The plates of the Earth's lithosphere (which includes the crust and the uppermost part of the mantle) slide over a layer called the asthenosphere.

Here's a breakdown:

* Lithosphere: This is the rigid, outermost layer of the Earth. It's broken into tectonic plates.

* Asthenosphere: This is a semi-solid, partially molten layer beneath the lithosphere. It's characterized by its plasticity, meaning it can deform under pressure but will return to its original shape when the pressure is released. This plasticity allows the lithospheric plates to move over it.

Think of it like a giant, very slow-moving conveyor belt!