Aftermath of a Cataclysm

Nine times in the past 4,000,000 years or so, the earth's magnetic field has completely reversed. The North Pole became the South Pole, the South Pole the North. During these reversals, scientists theorize, the intensity of the earth's magnetic field actually decreased to zero, then built back up with opposite polarity. During the temporary absence of a strong magnetic field, the earth was left without its protective shield; as a consequence, cosmic-ray particles that were normally deflected by the magnetic field could shower through the atmosphere. The result may have been the destruction, mutation or even creation of species. Scientists know that such reversals occur every 500,000 to 1,000,000 years. But they do not know exactly what causes them.

Clues to a possible explanation were recently uncovered at the bottom of the sea by Geologists Bruce Heezen and Bill Glass, of Columbia University's Lamont Geological Observatory, who were investigating some strange, glasslike fragments known as tektites. Many scientists believe that the tektites, which are found in several areas around the world, were formed when meteorites or comets collided with the earth. The en- counters were so catastrophic that bits of the earth, as well as chunks of the intruder, were hurled into space and then fell back. Heated both by the impact and their swift passage through the atmosphere, they were fused into glassy globules.

Flipped Field. One large meteorite is believed to have fallen in the area of southern China, the Philippines and Australia, where tektites found on land all appear to have had the same origin. Basing their estimates on the distribution and radioactive dating of these tek- tites, scientists had long assumed that the meteorite weighed a few thousand tons and struck about 700,000 years ago. While he was examining sediment cores taken from more widely separated locations on the floors of the Indian and Pacific oceans, Geologist Glass discovered tiny tektites, apparently from the same meteorite. To have littered so large an area, Glass and Heezen calculated, the meteorite could have weighed a billion tons and might have been as large as a mile in diameter. Even more intriguing, further examination of the sediment cores indicated that the tektites had been deposited around the time of the last reversal of the earth's magnetic field, which also occurred 700,000 years ago.

To Heezen and Glass, the coincidence implied that one phenomenon may have caused the other. The impact on the earth of a mile-wide meteorite might well have disturbed the complex motions of the earth's core that are believed to generate the magnetic field. As a result, the geologists suggest, the field may have flipped. It is also conceivable, they say, that at least some of the previous reversals of the magnetic field were caused by the catastrophic collision of huge meteorites or comets with the earth.

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