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{{short description|Repeated joining and separation of Earth's continents}}
[[File:Pangaea continents.svg|thumb|Map of [[Pangaea]] with modern continental outlines]]

The '''supercontinent cycle''' is the [[Quasiperiodicity|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&nbsp;million years. [[Continental collision]] makes fewer and larger continents while [[rift]]ing makes more and smaller continents.

==Theory==
[[File:Platetechsimple.png|thumb|right|Simplified representation of the proposed series of supercontinents to the modern day]]
The most recent [[supercontinent]], [[Pangaea]], formed about 300&nbsp;million years ago (0.3 [[billion years#SI_prefix_multipliers|Ga]]), during the [[Paleozoic]] era. There are two different views on the history of earlier supercontinents. 

=== Series ===
The first theory proposes a series of supercontinents: starting with [[Vaalbara]] (3.6 to 2.8&nbsp;Ga); [[Ur (continent)|Ur]] (c. 3&nbsp;Ga); [[Kenorland]] (2.7 to 2.1&nbsp;Ga); [[Columbia (supercontinent)|Columbia]] (1.8 to 1.5&nbsp;Ga); [[Rodinia]] (1.25&nbsp;Ga to 750&nbsp;Ma); and [[Pannotia]] ({{circa}} 600&nbsp;Ma), whose dispersal produced the continents that ultimately collided to form Pangaea.<ref name="Zhao1">{{Cite journal |doi=10.1016/S0012-8252(02)00073-9 |first1=Guochun |last1=Zhao |last2 = Cawood |first2=Peter A. |last3=Wilde |first3=Simon A. |last4=Sun |first4=M. |year=2002|title=Review of global 2.1–1.8&nbsp;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 |first1=Guochun |last1=Zhao |last2=Sun |first2=M. |last3=Wilde |first3=Simon A. |last4= Li |first4=S.&nbsp;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|url=http://www.gt-crust.ru/jour/article/view/518 }}</ref>

The kinds of [[Inclusion (mineral)|minerals found inside]] ancient [[diamond]]s suggest that the cycle of supercontinental formation and breakup began roughly 3&nbsp;Ga. Before 3.2&nbsp;Ga, only diamonds with [[peridot]]itic compositions (commonly found in the [[Earth's mantle]]) formed, whereas after 3.0&nbsp;Ga [[Eclogitization|eclogitic]] diamonds (rocks from the [[Earth's crust]]) became prevalent. This change is thought to have come about as [[subduction]] and [[continental collision]] introduced [[eclogite]] into subcontinental diamond-forming fluids.<ref>{{Cite journal | last1 = Shirey | first1 = S.&nbsp;B. | last2 = Richardson | first2 = S.&nbsp;H. | doi = 10.1126/science.1206275 | title = Start of the Wilson Cycle at 3&nbsp;Ga Shown by Diamonds from Subcontinental Mantle | journal = Science | volume = 333 | issue = 6041 | pages = 434–436 | year = 2011 | pmid =  21778395| bibcode = 2011Sci...333..434S }}</ref>

[[File:Wilson-cycle hg.svg|thumb|The supercontinent cycle and the [[Wilson cycle]] produced the supercontinents [[Rodinia]] and [[Pangaea]]]]
The hypothesized supercontinent cycle is concurrent with the shorter-term [[Wilson Cycle]] named after plate tectonics pioneer [[John Tuzo Wilson]], which describes the periodic opening and closing of [[oceanic basin]]s from a single plate rift. The oldest seafloor material found today dates to 170&nbsp;Ma, whereas the oldest continental crust material found today dates to 4&nbsp;Ga, showing the relative brevity of the regional Wilson cycles compared to the whole-planetary pulses seen in the arrangement of the continents.

=== Protopangea–Paleopangea ===
The second view, based on both [[Paleomagnetism|palaeomagnetic]] and geological evidence, is that supercontinent cycles did not occur before about 0.6&nbsp;Ga (during the [[Ediacaran]] period). Instead, the [[continental crust]] comprised a single supercontinent from about 2.7&nbsp;Ga until it broke up for the first time, somewhere around 0.6&nbsp;Ga. This reconstruction<ref name="piper">{{cite journal|last=Piper |first=J.&nbsp;D.&nbsp;A. |title=A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics |journal=Tectonophysics |volume=589 |date=2013 |pages=44–56 |doi= 10.1016/j.tecto.2012.12.042|bibcode=2013Tectp.589...44P }}</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 Ga; 1.5–1.25 Ga; and 0.75–0.6&nbsp;Ga.<ref name="piper2">{{cite journal|last=Piper |first=J.&nbsp;D.&nbsp;A. |title=Continental velocity through geological time: the link to magmatism, crustal accretion and episodes of global cooling |journal=Geoscience Frontiers |volume=4 |date=2013 |pages=7–36 |doi= 10.1016/j.gsf.2012.05.008|doi-access=free }}</ref> During the intervening periods, the poles appear to have conformed to a unified [[apparent polar wander]] path. 

