[ahr-kee-uh n]


or Ar·chae·an

[ahr-kee-uh n]
adjective Geology.
  1. noting or pertaining to rocks of the Archeozoic portion of the Precambrian Era.

Origin of Archean

1870–75; < Greek archaî(os) ancient (see archaeo-) + -an Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for archaean

Historical Examples of archaean

  • Directly or indirectly, the fish is a descendant of some Archaean Annelid.

  • This place is about five hundred yards west of the bridge in an alcove in the Archaean Rock which forms the Canyon wall.

    I Married a Ranger

    Dama Margaret Smith

  • Their lithological characteristics and the total absence of all organic remains point to the Archaean period.

  • It was laid down close to the igneous Archaean rocks when Mother Earth was in her girlhood and water first began to flow.

    Among the Forces

    Henry White Warren

  • The Archaean rocks still solidly underlie the lowest depth he has ever reached.

British Dictionary definitions for archaean


  1. any member of the Archaea, a domain of prokaryotic microorganisms, distinguished from bacteria on molecular phylogenetic grounds and often found in hostile environments, such as volcanic vents and hot springs


esp US Archean

  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


  1. a variant spelling (esp US) of Archaean
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for archaean



"of the earliest geological age," 1872, from Greek arkhaios "ancient," from arkhe "beginning" (see archon).

Online Etymology Dictionary, © 2010 Douglas Harper

archaean in Science


  1. Another spelling of Archean.


  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. See Chart at geologic time.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.