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.
If we discover a rock with 75% K-40 and 25% Ar-40, we know it has been solid for 0.625 billion years (half of a half-life).
Similarly, If we discover a rock with 50% K-40 and 50% Ar-40, we know it has been solid for 1.25 billion years. Note that we do not need to know how much material was present originally, only the ratios that are present today. There are three naturally occurring isotopes of carbon in the environment: carbon-12, carbon-13, and carbon-14.
Note that Carbon Dating uses a different method that I will discuss momentarily .
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.As the material decays it produces the decay product.By measuring the amount of decay product compared to the amount of material we can calculate how long since it was solidified.They all have six protons since they are carbon, and therefore they are identical chemically.The body cannot tell them apart, so they maintain approximately the same relative abundance in living tissue as it does in the atmosphere.