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" /> However, this approach is widely criticized as an incorrect application of paleomagnetic data.<ref name="li2009">{{cite journal |last1=Z.X |first1=Li |title=How not to build a supercontinent: A reply to J.D.A. Piper |journal=Precambrian Research |date=October 2009 |volume=174 |issue=1–2 |pages=208–214 |doi=10.1016/j.precamres.2009.06.007}}</ref>

==Effects on sea level==
{{Refimprove-section|date=May 2021}}
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.  

Major influences on sea level during the break up of supercontinents include: oceanic crust age, lost [[back-arc basin]]s, marine sediment depths, emplacement of large igneous provinces, and the effect of passive margin extension.  Of these, oceanic crust age, and marine sediment depths seem to play some of the largest roles in creating a sea level model. The addition of the other controlling parameters help stabilize models when data is sparse.<ref>Wright, N. M., Seton, M., Williams, S., Whittaker, J. M., & Müller, R. D. (2020). Sea-level fluctuations driven by changes in global ocean basin volume following supercontinent break-up. ''Earth-Science Reviews''. {{doi|10.1016/j.earscirev.2020.103293}}</ref> 

The age of the [[Lithosphere#Oceanic lithosphere|oceanic lithosphere]] provides a first order control on the depth of the ocean basins, and therefore on global sea level. Oceanic lithosphere forms at [[mid-ocean ridge]]s and moves outwards, [[thermal conduction|conductively cooling]] and [[thermal expansion|shrinking]], which decreases the thickness and increases the density of the oceanic lithosphere, and lowers the seafloor away from mid-ocean ridges. For oceanic lithosphere that is less than about 75&nbsp;Ma, a simple cooling [[Half-space (geometry)|half-space]] model of conductive cooling works, in which the depth of the ocean basins ''d'' in areas in which there is no nearby subduction is a function of the age of the oceanic lithosphere ''t''. In general,

<math display="block">d(t) = \frac{2}{\sqrt{\pi}} a_{\rm eff} T_1 \sqrt{\kappa t} + d_{\rm r}</math>

where ''κ'' is the [[thermal diffusivity]] of the mantle lithosphere ({{circa}} {{val|8|e=-7|u=[[metre|m]]<sup>2</sup>/[[second|s]]}}), ''a''<sub>eff</sub> is the effective thermal expansion coefficient for rock ({{circa}} {{val|5.7|e=-5|u=°C<sup>−1</sup>}}), ''T''<sub>1</sub> is the temperature of ascending magma compared to the temperature at the upper boundary ({{circa}} 1220&nbsp;°C for the Atlantic and Indian Oceans, {{circa}} 1120&nbsp;°C for the eastern Pacific) and ''d''<sub>r</sub> is the depth of the ridge below the ocean surface.<ref name="fundamentals">{{cite journal|last1=E.&nbsp;E.|first1=Davis|last2=Lister |first2=C.&nbsp;R.&nbsp;B.|title=Fundamentals of Ridge Crest Topography|journal=Earth and Planetary Science Letters|volume=21|issue=4|pages=405–413|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:

for the eastern Pacific Ocean:
<math display="block">d(t) = 350 \sqrt{t} + 2500</math>

and for the Atlantic and Indian Oceans:

<math display="block">d(t) = 390 \sqrt{t} + 2500</math>

where ''d'' is in meters and ''t'' is in millions of years, so that recently-formed crust at the mid-ocean ridges lies at about 2,500&nbsp;m depth, whereas 50-million-year-old seafloor lies at a depth of about 5,000&nbsp;m.<ref name="analysis">{{cite journal|last1=Parsons|first1=Barry|last2=Sclater |first2=John G.|title=An analysis of the variation of ocean floor bathymetry and heat flow with age|journal=Journal of Geophysical Research|volume=82|issue=B5|pages=802–827|bibcode=1977JGR....82..802P|year=1977|doi=10.1029/jb082i005p00803}}</ref> 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.

The surface area of the oceans can change when continents [[rift]] (stretching the continents decreases ocean area and raises sea level) or as a result of continental collision (compressing the continents increases ocean area and lowers sea level). Increasing sea level will flood the continents, while decreasing sea level will expose [[Continental shelf|continental shelves]]. 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.

