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 the rock to behave like a very viscous fluid over long periods. This is known as solid-state convection.
* Convection Currents: The slow, gradual movement of the mantle's material, driven by heat from the Earth's core, is what drives plate tectonics. The hot, less dense material rises, while cooler, denser material sinks, creating a cycle of movement.
Important Note: While the mantle is described as "flowing", it's not like the liquid water we're familiar with. It moves extremely slowly, on the order of centimeters per year.
