Convergent boundary
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In plate tectonics, a convergent boundary or convergent plate boundary, also known as a destructive plate boundary (because of subduction), is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another and collide. As a result of pressure and friction and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries.
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.
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[edit] Convergent margins
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 found where the denser plate is subducted underneath the other plate. There is water in the rocks of 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 denser 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.
[edit] Examples
- the collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.
- subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands.
- subduction of the Nazca Plate beneath the South American Plate to form the Andes.
- subduction of the Pacific Plate beneath the Australian Plate, and vice versa forming the complex New Zealand to New Guinea subduction/transform boundaries.
- collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey.
- Mariana Trench
- subduction of the Juan de Fuca Plate beneath the North American Plate.
