1. Interpreting Earth’s Rocks to Determine Its Past History

2. 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!

4. Geologic Time
Relative dating - sequence of geologic events in a region (order of past events)
(not dating your cousin )

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

6. Principle of Superposition
In horizontal rock layers, the oldest and first formed is on the bottom. 5 is the youngest because it is on the top, closest to the surface.

7. The Grand Canyon

9. Principle of Original Horizontality
Rock are originally deposited horizontal
Folding happened AFTER rocks were deposited

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. practice

13. Law of Inclusions

14. Index fossils
Fossils that lived for a short time can be used to date layers
Found world wide
Examples?

15. James Hutton: The father of modern geology (1726-1797)
Uniformitarianism:
“The present is the key to the past.”
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16. Examples of uniformitarianism
Petoskey stones
Striations
Rock formation
Paleoclimates
Earthquake location
What else?

17. Order the layers 1-7

19. Unconformities = “Missing Time”
Result of erosion

20. Angular Unconformities
Folded layers on bottom, flat layers on top
rock layers are eroded
new horizontal layers are deposited
angular unconformity
(rock layers have been eroded away)

21. Angular Unconformity F E D C B A

22. Nonconformity – igneous rocks on bottom; overlain by sedimentary rocks
sedimentary rock layers
Cambrian
Precambrian
Granite

23. Nonconformity

24. Disconformity DISCONFORMITY:Missing rock layers (sedimentary)
Very hard to see

25. Disconformities

26. Let’s Play…. Name the UNCONFORMITY!

27. Siccar Point, Scotland

32. granite shale sandstone limestone

35. Resources