Saturday, January 07, 2006
Mount St.Helen is still active
With the sticky molten rock comes a steady drumfire of small earthquakes.
The movement of lava up through the Southwest Washington volcano is "like a sticky piston trying to rise in a rusty cylinder," U.S. Geological Survey geologist Dave Sherrod said last week in a telephone interview from the agency's Cascades Volcano Observatory in Vancouver, Wash.
"These quakes are very small — we think they're associated with that sticking and slipping as the ground is deformed and relaxes."
The mountain is about 50 miles north of Vancouver and 100 miles south of Seattle.
St. Helens' violent May 18, 1980, eruption blasted 3.7 billion cubic yards of ash and debris off the top of the mountain. A torrent of scalding mud poured down the north fork of the Toutle River.
Fifty-seven people died in the blast, which left a gaping crater in place of the perfect, snowclad cone that had marked the original 9,677-foot peak known as "America's Mount Fuji."
St. Helens — now 8,325 feet — rumbled for six more years, extruding 97 million cubic yards of lava onto the crater floor in a series of 22 eruptions that built a 876-foot dome. The volcano fell silent in 1986.
Then in September 2004, the drumfire of low-level quakes began — occasionally spiking above magnitude 3, but generally ranging between magnitude 1 and 2. In the past 15 months, the mountain has squeezed out about 102 million cubic yards of lava.
Winter weather has prevented aerial monitoring of the crater since Oct. 24, "but we know what the rate has been. It's been relentless," Sherrod said, noting geologists also can rely on a network of remote monitoring equipment to tell them what's happening.
"One view of this eruption is that we're at the end of the eruption that began in 1980," he added. "If it hadn't been so cataclysmic ... it might instead have gone through 30 or 40 years of domebuilding and small explosions."
All the recent activity has remained within the crater, though scientists — keenly aware of the potential damage that silica-laced ash can pose to jet engines — monitor St. Helens closely for plumes of smoke and ash that can go as high as 30,000 feet.
"We haven't had that kind of plume since March 8, which is either a blessing or it leads us into complacency," Sherrod said, adding quickly, "We avoid complacency."
"This dome collapses and grows and collapses and grows. It changes its location. ... It can't seem to maintain its height at much more than it is now [about 1,300 feet]. Then it kind of shoves the sandpile aside and starts over."
It's not entirely clear where the lava is coming from. If it were being generated by the mountain, scientists would expect to see changes in the mountain's shape, with its sides compressing as lava is spewed out.
At the current rate of extrusion, "three or four months would have been enough time to exhaust what was standing in the conduit. ... The volume is greater than anything that could be standing in a narrow 3-mile pipe," Sherrod said.
That suggests resupply from greater depths, which normally would generate certain gases and deep earthquakes. Neither is being detected.
"That's one of the head-scratchers, I guess," Sherrod said.
St. Helens' unremitting, months-long pace is not common, he said. "It's not a characteristic feature of volcanism."
The mountain is the youngest and most restless of the Cascade volcanoes.
"Most of what we see today is 4,000 years old," Sherrod said. By comparison, Mount Hood is 30,000 to 50,000 years old. Parts of Mount Rainier date 200,000 years.