Sunday, September 03, 2006

Earth's Poles May Have Wandered

Earth's geographic poles may seem like immovable fixtures, but new evidence suggests that they weren't always where they are now. The findings support an old but controversial theory known as "true polar wander" that states that the poles moved as part of a planetary balancing act millions of years ago.

The idea behind true polar wander is that an unequal distribution of weight in Earth's mantle, the molten layer beneath the crust, would cause the planet to realign itself in relation to its spin axis. For instance, if a large mass such as a super-sized volcano formed away from the equator, the force of Earth's rotation would gradually pull the mass toward the equator. The entire planet would be reoriented, and so the poles-which are defined by where the spin axis passes through the surface-would also move to a new spot on Earth's surface. Something similar may have happened on Mars, where a collection of large volcanoes in the Tharsis region is believed to have caused the planet to realign to its present orientation with Tharsis on the equator.

A team led by geologists Adam Maloof of Princeton University and Galen Halverson of Paul Sabatier University in Toulouse, France, says that Earth indeed rebalanced itself around 800 million years ago during the Precambrian time period. They tested this idea by studying magnetic minerals in sedimentary rocks in a Norwegian archipelago. As the mineral grains were deposited or excreted by microbes, they aligned themselves with Earth's magnetic field. So they act as frozen compasses pointing to an ancient north pole.

Using these minerals, Maloof and Halverson found that the north pole shifted more than 50 degrees-about the current distance between Alaska and the equator-in less than 20 million years. Earth's tectonic plates move much more slowly than that, says Maloof, so the best explanation for this wandering pole is planetary rebalancing. This reasoning is supported by a record of changes in sea level and ocean chemistry in the Norwegian sediments that could be explained by true polar wander, the team reports in the September-October issue of the Geological Society of America Bulletin.

The results match other findings from Australia and China and could help determine how the continents fit together in a single supercontinent known as Rodinia, which existed at the time. But rocks this old are very hard to date accurately and may also have had their magnetic signatures disturbed over time, Van der Voo, of the University of Michigan, Ann Arbor cautions. "This is very tricky stuff," he says.


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