Alfred Wegener and Pangaea

1. Alfred Wegener and Pangaea • Pangaea is the name given to the single giant continent in a proposal by Wegener (1912). • The theory of continental drift states that the continents were once a single landmass that drifted apart and are still doing so.

2. Evidence for Continental Drift • Wegener tried to support his theory with evidence: • Maps - continents apparently fit together like a jigsaw-puzzle • Plant and animal fossils - coal in Antarctica

3. Evidence for Continental Drift • Why was his theory not accepted? • No explanation for how drift occurs • He was a meteorologist, not a geologist! • Plate tectonics theory - explanation for how continents move, making Wegener’s theory widely accepted.

4. New Technology and Seafloor Knowledge • Sonar – 1925 first seafloor map of ocean ridges • New features discovered • Mid-ocean ridges – mountain ridges on ocean floor • Rift valleys – valley in center of MOR • Trenches • Scientists observed patterns in features

5. Ridges and Trenches

6. Seafloor Spreading • Seafloor constantly being created and destroyed • New crust at rift valley • Crust destroyed at trenches

7. Evidence for Seafloor Spreading • Thin sediment in ridges • Age of rock at ridges much younger than continents • Patterns of magnetic polarity reversal

8. Seafloor Age

9. Types of Movement • Divergent – plates moving apart (rift valleys in MOR) • Convergent – plates coming together • Transform – plates slide past each other

10. Island Arcs • Convergent ocean plates • Subduction of denser plate

11. Subduction Zones • Ocean plate into continent • Trench, mountains, volcanoes formed

12. Convergent Continents • Mountains form

13. Hot Spots • Magma chambers in mantle push through surface of crust • Crust moves over mantle forming island chain

14. Plate Movement • Convection is the primary force driving seafloor spreading andcontinental drift. • Convection currents form as hot material rises and cold material sinks. This occurs in the mantle. • This creates a current that moves the plates away from each other at the divergent boundaries, toward each other at the convergent boundaries, and past each other at the transform boundaries. • A second driving force comes from seafloor spreading. • As new seafloor forms, the platestend to slide away fromthe elevated mid-ocean ridges. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-26 & 11-27

15. Predicted changes over the next 50 million years: The Baja Peninsula will have moved past and apart from the North American Plate. Southern California will pass San Francisco as it moves to the northwest. A new sea will form in eastern Africa. Australia will move toward Eurasia/the Equator. The Mediterranean Sea will close as Africa pushes towards Europe. The Atlantic and Indian Oceans will continue to grow while the Pacific will become smaller. Plate Movement (continued) The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-28 & 11-29