Either a whole rock or a single mineral grain can be dated.

Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme).

This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate.

The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time.

The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate.

The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.

This scheme was developed in 1937 but became more useful when mass spectrometers were improved in the late 1950s and early 1960s.

This technique is used on ferromagnesian (iron/magnesium-containing) minerals such as micas and amphiboles or on limestones which also contain abundant strontium.The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes.Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces.These are released as radioactive particles (there are many types).This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.All rely on the fact that certain elements (particularly uranium and potassium) contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age.