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

Isotopic geochemistry suggests the molten material came from Archean continental crust dating between 3.0 and 3.3 billion years old.

From Science Daily • Mar. 1, 2026

The study by Dr. Smit and his UBC-based team used data from all the TTG samples ever analyzed -- samples from Archean cratonic fragments exposed worldwide scrutinized by researchers over the past 30 years.

From Science Daily • Jan. 31, 2024

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

The first solid evidence of modern plate tectonics is found at the end of the Archean, indicating at least some continental lithosphere must have been in place.

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