3. Continental Drift Theory Flaws 1 st What force could push large mass over great distances 2 nd How the continents could move were through something solid like earth’s crust They questions were the reasons why Wegener’s theory was rejected until the 1950s

4. Seafloor Spreading Until 1900’s most people believed – that the oceans floors were flat – Ocean floor was not changing – Ocean floor was older than continental crust War and Technological advancement changed all misconceptions

5. WWII Sub Warfare Sonar tech was improved in order to create detailed maps and way to navigate Magnetometers: device that detect small changes in magnetic fields, towed behind boats to locate enemy subs Data from these sources revealed vast underwater mountain chains called ocean ridges and deep sea trenches Text p 474 figure 17.8

6. Ocean Rocks & Sediment Rock Analysis The age of the rocks varies and the variations were predictable Samples near the ocean ridges were younger than the samples taken near the trenches..

7. Ocean Rocks & Sediment Sediment Analysis Thin layers of sediment compared to continents Thickness of sediment increased with distance from an ocean ridge

8. Seafloor Spreading Theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep sea trenches Vital key that supports Wegener’s continential drift theory

9. In Class Activity Turn to p 477 figure 17.13 P. 478 read Visualizing Seafloor Spreading

10. ANY QUESTIONS?