Convergent Plate Boundary
Convergent plate boundary
In terms of plate tectonics , collision boundaries are sites where lithospheric plates move together and the resulting compression causes either subduction (where one or both lithospheric plates are driven down and destroyed in the molten mantle) or crustal uplifting that results in orogeny (mountain building).
Colliding plates create tremendous force. Although lithospheric plates move very slowly (low velocities of inches per each), the plates have tremendous mass. Accordingly, at collision, each lithospheric plate carries tremendous momentum (the mathematical product of velocity and mass) that provides the energy to cause subduction or uplifting. In addition, the buoyancy properties of the colliding lithospheric plates determine the outcome of the particular collision. Oceanic crust is denser than continental crust and is subductable. Continental crust, composed of lighter, less dense materials, is too light to undergo subduction and so overrides oceanic crust or uplifts.
Earth's crust is fractured into approximately 20 lithospheric plates. Each lithospheric plate is composed of a layer of oceanic crust or continental crust superficial to an outer layer of the mantle. Oceanic crust comprises the outer layer of the lithosphere lying beneath the oceans . Oceanic crust is composed of high-density rocks, such as olivine and basalt . Continental crust comprises the outer layer of the lithospheric plates containing the existing continents and some undersea features near the continents. Continental crust is composed of lower density rocks such as granite and andesite . Containing both crust and the upper region of the mantle, lithospheric plates are approximately 60 miles (approximately 100 km) thick. Lithospheric plates may contain various combinations of oceanic and continental crust in mutually exclusive sections (i.e., the outermost layer is either continental or oceanic crust, but not both except at convergent boundaries where subducting oceanic crust can make material contributions of lighter crustal materials to the overriding continental crust). Lithospheric plates move on top of the asthenosphere (the outer plastically deforming region of Earth's mantle).
At convergent boundaries, lithospheric plates move together in collision zones where crust is either destroyed by subduction or uplifted to form mountain chains . In zones of convergence, compressional forces (i.e., compression of lithospheric plate material) dominates.
When oceanic crust collides with oceanic crust, both subduct to form an oceanic trench (e.g., Marianas trench). When oceanic crust collides with continental crust, the oceanic crust subducts under the lighter continental crust and both pushes the continental crust upward into mountain chains (e.g., the Andes). The contribution of molten material from the subduction crust contributes to the volcanic arcs found along the Pacific Rim. Because continental crust does not subduct, when continental crust collides with continental crust, there is a uplift of both crusts (e.g., the ongoing collision of India with Asia that continues to push the Himalayas upward by about a centimeter a year. Given the expanse of geologic time , even modest geomorphologic changes—measured in inches or centimeters a year—can result in substantial changes over millions of years.
At triple points where three plates converge (e.g., where the Philippine sea plate merges into the North American and Pacific plate subduction zone ), the situation becomes more complex.
Convergent plate boundaries are, of course, three-dimensional. Because Earth is an oblate sphere, lithospheric plates are not flat, but are curved and fractured into curved sections akin to the peeled sections of an orange. Convergent movement of lithospheric plates can best be conceptualized by the movement together of those peeled sections over a curved surface.
Because Earth's diameter remains constant, there is no net creation or destruction of lithospheric plates and so the amount of crust created at divergent boundaries is balanced by an equal destruction or uplifting of crust at convergent lithospheric plate boundaries.
See also Divergent plate boundary; Earth, interior structure; Earthquakes; Geologic time; Mohorovicic discontinuity (Moho); Subduction zone