Dating

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Dating

• How can scientists determine the age of rocks
  and fossils?

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Geologic Principles

• Uniformitarianism
• The processes occurring today have been occurring
  since Earth formed
• Present Earth not caused by catastrophic events

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Relative vs. Absolute Dating

• Relative Age Dating
• Establishes the order of past geological events
  (ex A is older than B)
• A comparison, not an exact age
• Absolute Age Dating
• Scientists determine the numerical age of rocks
  and other objects (ex Sample A is 4.6 million
  years old)
• Exact age of object

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Relative vs. Absolute Dating
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Relative Age Dating

• Original Horizontality
• Sedimentary rocks are deposited in (nearly)
  horizontal layers
• Superposition
• In an undisturbed rock sequence, the oldest rocks
  are at the bottom and each layer above is younger
  than the rock layer below it

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Youngest
Oldest
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Relative Age Dating

• Cross-Cutting relationships
• States that an intrusion or a fault is younger
  than the rock it cuts across

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Absolute Age Dating

• Radioactive Decay
• Radioactive elements have unstable nuclei due to
  their number of neutrons
• To achieve stability, radioactive elements emit
  nuclear particles
• Parent the original radioactive element
• Daughter the new element

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Absolute Age Dating

• Radiometric dating
• Process used to determine the absolute age by
  determining the ratio of parent nuclei to
  daughter nuclei in a given sample
• Half-life
• Amount of time it takes a radioactive element to
  decay to one half of the original amount.

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Other Dating Methods

• Dendrochronology
• Science of comparing annual growth rings in trees
  to date events and environmental changes

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Other Dating Methods

• Key Beds
• Sediment layers that serve as a time marker in
  the rock record, result from volcanic ash or
  meteorite impact debris
• Index Fossils
• Remains of living things that are
• Easily recognized
• Abundant
• Short-lived

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Relative Dating

1. What principle states that sediments are
   deposited in horizontal layers?
2. Which layer is the oldest in this rock sequence?
   The Youngest?
3. What principle helped you answer questions 2?
4. Is the fault line in this rock sequence older or
   younger than layer D?