Relative and Absolute Geologic Time

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Relative and Absolute Geologic Time
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Whats the difference?

• Relative age- refers to the order in which rocks
  formed or geological event occur- expressed
  relative to another event/formation.
• Absolute age- time measured in years

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For example

• Your RELATIVE AGE
• Younger than me.
• Your ABSOLUTE AGE
• 17 years.

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Relative Dating- Methods

• Principle of original horizontality
• Principle of superposition
• Principle of crosscutting relationships
• Principle of faunal succession

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Principle of original horizontality

• Based on the observation that sediment usually
  accumulates in horizontal layers.
• If sedimentary rocks are folded or on an angle we
  can assume they were acted on by tectonic forces
  after they formed.

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Principle of superposition

• The bottom layer of sedimentary rocks is older
  than layers on top of it.

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Principle of crosscutting relationships

• Based on the obvious fact that a rock must exist
  before something can happen to it!

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Examples of Crosscutting

• An inclusion (sill or dike) is always younger
  than the rock layers it cuts through
• A fault is always younger than the rock layers it
  cuts
• Folding always happens after the rocks are in
  place.
• Pretty COMMON SENSE huh?

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Principle of faunal succession

• States that species evolved over time in a
  definite and recognizable order and that relative
  ages of sedimentary rocks can be recognized from
  their fossils.

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Index fossils

• Fossils used to accurately indicate the ages of
  sedimentary rocks.
• Must be an organism that
• Shows up a lot (abundantly preserved)
• Found all over the world
• Existed for only a short time
• Is easily identified in the field

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Interruptions in the rock record

• Unconformity- a period of time when no sediment
  was deposited, also during this time some rock
  layers may have been eroded. A long time
  interval with no geologic record!
• 3 types see diagram

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• Nonconformity- sedimentary layers rest upon an
  igneous or metamorphic rock formation, the
  boundary between these layers is called a
  nonconformity.

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• Disconformity- layers of sediments on the ocean
  floor are lifted above sea level and eroded, then
  much younger layers rest upon the older, eroded
  surface. This can represent a huge time gap.

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• Angular unconformity- rocks (deposited in
  horizontal layers) are folded or tilted, and
  erosion planes them off before other layers are
  deposited on top

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How can youtellthemapart?
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Absolute Age- Radioactive decay

• Radioactive decay- radioactive elements emit
  particles and energy, forming new isotopes or
  elements at a measureable rate, scientists
  determine the proportion of old, radioactive
  element and the newly formed element in a rock to
  figure out its age. (isotopes same of protons,
  different of nuetrons)
• Half life- time it takes for half the mass of a
  radioactive element to decay.
• Ex. Half life of U-238 to Pb-206 is 4.5 billion
  years.

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• Carbon dating- used to determine age of organic
  materials, uses a form of radioactive carbon-
  carbon-14
• Organisms take in C-12 and C-14 when alive and
  the ratio of these elements remains constant.
• When the organism dies, C-14 decreases as it
  decays to C-12, scientists measure the proportion
  of C-14 and C-12 to determine age.
• The half-life of C-14 is only 5,730 years.

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REVIEW

• Relative vs. absolute age?

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REVIEW

• Apply relative dating principles to this picture.
  
• (p. 64 FIGURE 3.33)

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REVIEW

• How can you recognize each type of unconformity?

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REVIEW

• What is carbon dating?
• What limitations does this method have?