When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.
Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.
The age of formations is marked on a geologic calendar known as the geologic time scale.
To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.