Geologic Time

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Geologic Time
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How Geologists Think about Time

• The big word Uniformitarianism
• Simply put If the geologic processes we
  observe today are representative of those that
  occurred in the past, then we can make important
  inferences about the past by observing Earth
  processes today.
• Even more simply put the present is the key to
  the past.

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Examples of Uniformitarian Inferences

• Sediment movement and deposition rates (now
  observed at mm/yr)
• So 1000 m of sedimentary rock thickness could
  represent 1 million years of deposition
• Uplift rates (mm/yr)
• Erosion rates (mm/yr)
• Plate speeds (cm/yr)

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Relative Age Inferences

• Assumptions / Principles
• Sediments deposited horizontally
• Younger sediments on top of older
• Units that cross-cut (e.g. faults or intrusions)
  came after (i.e., are younger than) those that
  they cut
• Units that include bits of another came later
  (are younger)

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Deduction, my dear Watson

• Geologists unravel complex stories using these
  simple principles

Example
Original Horizontality

• If layers are inclined at an angle, then
  something tilted them - they didnt form that way

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Original horizontality
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Correlation
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Relative Time Scale

• Worldwide changes in fossils give break points

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Fossils

• Single-celled organisms range?

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Fossils

• Clams (Mollusks) range?

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Fossils

• Dinosaur range?

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Absolute Age Determination

• Many methods have been devised to measure the
  Earths age
• Thickness of sedimentary rocks
• Cooling of the planet (assumed molten initially
  with no other heat source)
• Saltiness of the ocean
• Radioactivity proved to be the best

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Absolute Geologic Time Scale
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Absolute Geologic Time ScalePlate tectonics
animation
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The Divisions of Precambrian Time