Fossilized magnets and fickle rocks.

Fossilized magnets and fickle rocks

As the Pacific Ocean floor spread 65 million to 118 million years ago, magma from the earth's upper mantle rose to the surface to form unusually large numbers of seamounts similar to the Hawaiian Islands. The cooling lava took on the prevailing magnetic polarity of the earth.

This phenomenon is important to geologists, who use the fossilized magnetism of Pacific seamounts to deduce the movement of the Pacific plate and to understand global tectonics. This method is sometimes inaccurate, though, for two reasons: Moving plates carried seamounts into areas of different magnetic alignment, and the earth's magnetic poles have flip-flopped from north to south about every half-million years. As a result, an unknown portion of the magnetic rock gradually changed to realign itself with the prevailing magnetic direction, jeopardizing geologists' ability to chart plate movement.

Two researchers at the Scripps Institution of Oceanography in La Jolla, Calif., have found a way to use the magnetic polarity of seamounts more accurately. John Hildebrand and Hubert Staudigel studied Pacific seamounts with normal and reversed polarity. They found that, while most of the seamounts' magnetization remained stable throughout the changes in magnetic polarity, 25 percent of the magnetization changed. According to Staudigel, this change could be caused either by slowly changing magnetization or by quickly changing magnetization that changes with the prevailing magnetic field. Although researchers cannot distinguish between the two types from field observations, the total amount is relatively small when compared with a third type of magnetization that maintains the polarity of its formation.

This finding, says Staudigel, means researchers can still use ancient traces of magnetism in seamounts to accurately track the movement of plates in the Pacific basin.