The solid layer of Earth that can flow because of great heat and pressure is the mantle.

Here's why:

* Solid but flowing: The mantle is primarily composed of solid rock, but the immense pressure and heat deep within the Earth cause it to behave like a very viscous fluid over long periods. This slow, flowing movement is called convection.

* Convection drives plate tectonics: The convection currents in the mantle are what drive the movement of the Earth's tectonic plates, which in turn causes earthquakes, volcanoes, and mountain formation.

Let me know if you'd like to know more about the Earth's layers or the process of plate tectonics!