There is commonly an ocean trench along the boundary as the crust bends downwards. The significant volume of water within the subducting material is released as the subducting crust is heated.
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. 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.
Earthquakes occur relatively deep below the seafloor, where the subducting crust moves against the overriding crust. Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska creating the Aleutian Islands and under the Philippine Plate, where it creates the Marianas Trench, the deepest part of the ocean.
At an ocean-continent convergent boundary, the denser oceanic plate is pushed under the less dense continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the seafloor is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate Figure 4.
This is a difficult boundary to draw. First it is complex and second, it is poorly understood when compared to the other types of plate boundaries. In this type of convergent boundary, a powerful collision occurs. The two thick continental plates collide, and both of them have a density that is much lower than the mantle, which prevents subduction there may be a small amount of subduction, or the heavier lithosphere below the continental crust might break free from the crust and subduct.
Fragments of crust or continent margin sediments might be caught in the collision zone between the continents, forming a highly deformed melange of rock. The intense compression can also cause extensive folding and faulting of rocks within the two colliding plates.
This deformation can extend hundreds of miles into the plate interior. The Himalaya Mountain Range is the best active example of this type of plate boundary. Visit the Interactive Plate Boundary Map to explore satellite images of the Himalaya Range where the Indian and Eurasian plates are currently in collision. The Appalachian Mountain Range is an ancient example of this collision type and is also marked on the map.
Effects found at a convergent boundary between continental plates include: intense folding and faulting; a broad folded mountain range; shallow earthquake activity; shortening and thickening of the plates within the collision zone. Contributor: Hobart King Publisher, Geology. What is the San Andreas Fault? How did the Hawaiian Islands Form?
Find Other Topics on Geology. Maps Volcanoes World Maps. Teaching Plate Tectonics. Earth's Internal Structure. Divergent Boundary. Convergent Boundary. Transform Boundary.
Tectonic Features Map. This place where the denser plate subducts is called a subduction zone. Oceanic subduction zones almost always feature a small hill preceding the ocean trench itself. This hill, called the outer trench swell , marks the region where the subducting plate begins to buckle and fall beneath the more buoyant plate.
Some ocean trenches are formed by subduction between a plate carrying continental crust and a plate carrying oceanic crust. Continental crust is always much more buoyant than oceanic crust, and oceanic crust will always subduct. Ocean trenches formed by this continental-oceanic boundary are asymmetric al. On the inner slope continental side , the trench walls are much more steep.
The types of rocks found in these ocean trenches are also asymmetrical. The oceanic side is dominate d by thick sedimentary rock s, while the continental side generally has a more igneous and metamorphic composition. Some of the most familiar ocean trenches are the result of this type of convergent plate boundary. The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the Nazca plate subducting beneath the continental crust of the South American plate.
The Ryukyu Trench, stretching out from southern Japan, is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the Eurasian plate. More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Mariana Trench, in the South Pacific Ocean, is formed as the mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate.
In a subduction zone, some of the molten material—the former seafloor—can rise through volcanoes located near the trench. The volcanoes often build volcanic arc s—island mountain range s that lie parallel to the trench. The Aleutian Islands form a volcanic arc that swings out from the Alaskan Peninsula and just north of the Aleutian Trench. Not all ocean trenches are in the Pacific, of course.
The Puerto Rico Trench is a tectonically complex depression in part formed by the Lesser Antilles subduction zone. Here, the oceanic crust of the enormous North American plate carrying the western Atlantic Ocean is being subducted beneath the oceanic crust of the smaller Caribbean plate.
Accretionary wedge s form at the bottom of ocean trenches created at some convergent plate boundaries. Accretionary wedges form as sediment s from the dense, subducting tectonic plate are scraped off onto the less-dense plate.
Sediments often found in accretionary wedges include basalt s from the deep oceanic lithosphere, sedimentary rocks from the seafloor, and even traces of continental crust drawn into the wedge. The most common type of continental crust found in accretionary wedges is volcanic material from islands on the overriding plate.
Accretionary wedges are roughly shaped like a triangle with one angle pointing downward toward the trench. Because sediments are mostly scraped off from the subducting plate as it falls into the mantle , the youngest sediments are at the bottom of this triangle and the oldest are at the more flattened area above.
This is the opposite of most rock formations, where geologist s must dig deep to find older rocks. Active accretionary wedges, such as those located near the mouth s of river s or glacier s, can actually fill the ocean trench on which they form.
Rivers and glaciers transport and deposit tons of sediment into the ocean. The Caribbean island of Barbados, for example, sits atop the ocean trench created as the South American plate subducts beneath the Caribbean plate. Ocean trenches are some of the most hostile habitats on Earth. Pressure is more than 1, times that on the surface, and the water temperature is just above freezing. Perhaps most importantly, no sunlight penetrate s the deepest ocean trenches, making photosynthesis impossible.
Organisms that live in ocean trenches have evolve d with unusual adaptation s to thrive in these cold, dark canyon s. In general, life in dark ocean trenches is isolated and slow-moving. Pressure at the bottom of the Challenger Deep, the deepest spot on Earth, is about 12, tons per square meter 8 tons per square inch. Large ocean animals, such as sharks and whales, cannot live at this crushing depth. Many organisms that thrive in these high-pressure environments lack gas -filled organ s, such as lung s.
These organisms, many related to sea stars or jellies, are made mostly of water and gelatinous material that cannot be crushed as easily as lungs or bones. Many of these creatures navigate the depths well enough to even make a vertical migration of more than 1, meters 3, feet from the bottom of the trench—every day.
Even the fish in deep trenches are gelatinous. Several species of bulb-headed snailfish, for example, dwell at the bottom of the Mariana Trench. The bodies of these fishes have been compared to tissue paper. Shallower ocean trenches have less pressure, but may still fall outside the photic or sunlight zone , where light penetrates the water. Many fish species have adapted to life in these dark ocean trenches. Anglerfish, for instance, use a bioluminescent growth on the top of their heads called an esca to lure prey.
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