Why is the Pacific so big? Look down deep.

When geoscientists stare at a globe, they see a puzzle. Basic physics and simple computer models suggest that Earth should have tectonic plates no more than 3,000 kilometers wide. Yet the planet somehow bypassed that limitation.

The vast plate underneath the Pacific Ocean stretches 12,000 km across, and many other plates span two to three times the theoretical width.

A solution to this long-standing problem has now emerged from a massive computer model that simulates the movement of the surface plates and the underlying mantle. Hans-Peter Bunge and Mark A. Richards of the University of California, Berkeley report that characteristics of the deep mantle apparently govern the size of the plates.

"It really appears that the lower mantle is the unifying theme that makes plate tectonics work just exactly the way we observe it," says Bunge. He and Richards will publish their results in the Oct. 15 Geophysical Research Letters.

Their model, which ran on a supercomputer at Los Alamos (N.M.) National Laboratory, depicts the rising and sinking currents of rock in the mantle by breaking the region into 20 million blocks. Although the mantle is solid, pressures and temperatures reach such a feverish state that the rock flows, but at the sedate pace of a few centimeters per year.

Most previous models have suggested that the 3,000-km depth of the mantle naturally sets the scale for the rock currents. Convection circulates rock up from the deep mantle and carries it a few thousand kilometers horizontally just below the surface plates before taking it back down again. The surface plates would then, in theory, match the size of these convection cells.

To explain the reach of the Pacific and other large plates, some scientists suggested that the plates exert a controlling influence on mantle circulation, stretching the convection cells far beyond their natural breadth. In other words, the plates determine their own size and the mantle follows.

In Bunge and Richards' model, however, the mantle retains the dominant role.

Their modeling differs from previous experiments by including a more complex, realistic representation of the mantle's viscosity, or stickiness. When the scientists made the lower mantle more viscous than the upper mantle, the convection cells approached the size of Earth's cells. When they kept the viscosity uniform, the convection cells remained puny, the scientists reported in the Feb. 1 Nature.

In their more recent experiments, Bunge and Richards gauge the importance of Earth's plates by adding simplified versions of them. Without the increase in viscosity, the model with the plates failed to produce realistic convection patterns. It showed broad convection cells only when the lower mantle was 20 times more viscous than the upper mantle.

These results suggest that the lower mantle, rather than the plates, organizes the whole system. "Plates and their size basically reflect the scale of the mantle," says Bunge. "Since the mantle convects on such a large scale, the plates are allowed to reach large sizes."

According to Richard J. O'Connell of Harvard University, Bunge and Richards are the first to simulate plate motion in a three-dimensional model of a spherical Earth. Because the simplified plates they included are unbreakable, a next step would be to include more realistic plates, which can fracture.

This would provide further insight into why plates assumed the shapes and sizes they have today.