Continental drift

The theory of continental drift, also called continental drift, describes the slow movement, splitting and merging of continents. First hypotheses on continental drift led in the 18th and 19th centuries, a gradual shift away from Fixismus back to Mobilism. The movement of the continents is now part of the theory of plate tectonics.

• 5.1 Forecasts for continents
• 5.2 Extrapolation in seven steps

• 7.1 animations
• 7.2 videos
• 7.3 Scientific contributions

Demarcation

The causative forces are still in dispute, the geodynamic findings, however, clearly. Satellite geodesy and modern geodetic earth model show almost all continental margins annual movement rates ranging from 1 cm to about 10 cm. Also, some intra- continental fault lines (eg in the Mediterranean and East Africa) show recent shifts.

Unlike the concept of plate tectonics one speaks of plate kinematics, if it is purely geometrical models of crustal movements in which the causative forces are not considered. These kinematic models are currently subject to less discrepancies than their dynamic counterparts (eg NUVEL ) and are therefore already as reference systems for global coordinate changes. Best known is the international reference frame of the ITRF, which is adapted to the current continental drift since 2000, every 1-2 years.

History of Theory

First hypotheses ( 17-18. Century)

The most obvious and therefore recognized at the earliest reference to the continental drift is the similarity in the course of the west coast of Africa and the east coast of South America.

The oldest presumption of continental drift as the cause comes from the Flemish cartographer Abraham Ortelius. In the edition of his atlas Theatrum Orbis Terrarum of 1596, the similarity of the two coastlines is already clearly visible.

Often the first indication of Sir Francis Bacon is credited in 1620, but should Bacon have referred only to the similarity of the west coasts of both continents, so the Atlantic coast of Africa and the Pacific coast of South America By contrast, the theology professor Theodor Christoph Lilienthal given the similarity of the opposite coasts of South America and Africa in 1756 raised the possibility in Königsberg, that they were once close to each other. The break he brought the biblical deluge in conjunction.

Also in 1782, Benjamin Franklin made ​​in a letter to the French geologist Jean -Louis Giraud - Soulavie thoughts on continental drift:

" Such changes in the outer regions of the earth seemed to me to be unlikely if the earth were fixed to the center. I imagined, therefore, that the inner regions could be a liquid of much higher density and higher specific gravity than any of the solid substances that we know and that therefore float the outer areas on or in the liquid. Thus, the surface of the earth would be a bowl by the violent movements of the fluid on which it floats, and can break in disarray ... "

Alexander von Humboldt described 1801 and 1845, the similarity of the opposite coasts of South America and Africa and speculated that the Atlantic was washed out by a catastrophic power. In 1858, the American Antonio Snider - Pellegrini went a step further when he published the first map on which the Old and the New World were to be seen without being separated by ocean. He also speculated that it was the deluge which had separated the continents from each other. Around the turn of the century the drifting apart of the American and the African continent with the formation of the Moon from the Pacific has been associated.

The Austrian geologist Eduard Suess (1831-1914) advocated in his book series " The Face of the Earth" first the land bridge theory to explain the striking similarities between certain fossil plants and animals Socialization on different continents today. Later, however, he postulated the existence of two formerly contiguous large land masses. For the more southerly of the two Suess coined the previously introduced by another name Gondwana -Land. This continent was still in the Mesozoic includes all of today's continents of the southern hemisphere including India. At the beginning of the Cenozoic but were large parts of this continent has fallen and become oceans.

Alfred Wegener and his opponents

Wegener's immediate predecessor

This Fixismus for scoring models assume that the earth's crust is firmly connected to the ground. Especially through the work of Alfred Wegener ( 1912, 1915), these models were abandoned in favor of another as seen from the shift of the continents ( Mobilism ). Wegener himself stated in 1929 that the idea of ​​continental drift other than movement of parts of the earth's crust, though not had in so elaborate form like himself, before him. He emphasized that he had won his own ideas independent of those of his predecessors. Among those mentioned by Wegener predecessors Coxworthy Franklin ( 1848-1890 ), Roberto Mantovani ( 1889-1909 ), William Henry Pickering (1907 ) and Frank Bursley Taylor ( 1908), which postulated the breakup of Urkontinenten.

As Wegener noticed vote Roberto Mantovani card on which the original grouping of the continents is shown in part " amazing," with his own match. Wegener, however, did not mention that Mantovani did not emanate directly from a drift of the continents, but by an expansion of the oceanic areas between the continents in terms of the competing theory of Earth expansion. While the developed in the 1960s, the theory of plate tectonics supports the idea of the creation of new oceanic crust by seafloor spreading, however, takes a dive of the crust into the mantle at some continental margins ( subduction ) and thus a constant radius of the Earth at, although the conclusion ocean floor spreading and subduction would have to hold each other quantitatively the scale, from the fact of the subduction of certain continental margins may not necessarily be derived.

Particularly similar Wegener is portrayed by Frank Bursley Taylor, the theory of which claimed 29 December 1908 in a lecture to the Geological Society of America, the continents were slowly drifted apart as opposed to Mantovani at constant radius of the earth. Wegener pointed this out in particular on Taylor's account of the development of the Atlantic and the separation of Greenland from North America. The theory of continental drift was therefore formerly known in America as " Taylor - Wegener's theory," said Taylor became one of the first of Wegener's followers later.

