The Earth formed over tens of millions of years from an unknown
number of stochastic collisions with other planetary objects.
Its chemical and isotopic compositions provide an integrated
view of this accretion and early differentiation history.
Geochemical data for the Earth and other terrestrial planets can
be compared with the results of experimental simulations,
theory, and astronomical observations to elucidate the
conditions of planet and core formation. This chapter summarizes
the latest thinking in this area and particularly focuses on the
portion that we think is best understood, the state of the Earth
at around the time of the last major stage of accretion, the
Moon-forming Giant Impact. The age of the Moon is known to lie
in the range 30–55 My after the start of the solar system
because of evidence of decay of formerly live ¹⁸²Hf (half-life =
8.9 My) to ¹⁸²W within the lunar interior. As such, Earth’s
accretion and differentiation was more protracted than that of
Mars, Vesta, and magmatic iron meteo