Archean

or Ar·chae·an

[ ahr-kee-uhn ]

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.

adjective

1. of or relating to this eon.

Archean

/ ɑːˈkiːən /

adjective

1. a variant spelling (esp US) of Archaean

Archean

/ 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

Word History and Origins

Origin of Archean¹

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

Example Sentences

The researchers analyzed uranium, thorium and potassium concentrations from hundreds of samples of rocks from the Archean period, when the cratons formed, to assess the radiogenic heat productivity based on actual rock compositions.

From Science Daily

The majority of Ontario, Quebec, Manitoba, Saskatchewan, Northwest Territories and Nunavut is made up of Archean crustal fragments that are dominated by TTGs and their slightly younger and more evolved granite counterparts.

From Science Daily

So Venus's tectonic activity might resemble that on early Earth, during the Archean Eon, between four billion and 2.5 billion years ago, when the heat flow within the planet was higher and the lithosphere thinner.

From BBC

The rocks on which they have grown ultra-wealthy extracting are ancient, dating back to Precambrian times - specifically the Archean eon - and are about two-and-half billion years old.

From BBC

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

From Literature