<P> Earth has probably always had some form of basaltic crust, but the age of the oldest oceanic crust today is only about 200 million years . In contrast, the bulk of the continental crust is much older . The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield . Some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane . </P> <P> The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years . Most crustal rocks formed before 2.5 billion years ago are located in cratons . Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in Earth and so are not readily destroyed by subduction . Formation of new continental crust is linked to periods of intense orogeny; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana . The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle . </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> <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>

Whats the composition of the earth's crust