Geologic Time and Earth History

1. Geologic Time and Earth History

2. Two Conceptions of Earth History: Catastrophism • Assumption: Great Effects Require Great Causes • Earth’s History is Dominated by Violent Events Uniformitarianism • Assumption: We Can Use Cause And Effect to Determine Causes of Past Events • Finding: Earth’s 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 Dating Relative Dating- We Know the Order of Events But Not Dates • Civil War Happened Before W.W.II • Bedrock in Wisconsin Formed Before The Glaciers Came Absolute Dating – We Know the “Dates” • Civil War 1861-1865 • World War II 1939-1945 • Glaciers left about 11,000 Years Ago

9. Steno’s 3 Principles • Nicholas Steno (1600s) proposed some ideas on how we should look at sedimentary rocks. These are known as Steno’s 3 principles. • Law of Superposition – If sediments are not disturbed then the oldest sediments are at the bottom and the youngest ones are on top.

10. Superposition:Mindoro Cut, Wisconsin

11. Steno’s Principles continued • Principle of Original Horizontality – Sediments are originally laid down flat (horizontal).

12. Steno - example • Original Horizontality: • Layers of sediment in Whitewood Creek, South Dakota

13. Steno’s Principles continued • Principle of Cross-Cutting Relationships – • If a feature cuts across (or through) another rock layer, it is younger than the layer it cuts across.

14. Steno - example • Cross-Cutting Relationships: • Fault in Arizona

15. Geologic Map

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

17. Commonly Preserved: Hard Parts of Organisms: • Bones • Shells • Hard Parts of Insects • Woody Material: Bark, Limbs, Leaves

18. Rarely Preserved Soft or Easily Decayed Parts of Organisms: • Internal Organs • Skin • Hair • Feathers Jellyfish fossils are VERY rare – too soft.

19. 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

20. Dinosaur Tracks, Texas

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

22. Pseudofossils

23. Where Fossils Occur Almost always 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 (or they get eaten) • Preserved From Decay • Have hard parts: bones, teeth, scales, etc.

24. Fossil Tree in Lava Flow, Hawaii

25. Good Index Fossils • Abundant (common) • Widely-distributed (Global Preferred) • Short-lived or Rapidly Changing to a new form

26. Correlation: Matching Rocks Up

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 (1584)– Earth created at 9:00 a.m. Oct 29, 4004 B.C. • (Too Short a time for geologic processes)

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 – they change over time

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-about 11 billion actually: It has 5.5 billion years left.

31. Radiometric Dating: Half-Life

32. Present Radiometric Dating Methods For ‘Recent’ dates ONLY • C-14 5728 Yr. For Ancient dates • K-Ar (K-40) 1.25 B.Y. • Rb-Sr (Rb-87) 48.8 by • U-235 704 M.Y.

33. The Geologic Time Scale

34. Some Geologic Rates Cutting of Grand Canyon • 2 km/3 m.y. = 1 cm/15 yr = 0.028in./yr. Uplift of Alps • 5 km/10 m.y. = 1 cm/20 yr. = 0.02in./yr. Opening of Atlantic • 5000 km/180 m.y. = 2.8 cm/yr. Movement of San Andreas Fault • 1m/20yrs. = 5cm/yr. = 2in/yr.

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

36. Were The Dinosaurs Failures? Dinosaurs: 150,000,000 years Recorded Human History: 5000 years • For every year of our 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