Hadean Eon

The Hadean Eon began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. As of 2016, the ICS describes its status as "informal".

Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term: the "Priscoan period".

Other, older texts simply refer to the eon as the Pre-Archean.