<P> An age of 4.55 ± 0.07 billion years, very close to today's accepted age, was determined by Clair Cameron Patterson using uranium - lead isotope dating (specifically lead - lead dating) on several meteorites including the Canyon Diablo meteorite and published in 1956 . </P> <P> The quoted age of Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research . </P> <P> Most geological samples from Earth are unable to give a direct date of the formation of Earth from the solar nebula because Earth has undergone differentiation into the core, mantle, and crust, and this has then undergone a long history of mixing and unmixing of these sample reservoirs by plate tectonics, weathering and hydrothermal circulation . </P> <P> All of these processes may adversely affect isotopic dating mechanisms because the sample cannot always be assumed to have remained as a closed system, by which it is meant that either the parent or daughter nuclide (a species of atom characterised by the number of neutrons and protons an atom contains) or an intermediate daughter nuclide may have been partially removed from the sample, which will skew the resulting isotopic date . To mitigate this effect it is usual to date several minerals in the same sample, to provide an isochron . Alternatively, more than one dating system may be used on a sample to check the date . </P>

When an earth scientist says that the earth is differentiated it means that
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