Forms of radioactive dating
Carbon-14 dating: See Carbon 14 Dating in this web site.
Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.
In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon.
When they die no new carbon-14 is taken in by the dead organism.
Strontium-87 is a stable element; it does not undergo further radioactive decay.
(Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86.
By "age" we mean the elapsed time from when the mineral specimen was formed.
Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.