MOUNT ST. HELENS, Wash. - A small explosion of rocks, ash and steam could occur within the next few days within the crater of Mount St. Helens, where earthquake activity has been steadily building for nearly a week, scientists said Tuesday.
"It could certainly happen today; it might not happen for weeks or months," said seismologist Seth Moran of the U.S. Geological Survey's Cascade Volcano Observatory.
Scientists were keeping a close eye on the 925-foot-tall dome of hardened lava that has grown inside the crater since the May 18, 1980, eruption that blew the top off the mountain.
Swarms of tiny earthquakes - more than 1,000 since the mountain began stirring on Thursday - have gradually increased, cranking up to a level not seen since 1986, when the volcano's last dome-building eruption occurred.
Tuesday, the quakes were occurring at a rate of two or three a minute. The volcano was releasing three to four times the energy it was releasing Monday and "yesterday was hopping," said Jeff Wynn, chief scientist at the volcano observatory in Vancouver, Wash., about 50 miles south of the 8,364-foot mountain.
Moran said that in an eruption, rocks two or three feet in diameter could break off from the lava dome and possibly be tossed as far as the rim. But, he and other scientists emphasized, that's not unusual at Mount St. Helens.
Volcanologist Carl Thornber said scientists are baffled by the activity, unusual because the earthquakes are so shallow.
"Everything right now is within the dome or within a kilometer," Thornber said at the Johnston Ridge Observatory at the base of the mountain, where surrounding hillsides are still covered with trees toppled by the 1980 blast.
"Where's the energy to keep driving this system?" he said.
Largely unheralded steam explosions in 1989, 1990 and 1991 all broke pieces of lava off the dome, Moran said.
The likelihood of a significant eruption "is fairly small," Moran said. "There's a range of possibilities still for where this may go. It might go away and nothing happens. That becomes less likely as this continues to increase. At the other end, we could have a reactivation of the lava dome-building sequences."
Scientists are "not sure where this is going and it's really hard to communicate this succinctly," he said.
Seismologist George Thomas at the University of Washington said that on a scale of zero to 10, with 10 being the explosion at the mountain in 1980, the current activity would rate a one. Thomas said any rocks, ash or steam coming out of the volcano would most likely be contained within the crater itself.
"The alerts we're sending out are just to protect hikers and scientists doing research within the crater," he said.
Scientists are trying to determine if the quakes are caused by steam resulting from water seeping into the dome or more seriously, by magma moving beneath the crater.
Early tests of gas samples collected above the volcano by helicopter Monday did not show unusually high levels of carbon dioxide or sulfur.
"This tells us that we are probably not yet seeing magma moving up in the system," Wynn said.
Scientists were setting up new Global Positioning Satellite stations all over the mountain to track its movement, adding about six to the dozen or so already in place. Using GPS data, scientists can detect tiny movements by the Earth.
Observatory scientists are working 12-hour days, Wynn said. "This is just a hoot to them. This is what they live and breathe and now it's walking and talking to them."
The USGS issued a notice of volcanic unrest on Sunday, citing "increased likelihood of a hazardous event." U.S. Forest Service officials closed hiking trails above the tree line at 4,800 feet. The visitors center and most other trails at the Mount St. Helens National Monument remained open.
Mount St. Helens roared to life in 1980, when the massive explosion and landslide obliterated the top 1,300 feet of the volcano. The blast and subsequent mudflows killed 57 people, leveled hundreds of square miles of forests, spewed mud and debris for miles and spread volcanic ash across much of the Northwest.
In October 1980, five months after the volcano's devastating eruption, the lava dome began building in the crater. The last dome-building eruption came six years later, though steam explosions have periodically rocked the dome.
Earthquake swarms in 1998 and 2001 did not result in any surface activity.
On the Net
http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/
http://www.pnsn.org/HELENS/welcome.html