1. How do we determine geologic age?
Uniformitarianism = the processes that shape Earth have not changed.

2. Relative Age
2. Law of Superposition = rocks at bottom are oldest.

3. Relative Age
3. Law of Horizontality = sediments deposit in horizontal layers.

4. Explain This!

5. Folding and Faulting

6. Folding

8. Relative Age
4. Law of Crosscutting = features that cross other rock layer are younger than those layers.

11. Unconformities Uplift, erosion, and weathering
can leave a missing piece to puzzle.

12. Unconformities

13. Determining Relative Age

15. How do we correlate rock layers from different locations?

16. If locations are close together
Walk the outcrop:
Compare similarities like rock type, color, and mineral composition.

17. If locations are far apart
Volcanic Ash – unique composition, short time span, and broad distribution.

19. If locations are far apart
2. Asteroid Impact – unique composition, short time span, broad distribution.

20. K-T Boundary

22. If locations are far apart
3. Index Fossils – easily identifiable, short time span, and broad distribution.

24. Absolute Age
Radioactive decay = the decay of unstable isotopes (parent) into stable elements (daughter).
Isotope = element has different masses (or amount of neutrons).
Ex. Uranium 238 (radioactive) decays into Lead 206 (stable).

25. Why use radioactivity? Decay rate is predictable.
Immune to external forces: heat, pressure, chemical, and mass.
Decay begins:
rocks: when formed.
organic: when life ends.

26. Half-Life
time it takes for half or 50% of the parent to decay into the daughter.

27. Limitations
Air, Water, Life: C14 / C12 ratio is used for dating air, water, and organic material, but is limited to only ~50,000 years.
Rocks:
radioactive isotope must be in rock.
U238 date range: 10 million to 4.6 billion
K40 date range: 50,000 to 4.6 billion