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In general usage, an eon (sometimes spelled æon) is an arbitrarily designated period of time. Geologists and other scientific disciplines interested in earth history refer to an eon as the largest subdivision of time on the geologic time scale. For example, the Phanerozoic Eon, which is about 550 million years long, covers the period of time during which animals with hard shells that fossilize would have been abundant.
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The Eonothem is a corresponding system of classification of stratigraphy, a subdiscipline of geology, the multi-discipline analysis and classification of the rock layers laid down over the time passing during the specific eon. Eon's and Eonthems usually have the same names as established by the International Commission on Stratigraphy (ICS), then ratified by the parent organization the International Union of Geological Sciences (IUGS).
An eon is composed of several geologic eras, which in turn are composed of geologic periods, which are composed of geologic epochs, and so forth. The finest subdivisions in the system are geologic stages (Faunal stages), from which experts can give the most accurate dating estimates.
Terminology
We are currently in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene epoch. Formerly, only one eon existed besides the Phanerozoic: the Precambrian. More recently, the Hadean, Archean, and Proterozoic "eras" of Precambrian time have been considered eons by the parent body of the ICS, the International Union of Geological Sciences (IUGS). The geologic timescale in terms of eons, eras, and periods looks like this:
GSSA/GSSA calibration points
The International Commission on Stratigraphy has been since 1974 and is currently undertaking a world wide series of baseline comparisons and calibrations establishing basal Global Boundary Stratotype Section and Point (GSSP), a sort of benchmark in the rock layers calibrated region to region world wide. Whereas the ICS states that for Precambrian era and before (always earlier than 630 Mya, and many others before 542 Mya due to the spotty fossil record), similar reference points called Global Standard Stratigraphic Ages (GSSA), each used also in defining subdivisions are established by absolute age of the strata and related classification criteria. Global Standard and Selection GSSP and GSSA points will aid many disciplines going forward, and the calibration such data references enables has already benefited research dependent upon lithographic dating.
Greek root
The English word eon is derived from the Koine Greek word aion.
Although a proposal was made in 1957 to define an aeon to be a unit of time equal to one billion years (1 Ga), the idea was not approved as a unit of scientific measure and is seldom used for a specific period of time. Its more common usage is for any lengthy or indefinite period of time. The origin is from the Greek root "aion" for "age" or "life force." A similar Latin word "aevum" for age is still present in words such as longevity and medieval.[1]
See also
Multidiscipline comparison
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strata units of the geologic record |
units of the geological time system |
Main article: geologic time scale |
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4 total, largest spans of years |
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12 total, roughly span multiples of a hundred million years |
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Some familiar names, defined in 21 major divisions and 2 minor |
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48 units & each spanning tens of millions of years |
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100+ units & most all spanning millions of years |
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if the strata has good dating characteristics. |
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| Chronozone | Chron | only in more recent strata* |
| * Most and the most time specific time divisions[2] | ||
Related other topics
- Body form
- European Mammal Neogene
- Geologic time scale
- North American Land Mammal Age
- Fauna (animals)
- Type locality
- List of GSSPs
- List of GSSAs
External links
- The Global Boundary Stratotype Section and Point (GSSP): overview
- Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
- [ http://www.rocksandminerals.com/geotime/geotime.htm Geotime chart ] displaying geologic time periods compared to the fossil record.
Notes and references
References
- 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
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