Plate tectonics

Plate tectonics is a theory of geology underlying the observed phenomenon of continental drift. Based on models of the earth (well supported by the scientific evidence available), the theory of plate tectonics presupposes that the earth's outermost surface (crust) is made up of two sub-layers, the rigid lithosphere and the semi-molten asthenosphere. Beneath the crust is the mantle, a layer made up of molten rock (magma).

The rigid stone making up the lithosphere "floats" on the fluid-like asthenosphere, and, in areas where magma wells up from the mantle below the crust, may develop fractures. This results in the lithosphere being separated into continental and oceanic plates. Due to slow currents in the asthenosphere's material, the plates floating atop this material undergo slow motions in different directions. These floating plates regularly jostle one another, with interactions along these boundaries being responsible for earthquakes and volcanos.

The boundaries between plates are known as fault lines. Individual faults are regions where two plate sections are moving with respect to one another. Pressure builds up along these regions as the plates are drawn in different directions by the underlying currents; when the pressure is released by the abrupt motion of the plates, an earthquake results. One of the most famous systems of fault lines is the boundary of the Pacific plate, known as the Ring of Fire due to the amount of seismic activity along this boundary.

In some areas one plate will begin to overrun another. The plate being thus overrun will have its edge pressed downward below the leading egdge of the other plate. This process, called subduction, results in the edge of this plate being exposed to the greater pressures and temperatures within the asthenosphere, and the rock making it up may begin to liquify. The resulting build-up of magma under pressure may find explosive release through the lithosphere above it, resulting in a volcanic eruption.

In addition, the pressure exerted on both plates during a collision may cause the plates to buckle upward some distance from the plate boundaries. The resulting geographical features are known as upthrust mountain ranges.