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Wilson cycle
Wilson cycle
simplified sketch of the western part of Pangaea
simplified sketch of the western part of Pangaea

The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory is constantly being reconfigured. Continental collision makes fewer and larger continents while rifting makes more and smaller continents. Continental collision is a phenomenon of the Plate tectonics of Earth. In Geology, a rift is a place where the Earth 's crust and Lithosphere are being pulled apart and is an example of Extensional tectonics The last supercontinent, Pangaea, formed about 300 million years ago. In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. Pangaea, Pangæa or Pangea (pænˈdʒiːə from παν pan, meaning entire, and Γαῖα Gaea, meaning Earth in The previous supercontinent, Pannotia or Greater Gondwanaland, formed about 600 million years ago, and its dispersal formed the fragments that ultimately collided to form Pangaea. Pannotia, first described by Ian W D Dalziel in 1997 is a hypothetical Supercontinent that existed from the Pan-African orogeny about 600 million years ago to the Gondwana (ɡɒnˈdwɑːnə originally Gondwanaland) was a southern Supercontinent that existed about 500 to 200 Ma ago But beyond this the time span between supercontinents becomes more irregular. For example, the supercontinent before Gondwanaland, Rodinia, existed ~1. For the Genus of Metalmark butterflies, see Rodinia (butterfly. 1 billion to ~750 million years ago - a mere 150 million years before Gondwanaland. The supercontinent before this was Columbia: ~1. Columbia is the name of one of the Earth's oldest Supercontinents It was first proposed by J 8 to 1. 5 billion years ago. And before this was Kenorland: ~2. Kenorland was one of the earliest Supercontinents on Earth It is believed to have formed during the Neoarchaean Era ~2 7 to ~2. 1 billion years ago. Ur existed ~3 billion years ago and Vaalbara ~3. Ur is the first known continent that probably formed 3 billion years ago in the early Archean Eon. Vaalbara is the Earth's theorized first Supercontinent. According to radiometric data of the encompassing Cratons that constituted Vaalbara it is believed 6 to ~2. 8 billion years ago. One complete Supercontinent cycle is said to take 300 to 500 million years to occur.

The hypothetical supercontinent cycle is, in some ways, the complement to the Wilson cycle. The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth 's Continental crust. The latter is named after plate tectonics pioneer J. Tuzo Wilson and describes the periodic opening and closing of ocean basins. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere John Tuzo Wilson PhD, CC, OBE, DSc, FRS, FRSC, FRSE ( October 24, 1908 &ndash Hydrologically an oceanic basin may be anywhere on Earth that is covered by Seawater, but geologically ocean basins are large geologic basins Because the oldest seafloor is only 170 million years old, whereas the oldest bit of continental crust goes back to 4 billion years or more, it makes sense to emphasize the much longer record of the planetary pulse that is recorded in the continents.

Contents

Effects on sea level

d(t) = 2500 + (350t1/2)
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 2500 m depth, whereas 100 Ma-old seafloor lies at a depth of about 6000 m.

Just as the water level in a bathtub is controlled by the size of the person in the bath, sea level is controlled by the depth of the seafloor (neglecting complications resulting from glacial ice and temperature effects). The relationship between seafloor depth and sea level can be expressed as follows:

The mass (M) of water on the earth is a constant = K1, where
K1 = M(seawater) + M(freshwater) + M(ice) + M(atm. water)
We can neglect M(freshwater) + M(atm. water)
K1 = M(seawater) + M(ice)
Consider the ice-free world:
V(seawater) = K1/(mean density of seawater)

This volume fills the ocean basins to a depth determined by A x d′, where A = area of the ocean basins and d′ = mean depth of the ocean basins. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Seawater is Water from a Sea or Ocean. On average seawater in the world's oceans has a Salinity of about 3 d′ is determined by the mean age of the seafloor.

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). In Geology, a rift is a place where the Earth 's crust and Lithosphere are being pulled apart and is an example of Extensional tectonics Continental collision is a phenomenon of the Plate tectonics of Earth. Increasing sea level will flood the continents, while decreasing sea level will expose 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. The continental shelf is the extended perimeter of each Continent and associated Coastal plain, which is covered during interglacial periods such

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.

There will also be a climatic effect of the supercontinent cycle that will amplify this further:

Relation to Global Tectonics

There is a progression of tectonic regimes that accompany 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 is the situation that was observed during the Paleozoic Supercontinent Cycle and is being observed for the Mesozoic-Cenozoic Supercontinent Cycle, still in progress. The Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The Cenozoic (also Caenozoic or Cainozoic) Era (ˌsiːnəˈzoʊɪk/ /ˌsɛn- (meaning "new life" ( Greek ( kainos) "new"

Relation to climate

There are two types of global earth climates: Icehouse and Greenhouse. Icehouse is characterized by frequent continental glaciations and severe desert environments. We are now in the icehouse phase, moving towards Greenhouse. Greenhouse is characterized by warm climates. Both reflect the supercontinent cycle.

Periods of Icehouse Climate: Much of Neoproterozoic, Late Paleozoic, Late Cenozoic. An aragonite sea contains Aragonite and high-magnesium Calcite as the primary inorganic carbonate precipitates In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge A calcite sea is one in which low-magnesium Calcite is the primary inorganic marine Calcium carbonate precipitate The Neoproterozoic Era is the unit of geologic time from 1000 to 542 +/- 0 The Paleozoic or Palaeozoic Era (from the Greek palaio (παλαιο "old" and zoe (ζωη "life" meaning "ancient life" The Cenozoic (also Caenozoic or Cainozoic) Era (ˌsiːnəˈzoʊɪk/ /ˌsɛn- (meaning "new life" ( Greek ( kainos) "new"

Periods of Greenhouse Climate: Early Paleozoic, Mesozoic-Early Cenozoic. The Paleozoic or Palaeozoic Era (from the Greek palaio (παλαιο "old" and zoe (ζωη "life" meaning "ancient life" The Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The Cenozoic (also Caenozoic or Cainozoic) Era (ˌsiːnəˈzoʊɪk/ /ˌsɛn- (meaning "new life" ( Greek ( kainos) "new"

Relation to evolution

The principal mechanism for evolution is natural selection among diverse populations. Diversity is accomplished by isolation. Little isolation and thus little diverisification occurs when the continents are all together (One ocean, one continent). In Latest Neoproterozoic to Early Paleozoic times, when the tremendous burst of evolution of metazoa occurred, isolation of marine environments resulted from breakup of Greater Gondwanaland. An arrangement of N-S continents and oceans leads to much more diversity and isolation than E-W oceans and continents. This forms zones that are separated by water or land and that merge into climatically different zones along communication routes to the north and south. Formation of similar tracts of continents and ocean basins, only oriented E-W would lead to much less isolation, diversification, and slower evolution. Through the Cenozoic, isolation has been maximized by an arrangement of N-S ocean basins and continents. The Cenozoic (also Caenozoic or Cainozoic) Era (ˌsiːnəˈzoʊɪk/ /ˌsɛn- (meaning "new life" ( Greek ( kainos) "new"

Diversity as measured by the number of families follows the supercontinent cycle very well.

References

External links

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