Archean

Ar·che·an also Ar·chae·an  (är-kn)

adj.

1. Of or belonging to the earlier of the two divisions of Precambrian time, from approximately 3.8 to 2.5 billion years ago, marked by an atmosphere with little free oxygen, the formation of the first rocks and oceans, and the development of unicellular life. See Table at geologic time.

2. Of or relating to the oldest known rocks, those of the Precambrian Eon, that are predominantly igneous in composition.

n.

The Archean Eon. Also called Archeozoic.

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[From Greek arkhaios, ancient; see archaic.]

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Archean [ɑːˈkiːən]

adj

(Earth Sciences / Geological Science) a variant spelling (esp US) of Archaean

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Archean  (är-kn)

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. See Chart at geologic time.

Thesaurus

Noun	1.	Archean - the time from 3,800 million years to 2,500 million years ago; earth's crust formed; unicellular organisms are earliest forms of life Archaeozoic, Archaeozoic aeon, Archean aeon, Archean eon, Archeozoic, Archeozoic eon Precambrian, Precambrian aeon, Precambrian eon, Precambrian period - the eon following the Hadean time and preceding the Phanerozoic eon; from about 3,800 million years ago until 544 million years ago
Adj.	1.	archean - of or relating to the earliest known rocks formed during the Precambrian Eon archaean geology - a science that deals with the history of the earth as recorded in rocks early - at or near the beginning of a period of time or course of events or before the usual or expected time; "early morning"; "an early warning"; "early diagnosis"; "an early death"; "took early retirement"; "an early spring"; "early varieties of peas and tomatoes mature before most standard varieties"