“thickness of earth’s crust*
(in ‘kilometers’)
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*the continental crust is the layer of [‘igneous’ / ‘sedimentary’ / ‘meta-morphic’] rocks that forms the ‘continents’ + the areas of ‘shallow sea-bed’ close to their shores, known as “continental shelves”*
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This layer is sometimes called sial because its bulk composition is more felsic compared to the oceanic crust, called sima which has a more mafic bulk composition.
Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.
The continental crust consists of various layers, with a bulk composition that is intermediate to felsic.
The average density of continental crust is about 2.7 g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3.
Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3.
At 25 to 70 km, continental crust is considerably thicker than oceanic crust, which has an average thickness of around 7–10 km.
(about 40% of earth’s surface is currently occupied by ‘continental crust’)
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(it makes up about 70% of the ‘volume’ of earth’s ‘crust’)
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