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Continental crust

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The thickness of the Earth's crust (km).
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   The thickness of the Earth's crust (km).

   The continental crust is the layer of granitic, sedimentary and
   metamorphic rocks which form the continents and the areas of shallow
   seabed close to their shores, known as continental shelves. It is less
   dense than the material of the Earth's mantle and thus "floats" on top
   of it. Continental crust is also less dense than oceanic crust, though
   it is considerably thicker; mostly 35 to 40 km versus the average
   oceanic thickness of around 7-10 km. About 40% of the Earth's surface
   is now underlain by continental crust.

   As a consequence of the density difference, when active margins of
   continental crust meet oceanic crust in subduction zones, the oceanic
   crust is typically subducted back into the mantle. Because of its
   relative low density, continental crust is only rarely subducted or
   re-cycled back into the mantle (for instance, where continental crustal
   blocks collide and overthicken, causing deep melting). For this reason
   the oldest rocks on Earth are within the cratons or cores of the
   continents, rather than in repeatedly recycled oceanic crust; the
   oldest continental rock is the Acasta Gneiss at 4.01 Ga, while the
   oldest oceanic crust is of Jurassic age.

   The height of mountain ranges is usually related to the thickness of
   crust. This results from the isostasy associated with orogeny (mountain
   formation). The crust is thickened by the compressive forces related to
   subduction or continental collision. The buoyancy of the crust forces
   it upwards, the forces of the collisional stress balanced by gravity
   and erosion. This forms a keel or mountain root beneath the mountain
   range, which is where the thickest crust is found.

   The thinnest continental crust is found in rift zones, where the crust
   is thinned by detachment faulting and eventually severed, replaced by
   oceanic crust. The edges of continental fragments formed this way (both
   sides of the Atlantic Ocean, for example) are termed passive margins.

   It is a matter of debate whether the amount of continental crust has
   been increasing, decreasing, or remaining constant over geological
   time. One model indicates that at prior to 3.7 Bya continental crust
   constituted less than 10% of the present amount. By 3.0 Bya the amount
   was about 25% and following a period of rapid crustal evolution it was
   about 60% of the current amount by 2.6 Bya (Taylor and McLennan, 1995).
   The growth of continental crust appears to have occurred in spurts of
   increased activity corresponding to five episodes of increased
   production through geologic time (see graphic at Butler).
   Retrieved from " http://en.wikipedia.org/wiki/Continental_crust"
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