The Eoarchean ( ; also spelled Eoarchaean) is the first era of the Archean Eon of the geologic record. It spans 431 million years, from the end of the Hadean Eon 4031 Mya to the start of the Paleoarchean Era 3600 Mya. Some estimates place the beginnings of life on Earth in this era, while others place it earlier. Evidence of

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

and

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

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

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

was without

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and the pressure values ranged from 10 to 100 bar (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 Paleoarchean Era. The

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

now officially recognizes the Eoarchean Era as the first part of the Archaean Eon, 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 4.031 Gya (4031

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

) is officially recognized by the International Commission on Stratigraphy. The name comes from two Greek words: ' (dawn) and ' (ancient). The first

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...

candidate Vaalbara appeared around the end of this period at about .

Geology

The beginning of the Eoarchean is characterized by heavy

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

bombardment within the Inner Solar System: the Late Heavy Bombardment. The largest Eoarchean rock formation is the Isua Greenstone Belt on the south-west coast of

Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...

, which dates from 3.8 billion years. The

Acasta Gneiss The Acasta Gneiss Complex, also called the Acasta Gneiss, is a body of felsic to Ultramafic rock, ultramafic Archean Basement (geology), basement rocks, gneisses, that form the northwestern edge of the Slave craton, Slave Craton in the Northwes ...

within the

Canadian Shield The Canadian Shield ( ), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), th ...

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 in northern

Québec, Canada Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...

, which has been dated to be . These formations are presently under intense investigation. Oxygen isotope ratios show that the hydrological cycle had begun by the early Eoarchaean and possibly earlier. Carbonate precipitation (caused by heating of seawater by hydrothermal vents) acted as an important sink regulating the concentration of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

in the atmosphere during this era.

Atmosphere

3,850 million years old

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

from Greenland shows evidence of

Carbon-12 Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon ( carbon-13 being the other), amounting to 98.93% of element carbon on Earth; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-1 ...

enrichment. This has sparked a debate whether there might have been photosynthetic life before 3.8 billion years ago.

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