Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. The lithosphere and overlying crust make up tectonic plates, which move over the asthenosphere. The layer under the lithosphere is known as the asthenosphere. The outer layers of the Earth are divided into the lithosphere and asthenosphere. Similarly, Earth's lithosphere "floats" in the asthenosphere. Eclogite is denser than the surrounding asthenosphere. At that depth, craton roots extend into the asthenosphere. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. The asthenosphere erodes the lower levels of the Plateau. The release of water into the asthenosphere leads to partial melting. The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. Heat is also transferred to it from the surrounding asthenosphere. In some regions the asthenosphere could extend as deep as .