4.1 Geological Time Scale

Why would you want to name time?

When you refer to events in the history of your life you often link it to a time period. “When I was in kindergarten…” might be the start of your story. It helps scientists to have names to refer to events in Earth history. For this reason, they developed the geologic time scale.

The Geologic Time Scale

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.

Dividing Geologic Time

Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic period is named for the Jura Mountains in France and Switzerland, for example.

Putting Events in Order

To create the geologic time scale, geologists correlated rock layers. Steno’s laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. 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. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.

Divisions of the Geologic Time Scale

The largest blocks of time on the geologic time scale are called “eons.” Eons are split into “eras.” Each era is divided into “periods.” Periods may be further divided into “epochs.” Geologists may just use “early” or “late.” An example is “late Jurassic,” or “early Cretaceous.” Figure below shows you what the geologic time scale looks like.

The Geologic Time Scale.


  • geologic time scale: A division of Earth’s history into blocks of time distinguished by geologic and evolutionary events.


  • The geologic time scale divides earth history into named units. Naming time periods makes it easier to talk about them.
  • The units of the time scale are separated by major events in earth or life history.
  • In the geologic time scale, time units are divided and subdivided into smaller pieces.


Use this resource to answer the questions that follow.


  1. When did the world’s oldest rocks form?
  2. When did hard shelled mollusks appear?
  3. When did the earliest vertebrates appear?
  4. What appeared during the Permian period?
  5. When did Pangaea breakup?
  6. In which period did birds take flight?
  7. What period and era are we in now?


  1. Why do earth scientists need a geologic time scale?
  2. Why are some units of the geologic time scale longer and some shorter?
  3. What did the discovery of radioactivity do to change the geological time scale?

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