Geologists use a variety of techniques to date rocks.
Geologists are able to determine the age of rocks using a variety of techniques. Some are only able to provide a relative age for a rock, while others are able to determine the absolute age of a rock. Using techniques that provide an absolute age, geologists have been able to conclude that the oldest rocks on earth are four billion years old.
Relative Dating
Relative dating places rocks in a "chronologic sequence or order of occurrence" to date them. The rocks are dated in relation to other events or objects. According to the University of Alabama's department of geological sciences, the principle of superposition states that "... the youngest rocks are at the top, and the oldest are at the bottom."
"Faunal succession," the relation of rocks to the fossil record, "crosscutting," the idea that an intrusion or fault in a rock is younger than the rock itself, and "inclusions,"the premise that fragments are older than the rocks they are in, are all techniques used to determine the relative age of a rock.
Radiocarbon Dating
The radiocarbon method of rock dating was developed in the 1940s at the University of Chicago by a team of scientists led by William F. Libby. Radiocarbon dating is a technique used to date "young" rocks, less than 50,000 years old. It is based upon the rate of radioactive decay of the isotope Carbon-14.
Radiometric Dating
Radiometric dating relies on the use of isotope series. Because the isotope series used have relatively long half-lives, it is possible to look at the "subtle difference in the relative proportions of the two isotopes" to date rocks of any age, according to actionbioscience.org. Radiometric dating has been used to determine the ages of the earth and moon, meteorites and fossils and many other events, according to the United States Geological Survey.
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