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How is the asthenosphere different from the lithosphere?
Question Date: 2013-01-21
Answer 1:

The difference between asthenosphere and lithosphere is how the materials in these layers can flow. Rocks in the lithosphere are "rigid", meaning that they can bend but they cannot flow. Rocks in the asthenosphere are "plastic", meaning that they can flow in response to deformation. Even though it can flow, the asthenosphere is still made of solid (not liquid) rock; you can think of it kind of like Silly Putty. What determines whether rocks act in a rigid manner (bending or breaking) or a plastic manner (flowing) is temperature. Deep in the Earth, hot rocks (above about 1300°C) can flow, whereas cold rocks cannot. The lithosphere is broken up into rigid plates that ride on top of the flowing asthenosphere. In terms of chemical composition, there is no difference between the upper part of the asthenosphere and the lower part of the lithosphere. In fact, if the upper part of the asthenosphere cools down it becomes part of the lithosphere.

In addition to layers with different mechanical properties (lithosphere vs. asthenosphere), we can talk about layers with different chemical composition. The outer most layer of the Earth is the crust, which varies in thickness from about 7-70 km. Below that is the mantle, which of made up of denser rocks than the crust. At a depth of almost 3000 km, you reach the core, which is made of iron and nickel and is even denser than the mantle. The lithosphere is made up the crust plus the very upper part of the mantle, whereas the asthenosphere is only upper mantle material.

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