Because of their unique decay rates, different elements are used for dating different age ranges.For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.Each isotope is identified with what is called a ‘mass number’.When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.These use radioactive minerals in rocks as geological clocks.
Geologists often need to know the age of material that they find.
They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.
These break down over time in a process scientists call radioactive decay.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
This is different to relative dating, which only puts geological events in time order.
Most absolute dates for rocks are obtained with radiometric methods.