Mount St. Helens finally erupts, but without major disruptions Scientists at odds over prospect of more explosions

10/1/2004

'A SEISMIC BUrP'

MOUNT ST. HELENS, Wash. - After eight days of seismic rumbling, Mount St. Helens yesterday erupted in a giant belch of steam and ash that rose 16,000 feet into the air before dissipating into a hazy cloud, delivering as promised for curious scientists and eager tourists.

The event thrilled hundreds of tourists at the Johnston Ridge Observatory, five miles north of the crater, but did little to disrupt life in southwest Washington. Airplanes flew around the plume, cars whizzed down I-5, and locals went about their business.

Officials kept the alert level at "volcano advisory," the second of a three-step scale, with the third level indicating a major volcanic event.

Seismologists groped for words to describe the event - calling it a hydro-thermal explosion or a seismic burp - because it didn't fit the definition of a classic eruption involving lava and mudflows.

The eruption was nothing compared to what happened 24 years ago, when 57 people were killed and towns up to 250 miles away were showered with rock and ash. The 1980 eruption leveled 150 square miles of forest, and sent ash around the globe. The explosion blew off the top 1,300 feet of the peak, reducing the mountain to its current height of 8,364 feet.

Yesterday's eruption may not be all the volcano has to offer, however. A few hours later, small earthquakes started again at a rate of about one every 4 minutes. Within an hour they hit a one-per-min-

ute pace, said Bill Steele at the University of Washington seismic laboratory.

A couple were larger, exceeding magnitude 2.

He said there are likely to be a few more steam explosions "until enough debris is cleared, and then there is a significant chance that lava could be extruded at the surface."

At 12:03 p.m. yesterday, without detectable noise or tremor, a small puff of steam that looked like a cotton ball emerged from a spot inside the crater.

The puff grew in slow motion into a column, and continued to grow for another 20 minutes. The wind blew the column toward Portland, Ore., 50 miles southwest, but within an hour, the column was gone and scientists doubted any ash would reach the city.

As the cloud cleared, scientists in helicopters were able to photograph the "source hole" where the steam punched through.

The hole, about 100 feet in diameter, was on the southeast edge of a 900-foot-tall lava dome inside the crater.

The event culminated more than a week of small but increasingly intense earthquakes that originated deep inside the crater.

The largest quakes occurred Thursday and yesterday morning, reaching a magnitude 3.3 on the Richter scale.

Scientists differed on what might happen next.

University of Washington seismologist Tony Qamar said that even if small quakes continue, it's possible the most dramatic part of this episode might be over because most of the pressure has been released.

But Jon Major, a geologist with the U.S. Geological Survey, which runs the observatory, said, "My guess is this is the opening salvo. We'll probably see more like this."

The prospect of more explosions likely means a continuing flow of tourists to the mountain, which sits in the middle of the Gifford Pinchot National Forest.

This report includes information from the Associated Press.