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The Year in Science: Earth 1997

Toasty Down There

By Lybi MaJanuary 1, 1998 6:00 AM


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Geologists have long wondered about the temperature of Earth’s mantle—the thick layer of rock that stretches from 20 to 1,800 miles underground, sandwiched between the core and crust of the planet. Sticking a thermometer to that depth hasn’t exactly been feasible, so researchers have used indirect methods. This year two geophysicists announced they may have found the answer in the barrel of a gun.

Kathleen Holland of Sandia National Laboratories and Thomas Ahrens of Caltech put pieces of olivine—a translucent gem—into an apparatus that is essentially a sealed giant high-powered cannon. They fired projectiles at the olivine at velocities up to four miles a second, creating a brief shock wave so intense it transformed the crystal structure of the olivine into two minerals thought to be major ingredients of the mantle—magnesiowüstite and perovskite. Seismic-wave data suggest that the lower mantle is partially molten, so the researchers blasted the rock to different pressures, looking for the levels at which it would start to melt. Every time the rock was hit, it gave off a heat-induced glow that the researchers could measure, and from that calculate its temperature.

From these readings the researchers estimated the temperature of the lower mantle at no more than 7,300 degrees Fahrenheit—about three-quarters as hot as the surface of the sun. Knowing that temperature will help geophysicists understand how plumes of hot rock rise through the mantle, manifesting themselves at the surface as volcanoes and perhaps affecting the motion of the tectonic plates. We’re getting into explaining how such a layer of molten rock could exist at great depth, says Ahrens. This layer may be the layer that feeds the volcanic edifices such as the Hawaiian chain and Iceland.

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