# Plate Tectonics

Plate tectonics is a consequence of the gravitational body forces acting on the solid mantle and crust. Gravi- tational forces result in an increase of pressure with depth in the Earth; rocks must support the weight of the overburden that increases with depth. A static equilibrium with pressure increasing with depth is not possible, however, because there are horizontal variations in the gravitational body forces in the Earth’s interior. These are caused by horizontal variations in density associated with horizontal differences in temperature. The horizontal thermal contrasts are in turn the inevitable consequence of the heat release by radioactivity in the rocks of the mantle and crust. The horizontal variations of the gravitational body force produce the differential stresses that drive the relative motions associated with plate tectonics. One of the main purposes of this chapter is to introduce the fundamental concepts needed for a quantitative understanding of stresses in the solid Earth. Stresses are forces per unit area that are transmitted through a material by interatomic force fields. Stresses that are transmitted perpendicular to a surface are normal stresses; those that are transmitted parallel to a surface are shear stresses. The mean value of the normal stresses is the pressure. We will describe the techniques presently used to measure the state of stress in the Earth’s crust and discuss the results of those measurements.

Stress in an elastic solid results in strain or deformation of the solid. The simplest example of strain is the decrease in volume accompanying an increase in pressure due to the compressibility of a solid. Normal strain is defined as the ratio of the change in length of a solid to its original length. The shear strain is defined as one-half of the decrease in a right angle in a solid when it is deformed. The surface of the Earth is continually being 128 Stress and Strain in Solids strained by tectonic processes. These changes in strain can be measured directly by geodetic techniques. This chapter also discusses the basic concepts required for a quantitative understanding of strain and changes in strain in the solid Earth.

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