May 19, 2006
By Larry O'Hanlon
Discovery News
Researchers have captured three-dimensional images of the gigantic plume of molten rock welling up from the depths of the Earth under Yellowstone National Park.
Photo: What Lies Beneath? — New research suggests cleaning up toxic air -- like this haze over the Swiss Alps -- could lead to local spikes in warming. (AP Photo/Kevork Djansezian)
Using seismic tomographic imaging, which pieces together data from scores of seismic stations around Yellowstone, geologists have created a 3-D image showing where the rock has melted to become magma. The magma reveals itself to seismologists because it slows down seismic waves from distant earthquakes that pass through it.
"This result is from a focused study deploying a dense array of 80 seismographs around Yellowstone," reports Robert Smith, a Yellowstone geologist at the University of Utah.
The seismographs were installed from 2000 to 2002, explicitly to look for such a plume under the park. The results were published in the April issue of the Journal of Geophysical Research.
The multi-million-year-old plume is the source of heat that caused a series of giant eruptions at Yellowstone, the last one 600,000 years ago. There is no evidence another eruption is on the way.
Oddly enough, the image shows the plume does not rise from the Earth’s core-mantle boundary at 1,700 miles (2,700 km) down.
Instead, the column of magma appears out of nowhere some 310 to 400 miles (500-650 km) down, under the Montana-Idaho border northwest of Yellowstone National Park. From there it tilts to the southeast as it rises through the mantle until it’s directly under Yellowstone.
Graphic: Map of Yellowstone Region. The Yellowstone/Eastern Snake River Plain volcanic system, showing earthquake epicenters with black circles. Massive plumes of molten rock lie beneath the surface. (Robert Smith)
Smith explains the tilt is probably caused by a "wind" or current in the mantle through which the plume is rising. But just what melted the rock 310 to 400 miles down remains a big unknown, said geologist Eugene Humphreys of the University of Oregon.
"It’s tilted, which is interesting," said Humphreys. But how it got that hot, he said, "is still a mystery."
A clue might be found to the northwest, a region with 40,000 cubic miles of volcanic Columbia River Flood Basalts. That rock poured out of the earth to cover parts of Idaho, Oregon and Washington at the same time the Yellowstone plume was melting the crust and making a series of earlier "Yellowstones" in a line to the southwest from today’s Yellowstone, all the way to Nevada.
Because the North American plate is still moving southwest at Yellowstone and the plume shows no sign of abating, the next place to erupt would most likely be to the north of the park.
In fact, some researchers have already found evidence that the switch is underway – although no mega-eruptions are due anytime within the next million years.
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