Here's why:
* Solid but flows: The mantle is primarily composed of solid rock, but the immense pressure and heat cause it to behave in a way similar to a very viscous fluid. It can deform and flow over extremely long periods.
* Convection currents: The heat from the Earth's core creates convection currents within the mantle. Hotter, less dense material rises, while cooler, denser material sinks. This movement drives tectonic plate movement at the Earth's surface.
* Seismic wave behavior: Seismic waves, which travel through the Earth, move slower through the mantle than through the rigid lithosphere. This indicates that the mantle is not a solid, but rather a material that can deform under stress.
While the outer core is completely liquid, the mantle's fluidity is a more complex concept, often described as "solid-state convection" or "plasticity."
