The continental crust is the layer of granitic and sedimentary rock which forms 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 mantle and thus "floats" on top of it. The relative low density keeps the continental crust from being re-cycled or subducted back into the mantle. For this reason the oldest rocks on Earth are within the continents rather than in repeatedly re-cycled oceanic crust. It is also less dense than oceanic crust and considerably thicker averaging 20 to 80 km versus the average oceanic thickness of around 5-10 km. Continental crust is thickest beneath mountain ranges with a deep root. About 40% of the Earth's surface is underlain by continental crust.
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