Hadean The first Precambrian ‘system’ ( Harland et al., 1989), extending from about 4500 Ma to about 3875 Ma ago, and best preserved in lunar rocks. See also PRISCOAN.

Hadean The earliest eon in the history of the earth, from the time of the accretion of planetary material, around 4600 million years ago, to the date of the oldest known rocks – and hence the beginning of the geological record – about 3800 million years ago. The young earth was probably a rocky planet with a hot interior and a moist surface with oceans of liquid water. No evidence of life has been found. Compare Archaean.