Geological Glossary

Half Life and Radiometric dating

Spontaneous breakdown or decay of atomic nuclei, termed radioactive decay, is the basis for all radiometric dating methods. Radioactivity was discovered in 1896 by French physicist Henri Becquerel. By 1907 study of the decay products of uranium (lead and intermediate radioactive elements that decay to lead) demonstrated to B. B. Boltwood that the lead/uranium ratio in uranium minerals increased with geologic age and might provide a geological dating tool.

As radioactive Parent atoms decay to stable daughter atoms (as uranium decays to lead) each disintegration results in one more atom of the daughter than was initially present and one less atom of the parent. The probability of a parent atom decaying in a fixed period of time is always the same for all atoms of that type regardless of temperature, pressure, or chemical conditions. This probability of decay is the decay constant. The time required for one-half of any original number of parent atoms to decay is the half-life, which is related to the decay constant by a simple mathematical formula.

All rocks and minerals contain long-lived radioactive elements that were incorporated into Earth when the Solar System formed. These radioactive elements constitute independent clocks that allow geologists to determine the age of the rocks in which they occur. The radioactive parent elements used to date rocks and minerals are: