Erathem

	Eras mapped into Eons
	Eras in the Phanerozoic Eon
	Eras in the Proterozoic Eon; 542.0 (+/- 1.0) Mya – 2500 Mya
	Eras in the Archean Eon; 2500 Mya – years > 3600 Mya ; rocks older than 2.5 Billion years – rocks older than 3.6 Billion years
	Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the Earth's crust.

Short description: Total stratigraphic unit deposited during a certain span of time in an era in the geologic timescal

In stratigraphy, paleontology, geology, and geobiology, an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years – less than a geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). By 3,500 million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure).^[3] The atmosphere was a mix of noxious and poisonous gases (methane, ammonia, sulfur compounds, etc.^[3] – a so-called reducing atmosphere^[4] lacking much free oxygen which was bound up in compounds).

These simple organisms, cyanobacteria ruled the still cooling earth for approximately a billion years^[3] and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines.

Erathems are not often used in practice. While they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata.

Erathems have the same names as their corresponding eras. The Phanerozoic eonothem can thus be divided into Cenozoic, Mesozoic, and Paleozoic erathems. Similarly, the Proterozoic eonothem is divided youngest to oldest into the Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, and the Archean eon and eonothem are divided similarly into the Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.^[1]^[2]

References

Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne, Guy (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138 (2): 213–218. doi:10.1017/S001675680100509X. 1. Bibcode: 2001GeoM..138..213G. http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669.
Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976 ^{[ISBN missing]}
International Stratigraphic Chart from the International Commission on Stratigraphy
US National Park Service description of geographic time
Washington State University description of geographic time
Web Geological Time Machine
Eon or Aeon, Math Words – An alphabetical index

External links

The Global Boundary Stratotype Section and Point (GSSP): overview
Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
Geotime chart displaying geologic time periods compared to the fossil record – Deals with chronology and classifications for laymen (not GSSPs)

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Original source: https://en.wikipedia.org/wiki/Erathem. Read more

[ICS_Chart-1] 1.0 ^1.1 ^1.2 ^1.3 International Commission on Stratigraphy, by Gabi Ogg. "International Stratigraphic Chart". Archived from the original on 2008-05-29. https://web.archive.org/web/20080529054437/http://www.stratigraphy.org/chus.pdf. Retrieved 2008-06-17.

[listed_ages-2] 2.0 ^2.1 ^2.2 ^2.3 F.M. Gradstein, J.G. Ogg, A.G. Smith, et al., "A Geologic Time Scale", (2004; Cambridge University Press).

[Rockmans-3] 3.0 ^3.1 ^3.2 ^3.3 "Rockman's Geologic Time Chart". http://www.rocksandminerals.com/geotime/geotime.htm. Retrieved 2008-06-17.

[4] sis for the Miller–Urey experiment

[5] Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart, International Commission on Stratigraphy, http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf .

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Eras mapped into Eons
Eras in the Phanerozoic Eon^[1]^[2]
Geologic Era	Span of Years	Notes:
Cenozoic	present – 065.5 (+/- 0.3) Mya	many GSSP points
Mesozoic	065.5 (+/- 0.3) Mya – 251.0 (+/- 0.4) Mya	many GSSP points
Paleozoic	251.0 (+/- 0.4) Mya – 542.0 (+/- 1.0) Mya	mostly GSSP points
Eras in the Proterozoic Eon 542.0 (+/- 1.0) Mya – 2500 Mya^[1]^[2]
Neoproterozoic	542.0 (+/- 1.0) Mya – 1000 Mya	few GSSP points
Mesoproterozoic	1000 Mya – 1600 Mya	all GSSA points
Paleoproterozoic	1600 Mya – 2500 Mya	all GSSA points
Eras in the Archean Eon 2500 Mya – years > 3600 Mya rocks older than 2.5 Billion years – rocks older than 3.6 Billion years^[1]^[2]
Neoarchean	2500 Mya – 2800 Mya	(only GSSA points)
Mesoarchean	2800 Mya – 3200 Mya
Paleoarchean	3200 Mya – 3600 Mya
Eoarchean	3600 Mya – 4000 Mya	Earth's crust solidifies ca 3800 Mya^[3]
Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the Earth's crust.

Segments of rock (strata) in chronostratigraphy	Time spans in geochronology	Notes to geochronological units
Eonothem	Eon	4 total, half a billion years or more
Erathem	Era	10 defined, several hundred million years
System	Period	22 defined, tens to ~one hundred million years
Series	Epoch	34 defined, tens of millions of years
Stage	Age	99 defined, millions of years
Chronozone	Chron	subdivision of an age, not used by the ICS timescale

Erathem

Topic: Earth

See also

Multidiscipline comparison

Related other topics

Notes

References

External links