<P> Because both continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle . This is isostasy, and it's also one of the reasons continental crust is higher than oceanic: continental is less dense and so "floats" higher . As a result, water pools in above the oceanic crust, forming the oceans . </P> <P> The temperature of the crust increases with depth, reaching values typically in the range from about 200 ° C (392 ° F) to 400 ° C (752 ° F) at the boundary with the underlying mantle . The temperature increases by as much as 30 ° C (54 ° F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust . </P> <P> The continental crust has an average composition similar to that of andesite . The most abundant minerals in Earth's continental crust are feldspars, which make up about 41% of the crust by weight, followed by quartz at 12%, and pyroxenes at 11% . Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle . Although the continental crust comprises only about 0.6 weight percent of the silicate on Earth, it contains 20% to 70% of the incompatible elements . </P> <Table> <Tr> <Td> <Table> <Tr> <Th> Most Abundant Elements of Earth's Crust </Th> <Th_colspan="1.5"> Approximate% by weight </Th> </Tr> <Tr> <Td> O </Td> <Td> 46.6 </Td> </Tr> <Tr> <Td> Si </Td> <Td> 27.7 </Td> </Tr> <Tr> <Td> Al </Td> <Td> 8.1 </Td> </Tr> <Tr> <Td> Fe </Td> <Td> 5.0 </Td> </Tr> <Tr> <Td> Ca </Td> <Td> 3.6 </Td> </Tr> <Tr> <Td> Na </Td> <Td> 2.8 </Td> </Tr> <Tr> <Td> </Td> <Td> 2.6 </Td> </Tr> <Tr> <Td> Mg </Td> <Td> 1.5 </Td> </Tr> </Table> <P> </P> </Td> <Td> <Table> <Tr> <Th> Oxide </Th> <Th> Percent </Th> </Tr> <Tr> <Td> SiO </Td> <Td> 60.6 </Td> </Tr> <Tr> <Td> Al O </Td> <Td> 15.9 </Td> </Tr> <Tr> <Td> CaO </Td> <Td> 6.4 </Td> </Tr> <Tr> <Td> MgO </Td> <Td> 4.7 </Td> </Tr> <Tr> <Td> Na O </Td> <Td> 3.1 </Td> </Tr> <Tr> <Td> Fe as FeO </Td> <Td> 6.7 </Td> </Tr> <Tr> <Td> K O </Td> <Td> 1.8 </Td> </Tr> <Tr> <Td> TiO </Td> <Td> 0.7 </Td> </Tr> <Tr> <Td> P O </Td> <Td> 0.1 </Td> </Tr> </Table> <P> </P> </Td> <Td> <P> </P> </Td> </Tr> </Table>

How many types of rocks make up the earth's crust
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