Geologists have at their disposal an accurate method of dating rocks that can even cope with ages as far back as the origin of the Earth some 4.7 billion years ago.

The method is based on the fact that radioactive isotopes of chemical elements decay at a constant rate which is entirely independent of any environmental influences such as temperature or pressure.

A fundamental concept in this work is that of the half-life which refers to the length of time that elapses before half of a quantity of isotope has decayed to its end-product.

The most commonly used radioactive isotopes are shown in the table below, with their decay products and half-lives.

The ratio of decayed to undecayed isotope in a rock gives a measure of its age. It is evident from the huge figures for the half-lives of the isotopes that this technique is well able to produce a time-scale to include even the Earth’s oldest rocks.

Radioactive isotopes

Radioactive isotope	Isotope name	Decay product	Decay product name	Half-life (billion years) One billion is one thousand million: 1,000,000,000
40K	Potassium	40A	Argon	11.9
87Rb	Rubidium	87Sr	Strontium	50.0
232Th	Thorium	208Pb	Lead	13.9
235U	Uranium	207Pb	Lead	0.7
238U	Uranium	206Pb	Lead	4.5