Narrator: This is Science Today. Geologist Mark Richards of the University of California, Berkeley took a glimpse inside the earth without going there. He and his fellow researchers made a supercomputer model of the planet's interior -- the mantle on which the continents ride.

Richards: What's going on is that the pressure inside the earth increases tremendously from the surface, where you have practically zero pressure, to enormous pressures in the interior. And the minerals undergo all sorts of changes under these changes in pressure.

Narrator: But Richards discovered that the change in pressure isn't gradual, but abrupt.

Richards: And what we think is that as you squeeze these rocks more and more, somewhere in what we call the transition zone of the mantle between about 400 and 1000 kilometers depth, that the changes that are occurring in the mineral structure give rise to a higher strength by probably one or two orders of magnitude.

Narrator: Which means the mantle suddenly becomes thirty times thicker below the transition zone -- like a drink that's half water, half molasses. But the upper mantle is hardly a sloshing liquid.

Richards: The rocks are still rocks, and it's only on long geological time scales that they move.

Narrator: For Science Today, I'm Steve Tokar.