The mantle engine

Heat from the core and from radioactive decay keeps the mantle churning in slow convection cells: hot rock rises, spreads sideways near the surface, cools, and sinks again — the same pattern as bubbles in a pot of thick soup.

These currents drag the plates along, but they are not the whole story. Two other forces matter: ridge push (gravity sliding plates off the high ridge) and slab pull (a cold, dense subducting slab yanking the rest of the plate behind it). Slab pull is thought to be the strongest.