The basic idea of plane tectonics is that the lithosphere consists of several large and fairly stable slabs of solid, relatively rigid rock called plates, which extend over the globe like curved caps on a sphere.
Because plates are part of the lithosphere, they extend to a depth of 100 to 200km. Each plate moves horizontally, relative to neighbouring plates on the softer rock immediately below. At the edge of a plate, where there is contract with adjoining plates, plate movement creates large drag forces that operate on the rocks of the lithosphere, causing physical and even chemical changes in them. Because the lithospheric plates are generally rigid and strong, they transmit forces throughout their interiors without buckling; the relative motion between plates is taken up almost entirely along plate boundaries, which themselves can be rather broad. At these plate edges, the Earth's geological structure is affected by the forces of reaction between the plates, and this is where the most massive and radical geological changes occur.
The edges of the plates are clearly belts of high seismicity. The slip rate (the relative motion of one face of a fault relative to the other) for large scale faults at plate boundaries is about 5 cm a year. The present global plate structure is not permanent but is undergoing constant, gradual change. At divergent boundaries in mid-ocean, plates are spreading apart from each other. Lava is continually upwelling at ridges along mid-oceanic plate boundaries, adding new plate material, derived from the rocks of the asthenosphere, to the lithosphere.
The theory of plate tectonics, which predicts the interactions and consequences of plates, is based on four assumptions:
1. New plate material is generated by seafloor spreading; new oceanic lithosphere is generated along active mid-oceanic ridges.
2. The new oceanic lithosphere forms part of a moving plate; this plate may or may not include continental material.
3. The Earth's surface area remains constant; therefore, the growth of the lithosphere must be balanced by the consumption of plates elsewhere.
4. Because the plates transmit stresses over great horizontal distances without buckling, the relative motion between plates occurs almost entirely along their boundaries