|Eras mapped into Eons|
|Eras in the Phanerozoic Eon|
|Geologic Era||Span of Years||Notes:|
||many GSSP points|
||many GSSP points|
||mostly GSSP points|
|Eras in the Proterozoic Eon
542.0 (+/- 1.0) Mya — 2500 Mya
||few GSSP points|
||all GSSA points|
||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
||(only GSSA points)|
||Earth's crust solidifies
ca 3800 Mya
|Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the earths crust.|
It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an 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. The atmosphere was a mix of noxious and poisonous gases (Methane, Ammonia, Sulphur compounds, etc.— a so-called reducing atmosphere lacking much free oxygen which was bound up in compounds).
These simple organisms, Cyanobacteria ruled the still cooling earth for approximately a thousand million (over a billion) years 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 a
Similarly, the Proterozoic eonothem is divided youngest to oldest into the
- and the Archean eon and eonothem are divided similarly into the
|<span style="float:right;"><span class="plainlinksneverexpand">e  h</span></span><center><big>Units in geochronology and stratigraphy</big><ref>Cohen, K.M., Finney, S., Gibbard, P.L. (2013), International Chronostratigraphic Chart, International Commission on Stratigraphy.</ref></center>|
|Segments of rock (strata) in chronostratigraphy||Time spans in geochronology||Notes to<br/>geochronological units|
|<center>Eonothem</center>||<center>Eon</center>||4 total, half a billion years or more|
|<center>Erathem</center>||<center>Era</center>||10 total, several hundred million years|
|<center>Series</center>||<center>Epoch</center>||tens of millions of years|
|<center>Stage</center>||<center>Age</center>||millions of years|
|<center>Chronozone</center>||<center>Chron</center>||subdivision of an age, not used by the ICS timescale|
Related other topics
- Body form
- European Mammal Neogene
- Geologic time scale
- North American Land Mammal Age
- Fauna (animals)
- Type locality
- List of GSSPs
- 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)
Notes and references
- International Commission on Stratigraphy, by Gabi Ogg. "International Stratigraphic Chart" (PDF). Archived from the original on 2008-05-29. Retrieved 2008-06-17.
- F.M. Gradstein, J.G. Ogg, A.G. Smith, et al., "A Geologic Time Scale 2004", (2004; Cambridge University Press).
- "Rockman's Geologic Time Chart". Retrieved 2008-06-17.
- basis for the Miller-Urey experiment
- 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.
- Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976
- International Stratigraphic Chart from the International Commission on Stratigraphy
- USA National Park Service
- Washington State University
- Web Geological Time Machine
- Eon or Aeon, Math Words - An alphabetical index
|This geochronology article is a stub. You can help Wikipedia by expanding it.|