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Last time I looked, north was that way.

Last time I looked, north was that way

When volcanic rocks cool, the direction of the earth's ever-shifting magnetic field at that moment becomes frozen into the rocks' atomic structure. Geophysicists have long used this leftover magnetization, called thermoremanent magnetization (TRM), as a tool to trace changes in the earth's magnetic field and as a yardstick to measure the drift of the continents. But recent basic tests of TRM are throwing a wrench into the geophysicists' toolbox.

Joyce Castro and Laurie Brown of the University of Massachusetts at Amherst had originally set out to test the theory behind TRM by measuring the remanent magnetizations in Hawaiian lava flows from 1950 and 1972 and comparing those to the direction of the present magnetic field in Hawaii. Because scientists believe the field changes slowly, Castro and Brown expected all the directions to match within a degree or two. But the lavas turned out to be skewed by as much as 6 degrees, they report in the December GEOPHYSICAL RESEARCH LETTERS.

The researchers have yet to pin down the cause of this discrepancy. Some scientists have theorized that the shape of lava flows may affect the TRM, but this phenomenon would only account for a difference of a few degrees, says Brown. One possibility, she says, is that the field has changed rather quickly. To test this possible explanation and others, the researchers next plan to measure the TRM of lava within days of its eruption.
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Title Annotation:thermoremanent magnetization
Publication:Science News
Date:Jan 16, 1988
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