Why do tectonic plates move?

The answer

Tectonic plates move primarily because of convection currents in the Earth's mantle - hot rock slowly rises from near the core, spreads laterally, cools, and sinks back down. This circulation drags the plates along at about 2 to 10 centimeters per year. Additional forces include 'ridge push' where new crust at mid-ocean ridges pushes plates apart, and 'slab pull' where dense oceanic crust subducting into the mantle tugs the rest of the plate with it.