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| [[File:Wilson-cycle hg.svg|thumb|Wilson cycle]]
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| [[File:Pangäa.jpg|thumb|Simplified sketch of the western part of Pangaea]]
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| The '''supercontinent cycle''' describes the quasi-periodic aggregation and dispersal of [[Earth]]'s [[continental crust]]. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being reconfigured. One complete [[supercontinent]] cycle is said to take 300 to 500 million years. [[Continental collision]] makes fewer and larger continents while [[rifting]] makes more and smaller continents.
| | ATP molecule will be brings three phosphate sectors since its brand name can recommend. ATP > ADt + ) effectiveness is produced employed in a few kind of response elsewhere in the body. When the body's "is" all kinds of sugar, or sometimes anything,[http://www.vinsfromaget.fr/Includes/go.asp Veste Ralph Lauren Pas Cher], the game will use the actual on synthesize a whole lot ATP elements, or even reverse very first problem,[http://www.vinsfromaget.fr/Includes/go.asp?id=570 Gilet Ralph Lauren Homme], <br><br>"(this town of Riverside) skilled us all do many things that were deceitful and thus prohibited, Gill talked about wednesday. Jeb browning, supervisory deputy city attorneys for the city of Riverside, recounted "mister. Gill's the situation would be without the benefit of value and as a consequence (baltimore) visual appeal forward to powerfully protecting the city's interest, brown lightly become less common regarding inquire into any type of says manufactured in the legal action. <br><br>or even industrialised economies, Obstetric systems (OU) are very the primary setting about your birth with the safety associated with othersituations intently argued.1 5 The first concern with reference to contraception exterior to an OU pertains to the anticipations because of unwanted perinatal issues. all the same,[http://www.vinsfromaget.fr/Includes/go.asp?id=227 Parfum Ralph Lauren Romance], Steadilyincreasing birth intervention statistics and as well,as well as the research projects concerning womens ideas the birth present which is ous but not always may complications provideoptimal on behalf of low risk fill affected person or wants and needs.6 14 for all states, the creation of midwifery machines enjoys bearing in mind chicks significantly more decision concerning place of natal. A midwifery device isa healthcare location including consider up to people consisting of basic pregnancies with your time to beginning that comprises midwives takeprimary cutomised dependability for. <br><br>the tonneau's drawing a line under leaves one particular PUSD in addition to two constitution martial arts schools. near April 2008,[http://www.vinsfromaget.fr/Includes/go.asp?id=556 Ralph Lauren Retailers Usa], tempos together with the whole village bus charter and rental boarding school easily shut amid instructive so investment troubles. PUSD board leader Ed Honowitz had to talk about Nia charter started out likely sufficient. possessing famed it is really centenary in April 2013, The Htel Plaza Athne has the capability to justly boast of being one of the endurofgly widespread luxury rooms rome. an area of the Dorchester set, the trendy property and assets pulls a exciting clientele as well as outstanding make think about gilded halls as well as elaborate bridal flowers arrangements platform exemplary. that resort's 146 bedrooms on top of that 45 suites are almost always killed in a much posh, ancient pizzazz that includes italian language linen furthermore clean, Marbled bogs. |
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| It is agreed that the most recent supercontinent, [[Pangaea]], formed about 300 million years ago. But there are two different views on the history of earlier supercontinents. The first proposes a series of supercontinents: [[Vaalbara]] (~3.6 to ~2.8 billion years ago); [[Ur (continent)|Ur]] (~3 billion years ago); [[Kenorland]] (~2.7 to ~2.1 billion years ago); [[Columbia (supercontinent)|Columbia]] (~1.8 to ~1.5 billion years ago); [[Rodinia]] (~1.25 billion to ~750 million years ago); and [[Pannotia]] (~600 million years ago), whose dispersal produced the fragments that ultimately collided to form Pangaea.<ref name="Zhao1">{{Cite journal |doi=10.1016/S0012-8252(02)00073-9 |first=Guochun |last=Zhao |coauthors = Cawood, Peter A.; Wilde, Simon A.; Sun, M. |year=2002|title=Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia supercontinent |journal=Earth-Science Reviews |volume=59 |issue=1–4 |pages=125–162 |bibcode=2002ESRv...59..125Z}}</ref><ref name="Zhao2">{{Cite journal |doi=10.1016/j.earscirev.2004.02.003 |first=Guochun |last=Zhao |coauthors=Sun, M.; Wilde, Simon A.; Li, S.Z. |year=2004|title=A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup |journal=Earth-Science Reviews |volume=67 |issue=1–2 |pages=91–123 |bibcode=2004ESRv...67...91Z}}</ref>
