Thursday, May 19, 2005
Mount St.Helen is different and unique in the Cascade Range
Last fall, magma began rumbling up toward the blasted crater from a chamber 4 to 6 miles down. It's been welling up at a rate of as much as 10 cubic yards per second, though the current pace is about 3 cubic yards per second.
At that rate, 1,300 feet of mountain lost in the 1980 eruption could be replaced in a few decades, said Willie Scott, a scientist with the U.S. Geological Survey at the Cascades Volcano Laboratory in Vancouver, Wash."It's kind of great to have it happening nearby,'' Scott said. "We're kind of along for the ride.''Before 1980, St. Helens was promoted as "America's Mount Fuji'' and known for its conical symmetry — a perfection that sprang from its relative youth. It hadn't been through an Ice Age or subjected to millennia of erosion.
"The former summit of the volcano ... was a big lava dome that was sitting on the volcano like a scoop of ice cream,'' Scott said. It probably dated from the 16th century, when the peak was rebuilt after a period of explosive activity in the 1480s. "It was by far the youngest volcano in the Cascades.
''There'd been eruptions in the St. Helens area for hundreds of thousands of years, but for centuries they only produced small lava domes, Scott said.Volcanic eruptions don't always build a big central edifice, and St. Helens didn't start producing the lava flows that built the mountain until about 4,000 years ago, Scott said."It's done just about everything in those 4,000 years,'' Scott said. "Throughout its history, it's had this penchant for big explosive eruptions.
''The mountain emerged over a rift basin, "a crack through the crust,'' as geologist John Pallinger at the Cascade volcano lab put it.Far below the surface, movement in the St. Helens seismic zone — caused by shifting in the blocks that make up western Washington and Oregon — interacts with tectonic plates that form the earth's crust.To the north and west, the Juan de Fuca plate is diving under the continental shelf — creating enough friction and pressure to melt rock, said USGS scientist Craig Weaver at the University of Washington.
A big arc in the plate causes it to shove southeast under St. Helens while the fault line, trending north and west, is buffeted by action that starts with the Sierra Nevada plate to the south.Youth and an explosive temperament make St. Helens a special structure, Scott said."If we'd been around 12,000 years ago, we might think Mount Baker or Mount Adams were quite special.
That's when they were active,'' Scott said.The dome is a brittle stone skin over hot, fluid magma. If the lava extrusions continue, the dome will eventually fill the crater and magma pushing out to the surface will spill down the outer flanks of the mountain."It's not unlike piling gravel up. You get a nice cone with 25, 30-degree slopes. When the lava dome gets larger and material's cascading down all sides of it, it finally fills the crater rim and starts flowing down the flanks,'' Scott said.
"That's one way you create those smooth symmetrical cones.''But there's no way to tell what's going to happen next at the mountain, now the focus of a busy tourist season as a result of the anniversary and recent activity."Basically we have the same possible outcomes as from Day One,'' Scott said. "It could go away over the next six months — or it could go on for years, decades, centuries.''