Supercontinent cycle

The supercontinent cycle is 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.AfricaAntarcticaAsiaAustraliaEuropeNorth AmericaSouth AmericaAfro-EurasiaAmericaEurasiaOceaniaSubcontinents The supercontinent cycle is 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. The most recent supercontinent, Pangaea, formed about 300 million years ago (0.3 Ga). There are two different views on the history of earlier supercontinents. The first proposes a series of supercontinents: Vaalbara (c. 3.6 to c. 2.8 billion years ago); Ur (c. 3 billion years ago); Kenorland (c. 2.7 to 2.1 billion years ago); Columbia (c. 1.8 to 1.5 billion years ago); Rodinia (c. 1.25 billion to 750 million years ago); and Pannotia (c. 600 million years ago), whose dispersal produced the fragments that ultimately collided to form Pangaea. The second view (Protopangea-Paleopangea), based on both palaeomagnetic and geological evidence, is that supercontinent cycles did not occur before about 0.6 Ga (during the Ediacaran Period). Instead, the continental crust comprised a single supercontinent from about 2.7 Ga (Gigaannum, or 'billion years ago') until it broke up for the first time, somewhere around 0.6 Ga. This reconstruction 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. 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. The kinds of minerals found inside ancient diamonds suggest that the cycle of supercontinental formation and breakup began roughly 3.0 billion years ago (3.0 Ga). Before 3.2 billion years ago only diamonds with peridotitic 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 as subduction and continental collision introduced eclogite into subcontinental diamond-forming fluids. The hypothesized supercontinent cycle is overlaid by the Wilson cycle named after plate tectonics pioneer J. Tuzo Wilson, which describes the periodic opening and closing of ocean basins from a single plate rift. The oldest seafloor material found today dates to only 170 million years old, whereas the oldest continental crust material found today dates to 4 billion years, showing the relative brevity of the regional Wilson cycles compared to the planetary pulse seen in the arrangement of the continents. 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, conductively cooling and 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 million years old, a simple cooling 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, where κ is the thermal diffusivity of the mantle lithosphere (c. 8×10−7 m2/s), aeff is the effective thermal expansion coefficient for rock (c. 5.7×10−5 °C−1), T1 is the temperature of ascending magma compared to the temperature at the upper boundary (c. 1220 °C for the Atlantic and Indian Oceans, c. 1120 °C for the eastern Pacific) and dr is the depth of the ridge below the ocean surface. After plugging in rough numbers for the sea floor, the equation becomes: where d is in meters and t 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 5,000 m.