The asthenosphere is a semi-fluid layer within the Earth’s mantle, situated beneath the lithosphere. It extends from about 100 to 700 kilometers below the Earth’s surface and is composed of partially molten rock that allows tectonic plates to move. The asthenosphere plays a crucial role in plate tectonics and is involved in the convection currents that drive the movement of the Earth’s lithospheric plates.
Source Turcotte, D. L., & Schubert, G. (2014). Geodynamics. Cambridge University Press.