Structures at the surface of the Earth reflect processes occurring at deeper levels. We know that the Earth is divided into three concentric shells—the core, mantle, and crust. The core is a very dense iron-nickel alloy, comprising the solid inner core and the liquid outer core. The mantle is composed of lower-density, solid magnesium-iron silicates, and is actively convecting, transporting heat from the interior of the Earth to the surface. This heat transfer is the main driving mechanism of plate tectonics. The crust is the thin, low-density rock material making up the outer shell of the Earth.
Temperature increases with depth in the Earth at a gradient of about 54°F per half a mile (30°C/km) in the crust and upper mantle, and with a much smaller gradient deeper within Earth. The heat of the Earth comes from several different sources, including residual heat trapped from initial accretion, radioactive decay, latent heat of crystallization of the outer core, and dissipation of tidal energy of the sun-Earth-Moon system.
Heat flows out of the interior of the Earth toward the surface through convection cells in the outer core and mantle. The top of the mantle and the crust form a relatively cold and rigid boundary layer called the lithosphere, which is about 61 miles (100 km) thick. Heat escapes through the lithosphere largely by conduction, transport of heat in igneous melts, and in convection cells of water through mid-ocean ridges.
structural geologists predominantly study only the outer 12-18 miles (20-30 km) of the lithosphere. This puts into perspective how much structural geology infers a great deal about the interior of the Earth by examining only its skin.
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