What are U and Th series isotopes?

Rocks contain ppm quantities of the radioactive isotopes 232Th and 238U that form two chains of decay products of varying half lives, all less than those of the 232Th and 238U respectively. Given sufficient time (in the order of 1,000,000 years) and no losses from the system, all the isotopes within each chain will be in equilibrium, i.e. their activities equal to each other. However, a geochemical event can disrupt the equilibrium and this is utilised in many dating methods.

Relevant isotopes formula