The radiometric dating
A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.Scientists discovered that rocks could be timepieces -- literally.The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.
If we know the number of radioactive parent atoms present when a rock formed and the number present now, we can calculate the age of the rock using the decay constant.
Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.