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Chapter 17 Plate Tectonics Drifting Continents: Early Observations • Cartographers were the first people to notice matching continents Antonio Snider-Pelligrini (1858) Alfred Wegener • German meteorologist/climatologist • 1912: proposed the idea of continental drift – Continents started as “Pangea” 200 million years ago – Slowly moving apart to present positions Evidence for Continental Drift • Matching coastlines • Matching rock formations/mineral deposits separated by oceans • Climate evidence: fossil ferns Glossopteris) found in Antarctica, South America, and India • Matching fossils on widely separated continents (Cynognathus, Lystrasaurus, Mesosaurus) Fossil Evidence Flaws of Continental Drift • Did not explain the immense forces required to move continents long distances • No evidence of continents “plowing through” the ocean crust • Continental Drift was rejected until the early 1960s, when these flaws were resolved Seafloor Spreading • Technology advances in the 1940s and 1950s led to more detailed ocean floor mapping • Magnetometer: detects changes in magnetic fields • Sonar: found mid-ocean ridges and deep sea trenches • Interesting patterns in age of rock samples Magnetic Patterns and Seafloor Spreading Age of Ocean Crust and Seafloor Spreading Sediment Depth and Seafloor Spreading • Evidence for seafloor spreading corrected a flaw of Wegener’s model of continental drift • Continents ride along while ocean crust moves away from mid ocean ridges Types of Plate Boundaries: Divergent • Plates moving apart • Mid-ocean ridge • On land: rift valley Types of Plate Boundaries: Convergent • Continent-continent • Ocean-continent • Ocean-ocean Types of Plate Boundaries: Transform • Plates slide horizontally past each other What Causes Plates to Move?
