As oceanic lithosphere moves away from the ridge it cools and sinks deeper into

As oceanic lithosphere moves away from the ridge it

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As oceanic lithosphere moves away from the ridge, it cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge. z Convergent Plate Boundaries { When a plate of dense oceanic lithosphere moving in one direction collides with a plate moving in the opposite direction, one of the plates subducts beneath the other. Where this occurs an oceanic trench forms on the sea floor and the sinking plate becomes a subduction zone. The Wadati-Benioff Zone , a zone of earthquakes located along the subduction zone , identifies a subduction zone. The Page 7 of 13 Plate Tectonics 8/26/2015
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earthquakes may extend down to depths of 700 km before the subducting plate heats up and loses its ability to deform in a brittle fashion. { As the oceanic plate subducts, it begins to heat up causing the release water of water into the overlying mantle asthenosphere. The water reduces the melting temperature and results in the production of magmas. These magmas rise to the surface and create a volcanic arc parallel to the trench. { If the subduction occurs beneath oceanic lithosphere, an island arc is produced at the surface (such as the Japanese islands, the Aleutian Islands, the Philippine islands, or the Caribbean islands { If the subduction occurs beneath continental crust, a continental volcanic arc is produced (such as the Cascades of the western U.S., or the Andes mountains of the South America) { If one of the plates has continental lithosphere on its margin, the oceanic plate will subduct because oceanic lithosphere has a higher density than continental lithosphere. { Sediment deposited along the convergent margin, and particularly that in the trench will be deformed by thrust faulting. This will break the rocks up into a chaotic mixture of broken, jumbled, and thrust faulted rock know as an accretionary prism. Page 8 of 13 Plate Tectonics 8/26/2015
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z Transform Plate Boundaries { Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes. { Most transform faults occur where oceanic ridges are offset on the sea floor. Such offset occurs because spreading takes place on the spherical surface of the Earth, and some parts of a plate must be moving at a higher relative velocity than other parts One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere Triple Junctions occur at points where thee plates meet. Various combinations can exist, Two of these are shown in figure 4.10 in your text.
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