Note that Carbon Dating uses a different method that I will discuss momentarily .The radioactive material is alway decaying into something, but if the material is in a molten state, the decay products will not stay in place.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.
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.
Radiometric dating works by measuring how much a radioactive material has decayed, and using its known decay rate to calculate when the material was solidified.
There are a variety of ways of doing this; here is a common method.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Each isotope is identified with what is called a ‘mass number’.
Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in.