Which layer of the Earth is described as having varying viscosity due to changes in pressure and temperature?

The mantle of the Earth is characterized by its varying viscosity, which is influenced by changes in pressure and temperature. The mantle is located between the crust and the core and extends to a depth of about 2,900 kilometers. Within this layer, rocks can behave both as solid and as soft, flowing material depending on the conditions they are under.

At greater depths, the temperature increases, causing the rocks to become more ductile, which means they can flow more easily. Similarly, increased pressure can also affect the way materials behave, combining to create a zone where the viscosity is not constant but varies throughout, leading to the gradual movement of tectonic plates. This behavior is essential for understanding geological processes such as plate tectonics, as it allows for the movement of these plates over the more solid lower mantle and the outer core.