Archean

American  

[ahr-kee-uhn] / ɑrˈki ən /

Or Archaean

noun

1. the Precambrian eon that preceded the Proterozoic and during which the earliest datable rocks were formed and from which the oldest known fossil forms, including cyanobacteria, have been recovered.

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adjective

1. of or relating to this eon.

Archean British  

/ ɑːˈkiːən /

adjective

1. a variant spelling (esp US) of Archaean

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Archean Scientific  

/ är-kē′ən /

1. The earlier of the two divisions of the Precambrian Eon, from about 3.8 to 2.5 billion years ago. During this time the Earth had a reducing atmosphere consisting primarily of methane, ammonia, and other gases that would be toxic to most modern life forms. There was little free oxygen. Rocks from the earliest part of the Archean are predominantly volcanic and are similar to pillow basalts, suggesting that they formed underwater. Rocks from the later part of the Archean appear to have formed on continents. It is believed that about 70% of the continental masses formed during this time. Fossils preserved in rocks from this period of time include remains of cyanobacteria, the first single-celled forms of life. These organisms are preserved in the form of stromatolites and oncolites.

2. See Chart at geologic time

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Etymology

Origin of Archean

First recorded in 1870–75; from Greek archaî(os) “ancient” + -an adjective suffix; archaeo-, -an

Example Sentences

During the early Archean, abundant nickel and urea may have restricted cyanobacterial blooms, preventing sustained oxygen release.

From Science Daily • Nov. 2, 2025

By demonstrating how urea could form naturally under Archean conditions and showing that it acts as both a nutrient and an inhibitor, the researchers revealed how subtle chemical balances shaped Earth's early biosphere.

From Science Daily • Nov. 2, 2025

During Earth’s Archean period about 3 billion years ago, the mantle was hot and dry and some volcanoes erupted extremely hot lavas called komatiites, possibly similar to Io lavas.

From Slate • Sep. 27, 2023

Although there is current evidence that rocks and minerals existed during the Hadean Eon, the Archean has a much more robust rock and fossil record.

From Textbooks • Jan. 1, 2017

Others divide the Precambrian into two eras, the very ancient Archean and the more recent Proterozoic.

From "A Short History of Nearly Everything" by Bill Bryson