Summary:
- 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.