Archean
Encyclopedia
The Archean also spelled Archeozoic or Archæozoic) is a geologic
eon before the Paleoproterozoic
Era of the Proterozoic
Eon, before 2.5 Ga (billion years, or 2,500 ) ago. Instead of being based on stratigraphy
, this date is defined chronometrically. The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy
, but it is usually set to 3.8 Ga, at the end of the Hadean
Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek
"Αρχή" (Arkhē), meaning "beginning, origin".
history. When the Archean began, the Earth's heat flow was nearly three times higher than 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, heat from the formation of the Earth's core, and heat produced by radioactive elements.
Most surviving Archean rocks are metamorphic
or igneous
. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite
. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite
, diorite
, layered intrusions, anorthosite
s and monzonite
s known as sanukitoid
s.
The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material. This may have prevented craton
isation and continent formation until the mantle cooled and convection slowed down. Others argue that the oceanic lithosphere was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean is an active area of modern research.
There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. The other school follows the teaching of Richard Armstrong
, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.
Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics
operated in the Archean argue that the felsic
protocontinents formed at hotspots
rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapir
s, a variety of mafic
magmas produce intermediate and felsic rocks. Others accept that granite formation in island arc
s and convergent margin
s was part of the plate tectonic process, which has operated since at least the start of the Archean.
An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth's surface during the period. The similar age of the oldest surviving rocks and the "late heavy bombardment
" is probably not accidental.
. Astronomers think that the sun had about 70-75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling (the faint young sun paradox
). The presence of liquid water is evidenced by certain highly deformed gneiss
es produced by metamorphism of sediment
ary protolith
s. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history
. Alternatively, Earth's albedo
may have been lower at the time, due to less land area and cloud cover.
By the end of the Archaean c. 2500 Mya, plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basin
s, and evidence of volcanic arc
s, intracontinental rift
s, continent-continent collisions and widespread globe-spanning orogenic events
suggesting the assembly and destruction of one and perhaps several supercontinent
s. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formation
s, chert
beds, chemical sediments and pillow basalts.
are Archean or slightly older. Archean rocks are known from Greenland
, the Canadian Shield
, the Baltic Shield
, Scotland
, India, Brazil
, western Australia
, and southern Africa
. Although the first continents formed during this eon, rock of this age makes up only 7% of the world's current craton
s; even allowing for erosion and destruction of past formations, evidence suggests that continental crust
equivalent to only 5-40% of the present amount formed during the Archean.
In contrast to Proterozoic rocks, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstone
s, volcanic sediments, and banded iron formation
s. Carbonate
rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide
than during the Proterozoic. Greenstone belt
s are typical Archean formations, consisting of alternating units of metamorphosed mafic
igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arc
s, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin
. Greenstone belts represent sutures between protocontinents.
l fossils are known from chert
beds. In addition to the domain Bacteria
(once known as Eubacteria), microfossils of the domain Archaea
have also been identified.
Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (formerly known as Monera). There are no known eukaryotic
fossils, though they might have evolved during the Archean without leaving any fossils. No fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
eon before the Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
Era of the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
Eon, before 2.5 Ga (billion years, or 2,500 ) ago. Instead of being based on stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, this date is defined chronometrically. The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy
International Commission on Stratigraphy
The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...
, but it is usually set to 3.8 Ga, at the end of the Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
"Αρχή" (Arkhē), meaning "beginning, origin".
Archean Earth
The Archean is one of the four principal eons of EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
history. When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
(2,500 ). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements.
Most surviving Archean rocks are metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
or igneous
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
, layered intrusions, anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
s and monzonite
Monzonite
Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals...
s known as sanukitoid
Sanukitoid
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 with similar geochemical characteristics...
s.
The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material. This may have prevented craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
isation and continent formation until the mantle cooled and convection slowed down. Others argue that the oceanic lithosphere was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean is an active area of modern research.
There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. The other school follows the teaching of Richard Armstrong
Richard Lee Armstrong
Richard Lee “Dick” Armstrong was an American/Canadian scientist who was an expert in the fields of radiogenic isotope geochemistry and geochronology, geochemical evolution of the earth, geology of the American Cordillera, and large-magnitude crustal extension...
, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.
Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
operated in the Archean argue that the felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
protocontinents formed at hotspots
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapir
Diapir
A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress...
s, a variety of mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
magmas produce intermediate and felsic rocks. Others accept that granite formation in island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
s and convergent margin
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
s was part of the plate tectonic process, which has operated since at least the start of the Archean.
An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth's surface during the period. The similar age of the oldest surviving rocks and the "late heavy bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
" is probably not accidental.
Archean palaeoenvironment
The Archean atmosphere is thought to have lacked free oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Astronomers think that the sun had about 70-75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling (the faint young sun paradox
Faint young sun paradox
The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised...
). The presence of liquid water is evidenced by certain highly deformed gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es produced by metamorphism of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
ary protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
s. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history
History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...
. Alternatively, Earth's albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
may have been lower at the time, due to less land area and cloud cover.
By the end of the Archaean c. 2500 Mya, plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s, and evidence of volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...
s, intracontinental rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
s, continent-continent collisions and widespread globe-spanning orogenic events
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
suggesting the assembly and destruction of one and perhaps several supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
s. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
s, chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
beds, chemical sediments and pillow basalts.
Archean geology
Although a few mineral grains are known that are Hadean, the oldest rock formations exposed on the surface of the EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
are Archean or slightly older. Archean rocks are known from Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, the Canadian Shield
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
, the Baltic Shield
Baltic Shield
The Baltic Shield is located in Fennoscandia , northwest Russia and under the Baltic Sea. The Baltic Shield is defined as the exposed Precambrian northwest segment of the East European Craton...
, Scotland
Geology of Scotland
The geology of Scotland is unusually varied for a country of its size, with a large number of differing geological features. There are three main geographical sub-divisions: the Highlands and Islands is a diverse area which lies to the north and west of the Highland Boundary Fault; the Central...
, India, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, western Australia
Geology of Australia
Australia is a continent situated on the Indo-Australian Plate.The geology of Australia includes virtually all known rock types and from all geological time periods spanning over 3.8 billion years of the Earth's history.-Components:...
, and southern Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. Although the first continents formed during this eon, rock of this age makes up only 7% of the world's current craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s; even allowing for erosion and destruction of past formations, evidence suggests that continental crust
Crust
Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...
equivalent to only 5-40% of the present amount formed during the Archean.
In contrast to Proterozoic rocks, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s, volcanic sediments, and banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
s. Carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
than during the Proterozoic. Greenstone belt
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....
s are typical Archean formations, consisting of alternating units of metamorphosed mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
s, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin
Forearc
A forearc or forarc, also called arc-trench gap is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material...
. Greenstone belts represent sutures between protocontinents.
Archean life
Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l fossils are known from chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
beds. In addition to the domain Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
(once known as Eubacteria), microfossils of the domain Archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
have also been identified.
Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (formerly known as Monera). There are no known eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
fossils, though they might have evolved during the Archean without leaving any fossils. No fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses.