Earth's outer shell is broken into roughly a dozen rigid plates that drift a few centimeters a year over hot flowing rock beneath. In a single stroke the theory explains why mountains rise, where earthquakes strike, where volcanoes burn, and how the continents drifted apart from a single ancient landmass. Alfred Wegener proposed continental drift in 1912 and was ridiculed for decades; only in the 1950s and 1960s did seafloor spreading and magnetic evidence vindicate him, and the full theory was accepted around 1970. Plate boundaries also explain the unequal global geography of natural hazard.
For most of human history, the ground was the definition of permanence. Then a German scientist looked at a map and saw that the coastlines of Africa and South America fit together like torn pieces of paper. In 1912 Alfred Wegener argued the continents had once been joined and had since drifted apart. He was ridiculed for decades, because he could not say what force could possibly move a continent. He died on the Greenland ice in 1930, his idea still in disgrace. Then, in the 1950s and 1960s, scientists reading the magnetism frozen into the floor of the ocean found their answer. New crust was being born at the mid-ocean ridges and spreading outward, carrying the continents like luggage on a belt. Wegener, thirty years dead, was right. The single idea that grew from this — that Earth's surface is broken into drifting plates — turned out to explain almost everything geology had treated as separate puzzles: the quakes, the volcanoes, the mountains, the shape of the world.