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.
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, much of the water contained in the plate is released in dehydration reactions. The addition of water to the mantle causes partial melting of the mantle, generating magma, which then rises, and 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.
Some convergent margins have zones of active seafloor spreading behind the island arc, known as back-arc basins.
When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the oceanic plate is subducted, often forming an orogenic belt and associated mountain range. This type of convergent boundary is similar to the Andes or the Cascade Range in North America.
When two plates containing continental crust collide, both are too light to subduct. In this case, a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas.
When the subducting plate approaches the trench obliquely, the convergent plate boundary includes a major component of strike-slip faulting. The best example of this is the Sumatra convergent margin, where convergent action is occurring intermixed with a strike-slip boundary.
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