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Asthenosphere - The Asthenosphere In Plate Tectonic Theory

plates earth materials lithosphere

The asthenosphere is now thought to play a critical role in the movement of plates across the face of Earth's surface. According to plate tectonic theory, the lithosphere consists of a relatively small number of very large slabs of rocky material. These plates tend to be about 60 mi (100 km) thick and in most instances many thousands of miles wide. They are thought to be very rigid themselves but capable of being moved on top of the asthenosphere. The collision of plates with each other, their lateral sliding past each other, and their separation from each other are thought to be responsible for major geologic features and events such as volcanoes, lava flows, mountain building, and deep crustal faults and rifts.

In order for plate tectonic theory to make any sense, some mechanism must be available for permitting the flow of plates. That mechanism is the semi-fluid character of the asthenosphere itself. Some observers have described the asthenosphere as the 'lubricating oil' that permits the movement of plates in the lithosphere. Others view the asthenosphere as the driving force or means of conveyance for the plates.

Geologists have now developed theories to explain the changes that take place in the asthenosphere when plates begin to diverge from or converge toward each other. For example, suppose that a region of weakness has developed in the lithosphere. In that case, the pressure exerted on the asthenosphere beneath it is reduced, melting begins to occur, and asthenospheric materials begin to flow upward. If the lithosphere has not actually broken, those asthenospheric materials cool as they approach Earth's surface and eventually become part of the lithosphere itself. On the other hand, suppose that a break in the lithosphere has actually occurred. In that case, the asthenospheric materials may escape through that break and flow outward before they have cooled. Depending on the temperature and pressure in the region, that outflow of material (magma) may occur rather violently, as in a volcano, or more moderately, as in a lave flow. Both these cases produce crustal plate divergence, or spreading apart. Pressure on the asthenosphere may also be reduced in zones of divergence, where two plates are separating from each other. Again, this reduction in pressure may allow asthenospheric materials in the asthenosphere to begin melting and to flow upward. If the two overlying plates have actually separated, asthenospheric material may flow through the separation and form a new section of lithosphere.

In zones of convergence, where two plates are moving toward each other, asthenospheric materials may also be exposed to increased pressure and begin to flow downward. In this case, the lighter of the colliding plates slides upward and over the heavier of the plates, which dives down into the asthenosphere. Since the heavier lithospheric material is more rigid than the material in the asthenosphere, the latter is pushed outward and upward. During this movement of plates, material of the downgoing plate is heated in the asthenosphere, melting occurs, and molten materials flow upward to Earth's surface. Mountain building is the result of continental collision in such situations, and great mountain chains like the Urals, Appalachian, and Himalayas have been formed in such a fashion. When oceanic plates meet one another, island arcs (e.g., Japan or the Aleutians) are formed. Great ocean trenches occur in places of plate convergence. In any one of the examples cited here, the asthenosphere supplies new material to replace lithospheric materials that have been displaced by some other tectonic or geologic mechanism.

Therefore, whether scientists are considering the origin of compressed mountain ranges like the Himalayas, or the origin of the great ocean trenches (like the Peru-Chile trench), they also consider the activity of the asthenosphere, which keeps Earth's plates continually geologically active.

Resources

Books

Press, Frank, and Raymond Sevier. Understanding Earth. San Francisco: Freeman, 2000.

Tarbuck, Edward. J., Frederick K. Lutgens, and Dennis Tassa, eds. Earth: An Introduction to Physical Geology, 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Fuchs, Karl, and Claude Froidevaux. Composition, Structure, and Dynamics of the Lithosphere and Asthenosphere System. Washington, DC: American Geophysical Union, 1987.


David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Lithosphere

—The outer layer of Earth, that extends to a depth of about 60 mi (100 km).

Magma

—Molten material exuded from below Earth's surface, generally consisting of rock-like materials rich in silicon and oxygen.

Seismic wave

—A disturbance produced by compression or distortion on or within the earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g. earthquakes) or artificial (e.g. explosions) means.

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over 6 years ago

very nice!!!!!!!

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over 6 years ago

very nice!!!!!!!

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over 6 years ago

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about 2 years ago

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about 2 years ago

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almost 10 years ago

I think that this place is aweesome, but need s more info.

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almost 10 years ago

yous guys are da bomb keep up the good work

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almost 11 years ago

I think this passage gives a wonderful amount of information. Keep up the good work

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about 8 years ago

This is interesting material. You may consider that oil are the lubricating sources for the movement of the continental plates but if man keeps exploiting it and turning it into a gas in our atmosphere, this may make the planet spin unbalanced and throw us out of orbit. You may also consider that the entire universe moves in this fashion and that planets move in convection forces in the vacuum of space. Those stars you see above may actually be a compiliation of magma particles, just like smoldering ash flying around above a fire but on a cosmic scale. Our planet may just be a small ember. Think about it!!!

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almost 4 years ago

sorry i commented on the wrong thing

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over 4 years ago

This is interesting material. You may consider that oil are the lubricating sources for the movement of the continental plates but if man keeps exploiting it and turning it into a gas in our atmosphere, this may make the planet spin unbalanced and throw us out of orbit. You may also consider that the entire universe moves in this fashion and that planets move in convection forces in the vacuum of space. Those stars you see above may actually be a compiliation of magma particles, just like smoldering ash flying around above a fire but on a cosmic scale. Our planet may just be a small ember. Think about it!!!

Read more: Asthenosphere - The Asthenosphere In Plate Tectonic Theory - Plates, Earth, Materials, and Lithosphere - JRank Articles http://science.jrank.org/pages/566/Asthenosphere-asthenosphere-in-plate-tectonic-theory.html#ixzz2eE7PSGV7

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over 2 years ago

very nice...


keep up the good work....

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almost 3 years ago

What material?!?!?!?!?!?

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over 4 years ago

This is interesting material. You may consider that oil are the lubricating sources for the movement of the continental plates but if man keeps exploiting it and turning it into a gas in our atmosphere, this may make the planet spin unbalanced and throw us out of orbit. You may also consider that the entire universe moves in this fashion and that planets move in convection forces in the vacuum of space. Those stars you see above may actually be a compiliation of magma particles, just like smoldering ash flying around above a fire but on a cosmic scale. Our planet may just be a small ember. Think about it!!!

Read more: Asthenosphere - The Asthenosphere In Plate Tectonic Theory - Plates, Earth, Materials, and Lithosphere - JRank Articles http://science.jrank.org/pages/566/Asthenosphere-asthenosphere-in-plate-tectonic-theory.html#ixzz2eE7PSGV7

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almost 4 years ago

boring too long

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about 3 years ago

Anyone know what rollover astheneosphere means