Plate Tectonics: The Restless Crust

Earth's outer shell is broken into large plates that move a few centimetres a year, roughly as fast as fingernails grow. In 1912 Alfred Wegener proposed continental drift, pointing to matching coastlines, fossils and rocks on different continents, but many scientists rejected him because he could not explain what moved the continents. Evidence from the ocean floor changed that: in the early 1960s Harry Hess proposed seafloor spreading, and magnetic stripes on the seabed confirmed new crust forming at mid-ocean ridges.

Where plates pull apart, as in Iceland, new crust forms; where they collide, mountains rise or one plate sinks beneath another in a subduction zone, creating deep trenches and volcanoes; where they slide past each other, earthquakes strike along faults like the San Andreas. Plate tectonics explains why hazards cluster in particular places and why the Pacific is ringed by volcanoes.