Hawaii and Iceland's Deep Roots

The volcanoes of Hawaii and Iceland may be fueled by hot rock that has traveled thousands of kilometers through the Earth's mantle. The research, reported in tomorrow's Nature, may help to end a persistent debate about whether semimolten rock from just above the outer core can spark isolated eruptions at the surface, nearly 3000 kilometers away.

Most volcanoes spew magma that forms when crustal plates collide and melt rock that lies some tens of kilometers below the surface. But Hawaii, Iceland, and other "hot spots" are far from these collision zones. Geophysicists have suspected that the magma fueling those volcanoes bubbles up from deep within the planet, perhaps from the middle of the thick mantle or even deeper, just above Earth's swirling core of molten iron. Pinning down that location is a stiff challenge, however, because hot-spot plumes ascend in pipes that are too narrow for researchers to detect.

The two new studies favor the deep origins theory. A team led by seismologist Sara Russell of the University of California, Santa Cruz, found an odd pattern in earthquake waves that bounced off the outer core beneath Hawaii. The pattern--a change in wave orientation that reflects a journey through differently aligned materials--hints that rock flows horizontally toward the base of the plume, then rises vertically toward the hot spot.

Meanwhile, seismologist Don Helmberger of the California Institute of Technology in Pasadena and his colleagues saw seismic waves slow down dramatically in a patch at the bottom of the mantle under Iceland. "We think this is a dome-shaped pocket of partially molten rock surrounded by normal mantle," Helmberger says. The pocket, about 250 kilometers across, conducts heat from the core into the hot-spot plume, he believes.

The Hawaii research relies on a new seismic technique for detecting aligned flows of rock that has yet to be verified, says marine geophysicist Cecily Wolfe of the Woods Hole Oceanographic Institution in Massachusetts and the Carnegie Institution of Washington, D.C. However, the Iceland study is "very clear and compelling," she says, and consistent with a deep mantle origin for the plume. "We still haven't traced a plume all the way to the core-mantle boundary, but the evidence we have so far points to that region as the most logical source."

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