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The Tectonic Plates
The seven great slabs and dozens of smaller ones that make up Earth's moving shell
Earth's surface is not one solid shell but a mosaic of rigid plates, each about 100 kilometers thick, gliding over hotter, softer rock beneath. They move a few centimeters a year, roughly as fast as fingernails grow, yet over millions of years that motion opens oceans, closes them, and piles up mountain ranges. Almost every great earthquake, most volcanoes, and the deepest ocean trenches lie where plates meet. Plate tectonics, accepted only in the late 1960s, ties together why the Himalayas are rising, why Iceland is splitting, why the Pacific is ringed with fire, and why South America and Africa fit together like puzzle pieces.
Seven major plates carry most of the surface: the Pacific, the largest at about 103 million square kilometers, and the North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Many geologists now split the Indo-Australian into separate Indian and Australian plates. Smaller plates include the Nazca, Cocos, Caribbean, Arabian, Philippine Sea, Juan de Fuca, and Scotia, and detailed models recognize more than 50 in all. Plates meet at three kinds of boundary. At divergent boundaries such as the Mid-Atlantic Ridge and the East African Rift they pull apart. At convergent boundaries one plate dives beneath another, as at the Mariana Trench, or continents collide, as in the Himalayas. At transform boundaries such as the San Andreas they slide past each other.
In 1912 the German scientist Alfred Wegener proposed that the continents had drifted apart from a single landmass he later called Pangaea, but he could not explain how, and most geologists rejected the idea. Mapping of the seafloor after World War II changed that. Marie Tharp and Bruce Heezen charted the global ridge system, Harry Hess proposed seafloor spreading in 1962, and magnetic stripes on the ocean floor confirmed it. By 1968 the pieces had come together as plate tectonics. Today GPS measures plate motions directly, from about 2 centimeters a year across the Atlantic to more than 10 centimeters a year near Tonga. The same motions shape hazard maps, building codes, and tsunami warnings worldwide.