The asthenosphere is a layer of the Earth's mantle that lies beneath the lithosphere and is thought to be partially molten. It is also known as the asthenosphere mantle or the low-velocity zone. The asthenosphere is fundamental in the movement of plate tectonics and the formation of earthquakes. Other synonyms of the asthenosphere are the mesosphere, which is the lower part of the mantle, and the sub-lithospheric mantle. The rheosphere is a synonym for the asthenosphere, which emphasizes the layer's ability to flow and deform under stress. The upper mantle is also sometimes used as a synonym for the asthenosphere.