This mafic magma may then assimilate silicious rocks as it moves up through the overlying plate, forming a final andesitic composition that vents to form the island arc. Geologists think that at a depth of about 100 kilometers (60 miles) the asthenosphere just above the subduction zone partially melts. Modern‐day examples of island arcs are the Philippines and the Alaska Peninsula. Subduction probably occurs to a depth of at least 670 kilometers (400 miles), at which point the plate probably becomes plastic.Īndesitic volcanism often forms a curved chain of islands, or island arc, that develops between the oceanic trench and the continental landmass. Data from earthquakes along the subducting plate show that the angle of subduction increases with depth. The ocean trench curves outward toward the subducting plate over the subduction zone. One edge of ocean crust is subducted beneath the other at an ocean trench. Ocean‐ocean convergence occurs when two plates carrying ocean crust meet. The foreland basin fills with eroded material from the mountain ranges or occasionally with marine sediments if it becomes submerged. The additional weight of these rocks downwarps the inland area, forming a foreland basin. Volcanic arcs result from isostatic processes, compressional forces along the leading edge of the continent, and thrust faults that move slices of mountain‐belt rocks inward over the continental interior, creating backarc thrust belts. The Sierra Nevada in California and Nevada is a volcanic arc. These mountain ranges (also called volcanic arcs) are underlain by crust that has been thickened by intrusive batholiths that were generated by partial melting along the underlying subduction zone. Magmatic arc is a general term for belts of andesitic island arcs and inland andesitic mountain ranges that develop along continental edges. Earthquakes occur in the Benioff zones that dip underneath the continental edge. The leading edge of the continental plate is usually studded with steep andesitic mountain ranges. This forms an active continental margin between the subduction zone and the edge of the continent. Ocean‐continent convergence occurs when oceanic crust is subducted under continental crust. The crust is thickened along the suture zone, resulting in isostatic uplift, mountain‐building, and thrust faulting. One continent may override the other for a short distance, but the two continents eventually become welded together along a geologically complex suture zone that represents the original line of collision. Because the continents are less dense than the oceanic crust, they will not be pulled down the subduction zone. The continent overlying the subduction zone will develop a magmatic arc until the ocean floor becomes so narrow that the continents collide. The continents were separated at one time by oceanic crust that was progressively subducted under one of the continents. Three types ofĬonvergent boundaries are recognized: continent‐continent, ocean‐continent, and ocean‐ocean.Ĭontinent‐continent convergence results when two continents collide. Plates may converge directly or at an angle.
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