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The determination of the age of rocks and minerals, and hence of the fossils they contain, by measurement of the levels of certain radioactive elements. Two common methods are potassium-argon (K-Ar) dating and rubidium-strontium (Rb-Sr) dating. The technique employs the fact that radioactive elements decay to other stable elements at a constant rate. Hence by measuring the ratio of the stable daughter element to the radioactive parent element the age of the rock may be determined. The decay of potassium-40 to argon-40 has a half-life of 11.8 x 109 years, while that of rubidium-87 to strontium-87 is 48.8 x 109 years. Both techniques could theoretically be used on even the oldest of the earth's rocks since these are only some 4.6 x 109 years old. Such techniques have enabled geologists to construct an absolute geological time scale.
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