In
plate tectonics, a
convergent boundary – also known as a
convergent plate boundary or a
destructive plate boundary – is an actively deforming region where two (or more)
tectonic plates or fragments of
lithosphere move toward one another.
When two plates move toward one another, they form either a subduction zone or a continental collision. This depends on the nature of the plates involved. In a subduction zone, the subducting plate, which is normally a plate with oceanic crust, moves beneath the other plate, which can be made of either oceanic or continental crust. During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.
A subduction zone is formed at a convergent plate boundary when one or both of the tectonic plates is composed of oceanic crust. The denser plate, made of oceanic crust, is subducted underneath the less dense plate, which can be either continental or oceanic crust. When both of the plates are made of oceanic crust, convergence is associated with island arcs such as the Solomon Islands.
An oceanic trench is formed where the denser plate is subducted underneath the other plate. There is water in the rocks that are on the oceanic plate (because they are underwater) and as this plate moves further down into the subduction zone, the higher temperature causes the water to boil, melting the plate which turns into magma. In this process the magma eventually rises, which normally results in volcanoes. This normally happens at a certain depth, about 70 to 80 miles below the earth's surface, and so volcanoes are formed fairly close to, but not right next to the trench.
