Lesson Summary

  • The age of a rock in years is its absolute age. The main evidence for absolute age comes from radiometric dating methods, such as carbon-14 dating. These methods depend on radioactive decay.
  • Radioactive decay is the breakdown of unstable isotopes into stable elements. For example, carbon-14 is an unstable isotope of carbon that decays to the stable element nitrogen-14. The rate of decay of an isotope is measured in half-lives. A half-life is the time it takes for half a given amount of an isotope to decay.
  • Radiometric dating uses the rate of decay of unstable isotopes to estimate the absolute ages of fossils and rocks. Carbon-14 can be used to date recent organic remains. Other isotopes can be used to date igneous rocks that are much older.

Points to Consider

Scientists estimate the ages of rock layers in order to better understand Earth’s history and the history of life.

  • What do you already know about Earth’s history? For example, do you know how Earth formed?
  • How old is Earth? When did the planet first form? And when did life first appear?