1. Relative Dating of Rocks
What kind of rock is the best rock to find fossils?
Igneous
Sedimentary
Metamorphic
Why?

2. Edible Rocks Activity Sketch a cross section view of the “rock”.
What is the oldest layer of this “rock”? How do you know?
What is the youngest layer of this “rock”? How do you know?
Draw step-by-step pictures of how this “rock” formed. 1. 2. 4. 3.

3. What is Age? There are 2 kinds:
Absolute Age – The number of years since the rock formed. (150 million years old, 10 thousand years old.)
Relative Age – The age compared to the ages of other rocks. (older than this rock, younger than that rock.)

4. Age cont.
Relative Age – Much easier to figure out. Can be deduced from clues on the Earth.
Absolute Age – Much harder to figure out. Often impossible. (easier with radiometric dating…)

5. What are the ways to tell RELATIVE AGE.
1. Law of Superposition
2. Extrusion & Intrusion
3. Faults
4. Unconformities

6. What is the Law of Superposition?
In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

7. Age and Superposition

8. What is Extrusion?
When magma cools on the surface of the Earth it’s called an EXTRUSION.
Extrusions are always YOUNGER than the rock it is sitting on.
For extrusion think exit or outside.

9. What is an Intrusion?
When magma cools beneath the surface of the Earth it’s called an INTRUSION.
Intrusions are always YOUNGER than the rocks around it.
For intrusion think inside.

10. Here’s an Intrusion

11. Extrusions? Intrusions?F G
More rock layers are deposited on top of the extrusion.
First of all, which rock layer is the oldest?
What is this called?
EXTRUSION C D A B E
INTRUSION

12. Review: What is a Fault? A BREAK in the Earth’s crust.
A fault is always YOUNGER than the rock it cuts through.
Faults move over time, making determining the age of the layers confusing.

13. Here’s a Fault

14. Faults This crack in the rock layers is called a fault.
Faults move over time causing the rock layers to move.

15. Faults
As more time goes by, more rock layers are added on top. Making aging the rock layers difficult.

16. Faults A B C D E
Geologists must then label each of the rock layers. This can be tough because they weren’t around to see the whole process occur.

17. What is an Unconformity?
The surface where new rock layers meet a much older rock surface beneath them.
An unconformity shows where some rock layers have been lost because of erosion.

18. Unconformity

19. 3 Types of Unconformity

20. Someone took the last cookie in the cookie jar last night
Someone took the last cookie in the cookie jar last night. The last person to leave the scene is the culprit. Who was it?

21. Summary Relative vs. Absolute Age Determining Relative Age:
Law of Superposition – on top is younger
Intrusions – magma inside Earth.
Extrusions – magma on top of Earth.
Faults – breaks in the crust
Unconformities – signs of erosion.