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GEOL: CHAPTER 2

GEOL: CHAPTER 2. Plate Tectonics: A Unifying Theory. Learning Outcomes. LO1: Review early ideas about continental drift LO2: Explain the evidence for continental drift LO3: Describe Earth’s magnetic field LO4: Explain paleomagnetism and polar wandering

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GEOL: CHAPTER 2

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  1. GEOL: CHAPTER 2 Plate Tectonics: A Unifying Theory

  2. Learning Outcomes LO1: Review early ideas about continental drift LO2: Explain the evidence for continental drift LO3: Describe Earth’s magnetic field LO4: Explain paleomagnetism and polar wandering LO5: Explain magnetic reversals and seafloor spreading

  3. Learning Outcomes, cont. LO6: Explain plate tectonic theory: a unifying theory LO7: Identify the three types of plate boundaries LO8: Describe hot spots and mantle plumes LO9: Explain plate movement and motion

  4. Learning Outcomes, cont LO10: Explain the driving mechanism of plate tectonics LO11: Recognize the role of plate tectonics in the distribution natural resources LO12: Recognize the role of plate tectonics in the distribution of life

  5. Continental Drift: Early Ideas • Alfred Suess, late 19th C, noted similarities on different continents of: • Plant fossils • Glaciation • Rock sequences • Gondwanaland as supercontinent

  6. Alfred Wegener • Continental drift hypothesis, 1915 • Pangaea break-up • Presented geologic, paleomagnetic, paleontologic, and climatologic evidence • Not well received • Alexander du Toit, 1937: proposed supercontinent originally at South Pole

  7. Continental Drift Evidence • Continental fit • South America and Africa • Similarity of rock sequences and mountain ranges • Appalachian Mountains and mountains in Europe/Greenland

  8. Continental Drift Evidence, cont. • Glacial evidence • Deposits and striations • Fossil evidence • Same fossils on all continents • Paleomagnetic evidence • But no explanatory mechanism

  9. Earth’s Magnetic Field • Magnetism • Spinning electrons in atoms, especially iron • Moving electricity • Magnetic field • Lines of magnetic force • North and south poles: dipolar

  10. Earth’s Magnetic Field, cont. • Earth as giant dipole magnet • Magnetic field strength varies • Magnetic poles do not align with geographic poles • Magnetic pole positions vary over time

  11. Paleomagnetism and Polar Wandering • Paleomagnetism: remnant magnetism in rocks – gives direction and intensity of Earth’s field when rocks solidified • Curie point: temperature where iron-bearing minerals are magnetized • Polar wandering: continents moved while poles stayed relatively stationary • Continental drift explained the data

  12. Magnetic Reversals • Magnetic reversals • Magnetic field weakens and disappears • Field reappears with pole positions reversed • Discovered from studying paleomagnetism

  13. South magnetic pole (normal position) North magnetic pole (normal position) North magnetic pole (reversed) South magnetic pole (reversed) South magnetic pole (normal) North magnetic pole (normal) North magnetic pole (reversed) South magnetic pole (reversed) Stepped Art Fig. 2-9, p. 31

  14. Seafloor Spreading • Explains movement of continents • Harry Hess, 1962 • Continents and oceanic crust on same plate • Thermal convection cells move plates

  15. Seafloor Spreading, cont. • Magnetic anomalies • magnetic reversals explained • Plates form at mid-ocean ridges at divergent boundaries • Plates are destroyed at deep-sea trenches at convergent boundaries

  16. Major Tectonic Plates • Eurasian • Indian-Australian • Antarctic • North American • South American • Pacific • African

  17. Plate Tectonics • Seven major plates • Many smaller plates • Contain both continental and oceanic crust • Vary in thickness • Continent areas thicker than ocean areas

  18. Plate Tectonics, cont. • Heat transfer in asthenosphere causes plates to move • Separate at oceanic ridges and at some land masses – divergent boundaries • Collision and often subduction at convergent boundaries

  19. Plate Boundaries: 3 Types • Divergent plate boundaries • Convergent plate boundaries • Transform plate boundaries

  20. Divergent Boundaries • Mantle magma rises to surface • New oceanic lithosphere forms: basaltic • Magnetic orientation recorded as magma cools • Oceanic ridges most common: seafloor spreading

  21. Divergent Boundaries, cont. • Continental rifting • East African Rift Valley • Red Sea • Can create new ocean basins

  22. Volcanic activity Magma Continental crust Rift valley Narrow fault-bounded sea Coastal mountain range Continental “seaboard” (coastal mountains gone) Wide ocean Stepped Art Fig. 2-12, p. 35

  23. Convergent Boundaries • Oceanic-oceanic boundaries • Oceanic-continental boundaries • Continental-continental boundaries • Benioff zone: dipping plane of earthquake foci

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