Ch. 23.6: Interpreting the Rock Record

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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 Earths history.
• Key terms Law of Superposition Principle of
  Horizontality Unconformities Crosscutting
  Relationships Index fossils Radiometric dating
  isotopes half-life

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Earths Age

• Up until the 1700s Es age was estimated to be
  6,000 years old
• Today Es age is estimated to be 4.6 billion
  years old.
• Determined by absolute dating or radiometric
  isotopes (well get back to)

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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?
  Whats Earths normal?
• Document Evolution
• Fossil record
• Major Events Meteroid impact Mountain building
  (uplift) Rifting Glaciation

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Relative Dating of Earths Layers

• Allows you to determine the SEQUENCE OF EVENTS
• Order that rock layers formed (1st, 2nd, etc.)
• No specific date

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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.

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Relative Age2. Principle of Horizontality

• Sedimentary rock layers started out HORIZONTAL.
• If layers are TILTED or CURVED, tectonics
  deformed them (Mt. Building or Faulting)

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Relative Age3. Unconformities

• Breaks in geologic record Missing Time
• Deposition stopped or Rock layers were removed
  (usually after uplift and erosion)

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Relative AgeTypes 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.
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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
  --gt most recent).

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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

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Relative Age Index Fossils

• Example Ammonite fossils in layer 4 formed in
  rocks 108 - 206 mya

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Problem 1

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

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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)?

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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?

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Problem 3

• List sequence of events in relative order (oldest
  --gt youngest)

• Events may include
• Deposition of sedimentary layers
• Intrusion of igneous rock
• Tectonics Uplift folding faulting
• Erosion

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Problem 4

1. Put sedimentary layers in order.
2. Indicate when the intrusion happened.

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Absolute Age Radiometric Dating

• Uses Radioactive Isotopes
• Compares relative of parentdaughter
• Gives specific age of rock

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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

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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).

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Absolute Dating Radiometric Decay
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Absolute Age Half Life

• The time it takes for 1/2 the mass of PARENT --gt
  DAUGHTER.
• Half life of 14C 5,730 years
• 100 g 14 C -----gt 50g 14 C 50g 14N after 5,730
  years

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Half- Life of U 238 4.5 billion years
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Absolute Dating Half Life

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Complete the chart below
Time 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) 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
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Complete the chart below
Time Parent isotope (g) Daughter isotope (g) Remaining Parent Time (Years)
Rock cyrstallizes (forms) 100 0 0
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)
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Absolute Dating Carbon Dating

• Used for dating organic matter found in younger
  rocks (lt 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

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Answers to Quick Lab p.196
1. Parent Isotope After 3 intervals 12.5 After 6 intervals 1. 5 After 9 intervals 0.195
2. Daughter Isotopes created by decay After 3 intervals 12.5 After 6 intervals 1. 5 After 9 intervals 0.195
3. 20 seconds 5. No new parent (paper) added or removed cut at constant rate (half-life)