We need a geological stopwatch. Radioactivity Decay of radioactive elements can be used as a geological stopwatch. Certain isotopes of certain elements are unstable and change spontaneously into other elements through a process called radioactive decay. This decay also releases energy in the form of radiation and is the phenomenon which makes nuclear power plants possible. The atom which is produced when a parent atom decays.
Radioactive decay is a funny process.
Let's say you have a chunk of uranium and you have SuperVision and can see each and every individual atom in your uranium chunk all at the same time. However, the overall population of uranium atoms decays at a steady and predictable rate. The rate of decay is often expressed in terms of half-life , which is the time it takes for half of the parent atoms to decay away to child atoms. Below is a figure showing how a population of atoms changes through radioactive decay. You start off with 1000 parent atoms and no child atoms.