Chapter 21: Geologic Time

Geologic Time
• Separation =
• Correlate geologic
changes, and
changes among life
The Geologic Time Scale
• Record of Earth’s history
• Names of the time periods don’t
change but their boundaries are
always being changed.
Scale is cut into different sections:
1) Eon – longest time unit on the scale
Hadean (oldest)
Phanerozoic (youngest)
2) Era – next longest span of time
• Defined by the different life forms
that are found in the rocks.
 Precambrian (oldest)
 Paleozoic
 Mesozoic
 Cenozoic (youngest)
- Precambrian Era – microscopic life
- Paleozoic Era – Age of Trilobites
- Mesozoic Era – Age of Reptiles
- Cenozoic Era – Age of Mammals
3) Periods – defined by the life forms
that were abundant or became
extinct during the time in which the
rock was deposited.
4) Epochs – smaller division of
geologic time.