

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 potassiumargon (KAr) dating and rubidiumstrontium (RbSr) 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 potassium40 to argon40 has a halflife of 11.8 x 10^{9} years, while that of rubidium87 to strontium87 is 48.8 x 10^{9} years. Both techniques could theoretically be used on even the oldest of the earth's rocks since these are only some 4.6 x 10^{9} years old. Such techniques have enabled geologists to construct an absolute geological time scale.




