Convergent boundary
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In plate tectonics, a convergent boundary – also known as a convergent plate boundary, active margin or a destructive plate boundary – is an actively deforming region where two tectonic plates or fragments of lithosphere move towards one another.
The motion of two plates toward one another requires that the subducting plate move beneath the other, overriding plate. This forms either a subduction zone or a continental collision. This depends on the nature of the plates involved.
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[edit] Convergent margins
A subduction zone forms at a convergent plate boundary when one or both of the tectonic plates is composed of oceanic crust. The less dense plate, or lithosphere usually rides over the denser plate, which is subducted. This type of plate convergence is associated with island arcs such as the Solomon Islands. The island arc is formed above the subduction zone within the over-riding plate, with a deep oceanic trench formed by warping of the lithosphere before it descends into the subduction zone.
Some convergent margins have zones of active seafloor spreading, 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, often with an oceanic trench offshore. This type of convergent boundary is similar to the Andes or the Cascade Ranges 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 boundariy includes a major component of strike-slip faulting. The best examples 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 which is forming the Himalayas.
- subduction of the northern part of the Pacific Plate and the NW North American Plate which is forming the Aleutian Islands.
- subduction of the Nazca Plate beneath the South American Plate to form the Andes Mountains.
- subduction of the Pacific Plate beneath the Australian Plate, forming the complex New Zealand to New Guinea subduction/transform boundaries.
