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Absolute Dating
Absolute dating assigns fixed dates to the age of an object, people or intangible concepts, such as human language development. Absolute dating largely relies on scientific developments of the 20th century, but it also can derive absolute dates from history and archaeology.
Radiometry
In radiometry, the rate of radioactive decay of a specific element provides an absolute date. This absolute dating method does not provide a date that is 100 percent accurate, but it provides the scientist a fixed date that must then be confirmed with corroborative testing and evidence. It is absolute in the sense that the scientific method of radiometry provides a specific measurement leading to the determination of specific ages.
Potassium-Argon Dating
Potassium-argon dating is another absolute dating method that is used to determine the age of igneous or sedimentary rocks. This, in turn, should provide some evidence for the dates of the fossils within the rocks. In this method, an absolute date is determined by measuring the amount of decay of potassium-40, a radioactive isotope of the element potassium that has transformed into the stable isotope argon-40, and interpolating that amount of potassium-40 by the half-life, which is the amount of time that it will take half of the amount of the element to transform into another more stable element and is estimated to be 1,265,000,000 years. For rocks over 1 million years old, however, accuracy is not reliable.
Relative Dating
With relative dating, the order in which certain events occurred -- age relationships -- is determined, but the exact age of each event is not necessarily determined. Geologists and archaeologists have used relative dating methods in determining order of events in the formation of the earth and of land and rock formations. Archaeology, which may be considered to be a subdiscipline of anthropology, uses the principle of relative dating in verifying the sequence and events involving human cultures. Relative dating methods involving rock strata are based on the law of superposition; that is, the oldest layers are found on the bottom of rock strata.
Fluorine Dating
Fluorine dating is a relative dating method based on the principle that fluorine is absorbed by fossil bones from the soil. This method requires many samples from one area and may provide a general idea of the age of the fossil, but it needs substantial corroboration from other sources to verify the accuracy of the date.