An assumed process deep within the earth in which hotter materials move toward the surface while cooler materials are sent back down to the interior.

Mantle convection has been compared to the motions which occur inside a pot of boiling tar.

Heat which is supplied from below lowers the viscosity of the tar and causes it to rise slowly to the surface, where it cools and sinks o the bottom to be reheated.

The "skin" which forms on the top is similar to the earth's lithosphere.

The theory that the entire mantle, of hot land soft rocks, circulates and mixes in the course of bringing heat from the outer core to the surface.