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Geologic Time and Earth History . Two Conceptions of Earth History: . Catastrophism Assumption: Great Effects Require Great Causes Earth History Dominated by Violent Events Uniformitarianism Assumption: We Can Use Cause And Effect to Determine Causes of Past Events

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two conceptions of earth history
Two Conceptions of Earth History:

Catastrophism

  • Assumption: Great Effects Require Great Causes
  • Earth History Dominated by Violent Events

Uniformitarianism

  • Assumption: We Can Use Cause And Effect to Determine Causes of Past Events
  • Finding: Earth History Dominated by Small-scale Events Typical of the Present.
  • Catastrophes Do Happen But Are Uncommon
two kinds of ages
Two Kinds of Ages

Relative - Know Order of Events But Not Dates

  • Civil War Happened Before W.W.II
  • Bedrock in Wisconsin Formed Before The Glaciers Came

Absolute - Know Dates

  • Civil War 1861-1865
  • World War II 1939-1945
  • Glaciers Left Wisconsin About 11,000 Years Ago
absolute ages early attempts
Absolute Ages: Early Attempts

The Bible

  • Add up Dates in Bible
  • Get an Age of 4000-6000 B.C. For Earth
  • John Lightfoot and Bishop Ussher - 4004 B.C. (1584)
  • Too Short
absolute ages early attempts1
Absolute Ages: Early Attempts

Salt in Ocean

  • Rivers bring dissolved solids to ocean
  • If we know rate salt is added, and how much salt is in ocean, can find age of oceans.
  • Gave age of about 100 million years.
  • Problems
    • Is rate at which salt is added constant?
    • How much salt leaves ocean?
absolute ages early attempts2
Absolute Ages: Early Attempts

Sediment Thickness

  • Add up Thickest sediments for each period
  • Estimate rate of deposition to find age
  • Problem: rates of deposition very variable!
  • Indicated ages of at least 100 million years
age of the sun
Age of The Sun
  • If sun gets its heat from burning or other chemical reactions, could only last 10,000 years or so.
  • Best 19th century guess: sun was slowly contracting.
  • Problem: only 30 million years ago, sun would have extended out to earth's orbit!
  • Geologists wanted more time, but you can't fight the laws of physics...
  • Sun actually gets its energy from nuclear reactions and can keep going for billions of years
  • The Geologists were right after all. Go Team.
how do we measure half lives of billions of years
How Do We Measure Half-Lives of Billions of Years?
  • Number of Decays/Unit Time = k * Number of Atoms
  • k = Decay Constant
  • Leads to N = N0exp(-kt)(This is what the exp button on your calculator is for)
  • Half-Life = 0.693/k
present radiometric dating methods
Present Radiometric Dating Methods

Cosmogenic

  • C-14 5700 Yr.
  • Be-10 2.5 M.Y.

Primordial

  • K-Ar (K-40) 1.25 B.Y.
  • Rb-Sr (Rb-87) 48.8 by
  • U-235 704 M.Y.
  • Th-232 14 B.Y.
  • U-238 4.5 B.Y.

Primordial

  • Nd-Sm (Sm-147-Nd-143) 106 B.Y.
  • Re-187 43 B.Y.
  • Lu-Hf (Lu-176) 36 B.Y
  • Fission