The theory that Earth's rigid outer shell, the lithosphere, is broken into large and small plates that move slowly over the softer asthenosphere, and that their interactions at boundaries build mountains, open oceans and cause most earthquakes and volcanoes.
Deep-dive lesson:Earth's Interior, Rocks and Plate Tectonics. This page is a quick reference that connects the term across subjects; read the lesson for the full treatment.
The idea that continents move began with Alfred Wegener's theory of continental drift in 1912, which pointed to the matching coastlines of South America and Africa, similar fossils and rocks across oceans, and evidence of past glaciation in now-tropical lands. Wegener proposed a supercontinent, Pangaea, but could not explain a convincing mechanism, so many geologists rejected the idea. After the Second World War, mapping of the ocean floor revealed mid-ocean ridges, deep trenches and symmetrical magnetic stripes on either side of ridges. Harry Hess and others proposed seafloor spreading: new crust forms at ridges and moves away, eventually sinking into trenches.
By the late 1960s these findings were combined into plate tectonics. The theory explained why earthquakes and volcanoes cluster in belts such as the Pacific Ring of Fire, why young mountains such as the Himalaya and Andes form where plates converge, and why ocean floors are much younger than continents. For India, the theory explained a remarkable journey: after breaking from Gondwana, the Indian plate moved rapidly north across the ocean that once lay between it and Asia, the Tethys, and collided with Eurasia about 50 million years ago, raising the Himalaya and the Tibetan Plateau.
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