Radiometric Dating

Dating rocks and minerals by the steady decay of radioactive isotopes they contain, each decay series a clock running at a known rate. Uranium-lead, potassium-argon and radiocarbon methods each cover a different span of time, and together they turned the geologic time scale from a relative ordering into a calendar with numbers, including the 4.5 billion year age of Earth itself. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/