An end to the plume theory?

Standard fare in geology textbooks and school classrooms across the world is that the hot springs, geysers and volcanoes of Yellowstone National Park, Hawaii, Iceland, and many other volcanic regions were “created” by plumes of hot rock that rise from near the earth’s core. However, new results from recently published U.S. Geological Survey (USGS) research hint that such plumes may not exist at all.

Results from seismic tomography, a method that uses earthquake waves to “CAT-scan” the earth’s secretive goings-on, suggest that the magma system beneath Yellowstone is only skin-deep — shallower than 190 km, far less than the 2,800 km scientists would expect if the magma arose from near the earth’s molten core, as has been thought for decades. USGS scientists Robert Christiansen and Gillian Foulger have been researching what are purported to be mantle plumes beneath Yellowstone and Iceland, and their findings were published in the October edition of Bulletin of the Geological Society of America. In early December 2002, the scientists preseented their findings at the American Geophysical Union’s meeting in San Francisco, Calif.

“The seismic results don’t come as a surprise to me,” says Christiansen, a research geologist based in Menlo Park, Calif. “Those of us familiar with the geology of the Yellowstone region began to realize that although the plume model was at odds with many geological observations, there was nevertheless a steady stream of papers that speculatively attributed everything there as evidence for a plume. The new tomographic results are consistent with the geologic data in demonstrating the absence of a deep-seated mantle plume.”

The Yellowstone results have implications that range much farther than just the local geology of America’s first National Park. A progressively older trail of volcanic rock stretches westward across Wyoming, Idaho and Nevada for more than 320 km from Yellowstone, following the eastern Snake River Plain. Geologists have interpreted this “trail” as the “geologic remains” of past volcanism left behind as the North American plate slowly drifted over the fixed Yellowstone plume.

Similar trails elsewhere also have been attributed to mantle plumes. In particular, noted Christiansen, the spectacular chain of volcanic islands that stretches from the Big Island of Hawaii, northwest along the Hawaiian island chain, is commonly considered compelling evidence for a plume and is the “textbook example” of how mantle plumes “work” to create volcanic islands and other geologic features above the earth’s surface.

“If Yellowstone can leave a volcano trail without a plume, then other hot spots might also,” Christiansen said. “The implication is that Hawaii may not be underlain by a plume after all.”

As well, seismic tomography has been conducted at Iceland, also traditionally considered to be a landform created by a mantle plume. Geophysicist Gillian Foulger, working at the USGS while visiting from the University of Durham, U.K., is critically reappraising the evidence for a plume at Iceland.

“When I was a student, I was never introduced to the idea that plumes could just be a theory,” Foulger says. “When our seismic results from Iceland simply didn’t fit with that model, I was completely perplexed. I soon found that I was not alone in doubting the plume model.”

The results from Yellowstone and Iceland are drawing attention in the geological community. A growing number of earth scientists are thus taking a second look at their own data, and starting to cast around for alternatives to one of their favourite ideas.

— The preceding is from an information bulletin published by the U.S. Geological Survey.