Continental drift theory

Continental drift, in the context of the modern theory of plate tectonics , is explained by the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the earth's mantle). Precisely used, the term continental drift is actually rooted in antiquated concepts regarding the structure of the earth. Modern geophyicists and geologists explain the movement or drift of the continents within the context of plate tectonic theory. The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology, and nucleosynthesis in physics and chemistry .

The original theory of continental drift made the improbable assertion that the continents moved through and across an underlying oceanic crust much as ice floats and drifts through water . Eventually multiple lines of evidence allowed modern tectonic theory to replace continental drift theory.

Based upon centuries of cartographic depictions that allowed a good fit between the Western coast of Africa and the Eastern coast of South America—in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean.

In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust.

Wegener's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit who discovered a similarity in the fossils found on the coasts of Africa and South America that were seemingly derived from a common source. Other scientists also attempted to explain orogeny (mountain building) as resulting from Wegener's continental drift.

Wegener's initial continental drift assertions were based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas. Wegener also argued that the evidence of Paleozoic glaciation in South Africa, South America, India and Australia—sites far removed from estimates of the geographical extent of glaciation—argued strongly for continental drift

The technological advances necessitated by the Second World War made possible the accumulation of significant evidence regarding Wegener's hypothesis, eventually refining and supplanting Wegener's theory of continental drift with modern plate tectonic theory. Although Wegener's theory accounted for much of the then existing geological evidence, Wegener's hypothesis was specifically unable to provide a verifiable or satisfying mechanism by which continents—with all of their bulk and drag—could move over an underlying mantle that was solid enough in composition to be able to reflect seismic S-waves.

In his 1960 publication, History of Ocean Basins, geologist and U.S. Navy Admiral Harry Hess (1906–1969) asserted that thermal convection currents in the athenosphere provided the driving force behind plate tectonics. The degree with which the earlier geological community resisted acceptance of Wegener's theory of continental drift is clearly demonstrated by the fact that Hess's assertion of thermal currents was drawn from work done by Author Holmes in the 1930s.

See also Earth, interior structure; Hotspots; Sea-floor spreading; Seismology