Circular currents of magma in Earth's mantle carry molten rock upward to the lithosphere. Lava flows out onto Earth's surface at a divergent plate boundary. New lava flows push aside older rock at a seafloor spreading ridge. The oldest seafloor crust is eventually recycled - it "falls" back down into the mantle at trenches along the edges of seafloor plates. As the older rock sinks into the trench, it pulls the rest of the plate along ("slab pull") behind it.
Image courtesy of the USGS.