1. Geologic Time Scale

2. Uniformitarianism
Earth is an always changing place. Plate tectonics, weathering, erosion etc.
The same forces of change that are working today were at work in the past.
The Earth has changed gradually over time.

3. Geologic Time Scale
Shows Earth’s history divided into time units based on life forms that existed and major events.
Geologist piece
together rock
layers and fossils to
get this time line.

4. Earth is estimated to be 4.6 billion years old.
Oldest living things were single celled organisms (bacteria) that first showed up in the fossil record around 3.5 billion years ago.
Humans appeared in the fossil record 100,000 years ago.

5. Divisions of Geologic Time
Eons: Largest unit
Eras: eons are divided into eras
Paleozoic, Mesozoic, Cenozoic
Periods: eras are then divided into periods.
Epochs: periods are divided into epochs.

6. Precambrian Time Longest part of Earth’s history
About 4.5 billion years long
First igneous rocks formed 3.9 billion years ago.
First living things appeared in fossil record (bacteria)

7. Precambrian Time
Invertebrates (no backbone) animals appear at the end of the Precambrian.
This time ended with a global ice age.

8. Paleozoic Era 544 mya – 248 mya
Means “ancient life” and referred to as the age of fish.
At the beginning of this era there is an explosion of life in the warm, shallow seas.
Organisms with shells and vertebrates form (trilobites, mollusks, fish and amphibians)
1st land plants. Increase oxygen in atmosphere

9. Paleozoic Era
Life moves unto land at the end of this era due to increased oxygen.
First reptiles (able to reproduce on land) and large insects form.
Pangaea forms

10. Paleozoic Era
Largest mass extinction marks the end of this era. 95% of all life disappears in the fossil record.
One theory is global-climate change due to massive volcanic activity.

11. Mesozoic 248 mya – 65 mya
Means “middle life” and often know as the age of the reptiles. Dinosaurs ruled the Earth.
First flowering plants.
First birds and mammals.
Pangaea separates.

12. Mesozoic Another mass extinction at the end of this era.
Most believe that an asteroid hit the Earth causing global-climate change.

13. Cenozoic Era 65 mya - present
Means “recent life” and is considered the age of mammals.
Several ice ages occur during this time.
Fossil of 1st human shows up (100,000 years old).