Introduction

In the previous chapter we studied the elastic behavior of the outer shell of the Earth. Our studies of the bending of the lithosphere have shown that a near-surface region with a thickness of 25 to 50 km behaves elastically on geological time scales. Seismic studies have shown that the entire mantle of the Earth to a depth of 2885 km is a solid because it transmits shear waves. In order to understand the presence of a thin elastic shell, it is necessary to allow for variations in the rheology of the solid rock as a function of depth. Although the behavior of the near-surface rocks is predominantly elastic, the deeper rocks must exhibit a fluid or creep behavior on geological time scales in order to relax the stresses. The fluid behavior of mantle rock also results in mantle convection and the associated movement of the surface plates.

We know from both laboratory and theoretical studies that the rheology of solids is primarily a function of temperature. Therefore, to understand the mechanical behavior of the Earth, we must understand its thermal structure. The rheology of mantle rocks is directly related to the temperature as a function of depth. This, in turn, is dependent on the rate at which heat can be lost from the interior to the surface. There are three mechanisms for the transfer of heat: conduction, convection, and radiation. Conductive heat transfer occurs through a medium via the net effect of molecular collisions.