New experiments and computer simulations show that gigantic eruptions like those which blasted open the Yellowstone caldera are caused by vast pools of hot magma so buoyant they press incessantly on the rocks above until they break through with incredible eruptive force.
The new research, presented by two teams in two papers in the Jan. 5 issue of Nature Geoscience, suggests the magma's buoyancy triggers these rare super eruptions rather than some local trigger like an earthquake or an injection of more magma into the magma chamber from below.
The supervolcanic trigger appears to be markedly different from how smaller, more frequently erupting volcanoes blow their tops.
Volcanoes like Stromboli and Mount St. Helens have much smaller magma sources and are thought to erupt when, for instance, a quake shakes the magma up like a bottle of soda.
The eruption releases gases that dramatically increase the pressure -- or the shaking causes a large landslide that essentially uncaps the mountain that's holding the magma in, as happened at St. Helens in 1980.
“In general there may be some local triggers (for supervolcanic eruptions), but the magma chamber must be in critical condition,” said Luca Caricchi of the University of Bristol and University of Geneva. It's the buoyancy which makes it critical.
Caracchi, the lead author of one of the papers, compares the way magma buoys up through the crust to how a balloon buoys up in water -- so powerfully that it's difficult to keep it just under the surface. “It's got to come up,” he said.
There is also another factor which helps enlarge the magma chamber itself, at least for Yellowstone, according to the University of Utah's Robert B. Smith, chief seismologist for the Yellowstone Volcano Observatory.
“In Yellowstone we have made a case that (the magma chamber is) so big because it's in an area of lithospheric extension,” Smith said. This means the crust of the Earth there is being pulled apart.
“It allows magma to ascend much more easily," Smith said. "It allows it to rise and enlarge.”
In fact, GPS stations used to monitor Yellowstone show that it's stretching apart at a rate of 3.5 to 4 millimeters per year, Smith said. That's 3.5 to 4 meters per thousand years or 3.5 to 4 kilometers over a million years.
The last Yellowstone eruptions were 160,000 to 70,000 years ago. That was preceded by eruptions at 2.1 million, 1.3 million and 640,000 years ago. Understanding exactly what sets off these giant eruptions -- none have occurred in human history -- is critical for forecasting them, Smith said.
Read more at Discovery News
No comments:
Post a Comment