The main magnetic field generated by turbulent currents within the deep mass of molten iron of the Earth's outer core, periodically flips its direction. This is to say that a compass needle would completely reverse pointing south rather than north. Such polarity reversals have occurred hundreds of times at irregular intervals throughout the planet's history most recently about 780,000 years ago but scientists are still trying to understand how and why.

A new study of ancient volcanic rocks, reported in the Sept. 26 issue of the journal Science, shows that a second magnetic field source may help determine how and whether the main field reverses direction. This second field, which may originate in the shallow core just below the rocky mantle layer of the Earth, becomes important when the main north-south field weakens, as it does prior to reversing, says Brad Singer, a geology professor at the University of Wisconsin-Madison.

Current evidence suggests we are now approaching one of these transitional states because the main magnetic field is relatively weak and rapidly decreasing, he says. While the last polarity reversal occurred several hundred thousand years ago, the next might come within only a few thousand years.

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