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Crust (geology)

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Earth cutaway from core to exosphere.
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   Earth cutaway from core to exosphere.

   In geology, a crust is the outermost layer of a planet, part of its
   lithosphere. Planetary crusts are generally composed of a less dense
   material than that of its deeper layers. The crust of the Earth is
   composed mainly of basalt and granite. It is cooler and more rigid than
   the deeper layers of the mantle and core.

   On stratified planets, such as Earth, the lithosphere is floating on
   fluid interior layers. Because of convection in the plastic, although
   non-molten, upper mantle and asthenosphere, the lithosphere is broken
   into tectonic plates that move. Oceanic crust is different from that of
   the continents. The oceanic crust ( sima) is 5 to 10 km thick and is
   composed primarily of a dark, dense rock called basalt. The continental
   crust ( sial) is 20-70 km deep and is composed of a variety of less
   dense rocks. The crust's temperature ranges from the air temperature to
   about 900°C near the upper mantle.

Origin of the Earth's Crust

   The Earth is considered to have differentiated from an aggregate of
   planetesimals into its core, mantle and crust within ~100 million years
   of the formation of the planet, at 4.4 billion years ago. The
   primordial crust was very thin, and was likely recycled by much more
   vigorous plate tectonics and destroyed by significant asteroid impacts,
   which were much more common in the early stages of the solar system. Of
   particular note is a theory that the moon was formed by one such very
   large impact.

   The Earth has likely always had some form of basaltic oceanic crust,
   but there is evidence that it has also had continental style crust for
   as long as 3.8 to 3.9 billion years. The oldest crust on Earth is the
   Narryer Gneiss Terrane in Western Australia at 3.9 Ga, and certain
   parts of the Canadian Shield and the Fennoscandian Shield are also of
   this age.

   The majority of the current Earth's continental crust was formed
   primarily between 4.6 billion years and 3.9 billion years before
   present, in the Hadean. The vast majority of rocks of this age are
   located in cratons where the crust is up to 70km thick. The lower
   density of the continental crust as compared to the oceanic crust
   prevents it being destroyed by subduction. Crust formation is linked to
   periods of intense orogeny or mountain building; these periods coincide
   with the formation of the supercontinents such as Rodinia, Pangaea and
   Gondwana. The crust forms not so much by accumulation of granite and
   metamorphic fold belts, but by depletion of the mantle to form buoyant
   lithospheric mantle.

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