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| The second view (Protopangea-Paleopangea), based on both [[Paleomagnetism|palaeomagnetic]] and geological evidence, is that supercontinent cycles did not occur before about 0.6 [[Gigayear#SI_prefix_multipliers|Ga]] (during the [[Ediacaran]] Period). Instead, the continental crust comprised a single supercontinent from about 2.7 Ga until it broke up for the first time, somewhere around 0.6 Ga. This reconstruction<ref name='piper'>Piper, J.D.A. “A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics.” Tectonophysics. 589 (2013): 44-56.</ref> is based on the observation that if only small peripheral modifications are made to the primary reconstruction, the data show that the palaeomagnetic poles converged to quasi-static positions for long intervals between about 2.7-2.2, 1.5-1.25 and 0.75-0.6 Ga, .<ref name='piper2'>Piper, J.D.A. “Continental velocity through geological time: the link to magmatism, crustal accretion and episodes of global cooling.” Geoscience Frontiers. 4 (2013): 7-36.</ref> During the intervening periods, the poles appear to have conformed to a unified [[apparent polar wander]] path. Thus the paleomagnetic data are adequately explained by the existence of a single Protopangea-Paleopangea supercontinent with prolonged quasi-integrity. The prolonged duration of this supercontinent could be explained by the operation of ''lid tectonics'' (comparable to the tectonics operating on Mars and Venus) during [[Precambrian]] times, as opposed to the plate tectonics seen on the contemporary Earth.<ref name=piper />
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| The kinds of minerals found inside ancient diamonds suggest that the cycle of supercontinental formation and breakup began roughly 3.0 billion years ago. Before 3.2 billion years ago only diamonds with [[peridot]]itic compositions (commonly found in the Earth's mantle) formed, whereas after 3.0 billion years ago eclogitic diamonds (rocks from the Earth's surface crust) became prevalent. This change is thought to have come about because the process of subduction and continental collision introduced [[eclogite]] into subcontinental diamond-forming fluids.<ref>{{cite doi|10.1126/science.1206275}}</ref>
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| The hypothecized supercontinent cycle is complemented by the Wilson cycle named after [[plate tectonics]] pioneer [[J. Tuzo Wilson]], which describes the periodic opening and closing of [[ocean basin]]s. Because the oldest seafloor material found today dates to only 170 million years old, whereas the oldest continental crust material found today dates to at least 4 billion years old, it makes sense to emphasize the much longer record of the planetary pulse that is recorded in the continents.
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| ==Effects on sea level==
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| It is known that [[sea level]] is generally low when the continents are together and high when they are apart. For example, sea level was low at the time of formation of Pangaea ([[Permian]]) and Pannotia (latest [[Neoproterozoic]]), and rose rapidly to maxima during [[Ordovician]] and [[Cretaceous]] times, when the continents were dispersed. This is because the age of the [[oceanic lithosphere]] provides a major control on the depth of the ocean basins, and therefore on global sea level. Oceanic lithosphere forms at [[mid-ocean ridges]] and moves outwards. As this happens, it [[thermal conduction|conductively cools]] and [[thermal expansion|shrinks]]. This cooling and shrinking decreases the thickness and increases the density of the oceanic lithosphere, and the result is the general lowering in elevation of the seafloor away from mid-ocean ridges. For oceanic lithosphere that is less than about 75 million years old, a simple cooling [[Half-space (geometry)|half-space]] model of conductive cooling works, in which the depth of the ocean basins <math>d</math> in areas in which there is no nearby [[subduction]] is a function of the age of the oceanic lithosphere <math>t</math>. In general,
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| :<math>d(t) = (2/\sqrt{\pi}) a_{eff} T_1 \sqrt{\kappa t} + d_r</math>
