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In geology, a supercontinent is the assembly of most or all of Earth's continent or to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leaves room for interpretation and is easier to apply to Precambrian times. To separate supercontinents from other groupings, a limit has been proposed in which a continent must include at least about 75% of the continental crust then in existence in order to qualify as a supercontinent.
Moving under the forces of plate tectonics, supercontinents have assembled and dispersed multiple times in the geologic past. According to modern definitions, a supercontinent does not exist today; the closest is the current landmass, which covers approximately 57% of Earth's total land area. The last period in which the continental landmasses were near to one another was 336 to 175 million years ago, forming the supercontinent Pangaea. The positions of continents have been accurately determined back to the early Jurassic, shortly before the breakup of Pangaea.Fluteau, Frédéric. (2003). "Earth dynamics and climate changes". C. R. Geoscience 335 (1): 157–174. doi:10.1016/S1631-0713(03)00004-X Pangaea's predecessor Gondwana is not considered a supercontinent under the first definition since the landmasses of Baltica, Laurentia and Siberia were separate at the time.
A future supercontinent, termed Pangaea Proxima, is hypothesized to form within the next 250 million years.
For the period before Pangaea, there are two contrasting models for supercontinent evolution through geological time.
Although it contrasts the first model, the first phase (Protopangea) essentially incorporates Vaalbara and Kenorland of the first model. The explanation for the prolonged duration of the Protopangea–Paleopangea supercontinent appears to be that lid tectonics (comparable to the tectonics operating on Mars and Venus) prevailed during Precambrian times. According to this theory, plate tectonics as seen on the contemporary Earth became dominant only during the latter part of geological times. This approach was widely criticized by many researchers as it uses incorrect application of paleomagnetic data.
Secular trends such as , , , and greenstone belt deformation events are all possible indicators of Precambrian supercontinent cyclicity, although the Protopangea–Paleopangea solution implies that Phanerozoic style of supercontinent cycles did not operate during these times. Also, there are instances where these secular trends have a weak, uneven, or absent imprint on the supercontinent cycle; secular methods for supercontinent reconstruction will produce results that have only one explanation, and each explanation for a trend must fit in with the rest.
The following table names reconstructed ancient supercontinents, using Bradley's 2011 looser definition, with an approximate timescale of millions of years ago (Ma).
| Vaalbara | Also described as a supercraton or just a continent |
| Described as both a continent and a supercontinent | |
| Alternatively the continents may have formed into two groupings Superior craton and Sclavia craton | |
| Not generally regarded as a supercontinent, depending on definition | |
| Atlantica | Not generally regarded as a supercontinent, depending on definition |
| From the Carboniferous, formed part of Pangaea, not always regarded as a supercontinent | |
Besides having compositional effects on the upper mantle by replenishing the large-ion lithophile elements, volcanism affects plate movement. The plates will be moved towards a geoidal low perhaps where the slab avalanche occurred and pushed away from the geoidal high that can be caused by the plumes or superplumes. This causes the continents to push together to form supercontinents and was evidently the process that operated to cause the early continental crust to aggregate into Protopangea.Piper, J.D.A. "Protopangea: palaeomagnetic definition of Earth's oldest (Mid-Archaean-Paleoproterozoic) supercontinent." Journal of Geodynamics. 50 (2010): 154–165.
Dispersal of supercontinents is caused by the accumulation of heat underneath the crust due to the rising of very large or plumes, and a massive heat release resulted in the final break-up of Paleopangea.Piper, J.D.A., "Paleopangea in Meso-Neoproterozoic times: the paleomagnetic evidence and implications to continental integrity, supercontinent from and Eocambrian break-up." Journal of Geodynamics. 50 (2010): 191–223. Accretion occurs over geoidal lows that can be caused by avalanche slabs or the downgoing limbs of convection cells. Evidence of the accretion and dispersion of supercontinents is seen in the geological rock record.
The influence of known volcanic eruptions does not compare to that of . The timing of flood basalts has corresponded with a large-scale continental break-up. However, due to a lack of data on the time required to produce flood basalts, the climatic impact is difficult to quantify. The timing of a single lava flow is also undetermined. These are important factors on how flood basalts influenced paleoclimatology.
Phanerozoic (541 Ma to present) and Precambrian ( to ) had primarily passive margins and detrital (and orogenic ), whereas the tenure of Pangaea contained few. Matching edges of continents are where passive margins form. The edges of these continents may rift. At this point, seafloor spreading becomes the driving force. Passive margins are therefore born during the break-up of supercontinents and die during supercontinent assembly. Pangaea's supercontinent cycle is a good example of the efficiency of using the presence or lack of these entities to record the development, tenure, and break-up of supercontinents. There is a sharp decrease in passive margins between 500 and during the timing of Pangaea's assembly. The tenure of Pangaea is marked by a low number of passive margins during 336 to and its break-up is indicated accurately by an increase in passive margins.
Orogenic belts can form during the assembly of continents and supercontinents. The orogenic belts present on continental blocks are classified into three different categories and have implications for interpreting geologic bodies. Intercratonic orogenic belts are characteristic of ocean basin closure. Clear indicators of intracratonic activity contain and other oceanic materials that are present in the suture zone. Intracratonic orogenic belts occur as thrust belts and do not contain any oceanic material. However, the absence of ophiolites is not strong evidence for intracratonic belts, because the oceanic material can be squeezed out and eroded away in an intracratonic environment. The third kind of orogenic belt is a confined orogenic belt which is the closure of small basins. The assembly of a supercontinent would have to show intracratonic orogenic belts. However, interpretation of orogenic belts can be difficult.
