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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.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.
Some of the isotopic parents, end-product daughters, and half-lives involved are listed in Table 1.
They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.
Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.
These are also the methods most commonly criticized by creation “scientists.” For additional information on these methods or on methods not covered here, the reader is referred to the books by Faul (47), Dalrymple and Lanphere (35), Doe (38), York and Farquhar (136), Faure and Powell (50), Faure (49), and Jager and Hunziker (70), as well as the article by Dalrymple (32).
The K-Ar method is probably the most widely used radiometric dating technique available to geologists.
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For example, a method based on a parent isotope with a very long half-life, such as C method can only be used to determine the ages of certain types of young organic material and is useless on old granites.