How to date a fossil using radiometric dating
So if you try to run radiometric dating on a sedimentary rock, you won’t determine the age of that rock—that is, when the sediments were compacted and cemented to form a new layer. However, there is a problem, a really big one: Its half-life is only 5730 years.
See, the clock for radioactive elements starts at crystallization.
Carbon is naturally in all living organisms and is replenished in the tissues by eating other organisms or by breathing air that contains carbon.
At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues.
As the rock cools, there comes a point, called the closing temperature, when parent and daughter isotopes can no longer diffuse into or out of the rock system—at that point, the clock is set.
(It’s worth mentioning that this temperature varies by rock type and by isotope!