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Environment

The Riddles of Middle Earth

Charting seismic waves to map what's under the crust.

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In many ways, scientists know less about the interior of Earth than they do about distant stars. Seismologist Raffaella Montelli of Princeton University is trying to change that. She recently published striking computer-generated pictures that show trails of superhot rock shooting upward through Earth’s mantle—the vast expanse between the core and the crust—to create persistent “hot spot” volcanoes such as those in Hawaii and Iceland. This picture could finally explain the root cause of some of the world’s most intense volcanic regions. But in the uncertain and contentious world of geoscience, some of Montelli’s colleagues suspect she may be chasing shadows.

To produce her inner view, Montelli and her colleagues mapped the arrival of earthquake waves at seismic stations around the world. The waves slow down in relatively warm spots and speed up in cool ones. By charting when and where the various seismic waves arrived and plugging the data into a complex computer program, the researchers pinpointed 32 plumes, giant blobs of hot rock rising through the mantle. They then created three-dimensional renderings showing the paths of those blobs. “We isolated two families of volcanic hot spots, one with plumes originating at the base of the mantle and one with plumes roughly a quarter of the way down,” Montelli says.

Harvard University geophysicist Adam Dziewonski is unconvinced. “Most people who know what’s going on are skeptical about these results,” he says. He worries that Montelli’s team looked at only one type of seismic wave and that their analysis relied on “a lot of assumptions.” Other scientists dispute the very existence of mantle plumes (naturally they have a Web site, www.mantleplumes.org). The Plume-Lithosphere Undersea Mantle Experiment, a separate project set for later this year around Hawaii, will provide detailed seismic readings that may settle, once and for all, how the Aloha State was created.

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