/ Mount St. Helens 25th Anniversary

03:20 PM PDT on Thursday, May 12, 2005

From KING5.com Staff and Wire Reports

MOUNT ST. HELENS, Wash. - After 18 years of relative quiet, Mount St. Helens began shuddering again in October of last year.

Magma deep below the surface began moving up the throat of the volcano, spewing molten rock into the crater and rebuilding the volcano's dome - the plug in the throat that eventually becomes the mountain peak.

Most of the action has been inside the crater, with occasional plumes of gritty ash posing concerns to air traffic.

By March, a mammoth, smooth whaleback spine of new rock had formed inside the dome by, but surging seismic activity later that month took it's toll, leaving it more like the back of a stegosaurus, with bony plates.

Right now, it's being turned into just a rock pile, according to the USGS' Cascade Volcano Observatory spokeswoman Carolyn Driedger.

"That has been the principal activity at the dome for the past two months," she said.

Mount St. Helens erupted with devastating violence 25 years ago, on May 19, 1980, blasting open its once-perfect peak, leveling forests for miles and killing 57 people. It rumbled sporadically for several years afterward and then subsided, reawakening last fall.

USGS

Mount St. Helens' new dome from the northeast.

Of late, the mountain has settled back down to a steady drumbeat of temblors -- too small to be felt -- after weeks of increased seismic activity at the beginning April with quakes as large as magnitude 3.4.

The ups and downs have been somewhat cyclical since the volcano reawakened in October, with increases in December and again earlier this year.

But for much of this winter, clouds hid the crater from view and scientists weren't sure what was happening inside.

When the weather finally cleared in April, "Voila! The very smooth elongated whale back ... was getting all busted up, with big longitudinal cracks, rockfalls, with areas of broken rock all over the flanks," said David Sherrod at the U.S. Geological Survey's Cascades Volcano Observatory.

The former shape of the dome - like the underside of an aircraft carrier or perhaps a huge loaf of French bread, Sherrod said - was somewhat unusual.

"We don't have a lot of experience with a long, linear dome like this," he said.

It appears magma is still being extruded from the volcano's core - temperatures in cracks on the dome surface range from 1,100 to 1,300 degrees Fahrenheit, reflecting the core heat of about 1,650 degrees, Sherrod said in April.

"We know it's very hot as it comes out, which means the core is plastic," he said. "It now seems to be oozing onto the glacier" - which has emerged since the 1980 eruption, draped around the neck of the old dome like a collar.

The seismic activity also suggests continued magma movement, Sherrod said

The dome's new profile is visible from the Coldwater Canyon Visitor Center, about 8.5 miles from the volcano in the Mount St. Helens National Volcanic Monument.