• Earth formed 4.5 billion years ago.
  • Geologists divide this time span into smaller periods.
  • Many of the divisions mark major events in life history.
  • For example, the Cretaceous ended when the dinosaurs went extinct.
  • European geologists were the first to put together the geologic time scale.
  • The discovery of radioactivity in the late 1800s changed that.
  • Scientists could determine the exact age of some rocks in years.
  • They assigned dates to the time scale divisions.
  • Naming time periods makes it easier to talk about them.
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