Australia: The Land Where Time Began

A biography of the Australian continent 

The Hadean 4.5-3.8 Ga

The Hadean is the time when the Solar System and the Earth was forming from the accretion disk that is believed to have have existed that was probably a cloud of dust and gas. It is suggested by the relative abundances of the heavier elements in the Solar System that the material the Solar System, and the Earth, was derived from 1 or more supernovae, among an earlier generation of stars. The beginning of the Hadean has been set at 4.5 Ga because that is the oldest age determined for meteorites and lunar rocks, the assumption being that they were the first rocks to form from the condensing cloud of dust and gas.

It was assumed that the rocks of the Earth formed later than 4.5 Ga, because the Earth was being bombarded by rock chunks of various sizes, the heat generated by these impacts keeping the Earth molten for some time after it had coalesced. When these dates were set for the earliest stages of the formation of the Earth the oldest rocks known on Earth were 3.9 Ga. Since then detrital zircons have been found in Western Australia that were more than 4 Ga, the oldest at the time of writing being 4.4 Ga, a short time after the dates of the oldest known extraterrestrial material. These zircons where found in low hills at Mt Naryer and Jack Hills, the remnants of mountain ranges that have been eroded down to mere stubs, and are composed mostly of sand that resulted from the erosion of an earlier generation of ranges.

Sources & Further reading

1. Hadean Time