History of Plate Tectonics

1. History of Plate Tectonics starring Alfred Wegener’s Theory of Continental Drift… & the Theory of Plate Tectonics

2. How to start your timeline Plate Tectonics Timeline Get out a sheet of blank notebook paper Turn it so the holes are at the bottom Add the title “Plate Tectonics Timeline” (Note the red—when you see it, write those words down.) Draw a line along the long side of the paper, from one end to the other

3. Still starting Timeline • Add the 9 hatch marks where shown (they do not have to be exactlyas shown—try to spacethem this way) • You will add the years marked in red as we go along. • To write the info in red, you can write above or below the date—or you can number them #1-9 and write the info on back of your sheet Plate Tectonics Timeline

4. Alfred Wegener 1) In 1912 a German meteorologist, Alfred Wegener, had an idea about continental movement. He noticed that some continents fit together like puzzle pieces. His hypothesis was called Continental Drift.

5. His hypothesis proposed that the continents had once been joined as a single landmass, called PANGAEA. ‘Pangaea’ is a Greek word meaning ‘All the Earth’

6. PANGAEA Wegener believed that Pangaea began to break apart 250 million years ago. The continents have been continuously moving since then to where they are now.

7. Wegener’s Evidence • Rocks – on different continents were the same type! • Fossils – cases of the same animal and plant fossils being found on different continents! • Climate – evidences of climates in the past that were very different from today’s climate at some locations(We will go over this evidence in greater detail on another day.)

8. Was he believed? NO! • Other scientists rejected Alfred Wegener’s hypothesis because hecouldn’t explain what was causing the continents to move (a.k.a. HOW?) • He was a laughingstock in the sciencecommunity at that time. He returned to working as a meteorologist but continued to believe in his theory. Link to funny YouTube video about Alfred Wegener:https://biodeluna.wordpress.com/2014/02/20/wegener-song-by-the-amoeba-people/

9. Note that this is #3, not #2. That is on the next slide. Arthur Holmes • 3) 1929, a geoscientist, Holmes proposed that convection occurred in the mantle. • This convection could push and pull plates (continents) apart or together. • But, again, NO evidence!!!

10. FINDING EVIDENCEThe first way to use technology to map the ocean floor was done in the 2)1920’s & used an early type of Sonar.This way stunk because it didn’t accurately map the floor.

11. Then the 4)Magnetometer was invented in 1955. This could detect small changes in the magnetic field in rocks and map them.

12. So what? • Sailors had known for a long time that the ocean floor was not smooth. They used ropes tied with knots to get a rough estimate of the ocean floor near the coasts. • All of a sudden, with this new technology, an accurate map of mountains in the middle of the Atlantic Ocean floor were revealed…scientists called them a mid-ocean ridge!

13. Seafloor Spreading • Mid-Atlantic Ridge

14. Seafloor Spreading – new ocean crust is formed at ocean ridges and destroyed at deep sea trenches. 5) Robert Dietz, a geophysicist and oceanographer, proposed in 1961 a hypothesis he called “seafloor spreading”. He is the one who came up with that phrase.

15. Harry Hess • 6) In 1962, Harry Hess, a professor of geology and former Navy officer who had mapped ocean floors during WWII while his ship hunted submarines, formally published a similar hypothesis originallyin a 1959 paper– again, seafloor spreading!Both men are credited with this amazing discovery.

16. Drum Matthews & Fred Vine • In Cambridge, England 7)in1963, British Drum Matthews and his research assistant Fred Vinepublished a paper basedon magnetic surveysof the ocean floor that Matthews & and others had done. • Their findings about magnetic variations in ocean crust at mid-ocean ridges supported Hess’ & Dietz’ theory!

17. These new maps showed the different magnetism in different layers of rock. As new crust is created the iron bearing minerals point to where the magnetic pole is at the time of creation. This is called Paleomagnetism

18. Mid-Ocean Ridge • Isochron – line of same aged rock

19. Magnetic Pole Reversals What these results showed • Random reversal of Earth’s Polarity • Average irregular 300,000 year cycles • Last one was over 780,000 years ago = overdue • Unknown as to why • Maybe external, maybe internal…

20. This proved that new sea floor was being created at mid-ocean ridges.Identical magnetism on opposite sides of ridges showed the plates where moving away from each other…

21. Dan McKenzie • In 8) 1966, British scientist Dan McKenzie published a paper that provided a mathematical model for convection currents in the mantle • Now, finally, an explanation for why and how the tectonic plates move!

22. Plate Boundaries and Convection Currents

23. Another Driving Force for Plate Movement? • Recently, scientists have been lookingat the subduction end of the seafloor spreading model • In 9) 1994, Professor Seiya Uyeda of Japan, an expert in plate tectonics, concluded in a speech that the process of “Slab-Pull” may even be a bigger driving force to move tectonic plates than ridge pushing… but what is slab-pull?

24. Slab-Pull, a new theory about what causes tectonic plates move - As the denser ocean plate subducts under the other plate, the weight of the cold, dense slab “pulls” the platebehind it into the mantle. - The pacific plate, with a subduction zone, is destroyed at a faster rate than is the north american plate—which has no subduction zones!

25. Is slab-pull accepted? • This is a relatively new idea, so it is not yet widely accepted by all of the scientific community even though it helps explain things like why the Pacific Ocean is shrinking while the Atlantic Ocean is growing. • There is a lot of controversy among geophysicists about whether the mechanism of slab-pull is strong enough to move enormous tectonic plates. • We will have to wait and see how the debate goes. Some of you may find out in college whether this becomes an accepted theory or is discredited!

26. The End…for now • OK, that’s enough for now. • Next, we’ll further examine the evidence that Alfred Wegener found that supported his theory of continental drift. • Let’s hope we can do it in class. • If not, look for another wonderful flipped video!