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Geomagnetic switch: not impact caused.

Geomagnetic switch: not impact caused

A close examination of sediment cores from the deep sea is challenging the theory that comet or meteorite impacts have caused the Earth's magnetic field to reverse itself several times during geologic history. Scientists proposed this theory 20 years ago when they noted that a turnover in the geomagnetic field 730,000 years ago coincided in timing with an impact, marked in the geologic record by tiny, glassy grains called microtektites. Since then researchers have found two other close timings between impacts and reversals 15 million years ago and 900,000 years ago, seemingly bolstering the original theory. But a study of the event 900,000 years ago suggests the impact followed the reversal and could not have caused it, report David A. Schneider and Dennis V. Kent of the Lamont-Doherty Geological Observatory in Palisades, N.Y., in the February GEOPHYSICAL RESEARCH LETTERS.

The researchers studied a layer of microtektites buried in sediments near the Ivory Coast. They discovered the microtektite layer above the sediment layer that recorded the reversal and estimated the impact occurred 30,000 years after the beginning of the reversal.

Kent concedes that recent work suggests a reversal 730,000 years ago did directly follow an impact, but he notes neither the K-T impact 65 million years ago nor one 15 million years ago are clearly linked to a reversal. With only one of four cases supporting the theory that impacts cause reversals, Kent says that coincidence could well explain the one exception.
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Title Annotation:comet or meteorite impacts
Publication:Science News
Date:Mar 10, 1990
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