The Hadean Eon is a geologic eon of the Earth predating the Archean 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.
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