1. TOPIC XIII INTERPRETING GEOLOGIC HISTORY

2. How Old?? Absolute Dating Relative Dating Determine the actual age
Determine which thing is older

3. Original Horizontality
Sedimentary layers are formed horizontally
Any change from that position is evidence of crustal movement since their formation

4. Principle of Superposition
Older layers are on the bottom unless overturning has occurred

5. Intrusions & Extrusions
Magma squeezes into existing rock and crystallizes
Extrusion
Lava flows on the surface and solidifies

6. Dating Intrusions/Extrusions
Intrusions/Extrusions are always younger than any rock they cut through
Look for Contact Metamorphism

7. Intrusion or Extrusion???

8. Inclusions An older piece of rock inside an igneous rock
Inclusions are always older than the rock they are in

9. Structural Features Faulting, folding, or tilting
The rock is always older than the feature that affects it

10. Veins
Mineral deposit in a rock

11. Unconformity Buried erosional surface
You know that some of the rock is missing, but there is no way to know how much

12. Correlation
Showing that rocks from different locations are the same age
Correlation by exposed outcrops
Correlation by similarity
Correlation by use of fossils

13. Fossils Fossils Remains or evidence of once living things Bones Shells
Imprints

14. Index Fossils Must meet 2 criteria:
Must have lived over a wide geographical area
Must have existed for a relatively short time

15. Which of these fossils would make the best index fossil?

16. Other Ways to Correlate
Volcanic Ash
Spreads all over and falls in a short period of time
Meteorite Deposits
Also spread and fall quickly