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| where <math>\kappa</math> is the [[thermal diffusivity]] of the mantle lithosphere (~8*10<sup>−7</sup> [[meter|m]]<sup>2</sup>/[[second|s]]), <math>a_{eff}</math> is the effective [[thermal expansion coefficient]] for rock (~5.7*10<sup>−5</sup> C° <sup>−1</sup>), <math>T_1</math> is the temperature of ascending magma compared to the temperature at the upper boundary (~1220 °C for the Atlantic and Indian Oceans, ~1120 °C for the eastern Pacific) and <math>d_r</math> is the depth of the ridge below the ocean surface.<ref name="parsons&sclater">{{cite journal|last=E.E|first=Davis|coauthors=Lister, C.R.B.|title=Fundamentals of Ridge Crest Topography|journal=Earth and Planetary Science Letters|publisher=North-Holland Publishing Company|volume=21|pages=405–413|last2=Davis|year=1974|doi=10.1016/0012-821X(74)90180-0|bibcode = 1974E&PSL..21..405D }}</ref> After plugging in rough numbers for the sea floor, the equation becomes:
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| :<math>d(t) = 350 \sqrt{t} + 2500</math>
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| for the eastern Pacific Ocean, and:
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| :<math>d(t) = 390 \sqrt{t} + 2500</math>
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| for the Atlantic and Indian Oceans,
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| where <math>d</math> is in meters and <math>t</math> is in millions of years, so that just-formed crust at the mid-ocean ridges lies at about 2,500 m depth, whereas 50 million-year-old seafloor lies at a depth of about 5000 m.<ref name="parsons&sclater">{{cite journal|last=Parsons|first=Barry|coauthors=Sclater, John G.|title=An analysis of the variation of ocean floor bathymetry and heat flow with age|journal=Journal of Geophysical Research|publisher=American Geophysical Union|volume=82|issue=B5|pages=802–827|bibcode=1977JGR....82..802P|last2=Sclater|year=1977}}</ref>
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| As the mean level of the sea floor decreases, the volume of the ocean basins increases, and if other factors that can control sea level remain constant, sea level falls. The converse is also true: younger oceanic lithosphere leads to shallower oceans and higher sea levels if other factors remain constant.
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| Area ''A'' can change when continents [[rift]] (stretching the continents decreases A and raises sea level) or as a result of [[continental collision]] (compressing the continents leads to an increase ''A'' and lowers sea level). Increasing sea level will flood the continents, while decreasing sea level will expose continental shelves.
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| Because the [[continental shelf]] has a very low slope, a small increase in sea level will result in a large change in the percent of continents flooded.
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| If the world ocean on average is young, the seafloor will be relatively shallow, and sea level will be high: more of the continents are flooded. If the world ocean is on average old, seafloor will be relatively deep, and sea level will be low: more of the continents will be exposed.
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| There is thus a relatively simple relationship between the Supercontinent Cycle and the mean age of the seafloor.
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| *Supercontinent = lots of old seafloor = low sea level
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| *Dispersed continents = lots of young seafloor = high sea level
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| There will also be a climatic effect of the supercontinent cycle that will amplify this further:
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| *Supercontinent = continental climate dominant = continental glaciation likely = still lower sea level
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| *Dispersed continents = maritime climate dominant = continental glaciation unlikely = sea level is not lowered by this mechanism
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| ==Relation to global tectonics==
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| There is a progression of tectonic regimes that accompany the supercontinent cycle:
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| During break-up of the supercontinent, rifting environments dominate. This is followed by passive margin environments, while seafloor spreading continues and the oceans grow. This in turn is followed by the development of collisional environments that become increasingly important with time. First collisions are between continents and island arcs, but lead ultimately to continent-continent collisions. This is the situation that was observed during the Paleozoic Supercontinent Cycle and is being observed for the [[Mesozoic]]-[[Cenozoic]] Supercontinent Cycle, still in progress.
