What weaker layer lets Earth’s lithospheric plates move above the deeper mantle?

The asthenosphere is the weaker layer beneath Earth’s lithosphere that helps tectonic plates move.

It is a mechanically weak part of the upper mantle, generally found beneath the rigid lithosphere. Although it is solid rock, its high temperature allows it to deform slowly over geological time rather than behaving as a brittle layer.

Tectonic plates are not floating on a global ocean of liquid magma. Instead, they move over solid asthenospheric mantle through processes involving convection, sinking slabs, seafloor spreading, and forces at plate boundaries.

The asthenosphere’s depth varies by location and definition, and it is identified mainly by its physical behaviour and seismic properties. It is therefore different from the lithosphere, which is the stronger layer above it.