Supercontinent

In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, many earth scientists use a different definition: 'a clustering of nearly all continents', which leaves room for interpretation and is easier to apply to Precambrian times. AfricaAntarcticaAsiaAustraliaEuropeNorth AmericaSouth AmericaAfro-EurasiaAmericaEurasiaOceaniaSubcontinents In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, many earth scientists use a different definition: 'a clustering of nearly all continents', which leaves room for interpretation and is easier to apply to Precambrian times. Supercontinents have assembled and dispersed multiple times in the geologic past (see table). According to the modern definitions, a supercontinent does not exist today. The supercontinent Pangaea is the collective name describing all of the continental landmasses when they were most recently near to one another. The positions of continents have been accurately determined back to the early Jurassic, shortly before the breakup of Pangaea (see animated image). The earlier continent Gondwana is not considered a supercontinent under the first definition, since the landmasses of Baltica, Laurentia and Siberia were separate at the time. The following table names reconstructed ancient supercontinents, using a general definition, with an approximate timeline of millions of years ago (Ma). There are two contrasting models for supercontinent evolution through geological time. The first model theorizes that at least two separate supercontinents existed comprising Vaalbara (from ~3636 to 2803 Ma) and Kenorland (from ~2720 to 2450 Ma). The Neoarchean supercontinent consisted of Superia and Sclavia. These parts of Neoarchean age broke off at ~2480 and 2312 Ma and portions of them later collided to form Nuna (Northern Europe North America) (~1820 Ma). Nuna continued to develop during the Mesoproterozoic, primarily by lateral accretion of juvenile arcs, and in ~1000 Ma Nuna collided with other land masses, forming Rodinia. Between ~825 and 750 Ma Rodinia broke apart. However, before completely breaking up, some fragments of Rodinia had already come together to form Gondwana (also known as Gondwanaland) by ~608 Ma. Pangaea formed by ~336 Ma through the collision of Gondwana, Laurasia (Laurentia and Baltica), and Siberia. The second model (Kenorland-Arctica) is based on both palaeomagnetic and geological evidence and proposes that the continental crust comprised a single supercontinent from ~2.72 Ga until break-up during the Ediacaran Period after ~0.573 Ga. The reconstruction is derived from the observation that palaeomagnetic poles converge to quasi-static positions for long intervals between ~2.72–2.115, 1.35–1.13, and 0.75–0.573 Ga with only small peripheral modifications to the reconstruction. During the intervening periods, the poles conform to a unified apparent polar wander path. Because this model shows that exceptional demands on the paleomagnetic data are satisfied by prolonged quasi-integrity, it must be regarded as superseding the first model proposing multiple diverse continents, although 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. Plate tectonics as seen on the contemporary Earth became dominant only during the latter part of geological times. The Phanerozoic supercontinent Pangaea began to break up 215 Ma and is still doing so today. Because Pangaea is the most recent of Earth's supercontinents, it is the most well known and understood. Contributing to Pangaea's popularity in the classroom is the fact that its reconstruction is almost as simple as fitting the present continents bordering the Atlantic-type oceans like puzzle pieces. A supercontinent cycle is the break-up of one supercontinent and the development of another, which takes place on a global scale. Supercontinent cycles are not the same as the Wilson cycle, which is the opening and closing of an individual oceanic basin. The Wilson cycle rarely synchronizes with the timing of a supercontinent cycle. However, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia. Secular trends such as carbonatites, granulites, eclogites, 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 lack of 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 causes of supercontinent assembly and dispersal are thought to be driven by convection processes in the Earth's mantle. Approximately 660 km into the mantle, a discontinuity occurs, affecting the surface crust through processes like plumes and 'superplumes'. When a slab of subducted crust is denser than the surrounding mantle, it sinks to the discontinuity. Once the slabs build up, they will sink through to the lower mantle in what is known as a 'slab avalanche'. This displacement at the discontinuity will cause the lower mantle to compensate and rise elsewhere. The rising mantle can form a plume or superplume.