(World Geography) - Origin of Continents and Ocean Basins, Tectonics Plate
The science of lithospheric plates motion is called plate tectonics. The theory of plate tectonics identifies 7 major and 20 minor types of lithospheric plates. The major ones are Eurasian, African, Indo-Australian, Pacific, North American, South American and Antarctic plate. Important minor plates are Cocos, Nazca, Arabian, Philippines, Caroline, and Fuji etc. These plates are continuously in motion with respect to each other. It is not the continent that moves as believed by Wegener. Continents are part of a plate and what moves is the plate. Tectonically plate margins or plate boundaries are most important as all tectonic activities occur along the plate margins.
Plate margins are generally divided into three types which define three fundamental kinds of geological activities:
1) Constructive margin or Divergent plate margin or Accreting plate boundaries: constructive plate margin represent zones of divergence where there is continuous upwelling of lava. This lava spreads on the adjacent ocean floors thus creating new ocean floor, hence the name constructive plate margins. The pulling apart of plates, as occurs with seafloor spreading, is tectonic plate divergence.
2) Destructive plate margins or convergent plates or Consuming plate boundaries: The tectonic plate convergence involves movement of two plates toward each other, their collision, overriding of one plate by the edge of other plate and subduction and melting of overridden plate margin into the mantle.
The process in which one plate is carried beneath another is called subduction.
Ocean-ocean collision: Deep ocean trenches, such as the Peru-Chile trench and the Japanese trench, occur where oceanic crust is dragged downward in this way. The subducting plate is heated and rocks are melted as it plunges downward into the mantle. As the subducting plate grinds downward, enormous friction is produced, which explains the occurrence of major earthquakes in these regions.
Ocean-continent collision: Where oceanic crust collides with continental crust, the oceanic crust, which is denser, is subducted beneath the less dense continental crust. This is the situation along South America’s Pacific coast, where the Nazca plate subducts beneath the South American Plate, Juan de Fuca beneath the North American plate and, in Japan where the Pacific plate of which it forms a part, is subducted.
3) Conservative plate margins or Transform Plate Movement: Here lithospheric plates slide past each other, in opposite directions, without diverging or converging. Such a movement of plates is called transform movement. The two plates are in contact along a vertical fracture, called a transform fault. Such a boundary exists along the San Andreas Fault zone in California.