Final answer:
The force applied to plates at spreading centers is known as ridge push, a process involving the horizontal movement of tectonic plates caused by the formation and cooling of new crust at mid-ocean ridges.
Step-by-step explanation:
The force applied to plates at spreading centers is ridge push. Ridge push is a tectonic process associated with the formation of new oceanic crust at mid-ocean ridges, or spreading centers. As new crust is formed through volcanic activity, it cools and becomes denser as it moves away from the ridge axis. This process contributes to the horizontal force that drives the plates apart.
Ridge push is one of the driving forces of plate tectonics, along with slab pull, which is the force exerted by a sinking tectonic plate as it subducts into the mantle at a convergent boundary. This force plays a significant role in plate movements along mid-ocean ridges. Unlike slab pull, which mainly affects downward and inward movement, ridge push primarily involves horizontal movement away from the ridge. Convection currents in the mantle also facilitate the movement of tectonic plates but are not the direct force acting at spreading centers.