Yellowstone gives geologists an opportunity to look inside the Earth at an active hot spot.
Taking advantage of the park's size and location on dry land, Smith and other geophysicists have built a network of sensors that pick up the seismic waves from earthquakes as they pass under and are altered by the structures beneath Yellowstone.
The seismic network has recently revealed a 3,600-cubic-mile banana-shaped body located a half-mile to several miles underneath part of Yellowstone. This chamber slows down seismic waves, which means it could contain 10 to 20 percent melted rock, Smith says.
And a broader seismic array has just recently revealed an even deeper feature: a tilted pipe rising up through the Earth from the northwest, from 400 miles down.
"It's this pipe that's bending over in the wind," says Ihinger. "That is at the heart of the matter." The "pipe" appears to be the track of a slow, viscous upwelling of hot rock from below the crust. The upwelling could cause rocks in the crust to melt, creating the magma chamber below Yellowstone.
But what causes the upwelling, and why is the pipe tilted under Yellowstone? There are currently two schools of thought, says Smith. One argues that something happens down at the boundary between the Earth's core and the mantle to create a narrow, upwelling plume of hot material.
"One of the main controversies is the plume idea," says geophysicist Eugene Humphreys of the University of Oregon. "In the last 10 years, it's been under quite a bit of attack."
The second school of thought says the upwelling is caused by spreading of the Earth's crust.
Scientists know that a good portion of the western United States is spreading wider, at a rate of at least a couple of centimeters every year. The Yellowstone Volcano Observatory's GPS network has revealed a spreading rate at the park that's even faster, says Smith.
So much extending of the land means something has got to give somewhere. The crust gets thinner and weaker in some places. When that happens, rocks deeper down get a load lifted off them and decompress — which is another way rocks can shift from solid to a liquid, magma phase, says Ihinger. "And when it becomes liquid, it has to come up," he says.
Most geologists studying Yellowstone aren't convinced either scenario alone describes what is happening under Yellowstone. It may be, for instance, that the hot spot is caused by melting of the long-lost Farallon Plate, which was driven under North America's western edge 80 million years ago. Perhaps that has something to do with the odd tilt to the "pipe" under Yellowstone, Ihinger suggests.
"Often times we're stuck in this complicated middle ground," said Humphreys. It's the nature of a science where it's hard enough to observe what's happening, much less conduct experiments. "In Earth science it's interesting that nature does the experiments."
And that's exactly what makes Yellowstone one of the best laboratories in the world.
Larry O'Hanlon is a science writer, geologist and former National Park Service ranger based in New Mexico. Besides being a long-time science correspondent for
Discovery News, Larry has reported on Earth science research for numerous outlets and organizations, including
Nature,
New Scientist and the Geological Society of America. He also currently serves as the science writer for the Explora Science Center in Albuquerque.
