1. How to tell how old the layers of rock are.
Dating Rock Layers
How to tell how old the layers of rock are.

2. What is Relative Age? 1. Law of Superposition 2. Extrusion & Intrusion
Relative Age – The age compared to the ages of other rocks. (older than this rock, younger than that rock.)
The ways to tell RELATIVE AGE:
1. Law of Superposition
2. Extrusion & Intrusion
3. Faults
4. Unconformities

3. 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.

4. Age and Superposition

6. 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.

7. 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.

8. Here’s an Intrusion

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

10. 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.

11. Here’s a Fault

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

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

14. 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.

15. Unconformity

16. Summary
Relative Age – comparison of ages – youngest to oldest / oldest to youngest
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.