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. There is thus a relatively simple relationship between the supercontinent cycle and the mean age of the seafloor.
*Supercontinent = much old seafloor = low sea level
*Dispersed continents = much young seafloor = high sea level

There will also be a climatic effect of the supercontinent cycle that will amplify this further:
*Supercontinent = continental climate dominant = continental glaciation likely = still lower sea level
*Dispersed continents = maritime climate dominant = continental glaciation unlikely = sea level is not lowered by this mechanism

==Relation to global tectonics==
There is a progression of tectonic regimes that accompanies the supercontinent cycle:

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 was the situation during the Paleozoic supercontinent cycle; it is being observed for the [[Mesozoic]]–[[Cenozoic]] supercontinent cycle, still in progress.

==Relation to climate==
{{Main|Greenhouse and icehouse Earth}}
<!--{{unreferenced section|date=February 2011}}-->{{more citations needed section|date=August 2012}}
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. The Earth is currently in a short greenhouse phase of an icehouse climate.<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> Periods of icehouse climate include much of the [[Neoproterozoic]], late [[Paleozoic]], late [[Cenozoic]], while periods of greenhouse climate include early [[Paleozoic]], [[Mesozoic]]–early [[Cenozoic]].

*'''Icehouse climate'''
**Continents moving together
**Sea level low due to lack of seafloor production
**Climate cooler, arid
**Associated with [[aragonite sea]]s
**Formation of supercontinents
*'''Greenhouse climate'''
**Continents dispersed
**Sea level high
**High level of [[seafloor spreading]]
**Relatively large amounts of CO<sub>2</sub> production at oceanic rifting zones
**Climate warm and humid
**Associated with [[calcite sea]]s
==Relation to evolution==
The principal mechanism for evolution is [[natural selection]] among diverse populations. Diversity, as measured by the number of families, follows the supercontinent cycle very well.<ref>{{cite encyclopedia|last=Benton|first=Michael J.|title=eLS |chapter=Fossil Record: Quality|encyclopedia=Encyclopedia of Life Sciences|publisher=John Wiley & Sons, Ltd|doi=10.1038/npg.els.0004144|date=23 September 2005|isbn=978-0470016176}}</ref> As [[genetic drift]] occurs more frequently in small populations, diversity is an observed consequence of geographic isolation. Less isolation, and thus less diversification, occurs when the continents are all together, producing one continent, one continuous coast, and one ocean. In late Neoproterozoic to early Paleozoic, when the tremendous proliferation of diverse [[Animal|metazoa]] occurred, isolation of marine environments resulted from the breakup of Pannotia.

A north–south arrangement of continents and oceans leads to much more diversity and isolation than east–west arrangements. North-to-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.

==See also==
*[[History of Earth]]
*[[List of supercontinents]]

==References==
{{reflist|30em}}

==Further reading==
*{{cite journal | last1 = Gurnis | first1 = M. | year = 1988 | title = Large-scale mantle convection and the aggregation and dispersal of supercontinents | journal = Nature | volume = 332 | issue = 6166 | pages = 695–699 | doi=10.1038/332695a0 | bibcode=1988Natur.332..695G}}
*{{cite journal | last1 = Murphy | first1 = J.&nbsp;B. | last2 = Nance | first2 = R.&nbsp;D. | year = 1992 | title = Supercontinents and the origin of mountain belts | journal = Scientific American | volume = 266 | issue = 4| pages = 84–91 |bibcode = 1992SciAm.266c..84M |doi = 10.1038/scientificamerican0492-84 }}
*{{cite journal | last1 = Nance | first1 = R.&nbsp;D. | last2 = Worsley | first2 = T.&nbsp;R. | last3 = Moody | first3 = J.&nbsp;B. | year = 1988 | title = The supercontinent cycle | journal = Scientific American | volume = 259 | issue = 1| pages = 72–79 | doi = 10.1038/scientificamerican0788-72 |bibcode = 1988SciAm.259a..72N }}

==External links==
{{Commonscat|Supercontinents}}
* [http://www.scotese.com/earth.htm Reconstructions from "Paleomap Project"]
* [http://www-udc.ig.utexas.edu/external/plates/recons.htm Plate reconstructions and movies from UTIG 'PLATES' project]
* [http://csmres.jmu.edu/geollab/Fichter/Wilson/rkcytst.html A Tectonic Rock Cycle]

{{Continents of Earth}}

[[Category:Geologic modelling]]
[[Category:Plate tectonics]]
[[Category:Supercontinents]]