1. Geologic
Time

2. never lived at the same time
Why is this statement false: “The caveman had dinosaur for breakfast.”?
man and dinosaurs
never lived at the same time

3. Laws of geologic history
Superposition—Oldest rocks on the bottom of an undisturbed sequence of rock
Law of original horizontality—All rocks are initially deposited in flat (horizontal) rows
Cross-cutting relationships—Objects which cross-cut rocks are younger than the rocks themselves (ex: fault or intrusion)

4. Superposition

5. Using superposition, label the strata below from oldest to youngest.

6. Horizontality
All rock layers are originally deposited in flat (horizontal) layers

7. Faults are always (older, younger) than the rocks they cut through.
Cross-cutting relationships
Faults are always (older, younger) than the rocks they cut through.

9. there first (it's older)
Which is older:
F or S
How do you know?
F cuts through S
S must have been
there first (it's older)

10. Uniformitarianism
These laws are based on the assumption that the forces at work today are the same forces at work in history = uniformitarianism.

11. If a geologist finds an igneous sill, how can she determine if the sill is an intrusion or an extrusion?

13. Is “H” an intrusion or extrusion? How can you tell?

14. H is an intrusion. Contact metamorphism on top

15. On to the next question...

16. a buried erosional surface
What is an unconformity? How does it complicate the relative dating of rock layers?
a buried erosional surface
a part of the rock record
is missing

17. Using the diagram to the left, identify where the unconformity is located by drawing an arrow and writing the word “unconformity” next to it.

18. What are
Index fossils?

19. What characteristics must fossils have in order to be good index fossils?
lived over a large geographic area (large horizontal distribution)
lived for a short period of time (small vertical distribution)

20. In geology, this is referred to as CORRELATION
In the diagram below, a geologist has matched up rock layers based on index fossils.

21. On to the next question...

22. their half-lives are constant
Absolute Age
Why are radioactive isotopes useful in
determining the absolute age of a rock?
their half-lives are constant

23. NOTHING! it is reliable to calculate age
Absolute Age
What can be done to change the
half-life of a radioactive isotope?
Why is this important?
NOTHING!
it is reliable to calculate age

24. stable atoms unstable atoms Draw the generic graph for the
half-life of a radioactive isotope.
stable atoms
unstable atoms

25. carbon-14 What radioactive isotope could be
Absolute Age
What radioactive isotope could be
used to determine the absolute
age of material that was recently living?
carbon-14

26. What is the half life of uranium-238?
Please answer the two questions
on page 7 of your Notes
What is the half life of uranium-238?
4.5 x 109
4,500,000,000
4.5 billion years
choice (c)

27. 3 half-lives x (5.7 x 103) =1.71 x 104 = 17,100 years
If there is a 100g sample of C14, how many grams of
C14 would remain after three half-lives?
How long would this take? Show all work.
100g 
50g 
25g 
12.5g
3 half-lives x (5.7 x 103) =1.71 x 104
= 17,100 years