Wilson cycle

The Wilson cycle is an expansion to Wegener's theory of continental drift/plate tectonics. Wegener theorized that all the continents as they are today were once joined in single large landmass, Pangea. The Wilson cycle, named after J. Tuzo Wilson who first proposed it, takes this a step farther--not only were the present continents once joined, but the process has occured multiple times throughout the geologic history of the Earth. There may have been as many as 6-10 occurences of this cycle since the early Proterozoic period (see Rodinia).

The Wilson cycle describes ocean basins, and the stages that they go through from creation to elimination. There are six seperate stages of the Wilson cycle:

Embryonic: Thick continental crust blocks the flow of heat, and there is a change in convection currents of the asthenosphere (the soft plastic layer on which the continental and ocean plates 'float'). There is an upwelling of magma, which causes continental rifting to begin. The East African rift valley system is an example of this process.

Youth: The rift expands as magma continues to rise to the surface and creates new crust, and water fills the young ocean basin, creating a 'linear ocean' like the modern Red Sea.

Adolescence: Our new ocean grows wider begins to age. The passive boundaries between the continent and the oceans are accumulating sediment from the erosion of the continents. The Atlantic Ocean is one such maturing ocean basin.

Maturity: The weight of accumulating sediments at the margin where continental crust meets the ocean basin causes depression of the ocean crust. Eventually, a subduction zone is formed, where the thinner, dense ocean crust slips below the continental crust. See Pacific Ocean as an example of this stage in ocean development.

Old Age: Accretionary wedges are formed as sediments are scraped off the subducting ocean crust which creates a tectonic crest which can form offshore island arcs. The ocean basin continues to narrow. This terminal stage of development is exemplified by the Mediterranean Sea.

Death: All of the oceanic crust that seperated the two masses of continental crust has been subducted, and the continents collide. This collision causes a mountain range to form along the collision- a suture. Examples of this type of landform are abundant, and include the Indus-Yarlung Zangbo suture in the Himalayas where India collided with Asia, and the Ural mountains which mark the collision of the Asian landmass with Europe.

This theory is an attempt to explain the presence of ancient orogenic belts, or zones that have undergone techtonic compression. The Appalachian Mountains are one of these orogenic belts that are evidence of the Wilson cycle- a collisional mountain range whose age dates to before the theorized breakup of Pangea, therefore there must have been seperate continents to collide before the supercontinent's existence.

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Sources:
http://www.xrefer.com/entry.jsp?xrefid=620558&secid=.-&hh=1
http://geology.lssu.edu/NS116/lecture8.html
http://faculty.weber.edu/bdattilo//shknbk/notes/wlsn.htm
http://earth.leeds.ac.uk/dynamicearth/history/wilson/
http://mscserver.cox.miami.edu/msc101/harrison/Chap3a/tsld045.htm/

The Wilson Cycle and its relationship to Earth Cooling:

Plate Tectonics, and all crustal processes, can be viewed in the context of the cooling of the Earth. The Earth's mantle and core gain their heat from energy released by the radioactive decay of unstable isotopes (of most importance are ⁴⁰K, ²³⁵U, ²³⁸U and ²³²Th). This free heat is the driving process behind all of plate tectonics, as it eventually radiates from The Earth into space. The Wilson Cycle, as the grand process in plate tectonics, is extremely revealing when taken in this context.

The Earth is the only planet in the solar system known to release its energy via plate tectonics. Mars releases its heat via a series of mantle plumes; upwelling of hot, mantle material from the core-mantle boundary, which rise through the mantle (much like a 1970s lava lamp) until reaching the crust and resulting in hot-spot volcanism. Olympus Mons, the solar system's largest mountain is an example of a static mantle plume. Venus is an enigma, since its entire crust appears to be a uniform, relatively recent, age implying a catastrophic process is involved in its thermal release.

In the case of the Earth, heat is released continuously at spreading margins (such as the centre of the Atlantic) and in the physical motion of the continents. Earth is also subject to limited mantle-plume volcanism, as is evident in the case of Hawaii or Iceland, but due to the motion of the plate above the stationary plume our planet is not subject to the static mountain ranges of Mars. Heat is conducted easily through the thin oceanic crust, and is insulated by thicker continental crust. The Wilson Cycle therefore, with its distinct periods of activity has a huge impact on the continuous process of thermal release.

The ideal period for thermal release in the Wilson Cycle was passed relatively recently, where the spreading ridges are most active and the oceanic crust is young, thin and buoyant. However, since the continents act as an insulator, the process of orogeny (mountain formation) always leads to thicker crust, greater insulation and reduces the rate of continental motion; the formation of the Himalayas has resulted in the slowing of motion of the Indian plate, followed by a decrease in spreading rate at its oceanic margin. The formation of a supercontinent such as Pangea, Gondwanaland or Rodinia results in the Earth becoming thermally unstable. When a supercontinent is at maximum extent then oceanic spreading is likely to be occurring only on the opposite side of the planet to the area of most insulation, and its rate is likely to be low since it is not being propelled by the motion of the continents. This slows down the release of thermal radiation and forces the continents to rift.

The rifting of continents is a poorly understood subject, But is currently believed to be driven by dynamic rifting, an aggressive process involving mantle plumes. In dynamic rifting the impact of a mantle plume causes a point of weakness in the continental crust, this process causes the crust to dome and thin directly above the point of activity, accompanied by extensive volcanic activity. This thinning results in lines of weakness occurring in a Y shape (There are Y shapes across the globe, examine the Red Sea, Gulf of Aden, East African Rift for an example) and eventually two of these end members form an oceanic spreading centre (but usually following a Continental Flood Basalt eruption, the single most dramatic volcanic event that takes place on our Earth). The point of interest here is the requirement of mantle plumes to cause rifting.

There is currently strong evidence that there were few mantle plumes in the late stages of Rodinia, and that at the present times mantle plumes are much less active than at the height of Pangea. It is speculated that the insulation caused by a supercontinent causes a catastrophic overheating in the earths mantle, and that this leads to the formation of mantle plumes at the core-mantle boundary. The largest and therefore most buoyant of these then rise up towards the continental plate and tear it open. Support for this interpretation is given by the decreasing volume of lava seen at continental flood basalts since the first event in the break-up of Pangea (The Siberian Traps eruption). Note should also be taken that the Siberian Traps eruption occurred synchronously with the Permo-Triassic extinction event when 95% of all life on this planet ceased. Continuing this theme, all recent mass extinction (including the Cretaceous-Tertiary event that lead to the extinction of the Dinosaurs) events appear to be synchronous with a flood basalt eruption.

If we follow this interpretation through, then the active stage in the Wilson cycle is the pivot of all Earth processes, and that as the Earth drifts towards a new supercontinent forming where the Pacific is currently located, insulation will take place resulting in another cycle of continental shattering and extreme volcanism. The Wilson Cycle, and radioactive decay, drive all processes on our planet over their 600 million year cycle.