<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> 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> <P> The thickness of the crust ranges between about 20 and 120 km . Crust on the far side of the Moon averages about 12 km thicker than that on the near side . Estimates of average thickness fall in the range from about 50 to 60 km . Most of this plagioclase - rich crust formed shortly after formation of the moon, between about 4.5 and 4.3 billion years ago . Perhaps 10% or less of the crust consists of igneous rock added after the formation of the initial plagioclase - rich material . The best - characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago . Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago . There is no evidence of plate tectonics . </P> <P> Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth . Although the radius of the Moon is only about a quarter that of Earth, the lunar crust has a significantly greater average thickness . This thick crust formed almost immediately after formation of the Moon . Magmatism continued after the period of intense meteorite impacts ended about 3.9 billion years ago, but igneous rocks younger than 3.9 billion years make up only a minor part of the crust . </P>

Earth's crust contains a high concentration of