# Asthenosphere: Definition, Temperature & Density

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• 0:03 What Is the Asthenosphere?
• 1:18 Density
• 1:44 Lesson Summary

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Lesson Transcript
Instructor: Lindsey Klingele
The asthenosphere is a layer of the inner earth where the crust begins to melt into the mantle. This lesson will discuss the important properties of the asthenosphere.

## What Is the Asthenosphere?

The asthenosphere is part of the upper mantle located below the crust of the earth. The asthenosphere boundary is primarily defined by temperature, and it starts when the crust heats up to 1300° C. At this temperature the crust begins to melt and move more as a liquid. The asthenosphere is important in plate tectonics, as convection currents slowly move the tectonic plates that lie above.

Seismic waves pass relatively slowly through the asthenosphere. The boundary is characterized by a drop in velocity of earthquake seismic waves. When an earthquake occurs, two types of seismic waves are released: primary and secondary. The speed at which the seismic waves travel depends on the density of the material through which it is passing. When a primary wave travels from a solid to a liquid two things happen; it slows down, and it refracts.

Secondary waves have an even more pronounced changed at the asthenosphere. These are lateral waves, meaning they move from side to side. As a secondary wave moves from a solid to liquid, its velocity drops down to zero. Secondary waves are not able to pass through the asthenosphere.

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