Supercontinent

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Development of the continents 250 million years until today
Paleotectonic reconstruction for the Ediacaric period (550 mya), after the disintegration of Pannotia (view of the South Pole). Sibiria (pink), Laurentia (purple) and Baltica (green) have separated from Pannotia (now Gondwana, yellow). In this reconstruction, (East) Antarctica and Australia (together blue-gray, "Australo-Antarctica") are not yet a constituent of Gondwana.
Artist's impression of the Young Proterozoic Earth with the supercontinent Pannotia
Animation of the disintegration of Pangea (Triassic – today).

A supercontinent is a coherent land mass that unites all or at least almost all continental cores or cratons of the earth , which is created in geological time periods by the movement of the lithospheric plates and then disintegrates again ( Wilson cycle ). The most famous and also the youngest supercontinent is the Pangea , which existed in the Permian and Triassic (275–200 mya ).

The boundary between the supercontinent and the major continent is fluid. Sometimes the largely contiguous land mass, which currently consists of the continents of Europe, Asia and Africa, is referred to as the greater or supercontinent Africa-Eurasia .

The major and super continents of the earth's history

In addition to today's major continents Eurasia or Africa-Eurasia and possible future supercontinents ( Pangea Proxima , Aurica or Amasien in about 250 to 400 million years), there were several - scientifically more or less controversial - major and supercontinents:

  • Laurasia in the late Mesozoic and early Paleogene - about 200 to 55 mya. The northern major continent existed after the disintegration of Pangea until the opening of the North Atlantic and included the present-day continental blocks North America and Eurasia.
  • Pangea in the late Paleozoic and early Mesozoic times - around 275 to 200 mya - the youngest of the “real” supercontinents in Earth's history and the only one whose configuration is largely undisputed. It was created by the closure of the Rheic Ocean and Ural Ocean and the subsequent collision of Laurussia with Gondwana or Siberia-Kazakhstan (today's Northwest Asia). The collisions had a. the Variscan Orogeny result. The huge bay to the east of this C-shaped supercontinent is called the Tethys Sea . The southern part of the young Pangea - the old Gondwana - was affected by the so-called Permo Carboniferous Ice Age .
  • Laurussia in the Paleozoic Era - about 400 to 300 mya. Arose mainly from the closure of the Iapetus Ocean with the subsequent collision of the continents Laurentia ("Ur-North America") and Baltica ("Ur-Europe"). This collision as well as the collision of smaller island arcs and small continents ( Avalonia ) with Laurentia and / or Baltica led to the Caledonian mountain formation . The reddish Devonian Molassesedimenten the Caledonian Mountains in what is now Western Europe, the so-called Old Red Sandstone , Laurussia owes its name to Old Red continent .
  • Gondwana from the latest Neoproterozoic to the Mesozoic - about 550 to 150 mya. The long-lived large southern continent was created by the separation of the Phanerozoic northern continents Baltica, Sibiria and Laurentia from Pannotia and included the present-day continents South America, Africa (including the Arabian Peninsula), Antarctica and Australia (including New Guinea) as well as the Indian subcontinent. Whether Antarctica and Australia (Australo-Antarctica) were already part of Pannotia (and thus Gondwana) at the time the northern continents separated is a matter of dispute. In the following millions of years, smaller continental fragments repeatedly broke away from the northern edge of Gondwana (the so-called Perigondwani Terrane , including Avalonia), drifted north and collided with the northern continents.
  • Pannotia in the Younger Neoproterozoic - about 600 to 550 mya. Created by the collision of the fragments of Rodinia : North Rodinia , South Rodinia and the Congo continent . The mountain formations associated with the formation of Pannotia are summarized under the term Pan-African Orogeny (Brasiliano Orogeny, Cadomian Orogeny). It is a matter of dispute whether Northern Rodinia was involved in the formation of Pannotia as a large continent with a closed structure. It is possible that Australo-Antarctica was only "docked" later on the northwestern edge of the northern part of Pannotia. It is possible that this only happened after the northern Phanerozoic continents drifted away, so that Pannotia never existed as a “real” supercontinent.
  • Rodinia in the Younger Proterozoic - around 1,100 to 750 mya - is considered to be the first "real" supercontinent in the history of the earth. Its constellation and the timing of its formation are controversial. At the end of its existence there was the first secure, possibly even global, glaciation of the earth, the so-called snowball earth, with its climax in the cryogenium . The mountain formations that are related to the formation of Rodinia are summarized under the term Grenville orogeny (Svekonorwegische orogenesis, Sunsás orogenesis).
  • Columbia in the Paleoproterozoic - 1,800 to 1,500 mya - is considered hypothetical, as are its constituent parts Nuna or Nena and Atlantica .
  • Kenorland in the Paleoproterozoic - 2,450 to 2,110 mya - is considered to be paleomagnetic likely. Traces point to an icing, the so-called Huron Ice Age .
  • Ur in the early Archean - 3,000 to 1,000 mya - is considered hypothetical - as is its possible brother continent Arktica - 2,500 mya.

The last-listed major and supercontinents of the older Precambrian (Ur, Kenorland, Columbia) were significantly lower in size than later formations of this type, as the earth's crust only had a few small areas with differentiated continental crust that could collide with one another. The oldest rocks on earth - the Nuvvuagittuq greenstone belt from the superior craton and the acasta gneiss from the slave craton of the Canadian Shield - are more than 4 billion years old and show that mainland islands already existed in the Hadean .

Influence of the supercontinents on the climate and the living world

When all continents are united into one landmass, this has an impact on the climate: there are fewer irrigated coastlines and more arid areas in the interior of the continent. The arid regions of Central Asia ( Gobi , Taklamakan ) are an example of such a development in the interior of a large continent .

