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72: Inner Earth Revealed

By Anne SassoJanuary 3, 2005 6:00 AM


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The center of our planet is 3,958.7 miles beneath our feet. But so far geologists know for sure only what lies within the top 100 miles or so. Everything below can best be surmised from seismic waves from earthquakes as they travel through the inner earth. This year geologists uncovered another clue. A newly discovered mineral phase may solve a long-standing enigma: the composition of the D double prime layer, or D".

First defined in 1950, the D" layer is 1,700 miles beneath Earth’s surface and forms the boundary between the silicate-rock mantle and the molten iron core. There, seismic waves dramatically change speed and direction. “Many kinds of large seismic anomalies are observed, but their cause has been unclear,” says Kei Hirose, a mineralogist at the Tokyo Institute of Technology.

Since Hirose and his colleagues could not drill into the D" layer, they experimented in the lab. Geologists have long suspected that the lower mantle is composed largely of perovskite, a magnesium silicate mineral. Hirose squeezed perovskite between two gem-quality diamonds, called anvils, and heated it with a powerful laser beam to duplicate the high temperatures and pressures thought to exist in the 125- to 185-mile-thick D" layer. The density and compressibility of the squashed rock was then measured with X-rays.

What they found was totally new: an unknown crystal structure with a chemical composition similar to perovskite. They called it post-perovskite. The mineral has a unique layered structure that causes seismic waves to change velocity depending on their direction of travel—just as in the D" layer. “This phase can explain many of the seismic anomalies that have never been well explained,” says Hirose. “We finally opened the door at the bottom of the mantle and discovered the secret there.”

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