Radiometric Dating

A method of dating a rock by measuring the proportions of radioactive parent istopes to daughter products.

Isotope Half-Life(yr) Daughter Product Parent abundant in:
40K 1300 Myr 40Ar Feldspar, mica
238U 4500 Myr 206Pb Uranium ores, zircon
235U 704 Myr 207Pb Uranium ores, zircon
232Th 14100 Myr 208Pb Zircon
87Rb 48800 Myr 87Sr Potassium-rich minerals
14C 5730 yr 14N Organic matter, dissolved limestone

Atoms have a nucleus which contains protons and neutrons which are surrounded by electrons. An element may have a varying number of neutrons, producing isotopes. Some of the isotopes produced may be unstable- radioactive isotopes. Radioactive isotopes change over time to become stable (radioactive decay). For example, 235U decays to form 207Pb.

A half life is the time it take for half of the parent product to decay into its daughter product.

image from

The half life for a particular element is constant. If the number of radioactive parents atoms present when a rock was formed is known and the number present now, the age of the rock can be calculated using the decay constant.

Radioactive decay is random and is unaffected by temperature or pressure.

Errors in Radiometric Dating