Ocean sea floor spreading. Block artwork showing the process of sea floor spreading in plate tectonics. New sea floor (centre) is formed at a mid-ocean ridge (divergent plate boundary) by upwelling lava (yellow). Over millions of years, new rock (parallel ridges) forms as the continents and tectonic plates move apart. Hot spots of volcanism create underwater mountains that can reach the surface and form volcanic islands (upper left). Other features shown include mountains on land (continental crust), a sediment-laden continental shelf (top left) and a subduction zone (convergent plate boundary, bottom right) where oceanic crust is destroyed.