Chapter 17 Plate Tectonics Drifting Continents Early Observations
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 1960 s, when these flaws were resolved
Seafloor Spreading • Technology advances in the 1940 s and 1950 s 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-ocean
Types of Plate Boundaries: Transform • Plates slide horizontally past each other
What Causes Plates to Move?
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