1) Alfred Wegener first proposed the theory of continental drift in the early 20th century, suggesting that the continents were once joined together in a supercontinent called Pangaea before drifting apart. 2) Wegener provided geological, fossil, and climate evidence to support his theory, but his ideas were rejected by many scientists because he could not explain the force driving the continental movement. 3) In the 1960s, the theory of plate tectonics was developed, explaining that convection currents in the mantle cause the plates to move and providing the mechanism that Wegener could not.

1. Continental Drift and Plate Tectonics

4. Wegener searched and found three main pieces of evidence. Geologic - evidence in the layers of rocks across continents Fossil - evidence in the places certain fossils are found Climate - evidence in the changing climates during the past

5. Fit of Continents Across the Atlantic Mountain ranges in South America line up exactly with those in Africa! Geologic evidence

6. Fossil evidence Notice how fossils lined up across continents! http://evolution.berkeley.edu/evolibrary/images/history/fossils3.gif

7. Amazing Facts: Did you know... ...that India was once in the Southern Hemisphere connected to Antarctica? ...that North America was once surrounded by warm, tropical seas? ...that Africa was once covered by glaciers, which were kilometers in thickness? ...that the Sahara desert was once a tropical rain forest? Climate change

8. As North America has drifted to different latitudes, changing climates were experienced. This shows the changes in the state of New Mexico. An example of the proof for changing climate: At one time, the area that is New Mexico was below the equator!

9. Wegener's problem He could not find the force that was causing the continents to drift. Because of this, he could not convince anyone that continents could move. He died in Greenland on an expedition. At the time of his death, no one believed his hypothesis! Technology developed during the 1940’s changed all that!

10. Discovery of Sea-Floor Spreading Sea floor spreading video clip

11. As the sea floor spreads, the lava cools according to the magnetic poles at the time. The rocks on the ocean floor have proved that the earth’s magnetic field sometimes reverses. The inner core flips and so the north pole moves to the southern hemisphere! The earth itself does not flip. Amazing proof of that the inner core flips! http://platetectonics.pwnet.org/img/blocks.gif

12. What causes sea floor spreading? Convection currents!

13. Can you explain this diagram? Click here for a hyperlink to an animation of convection Be sure to click the play button!

14. Animation of sea floor spreading Can you explain this diagram!?

16. This is a model of sea floor spreading at a divergent boundary called a mid ocean ridge .

17. Did you know that the Earth’s longest mountain range is underwater and is called the mid-ocean ridge? : www.ocean.udel.edu The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. It circles the earth like the stitching on a baseball!

20. There are three conbinations of how earth's crust can come together. Continental crust to continental Continental crust to oceanic Oceanic crust to oceanic

21. from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270 Continental crust to continental crust collision India-Asia (Himalayas) Before collision After collision

22. This is how the Himalaya Mountains formed Also the way the Appalachian Mountains formed!

23. Because one plate gets pushed under another, it is called subduction . This is where volcanoes form! Oceanic crust colliding with oceanic crust Oceanic crust colliding with continental crust all from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

24. The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle is called subduction

25. Plate tectonics video clip Here is an great link for seeing where each type of boundary is located. Click here for a great hyperlink! Make sure you click on each type of boundary in the key!

26. Worldwide Earthquake patterns help us know where the edges of plates are located

27. The Earth’s Plates

28. Here you see the two slides together. What do you see?

29. http://www.livescience.com/images/pangea_animation_03.gif