According to the plate tectonic model, the surface of the earth consists of a series of relatively thin but rigid plates which are in constant motion. The surface layer of each plate is composed of oceanic crust, continental crust or a combination of both. The plates maybe up to 70 km thick if composed of oceanic crust or 150 km thick if incorporated of continental crust.
The lines where these plates meet are called plate boundaries. There are usually three types of plate boundaries.
Divergent boundaries– A divergent boundary occurs when two tectonic plates move away from each other. This leads to frequent earthquakes which strike along the rift. At this type of boundary new oceanic crust is formed in the gap between the two diverging plates. Presently, most divergent margins occur along the central zone of the world’s major ocean basins.
Convergent boundaries– A convergent boundary occurs when two tectonic plates come together. The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bend the other down into a deep seafloor trench at the angle of around 45 degrees and is incorporated into the earth’s mantle along a subduction zone. When two plates of oceanic crust collide a volcanic island, the arc may form.
Conservative or transform margins– A transform boundary occurs where two plates slide horizontally against each other, neither create nor destroy lithosphere. However, at these boundaries or transform faults, powerful earthquakes may occur. The San Andreas fault system is the most famous example of this type of boundary.