1. A View of the Earth’s
Past
The Geologic Time Scale
2. Rock Record holds a fascinating story of the
evolution of life on earth
3. The Geologic Time Scale
Some organisms flourish then become extinct with
the changes that occur on earth such as mountains
forming and eroding, oceans rising and receding
Scientists use the geologic time scale – gives the
sequence and length of time these changes occurred
4. The Geologic Time Scale:
The Geologic Column
Review
Law of Superposition
Index fossils
Relative ages of rocks
Geologic Column – the ordered arrangement of rock
layers based on the ages of rocks
6. The Geologic Time Scale:
The Geologic Column
Rock layers distinguished from each other mainly
by the types of fossils and the kinds of rocks found
in the rock layers
Upper layers of rock layers contain modern types of
plants and animals
Lower layers have older plants and animal fossils
Many contain EXTINCT animals and plants
7. The Geologic Time Scale:
The Geologic Column
Original theory of geologic column thought the
rates of deposition were constant
Later it was discovered that the rates of deposition
change over time
Radioactive dating development also helped refine
the geologic column theory
8. The Geologic Time Scale:
The Geologic Column
According to the law of superposition, the position
of a given rock layer is a clue to its relative age
10. The Geologic Time Scale:
Divisions of Geologic Time
Geologic history is divided into sections, like a
calendar, that represent major changes in the earth
surface or climate changes
Geologists use these events to divide the geologic time
scale into smaller units
11. The Geologic Time Scale:
Eras
Era – a very large unit of geologic time
Four geologic eras
1. Precambrian
2. Paleozoic
3. Mesozoic
4. Cenozoic
12.
13. The Geologic Time Scale:
Eras
1. Precambrian Time
- OLDEST layer
- much larger than other eras – 4 billion years
- not many fossils so difficult to divide into
sections
14. The Geologic Time Scale:
Eras
2. Paleozoic Era
- Greek word meaning “ancient life”
- lasted 325 million years (570 million years ago
to 245 million years ago)
- large variety of marine, plant, and animal
fossils
15. The Geologic Time Scale: Eras
3. Mesozoic Era
- Greek for “middle life”
- existed for 180 million years
- more-complex life forms found like reptiles and
birds
16. The Geologic Time Scale:
Eras
4. Cenozoic Era
- Greek meaning “recent life”
- fossils of mammals found in the rocks
- earlier forms of life are lower than more-
complex forms
17. The Geologic Time Scale:
Periods and Epochs
Periods – shorter time units of each Era
- defined by specific fossils
- named by location where fossils were first
found in geologic column
Example: Devonian Period named after Devonshire,
England
18.
19. The Geologic Time Scale:
Periods and Epochs
Epochs - shorter time units of each era defined by
diversity of fossils
- Cenozoic Era has the most diverse and
detailed fossils so easy to divide rock layers from
this era into periods
