Continental Drift proposed

In 1912, the German meteorologist and geophysicist Alfred Wegener publicly proposed that the continents had once been joined and had slowly moved apart over geological time. Known as continental drift, the idea was dismissed or resisted by most scientists for decades. It was later vindicated in modified form by plate tectonics, which now underpins the whole of modern Earth science. The summary's phrasing is a common shorthand, but Wegener did not propose plate tectonics itself. He proposed the drifting of continents, and he lacked a convincing mechanism to explain it.

The idea had precursors. Since the sixteenth century, mapmakers and naturalists had remarked on how the coasts of South America and Africa seem to fit together like pieces of a puzzle. In the nineteenth century, geologists noticed striking similarities between rock formations and fossils on opposite sides of the Atlantic. The standard explanation held that land bridges, since sunk beneath the sea, had once connected distant continents, and that the Earth had gradually cooled and wrinkled, raising mountains much as a drying apple shrivels.

Wegener first presented his hypothesis in lectures in early 1912, and expanded it in his book The Origin of Continents and Oceans, first published in 1915 and revised in later editions. He gathered evidence from several fields: matching fossils of land animals and plants found on widely separated continents, continuous rock strata and mountain belts that lined up when the continents were reassembled, and signs of ancient glaciation in regions now tropical, alongside coal deposits in polar areas. He argued these pointed to a single supercontinent, which he called Pangaea, that later broke apart.

Reception was largely hostile. Critics objected that Wegener could not explain what force could push continents through solid ocean floor. His proposals involving centrifugal force and tidal effects were shown by physicists to be far too weak. Some of his figures for the speed of drift were also inaccurate. Many geologists, especially in North America, also resented a proposal from someone outside their discipline. Wegener died on an expedition to Greenland around 1930, still without wide acceptance.

Support revived in the mid-twentieth century. Studies of rock magnetism showed that continents appeared to have shifted position relative to the magnetic poles. Mapping of the ocean floor revealed mid-ocean ridges, and by the 1960s the theory of seafloor spreading explained how new crust forms and moves. Together these findings led to the theory of plate tectonics, in which the rigid outer shell of the Earth is divided into plates driven by processes in the mantle. Wegener is now honored as a pioneer who was right about the central fact of moving continents, even though he was wrong about how it happened.