1. Aim: How can we explain relative dating?

2. Relative Dating
Determines age of a rock or event by comparing it to other rocks or events

3. The Principle of Superposition
The youngest rock layer is found on top and the rock ages increase with depth

4. Youngest
Oldest

6. Exceptions to P. of S.
When rock layers are folded, overturned, or faulted, older layers are moved over younger layers.

7. Younger
Older

10. Intrusion Molten rock squeezes into preexisting rocks and solidifies.
YOUNGER than any rock it cuts through.

11. Extrusion Molten rock flows on surface and solidifies.
Younger than rock beneath it, older than any layers above it.

12. Inclusion A body of older rock within younger rock.
Ex. Pebbles held together by cement – pebbles are older than the cement

13. Contact Metamorphism
Both intrusions and extrusions form contact metamorphism with layers they touch.
If you see contact metamorphism, it is YOUNGER than that layer.

15. Folds, faults, and joints
These rock features are YOUNGER than the layers faulted or folded

17. Using Relative Dating

18. Correlation Used to reconstruct geologic history of a location
Determines if rock layers or events in 2 separate areas are the same

19. 1. Similarity of Rocks
Rocks matched on similarity in appearance, color, and composition.
Ex. Two sides of a valley

21. 2. Fossil Evidence
Index fossils useful in determining if two rocks are from same time period
They must have
Lived over a wide geographic area
Lived for a relatively short time

23. 3. Volcanic Time Markers
Volcanic eruptions are short and eject layer of ash over a large area
Useful as time markers

24. Unconformities
Eroded surface that has been buried – part of rock record is missing
Rocks above unconformity are younger, rocks below are older
Associated with orogenies

25. Commonly caused by:
Uplifterosion
submergencedeposition