Plate tectonics. Artwork showing the processes of plate tectonics around an ocean basin. The Earth's crust is made up of tectonic plates that move on top of molten rock (red/yellow). Continental rock (light brown) rides on top of heavier rocks (pink) that also make up the ocean basins. New ocean rock is formed at mid-ocean ridges (centre, red arrow). This leads to sea-floor spreading (yellow arrows) and the drifting apart of the continents. At right is an active continental margin, with a subduction zone where oceanic rock is being pushed under the continental plate and destroyed, forming volcanoes along the margin (lower right). Mountains form along margins, and a hotspot has created volcanic islands (upper left) on the oceanic plate.