Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and minerals contain tiny amounts of these radioactive elements.
Radioactive decay occurs at a constant rate, specific to each radioactive isotope .
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history. Precise measurements of the amount of 40 P relative to 40 Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized. If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism. This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things. Any dead material incorporated with sedimentary deposits is a possible candidate for carbon-14 dating.