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; see archaeo-, -an

Example Sentences

The UBC researchers' discovery of a stand-alone "intra-crustal" mechanism to make TTGs dispels the long-standing theory that Archean TTGs are formed in Earth's first subduction zones and mark the start of plate tectonics.

From Science Daily • Jan. 31, 2024

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 • 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

Geological time has been divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic, and as shown in Figure 8.3, the first three of these represent almost 90% of Earth’s history.

From Textbooks • Jan. 1, 2015

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