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Quick flip-flop in the magnetic world.

Quick flip-flop in the magnetic field

Boy Scouts learn that compass needles point to the north, but this survival fact will not always hold true. Over geologic time, Earth's magnetic field has flipped back and forth, sometimes pointing north and other times pointing south. Researchers have now found a relatively recent reversal in the magnetic field that had escaped almost all attention.

The evidence comes from rocks drilled from the ocean floor between the Philippines and Borneo as part of the Ocean Drilling Program's Leg 124, which ended in January. By examining magnetic grains buried in te sediments, the investigators found that the magneti field reversed itself for a 10,000-year period about 1.1 million years ago. Signs of this switch had previously shown up in rock from an Oregon mountain, but scientists say they need evidence from more than one location to document a reversal.

The investigators found the reversal in both the Celebes Sea and the Sulu Sea, the two ocean basins they examined, says Eli Silver from the University of California at Santa Cruz, co-chief scientist on the cruise. Because sediments were rapidly accumulating in these basins at the time of the reversal, the sedimentary record from this region may help researchers study fine details about the magnetic flip-flop, he says. Geoscientists still do not know how or why the magnetic field switches polarity.

Leg 124's magnetic find came as a surprise, says Silver. The investigators were actually drilling in the area to learn how and when the two ocean basins formed.
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Title Annotation:reversals in earth's magnetic field
Publication:Science News
Date:Mar 25, 1989
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