EARTH STRUCTURE AND ORIGINS 427 Figure 11 A comparison between the eons and eras of the geological column and sequences of events that have been deduced for the other inner planets, the Moon and satellites of the outer planets on the basis of the very limited evidence available from Space and meteoritical research (modified from G J H McCall (2000) Age and early evolution of the Earth and Solar System In: Hancock PL and Skinner BJ (eds.) Oxford Companion to the Earth, pp 11 Oxford: Oxford University Press) The Origin of the Earth The Earth was formed from the nebula that produced the Solar System It is almost universally accepted that the Sun, the planets and their satellites, the asteroids, and the comets of the Oort ‘cloud’ grew from a cloud of gas and dust that contracted under its own gravity The cloud of gas and dust had some degree of rotation (Figure 12) and, as the centre contracted, the angular momentum forced the remains into a flattened disk, in the plume perpendicular to the axis of rotation of the proto-Sun This was apparently a very rapid process, perhaps taking place over 10 000 years The dust particles accreted heterogeneously to produce lumps that formed planetesmals, the first small solid bodies, and cooling occurred We can estimate the composition of the original nebula from the solar abundances of elements, obtained spectrographically, and analysis of primitive meteorites (e.g carbonaceous chondrites) (see Solar System: Meteorites); however, these are only estimates, and a second method is based on an assumption that may not be entirely accurate Radioactivity-based dating methods (relying on the decay of radiogenic isotopes), as applied to terrestrial igneous rocks, allow us to date the condensation to 4500 Ma ago (the so-called ‘age of the Earth’), and it is believed that condensation was rapid, taking about 100 000 years Accretion of the Earth and the other planets may have been slow and heterogeneous or rapid and homogeneous Which of these two models is correct remains uncertain, but the first model, if correct, could have produced a layered mantle (see Earth: Mantle) After condensation and accretion, the Earth evolved rapidly into a planet with most of its present properties This could have taken no more than 500 Ma and was probably effected more rapidly: the oldest known minerals are zoned zircons from the Mount Narryer gneiss terrain, Western Australia, which give spot SHRIMP ages of 4400 Ma and are believed to have been formed by partial melting of preexisting granitic crust (see Minerals: Zircons) This evidence indicates that there was solid rock at the Earth’s surface astonishingly soon after condensation and accretion The oldest actual rock dated is a component of the Acasta Gneiss in the Slave Province of Canada, which has an age of 4000 Ma Despite the existence of the Mount Narryer minerals, there is no other really significant evidence from geology of the Earth pertaining to the 500 Ma prior to the formation of the Acasta Gneiss, and this is called the ‘Hadean Eon’,