ch. 23.6: interpreting the rock record objective: use principles of relative and absolute dating to...
TRANSCRIPT
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
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)
• Paleoenvironment & Climate Was this place a swamp? Coral reef? Desert? Tropical forest? Covered in ice?
• Rates of Climate ChangeHas Earth rapidly warmed or cooled before? What’s Earth’s normal?
• Document EvolutionFossil record
• Major Events: Meteroid impact; Mountain building (uplift); Rifting; Glaciation
Importance of Rock Record
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
Relative Age1. 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.
Relative Age2. Principle of Horizontality
Sedimentary rock layers started out HORIZONTAL.
If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)
Relative Age3. Unconformities
Breaks in geologic record = Missing Time
Deposition stopped or Rock layers were removed (usually after uplift and erosion)
Relative AgeTypes of UnconformitiesLook 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.
Relative Age4. 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).
Relative Age: Index Fossils
Fossils that narrow age of rock to a geologic period or era (millions of years)
Requirements:
1. Abundant - found in many regions
2. Lived during “short” , specific span of time
3. Distinguishing features
Relative Age: Index Fossils
Example: Ammonite fossils in layer 4 formed in rocks 108 - 206 mya
Problem 1
1. Sequence the order of rock layers (oldest --> youngest)
2. All of the numbered layers are sedimentary except for ___ and _____.
3. There is an unconformity present. Where is it? What does this mean?
Problem 1
4. 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)?
Problem 21. Label youngest and oldest
sedimentary layers (bottom drawing).
2. 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?
Problem 3 1. 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
Problem 4
1. Put sedimentary layers in order.
2. Indicate when the intrusion happened.
Absolute Age: Radiometric Dating
• Uses Radioactive Isotopes
• Compares relative amounts of parent:daughter
• Gives specific age of
rock
Isotopes = ______________________________________________________________________________________________
Ex: 12 Carbon = _________________ 14 Carbon = _________________Radioactive Isotopes = Atoms that have nuclei that break apart
(unstable) naturally.Release …_____________________________________
Absolute Age: Radiometric Dating
Nucleus = Particles w/Mass
Protons (+), determine element identity
Neutrons (no charge), can vary
• ____________________________________________________________________________________________
Decay happens at a _____________________(not changed by Temp., Pressure, or environmental conditions).
Absolute Age: Radioactive Decay
Absolute Dating: Radiometric Decay
• __________________________________________
Half life of 14C = 5,730 years
100 g 14 C -----> 50g 14 C + 50g 14N after 5,730 years
Absolute Age: Half Life
Half- Life of U 238 = 4.5 billion years
Absolute Dating: Half Life
Complete the chart belowTime Parent
isotope (g)Daughter isotope (g)
Remaining Parent
Time (Years)
Rock cyrstallizes
(forms)
100 0 0
1 half-life 50% (1/2) 15 million
2 half - lives 25 25% (1/4)
3 half-lives 87. 5 45 million
4 half-lives 6.25 6.25% (1/16)
Absolute Dating: Carbon Dating
______________________________________________________________________________________
Example: __________________________________
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
Half- Life ActivityStart with a whole piece of paper. This represents the amount of PARENT ISOTOPE in a rock sample when the rock first crystallizes.1.Cut the paper in half. Put one half to the side. Cut the paper in half again after 20 seconds. 2.Continue step 1 eight more times. 3.If the original paper represents your PARENT ISOTOPE, What do the pieces of paper you set aside in each step represent? __________________________.4.What is the half-life you your paper isotope? ________________5.What happens to the amount of parent isotope over time? _____________________________________________________6.What happens to the amount of daughter isotope over time?7.How much of the original paper isotope was left after …
1 “half-life” (20 second interval): ________%2 “half-lives”: ________%3 “half –lives”: ________%
8.Why would it be difficult to calculate the “age” of your rock after 15 half-lives?
Complete the chart belowTime Parent
isotope (g)Daughter isotope (g)
Remaining Parent
Time (Years)
Rock cyrstallizes
(forms)
100 0 100% 0
1 half-life 50 50 50% (1/2) 15 million
2 half - lives 25 75 25% (1/4) 30 million
3 half-lives 12. 5 87. 5 12. 5% (1/8) 45 million
4 half-lives 6.25 93.75 6.25% (1/16)
60 million
Answers to Quick Lab p.196
1. Parent Isotope 4. After 3 intervals: 12.5%
After 6 intervals: 1. 5%
After 9 intervals: 0.195%2. Daughter Isotopes created by decay
3. 20 seconds 5. No new parent (paper) added or removed; cut at constant rate (half-life)