What is the Archean Eon?

Archean Eon was the earlier of the two formal divisions of Precambrian time (about 4600 MA to 541 million years ago). The Archean Eon began about 4000 million years ago with the formation of Earth’s crust and extended to the start of the Proterozoic Eon 2500 million years ago; the latter is the second formal division of Precambrian time. The Archean Eon was preceded by the Hadean Eon, an informal division of geologic time spanning from 4600 million to 4000 million years ago and characterized by Earth’s initial formation. Records of Earth’s primitive atmosphere and oceans emerge in the earliest Archean (Eoarchean Era), and evidence of the earliest primitive life-forms—bacteria and blue-green algae—appears in rocks about 3500–3700 million years old. Archean greenstone-granite belts contain many economic mineral deposits, including gold and silver.

The start of the Archean Eon is only defined by the isotopic age of the earliest rocks. Prior to the Archean Eon, the Earth was in the astronomical (Hadean) stage of planetary accretion that began about 4600 million years ago; no rocks are preserved from this stage. The earliest terrestrial materials are not rocks but minerals. In Western Australia some sedimentary conglomerates, dated to 3300 million years ago, contain relict detrital zircon grains that have isotopic ages between 4200 and 4400 million years. These grains must have been transported by rivers from a source area, the location of which has never been found; it was possibly destroyed by meteorite impacts—quite frequent on both the Earth and the Moon before 4000 million years ago.