The asthenosphere is the layer of the Earth that lies below the lithosphere. It is a layer of solid rock that has so much pressure and heat the rocks can flow like a liquid. The rocks are less dense than the rocks in the lithosphere. This allows the tectonic plates in the lithosphere to move around on the Earth's surface by "floating" on the rocks that are slowly flowing like a liquid.

Asthenosphere, USGS

Tectonic plate interaction with the
upper layers of the Earth

Scientists use the speed and direction of earthquakes to study parts of the Earth beneath the lithosphere. Earthquake waves traveling through the Earth change speed and direction when there is a change in density of rocks within the earth. Not only do the rocks in this layer flow like a liquid they also break apart.  Deep earthquake are caused by rocks breaking in this layer.

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Rocks in upper mantle
The rocks in the upper mantle can be part of the lithosphere or the asthenosphere. Lithosphere rocks are solid and rigid. Rocks below the lithosphere flow and break apart. They also rise through the lithosphere in areas where tectonic plates are separating.

How magma chambers form
The decrease in pressure allows rocks in this part of the Earth to melt and collect in magma chambers where they erupt as pillow basalt on the ocean floors.

Boundaries of the lithosphere
The boundary with the lithosphere lies between 100 and 200 kilometers deep. Scientists do not know for certain the depth of the lower boundary but it is approximately 700 kilometers deep. Earthquake waves in this part of the Earth travel much slower than they do in the lithosphere because of the density of the rocks.

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