dating reliable? Because the rates of decay for many isotopes have been precisely measured and do not vary under the physical con ditions that exist in Earth's outer layers. Therefore, each radioac- tive isotope used for dating has been decaying at a fixed rate since the formation of the rocks in which it occurs, and the products of decay have been accumulating at a corresponding rate. For exam ple, when uranium is incorporated into a mineral that crystallizes from magma, there is no lead(the stable daughter product from previous decay. The radiometric"clock" starts at this point. As the uranium in this newly formed mineral disintegrates, atoms of the daughter product are trapped, and measurable amounts of lead eventually accumulate