Greenlandite (fuchsite-quartz gneiss), Greenland

Garnet paragneiss Nuvvuagittuq Greenstone Belt, 4

The Eoarchean (; also spelled Eoarchaean) is the first

era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Comp ...

of the

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

Eon of the

geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...

. It spans 400 million years, from the end of the Hadean Eon 4 billion years ago (4000 Mya) to the start of the

Paleoarchean The Paleoarchean (), also spelled Palaeoarchaean (formerly known as early Archean), is a Geologic time scale#Terminology, geologic era within the Archean, Archaean Eon. The name derives from Greek "Palaios" ''ancient''. It spans the period of time ...

Era 3600 Mya. The

beginnings Beginnings may refer to: Literature * ''Beginnings'' (collection), a 1988 collection of short stories and poems by Gordon R. Dickson * ''Beginnings'' (Honorverse), a 2013 collection of short stories in the Worlds of Honor series * ''Beginnings ...

life on Earth Life on Earth may refer to: Science * Life * Earliest known life forms * Evolutionary history of life ** Abiogenesis Film and television * ''Life on Earth'' (film) (''La Vie Sur Terre''), a 1998 Malian film * ''Life on Earth'' (TV series), a 197 ...

have been dated to this era and evidence of

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

date to 3500 Mya, comparatively shortly after the Eoarchean. At that time, the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

was without oxygen and the pressure values ranged from 10 to 100

bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar (u ...

(around 10 to 100 times the atmospheric pressure today).

Chronology

The Eoarchean Era was formerly officially unnamed and informally referred to as the first part of the ''Early Archean'' Eon (which is now an obsolete name) alongside the

Era. The

International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...

now officially recognizes the Eoarchean Era as the first part of the Archaean

Eon Eon or Eons may refer to: Time * Aeon, an indefinite long period of time * Eon (geology), a division of the geologic time scale Arts and entertainment Fictional characters * Eon, in the 2007 film '' Ben 10: Race Against Time'' * Eon, in the ...

, preceded by the Hadean Eon, during which the Earth is believed to have been essentially molten. The Eoarchaean's lower boundary or starting point of 4

Gya A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to seconds (or simply 1,000,000,000 years). It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or ...

billion years ago bya or b.y.a. is an abbreviation for "billion years ago". It is commonly used as a unit of time to denote length of time before the present in 109 years. This initialism is often used in the sciences of astronomy, geology, and paleontology. The " ...

) is officially recognized by the International Commission on Stratigraphy. The name comes from two

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

words: ' (dawn) and ' (ancient). The first supercontinent candidate

Vaalbara Vaalbara was an Archean supercontinent consisting of the Kaapvaal Craton (now in eastern South Africa) and the Pilbara Craton (now in north-western Western Australia). E. S. Cheney derived the name from the last four letters of each craton's name ...

appeared around the end of this period at about .

Geology

The beginning of the Eoarchean is characterized by heavy asteroid bombardment within the

Inner Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

: the

Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypoth ...

. The largest Eoarchean rock formation is the Isua Greenstone Belt on the south-west coast of Greenland, which dates from 3.8 billion years. The Acasta Gneiss within the Canadian Shield have been dated to be 4,031 Ma and are therefore the oldest preserved rock formations. In 2008, another rock formation was discovered in the

Nuvvuagittuq Greenstone Belt The Nuvvuagittuq Greenstone Belt (NGB; Inuktitut: ) is a sequence of metamorphosed mafic to ultramafic Volcanic rock, volcanic and associated sedimentary rocks (a greenstone belt) located on the eastern shore of Hudson Bay, 40 km southeast of ...

in northern

Québec, Canada Quebec ( ; )According to the Government of Canada, Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is ...

which has been dated to be . These formations are presently under intense investigation.

Atmosphere

3,850 million years old Greenland

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

shows evidence of Carbon-12 enrichment. This has sparked a debate whether there might have been photosynthetic life before 3.8 billion years.

Proposed subdivisions

*Eoarchean Era — 4031–3600 Mya **Acastan Period — 4031–3810 Mya **Isuan Period — 3810–3600 Mya

References

External links

*
Taxonconcept.stratigraphy.ne: A short fact sheet on the Eoarchean
{{Geological history, c *01 Geological eras Precambrian geochronology