<P> When two plates with oceanic crust converge, they typically create an island arc as one plate is subducted below the other . The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it . The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight - edge of the knife). A deep oceanic trench is located in front of such arcs where the descending slab dips downward, such as the Mariana Trench near the Mariana Islands . </P> <P> Some convergent margins have zones of active seafloor spreading behind the island arc, known as back - arc basins . When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the denser oceanic plate is subducted, often forming an orogenic belt and associated mountain range . This type of convergent boundary is similar to the Andes or the Cascade Range in North America . </P> <P> An oceanic trench is found where the denser plate is subducted underneath the other plate . There is water in the rocks of the oceanic plate (because they are underwater), and as this plate moves further down into the subduction zone, much of the water contained in the plate is squeezed out when the plate begins to subduct . However, the recrystallization of ocean floor rocks, such as serpentinite, which are unstable in the upper mantle, recrystallize into olivine, causing dehydration through loss of hydroxyl groups . This addition of water to the mantle causes partial melting of the mantle, generating magma, which then rises, and which normally results in volcanoes . This normally happens at a certain depth, about 70 to 80 miles below the Earth's surface, and so volcanoes are formed fairly close to, but not right next to, the trench . </P> <Ul> <Li> The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas . </Li> <Li> The collision between the Australian Plate and the Pacific Plate that formed the Southern Alps in New Zealand </Li> <Li> Subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands . </Li> <Li> Subduction of the Nazca Plate beneath the South American Plate to form the Andes . </Li> <Li> Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate, forming the complex New Zealand to New Guinea subduction / transform boundaries . </Li> <Li> Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey . </Li> <Li> Mariana Trench </Li> <Li> Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range . </Li> </Ul>

Do volcanoes usually form at convergent continental/continental boundaries
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