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Archean

[ahr-kee-uh n] /ɑrˈki ən/
adjective, Geology
1.
noting or pertaining to rocks of the Archeozoic portion of the Precambrian Era.
Also, Archaean.
Origin
1870-1875
1870-75; < Greek archaî(os) ancient (see archaeo-) + -an
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for Archean

Archaean

/ɑːˈkiːən/
adjective
1.
of or relating to the highly metamorphosed rocks formed in the early Precambrian era
2.
the earlier of two divisions of the Precambrian era, during which the earliest forms of life are assumed to have appeared
Compare Proterozoic

Archean

/ɑːˈkiːən/
adjective
1.
a variant spelling (esp US) of Archaean
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Archean in Science
Archean
  (är-kē'ən)   
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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