1. Chapter 9: A View into Earth’s Past
Geologic Time
Chapter 9: A View into Earth’s Past

2. Geologic Column
A geologic column is an ordered arrangement of rock layers that is based on relative age
older rocks on bottom, newer rocks on top

3. Fossils in Rock Layers
Rock layers can be distinguished by the types of fossils the layers contain
Fossils in the upper layers of Earth contain remains of modern plants and animals
Fossils in lower, older layers are different than those living today; some may even be extinct

4. Index Fossils
Index fossils are used to help geologists match different rock layers that may be located far apart
To be an index fossil it must be…
Abundant
Widespread around the world
Remains of organisms that lived for a short period of time
trilobite

5. Geologic Time Scale A summary of the major events in Earth’s history
Divided into units of time (eon, era, period, epoch)
Changes in these time periods are typically marked by major changes in Earth’s surface, climate, and types of organisms
Mass extinction (dinosaurs)
Ice ages

6. Geologic Time Scale EON – largest segment of geologic time ERA PERIOD
EPOCH – smallest segment of geologic time

7. Geologic Time Scale
see page 213

8. Geologic Time compared to 1 Day

9. Precambrian Time
The time period from the formation of Earth until about 542 million years ago
Accounts for 88% of Earth’s history!
We know very little about what happened during that time
Rock record from this time difficult to interpret due deformed rock from disturbing forces within Earth
Fossils are rare from this time, most of Earth was covered by ocean

10. Precambrian Time

11. Paleozoic Era 542 million years ago (mya) to 251 mya
Earth’s atmosphere becomes oxygen rich at this point
At the end of this era Pangaea existed
Large fossil record
Trilobites was the common marine invertebrate used as an index fossil.

12. Mesozoic Era 251 mya to 65.5 mya Age of the reptiles
There is a rich fossil record for this era
Divided into 3 periods:
Triassic
Jurassic
Cretaceous
Ended with a mass extinction

13. Cenozoic Era Current era Age of the mammals 2 periods:
Tertiary – before the last ice age
Quaternary – after the last ice age

14. Geologic Time Video