Some of the methods have internal checks, so that the data themselves provide good evidence of reliability or lack thereof.
Commonly, a radiometric age is checked by other evidence, such as the relative order of rock units as observed in the field, age measurements based on other decay schemes, or ages on several samples from the same rock unit.
The main point is that the ages of rock formations are rarely based on a single, isolated age measurement.
On the contrary, radiometric ages are verified whenever possible and practical, and are evaluated by considering other relevant data.
There are a number of long-lived radioactive isotopes used in radiometric dating, and a variety of ways they are used to determine the ages of rocks, minerals, and organic materials.
Some of the isotopic parents, end-product daughters, and half-lives involved are listed in Table 1.
The geochronologist takes this factor into account when assigning experimental errors to the calculated ages. First, there must be no argon other than that of atmospheric composition trapped in the rock or mineral when it forms.Lord Kelvin and Clarence King calculated the length of time required for the Earth to cool from a white-hot liquid state; they eventually settled on 24 million years.James Joly calculated that the Earth’s age was 89 million years on the basis of the time required for salt to accumulate in the oceans.My purpose here is not to review and discuss all of the dating methods in use.Instead, I describe briefly only the three principal methods. These are the three methods most commonly used by scientists to determine the ages of rocks because they have the broadest range of applicability and are highly reliable when properly used.The point is that not all methods are applicable to all rocks of all ages.One of the primary functions of the dating specialist (sometimes called a geochronologist) is to select the applicable method for the particular problem to be solved, and to design the experiment in such a way that there will be checks on the reliability of the results.Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.Sometimes these decay schemes are used individually to determine an age (e.g., Rb-Sr) and sometimes in combinations (e.g., U-Th-Pb).Each of the various decay schemes and dating methods has unique characteristics that make it applicable to particular geologic situations.