Identify current methods for measuring the age of earth and its parts, including the law of superposition and radioactive dating

In the early 1900's the concept of radioactivity surfaced and offered a new and much more accurate way to date rocks. Radiometric dating is a method commonly used in modern times to help date rocks and fossils. New rocks and fossils contain a certain amount of radioactive isotopes. As these rocks age, the isotopes lose energy at a regular and predictable rate. When half of the once-radioactive isotopes have become stable (not radioactive), this is referred to as the half life. Scientists know what many common elements' half-lives are. They are then able to count the percent of unstable (parent) isotopes compared to the stable (daughter) isotopes and have a very accurate date as to when the rock or fossil formed or was deposited.