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The boundary between the crust and mantle is conventionally placed at the Mohorovi? discontinuity, a boundary defined by a contrast in seismic velocity.The crust occupies less than 1% of Earth's volume.[1] The oceanic crust of the sheet is different from its continental crust.Some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane.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 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.[9] The underlying mantle is denser and olivine-rich.The thickness of the crust ranges between about 20 and 120 km.

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Partly by analogy to what is known about the Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago.The primordial crust was very thin and was probably recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system.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 [4][5] 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.Highest customer-rated video chat script, including language support, Speed Dating, Geo IP location and more...