Wilson cycle

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The stages of the Wilson cycle using the example of recent manifestations of plate tectonics

A Wilson cycle is understood to mean the formation and subsequent breakup of supercontinents as well as the associated closure or opening of ocean basins in the course of plate tectonics . The Wilson cycle is named after the Canadian geoscientist John Tuzo Wilson , who first described these cycles , which run on a time scale of several hundred million years, in the late 1960s.

Phases

Wilson cycles over the past 1 billion years, from a North American-European perspective. Red = rift stages, pink = Atlantic stages, light blue = Mediterranean and Himalayan stages.

Resting phase

At the beginning of the development, Wilson places a (large) continental block, which is in a tectonic dormant state. During this rest phase, the folds that emerged from the final phase of the preceding Wilson cycle are removed .

Ditch or rift stage

The continental rift stage follows the resting phase. As the founder of the hotspot hypothesis, Wilson suggests that hotspots are responsible for the breaking of the continental lithosphere and the formation of rifts .

A hotspot is an area of ​​elevated temperature in the Earth's mantle. If such a hotspot is located under the oceanic crust , oceanic volcanic islands can form (for example the Hawaiian archipelago). Active hotspots under a continental block can cause the crust to be thinned and bulged until it finally gives way, collapses and gives way to the side (stretching tectonics). A trench system is created, such as B. the East African Trench . Mostly alkaline volcanism occurs along the faults in the earth's crust .

Oceanic Young Stage (Red Sea Stage)

At this stage, the inner-continental rift has expanded so much that the expansion can no longer be compensated for by the sinking and tilting of the rift clods from the continental crust and oceanic crust is formed along the expansion joint by rising basaltic magma . A mid-ocean ridge (MOR) is formed along the expansion joint, now referred to as the oceanic expansion zone . The large continental block has been divided into two smaller, drifting apart continental blocks. The edges of the continental blocks facing the young ocean basin, now again increasingly tectonically calm, are called passive continental margins . Even before the formation of the oceanic crust, the rift zone has often sunk so far that it is below sea level and, with sufficient proximity to the coast, has turned into an arm of the sea. This stage is named after the Red Sea , a very young and narrow ocean basin between the Arabian Plate and the African Plate . The Baffin Bay , Davis Strait and Labrador Sea together form a basin, the opening of which stopped in an advanced phase of the Red Sea stage. Because the corresponding spreading zone has not been active for more than 30 million years, Greenland is part of the North American plate .

Oceanic stage of maturity (Atlantic stage)

This stage describes a relatively wide open ocean basin with passive continental margins and a MOR that extends along the longitudinal axis of the basin. Under special circumstances, so much magma can be extracted at the MOR in places that it rises above sea level. The best known example of such a special case is the island of Iceland .

The passive continental margins - now a long way from the MOR - form shelves on which massive sediment sequences are deposited.

Inversion of the ocean basin (Pacific stage)

Since the surface of the earth's body (including the sea floor) does not increase, since the earth's crust is newly formed at MOR, the earth's crust has to disappear from the surface at another point. This takes place in so-called subduction zones , where oceanic earth crust dips under the continental earth crust ( subduction ). A volcanic arch is formed over the subduction zone and, under certain conditions, an Andean-type mountain range of folds.

If, in the course of time, at least one of the edges of the Atlantic Stadium ocean basin changes from a passive continental margin to a subduction zone, if the subduction rate there is higher than the spread rate at the MOR, the ocean basin will gradually become narrower as the continental blocks adjoin the Now move the edges of the pelvis towards each other. This narrowing process is also called ocean basin inversion and the corresponding stage of inversion .

The designation Pacific stage , on the other hand, is somewhat misleading, since the Pacific, although almost completely flanked by subduction zones, is in no way in danger of being closed. Strictly speaking, the Pacific, as a purely morphological basin, is extremely long-lived and has existed for at least 500 million years (mostly referred to as Panthalassa in paleogeographical and tectonic maps of the Paleozoic and Mesozoic Era ).

Oceanic end stage (Mediterranean stage)

The continued subduction of the oceanic crust of the Pacific-type basin, if it proceeds faster than the spreading at the MOR, leads to a progressive closure of the ocean basin. After subduction of the MOR, the pelvic inversion is additionally accelerated by the elimination of the splay. Eventually the continental blocks that delimit the basin come into contact with one another. This collision leads to a mountain formation (orogenesis) of the collision type (formation of a so-called alpinotype fold mountain range ), whereby at first cover thrusts dominate, in which the shelf sediments of the ocean basin and the rocks of the volcanic arc are folded and pushed over one another. Recent examples of this stage include the Eastern Mediterranean , the Black Sea, and the Caspian Sea , which are relic basins of the Neotethys Ocean, which was inverted by subduction under the Eurasian Bloc .

Collision stage (Himalaya stage)

At this stage the ocean basin is completely closed and the (alpinotype) mountain formation reaches its climax. The thrust tectonics has led to an enormous thickening of the earth's crust involved. The thickened crust area is lifted out a little by mass balancing movements and a high mountain range is created. The continental blocks, which were previously separated by the ocean basin, are now united to form a larger continental block (large or supercontinent).

Resting phase

After the mountain formation has ended, the newly formed continental block enters a tectonic rest phase and the piled up mountains are removed again. The cycle has arrived at its starting point.

Web links

literature

  • Andrew Goudie: Physical Geography: An Introduction. Lorenz King, Elisabeth Schmitt (eds.), Spektrum Akademischer Verlag, Munich 2004, pp. 23–24.
  • Volker Kaminske, Christian Keipert: Construction and dynamics of the earth. Basic geological knowledge from the formation of the Alps to Zechstein. Westermann Printing and Publishing Group, Braunschweig 2006, pp. 80–81.
  • Wolfgang Frisch, Martin Meschede: Plate tectonics and mountain formation . 3rd edition, Wissenschaftliche Buchgesellschaft, Darmstadt 2009, pp. 153–164.

Individual evidence

  1. ^ J. Tuzo Wilson: Static or Mobile Earth: The Current Scientific Revolution. Proceedings of the American Philosophical Society. Vol. 112, No. 5 ( Gondwanaland Revisited: New Evidence for Continental Drift ), 1968, pp. 309-320 ( JSTOR 986051 ), pp. 312 f.
  2. ↑ M. Hosseinpour, RD Müller, SE Williams, JM Whittaker: Full-fit reconstruction of the Labrador Sea and Baffin Bay. Solid Earth. Vol. 4, 2013, pp. 461-479, doi: 10.5194 / se-4-461-2013 , p. 462.