Estimating Geological Age

Though geological age can be determined by the fossils found in the earth, that age is a relative one. How, then, can the absolute age of rocks be determined? Elements from which rocks are made have isotopes with the same atomic number but a different atomic mass. The unstable, radioactive isotopes among them change into stable isotopes at a constant rate while emitting radiation. The age of a given rock can be determined by measuring the ratio of radioactive element and its decay product.