Y’stone magma plume bores deep to Earth’s core

March 28, 2018 GMT

Scientists have mapped the precise route of the magma plume feeding the famed and sometimes-feared Yellowstone hotspot from the surface of the Earth all the way to its outer core.

The source of heat slowly swelling the Yellowstone Plateau is a 215-mile-wide cylindrical plume that originates 1,800 miles beneath the northern reaches of Baja California, according to new University of Texas at Austin research. The decades-in-the-making finding confirms geophysicists’ longtime suspicions and explains why a dormant supervolcano and geysers, hot springs, mud pots and fumaroles are located in northwest Wyoming.

“Why do we have the largest volcano?” asked Peter Nelson, a University of Texas geophysics PhD student. “That was a question people have been wondering about for 40, 50 years, and what we showed is it comes from pretty much all the way at the core — really deep in the Earth.”

Nelson, who wrote a paper with his advisor, Stephen Grand, published his findings in the latest edition of Nature Geoscience.

Until now researchers were able to trace the magma plume feeding the Yellowstone hotspot down to only about 600 or so miles underground. Nelson’s work built upon existing tomography techniques, mapping how seismic “S” waves from earthquakes pass through Earth’s mantle. When the waves reach a hotspot, like a magma plume, they slow down.

The find was made possible by analyzing data from 71 magnitude 5 or larger earthquakes that rumbled all around the world between 2005 and 2012. Those quakes were part of the “USAArray” dataset from the National Science Foundation’s Earthscope experiment, which swept a network of 400 seismometers across the continental United States.

“Before 2012 you just didn’t have the resolution to see down that deep,” Nelson said. “Before the USAArray, nobody had ever put that many seismometers at that dense of spacing or that large an area. It really has revolutionized our understanding of the Earth, at least in the North American continent.”

It was “tedious” work assessing minute differences in how waves traveled through the area under Yellowstone, Nelson said. Waves that would otherwise take about 15 minutes to navigate the mantle, he said, would get caught up for just a second or two.

“No one has ever looked and measured as many of these waves that travel to the core as we did,” Nelson said.

Moose resident Bob Smith, a University of Utah scientist behind much of the Yellowstone hotspot imaging to date, said Nelson and Grand’s research was to be commended.

“What they did was a great job,” Smith said. “That was the early theory way, way before us. ... And it confirms what we had originally hypothesized. We started doing this in ’85 with old methods, old computers and old seismographs.”

Over the last 30-some years Smith and his colleagues have mapped an upper crustal magma chamber nearest Yellowstone’s surface and a larger magma reservoir beneath it. The plume feeds both. The 2,050-mile-long plume dwarfs the other features.

Nelson leapt into his doctoral studies hoping to determine how deep the Yellowstone magma plume goes. The hypothesis was that it would be a more vertical structure.

“What we found was a plume-like structure, but it was tilted more than we expected,” Nelson said. “We didn’t expect it to be under the Mexico-California border. We might have expected it to be under Utah, but not for it to be that far south.”

Where it originates, at the core-mantle boundary, the plume is estimated to be about 1,100 to 1,500 degrees Fahrenheit warmer than the surrounding mantle. The structure is pulled to the surface by its buoyancy, and as it rises it loses its temperature, running only 750 degrees warmer than the mantle by the time it’s 620 miles away from the Earth’s surface. Its content is hot rock, Nelson said, not molten or liquid material.

Nelson’s research doesn’t turn anything that’s already known about the Yellowstone hotspot on its head. Yellowstone’s supervolcano, for example, isn’t any more hazardous now that it’s known there’s a direct tie to the Earth’s core.

“It doesn’t really mean anything for risk hazard, or volcanic cycles or eruption cycles,” Nelson said. “Really the question was, Where was the energy driving Yellowstone coming from?”