Here's why:
* Convection Cells: The Earth's mantle is made of hot, semi-solid rock. Heat from the Earth's core creates convection currents within the mantle. Hotter, less dense material rises, while cooler, denser material sinks, creating a circular flow.
* Asthenosphere: This layer of the mantle is partially molten and behaves like a viscous fluid. The convection currents in the asthenosphere drag the tectonic plates, which sit on top of it, along for the ride.
* Crust: The Earth's crust is the outermost layer and is broken into tectonic plates. These plates move very slowly, driven by the convection currents in the asthenosphere.
So, while the crust is where we see the effects of plate movement (earthquakes, volcanoes, mountain ranges), the actual movement is driven by the convection cells in the mantle's asthenosphere.