Biodiversity is also influenced by the transition from a large continent to several smaller ones: the spread of terrestrial organisms on a single continent is easy and therefore the biodiversity there is rather low. Only the division into several continents leads to the complete isolation of individual populations of the species, from which new species then emerge.

Evidence of the existence of supercontinents

The verification of the supercontinent or Wilson cycle hypothesis can e.g. B. be done by examining the isotope geochemistry of sedimentary rocks. Two examples are given below:

  • Sulfur : Heavy isotopes of an element precipitate out earlier in a supersaturated solution than light isotopes. Therefore it is expected that in geological eras, in which there are many evaporite in which are pools sulphate rich deposits (i. E. L. gypsum ) form and but the water is still to some extent in exchange with the ocean, the proportion of lighter 32 S isotope in the ocean is higher than the proportion of the heavier 34 S isotope. If one also assumes that the number of evaporite basins is particularly high when a supercontinent is in the first phases of the Wilson cycle (advanced continental rifts or narrow ocean basins connected to the sea), a high 32 S / 34 S- Ratio (δ  34 S) in open marine sediments indicate the existence of a disintegrating supercontinent. Corresponding investigations on open marine sediments actually showed increased δ  34 S values ​​for the period about 200 million years and 600 million years ago, for which the beginning of the disintegration of Pangea or Pannotias is assumed.
  • Strontium and osmium : During the chemical weathering of calcium and magnesium silicates (e.g. plagioclases , amphiboles and pyroxenes ), the greenhouse gas carbon dioxide (CO 2 ) is bound from the air in the form of hydrogen carbonate (HCO 3 - ) and into the sea via rivers where it precipitates again mainly in the form of calcium carbonate ( calcite ) and is thus removed from the atmosphere for geological periods of time. This in turn influences the world climate. In said case there is a weakening of the greenhouse effect, i. H. a cooling of the world climate (English: icehouse ), instead, as less CO 2 in the atmosphere can hold back less heat radiated from the earth's surface. An important triggering factor for the Sturtic Ice Age is therefore suspected in the beginning of the breakup of Rodinia and the formation of the so-called Laurentian Magmatic Province in low geographical latitudes ("fire-and-ice" hypothesis): The rift tectonics in the course of the opening of the Proto- The Pacific Basin was associated with volcanism that brought large amounts of basic magma to the surface. The weathering of the corresponding, geologically relatively young, strongly Ca- and Mg-silicate-containing rocks ( basalt , etc.) in this aggressive equatorial climate removed a lot of CO 2 from the atmosphere . This assumption is supported by the low 187 Os / 188 Os- and 87 Sr / 86 Sr ratios in carbonate rocks below and above the glacial deposits of the Sturtic glaciation. 187 Os and 87 Sr are stable decay products of 187 Re and 87 Rb, respectively . The latter have an extremely long half-life (approx. 41 or 48 billion years). In addition, when magma is formed deep in the earth's interior, the mother isotopes 187 Re and 87 Rb go into the melt preferentially over their decay products. Therefore, primary 187 Os and 87 Sr can only accumulate in sediments through the weathering of very old continental crust, which in turn means that low 187 Os / 188 Os- and 87 Sr / 86 Sr ratios indicate increased continental weathering of relatively young igneous rocks in the Can indicate the deposition period of the sedimentary rocks examined.

See also

literature

Popular scientific literature

  • Ted Nield: Super Continent . The Secret Life of Our Planet: An Adventurous Journey Through Earth's History. Verlag Antje Kunstmann, Munich 2008, 287 pages, ISBN 978-3-88897-526-4 .

Web links

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Individual evidence

  1. ↑ Christopher R. Scotese: Late Proterozoic plate tectonics and palaeogeography: a tale of two supercontinents, Rodinia and Pannotia. Geological Society, London, Special Publications. Vol. 326, pp. 67-83, doi : 10.1144 / SP326.4
  2. ↑ Note: Only Pangea and its early constituents (from around 444 mya) offered unrestricted spreading possibilities for land creatures , since the flora only spread on the mainland since the Ordovician and the fauna since the Silurian . It took about 100 million years of development in the marginal seas of the primordial ocean to allow the oxygen content of the earth's atmosphere to rise to 2% through photosynthesis or to form a stable, protective ozone layer as a prerequisite for “going ashore” . The ancestors of modern land creatures developed from the first macroscopic life forms of the Ediacarium and above all the Cambrian Explosion (from 542 mya) . The earlier supercontinents had only undergone geological development; their climate had no direct impact on the evolution of living beings.
  3. ^ R. Damian Nance, Thomas R. Worsley, Judith B. Moody: The Supercontinent Cycle. Scientific American. Vol. 259, 1988, pp. 72–79, doi : 10.1038 / scientificamerican0788-72 ( alternative download , PDF, 1.5 MB)
  4. ↑ Yannick Godderis, Yannick Donnadieu, A. Nédélec, B. Dupré, C. Dessert, A. Grard, G. Ramstein, LM François: The Sturtian 'snowball' glaciation: fire and ice . In: Earth and Planetary Science Letters . 211, No. 1-2, 2003, pp. 1-12. doi : 10.1016 / S0012-821X (03) 00197-3 .
  5. ↑ G. E. Ravizza, J. C. Zachos: Records of Cenozoic Ocean Chemistry. Pp. 551-581 in: H. Elderfield (Ed.): The Oceans and Marine Geochemistry. Treatise on Geochemistry, Volume 6, Elsevier, 2003, doi : 10.1016 / B0-08-043751-6 / 06121-1 .
  6. ^ Alan D. Rooney, FA Macdonald, JV Strauss, FO Dudas, C. Hallmann, D. Selby: Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth . In: Proceedings of the National Academy of Sciences . 2013. doi : 10.1073 / pnas.1317266110 .