What is the name of the ductile layer beneath Earth's lithosphere?

The ductile layer beneath Earth's lithosphere is the asthenosphere. It is a mechanically weak part of the upper mantle that allows the rigid tectonic plates above it to move.

The asthenosphere is made mostly of solid peridotite, not a global ocean of liquid rock. Its intense heat and pressure let the mantle deform slowly over geological time, while a very small amount of partial melting may further weaken it. This behavior is why the layer can flow gradually even though most of its material remains solid.

Its upper boundary is commonly placed roughly 80–100 kilometers beneath the surface, while its lower boundary is often estimated near 700 kilometers. Those boundaries are not perfectly sharp: geologists define them using changes in temperature, seismic-wave speed, composition, and mechanical behavior.

A common mix-up is confusing the asthenosphere with the lithosphere. The lithosphere includes the crust and rigid uppermost mantle; the asthenosphere lies beneath it. Slow movement in the asthenosphere helps drive plate motion, feeding processes such as seafloor spreading, subduction, and volcanic activity at mid-ocean ridges.