Plate tectonics. Cutaway artwork showing sea floor spreading and subduction in plate tectonics. New sea floor (right) is formed at a mid-ocean ridge (divergent plate boundary) by upwelling lava (red arrows). 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. Other features shown include mountains on land (continental crust), a continental shelf (left) and a subduction zone (convergent plate boundary, left) where oceanic crust is pushed downwards (red arrow) and destroyed, fuelling volcanoes.