The lithosphere extends to the depth where mantle rock becomes brittle and viscous. The lithosphere is strong compared to the layer below: the asthenosphere. At ~80–200 km, the asthenosphere is the top layer of the mantle and is relatively ductile, owing to temperature and pressure.
Although comparatively thin, the asthenosphere has a remarkable impact on the mantle. ~ American geophysicists Don Anderson & Scott King
The asthenosphere is a region of concentrated shear which distributes heat in the lithosphere according to geological patterns in both the asthenosphere and lithosphere. The asthenosphere is hotter than the mantle layer below it: so sizzling that it is close to, or even at, its melting point. How and why that is so is not fully understood.
The relatively low viscosity of the asthenosphere facilitates the movement of continents. It is impossible to imagine tectonics with an asthenosphere otherwise.
The low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics. ~ French geophysicist David Sifré et al
Convection – the flow of heat in mantle material – drives tectonics. The mantle transfers heat from the core.
Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics. ~ American geophysicist David Rowley