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What is the plate tectonic theory?
Magma generation, igneous intrusions, metamorphism, volcanic action, earthquakes, faulting, and folding are usually the result of plate tectonic activity. The plate tectonics theory is simple. Briefly, it describes the Earth‟s outer layer, called the lithosphere, as a shell of hard, strong rock. This shell is broken into seven large (and several smaller) segments called tectonic plates. They are also called lithospheric plates .
The tectonic plates float on the layer below, called the asthenosphere. The asthenosphere, like the lithosphere, is rock. But the asthenosphere is so hot that 1 to 2 percent of it is melted. As a result, it is plastic, and weak. The lithospheric plates glide slowly over the asthenosphere like sheets of ice drifting across a pond .
Continents and ocean basins make up the upper parts of the plates. As a tectonic plate glides over the asthenosphere, the continents and oceans move with it. Volcanoes, earthquakes, mountain building, and oceanic trenches occur near plate boundaries. Interior parts of lithospheric plates are tectonically stable. Tectonic plates move horizontally at rates that vary from 1 to 16 centimeters per year.
Different types of plate boundaries
Three types of plate boundaries exist:
1. Divergent plate boundaries or spreading centers: At a divergent plate boundary, also called a spreading center and a rift zone, two lithospheric plates spread apart . The underlying asthenosphere then flows slowly upward to fill the gap between the separating plates. As the asthenosphere rises between separating plates, some of it melts to form molten rock called magma. Most of the magma rises to the Earth‟s surface, where it cools to form new crust, the top layer of the lithosphere. Most of this activity occurs beneath the seas because most divergent plate boundaries lie in the ocean basins. The sea floor at a spreading center floats to a high elevation, forming an undersea mountain chain called the midoceanic ridge . The mid-oceanic ridge system is the Earth‟s longest mountain chain. The basaltic magma that flows slowly onto the sea floor at the ridge creates approximately 6.5×1018 (6,500,000,000,000,000,000) tons of new oceanic crust each year. Moreover, a divergent plate boundary can rip a continent called continental rifting. Continental rifting is now taking place along a zone called the East African rift .
2. Convergent plate boundaries: At a convergent plate boundary, two lithospheric plates move toward each other. Convergence can occur:
a) Between a plate carrying oceanic crust and another carrying continental crust,
b) Between two plates carrying oceanic crust,
c) Between two plates carrying continental crust.
When two plates converge, the denser plate enter beneath the lighter one and sinks into the mantle. This process is called subduction. A subduction zone is a long, narrow belt where a lithospheric plate is sinking into the mantle . Oceanic crust, composed mainly of olivine and pyroxene, can be subducted along Benioff zones and recycled back into the mantle. Continental crust is composed dominantly of granites, gneisses, and upper crustal sedimentary rocks. The quartz and feldspar-rich continental crust is weaker and more buoyant, and cannot easily subduct into the denser mantle. Subduction zones are often marked by overlying chains of volcanic islands called island arcs. Magmatic arc is a general term for belts of andesitic island arcs and inland andesitic mountain ranges that develop along continental edges.
3. Transform plate boundaries: A transform plate boundary forms where two plates slide horizontally past one another as they move in opposite directions . California‟s San Andreas Fault is the transform boundary between the North American plate and the Pacific plate. This type of boundary can occur in both oceans and continents.
Mechanisms of plate movement and subduction
Mantle convection may cause plate movement. Alternatively, a plate may move because it slides downhill from a spreading center, as itscold edge sinks into the mantle and drags the rest of the plate along.
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