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Environment

Earth's Big Balancing Act

Once, Alaska was near the equator — possibly due to so-called true polar wander.

By Stephen OrnesNovember 17, 2006 6:00 AM

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The Earth we know generally moves in nonmysterious ways, but the latest evidence from an international team of geoscientists shows that about 800 million years ago, our planet executed a tricky balancing act that changed the course of the continents. Princeton geologist Adam Maloof says that over the course of 2 million to 10 million years, the North Pole appeared to shift a staggering 55 degrees—roughly the distance from the current north pole to San Francisco. This phenomenon, confusingly called true polar wander, is known to happen on Mars, where huge volcanic eruptions change the planet's weight distribution, but it is controversial whether that could happen on Earth.

In reality, it is not the poles that wander on Mars or on Earth; rather, all the continents shift their positions relative to the poles. "Any spinning object wants to adjust itself until it's rotating around its shortest axis, and all the extra weight is around the equator," Maloof says. "The Earth is no different—it is constantly adjusting itself to be in equilibrium." For example, if a large mass suddenly appeared near the pole, the continents would shift to move the extra weight toward the equator.

Maloof's team isn't sure why Earth's surface shifted so significantly, but they conclude that it did from an abrupt change in the orientation of the magnetic field preserved in sedimentary rocks in Svalbard, Norway. Now they are analyzing similar data from Australia. "Of all the possible hypotheses, the seemingly simplest is true polar wander," Maloof says. "Plus, it's imminently testable. All we have to do is go to another continent that has sediments of the same age."

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