Circular convection currents in Earth's mantle carry magma upward to the base of the lithosphere at a divergent plate boundary. The combined forces of "slab pull" (from the oldest part of an oceanic plate), "ridge push" (as new lava wells up at the mid-ocean ridge), and drag on the plate's underside from the "conveyor belt" motion of the convective mantle continuously usher newly created seafloor away from the spreading ridge. Oceanic crust material created at spreading ridges is later subducted back into the mantle at trenches along plate boundaries.
Image courtesy of the USGS.