1. Geologic time Relative dating
Interpreting Earth’s Rocks to Determine Its Past History

2. Geologic Time
Relative Time - sequence of geologic events in a region (order of past events)
Absolute Time – numeric- time in years; use radioactive isotopes.
How do we know how old something is?

3. James Hutton: The father of modern geology (1726-1797)
Uniformitarianism:
“The present is the key to the past.”

4. Examples of uniformitarianism
Petoskey stones and past climates
How sandstone forms
Grand Canyon formation
Dino reconstruction

5. Determining Geologic History:
Geologists are like CSI
detectives wanting to know
Where?, When?, Why?, How?
Sometimes very little “evidence”
remains (exposure, erosion)
Evidence has been moved or
tampered with! Rocks have
been moved (faults, folds)
Usually no witnesses!

6. Principle of Superposition
Oldest layer on bottom (if tectonically undisturbed)
Younger
Older

7. The Grand Canyon

8. Principle of Original Horizontality
Sediments are originally deposited as horizontal (or near horizontal) beds
Use for tilted or bent layers

9. POH
What can cause rock to bend?

10. folded rock layers = compression by tectonic activity

11. Cross-Cutting Relationships
An igneous intrusion (or fault) is younger than the rock layers that it cuts across.
These are younger than the surrounding rock
youngest
fault
oldest

12. Write the order of events that have taken place above
Write the order of events that have taken place above List (oldest - 1 to youngest - 6)

13. Which came first? Order the rock layers above from oldest to youngest.
Is this an example of absolute or relative dating?

14. Which came first? A C B C D B D D E
The first layer deposited and the oldest is D, followed by C, A, B, and E.

15. List the order of events that took place:
earthquake
deposit D
deposit B
deposit G
intrusive lava
rocky deposit R
river cuts through
deposit E
deposit A

16. Unconformities = “Missing Time”

17. Angular Unconformities
Folded layers below horizontal layers.
rock layers are eroded
new horizontal layers are deposited
angular unconformity
(rock layers have been eroded away)

19. Angular Unconformity
sedimentary rocks above and below unconformity surface with rocks below tilted at an angle F E D C B A

20. Nonconformity – younger sedimentary rocks rest on older igneous rocks
sedimentary rock layers
Cambrian
Precambrian
Granite

21. Nonconformity

22. Disconformity DISCONFORMITY:Missing rock layers (sedimentary)
Very hard to pick out

23. Disconformities

24. Type of unconformity?

25. Siccar Point, Scotland

26. granite shale sandstone limestone

31. Resources