The collision of Gondwana and Laurasia occurred in the late Palaeozoic. By this collision, the Variscan orogeny was created, along the equator. This 6000-km-long mountain range is usually referred to in two parts: the Variscan orogeny of the late Carboniferous makes up the eastern part, and the western part is the Appalachian Mountains, uplifted in the Cisuralian. (The existence of a flat elevated plateau like the Tibetan Plateau is under debate.) The locality of the Variscan range made it influential to both the northern and southern hemispheres. The elevation of the Appalachians would greatly influence global atmospheric circulation.
In contrast, the Protopangea–Paleopangea theory shows that these glaciations correlated with periods of low continental velocity, and it is concluded that a fall in tectonic and corresponding volcanic activity was responsible for these intervals of global frigidity. During the accumulation of supercontinents with times of regional uplift, glacial epochs seem to be rare with little supporting evidence. However, the lack of evidence does not allow for the conclusion that glacial epochs are not associated with the collisional assembly of supercontinents. This could just represent a preservation bias.
During the late Ordovician (~458.4 Ma), the particular configuration of Gondwana may have allowed for glaciation and high CO2 levels to occur at the same time.Crowley, Thomas J., "Climate Change on Tectonic Time Scales". Tectonophysics. 222 (1993): 277–294. However, some geologists disagree and think that there was a temperature increase at this time. This increase may have been strongly influenced by the movement of Gondwana across the South Pole, which may have prevented lengthy snow accumulation. Although late Ordovician temperatures at the South Pole may have reached freezing, there were no ice sheets during the Llandovery epoch through the late Mississippian Agreement can be met with the theory that continental snow can occur when the edge of a continent is near the pole. Therefore Gondwana, although located tangent to the South Pole, may have experienced glaciation along its coasts.
Cold winters in continental interiors are due to rate ratios of radiative cooling (greater) and heat transport from continental rims. To raise winter temperatures within continental interiors, the rate of heat transport must increase to become greater than the rate of radiative cooling. Through climate models, alterations in atmospheric CO2 content and ocean heat transport are not comparatively effective.
CO2 models suggest that values were low in the late Cenozoic and Carboniferous-Permian glaciations. Although early Paleozoic values are much larger (more than 10 percent higher than that of today). This may be due to high seafloor spreading rates after the breakup of Precambrian supercontinents and the lack of land plants as a carbon sink.
During the late Permian, it is expected that seasonal Pangaean temperatures varied drastically. Subtropic summer temperatures were warmer than that of today by as much as 6–10 degrees, and mid-latitudes in the winter were less than −30 degrees Celsius. These seasonal changes within the supercontinent were influenced by the large size of Pangaea. And, just like today, coastal regions experienced much less variation.
During the Jurassic, summer temperatures did not rise above zero degrees Celsius along the northern rim of Laurasia, which was the northernmost part of Pangaea (the southernmost portion of Pangaea was Gondwana). Ice-rafted sourced from Russia are indicators of this northern boundary. The Jurassic is thought to have been approximately 10 degrees Celsius warmer along 90 degrees East paleomagnetism compared to the present temperature of today's central Eurasia.
Oxygen levels of the Archaean were negligible, and today they are roughly 21 percent. It is thought that the Earth's oxygen content has risen in stages: six or seven steps that are timed very closely to the development of Earth's supercontinents.Campbell, Ian H., Charlotte M. Allen. "Formation of Supercontinents Linked to Increases in Atmospheric Oxygen." Nature. 1 (2008): 554–558.
At there was an increase in molybdenum isotope fractionation. It was temporary but supports the increase in atmospheric oxygen because molybdenum isotopes require free oxygen to fractionate. Between 2.45 and the second period of oxygenation occurred, which has been called the 'great oxygenation event.' Evidence supporting this event includes red beds appearance (meaning that Fe3+ was being produced and became an important component in soils).
The third oxygenation stage approximately is indicated by the disappearance of iron formations. Neodymium isotopic studies suggest that iron formations are usually from continental sources, meaning that dissolved Fe and Fe2+ had to be transported during continental erosion. A rise in atmospheric oxygen prevents Fe transport, so the lack of iron formations may have been the result of an increase in oxygen. The fourth oxygenation event, roughly is based on modeled rates of sulfur isotopes from marine carbonate-associated . An increase (near doubled concentration) of sulfur isotopes, which is suggested by these models, would require an increase in the oxygen content of the deep oceans.
Between 650 and there were three increases in ocean oxygen levels, this period is the fifth oxygenation stage. One of the reasons indicating this period to be an oxygenation event is the increase in redox-sensitive molybdenum in black . The sixth event occurred between 360 and and was identified by models suggesting shifts in the balance of 34S in and 13C in , which were strongly influenced by an increase in atmospheric oxygen.
Some issues exist with relying on granite sourced zircons, such as a lack of evenly globally sourced data and the loss of granite zircons by sedimentary coverage or consumption. Where granite zircons are less adequate, detrital zircons from appear and make up for the gaps. These detrital zircons are taken from the sands of major modern rivers and their . Oceanic magnetic anomalies and paleomagnetic data are the primary resources used for reconstructing continent and supercontinent locations back to roughly 150 Ma.
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