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Relative & Absolute Dating

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Relative & Absolute Dating

  1. 1. Geologic Time and Earth History
  2. 2. Two Conceptions of Earth History: <ul><li>Catastrophism </li></ul><ul><li>Assumption: Great Effects Require Great Causes </li></ul><ul><li>Earth History Dominated by Violent Events </li></ul><ul><li>Uniformitarianism </li></ul><ul><li>Assumption: We Can Use Cause And Effect to Determine Causes of Past Events </li></ul><ul><li>Finding: Earth History Dominated by Small-scale Events Typical of the Present. </li></ul><ul><li>Catastrophes Do Happen But Are Uncommon </li></ul>
  3. 3. Uniformitarianism <ul><li>Continuity of Cause and Effect </li></ul><ul><li>Apply Cause and Effect to Future - Prediction </li></ul><ul><li>Apply Cause and Effect to Present - Technology </li></ul><ul><li>Apply Cause and Effect to Past – Uniformitarianism </li></ul><ul><li>The Present is the Key to the Past </li></ul>
  4. 4. Ripple Marks, Bay Beach
  5. 5. Fossil Ripple Marks, Baraboo Range
  6. 6. Modern Mud Cracks
  7. 7. Fossil Mud Cracks, Virginia
  8. 8. Two Kinds of Ages <ul><li>Relative - Know Order But Not Dates </li></ul><ul><li>Civil War Happened Before W.W.II </li></ul><ul><li>Bedrock in Wisconsin Formed Before The Glaciers Came </li></ul><ul><li>Absolute - Know Dates </li></ul><ul><li>Civil War 1861-1865 </li></ul><ul><li>World War II 1939-1945 </li></ul><ul><li>Glaciers Left Wisconsin About 11,000 Yrs Ago </li></ul>
  9. 9. Superposition: Mindoro Cut, Wisconsin
  10. 10. Geologic Map
  11. 11. Fossils Remains of Ancient Plants And Animals, Evidence of Life
  12. 12. Commonly Preserved: <ul><li>Hard Parts of Organisms: </li></ul><ul><li>Bones </li></ul><ul><li>Shells </li></ul><ul><li>Hard Parts of Insects </li></ul><ul><li>Woody Material </li></ul>
  13. 13. Rarely Preserved <ul><li>Soft or Easily Decayed Parts of Organisms: </li></ul><ul><li>Internal Organs </li></ul><ul><li>Skin </li></ul><ul><li>Hair </li></ul><ul><li>Feathers </li></ul>
  14. 14. Types of Fossils <ul><li>Original Material </li></ul><ul><li>Casts & Molds </li></ul><ul><li>Replacement (Petrified Wood) </li></ul><ul><li>Carbonized Films (Leaves) </li></ul><ul><li>Footprints, Tracks, Etc. </li></ul><ul><ul><li>“ Trace Fossils” – Our only preserved record of behavior of fossil organisms </li></ul></ul>
  15. 15. Dinosaur Tracks, Texas
  16. 16. Where Fossils Occur <ul><li>Almost Exclusively in Sedimentary Rocks </li></ul><ul><li>Heat of Melting or Metamorphism Would Destroy Almost Every Type of Fossil </li></ul><ul><li>Rare Exceptions : </li></ul><ul><ul><li>Some Fossils in Low-grade Metamorphic Rocks </li></ul></ul><ul><ul><li>Trees Buried by Lava Flow </li></ul></ul><ul><li>To Be Preserved, Organisms Have to Be: </li></ul><ul><li>Buried Rapidly After Death </li></ul><ul><li>Preserved From Decay </li></ul>
  17. 17. Fossil Tree in Lava Flow, Hawaii
  18. 18. Some Geologic Rates <ul><li>Cutting of Grand Canyon </li></ul><ul><li>2 km/3 m.y. = 1 cm/15 yr </li></ul><ul><li>Uplift of Alps </li></ul><ul><li>5 km/10 m.y. = 1 cm/20 yr. </li></ul><ul><li>Opening of Atlantic </li></ul><ul><li>5000 km/180 m.y. = 2.8 cm/yr. </li></ul><ul><li>Uplift of White Mtns. (N.H.) Granites </li></ul><ul><li>8 km/150 m.y. = 1 cm/190 yr. </li></ul>
  19. 19. Some Geologic Rates <ul><li>Movement of San Andreas Fault </li></ul><ul><li>5 cm/yr = 7 m/140 yr. </li></ul><ul><li>Growth of Mt. St. Helens </li></ul><ul><li>3 km/30,000 yr = 10 cm/yr. </li></ul>
  20. 20. The Fundamental Rule of Absolute Ages The Earth is older than everything on or in it -Except its atoms -All ages are minimum ages
  21. 21. Radiometric Dating: Half-Life
  22. 23. <ul><li>If I had a sharks tooth that I estimate to be 15,000 years old based on the sediments I found it in… what could I use to determine the absolute age? U 238 or C 14 ? </li></ul><ul><li>(*go back one slide to check your answer) </li></ul>
  23. 24. <ul><li>If I had a sharks tooth that I estimate to be 15,000 years old based on the sediments I found it in… what could I use to determine the absolute age? U 238 or C 14 ? </li></ul><ul><li>Carbon-14 Dating </li></ul><ul><li>(*go back one slide to check your answer) </li></ul>

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