Radioactive isotope used in geological dating
The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.
These break down over time in a process scientists call radioactive decay.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 rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.Because of their unique decay rates, different elements are used for dating different age ranges.