Alfred Wegener

In his 1915 published book The Origin of Continents and Oceans Wegener concluded from the exact fit of the coastlines of South America and Africa that these fragments might have been a once larger continent that broke up in the geological past. The fit is more accurate, if one considers not the shorelines, but the shelf edges, ie the submerged parts of a continent. Unlike its predecessors, however, succeeded in Wegener to support his theory by a variety of studies in the various branches of the geosciences. These include the following data collected by Wegener arguments:

• Fold belt and shear zones from South America can be compared with very similar rock sequences and deformation patterns in Africa.
• Diamond deposits in South America and West Africa have geological similarities.
• On all the southern continents to find air - witnesses of the Permo - Carboniferous glaciation, including rectified glacial grinding marks.
• Certain fossil and recent floras and faunas on both sides of the Atlantic agree: Fossils cold-loving land plants with tongue-shaped leaves ( Glossopteris flora) were common to all southern continents.
• Fossil remains of Mesosaurus, a living in freshwater reptile could be detected both in Africa and in South America.
• The manatees occur in both West Africa and in Central and South America.

Based on such evidence Wegener reconstructed a supercontinent, which he Pangaea - All Country - called, which included not only the southern continents, but all known continental masses. According to his theory should, which consists of predominantly granitic rock, specifically lighter continental crust or " SiAl " - in addition to silicon, aluminum is the predominant element of the granite - on the denser, basaltic underground, the " SiMa " - the aluminum is present in the basalt by magnesium - " float" a bit like an iceberg in the sea drives.

As a possible force that could shatter and disperse the continents, Wegener suggested various astronomical forces before: for example, the deceleration of the Earth's rotation by tidal friction of the Moon, precession and tidal forces. The " Polflucht ", ie the centrifugal force generated by the rotation of the earth should slow to move on the earth's mantle "floating" continental masses toward the equator. This hypothesis was supported by P. S. Epstein 1920 elaborated, but soon disproved.

The theory of continental drift was not long prevail, what was the absence of a plausible explanation of the shifts. The explanations of Mantovani ( movement as the result of Earth expansion due to thermal expansion ), Taylor (motion due to the tidal forces by the approach of the moon 100 million years ago ), and Wegener ( movement due to the centrifugal and tidal power ) proved to be wrong. Only the study of plate tectonics in 1960 brought the general recognition of the continental drift. A less scientific motive of Wegener's opponents was possibly in the conflicts between the then strictly separate branches of geosciences. As Wegener had originally occupied with astronomy, meteorology and climatology, he was considered by many "real" geologists as an unqualified " career changers ".

Basic concept of continental drift

Of particular interest are those periods of Earth's history, in which all continents were united into a landmass or where the so-called super-continent broke apart again. One also speaks of supercontinent cycles, of which five or six are postulated in the course of the earth. However, only the last (named Pangaea designated ) and the penultimate ( under the name of Rodinia or Vendian known ) supercontinent generally accepted. If the current movement of the continents continues unabated, Australia is in about 60 million years ago abut Asia and one can roughly 300 million years ago, the formation of a new supercontinent, Pangaea ultima sometimes called expect.

The geological layers that emerged between the Permian and Jurassic, allow the reconstruction of the broken apart at the time the supercontinent Pangaea. In contrast, the reconstruction of Rodinia, broke up the end of the Precambrian, less well secured.

→ The mechanisms of the process see plate tectonics

Plate shifts in the past

It is known how fast and where the large plates are currently moving, and various clues allow their paths to reconstruct the past. Because of their inertia they need tens of millions of years to come to a halt, and even longer to reverse their motion.

• It is assumed that the land mass of the earth from about 320 million years ago, essentially comprised two continents, Gondwana and Laurasia namely.
• Before about 250 million years, the two had grown together into a giant continent Pangaea, which was surrounded by giant ocean Panthalassa, and in the east from the Tethys as a huge bay stretched into it.
• Before about 135 million years ago, the continental mass broke apart. The Tethys opened further to the west and separated from a southern continent, which is again called Gondwana. The northern continent was divided by the opening of the North Atlantic in the two parts of North America and Eurasia.
• Until about 100 million years, the decay process of the continents has continued. Above all, the great southern continent has split in South America, Africa, India, Antarctica and Australia. The Tethys separated to be the northern continents of the southern continents.

470 Ma - Ordovician

430 Ma - Silurian

370 Ma - Devon

300 Ma - Carbon

260 Ma - Perm

220 Ma - Triassic

150 Ma - Jura

90 Ma - Chalk

50 Ma - Eocene

20 Ma - Neogen

Plate shifts in the future

All geological observations indicate that the sheets are more dynamic.

Forecasts for continents

India is still pushing for some time under the Himalayas and perhaps disappear entirely under Tibet. But will continue to grow in the north, the elevations in Mongolia and finally an extensive mountain range will advance to Lake Baikal.

Big changes are expected before Australia, the drifting rapidly to the north and is likely to slide under the Sunda Islands. These then grow up to a new mountain range that slides onto the Australian plate.

Along the Lakes of the East African Rift, the southern part of the Great African grave breach, the emergence of a new ocean is observed, which will connect to the Red Sea. As a result, a new continental plate of Africa will split off and continued to drift eastward. A new mid oceanic ridge will then be out of the grave breach.

The Atlantic will continue to open along the mid-Atlantic ridge. In return, the Pacific will gradually shrink and disappear completely in the future. The Pacific is a remnant of Panthalassa - ocean that is said to have once surrounded the supercontinent Pangaea, however, be of whose former existence no geological evidence. The mid-ocean ridges of the North Pacific, the East Pacific Rise was subducted under North America. Prior to South America, this process is imminent. It is assumed that this will continue.

Extrapolation in seven steps

It is impossible to predict exactly how the land masses will be distributed in 200 million years on Earth's surface. An extrapolation of the current movements is as follows: