1. We know it’s BIG…but HOW do we know that?
Geologic Time
We know it’s BIG…but HOW do we know that?

2. BIG QUESTIONS:
What methods do we use to figure out the age of our earth?
How do fossils help
us to learn things
about the past?

3. Uniformitarianism
Uniformitarianism is the assumption that the same processes that operate on the earth now have always operated on the earth in the past.
For example:
Given a long period of time, a river can erode a valley. Based on the principle of uniformitarianism, all valleys in the past have been formed by rivers eroding them and all future valleys will be formed this same way

4. Relative Dating
Relative dating refers to the relative order in which things occurred, without knowing the actual date of each event. Relative dating relies on several principles.

5. Principle of Original Horizontality
This principle states that layers of sediment are originally deposited horizontally under the action of gravity.
This means that any “sloping” of layers must have occurred AFTER the layers formed

6. Principle of Superposition
Layers of earth are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

7. Cross-Cutting Relationships
 The principle of cross-cutting relationships states that the geologic feature which cuts another is the younger of the two features.
Younger

8. Inclusions
This principle states that fragments included in a host rock are older than the host rock itself.
This rock (inclusion) must have existed BEFORE the lava (host rock) formed…therefore it is older.

9. Practice… Identify the layers from oldest to youngest

10. Absolute Dating
Absolute dating refers to the exact age in years of an object Several methods exist for absolute dating.

11. Radiometric Dating
Technique used to date materials such as rocks, using radioactive materials contained in the rocks/fossils This exact age can then help us with some aspects of relative dating

12. Fossils
The preserved remains or traces of animals, plants, and other organisms from the past Many fossils can be dated using relative AND absolute dating

13. Index Fossils
A particular plant/animal that only existed for a very short period of time on the earth Useful for relative dating purposes.