Plates move due to combined forces: ridge-push (gravity-driven flow at mid-ocean ridges), slab-pull (dense oceanic lithosphere sinking into mantle), and mantle-drag from convection. These forces explain plate velocities and motion directions across plate boundaries.
Map global plate velocities and relate to ridge-push and slab-pull forces. Use force balance models to predict plate motion.
From your understanding of the lithosphere-asthenosphere system and isostasy, you know that Earth's rigid outer shell floats on a weaker, ductile layer beneath it, and that density differences control vertical positioning. Plate tectonics driving forces extend this picture to explain *horizontal* motion — why plates move at all, and why they move at different speeds and in different directions.
Three primary forces drive plate motion: slab-pull, ridge-push, and mantle drag. Slab-pull is the gravitational force exerted by a dense oceanic plate sinking into the mantle at a subduction zone. As oceanic lithosphere ages and moves away from the ridge where it formed, it cools, thickens, and becomes denser than the underlying asthenosphere. At a subduction zone, this cold, dense slab descends into the mantle, pulling the rest of the plate behind it like a tablecloth sliding off a table. Slab-pull is the dominant force — plates attached to large subducting slabs (like the Pacific plate) move fastest, typically 5–10 cm/year, while plates without significant subduction zones (like the African plate) move slowly.
Ridge-push is a gravity-driven force arising from the elevated topography of mid-ocean ridges. Hot, buoyant mantle material rises beneath ridges, creating a topographic high. The newly formed lithosphere at the ridge crest sits several kilometers above the surrounding abyssal plains. This elevation difference creates a lateral pressure gradient — the elevated ridge material pushes outward on the adjacent plate, like a ball rolling downhill. Ridge-push is weaker than slab-pull (roughly an order of magnitude smaller), but it acts over the entire length of a ridge system and contributes to plate motion even where no subduction is occurring.
Mantle drag is the most debated of the three. The flowing asthenosphere exerts shear stress on the base of the lithosphere. If the mantle flows faster than the plate, it drags the plate along; if the plate moves faster than the mantle beneath it, drag acts as resistance. The direction and magnitude of mantle drag vary spatially, making it difficult to quantify globally. The old "conveyor belt" model — in which mantle convection cells carry plates passively — overstates mantle drag's role. Modern understanding treats plates as active participants in convection: the sinking slab *is* the downgoing limb of convection, not something pushed along by it. The balance of these forces at any given plate boundary determines local plate velocity, convergence rate, and the stress regime within the plate interior, connecting the physics of Earth's deep interior to the surface geology you observe in the field.