Geologists analyze geologic time in two different ways: in terms of relative geologic age, and in terms of absolute (or numeric) geologic age.The combination of these two types of geologic ages makes a complete record of earth's geologic history in terms of the order of events and in terms of how many years ago each event occurred.Absolute age-dating tells how long ago an event occurred.
As minerals crystallise in igneous and metamorphic rocks they trap certain isotopes in their crystal structure that begin to decay radioactively as soon as the mineral forms.
These radioactive isotopes are parent isotopes, which decay slowly to daughter isotopes, changing the rock’s isotopic character.
The rate at which the isotopes decay is in effect our "geological clock".
By measuring the amount of the parent and daughter isotopes in a crystal, and then applying the decay rate, the actual age in years since the rock crystallized can be calculated.
Further confusion arises when one publication or geologist says, for example, that the Kaibab Formation is 270 m.y. The same questions arise for the other rock units at Grand Canyon. This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks.