1. Review Session 6
Geologic History

2. RELATIVE DATING:
Putting rock layers in order of what came first,
then second, etc.
In order to RELATIVELY DATE rocks a number of rules have to be followed:
1. LAW OF SUPERPOSITION-
Normally, the oldest layer in a series of
sedimentary rocks is the bottom layer.
Each overlying layer is younger, so that the top layer is the youngest.

3. 2. ORIGINAL HORIZONTALITY:
Sedimentary rocks and lava flows are originally
deposited in horizontal layers
3. IGNEOUS INTRUSIONS AND EXTRUSIONS-
Contact metamorphism occurs when magma or lava
burns a rock.
Therefore:
An intrusion is younger than the rock it cuts through.
An extrusion (lava flow) is younger than rocks below it, but older than any rocks above it.

4. 4. FOLD- A rock is older than folding.
Sometimes folding can overturn rock strata so that older rocks lie on top and younger rocks are on bottom.
5. FAULT- The rock layers are older than the fault.
Sometimes extreme faulting can push older rocks from below over younger rocks on top.
6. The sediment in a sedimentary rock is older than the rock.

5. # 1 What is the order of events?

6. # 2 What is the order of Events

7. Igneous Intrusion – Younger than all the rock layers Lines touching layers = contact metamorphism # 3 What is the order of events?

8. # 4 What is the order of events in the following rock strata?

9. Fossils

10. Formation Organism dies Soft parts decay and/or are eaten
Hard parts buried in sediment
Sediment turns into rock
Uplift, weathering, and erosion expose the fossil

11. ESRT pages 8 & 9

12. Radioactive Dating

13. Radioactive Dating
Radioactive ISOTOPES in rocks DECAY away at a predictable rate.

14. Half Life
The amount of time it takes for HALF of the radioactive isotope to DECAY is called its HALF LIFE.
Half life DOES NOT CHANGE. (That’s why it’s on the ESRT!)

16. Since you don’t know how many atoms you started with, a ratio between parent to daughter will tell you how many half-lives have gone by.
100% parent
0% daughter
0 half lives

17. One half-life.
50% parent
50% daughter
1 half-life

18. Two half-lives.
25% parent
75% daughter
2 half-lives

19. Three half-lives.
12.5% parent
87.5% daughter
3 half-lives

20. Four half-lives.
6.25% parent
93.75% daughter
4 half-lives