Plate Tectonics

1. Plate Tectonics Or Do the continents move because they don’t like their neighbors?

2. 19th century geologists wondered about the origin of mountains. • How were they formed? • What process folded rocks like putty? • What made the Earth’s surface move?

3. Contraction Theory Edward Suess (1831-1914) • Thought that the Earth has been cooling since its formation. • And as the planet contracted its surface wrinkled like a drying apple.

4. Or maybe prunes. • Suess believed that: • Continents over time would collapse and become ocean basins. • Ocean basins would then be higher and become continents. • This would explain marine fossils on dry land.

5. The Theory of Continental Drift • In 1915, Alfred Wegener, a German meteorologist and geophysicist, published The Origin of Continents and Oceans. • This book set forth the hypothesis of Continental Drift.

6. Pangaea - “All Land” • Wegener believed that 200 million years ago all the present-day continents combined to form a single super continent called Pangaea.

7. The Evidence • If the continents were cut out like a jig-saw puzzle, which ones would you match up first?

8. Since the 16th century, scientists have puzzled over the fit of South America and Africa.

10. Evidence: Jigsaw Puzzle Match • Wegener also noticed the fit. • Challenged by other scientists citing coastal erosion. • In fact at a depth of 900 meters at the edge of the continental shelf, the continents match perfectly.

11. Evidence: Fossil Correlation • Wegener cited several documented cases where fossils were found on different landmasses but that could not have crossed present day oceans. • Classic example Mesosaurus.

12. Evidence: Rock Type and Structure Match • Several mountain belts of similar rock structure terminate at a coastline only to reappear on a land mass across the ocean. • Example: Appalachians and Caledonian Mts.

13. Evidence: Ancient Climates • Evidence of glaciation in areas presently in subtropical climates.

14. Layers of glacial till and scratches found in southern Africa and South America, also in India and Australia.

15. Wegener’s Theory Rejected • The theory was widely discussed in the 1920’s and 30’s. • Hotly rejected, especially by geologists in the U.S. who labeled it bad science. • Reason: Wegener could not give clear causal mechanism.

16. Earth’s Internal Heat • As Earth’s internal heat became better understood, new theories arose. • Arthur Holmes (1890-1965)believed the area under the continents could be plastic and ductile due to heat. • If it was, then continents could move with it.

17. Upper Mantle Convection • Holmes believed that continental drift was driven by mantle convection currents. • At mid-ocean ridges new crust is being made and at deep ocean trenches old crust pushed back into mantle. • Continents dragged along conveyor belt style.

18. More Evidence

19. Sea floor spreading • Mid-ocean ridges were not known until about 55 years ago because they are under water. • Rift valleys in the middle of long mountain ranges. • Upwelling of lava forms the igneous rock basalt

20. Mid-Atlantic Ridge

21. Age of Sea Floor • The sea floor (oceanic crust) is younger close to mid-ocean ridges and much older farther away. Red – young Dark Blue - old

22. Magnetic Reversals • Atoms of iron in molten lava arrange themselves in the direction of Earth’s magnetic field. • When the lava hardens the iron atoms are locked in place. • It was found that at times they pointed north and other times pointed south. • The magnetic bands on one side of the mid-ocean ridge are a mirror image of the bands on the other side.

23. Wegener’s Theory Vindicated

24. “I’ve heard of continental drift, but this is ridiculous.”