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.
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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.For more information on geochronology, visit Britannica.com.
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.
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.
For instance, with reference to the Geologic time scale, the Upper Permian (Lopingian) lasted from 270.6 +/- 0.7 Ma 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.
By measuring the amount of radiocative 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.
Other radiogenic dating techniques include:
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.
The geochronologic and chronostratigraphic units can be mixed up[2].
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