![]() ![]() The rifting may be caused by a mantle plume that has come near to the surface and is spreading out, creating convection currents that stretch the crust to the point of breakage. If it develops along the continental edge, it can also split off a strip of the continent and push it seaward toward the subduction zone-Japan is a modern‐day example. This backarc spreading can push the island arc away from the continent toward the subduction zone. In other words, a “mini” spreading center develops as an equilibrium response to changes in the way the plate is being subducted. The backarc basin, the basin that occurs between the island arc and the continental mass, is occasionally split by new extensional forces into two parts that migrate in different directions ( backarc rifting). The forearc basin is the relatively undisturbed expanse of ocean floor between the accretionary wedge and island arc the area on the continental side of the arc is called the backarc. The accretionary wedge is continuously pushed up to form a ridge along the surface of the trench over the subducted crust. This accumulation is called the accretionary wedge or subduction complex. The trench becomes filled with folded marine sediments that slide off the descending plate and pile up against the wall of the trench. The distance the island arc forms from the oceanic trench is dependent on the steepness of the subduction zone-the steeper the angle of subduction, the more quickly the subducted material reaches the magmaforming depth of 100 kilometers, and the closer the arc will be to the oceanic trench. 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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