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 geologic time Interval of time occupied by the Earth's geologic history, extending from c. 3.9 billion years ago (corresponding to the age of the oldest known rocks) to the present day. It is, in effect, the part of the Earth's history that is recorded in rock strata. , Earth's magnetic field Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with one pole near the north pole (see Magnetic North Pole) and the other near the geographic south pole (see Magnetic South Pole). 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 Celebes Sea Part of the western Pacific Ocean. It is bordered by the Sulu Archipelago, Mindanao, the Sangihe Islands, Celebes, and Borneo. It extends 420 mi (675 km) north-south and 520 mi (837 km) east-west, occupying about 110,000 sq mi (280,000 sq km). and the Sulu Sea Sulu Sea An arm of the western Pacific Ocean between the Philippines and Borneo. The Sulu Archipelago, a chain of small islands belonging to the Philippines, separates the Sulu Sea from the Celebes Sea southwest of Mindanao. , the two ocean basins they examined, says Eli Silver from the University of California The University of California has a combined student body of more than 191,000 students, over 1,340,000 living alumni, and a combined systemwide and campus endowment of just over $7.3 billion (8th largest in the United States). at Santa Cruz Santa Cruz, city, United States Santa Cruz (săn`tə kr  z), city (1990 pop. 49,040), seat of Santa Cruz co., W Calif., on the north shore of Monterey Bay; inc. 1866. , 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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