New Yellowstone supervolcano study stirs debate about its deep past

Ying Zhou teaches a Geology 101 class to students at Virginia Tech, but after peering deeply into the subterranean world beneath the Yellowstone volcano, Zhou says she needs to modify her lesson plans — for Yellowstone, anyhow. 

Yellowstone is one of the most heavily-researched volcanoes in the world, and it’s believed to be responsible for few mega-eruptions in the last two million years. Volcanologists understand its behavior quite well, and there’s agreement that any sort of eruption would be likely be proceeded by months or even years of warning. 

But scientists still aren’t sure exactly how the volcano came to be, though many suspect its hundreds of geysers and steaming springs are fueled by a great plume of hot rock emanating from the deep Earth, which rises up to a surface “hotspot.”

Yet Zhou, a theoretical seismologist, has found otherwise.

In a study last week in the journal Nature Geoscience, Zhou describes how she used seismic measurements from earthquakes around North American and beyond to build a visual representation of an inaccessible, mysterious place: the world deep beneath Yellowstone. And what Zhou found, was unexpected.

Yellowstone National Park’s Great Fountain Geyser.

“I was puzzled,” said Zhou in an interview.

She thought she might see the great plume of intensely hot rock, similar to the plume feeding Hawaii’s Kilauea volcano. 

“But we saw something different,” said Zhou. 

She found a massive slab of ancient Earth crust, some 250 to 270 miles beneath the ground. 

It had broken into giant pieces, but one slab dropped deep beneath Yellowstone, digging into the deep Earth and apparently triggering a welling up of the Earth’s mantle — the hot rock beneath the crust.

There was no clear sign of a single, large plume feeding Yellowstone, and it’s famous national park.

“The argument has been going on for a long time,” John Wolff, a volcanologist at Washington State University who had no role in the research, said in an interview. “While its true that the plume hypothesis tends to dominate, there’s always been this alternative view.” 

“I’m really on the fence,” Wolff added.

But Zhou isn’t.

“If there is a plume, it would explain things nicely,” said Zhou. “But here we don’t see the plume — so in that sense, the plume theory is open to challenge.”

The triangles show a track of volcanoes formed in the last 16.5 million years, with ages shown in millions of years. Yellowstone is the youngest.

In fact, no one has ever actually seen the plume, nor the chunk of Earth’s crust Zhou thinks is there. 

Earth’s deep underworlds are mostly distant and inaccessible. So what any geologist knows is largely based upon studying rocks on the surface and detecting seismic activity beneath the ground to build a picture of what might be transpiring below. 

That’s why the science isn’t yet settled. 

“Both theories involve explaining a feature that formed over the past 15 to 20 million years ago when there was nobody around to take notes or images of what was going on,” Stanley Mertzman, a geoscientist at Franklin and Marshall College who had no involvement in the research, said over email. 

“So scientists are doing their level best to explain this feature based on rather indirect evidence.”

Mertzman speculated that this could potentially be “the opening salvo in a debate” between Zhou’s recent findings and researchers persuaded by the hotspot theory, which has decades of evidence behind it.

“This paper is not going to end the argument,” said Wolff. 

An illustration of a plume (red) feeding Yellowstone’s volcanic activity.

Stephen Grand, a University of Texas at Austin geologist who specializes in studying Earth’s mantle, recently added even more evidence to the hotspot theory. 

In research also published in Nature Geoscience back in March, Grand concluded that a “single narrow, cylindrically shaped” structure some 250 miles diameter — which is the great plume feeding the volcano — sits beneath Yellowstone.

Like Zhou — and every other geologist — Grand and his team have no hope of digging a tunnel to depths beneath Yellowstone, so they used a method similar to Zhou’s to measure how seismic waves moved through the ground. They found that in this peculiar, cylinder-like area, seismic waves traveling through the Earth move notably slower than the rock around it. 

“It’s like doing a CT scan of peoples’ heads,” said Grand in an interview, referencing the medical imaging that reveals different internal structures by analyzing how X-ray waves move through tissue, like the brain.

And Grand’s scans of the deep Earth support a plume of hot rock “extending from the core of the Earth all the way up to Yellowstone,” he said.

But Zhou’s technique — which she described as “putting special glasses on the seismometers so you can see the detail better” — found a subterranean structure, seemingly like a chunk of Earth’s crust, that extended all the way from Yellowstone to Oregon. 

Yellowstone’s Lone Star Geyser erupting.

“If it was a plume, you would just see an unusual structure just right under Yellowstone, not all the volcanoes extending to Oregon,” she said, referencing a curving line of volcanoes reaching southwest from Yellowstone.

This healthy scientific debate will only continue, as Mertzman noted scientists are trying to make sense  of a “long and complicated” geologic history. 

“Zhou’s work is really the tip of the iceberg,” he said. 

Though, a passageway leading 400 miles beneath the surface — if it were ever to exist — would certainly clear up a lot of debate, mused Mertzman.

“What does it really look like in the walls of that descending passageway through rock that have been pushed and pulled through a very active 500 million years of Earth history?” he wonders.