Why is there a continental drift

Continental drift

The continental drift is the name given to the very slow horizontal drift (epirophoresis) of the earth's crustal parts, the continental tables, made of relatively light material (sial), on the heavy plastic subsurface (Sima), which was first assumed independently of one another by Alfred Wegener and the American Frank B. Taylor for the first time in 1912. , in particular the drifting apart of the American double continent and the Old World, which led to the formation of the Atlantic Ocean, and the divergence of Africa, Antarctica, Australia and India. The Drift- or K. theory assumes a great primordial ocean (primeval Pacific) and a large contiguous primeval continent (Pangea) which is said to have existed up to the Upper Paleozoic or the Mesozoic Era and initially broke into two parts, from which North America and Eurasia on the one hand and South America, Africa, Antarctica, Australia and India on the other hand emerged, which have since diverged in different directions (or towards one another ) drift; z. B. the fold mountains on the west coast of America were formed by traffic jams and India was pushed onto the Eurasian floe.

Arguments for a C. are the correspondence of the opposite continental margins of Africa and South America and these lines with the South Atlantic Ridge, the geological, palaeontological and biogeographical correspondences as well as paleoclimatological indications (especially the permocarbonic glaciation of the southern continent parts). After initially widespread rejection by experts, the theory, which makes the assumption of land bridges or submerged intercontinents superfluous, has been based on paleomagnetic measurements on pre-tertiary rocks on both sides of the Atlantic (paleomagnetism) and the theories of plate tectonics (e.g. magnetic profile recordings in the area of ​​the submarine ridge) receive considerable support.

According to the modern conceptions, the K. v. a. from the plate boundaries formed as mid-ocean ridges. The magmas (material of the earth's mantle) emerging from the central trenches spread sideways (Sea-floor spreading), thereby pushing the entrained continents apart (thus causing the K.) and are finally sucked back into the underground in the area of ​​the large deep-sea trenches in front of continents or island arcs (subduction). Today, the drift speed in the South Atlantic is at least 2 cm annually, in the North Atlantic 1 cm, in the Indian Ocean up to 6 cm, in the central Pacific 5-6 cm, e.g. D. 8-10 cm. These assumptions are confirmed by the age of the (volcanic) rocks covered by thin sediments: The oldest oceanic soils (in the northwestern Pacific) were formed in the Jurassic, 180-200 million years ago. Every year a total of 2.5-3 km3 new oceanic crust formed; over half of today's seabed, d. H. around a third of the earth's surface has only been formed since the beginning of the Cenozoic, for 60 million years.

Instead of the evacuation force used by Wegener, which strives for a mass-symmetrical position of the earth's axis, the driving force of the K. is nowadays assumed to be convection currents in the earth's mantle, which are based on the decay of radioactive elements: The resulting heat accumulates under the continents, flows laterally to the Oceans and here, perhaps starting at hot spots, causes hot lavas to escape.