other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.
Measuring the ratio of C14 to C12 and C13 therefore dates the organic matter for periods back to about eight half-lives of the isotope, 45,000 years.
After a long enough time the minority isotope is in an amount too small to be measured.
There are about two dozen decay pairs used for dating.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. It decays to nitrogen 14 with a half life of 5730 years.
Carbon 14 is created by cosmic rays in the upper atmosphere.
C14 is continually being created and decaying, leading to an equilibrium state in the atmosphere.
When the carbon dioxide, containing C14 as well as stable C12 and C13, is taken in by plants it is no longer exposed to the intense cosmic ray bombardment in the upper atmosphere, so the carbon 14 isotope decays without being replenished.
Other methods do not require knowing the initial quantities.
For example, potassium decays into two different isotopes of argon having different half-lives.