Plate boundaries are zones where plates collide or separate resulting in the crust being created or destroyed. The plates move due to the movement of convectional currents in the mantle below the crust. This is called as the endogenetic force as the energy comes from within the earth.
At a constructive plate boundary (divergent boundaries) a new crust is created as plates move away from each other. An example of this can be found beneath the middle of the Atlantic ocean – the Mid Atlantic Ridge. At this boundary, convection currents of magma cause the North and South American plates to move away from the Eurasian and African Plates. As the plates separate, molten magma pours out into the surface and cools quickly as it comes in contact with the cold Atlantic seawater; thus forming a new crust. This new crust becomes gradually wider as the plates continue to move apart.
At a destructive plate boundary (convergent boundaries) the crust is destroyed as the plates converge or collide. An example of this can be found on the west margin of South America. A this boundary, convection currents of magma cause the west-moving South American Plate to collide with the East-moving Nazca Plate. The Nazca Plate is made up of oceanic crust which is heavier than the South American Plate which is made up of continental crust. This results in the Nazca plate sinking beneath the South American Plate at the point of collision. This sinking process is called subduction. The oceanic plate is destroyed as it sinks into the mantle and melts. Molten magma from the batholith, created by the molten plate materials, sometimes forces its way up to the surface to eventually form volcanic mountains.