1. Dating Rock Layers

2. Relative vs. Absolute Age
What is Relative Age?
The age of an object or event in comparison to another object or event.
What is Absolute Age?
The actual age, in years, of a rock or fossil.
Tells how many years ago a rock or fossil was formed.

3. Relative Age Methods of determining Relative Age: Law of Superposition
Extrusion & Intrusion
Faults
Unconformities

4. What is the Law of Superposition?
The Law of Superposition- in
undisturbed SEDIMENTARY
rock layers, the older layers
are at the bottom and
younger layers are
above it.

5. What is Extrusion?
Extrusion is when magma cools on the surface of the earth.
Forms a YOUNGER layer than the rock below.

6. Here’s an Extrusion

7. Another Extrusion

8. What is Intrusion?
Intrusion is when magma is forced into older rock layers and cools.
Forms a YOUNGER layer within the layers of rock.

9. Here’s an Intrusion

10. 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

11. What is a Fault?
Faults are a BREAK in the Earth’s crust due to tectonic plates colliding and rock layers being squeezed together.
Faults are YOUNGER than the rock they cut through.
Faults move over time, which causes rock layers to move.
This makes it difficult to determine the age or the surrounding rock.

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. What is Unconformity
Unconformity is when new rock layers meet a much older rock surface beneath.
Shows where
some rock
layers have
been lost due
to erosion.