The lower mantle is not made of plastic-like rock. While it's true that the lower mantle is very hot and under immense pressure, it's not in a liquid state like magma. Instead, it's in a solid state but behaves more like a very viscous fluid over long periods. This is known as solid-state convection.

The main minerals found in the lower mantle are bridgmanite (formerly known as perovskite) and ferropericlase. These are extremely hard and dense minerals, formed under the immense pressure and temperatures found in the lower mantle.

It's important to understand that the term "plastic" in this context refers to the rock's ductility and deformability under extreme pressure, not its state of matter.