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Jonathan Newton/The Washington Post via Getty Images
Yellowstone is one of the few volcanoes in the world that’s not located along a plate boundary, the seismically active zones where two tectonic plates meet. Like the volcanoes that formed the Hawaiian Islands, Yellowstone is situated over an intraplate hotspot, a seemingly random upwelling of magma from the simmering mantle below.
Back in the 1970s, geophysicist Jason Morgan — the father of plate tectonics — posited that hotspots were caused by rising plumes of superheated magma that originated deep in the outer core. But despite widespread acceptance of the deep plume theory, geoscientists have failed to produce seismic imaging of just such a mushroom-shaped plume extending 3,000 kilometers down to the core-mantle boundary.
Until now.
A Ph.D. student from the University of Texas used an innovative method for analyzing deep seismic waves from large earthquakes to reveal a cylindrical column of hotter-than-normal magma rising from deep below Mexico and surfacing right below the Yellowstone supervolcano, according to a paper published in Nature Geoscience.
Peter Nelson is a fourth-year doctoral student in the Jackson School of Geosciences at the University of Texas at Austin and co-author of the new paper, which provides the first convincing evidence of a deep-mantle plume beneath Yellowstone.
“Plumes are analogous to a lava lamp,” Nelson told Seeker. “As the denser material on the bottom heats up, it rises in these upwellings that have a mushroom head followed by a long tail.”
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The plumes that originate at the core-mantle boundary may have greater density or may simply be caused by thermal instability between the molten iron core and the cooler mantle material surrounding it. To capture an image of such a plume, geologists like Nelson use something called seismic tomography — the Earth’s equivalent of a CAT scan.
“When large earthquakes happen, they create waves that propagate through the whole Earth, and those can be sensed by seismometers,” explained Nelson. “What seismic tomography does is let you determine the travel times of these waves. Waves travel at different speeds through rock depending on its temperature and composition.”
Nelson knew he was looking for a magma plume that was hotter than the surrounding mantle and vertically oriented. So he chose a specific type of deep seismic wave called an SKS wave that’s only produced by the biggest earthquakes. These waves are powerful enough to originate on one side of the planet, bend through the Earth’s core, and trace a nearly vertical path toward the surface on a completely different side of the planet.
SKS waves travel slower when they pass through hotter material, but the time difference isn’t huge.
“It’s only 1 to 2 percent slower at max,” said Nelson. “These waves take over 10 minutes to travel a quarter of the way around the earth and we’re looking at a one-second time difference.”
To capture subtle timing shifts occurring thousands of kilometers below the surface, Nelson relied on data from a dense cluster of seismometers called the USArray. By carefully timing the travel speeds of SKS waves as they rose vertically underneath Yellowstone, Nelson hoped to snap an image of the mushrooming plume in action.
Incredibly, he got his shot.
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The SKS data paints a clear tomographic picture of a plume of hotter material rising from 3,000 kilometers below the Mexico-California border and swelling upward on a slight angle toward Yellowstone National Park in western Wyoming.
The big question is whether the confirmed existence of this deep plume of magma proves that Yellowstone is due for another supereruption that could reshape the landscape of North America.
Nelson said that while his research doesn’t say anything directly about the timing or danger of a Yellowstone supereruption, it does prove that the massive energy driving the volcano ultimately comes from the core of the Earth itself, and that the plume feeding the Yellowstone hotspot isn’t going away anytime soon.
“It’s been around for at least 17 million years,” he said, “and it’s going to be around much longer than that.”