Convergent Zones in a Nutshell
Where lithospheric plates come together
The material of the Earth's plates comes in two types: continental lithosphere and oceanic lithosphere (more on the lithosphere here). A plate can be part oceanic and part continental. When two plates move together or converge, then, there are three possible types of convergence: oceanic-oceanic, oceanic-continental and continental-continental.
In the first two cases, one plate turns downward and sinks, or subducts. (more about subduction) The material carried into the mantle by subduction triggers the formation of magma. The magma erupts, and volcanoes form in long curving chains. (more about arc volcanism)
U.S. Geological Survey images
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Oceanic-oceanic convergent zones are mostly hidden under the sea. Only arcs of volcanic islands mark them, made of dark and heavy basaltic lavas. The western Pacific Ocean is full of these—from north to south they include the Aleutian, Kuril, Japanese, Ryukyu, Izu-Bonin, Philippine, Mariana, Solomon and Tonga-Kermadec island arcs. In the Atlantic are the Caribbean and South Sandwich island arcs. In the Indian Ocean is the tangle of arcs that makes up the Indonesian archipelago.
Oceanic-continental convergent zones are the classic case. The oceanic plate undergoes subduction and the volcanic arcs arise on land, their less-dark and less-dense andesitic lavas bearing chemical traces of the continental crust they rise through. The west coast of the Americas is the major example, with volcanic zones in Alaska, the Pacific Northwest states, and a continuous stretch from Central America to Tierra del Fuego ("land of fire"). Italy, Greece, Kamchatka and New Guinea also fit this type.
Continent-continent convergent zones are tectonic sumo wrestling matches in which neither plate is subducted, because continental rock is too light to be carried very far into the dense mantle (about 150 km down at most). Instead the continental crust crumples into thick knots—tectonic mountain ranges—exposing deep-seated granites and gneisses with relatively little volcanism. The continental crust may also be cracked in pieces and shoved aside. Both of these responses are seen to perfection in the great foldbelt that stretches from Turkey to China. There the African, Arabian and Indian plates are moving northward into the Eurasian plate, respectively raising the Anatolian, Makran (Iranian) and Himalayan/Tibetan highlands. A small example occurs in southeastern Alaska as well. These places, especially Tibet, are sites of
extreme tectonics.
See these localities on the Plate Boundaries map.
Back to Plate Tectonics in a Nutshell
