Geochronometry
Geochronometry is a branch of stratigraphy aimed at the quantitative measurement of geologic time. It is considered a branch of geochronology.
Brief history
The measurement of geologic
Radiometric dating
All methods based on the radioactive decay belong to this category. The principle at the base of radiometric dating is that natural unstable isotopes, called 'parent isotopes', decay to some isotope which is instead stable, called the 'daughter isotope'.
Under the assumptions that:
(1) the initial amount of parent and daughter isotopes can be estimated, and
(2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of radioactive decay. Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are:
Each of these methods perform better in different time ranges and has different limitations. However, uranium–lead dating on
Other methods of radiometric dating are also available, that are based on slightly or largely different principles, but always rely on the phenomenon of radioactive decay. These alternative radiometric methods are:
- 14C, or radiocarbon
- Fission track dating
- Optical luminescence dating and Thermoluminescence dating
- Cosmic ray exposure dating
These methods, especially
Incremental dating
These methods are based on the building of incremental chronologies from a point of known age, which is usually the present. When a chronology is not tied to such a known age point, it is called a floating chronology. Incremental dating methods include:
- Dendrochronology
- Lichenometry
- mollusc shells or coralskeletons
- ice cores)
- Lamina counting in speleothems
- Milankovitchcycles
Geologic time scale
A major achievement of geochronometry is the documentation of geologic
See also
References
- Hallam, Anthony, 1983 – Great geological controversies. Oxford University Press, Oxford, UK.
- ^ Boltwood B.B., 1907, On the ultimate disintegration products of the radioactive elements. Part II. The disintegration products of uranium. American Journal of Science, v. 23, p. 77-88.
- ^ Strutt R.J., 1909, The accumulation of helium in geologic time III. Proceedings of the Royal Society of London A, v. 83, p. 298-301.
- ^ Hanchar J.M, Hoskin P.W.O. (editors), 2003, Zircon. Reviews in Mineralogy and Geochemistry, v. 53, Mineralogical Society of America. ISSN 1529-6466
- ^ a b c Erwin D.H., 2006, Dates and Rates: Temporal resolution in the deep time stratigraphic record. Annual Review of Earth and Planetary Sciences, v. 34, p. 569-590.
- ^ Gradstein F.M., Ogg J.G. and Smith A.G., 2004, A Geologic Time Scale 2004, Cambridge University Press, Cambridge, UK.
External links
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