The chronology of the earth's history as determined by geologic events.
geochronologic ge'o·chron'o·log'ic (-krŏn'ə-lŏj'ĭk) or ge'o·chron'o·log'i·cal adj.geochronologically ge'o·chron'o·log'i·cal·ly adv.
geochronologist ge'o·chro·nol'o·gist n.
Dictionary:
ge·o·chro·nol·o·gy (jē'ō-krə-nŏl'ə-jē) ![]() |
The chronology of the earth's history as determined by geologic events.
geochronologic ge'o·chron'o·log'ic (-krŏn'ə-lŏj'ĭk) or ge'o·chron'o·log'i·cal adj.| 5min Related Video: geochronology |
| Britannica Concise Encyclopedia: geochronology |
For more information on geochronology, visit Britannica.com.
| Wikipedia: Geochronology |
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| Segments of rock (strata) in chronostratigraphy | Periods of time in geochronology | Notes |
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4 total, half a billion years or more |
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12 total, several hundred million years |
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tens of millions of years |
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millions of years |
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smaller than an age/stage, not used by the ICS timescale |
In the natural sciences under the umbrella of natural history, Geochronology is the science of determining the absolute age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent within the method used. A variety of dating methods are used by geologists to achieve this. The interdisciplinary approach of using several methods can often achieve best results. [2]
Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, merely places it within an interval of time at which that fossil assemblage is known to have coexisted. Both disciplines work together hand in hand however, to the point they share the same system of naming rock layers and the time spans utilized to classify layers within a strata. (See table at right for terminology.)
For instance, with reference to the Geologic time scale, the Upper Permian (Lopingian) lasted from 270.6 +/- 0.7 Ma (Ma = millions of years ago) until somewhere between 250.1 +/- 0.4 Ma (oldest known Triassic) and 260.4 +/- 0.7 Ma (youngest known Lopingian) - a gap in known, dated fossil assemblages of nearly 10 Ma. While the biostratigraphic age of an Upper Permian bed may be shown to be Lopingian, the true date of the bed could be anywhere from 270 to 251 Ma.
On the other hand, a granite which is dated at 259.5 +/- 0.5 Ma can reasonably safely be called "Permian", or most properly, to have intruded in the Permian.
The science of geochronology is the prime tool used in the discipline of chronostratigraphy, which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies.
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By measuring the amount of Radioactive decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the radiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope.[3][4][5] Two or more radiometric methods can be used in concert to achieve more robust results.[6] Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the 40Ar/39Ar dating method can be extended into the time of early human life [7] and into recorded history.[8]
Some of the commonly-used techniques are:
Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, including optically stimulated luminescence (OSL), cathodoluminescence (CL), and thermoluminescence (TL). Thermoluminescence and optically stimulated luminescence are used in archaeology to date 'fired' objects such as pottery or cooking stones, and can be used to observe sand migration.
Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be fixed (i.e. linked to the present day and thus calendar or sidereal time) or floating.
Marker horizons are geological units in different geographic locations but which are of the same age. This allow age-equivalence to be established between different sites. [10] For example, tephra is often used in archaeology.
It is important not to confuse geochronologic and chronostratigraphic units. [11] Geochronological units are periods of time, thus it is correct to say that Tyrannosaurus rex lived during the the Late Cretaceous Epoch. [12] Chronostratigraphic units are geological material, so it is also correct to say that fossils of the genus Tyrannosaurus have been found in the Upper Cretaceous Series. [13] In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit - such as the Egyptian mangrove deposit where the Tyrannosaurus fossils were found - but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time.
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| orthochronology (geology) | |
| parachronology (geology) | |
| relative chronology (geology) |
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| How did geochronology begin? | |
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![]() | Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more | |
![]() | Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Geochronology". Read more |
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