Heat Transfer

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. It is a diffusive process wherein molecules transmit their kinetic energy to

other molecules by colliding with them. Heat is conducted through a medium in which there is a spatial variation in the temperature. Convective heat transport is associated with the motion of a medium. If a hot fluid flows into a cold region, it will heat the region; similarly, if a cold fluid flows into a hot region, it will cool it. Electromagnetic radiation can also transport heat. An example is the radiant energy from the Sun. In the Earth, radiative heat transport is only important on a small scale and its influence can be absorbed into the definition of the thermal conductivity.

Don't use plagiarized sources. Get Your Custom Essay on
Heat Transfer
Just from $13/Page
Order Essay

As the discussion of this chapter shows, both conduction and convection are important heat transport mechanisms in the Earth. The temperature distribution in the continental crust and lithosphere is governed mainly by the conductive heat loss to the surface of heat that is generated internally by the decay of radioactive isotopes in the rocks and heat that flows upward from the subcontinental mantle. The loss of the Earth’s internal heat through the oceanic crust and lithosphere is controlled largely by conduction, although convective heat transport by water circulating through the basaltic crustal rocks is also important, especially near ridges. Intrusive igneous bodies cool by both conduction and the convective effects of circulating groundwater.

The heating of buried sediments and the adjustment of subsurface temperatures to effects of surface erosion and glaciation occur via the process of conduction. Convection plays the dominant role in the transport of heat from the Earth’s deep mantle and in controlling the temperature of its interior. This chapter discusses mainly heat conduction and its application to geological situations. Because convective heat transfer involves fluid motions, we will postpone a detailed discussion of this subject to Chapter 6, where we will develop the fundamentals of fluid mechanics. However, the consequences of convective heat transport are incorporated into our discussion of the Earth’s temperature toward the end of this chapter.