Plate Tectonics and the Ocean Floor

1. Plate Tectonics and the Ocean Floor

2. Continental Drift • Theory proposed by Alfred Wegener in 1912 • Continents once formed a single landmass

3. Early Evidence

4. Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov

5. Paleomagnetism

6. Plate Tectonics – 1960’s • Explains HOW the plates moved

7. The Crust • Continental Crust • thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km)- dense (sinks under continental crust)- young

8. Plate Movement • “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

9. Three types of plate boundary • Divergent • Convergent • Transform

10. Divergent Boundaries • Spreading ridges • As plates move apart new material is erupted to fill the gap

11. Mid-Ocean Ridge • Underwater mountain ranges • Due to plates pulling away from each other • A Rift Valley forms in the middle where magma comes out • Have many fracture zones which break the ridge up

12. Black smoker at a mid-ocean ridge hydrothermal vent in the Atlantic.

13. Close up at the mid-ocean ridge

14. Iceland: An example of continental rifting • Iceland has a divergent plate boundary running through its middle

15. Convergent Boundaries • There are three styles of convergent plate boundaries • Continent-continent collision • Continent-oceanic crust collision • Ocean-ocean collision

16. Continent-Continent Collision • Forms mountains,e.g. European Alps, Himalayas

17. Himalayas

18. Continent-Oceanic Crust Collision • Called SUBDUCTION TRENCH

19. Subduction • Oceanic lithosphere subducts underneath the continental lithosphere • Oceanic lithosphere heats and dehydrates as it subsides • The melt rises forming volcanism • E.g. The Andes

20. Trenches • Due to one plate subducting (going below) another plate • Earthquakes • Many volcanoes and volcanic island arcs form here

21. Ocean-Ocean Plate Collision • When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. • The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. • The worlds deepest parts of the ocean are found along trenches. • E.g. The Mariana Trench is 11 km deep!

23. Transform Boundaries • Where plates slide past each other Above: View of the San Andreas transform fault

25. Pacific Ring of Fire Volcanism is mostly focused at plate margins

26. Other Ocean Features

27. Continental Shelf • Part of a continent covered by water • Gentle slope • Average depth is 60 m • Amount exposed changes with sea level

28. Continental Slope • Steep • May be cut by submarine canyons • Sediment piles up at the bottom and forms continental rise

29. Abyssal Plains • Flattest area on earth • Cover ½ of deep ocean • Covered with fine sediment

30. Seamounts • Submerged volcanic mountains • Called oceanic island if they rise above water

31. Guyot • Seamounts that have been eroded and are now flat on top