The geologic time scale divides Earth's history into units of time called eons, eras, periods, and epochs. Eons are the largest unit of time and are divided into shorter eras, which are further divided into periods and epochs to create smaller and more specific time spans. These divisions make up the geologic time scale and represent changes in life forms and rock formations over the planet's 4.56 billion year history.

1. 12.4 The Geologic Time Scale

2. Geologic time scale
• The division of Earth history into blocks of
time- eons, eras, periods, and epochs; The
time scale was created using relative dating
principles.

3. Eon
• The largest time unit on the geologic time
scale, next in order of magnitude above era

4. Era
• A major division on the geologic time scale;
Eras are divided into shorter units called
periods

5. Period
• A basic unit of the geologic time scale that is a
subdivision of an era; Periods may be divided
into smaller units called epochs

6. epoch
• A unit of the geologic time scale that is a
subdivision of a period

7. Key Concept
• Based on their interpretations of the rock
record, geologists have divided Earth’s 4.56-
billion-year history into units that represent
specific amounts of time. Taken
together, these time spans make up the
geologic time scale.

8. Key Concept
• Eons represent the greatest expanses of time.
Eons are divided into eras. Each era is
subdivided into periods. Finally, periods are
divided into still smaller units called epochs

9. Key Concept
• There are three eras within the Phanerozoic.
The Paleozoic, which means “ ancient life”, the
Mesozoic, which means “ middle life”, and the
Cenozoic, which means “recent life”.

10. Key Concept
• Each era is subdivided into periods, each of
which is characterized by a somewhat less
profound change in life forms as compared
with the eras.

11. Key Concept
• The periods of the Cenozoic are divided into
still smaller units called epochs.

12. Key Concept
• During Precambrian time, there were fewer
life forms. These life forms are more difficult
to identify and the rocks have been disturbed
often.

13. Key Concept
• A sedimentary rock may contain particles that
contain radioactive isotopes, but these
particles are not the same age as the rock in
which they occur.

14. Key Concept
• The age of a particular mineral in a
metamorphic rock does not necessarily
represent the time when the rock first formed.
Instead, the date may indicate when the rock
was metamorphosed.