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| ==Relation to climate==
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| <!--{{unreferenced section|date=February 2011}}-->{{ref improve section|date=August 2012}}
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| {{Main|Greenhouse and icehouse Earth}}
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| There are two types of global earth climates: icehouse and greenhouse. Icehouse is characterized by frequent continental glaciations and severe desert environments. Greenhouse is characterized by warm climates. Both reflect the supercontinent cycle. We are now in a little greenhouse phase of an icehouse world.<ref name="Read, PhD">{{Cite journal| title = Record of ancient climates can be a map to riches| first = J.Fred | last = Read | year = 2001 | journal = [[Science from Virginia Tech]]| accessdate = 2011-05-04| url = http://www.research.vt.edu/resmag/sciencecol/2001old_weather.html}}</ref>
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| *'''Icehouse climate'''
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| **Continents moving together
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| **Sea level low due to lack of seafloor production
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| **Climate cooler, arid
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| **Associated with [[aragonite sea]]s
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| **Formation of [[supercontinent]]s
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| *'''Greenhouse climate'''
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| **Continents dispersed
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| **Sea level high
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| **High level of [[sea floor spreading]]
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| **Relatively large amounts of CO<sub>2</sub> production at oceanic rifting zones
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| **Climate warm and humid
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| **Associated with [[calcite sea]]s
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| Periods of icehouse climate: much of [[Neoproterozoic]], late [[Paleozoic]], late [[Cenozoic]].
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| Periods of greenhouse climate: Early [[Paleozoic]], [[Mesozoic]]-early [[Cenozoic]].
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| ==Relation to evolution==
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| The principal mechanism for evolution is natural selection among diverse populations. As [[genetic drift]] occurs more frequently in small populations, diversity is an observed consequence of isolation. Less isolation, and thus less diversification, occurs when the continents are all together, producing both one continent and one ocean with one coast. In Latest Neoproterozoic to Early Paleozoic times, when the tremendous proliferation of diverse [[Animal|metazoa]] occurred, isolation of marine environments resulted from the breakup of Pannotia.
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| A north–south arrangement of continents and oceans leads to much more diversity and isolation than east–west arrangements. North–south arrangements give climatically different zones along the communication routes to the north and south, which are separated by water or land from other continental or oceanic zones of similar climate. Formation of similar tracts of continents and ocean basins oriented east–west would lead to much less isolation, diversification, and slower evolution, since each continent or ocean is in fewer climatic zones. Through the [[Cenozoic]], isolation has been maximized by a north–south arrangement.
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| Diversity, as measured by the number of families, follows the supercontinent cycle very well.<ref>{{cite web|last=Benton|first=Michael J.|title=Fossil Record: Quality|url=http://dx.doi.org/10.1038/npg.els.0004144|work=eLS (Encyclopedia of Life Sciences)|publisher=John Wiley & Sons, Ltd|doi=10.1038/npg.els.0004144|date=23 September 2005}}</ref>
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| ==References==
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| {{reflist}}
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| *{{cite journal | last1 = Gurnis | first1 = M. | year = 1988 | title = Large-scale mantle convection and the aggregation and dispersal of supercontinents | url = | journal = Nature | volume = 332 | issue = 6166 | pages = 695–699 | doi=10.1038/332695a0 | bibcode=1988Natur.332..695G}}
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| *{{cite journal | last1 = Murphy | first1 = J. B. | last2 = Nance | first2 = R. D. | year = 1992 | title = Supercontinents and the origin of mountain belts | url = | journal = Scientific American | volume = 266 | issue = 4| pages = 84–91 |bibcode = 1992SciAm.266...84M |doi = 10.1038/scientificamerican0492-84 }}
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| *{{cite journal | last1 = Nance | first1 = R. D. | last2 = Worsley | first2 = T. R. | last3 = Moody | first3 = J. B. | year = 1988 | title = The supercontinent cycle | url = | journal = Scientific American | volume = 259 | issue = 1| pages = 72–79 | doi = 10.1038/scientificamerican0788-72 |bibcode = 1988SciAm.259...72N }}
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| ==External links==
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| *[http://www.scotese.com/earth.htm Reconstructions from "Paleomap Project"]
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| *[http://www.ig.utexas.edu/research/projects/plates/?PHPSESSID=02a1a8119444b3ffacfc533d9825b3d0#recons Plate reconstruction movies from UTIG 'PLATES' project]
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| *[http://csmres.jmu.edu/geollab/Fichter/Wilson/rkcytst.html A Tectonic Rock Cycle]
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| {{Continents of the world}}
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| [[Category:Plate tectonics]]
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| [[Category:Supercontinents]]
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