Study finds plume extends far under park

By MIKE STARK
Billings Gazette

BILLINGS, Mont. -- The fiery engine that drives the spectacular geysers and other features at Yellowstone National Park extends at least 240 miles beneath the Earth's surface, according to a new study.

The findings lend weight to the argument that a plume of hot rock rising from deep within the Earth fuels Yellowstone's churning geothermal features.

"It's a step in that direction," said Greg Waite, a scientist with the U.S. Geological Survey who worked on the study.

The research, recently published in the Journal of Geophysical Research, indicates that the giant plume of magma beneath Yellowstone tilts to the northwest beneath Dillon, Mont.

The plume is not a textbook example that dives more than 1,000 miles into the Earth's core mantle, but appears to be situated more in the upper mantle.

"It's usually the case that the real world is more complicated," Waite said.

There has been intense debate about what created and drives volcanic areas including Yellowstone, Hawaii and Iceland.

In recent years, some have theorized that that the traditional idea of a deep plume of molten rock may not be the case beneath Yellowstone. Instead, they said, there may simply be a shallow skin of magma beneath the Earth's crust. As continental plates shift and stretch, the magma bubbles up to fill in the gaps, according to the theory.

The competing theories divided scientists.

Waite, along with Bob Smith of the University of Utah and Richard Allen of the University of California at Berkeley, decided to help inform the debate by collecting earthquake data at Yellowstone to try to delineate the boundaries of the magma beneath the park.

"We figured this was probably the best way to try to answer that question," Waite said.

The scientists set up about 80 seismographs around Yellowstone in late 1999 and 2000. For two years, the instruments tracked earthquake waves from around the world as they traveled beneath the surface.

Because waves change as they pass through hotter material, researchers are able to map what they think is happening below Yellowstone.

"There is a plume of material that goes down as least as deep as we can accurately see," Waite said.

The findings jibe with another study from the same time period by Ken Dueker of the University of Wyoming and a team of researchers.

It's possible that the plume extends farther than the seismographs could read, Waite said, but definitive proof would require additional study.

The latest findings should help further the discussion about the origin of hot spots around the world, Waite said. Yellowstone's hot spot is unique, though, because it's not surrounded by water like those in Iceland and Hawaii.

Even though that makes it easier to study, there are still enough lingering questions about Yellowstone's furnace -- including what might be happening in the upper portion -- to keep geologists busy for a long time.

"There are still lots of puzzles," Waite said. "This is really just one little piece."