![]() A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The significant volume of water within the subducting material is released as the subducting crust is heated. ![]() The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). There is commonly an ocean trench along the boundary as the crust bends downwards. Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. The types are ocean-ocean, ocean-continent, and continent-continent.Īt an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure 4.6.1). It does not store any personal data.Modified from "Physical Geology" by Steven Earle*Ĭonvergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary - oceanic or continental. The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. The cookie is used to store the user consent for the cookies in the category "Performance". This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other. ![]() The cookies is used to store the user consent for the cookies in the category "Necessary". The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". The cookie is used to store the user consent for the cookies in the category "Analytics". These cookies ensure basic functionalities and security features of the website, anonymously. Necessary cookies are absolutely essential for the website to function properly. A graben or sunken valley has been formed where the crust has been stretched, causing faulting. This is where the North American Plate and the Eurasian plate are separating. One of the best examples is Iceland’s rift valley, þingvellir. The continental crust must be thin for rifting to happen. Rift zones also occur on land and help explain how continents break up. This forms the underwater rift valleys found along mid-oceanic ridges. ![]() The lithosphere is put under great stress and eventually fractures along faults. In the North Atlantic the extrusion of magma has been so great it created the largest volcanic island in the world, Iceland.Īs magma rises the rocks above often form a dome. Pillow lavas are formed as lava is rapidly cooled on the sea floor. Due to the varying amount and rate of magma released mid-oceanic ridges vary in shape.Įruptions along constructive plate margins mainly occur underwater. Long chains of mountains form along these ridges. This typically occurs along a mid-oceanic ridge, such as the mid-Atlantic rift that extends from the north to the south of the Atlantic ocean. Where this occurs magma rises through the asthenosphere to the surface of the Earth. Constructive (divergent) plate marginsĬonstructive plate margins involve two plates moving away from each other. Fold mountains, such as the Alps and the Himalayas form. Where two continental plates meet there is typically no subduction. The north-west Pacific Ring of Fire has a series of island arcs including the Aleutian Islands. As the oceanic plate descends it melts, and the magma rises forming a volcanic island chain, known as an island arc. Where two oceanic plates converge the denser crust subducts the other. ![]() Volcanoes are formed where magma reaches the surface of the Earth. Large intrusions of magma create uplift, further contributing to the formation of fold mountains. The magma rises as it is less dense than the material around it. As the oceanic crust subducts the continental crust it melts. ![]()
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