A convergent plate boundary is a tectonic boundary where two lithospheric plates move toward one another. Convergence can occur between an oceanic plate and a continental plate, between two oceanic plates or between two continental plates. The geological result depends strongly on the relative densities, thicknesses and compositions of the colliding lithospheres. Oceanic lithosphere is commonly dense enough to subduct beneath another plate, while continental crust is comparatively buoyant and generally resists being subducted deeply as a whole. Convergent boundaries can therefore produce subduction zones, volcanic arcs, deep earthquakes, trenches, mountain belts and intense crustal deformation.
At an oceanic continental convergent boundary, the denser oceanic lithosphere commonly sinks beneath the less dense continental lithosphere in a process called subduction. A deep ocean trench forms near the boundary, while earthquakes occur along the descending slab. Fluids released from the subducting slab can enter the overlying mantle and promote partial melting. Magma rises through the continental crust and can produce a chain of volcanoes called a continental volcanic arc. The Andes provide a major example associated with subduction of oceanic lithosphere beneath the South American Plate along the western margin of South America.
At an oceanic oceanic convergent boundary, one oceanic plate generally subducts beneath the other, often producing a deep ocean trench and a volcanic island arc. The older or colder oceanic plate is commonly denser and therefore more likely to descend, although the precise dynamics depend on multiple factors. Examples of island arc systems include the Mariana arc and parts of the Aleutian system. Subduction generates earthquakes along the descending slab and can produce large earthquakes capable of generating tsunamis when seafloor displacement occurs.
Continental continental convergence produces a different geological outcome because neither continental block readily subducts into the mantle as an entire continent. Instead, crust is shortened, folded, faulted and thickened, producing major mountain systems. The Himalayas formed through the continuing collision between the Indian Plate and Eurasian Plate. Continental collision can create very thick crust and high elevations while generating earthquakes across broad regions. Volcanism may be less directly associated with the collision zone than at typical oceanic subduction boundaries because there is no straightforward oceanic slab supplying fluids to a mantle wedge in the same way.
Convergent boundaries are major sites of earthquake activity. Subduction zones can generate shallow, intermediate and deep earthquakes along the descending slab, with some reaching several hundred kilometres beneath the surface. The largest known earthquakes occur at some subduction interfaces, where enormous areas of the plate boundary can rupture. If the seafloor is displaced vertically, the resulting movement of seawater can generate a tsunami. Earthquake depth patterns in subduction zones provide important evidence for the geometry of descending slabs and the mechanics of plate convergence.
Convergence is closely connected with the recycling of oceanic lithosphere. At subduction zones, old oceanic lithosphere descends into the mantle, carrying crustal rocks, sediments, water and other chemical components downward. Some material is incorporated into the mantle, while some contributes to melting and volcanism above the slab. Over geological time, subduction helps regulate the creation and destruction of oceanic lithosphere. It also participates in large scale chemical cycles involving water, carbon and other elements between the surface and deep Earth.
Convergent boundaries should not be confused with every location where mountains or earthquakes occur. Mountains can also form through other processes, and earthquakes occur at divergent and transform boundaries as well. Convergence specifically refers to relative plate motion toward one another. Similarly, subduction is one possible consequence of convergence rather than a synonym for convergence. Transform boundaries involve lateral motion, while divergent boundaries involve separation. Real plate boundaries can be geometrically complex and may contain segments with different styles of deformation.
Convergent plate boundaries are among the most geologically consequential features on Earth because they create and destroy lithosphere, build mountains, generate volcanoes, produce powerful earthquakes and influence global chemical cycles. Their effects can extend hundreds of kilometres from the actual plate interface because stress, magma and deformation propagate into surrounding crust and mantle. The different outcomes of oceanic continental, oceanic oceanic and continental continental convergence demonstrate that plate tectonics is controlled by density, buoyancy, temperature, composition, plate age, geometry and the mechanical properties of rocks rather than by plate motion alone.
