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Plume theory fight erupts:
Rumblings of dissent over tenet of geology are coming to a head

Alexandra Witze

The Dallas Morning News, 10th November, 2003

PLUMES UNDER FIRE

For the last 30 years, most earth scientists have accepted that “plumes” of hot rock (as shown in the diagram below, welling up from deep inside the Earth) were responsible for certain volcanoes. But now some geologists are challenging this notion, saying there is no good evidence for the existence of plumes. Instead, they advocate alternate explanations for how volcanoes form at “hotspots” – places like the Hawaiian Islands, Iceland and Yellowstone, where eruptions take place far from the boundaries of tectonic plates.

There's a battle taking place over science textbooks, but for once it doesn't involve evolution.

Rather, a few geologists are working to topple a long-established theory about how the Earth works. They say that mantle plumes – columns of hot rock that are thought to rise from deep inside the Earth, fueling volcanoes such as those in Hawaii – don't exist.

To many earth scientists, such a statement may qualify as near-heresy. Mantle plumes appear in nearly every introductory geology textbook, right next to the theory of plate tectonics from which they sprung. The man who first proposed them, W. Jason Morgan of Princeton University, even won the country's highest scientific honor, the National Medal of Science, last month for his work.

For decades, plumes were the favored way to explain “hotspot” volcanoes, which are fueled from below with a steady source of heat like a blowtorch. Hotspots aren't easily explained by plate tectonics, which describes how rigid plates of the Earth's crust grind against one another to create volcanoes, earthquakes and other geological phenomena.

Dr. Morgan came up with plumes as a way to explain volcanoes that erupt away from the boundaries of tectonic plates. The idea quickly caught on, and challenging the existence of plumes soon became unthinkable.

“It was a possibility that hadn't even crossed my mind before,” says Gillian Foulger, a seismologist at the University of Durham in England. “It's like questioning whether your parents are yours or not.”

But now she is heading the charge against plumes, inspired by new work that looked for but wasn't able to find convincing evidence of them. Longtime plume critic Don Anderson, of the California Institute of Technology, and many others have joined the fray.

“We're now looking at hundreds of people who think it's good science to question whether there's a plume there or something else that is causing their volcano,” says Dr. Foulger. She has set up a Web site (www. mantleplumes.org) and helped organize an anti-plume meeting in Iceland in August.

Yet plumes aren't about to go quietly. Pro-plumers have fired back in an online debate hosted by England's Geological Society (www.geolsoc.org.uk) and in their own meeting in Wales in September. Perhaps most significant, they say they have new evidence that shows at least a dozen deep mantle plumes in places such as Hawaii, Samoa and Tahiti.

For now, neither side can claim victory. But the debate is causing many geologists to rethink their ideas about the Earth's interior.

“Part of the source of the controversy is that 'plume' ... means something different to practically every earth scientist,” says Millard Coffin, a geologist at the University of Tokyo.

Ever since Dr. Morgan proposed plumes in 1971, researchers have disagreed on almost every aspect. They debated, for instance, how many plumes might exist (estimates have ranged from less than a dozen to several thousand) and how deep they might reach (anywhere from several hundred to several thousand miles into the planet). But they remained generally well-accepted – until now.

For Dr. Foulger, the plume idea began to falter in the late 1990s, when she and her colleagues went to look for seismic evidence of the plume that was thought to exist under Iceland.

If plumes exist, seismology would be expected to detect them because seismic waves travel more slowly through hot, less dense rock than they do through cooler, denser rock. By bouncing enough waves through the ground, scientists can build up a three-dimensional picture of the underlying rock and see whether there is a narrow plume where the waves travel slowly.

Depth doesn't jibe

The studies beneath Iceland revealed no deep plume, Dr. Foulger says. Whatever heat source fuels Iceland's volcanoes must be coming from a broad reservoir no more than 250 miles down. Plumes were traditionally thought to come from 1,800 miles deep, at the boundary between the Earth's core and mantle.

“This took me completely by surprise,” Dr. Foulger says. “I started telling people that the plume seems to be shallow. ... And it suddenly occurred to me that it's not a plume if it's shallow.”

Iceland isn't the only place where plume evidence has come up lacking. Similar work in and around Yellowstone National Park found no traces of a plume, even though the biggest hotspot in North America lies under the region.

But other seismologists say their research confirms the presence of plumes elsewhere. Raffaella Montelli of Princeton will present such pro-plume findings in December, at a meeting of the American Geophysical Union.

Dr. Montelli used a new technique to further clarify how seismic waves bounce around narrow objects such as plumes. The new analysis found evidence for about 30 plumes, including a dozen reaching deep to the core-mantle boundary, says team member Guust Nolet.

In his view, the work supports deep plumes. “We scientists are skeptical by nature, and it is not surprising that the plume hypothesis came under fire,” he wrote in an e-mail interview. “However, much of the skepticism is now obsolete.”

Obsolete or not, the skepticism remains. Dr. Foulger, for instance, says that the seismic data is too patchy to be able to confirm the Princeton observations as true plumes.

In the meantime, anti-plumers come up with a few alternative ideas to explain hotspot volcanism. One such notion is the “crack” theory, which originated in the 1840s as a way to explain chains of volcanic islands in the Pacific, says James Natland, a marine geologist at the University of Miami.

In this scenario, plates fracture, allowing a blob of material to rise upward, melt and fuel a volcano. The crack theory doesn't require a deep source of hot rock, says Dr. Natland - just shallow mantle rock, primed to melt if pressure is relieved from above.

Chemistry helps determine at what depth and pressure a piece of rock will melt. In order for crack theory to work, the mantle would have to be near its melting point at relatively low pressure.

New experiments support this idea, says Dean Presnall, a petrologist at the Carnegie Institution of Washington who recently retired from the University of Texas at Dallas.

His work, which simulates the high temperatures and pressures inside the Earth's mantle, suggests that rocks don't have to be that hot – and thus deep – in order to melt. In other words, given just a crack, the mantle could rise upward and melt due to decompression without relying on plumes from deep down.
“ I think plumes have been vastly overplayed,” says Dr. Presnall.

Perhaps the biggest unknown in the plumes debate is Hawaii, where the seismology data can be interpreted as either pro-plume or anti-plume. To settle the question, next year researchers will begin the Plume-Lithosphere Undersea Mantle Experiment, or PLUME. It will use 64 seismic instruments on the seafloor, and 10 on the surface, to take pictures of the upper mantle beneath the Hawaiian Islands.

The 15-month project may finally be able to determine whether Hawaii is in fact a plume or not. But no matter what its results are, Dr. Foulger says the debate has succeeded in getting scientists to think of plumes in a new way.

“If we want to hang on to our dear old friend plumes,” she says, “then we need to acknowledge that they're rather different than what we thought.”

E-mail: awitze@dallasnews.com

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