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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.
Throughout the history of life, different organisms have appeared, flourished and become extinct.
Many of these organisms have left their remains as fossils in sedimentary rocks.
The table below shows characteristics of some common radiometric dating methods.
That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.
Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.
Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.
Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
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.