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Rodinia
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In geology, Rodinia (from the Russian ??????, or "motherland") is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1100 and 750 million years ago, in the Neoproterozoic era.
In contrast with Pangaea, the last supercontinent about 300 million years ago, little is known yet about the exact configuration and geodynamic history of Rodinia.
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In geology, Rodinia (from the Russian ??????, or "motherland") is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1100 and 750 million years ago, in the Neoproterozoic era.
In contrast with Pangaea, the last supercontinent about 300 million years ago, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing similar geologic features, often now widely dispersed.
The extreme cooling of the global climate around 700 million years ago (the so called snowball Earths of the Cryogenian period) and the rapid evolution of primitive life during the subsequent Ediacaran and Cambrian periods are often thought to have been triggered by the breaking up of Rodinia.
Geodynamics
Paleogeographic reconstructions
The idea that a supercontinent existed in the early Neoproterozoic arose in the 1970s, when geologists mentioned that orogens of this age exist on virtually all cratons. Examples are the Grenville orogeny in North America, the Uralian orogeny in Siberia and the Dalslandian orogeny in Europe.
Since then many alternative reconstructions have been proposed for the configuration of the cratons in this supercontinent. Most of these reconstructions are based on the correlation of the orogens on different cratons. Though the configuration of the core cratons in Rodinia is now reasonably well known, recent reconstructions still differ in many details. Geologists try to decrease the uncertainties by collecting geological and paleomagnetical data.
Rodinia's landmass was probably centered south of the equator. Most reconstructions show Rodinia's core was formed by the North American craton (the later paleocontinent of Laurentia), surrounded in the southeast with the East European craton (the later paleocontinent of Baltica), the Amazonian craton and the West African craton; in the south with the Rio de la Plata and São Francisco cratons; in the southwest with the Congo and Kalahari cratons; and in the northeast with Australia, India and eastern Antarctica. The positions of Siberia and North and South China north of the North American craton differ strongly depending on the reconstruction.
Little is known about the paleogeography before the formation of Rodinia. Paleomagnetic and geologic data is only from the break up of Rodinia onward definite enough to form reconstructions that are generally agreed on. Rodinia was probably formed between 1100 and 1000 million years ago and broke up again before 750 million years ago. Rodinia was surrounded by the superocean geologists are calling Mirovia (from mir, the Russian word for "globe").
Break up
In contrast to Rodinia's formation, the movements of continental masses during and since its breakup are fairly well understood. Rifting did not start everywhere simultaneously. Extensive lava flows and volcanic eruptions of Neoproterozoic age are found on most continents, evidence for large scale rifting about 750 million years ago. As early as 850 and 800 million years ago, a rift developed between the continental masses of present-day Australia, eastern Antarctica, India and the Congo and Kalahari cratons on one side and later Laurentia, Baltica, Amazonia and the West African and Rio de la Plata cratons on the other. This rift developed into the Adamastor Ocean during the Ediacaran.
The first group of cratons would eventually, around 550 million years ago (on the boundary between the Ediacaran and Cambrian), fuse again with Amazonia, West Africa and the Rio de la Plata craton. This tectonic phase is called the Pan-African orogeny. It created a configuration of continents that would remain stable for hundreds of millions of years in the form of the continent Gondwana.
In a separate rifting event about 610 million years ago (halfway in the Ediacaran period), the Iapetus Ocean formed. The eastern part of this ocean formed between Baltica and Laurentia, the western part between Amazonia and Laurentia. Because the exact moments of this separation and the partially contemporaneous Pan-African orogeny are hard to correlate, it might be that all continental mass was again joined in one supercontinent between roughly 600 and 550 million years ago. This hypothetical supercontinent is called Pannotia.
Influence on paleoclimate and life
Unlike later supercontinents, Rodinia itself was entirely barren. It existed before life colonized dry land, and, since it predated the formation of the ozone layer, it was too bathed in ultraviolet sunlight for any organism to inhabit it. Nevertheless, its existence did significantly influence the marine life of its time.
In the Cryogenian period the Earth experienced large glaciations, and temperatures were at least as cool as today. Substantial areas of Rodinia may have been covered by glaciers or the southern polar ice cap.
Low temperatures may have been exaggerated during the early stages of continental rifting. Geothermal heating peaks in crust about to be rifted; and since warmer rocks are less dense, the crustal rocks rise up relative to their surroundings. This rising creates areas of higher altitude, where the air is cooler and ice is less likely to melt with changes in season, and it may explain the evidence of abundant glaciation in the Ediacaran period.
The eventual rifting of the continents created new oceans, and seafloor spreading, which produces warmer less-dense oceanic lithosphere. Due to its smaller density, hot oceanic lithosphere will not lie as deep as old, cool oceanic lithosphere. In periods with relatively large areas of new lithosphere, the ocean floors come up, causing the eustatic sea level to rise. The result was a greater number of shallow seas.
The increased evaporation from the larger water area of the oceans may have increased rainfall, which, in turn, increased the weathering of exposed rock. By inputting data on the ratio of stable isotopes 18O:16O into computer models, it has been shown that in conjunction with quick-weathering of volcanic rock, this increased rainfall may have reduced greenhouse gas levels to below the threshold required to trigger the period of extreme glaciation known as Snowball Earth.
Increased volcanic activity also introduced into the marine environment biologically important nutrients, which may have played an important role in the development of the earliest animals.
See also
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