Evidence for Continental Drift

1. Evidence for Continental Drift • Jigsaw Puzzle fit of continents Alfred Wegener during Greenland expedition

2. More evidence • Matching fossils on continents now located thousands of miles apart. Example = Mesosaurus, a freshwater reptile • Many others

3. Matching mountain ranges

4. More evidence • Matching geologic structures including: • Mountain chains • Ore deposits • Same rocks of same age

5. More evidence • Climate change evidence • Glacial deposits at current equator • Fossilized palm trees in Greenland • Map shows why according to the placements of current continents within Pangaea

7. Wegener not believed • Why? - • What could possibly force the continents to move across the ocean floor in this way. They would be crushed. • He was a meteorologist, not a geologist

8. Developments 1950s and 1960s • World War II submarines using sonar found mountains under the oceans – the mid-ocean ridges • Sea floor drilling showed rocks younger than expected and youngest towards the center of the mid-ocean ridge • Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess

9. Seafloor spreading First look at the earth’s layers as shown here.

10. Evidence for sea floor spreading • Alvin and other submersibles found cracks and lava showing spreading and volcanism at mid ocean ridges and odd life forms • Matthews and Vine’s survey of the Indian Ocean sea floor showed matching stripes of reversing polarities on either side of ridge – what caused these? • Lava spreading during alternating magnetic periods

11. The Earth’s magnetic field • It is produced by the outer core of the earth which is made of liquid iron and nickel • This moving magnetic material produces a moving magnetic field, which in turn produces a moving electric field. It is a dynamo! • Earth’s magnetic field varies over time and it protects us from cosmic radiation

12. Seafloor spreading and paleomagnetism

13. Pangaea revisited • By piecing together this information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading

14. Plate tectonics • Sea floor spreading provides the driving mechanism for movement • However, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere • The lithosphere is made up of about 20 plates which move relative to each other in several ways • Let’s look at a generalized sketch

15. Confirmation of the Theory of Plate Tectonics Evidence supporting the theory of plate tectonics: Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted. The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.

16. Confirmation of Plate Tectonics Theory Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

17. The Plates

19. Sea-Floor Spreading and Plate Boundaries

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

21. Mid Atlantic Ridge Mid Atlantic Ridge

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

23. Converging Margins: India-Asia Collision

24. Transform Faults and Seafloor Spreading

25. What causes plate tectonics? • Convection in the mantle, as the plastic asthenosphere flows, carrying the plates with it. • This is probably aided by slab pull at subduction zones and ridge push at mid ocean ridges and rising plumes in the mantle • This diagram shows several different model hypotheses

26. “Plates” of lithosphere are moved around by the underlying hot mantle convection cells