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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. Uniformitarianism Continuity of Cause and Effect • Apply Cause and Effect to Future - Prediction • Apply Cause and Effect to Present - Technology • Apply Cause and Effect to Past – Uniformitarianism The Present is the Key to the Past

  4. Ripple Marks, Bay Beach

  5. Fossil Ripple Marks, Baraboo Range

  6. Modern Mud Cracks

  7. Fossil Mud Cracks, Virginia

  8. 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

  9. Superposition:Mindoro Cut, Wisconsin

  10. Geologic Map

  11. Fossils Remains of Ancient Plants And Animals, Evidence of Life

  12. Commonly Preserved: Hard Parts of Organisms: • Bones • Shells • Hard Parts of Insects • Woody Material

  13. Rarely Preserved Soft or Easily Decayed Parts of Organisms: • Internal Organs • Skin • Hair • Feathers

  14. Types of Fossils • Original Material • Casts & Molds • Replacement (Petrified Wood) • Carbonized Films (Leaves) • Footprints, Tracks, Etc. • “Trace Fossils” – Our only preserved record of behavior of fossil organisms

  15. Dinosaur Tracks, Texas

  16. Rubbing Rock? Wisconsin

  17. Rubbing Rock? California

  18. Pseudofossils Look Like Fossils But Aren't • Dendrites • Concretions

  19. Pseudofossils

  20. Natural or Sculpture?

  21. Johannes Beringer’s “Fossils”

  22. Beringer’s Book

  23. Where Fossils Occur Almost Exclusively in Sedimentary Rocks • Heat of Melting or Metamorphism Would Destroy Almost Every Type of Fossil • Rare Exceptions: • Some Fossils in Low-grade Metamorphic Rocks • Trees Buried by Lava Flow To Be Preserved, Organisms Have to Be: • Buried Rapidly After Death • Preserved From Decay

  24. Fossil Tree in Lava Flow, Hawaii

  25. Good Index Fossils • Abundant • Widely-distributed (Global Preferred) • Short-lived or Rapidly Changing

  26. Correlation

  27. The Geologic Time Scale

  28. 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

  29. Absolute Ages: Early Attempts Salt in Ocean • If we know rate salt is added, and how much salt is in ocean, can find age of oceans. Sediment Thickness • Add up thickest sediments for each period, estimate rate. Both methods gave age of about 100 million years • Problem: Rates Variable

  30. 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.

  31. The Fundamental Rule of Absolute Ages The Earth is older than everything on or in it -Except its atoms -All ages are minimum ages

  32. Radiometric Dating: Half-Life

  33. Present Radiometric Dating Methods Cosmogenic • C-14 5700 Yr. Primordial • K-Ar (K-40) 1.25 B.Y. • Rb-Sr (Rb-87) 48.8 by • U-235 704 M.Y.

  34. The Geologic Time Scale

  35. Some Geologic Rates Cutting of Grand Canyon • 2 km/3 m.y. = 1 cm/15 yr Uplift of Alps • 5 km/10 m.y. = 1 cm/20 yr. Opening of Atlantic • 5000 km/180 m.y. = 2.8 cm/yr. Uplift of White Mtns. (N.H.) Granites • 8 km/150 m.y. = 1 cm/190 yr.

  36. Some Geologic Rates Movement of San Andreas Fault • 5 cm/yr = 7 m/140 yr. Growth of Mt. St. Helens • 3 km/30,000 yr = 10 cm/yr. Deposition of Niagara Dolomite • 100 m/ 1 m.y.? = 1 cm/100 yr.

  37. 1 Second = 1 Year • 35 minutes to birth of Christ • 1 hour+ to pyramids • 3 hours to retreat of glaciers from Wisconsin • 12 days = 1 million years • 2 years to extinction of dinosaurs • 14 years to age of Niagara Escarpment • 31 years = 1 billion years

  38. Were The Dinosaurs Failures? Dinosaurs: 150,000,000 years Recorded History: 5000 years • For every year of recorded history, the dinosaurs had 30,000 years • For every day of recorded history, the dinosaurs had 82 years • For every minute of recorded history, the dinosaurs had three weeks

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