1. Ch. 23.6: Interpreting the Rock Record
OBJECTIVE:
Use principles of relative and absolute dating to determine a sequence of events (climate, tectonic, & environmental) in Earth’s history.
Key terms: Law of Superposition; Principle of Horizontality; Unconformities; Crosscutting Relationships; Index fossils; Radiometric dating; isotopes; half-life

2. Earth’s Age
Up until the 1700s E’s age was estimated to be ~ 6,000 years old
Today: E’s age is estimated to be 4.6 billion years old.
Determined by absolute dating or radiometric isotopes (we’ll get back to)

3. Importance of Rock Record
Paleoenvironment & Climate
Was this place a swamp? Coral reef? Desert? Tropical forest? Covered in ice?
Rates of Climate Change
Has Earth rapidly warmed or cooled before? What’s Earth’s normal?
Document Evolution
Fossil record
Major Events: Meteroid impact; Mountain building (uplift); Rifting; Glaciation

4. Relative Dating of Earth’s Layers
Allows you to determine the SEQUENCE OF EVENTS
Order that rock layers formed (1st, 2nd, etc.)
No specific date

5. Relative Age 1. Law of Superposition
A sedimentary rock layer is older than the layer above; younger than layer below
* Undeformed layers
Sediments are deposited on top of existing layers and lithified.

6. Relative Age 2. Principle of Horizontality
Sedimentary rock layers started out HORIZONTAL.
If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)

7. Relative Age 3. Unconformities
Breaks in geologic record = Missing Time
Deposition stopped or Rock layers were removed (usually after uplift and erosion)

8. Relative Age Types of Unconformities
Look for erosional surfaces; tilted layers; or igneous intrusions
Left: Nonconformity = Igneous or metamorphic rock is uplifted, exposed, and eroded. Sed layers deposited on top.
Middle: Angular Unconformity = layers are folded or tilted, then eroded. New layers sed layers deposited on top.
Right: Disconformity = Horizontal layers are uplifted and eroded. New sed. Layers deposited on top.

9. Relative Age 4. Crosscutting Relationships
If a fault or igneous intrusion cuts across a layer … it happened after that layer
Which happened first: faulting or igneous intrusion?
Write a summary of events for this region (oldest --> most recent).

10. Relative Age: Index Fossils
Fossils that narrow age of rock to a geologic period or era (millions of years)
Requirements:
Abundant - found in many regions
Lived during “short” , specific span of time
Distinguishing features

11. Relative Age: Index Fossils
Example: Ammonite fossils in layer 4 formed in rocks mya

12. Problem 1
Sequence the order of rock layers (oldest --> youngest)
2. All of the numbered layers are sedimentary except for ___ and _____.
There is an unconformity present. Where is it? What does this mean?

13. Problem 1
What evidence is there that a tectonic event affected this area in the past? Describe and interpret this evidence.
5. What happened first: Faulting (B) or Intrusion (3)?

14. Problem 2
Label youngest and oldest sedimentary layers (bottom drawing).
Describe the tectonic setting that would produce the folded layers.
3. Why are the tops of the folded layers cut off? How did this happen?

15. Problem 3
List sequence of events in relative order (oldest --> youngest)
Events may include:
Deposition of sedimentary layers
Intrusion of igneous rock
Tectonics: Uplift; folding; faulting
Erosion

16. Problem 4 Put sedimentary layers in order.
Indicate when the intrusion happened.

17. Absolute Age: Radiometric Dating
Uses Radioactive Isotopes
Compares relative % of parent:daughter
Gives specific age of rock

18. Absolute Age: Radiometric Dating
Nucleus = Particles w/Mass
Protons (+), determine element identity
Neutrons (no charge), can vary
Isotopes = Atoms of the same element with different # of neutrons.
Ex: 12 C (6 protons + 6 neutrons), 14 C (6 protons + 8 neutrons)
Radioactive Isotopes = Atoms that have nuclei that break apart (unstable) naturally.
Release energy & particles

19. Absolute Age: Radioactive Decay
Unstable PARENT Isotope breaks down to stable DAUGHTER Isotope (& releases energy)
Decay happens at a constant rate (not changed by Temp., Pressure, or environmental conditions).

20. Absolute Dating: Radiometric Decay

21. Absolute Age: Half Life
The time it takes for 1/2 the mass of PARENT --> DAUGHTER.
Half life of 14C = 5,730 years
100 g 14 C -----> 50g 14 C + 50g 14N after 5,730 years

22. Half- Life of U 238 = 4.5 billion years

24. Absolute Dating: Half Life

25. Complete the chart below
Time
Parent isotope (g)
Daughter isotope (g)
Remaining Parent
Time (Years)
Rock cyrstallizes (forms)
100
100%
1 half-life 50
50% (1/2)
700
2 half - lives 25 75
25% (1/4)
1400
3 half-lives
12. 5
87. 5
12. 5% (1/8)
2100
4 half-lives
6.25
93.75
6.25% (1/16)
2800

26. Complete the chart below
Time
Parent isotope (g)
Daughter isotope (g)
Remaining Parent
Time (Years)
Rock cyrstallizes (forms)
100
1 half-life
50% (1/2)
700
2 half - lives 25
25% (1/4)
3 half-lives
87. 5
2100
4 half-lives
6.25
6.25% (1/16)

27. Absolute Dating: Carbon Dating
Used for dating organic matter found in younger rocks (< 70,000 years)
Wood, bones, shells
14 C made by cosmic radiation & incoporated into plants via photosynthesis (plants take in CO2 from air)
Alive - Organisms have constant ratio of 12C: 14C
Dead - 14C decays and 14N increases

29. Answers to Quick Lab p.196 1. Parent Isotope
After 3 intervals: 12.5%
After 6 intervals: 1. 5%
After 9 intervals: %
2. Daughter Isotopes created by decay
seconds
5. No new parent (paper) added or removed; cut at constant rate (half-life)