Continental drift is the hypothesis that continents have moved relative to one another over geological time rather than remaining permanently fixed in their present positions. The idea became especially influential through the work of Alfred Wegener, who argued in the early twentieth century that the continents had once been joined into a much larger landmass and subsequently separated.
Continental drift was historically distinct from the modern theory of plate tectonics because Wegener correctly recognised large scale continental movement but did not have a satisfactory physical mechanism capable of explaining how continents could move through oceanic crust. Modern plate tectonics provides that missing framework by showing that continents are embedded within moving lithospheric plates.
One of the most striking pieces of evidence considered in support of continental drift was the apparent geometric fit between continental margins, especially South America and Africa. The match is more meaningful when the edges of continental shelves rather than present coastlines are compared because coastlines can change through erosion, sedimentation and sea level variation. Geological evidence provided additional support. Rock sequences and mountain belts of similar age and composition occur on continents now separated by oceans, suggesting that these regions were once connected. The distribution of certain fossils across continents also indicated former geographical connections that could not be easily explained by migration across modern oceans.
Palaeoclimatic evidence supplied another important argument. Geological deposits show that regions now located in tropical or temperate climates once experienced very different conditions. Glacial deposits and striations occur in several southern continents in patterns that make much more sense when the continents are reconstructed into a former connected landmass. Coal deposits occur in areas that are not currently tropical, reflecting the movement of continents through different climate zones over geological time. These observations demonstrate that continental positions have changed relative to Earth's climate belts. They also helped establish that geological history must be interpreted in terms of changing geography rather than assuming that present continental positions have always existed.
Wegener proposed that the continents had once formed a supercontinent commonly called Pangaea. In his reconstruction, Pangaea existed roughly hundreds of millions of years ago and later fragmented into continental masses that continued moving. The exact timing and configuration of ancient supercontinents are now understood through much more detailed evidence than was available to Wegener. Pangaea itself is only one stage in a much longer tectonic history. Earlier supercontinents and continental assemblies formed and broke apart repeatedly, and continents have occupied many different positions through geological time. Continental drift is therefore best understood as a broad concept within the history of reconstructing ancient geography.
The major weakness of early continental drift theory was its mechanism. Wegener considered possibilities involving centrifugal effects associated with Earth's rotation and tidal forces, but these explanations were not strong enough to account for the enormous forces required to move continents through oceanic crust. Many geologists therefore accepted some of the evidence for continental movement while rejecting Wegener's proposed mechanism. The later discovery of seafloor spreading and the recognition that the ocean floor itself could move transformed the situation. Continents did not need to plough through stationary oceanic crust. Instead, they could move as components of larger lithospheric plates.
Paleomagnetism became especially important in the development of modern understanding. Magnetic minerals in rocks can preserve information about the direction of Earth's magnetic field when the rocks formed. Because the magnetic poles have changed position relative to continents and the field has reversed repeatedly, ancient rocks can provide evidence about past continental locations and orientations. Apparent polar wander paths from different continents could be reconciled more naturally when the continents were moved relative to one another. Marine magnetic anomalies subsequently provided direct evidence for seafloor spreading and helped establish the plate tectonic framework that incorporated continental drift.
Continental drift has consequences far beyond the positions of landmasses. Changing continental arrangements affect ocean currents, atmospheric circulation, climate, sea level, habitats, species distributions and the development of mountain ranges. When continents assemble into large landmasses, interior regions can become more climatically extreme because they are distant from moderating ocean influence. Opening ocean gateways can alter currents and global climate patterns. Continental collisions create mountain systems and can influence atmospheric and erosional processes. The movement of continents also affects the geographical isolation of organisms and therefore plays an important role in biogeography and evolution.
Continental drift is now considered an important historical component of the broader plate tectonic theory rather than a competing explanation. The essential idea that continents move is firmly supported, but modern geology explains the movement through lithospheric plates and their interactions with the mantle. The term can therefore refer either to the historical hypothesis associated particularly with Wegener or, more generally, to the geological phenomenon of long term continental movement. It should not be confused with daily or seasonal movements of land caused by earthquakes or elastic deformation. Continental drift occurs over millions of years and reflects the large scale reorganisation of Earth's lithosphere.
