The Archean Eon ( /ɑːrˈkən/ ar-KEE-ən, also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from 4,000 to 2,500 million years ago. In this time, the Earth's crust had cooled enough for continents to form and for the earliest known life to start. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen. The Archean was preceded by the Hadean Eon and followed by the Proterozoic.

Artist's impression of an Archean landscape.
Name formalityFormal
Alternate spelling(s)Archaean, Archæan
J.W. Dawson, 1865
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Chronological unitEon
Stratigraphic unitEonothem
Time span formalityFormal
Lower boundary definitionDefined Chronometrically
Lower boundary GSSPN/A
GSSP ratifiedN/A
Upper boundary definitionDefined Chronometrically
Upper boundary GSSPN/A
GSSP ratifiedN/A

Etymology and changes in classificationEdit

The word Archean comes from the Greek word arkhē (αρχή), meaning 'beginning, origin.'[1] It was first used in 1872, when it meant 'of the earliest geological age.'[a] 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.[citation needed]

Instead of being based on stratigraphy, 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.[3]


The evolution of Earth's radiogenic heat flow over time

When the Archean began, the Earth's heat flow 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 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, from the formation of the metallic core, and from the decay of radioactive elements.

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, Siberia, the Canadian Shield, Montana and Wyoming (exposed parts of the Wyoming Craton), the Baltic Shield, the Rhodope Massif, Scotland, India, Brazil, western Australia, and southern Africa.[citation needed] 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, anorthosites and monzonites known as sanukitoids. Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite.[4] Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.[5] Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks. The metamorphosed igneous rocks were derived from volcanic island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents.[6]: 302–303 

The Earth's continents started to form in the Archean, although details about their formation are still being debated, due to lack of extensive geological evidence. 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.[7] A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma.[8] Although the first continents formed during this eon, rock of this age makes up only 7% of the present world's cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40% of the present area of continents formed during the Archean.[6]: 301–302 

By the end of the Archean around 2,500 Ma, plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert 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 layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.[9]


The pale orange dot, an artist's impression of the early Earth which is believed to have appeared orange through its hazy methane rich prebiotic second atmosphere. Earth's atmosphere at this stage was somewhat comparable to today's atmosphere of Titan.[10]

The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the Sun had about 70–75 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 presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history.[11][12] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.[13]

Early lifeEdit

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 of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland.[14]

Lithified stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first direct fossil traces of life on Earth.

The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.[15][16] Stromatolites are found throughout the Archean[17] and become common late in the Archean.[6]: 307  Cyanobacteria were instrumental in creating free oxygen in the atmosphere.[18]

Further evidence for early life is found in 3.47 billion-year-old baryte, in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%,[19] which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34.[20]

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.[21][22]

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.[23]

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during the Archean without leaving any.[6]: 306, 323  Fossil steranes, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter.[24] No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.[25]

See alsoEdit


  1. ^ The name Archean was coined by American geologist James Dwight Dana (1813–1895).[2] The Pre-Cambrian eon had been believed to be without life (azoic); however, because fossils had been found in deposits that had been judged to belong to the Azoic age, "... I propose to use for the Azoic era and its rocks the general term Archæn (or Arche'an), from the Greek άρχαιος, pertaining to the beginning."[2]: 253 


  1. ^ Harper, Douglas. "Archaean". Online Etymology Dictionary.
  2. ^ a b Dana JD (1872). "Green Mountain geology. On the quartzite". American Journal of Science and Arts. 3rd series. 3 (16): 250–257.
  3. ^ "International Chronostratigraphic Chart v.2013/01" (PDF). International Commission on Stratigraphy. January 2013. Retrieved 6 April 2013.
  4. ^ Dostal J (2008). "Igneous Rock Associations 10. Komatiites". Geoscience Canada. 35 (1).
  5. ^ Cooper JD, Miller RH, Patterson J (1986). A Trip Through Time: Principles of historical geology. Columbus: Merrill Publishing Company. p. 180. ISBN 978-0675201407.
  6. ^ a b c d Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. ISBN 978-0716728825.
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  10. ^ Trainer, Melissa G.; Pavlov, Alexander A.; DeWitt, H. Langley; Jimenez, Jose L.; McKay, Christopher P.; Toon, Owen B.; Tolbert, Margaret A. (28 November 2006). "Organic haze on Titan and the early Earth". Proceedings of the National Academy of Sciences. Proceedings of the National Academy of Sciences. 103 (48): 18035–18042. doi:10.1073/pnas.0608561103. ISSN 0027-8424.
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  15. ^ Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013.
  16. ^ Noffke N, Christian D, Wacey D, Hazen RM (December 2013). "Microbially induced sedimentary structures recording an ancient ecosystem in the ca. 3.48 billion-year-old Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–1124. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812.
  17. ^ Garwood, Russell J. (2012). "Patterns In Palaeontology: The first 3 billion years of evolution". Palaeontology Online. 2 (11): 1–14. Retrieved 25 June 2015.
  18. ^ "Early life: Oxygen enters the atmosphere". BBC. Retrieved 20 September 2012.
  19. ^ Shen Y, Buick R, Canfield DE (March 2001). "Isotopic evidence for microbial sulphate reduction in the early Archaean era". Nature. 410 (6824): 77–81. Bibcode:2001Natur.410...77S. doi:10.1038/35065071. PMID 11242044. S2CID 25375808.
  20. ^ Seal RR (2006). "Sulfur isotope geochemistry of sulfide minerals". Reviews in Mineralogy and Geochemistry. 61 (1): 633–677. Bibcode:2006RvMG...61..633S. doi:10.2138/rmg.2006.61.12.
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External linksEdit

  • "Archean". GeoWhen Database. Archived from the original on 22 August 2010. Retrieved 17 September 2010.
  • "When did plate tectonics begin?". University of Texas – Dallas.
  • "Archean (chronostratigraphy scale)".