for National Geographic News
A new study lends weight to the controversial theory that Earth became massively imbalanced in the distant past, sending its tectonic plates on a mad dash to even things out.
Bernhard Steinberger and Trond Torsvik, of the Geological Survey of Norway, analyzed rock samples dating back 320 million years to hunt for clues in Earth's magnetic field about the history of plate motions.
The researchers found evidence of a steady northward continental motion and, during certain time intervals, clockwise and counterclockwise rotations.
That pattern matches the predictions of a phenomenon known as true polar wander, a theory first proposed in the 1950s.
The theory states that at times Earth's surface mass becomes imbalanced. The continents become dramatically offset from the planet's spin axis and so move rapidly to right themselves.
The new study shows evidence for such motion within the past 320 million years that would have been enough to shift the continents by about 18 degrees latitude.
A change like that today would put Richmond, Virginia, where Mexico City is now. (See a map of the region.)
Island Hot Spots
"I am surprised that our results clearly indicate those episodes of true polar wander at all," Steinberger said.
"Up until now, there wasn't really any agreement in the community about the existence and amount of true polar wander."
That's because the phenomenon has been difficult to distinguish from the slower motion of tectonic plates traveling over the underlying mantle, Steinberger said.
Scientists often use hot spots, relatively fixed thermal plumes of material that rise up from the deep mantle, to track the paths of plates. The Hawaiian island chain is thought to be an example of a hot spot.
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