Plate tectonics. Diagram showing 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 (purple-brown) that also make up the ocean basins. New ocean rock is formed at mid-ocean ridges (centre). This leads to sea-floor spreading (yellow arrows) and the drifting apart of the continents. Two subduction zones, one offshore (left) and one coastal (right) show where oceanic rock is being pushed under the continental plate and destroyed, forming volcanoes, volcanic islands (hotspot, upper left), and mountains.
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