1. The Theory of Continental Drift
The Earth’s Crust
The Theory of
Continental Drift
Wednesday, January 02, 2019Wednesday, January 02, 2019

2. Introduction:

3. Pangaea
Alfred Wegener – a German scientist who proposed (in 1915) that the continents were slowly drifting apart (the theory of continental drift)

4. Wegener said that all of the continents were once joined together in a supercontinent (which he called Pangaea) which existed about 200 million years ago.
The continents then broke apart and drifted to where they are today.

5. Wegener’s ideas about continental
drift were controversial
Was not until after Wegener’s death
that his hypothesis was accepted
He proposed that continents plowed
through the ocean foor driven by
the spin of the Earth

6. Evidence 1. Fit of the Continents
The coastlines of Africa and South America looked like they fit together.

7. Wegener then noticed that the other continents looked like they could have fit together too.

8. 2. Fossil Clues
- fossils of the same plants and animals are found today on widely separated continents (including Africa, South America, Antarctica, Australia, and Asia – India)

9. Mesosaurus - fossils have only been found in South America and Africa.
Mesosaurus was a fresh water reptile so it’s difficult to imagine him migrating across the ocean.

10. Lystrosaurus - fossils of this little land reptile were found in Africa, India and Antarctica!
It would have been even harder to imagine Lystrosaurus swimming across an ocean.

11. Cynognathus – fossils of this creature are found in Africa and South America
It’s unlikely Cynognathus could have made it across the ocean either.

12. Glossopteris – fossils of this tropical tree are found in South America, Africa, India, Australia and Antarctica!
That’s one mobile tree!

13. Of course, none of these living things had to move anywhere if Wegener were right.
Fossils of both plants and animals supported his hypothesis of continental drift

14. This animation shows the pre-drift
locations of Glossopteris and Lystrosaurus.

15. Appalachian Mountains
3. Mountain chains - match up on each side of the Atlantic Ocean
Appalachian Mountains
The Appalachian Mountains seem to end in North America…
…but mountains in Europe are the same age.

16. And if the continents are put back to their pre-drift locations the mountains form a continuous chain.

17. 4. Climate Evidence
- evidence of glaciers on the equator and tropical forests in the arctic.

18. We find evidence of glaciers along the equator (the warmest area of the planet)….

19. …and fossils of tropical rain forests in New Brunswick, northern Canada and northern Europe.

20. Again, it doesn’t make much sense unless the continents are moved back to their pre-drift location.

21. Now the glaciers are near the pole and the palm trees are on the equator.

22. In spite of this evidence, the scientific community rejected Wegener’s ideas and he was forgotten for almost fifty years.
(He could not explain how the continents moved)

23. Mapping the Ocean Floor
Seafloor Spreading
Mapping the Ocean Floor
During WW1, German scientists introduced the use of sound waves to detect submarines
Sound waves echo off the ocean bottom
The longer the sound waves take to return to the ship, the deeper the water is

24. Researchers discovered an underwater system of ridges, or mountains, and valleys like those found on the continents

25. In some of these underwater ridges are rather long rift valleys where volcanic eruptions and earthquakes occur
In the Atlantic, Pacific, and other oceans these ridges called the mid-ocean ridges

26. The Seafloor Moves
In the 1960’s Harry Hess suggested a theory called seafloor spreading: a hot, dense material below earth’s crust rises toward the surface at the mid-ocean ridges
Princeton University scientist, Hess

27. Seafloor spreading
Then if flows sideways, carrying the seafloor away from the ridge in both directions; creating new seafloor
Older seafloor moves away from the ridge in opposite direction

28. Evidence for spreading
1968 scientist began gathering information about the rocks on the seafloor
Found youngest rocks are located at mid-ocean ridge
Older rocks were found farther from the ridges

29. As molten material is forced upward, it brings heat and chemicals that support exotic life-forms in deep ocean water

30. Magnetic Clues
The Earth can change its magnetic poles; during a magnetic reversal, the lines of magnetic force run the opposite way
Reversals are recorded in rocks forming along the mid-ocean ridges
Newly forming iron minerals will record the magnetic reversal
Using a magnetometer (used to detect magnetic fields, scientist found that rocks on the ocean floor show many periods of magnetic reversal
Scientist have determined that Earth’s magnetic field has reversed itself many times in the past