Can you think of a solid that can flow?

You use one twice a day! Toothpaste is a solid that can flow. Is the asthenosphere made of toothpaste? Only if the toothpaste is ultramafic in composition, and then it would only be able to flow if it were really, really hot. Still the toothpaste analogy gives you a good image of how the asthenosphere might behave if you squeezed it!

Lithosphere

The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle, rigid solid. The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick. How are crust and lithosphere different from each other?

The definition of the lithosphere is based on how Earth materials behave, so it includes the crust and the uppermost mantle, which are both brittle. Since it is rigid and brittle, when stresses act on the lithosphere, it breaks. This is what we experience as an earthquake.

Although we sometimes refer to Earth’s plates as being plates of crust, the plates are actually made of lithosphere.

Asthenosphere

The asthenosphere is solid upper mantle material that is so hot that it behaves plastically and can flow. The lithosphere rides on the asthenosphere.

Summary

• The lithosphere is the brittle crust and uppermost mantle.
• The asthenosphere is a solid but it can flow, like toothpaste.
• The lithosphere rests on the asthenosphere.

Explore More

Watch this video to answer the questions that follow:

1. What does he mean by the mechanical properties of a layer?
2. In the compositional model: What is the outermost layer? What are the two types of this layer and what are their main features?
3. What is the next layer down? What are its main features?
4. What is the deepest layer? Why is this the densest layer?
5. What is the composition of this layer?
6. What is the lithosphere?
7. What are the mechanical properties of the material below the lithosphere and what is the layer called?
8. What is the composition and mechanical property of the mesosphere relative to the asthenosphere?

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