption could occur.
Geologists at the Cascades Volcano Observatory said the area could remain under the volcanic advisory for days or even weeks.
"We think there is a heightened possibility that we could see an explosion," said Cynthia Gardner, acting scientist in charge at the observatory. "We are expecting that either nothing could happen or perhaps we could have an explosive event."
She said if a small to moderate eruption occurred, rocks could be hurled up to 3 miles from the volcanic dome and ash could spew up to 10,000 feet in the air and be carried about 10 miles downwind.
"The concern there is an ash and aviation hazard," she said, noting that volcanic ash can clog a plane's engine and cause other problems.
The U.S. Forest Service has closed access into the crater and access to areas above 4,800 feet around Mount St. Helens. Trails on the north side of the crater have been closed.
The advisory that was issued is the third of four levels -- with the fourth being eruption.
Late last week, seismologists began recording "swarms" of earthquake activity from the volcano -- and the activity has increased since then.
Seismologist Seth Moran said the quakes will occur in great frequency, then taper off before ramping up again. The strongest quakes so far have been around magnitude 2.5, recorded earlier Wednesday.
"It's hard to say where this is going to go," Moran said. "There's nothing like this that has happened in the last 18 years."
He said the seismic activity is similar to activity in 1985 and 1986 when there was a rapid activity of quakes before minor eruptions. In those cases, the quakes occurred consistently, without tapering off.
There were other minor eruptions from 1989 to 1991 that occurred with very little warning.
Scientists currently are monitoring the seismic activity, as well as volcanic gasses being emitted and the swelling and contracting of the volcanic dome.
Gardner said the volcano is emitting "very little gas" -- an indication that an eruption would be small to moderate.
"It's a puzzle, and it's keeping us on our toes," said U.S. Geological Survey geologist John Major. "It's showing signs of an eruption, but not to the size and magnitude of 1980."
With an earthquake measuring a magnitude of 5.1, Mount St. Helens erupted on May 18, 1980. The north face collapsed in a massive rock debris avalanche.
The nine-hour eruption blew over or killed nearly 230 square miles of forest and sent a mushroom cloud of ash thousands of feet into the sky.
That eruption killed 57 people.
Groups of very small, shallow earthquakes -- most under magnitude 2 and called "swarms" by seismologists -- are now taking place on the mountain, the scientists said.
Small quakes are not uncommon in the fall, as rainwater seeps into the ground and turns into steam when it reaches hot magma, scientists said. Magma is lava that has not yet escaped from the volcano.
If the steam has no outlet, they said, it can build until a small explosion occurs. But they can't rule out the involvement of something other than steam.
Bill Steele, with the University of Washington's seismology lab, said scientists are looking at seismological recordings from stations off the mountain to try to determine what the latest swarms might mean.
Mount St. Helens has been an active volcano for about 40,000 years with intermittent periods of dormancy.
The last 2,500 years, however, have seen shorter periods of dormancy and a change in the type of rocks expelled by the volcano.
CNN correspondent Kimberly Osias and meteorologist Orelon Sidney contributed to this report.
Copyright 2004 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.
Associated Press contributed to this report.
