Emerging dome evolving on St. Helens

MOUNT ST. HELENS, Wash. – Surging seismic activity at Mount St. Helens has changed the whale-back shape of the volcano’s emerging new dome, leaving it more like the back of a stegosaurus – a dinosaur with bony plates of armor along its spine.

The outer edge of the jagged, plate-like protrusions matches the smooth old whale-back profile, so it appears the new look reflects a sinking away of some segments of the dome, said David Sherrod at the U.S. Geological Survey’s Cascades Volcano Observatory, 50 miles away in Vancouver, Wash.

New snow blanketed the simmering volcano Monday as scientists logged a scaling back of seismic movement over the past week or so after intensified activity in February and March.

The mountain has been shuddering with quakes since October, when 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.

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

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.

Now, after weeks of increased seismic activity with quakes as large as magnitude 3.4, the mountain is settling back down to a drumbeat of temblors magnitude 1 or smaller – too small to be felt – every minute or two, Sherrod said.

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 cleared last week, “Viola! The very smooth elongated whale back … is getting all busted up, with big longitudinal cracks, rockfalls, with areas of broken rock all over the flanks,” Sherrod said.

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. Now, it appears the dome is spreading out or “pancaking.”

“It’s starting to sag a little bit” on the sides, Sherrod said.

It appears the jutting slabs “are parts that aren’t dropping yet,” he said. The outer edge of the dome’s formerly smooth profile “is now preserved only as those jagged peaks.”

The bulk of the dome has dropped by 30 to 90 feet except for these high-standing remnants, said a USGS news release.

It appears magma is still being extruded from the volcano’s core – temperatures in cracks on the dome surface range from 1100-1300 degrees Fahrenheit, reflecting the core heat of about 1650 degrees, Sherrod said.

“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.

Scientists observe the mountain with instruments and via helicopter.

Action inside the crater keeps destroying instruments placed on the new dome, so it’s not clear how much movement has occurred there.

On Sunday, a helicopter retrieved a couple of non-functioning instruments – Global Positioning System and seismic monitors – from the crater. Meanwhile, a GPS receiver about 500 feet north of the new dome continues to creep north-northwest at a rate of about 4 inches a day, suggesting the magma is budging rocks around the dome.