Plate tectonics

The plate tectonics (initially called continental drift ) is the model current inner working of the Ground. It is the expression on the surface of the Convection which is held in the terrestrial coat.

The Lithosphere, layer external of the Ground is cut out in rigid plates which float and move on the Asthénosphère, more ductile. The first concepts, stammered as of the 18th century, were scientifically formulated in 1912 by the German climatologist Alfred Wegener starting from considerations cartographic, structural, paleontological and paleoenvironmental.

A little history…

Alfred Wegener proposes in 1912 that all the continents known today were gathered into only one Supercontinent (the Pangée); this one would have been fractured in blocks which, such of the rafts, derived far from/to each other to lead to the current distribution of the continents. The theory of Wegener is put on side by the scientists of the time because it missed a justification as for the " moteur" movements of the aforesaid plates. The origin of the forces necessary to these displacements today is known and argued: it is the Convection terrestrial intern. It is only as from years 1960 that new data, coming in particular from the study of the underwater funds, made it possible to build and to validate the model of the plate tectonics . The synthetic theory of the plate tectonics was stated finally in 1967 by American Jason Morgan, the British daN McKenzie and French Xavier Pichon.

The current model

It is admitted now that the tectonic plates are carried by the movements of the subjacent coat asthenospheric and undergo interactions whose three principal types are:

  • the divergence : of a movement moving away two plates one is said from the other, letting the coat go up between them. Their divergent border corresponds to a oceanic wrinkle or dorsal, place of creation of oceanic Lithosphère and theater of intense volcanicity. Volcanicity on the level of the dorsals is generally basaltic, with a tholeiitic Géochimie . It is through this movement that creates for itself the crust lithospheric (rigid layer located at the top of the higher coat):

  • the convergence : of a movement bringing closer two plates one is said to the other, thus compensating for the oceanic expansion in other zones of the sphere. Three types of convergent border of plates adapt the bringing together:

    • a zone of Subduction where a plate (in general densest) plunges under another, less dense; the Volcan ism above the zones of subduction is generally andesitic, with a calc-alkali Géochimie. The west coast of the South America in is an example.
    • a zone of collision, where two plates are confronted. It is the case in particular of the chain of the the Himalayas, at the border between the Indian Plaque and the Eurasian Plaque.
    • a zone of Obduction, where a oceanic lithosphere is transferred onto a continent. One does not know a currently active obduction on the surface of the terrestrial sphere.

  • the transcurrence : horizontal slip of two plates is said, one at side and along the other.

With these three types of interaction the three big families with Faille S are associated:

  • a normal Faille is divergent (extensive);

  • a Faille reverses is convergent (compressive);
  • a Décrochement east transcurrent (the axes of extension and compression are in the horizontal plane).

The origin of the force which makes the plates mobile is discussed: it can be related on the constraint shearing between the crust and the asthenosphere (related to the importance of the coupling between the two), to the weight of the crust subductante (which “draws” all the plate) or with pushed with the wrinkle (the weight of the young crust in top of the wrinkle “pushes” all the plate). These possibilities are not exclusive, but the relative contributions in the movement are very discussed and depend on the studies.

Plate tectonics and convection in the coat

The Earth has an important heat because of Radioactivité (disintegration of the Potassium, the Uranium and the Thorium) and of the initial accretion heat. It cools by evacuating heat on its surface. For that, one knows three mechanisms: thermal Conduction, Convection and radiative Transfer. On the level of the terrestrial coat, the major part of the Heat flow is evacuated by the actuation of the rocks (convection). The convection is induced by the presence of hot material (thus less dense) under of the less hot material (thus denser). These movements, very slow (about 1 to 10 cm/an) are at the origin of the movements of the tectonic plates and the hot spots.

Limits of the model

The plate tectonics is perfectly valid for the oceanic plates (or for the oceanic parts of the mixed plates). Indeed, the oceanic plates are thin and rigid; their limits are very clear (médio-oceanic Ride, Faille S transforming or zones of Subduction). On the other hand, the continental plates are much thicker and less rigid. The limits of plates are thus much fuzzier, and one can regard as limit the paleogeographic joining (the old ocean), or the zone which currently becomes deformed (in the cases of the the Himalayas - Tibet, the difference is of several thousands of km).
De more, this lack of rigidity induces the presence of a multitude of " blocs" or of " microplaques" , more or less independent. The mosaic in the Mediterranean in is an good example, with a multitude of systems in extension (Tyrrhenian Sea, Mer of Alboran, Aegean Sea) in a compressive context (bringing together of the African and European plates). The model of plate tectonics finds its limits here, and certain model can better explain certain structures without the plate tectonics (in the East of the plate Tibetan, for example).

List tectonic plates

The list of the current plates is, alphabetically (smaller units exist; they are called “blocks” or “microplaques”):

References

See too

Internal bonds

  • Tectonic Tectonic plate
  • List of tectonic plates
  • Not hot

External bonds

  • Plate tectonics and continental drift
  • Objective Ground, geodynamics
  • the lower part of the charts: plate tectonics (video on DailyMotion)
  • Animation in Flash.

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