What is the Archean Eon?

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Archean eon.

The Archean, also known as Archeozoic is a geologic eon (The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages) before the Paleoproterozoic (first of the three sub-divisions (eras) of the Proterozoic occurring between 1,600 to 2,500 million years ago) Era of the Proterozoic (a geological eon representing a period before the first abundant complex life on Earth) Eon, before 2.5 Ga (billion years ago, or 2,500 Ma). Instead of being based on stratigraphy (studies rock layers), 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 (geologic eon before the Archean) Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek “Arkhe”, meaning "beginning / origin". Differing from most recent eons when life was abundant, the Archean is defined by random date limits rather than the presence or absence of certain fossils. It would be precise to consider that the Archean eon was a very long time ago, as is reflected in its name, which means old.


What was Archean Eon environment like?

  • The Archean atmosphere is assumed to have lacked free oxygen. Astronomers think that the sun was about one-third dimmer than at present, 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 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 presence of liquid water is evidenced by certain highly deformed gneisses (a common and widely distributed type of rock ) produced by metamorphism of sedimentary protoliths. Protolith refers to the precursor lithology of a metamorphic rock. The lithology of a rock unit is a description of its physical characteristics. Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form".
  • The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history.
  • Alternatively, Earth's fraction of power may have been lower at the time, due to less land area and cloud cover.
  • By the end of the Archaean, 2600 Mya, plate tectonic activity may have been similar to that of the modern Earth.
  • There are well-preserved sedimentary basins (refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation), and evidence of volcanic arcs (a chain of volcanoes parallel to a mountain belt), intracontinental rifts (a place where the Earth's crust and lithosphere are being pulled apart), continent-continent collisions suggesting the assembly and destruction of one and perhaps several supercontinents, a landmass comprising more than one continental core, or craton..
  • Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert (a fine-grained silica-rich microcrystalline, or microfibrous sedimentary rock that may contain small fossils) beds, chemical sediments and pillow basalts.


How was life like in Archean Eon?

  • 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 bacterial fossils are known from chert beds. Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae).
  • 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. 


What was the Geology of Archean Eon?

  • Even if a few mineral grains are known Hadean, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older.
  • Archean rocks are known from Greenland (an autonomous country within the Kingdom of Denmark), 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 cratons; 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 (a variety of sandstone), mudstones (a fine grained sedimentary rock), volcanic sediments, and banded iron formations (a distinctive type of rock often found in primordial (precambrian)sedimentary rocks.).
  • Carbonate (presence of the carbonate ion) rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.
  • Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks.
  • The meta-igneous rocks were derived from volcanic island arcs, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. A forearc is a depression in the sea floor located between a subduction zone and an associated volcanic arc. Greenstone belts represent sutures between protocontinents.


Did Plate tectonic exist in Archean Earth?

The Archean is one of the four principal eons of Earth 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 Ma). 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. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as “komatiite”. Granitic (a common and widely occurring type of intrusive, felsic, igneous rock) rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite (a grey to dark grey intermediate intrusive igneous rock), layered intrusions (a large sill-like body of igneous rock). 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 cratonisation (craton is an old and stable part of the continental lithosphere) and continent formation until the mantle cooled and convection slowed down. The query of whether plate tectonic activity existed in the Archean is an active area of modern research.


Did Continents exist in Archean?

There are two discipline of thought concerning the amount of continental crust that was present in the Archean.

  • One discipline believes that no large continents existed until late in the Archean: small “protocontinents” were the norm, prevented from combining into larger units by the high rate of geologic activity.
  • The other discipline school go behind the ideas of Richard Armstrong, an American/Canadian scientist, 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. Subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. Beliefs are also divided about the mechanism of continental crustal growth.

    • I. Those scientists who doubt that plate tectonics operated in the Archean argue that the felsic (term used in geology to refer to silicate minerals, magma, and rocks) proto continents formed at hotspots (are volcanic regions) rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapirs (a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks), a variety of mafic (a silicate mineral or rock) magmas produce intermediate and felsic rocks.
    • II. Others accept that granite formation in island arcs (composed of a chain of volcanoes) and convergent margins (an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another and collide) was part of the plate tectonic process, which has operated since at least the start of the Archean.


What happened to the rocks in the continental crust?

An elucidation for the general shortage of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or wiped out any isotopic record of their ancient times. 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" (a period of time approximately 4.1 to 3.8 billion years ago (Ga) 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.

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