Explosion possible at St. Helens

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.

On Tuesday morning, 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.

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 1. 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.