<Tr> <Td> TiO </Td> <Td> 0.7 </Td> </Tr> <Tr> <Td> P O </Td> <Td> 0.1 </Td> </Tr> <P> All the other constituents except water occur only in very small quantities and total less than 1% . Estimates of average density for the upper crust range between 2.69 and 2.74 g / cm and for lower crust between 3.0 and 3.25 g / cm . </P> <P> A theoretical protoplanet named "Theia" is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon . As the Moon formed, the outer part of it is thought to have been molten, a "lunar magma ocean ." Plagioclase feldspar crystallized in large amounts from this magma ocean and floated toward the surface . The cumulate rocks form much of the crust . The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase . The underlying mantle is denser and olivine - rich . </P>

What is the importance of the earth's crust