Gravity lines linked to mantle motion.

Gravity lines linked to mantle motion

In the 20 years since the theory of plate tectonics was proposed, scientists have unraveled the kinematics or relative motions of the dozen or so plates that make up the outer shell of the earth. But the dynamics of this motion -- what convection patterns in the mantle might drive plate tectonics, for example -- remain elusive, primarily because so few of the observations made at the earth's surface directly relate to the mantle motions below.

Now two researchers at Lamont-Doherty Geological Observatory in Palisades, N.Y., report in the March 10 JOURNAL OF GEOPHYSICAL RESEARCH on a new surface observation that may directly reflect small-scale mantle processes. William F. Haxby and Jeffrey K. Weissel discovered lineated patterns of variation in the gravitation field over the Pacific and Indian oceanic plates. The discovery of these lines, which in the east central Pacific span an area the size of the continental United States, could not have been made without the large-scale view available for the first time from Seasat, says Haxby. Last summer, Weissel and others aboard a research ship confirmed the satellite findings and studied the patterns in finer detail.

Because the lines in the eastern Pacific run parallel to the direction of plate motion, the researchers believe they directly overlie 100-kilometer-deep cells of mantle convection, which, as predicted by one model, are thought to be shaped by plate motion into longitudinal rolls. One possibility being explored, says Haxby, is that the upwelling and downwelling of mantle material pushes up and pulls down the overlying crust, making ripples in the seafloor topography and the gravity field. An unexpected aspect of the lineations, he notes, is that they begin in 5-million- to 10-million-year-old crust -- much younger than had been predicted by other models simulating the onset of mantle convection under the cooling ocean crust.