Granite batholith radiometric dating
By the early 1960s, most of the major radiometric dating techniques now in use had been tested and their general limitations were known.No technique, of course, is ever completely perfected and refinement continues to this day, but for more than two decades radiometric dating methods have been used to measure reliably the ages of rocks, the Earth, meteorites, and, since 1969, the Moon.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 K-Ar clock works primarily on igneous rocks, i.e., those that form from a rock liquid (such as lava and granite) and have simple post-formation histories.It does not work well on sedimentary rocks because these rocks are composed of debris from older rocks.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.
Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology.
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.