The earth's dynamic surface is divided into a dozen main crustal plates whose slipping and sliding unleash volcanoes, build mountain ranges and trigger earthquakes. But the workings of the inner world are invisible and frustratingly hard to decipher. Now Mark Richards of the University of California at Berkeley and Hans-Peter Bunge of Los Alamos National Laboratory have created computer simulations of the earth's interior that show in vivid detail the churning, convective processes that--literally--rock our world.

These images help to solve a long-standing question of geophysics. In the simplest earth models, each convective loop or "cell" is only about as wide as it is deep. That pattern would indicate that the largest of the plates should be about as wide as the depth of the earth's mantle, or about 3,000 kilometers. But the Pacific plate spans more than four times that distance. Richards and Bunge suspected that the models were ignoring a key factor: the way that the viscosity, or stickiness, of the hot-mantle rocks changes with depth.

To test this theory, the researchers developed a revised model in which the viscosity of the rock increases by a factor of 30 from the top to the bottom of the mantle. They then used a massively parallel Cray T3D computer at Los Alamos National Laboratory to see how well their model accords with reality. The resulting simulation matches much better with the real dimensions of the earth's tectonic plates. Another simulation showing conditions 300 kilometers below the surface shows how the variable-viscosity model (at bottom in this image) accurately depicts the subduction of cool material in the mantle, whereas the constant-viscosity model (at top) does not. A full report on this research will appear in the October 1 issue of Geophysical Research Letters.

The work of Richards and Bunge comes on the heels of a number of other breakthroughs in understanding the earth's deep insides--detailed models of the dynamo that drives the earth's magnetic field, for instance, and the discovery of the crystal structure and independent motion of the inner core. Armed with faster computers and more complete seismic data, geophysicists are making remarkable progress in unveiling the once secret realms beneath our feet.