Geology/Chronology

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This is a geochronolgy time spiral. Credit: United States Geological Survey.

Geochronology is the science of applying dates in the past to rocks. These rocks receive dates because they contain constituents that can be used as chronometers.

Geologic time[edit | edit source]

This clock representation shows some of the major units of geological time and definitive events of Earth history. Credit: Woudloper.

On the right is a geologic clock representation. It shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).[1] Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.

Units in geochronology and stratigraphy[2]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Supereonothem Supereon 1 total, four billion years or more (e.g. Precambrian)
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

Notations[edit | edit source]

Let

  1. ALMA represent the Asian Land Mammal Age,
  2. b2k represent before AD 2000,
  3. BP represent before present, as the chart is for 2008, this may require an added -8 for b2k,
  4. ELMMZ represent the European Land Mammal Mega Zone,
  5. FAD represent first appearance datum,
  6. FO represent first occurrence,
  7. Ga represent Gegaannum, billion years ago, or -109 b2k,
  8. GICC05 represent Greenland Ice Core Chronology 2005,
  9. GRIP represent Greenland Ice Core Project,
  10. GSSP represent Global Stratotype Section and Point,
  11. HO represent highest occurrence,
  12. ICS represent the International Commission on Stratigraphy,
  13. IUGS represent the International Union of Geological Sciences,
  14. LAD represent last appearance datum,
  15. LO represent lowest occurrence,
  16. Ma represent Megaannum, million years ago, or -106 b2k,
  17. NALMA represent the North American Land Mammal Age,
  18. NGRIP represent North Greenland Ice Core Project, and
  19. SALMA represent South American Land Mammal Age.

"The term b2 k [b2k] refers to the ice-core zero age of AD 2000; note that this is 50 years different from the zero yr for radiocarbon, which is AD 1950 [...]."[3]

Stratigraphy[edit | edit source]

Main articles: Geochronology/Stratigraphy and Stratigraphy

Orbitally forced cyclicity[edit | edit source]

Main article: Geochronology/Orbitally forced cyclicity

Argon–argon dating[edit | edit source]

Main article: Geochronology/Argon–argon dating

Cathodoluminescences[edit | edit source]

Main article: Geochronology/Cathodoluminescences

Chemostratigraphy[edit | edit source]

Main article: Geochronology/Chemostratigraphy

Cosmogenic radionuclide dating[edit | edit source]

Main article: Geochronology/Cosmogenic radionuclide dating

Dendrochronology[edit | edit source]

Main article: Geochronology/Dendrochronology

Electron spin resonances[edit | edit source]

Fission track dating[edit | edit source]

Geomagnetic Polarity Time Scale[edit | edit source]

Main article: Geochronology/Geomagnetic Polarity Time Scale

Ice cores[edit | edit source]

Main article: Geochronology/Ice cores

Lichenometry[edit | edit source]

Magnetostratigraphy[edit | edit source]

Marker horizons[edit | edit source]

Main article: Geochronology/Marker horizons

Optically stimulated luminescences[edit | edit source]

Main article: Geochronology/Optically stimulated luminescence

Paleomagnetic dating[edit | edit source]

Palynology[edit | edit source]

Main article: Geochronology/Palynology

Potassium–argon dating[edit | edit source]

Main article: Geochronology/Potassium–argon dating

Radiocarbon dating[edit | edit source]

Main article: Geochronology/Radiocarbon dating

Tephrochronology[edit | edit source]

Main article: Geochronology/Tephra layers

Thermoluminescences[edit | edit source]

Main article: Geochronology/Thermoluminescences

Uranium–lead dating[edit | edit source]

Main article: Geochronology/Uranium-lead dating

Uranium-thorium dating[edit | edit source]

Main article: Geochronology/Uranium-thorium dating

Varves[edit | edit source]

Phanerozoic[edit | edit source]

The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic.

Cenozoic[edit | edit source]

Main article: Geochronology/Cenozoic

Mesozoic[edit | edit source]

Main article: Geochronology/Mesozoic

Paleozoic[edit | edit source]

Main article: Geochronology/Paleozoic

Precambrian[edit | edit source]

Main article: Geochronology/Precambrian

Hypotheses[edit | edit source]

Main article: Hypotheses
  1. Each time frame or span of time in geochronology has at least one dating technique.
  2. Late Jurassic is a time frame and Upper Jurassic is a stratigraphic frame so both are different time frames.
  3. The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).

See also[edit | edit source]

References[edit | edit source]

  1. ↑ stratigraphy.org. International Commission on Stratigraphy 2008. http://www.stratigraphy.org/column.php?id=Chart/Time%20Scale. Retrieved 9 March 2009. 
  2. ↑ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015). "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy.
  3. ↑ Mike Walker, Sigfus Johnsen, Sune Olander Rasmussen, Trevor Popp, Jørgen-Peder Steffensen, Phil Gibbard, Wim Hoek, John Lowe, John Andrews, Svante Björck, Les C. Cwynar, Konrad Hughen, Peter Kershaw, Bernd Kromer, Thomas Litt, David J. Lowe, Takeshi Nakagawa, Rewi Newnham and Jakob Schwander (2009). "Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records". Journal of Quaternary Science 24 (1): 3-17. doi:10.1002/jqs.1227. http://www.stratigraphy.org/GSSP/Holocene.pdf. Retrieved 2015-01-18. 

External links[edit | edit source]

{{Archaeology resources}}

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