Newly published research contradicts what has been a long-held — although fervently debated — theory about the hotspot underlying the Yellowstone supervolcano.
Geologists at the University of Illinois used seismic waves that travel through the earth after earthquakes and explosions to produce an almost X-ray-like view of what’s going on underground. The information was then fed through a supercomputer to mimic different geologic scenarios that are known to have occurred over the past 20 million years in an attempt to come up with an explanation for the Yellowstone hotspot.
Their conclusion? Yellowstone’s heat is being funneled east from the geologically active Pacific Coast.
"A robust result from these models is that the heat source behind the extensive inland volcanism actually originated from the shallow oceanic mantle to the west of the Pacific Northwest coast," said Lijun Liu, a geology professor who led the research, in a statement. "This directly challenges the traditional view that most of the heat came from the plume below Yellowstone."
Not everyone agrees with Liu’s theory, though.
Similar seismic tomography research by the University of Utah, published in 2015 and which can be found on the Yellowstone Volcano Observatory’s website, came to a different conclusion. In that study scientists discovered one shallow magma reservoir under Yellowstone’s hotspot that is fed by a much deeper mantle plume.
“Mantle plume theory has its problems, but it makes more sense than any other theories, which is why it remains the accepted explanation for places like Hawaii and, yes, even Yellowstone,” said Michael Poland, the Yellowstone Volcano Observatory scientist-in-charge, in an email.
Poland added that Liu’s research is not “the final word on the story” and “is a work in progress.
“The chemistry is a major sticking point — the chemical signature of Yellowstone is very much reflective of a deep mantle source (a plume),” Poland said. “There are also other seismic tomography studies that do see a plume. So I think these researchers still have a way to go before they are able to demonstrate that their model satisfies more of the observations than other previously proposed models. It will be interesting to see how their research develops.”
Poland added that he hopes the paper prompts geologists who really know the rocks to talk with the computer modelers.
“It's when these disparate groups get together that some of the more exciting science happens,” he said. “Often the result is something that no one anticipated.”
Liu sees the issue differently.
“For Yellowstone, the plume is not a big deal at all,” he said in 2016 following the publication of earlier research.
That study concluded that the mantle plume powering Yellowstone’s supervolcano had been capped by a cold, sinking tectonic plate that had slid under the North American landmass millions of years ago, in effect putting a lid on the mantle plume.
"The heat needed to drive volcanism usually occurs in areas where tectonic plates meet and one slab of crust slides, or subducts, under another,” Liu said. “However, Yellowstone and other volcanic areas of the inland western U.S. are far away from the active plate boundaries along the West Coast."
Liu said his most recent research shows that the hot Pacific mantle “has been ‘pumped' eastward toward Yellowstone since 16 (million years ago) through tears within the down-going oceanic plate … this hot horizontal mantle flow should be the force that formed the hotspot track leading toward Yellowstone today.”
The mantle plume theory, advocated by other scientists, explains that the volcanic activity over the past 17 million years — which has stretched across the Snake River Plain in northern Nevada and southern Idaho to what is now Yellowstone — has been the result of the North American landmass slowing moving across this shallow hotspot. That region of the Earth’s crust was pulled and stretched until it was thinned and fractured, making it easier for the magma to push through and create a series of about 150 volcanic eruptions in the region.
In a 2009 study, Bob Smith, of the University of Utah, published research theorizing that the head of the original mantle plume rose to the east of the Juan de Fuca plate, which is being swallowed under the much larger North American plate.
Theories on theories
“Since 1971, the plume hypothesis, although never universally accepted, has become the most widely held explanation for so-called anomalous volcanism — the type that occurs far from plate boundaries, like in Hawaii and Yellowstone, or in excessive amounts along mid-ocean ridges, as in Iceland,” wrote Sara Platt in a 2015 Earth Magazine article, which does a good job of explaining the competing theories in laymen’s terms.
“If the vast body of mantle plume research has done nothing else, it has revealed the difficulties inherent in trying to plumb the depths of Earth’s interior,” Platt wrote. “Reaching to a depth of 1,800 miles, the mantle cannot be sampled by fieldwork; it must be remotely sensed and modeled.”
That may be so, but aspects of plume theory still do the best job of explaining Yellowstone’s complicated geology, Poland said.
“There is a segment of the geoscience community that thinks mantle plumes may not be real — that science is being twisted to fit that hypothesis, and that plumes are too widely used to explain anomalous volcanism” Poland said. “And there are a lot of things wrong with plume theory, although most of these things would probably qualify as ‘details’ to many in the field. Point being, the story has not been put to bed, although the evidence for some deep component to the melt feeding Yellowstone is pretty compelling.”
Platt’s article further revealed that some scientists refer to the mantle plume theory as “'zombie science’ — a hypothesis that, despite contradictory evidence and the lack of supporting evidence, will not die.’”
Poland said those voices tend to be from a small, yet vocal group, but that these contradictory ideas are needed to move the science forward, and keep scientists from being complacent.
“Controversy in science is a good thing. That’s when we learn.”