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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. 11 - 1 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 11 - 2 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. 11 - 3 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 11 - 4 Ridges and Trenches 11 - 5 Seafloor Spreading Seafloor constantly being created and destroyed New crust at rift valley Crust destroyed at trenches 11 - 6 Evidence for Seafloor Spreading Thin sediment in ridges Age of rock at ridges much younger than continents Patterns of magnetic polarity reversal 11 - 7 Seafloor Age 11 - 8 Types of Movement Divergent – plates moving apart (rift valleys in MOR) Convergent – plates coming together Transform – plates slide past each other 11 - 9 Island Arcs Convergent ocean plates Subduction of denser plate 11 - 10 Subduction Zones Ocean plate into continent Trench, mountains, volcanoes formed 11 - 11 Convergent Continents Mountains form 11 - 12 Hot Spots Magma chambers in mantle push through surface of crust Crust moves over mantle forming island chain 11 - 13 Plate Movement 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. Chapter 11 Pages 11-26 & 11-27 The Unifying Theory: Plate Tectonics Convection is the primary force driving seafloor spreading and continental drift. 11 - 14 As new seafloor forms, the plates tend to slide away from the elevated mid-ocean ridges. Plate Movement (continued) Chapter 11 Pages 11-28 & 11-29 The Unifying Theory: Plate Tectonics Predicted changes over the next 50 million years: 11 - 15 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.