Continental drift

Continental drift is the idea that the continents have not always occupied their present positions. Alfred Wegener developed a comprehensive version of the hypothesis in the early twentieth century, arguing that the continents had once formed a larger landmass, commonly called Pangaea, and later separated. Earlier writers had noticed that some continental outlines appear to fit, but Wegener's argument drew together several types of geological and paleontological evidence. Continental drift is now regarded as an important precursor to plate tectonics, rather than a complete equivalent of the modern theory.

Wegener pointed to the apparent correspondence between the margins of South America and Africa, but shoreline resemblance alone was not decisive. He also compared geological structures and rock sequences on now-separated continents, along with fossils of organisms found on both sides of the Atlantic. Some of these organisms were terrestrial or freshwater forms, making a continuous ancient land connection a more plausible explanation than crossing a wide ocean. Evidence of past climates also mattered: geological deposits and fossil plants in regions whose present climates differ sharply suggested that continents had once occupied different latitudes. These clues were mutually reinforcing, though at the time they did not establish a physical mechanism for continental motion.

Many geologists rejected Wegener's proposal while he was alive. A central weakness was that he could not provide a convincing account of the forces capable of moving continents through the oceanic crust. The dispute was not settled simply by recognizing matching coastlines or fossils; a viable explanation had to account for the behavior of the Earth's outer layer and the ocean basins as well. Wegener continued to defend and refine his hypothesis, but died in 1930 before the later evidence that transformed the debate was available.

Ocean-floor mapping and geophysical research in the 1950s and 1960s revealed features such as mid-ocean ridges and deep-sea trenches. Work on the ocean floor supported the idea that new oceanic crust forms at spreading centers and is recycled at subduction zones. Symmetrical patterns of magnetic anomalies on either side of spreading ridges provided another line of evidence for seafloor spreading. The modern theory of plate tectonics brought these findings together: continents are embedded in lithospheric plates that move relative to one another. The theory explains continental motion within a framework that Wegener's original model lacked, while preserving his central insight that continents change position over geological time.