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The Archean Eon ( , also spelled Archaean or Archæan) is the second of four
geologic eon The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochron ...
s of
Earth's history The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geologic ...
, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
during the Archean was mostly a water world: there was
continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' be ...
, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water
microbial mat A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. ...
s called
stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...
s, and the atmosphere lacked free oxygen.


Etymology and changes in classification

The word ''Archean'' comes from the Greek word (), meaning 'beginning, origin'. It was first used in 1872, when it meant 'of the earliest geological age'. Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago. Instead of being based on
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...
, the beginning and end of the Archean Eon are defined chronometrically. The eon's lower boundary or starting point of 4 billion years ago is officially recognized by 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 ...
.


Geology

When the Archean began, the Earth's
heat flow Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 ). The extra heat was the result of a mix of remnant heat from
planetary accretion In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are forme ...
, from the formation of the metallic core, and from the decay of
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
elements. As a result, the Earth's mantle was significantly hotter than today. Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
,
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
, the
Canadian Shield The Canadian Shield (french: Bouclier canadien ), also called 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), the anc ...
,
Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
and
Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
(exposed parts of the
Wyoming Craton The Wyoming Craton is a craton in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah. Also called the Wyoming Province, it is the init ...
), the
Baltic Shield The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's crust belonging to the East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed mostly of Archean and ...
, the Rhodope Massif,
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
,
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
,
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, western
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, and southern
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
.
Granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...
rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of
granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
,
diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...
,
layered intrusion A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to ...
s,
anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...
s and
monzonite Monzonite is an igneous intrusive rock, formed by slow cooling of underground magma that has a moderate silica content and is enriched in alkali metal oxides. Monzonite is composed mostly of plagioclase and alkali feldspar. Syenodiorite is an o ...
s known as
sanukitoid {{No footnotes, date=December 2021 Sanukitoids are a variety of high-Mg granitoid found in convergent margin settings. The term "sanukitoid" was originally used to define a variety of Archean plutonic rock, but now also includes younger rocks w ...
s. Archean rocks are often heavily metamorphized deep-water sediments, such as
graywacke Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...
s,
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s, volcanic sediments, and
banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness a ...
s.
Volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...
activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite.
Carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
rocks are rare, indicating that the oceans were more acidic due to dissolved
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
than during the Proterozoic.
Greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from the green ...
s are typical Archean formations, consisting of alternating units of metamorphosed
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
igneous and sedimentary rocks, including
Archean felsic volcanic rocks Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon (4 to 2.5 billion years ago). The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ag ...
. The metamorphosed igneous rocks were derived from volcanic
island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
s, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a
forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...
basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents.
Plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
likely started vigorously in the Hadean, but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean. Due to recycling and metamorphosis of the Archean crust, there is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called
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 ...
as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled. By the
Neoarchean The Neoarchean (; also spelled Neoarchaean) is the last geologic era in the Archean eon that spans from 2800 to 2500 million years ago—the period being defined chronometrically and not referencing a specific level in a rock section on Earth. ...
, plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of
slab detachment In plate tectonics, slab detachment or slab break-off may occur during continent-continent or arc-continent collisions. When the continental margin of the subducting plate reaches the oceanic trench of the subduction zone, the more buoyant contin ...
resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
into
eclogite Eclogite () is a metamorphic rock containing garnet (almandine- pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, ...
as they sank. There are well-preserved
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
s, and evidence of
volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...
s, intracontinental
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
s, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several
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", ...
s. Evidence from banded iron formations,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed. Asteroid impacts were frequent in the early Archean. Evidence from
spherule A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ...
layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than every 15 million years. This is about the size of the
Chicxulub The Chicxulub crater () is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore near the community of Chicxulub, after which it is named. It was formed slightly over 66 million years ago when a large a ...
impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.


Environment

The Archean atmosphere is thought to have nearly lacked free oxygen; oxygen levels were less than 0.001% of their present atmospheric level, with some analyses suggesting they were as low as 0.00001% of modern levels. However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980-2,960 Ma and around 2,501 Ma. Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present. Astronomers think that the Sun had about 75–80 percent of the present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the
faint young Sun paradox The faint young Sun paradox or faint young Sun problem describes the apparent contradiction between observations of liquid water early in Earth's history and the astrophysical expectation that the Sun's output would be only 70 percent as intense d ...
). The presence of liquid water is evidenced by certain highly deformed
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
es produced by metamorphism of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
ary
protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thu ...
s. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Alternatively, Earth's
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
may have been lower at the time, due to less land area and cloud cover.


Early life

The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon. The earliest evidence for life on Earth is
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
of
biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...
origin found in 3.7 billion–year-old
metasedimentary rock In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and er ...
s discovered in
Western Greenland Kitaa, originally Vestgrønland ("West Greenland"), is a former administrative division of Greenland. It was by far the most populated of the divisions, being home to almost 90% of the total population. The divisions were de facto replaced by st ...
. The earliest identifiable fossils consist of
stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...
s, which are
microbial mats A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. ...
formed in shallow water by
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 ...
. The earliest stromatolites are found in 3.48 billion-year-old
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
discovered in
Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...
. Stromatolites are found throughout the Archean and become common late in the Archean. Cyanobacteria were instrumental in creating free oxygen in the atmosphere. Further evidence for early life is found in 3.47 billion-year-old
baryte Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
, in the
Warrawoona Group The Warrawoona Group is a geological unit in Western Australia containing putative fossils of cyanobacteria cells. Dated 3.465 Ga, these microstructures, found in Archean chert, are considered to be the oldest known geological record of life on E ...
of Western Australia. This mineral shows sulfur
fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the ...
of as much as 21.1%, which is evidence of
sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
that metabolize sulfur-32 more readily than sulfur-34. Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
s dated to 4.1 billion years ago, but this evidence is preliminary and needs validation. Earth was very hostile to life before 4.2–4.3 Ga and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called
prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s. In addition to the domain
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, microfossils of the domain
Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
have also been identified. There are no known
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
fossils from the earliest Archean, though they might have evolved during the Archean without leaving any. Fossil
sterane Sterane (cyclopentanoperhydrophenanthrenes) compounds are a class of tetracyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. Steranes have an androstane skeleton with a side chain at carb ...
s, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
replicators such as
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...
es. Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.


See also

* * * * * * * * *


Footnotes


References


External links

* * * {{Authority control Precambrian geochronology