Dating by measurements
It is based on the radioactivity of Ar, however, is an inert gas that escapes easily from rocks when they are heated but is trapped within the crystal structures of many minerals after a rock cools. This correction can be made very accurately and has no appreciable effect on the calculated age unless the atmospheric argon is a very large proportion of the total argon in the analysis.
These parent isotopes decay to stable daughter isotopes at rates that can be measured experimentally and are effectively constant over time regardless of physical or chemical conditions.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.Second, the rock or mineral must not lose or gain either potassium or argon from the time of its formation to the time of analysis.By many experiments over the past three decades, geologists have learned which types of rocks and minerals meet these requirements and which do not.
James Hutton, a physician-farmer and one of the founders of the science of geology, wrote in 1788, “The result, therefore, of our present inquiry is, that we find no vestige of a beginning, — no prospect of an end.” Although this may now sound like an overstatement, it nicely expresses the tremendous intellectual leap required when geologic time was finally and forever severed from the artificial limits imposed by the length of the human lifetime.