The movement of the Earth's continents relative to one other, causing them to seem to drift together over the sea bed, is characterised as the Theory of Continental Drift. Although Alfred Wegener was able to establish a plausible hypothesis with data and precisely articulate the idea, it should be remembered that Wegener was not alone in his thinking. Between 1889 and 1909, for example, Roberto Mantovani theorised that all continents had previously been linked in a "supercontinent," and even created an expanding earth idea.
Continental Drift Theory of Wegener
In 1910, Alfred Wegener became interested in the notion of continental drift after noticing how Earth's continents resembled jigsaw puzzle pieces. He observed how the coast of South America perfectly aligned with the coast of Northwest Africa. Wegener's true love for the subject came to light in 1911, when he came across multiple scientific records documenting fossils of similar plants and animals discovered on different sides of the Atlantic.
In the 1920s, Alfred Wegener proposed the continental drift theory. According to Continental Drift Theory, there was once a single large continent known as Pangaea, which was surrounded by a single large ocean known as Panthalassa. Pangaea was separated into two large landmasses by a sea named Tethys: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys. Drift began some 200 million years ago (Mesozoic Era, Triassic Period, Late Triassic Epoch), with continents breaking up and drifting apart.
The drift, according to Wegener, was in two directions: equatorwards owing to the interplay of gravity, pole-fleeing force (produced by the earth's rotation) and buoyancy (ship floats in water due to buoyant force given by water), and westwards due to tidal currents induced by the earth's motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener). Tidal force (the gravitational attraction of the moon and, to a lesser extent, the sun) was also hypothesised by Wegener to have a significant effect.
The polar-fleeing force is related to the earth's rotation. The Earth is not a perfect spherical; it has an equatorial bulge. This bulge is caused by the earth's rotation (greater centrifugal force at the equator). As we go from the poles to the equator, the centrifugal force rises. According to Wegener, the rise in centrifugal force has resulted in pole fleeing. Tidal force is caused by the moon's and sun's attraction, which causes tides to form in marine seas. These pressures, according to Wegener, would become effective if applied over several million years, and the drift is ongoing.
Evidences and Drawbacks of Continental Drift Theory
Aside from the way the continents fit together, Wegener and his allies gathered a substantial amount of data to support the continental drift idea. For example, similar rocks of the same sort and age can be found on both sides of the Atlantic. Wegener stated that the rocks formed side by side and that the earth has since separated. On opposite sides of the Atlantic Ocean, mountain ranges with comparable rock types, structures, and ages can be found.
Mountain ranges in eastern Greenland, Ireland, the United Kingdom, and Norway, for example, are similar to those in the eastern United States and Canada's Appalachians. Wegener came to the conclusion that they began as a single mountain range that was divided when the continents drifted apart. Ancient fossils of the same extinct plant and animal species have been discovered in rocks of the same age but on continents that are currently widely distant. Wegener argued that the species had coexisted, but that the lands had separated after they died and were petrified.
He hypothesised that the creatures could not have travelled across the oceans. The fossils of the seed fern Glossopteris, for example, were too heavy to be transported thus far by wind. The swimming reptile Mesosaurus could only swim in freshwater. Cynognathus and Lystrosaurus were terrestrial reptiles that couldn't swim. Grooves and rock deposits left by past glaciers may still be observed today on several continents around the equator. This would imply that the glaciers developed in the midst of the ocean and/or covered the majority of the Earth. Glaciers now develop exclusively on land and near the poles.
Wegener hypothesised that glaciers were focused over the southern land mass around the South Pole, and that the continents shifted to their current locations later. Although coral reefs and coal-forming wetlands are located in tropical and subtropical habitats, ancient coal seams and coral reefs are discovered in regions that are far too cold now. Wegener hypothesised that these organisms existed in warm temperature zones and that the fossils and coal had later moved to other sites on the continents.
Despite Wegener's compelling data, most geologists at the time dismissed his theory of continental drift. Scientists contended that there was no explanation for how solid continents could smash through solid oceanic crust. Wegener's theory was practically forgotten until technology developments provided further proof that the continents migrated and provided scientists with the means to devise a mechanism for Wegener's floating continents.
Drawbacks of the Theory
Wegener did not explain why the drift started only in the Mesozoic era and not earlier.
Oceans are not taken into account in the hypothesis.
Proofs rely significantly on generalizable assumptions.
Buoyancy, tidal currents, and gravity forces are insufficient to move continents.
Modern theories (Plate Tectonics) acknowledge the existence of Pangaea and comparable land masses but explain the reasons for drift in a fundamentally different way.
Despite being factually incorrect on several counts, Wegener's hypothesis represents a key milestone in the study of tectonics, laying a solid framework for future hypotheses such as seafloor spreading and plate tectonics.