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Geologic Time and Earth History

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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Geologic Time and Earth History

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  1. Geologic Time and Earth History

  2. 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

  3. 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

  4. 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

  5. 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?

  6. 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

  7. 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.

  8. Radiometric Dating: Half-Life

  9. 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

  10. 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

  11. The Geologic Time Scale

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