Geologic Time
Introduction
• Nearly 4.6 billion years have passed since Earth’s
formation.
• In that time, life has exploded from a few simplecelled organisms to a great variety of single-celled
and multi-celled forms.
• Scientists know about life’s history from studying the
fossil record and rock layers worldwide.
• This record can be arranged in a geologic time scale.
Organizing Earth’s History
• The geologic time scale is a timeline that organizes the events in
Earth’s history.
4.6 Billion
Years Ago
4
3
Prokaryotes
2
Eukaryotes
1
Today
Humans
• It reveals that algae, bacteria, and protozoa dominated most of
Earth’s history.
• More complex organisms, such as land plants and fish, evolved only
within the last 500 million years.
• Humans evolved only about 110,000 year ago.
• If the entire history of earth were squeezed into a single day,
humans would not evolve until the last few seconds.
Geologic Time Scale
•The geologic time scale is like
a calendar extending from
Earth’s formation to the present.
The scale is divided into eons,
eras, period and epochs.
•Eon: The largest group:
billions of years long
•Era: mass extinctions mark
the boundaries between the
eras; hundreds of millions of
years long
•Period: tens of millions of
years long
•Epoch: divisions of the most
recent periods; several
million years long.
Your Geologic Time Scale
• The back of your paper is divided into 4 columns
• In the last or fourth column, list six to ten events in the school year
in the order they will happen. For example, you may include a
particular soccer game, dance, or event.
• In the third column, organize those events into larger time periods,
such as soccer season, rehearsal week, or whatever you choose.
• In the second column, organize those time periods into even larger
ones.
• In the first column, organize the time period into the largest group
• Go back to the 4 columns and label them as eon, era, period, and
epoch
• Conclusion:
– List in order of smallest to largest. Epoch, Eon, Era and Period.
– How does putting events into categories help you and
geologists see the relationship among events?
Geologic Time Scale in a Calendar Year
Review
• Why is a time scale used to represent
Earth’s history instead of a calendar?
• When did geologic time begin?
• When does geologic time end?
• Put the following words in order of
smallest to largest: epoch, era, period,
eon.