( mya )
Duration: 300 Ma
Duration: 400 Ma
Duration: 400 Ma
Duration: 400 Ma
The Archean (also Archäikum) is an eon of geological time scale . It extends from around 4000 to around 2500 million years ago. Outdated (partial) synonyms for this geological age are Azoic and Archaeozoic .
The term Archean or rather its English counterpart Archean and the partially used today term Archaeozoic ( Archeozoic ) were American US by the geologist in 1872. James Dwight Dana coined. In the original Danaian sense, the Archaic and Archaeozoic Era referred to the entire geological period before the beginning of the Paleozoic Era, so they were synonymous with the term Precambrian, which is commonly used today . Dana had discovered that there are also indications in pre-Paleozoic sedimentary rocks that life already existed on Earth at this time. He therefore created the terms Archaeozoic (“time of the first life”) or Archaic to replace the term Azoic (“time without life”) introduced in 1845 for the pre-Paleozoic period, with the restriction that only those archaic rocks belong to the Archeozoic Era that can be shown to come from a period in which there was life:
"Whatever part of the Archæan beds are proved to belong to an era in which there was life, will be appropriately styled the Archeozoic ."
It was not until more than 50 years later that the distinction between the Archaic and Proterozoic Era was first made within the Precambrian, whereby the Archaic Era was again assigned the meaning of the “Azoic” era and the Proterozoic Era was identical to the Archaeozoic Era in the original Danaian sense. In the second half of the 20th century, the Hadaikum was finally separated as the period in early geological history for which there is no geological evidence.
- Eon : Phanerozoikum (541-0 mya )
- Eon : Proterozoic (2500-541 mya)
- Eon: Archean (4000-2500 mya)
- Eon: Hadean (4600-4000 mya)
Geological evidence and implications for the biosphere and atmosphere
The Isua gneiss from the Nuuk area in Greenland is dated to the beginning of the Archean . Only since 1999 are rocks known that were formed before this eon , i.e. in the Hadean. The oldest definitely dated rock to date is the Acasta gneiss with 4030 mya from the western Canadian shield . In 2008, further dates from the Hadean became known, which come from the Nuvvuagittuq greenstone belt in the eastern Canadian shield. The first - hypothetical - major continent Ur could have formed 3 billion years ago from these first hadaic mainland islands ( cratons ).
Some of the largest hitherto recorded impacts from meteorites occurred within the Archean. In 2014, for example, spherules ( impact glasses ) were found in the Barberton greenstone belt (South Africa) , which suggest the impact of an impactor with a diameter of 37 to 58 km, which hit our planet around three billion years ago and had far-reaching consequences for the ecosphere at that time should.
Although it was assumed for a long time that no fossils have survived from the Archean , it has been found that evolution processes - initially the chemical evolution from 3,800 mya - began within a very short time after the formation of stable crustal components : Macromolecules were able to get through To enlarge the attachment of other molecules and to reproduce itself . In the early Archean, this point in time can be taken as the beginning of life on earth.
The oldest chemofossils found so far, i.e. fossil traces of living beings , are microscopic 'threads' in rocks, which, first discovered in South Africa , could be considered the remains of 3.5 billion year old cyanobacteria - or blue-green algae . This is followed later by clearly biogenic fossil remains in the form of stromatolites , which originate from the Archean.
Naturally, the amount of findable fossils and rocks from this eon is minimal, since most of the land masses from this time that could bear such traces have completely eroded into fine sand , metamorphosed , became the basis of sedimentary rocks or melted in the earth's mantle . Only in the oldest cratons that have not changed since their formation - the archons - is there a chance of making such discoveries by means of deep drilling in several thousand meters. Such archons are, for example, the Kola Peninsula, Zimbabwe , the Canadian Shield , parts of China or Western Australia .
The atmosphere in the early Archean did not yet contain free oxygen . The photosynthesis of the first prokaryotes initially oxidized the minerals of the primeval ocean and only towards the end of the Archean - 2500 mya - was oxygen released into the atmosphere.
Large parts of the subcontinental lithospheric mantle that formed at that time seem to be in place today. This fact is surprising in two respects, on the one hand, because the rock layers that are formed today are too dense to remain directly under the continents and sink further into the earth's mantle, and on the other hand, because they have been in this position for so long have held. These parts of the lithospheric mantle probably originated on the archaic mid-ocean ridges or by mantle diapirs, which were significantly hotter than today and therefore have a mineralogy richer in forsterite (i.e. magnesium). However, these rigid rock strata cannot - as is the case today - be subducted and were therefore simply pushed under the existing lithosphere.
- Wolfgang Oschmann: Evolution of the Earth. History of the earth and life. Haupt Verlag, Bern 2016, ISBN 978-3-8252-4401-9 (UTB; 4401), pp. 53-66 (introduction).
- Emergence of an aerobic biosphere during the Archean-Proterozoic transition: Challenges of future research . By VA Melezhik et al. In: GSA Today , Volume 15, Issue 11, November 2005.
- International Stratigraphic Chart 2018 , International Commission on Stratigraphy
- James D. Dana: Corals and coral islands. Dodd & Mead Publishers, New York 1872, p. 373 ( HathiTrust ).
- James D. Dana: Green Mountain Geology. On the quartzite. American Journal of Science and Arts. 3rd series, 3rd volume, numbers 13-18, 1872, pp. 250-256 ( BHL ).
- Roderick Impey Murchison, Edouard de Verneuil, Alexander von Keyserling: The geology of Russia in Europe and the Ural Mountains. Vol. 1. John Murray [u. a.], London [u. a.] 1845, p. 10 * ( HathiTrust )
- Wilmarth M. Grace: The geologic time classification of the United States Geological survey compared with other classifications. Government Printing Office, Washington DC 1925, p. 127 ( HathiTrust )
- Primeval impact: boiling seas, liquid rock. Retrieved March 25, 2017 .
- Coupled evolution of Archean continental crust and subcontinental lithospheric mantle Hugh Rollinson *