1. The Theory of Plate Tectonics

2. Earth is a dynamic, ever changing planet!

3. If we could go back in time 200 million years, there would be no Grand Canyon…

4. Or Rocky Mountains!

5. Moreover, we would find landmasses with unfamiliar shapes and located in different positions from today’s continents.

6. Earth scientists have shown that the continents are not fixed, but slowly migrate across the globe.

7. Large landmasses have split apart resulting in the formation of oceans…

8. …while the floor of the ocean has been recycled back into Earth’s interior.

9. Further, landmasses that were once separated by vast oceans have since collided and formed larger continents.

10. The movement of Earth’s outer layer continues today
The movement of Earth’s outer layer continues today. Results of this movement include earthquakes…

11. …volcanoes,…

12. …and building of Earth’s mountains.

13. The modern scientific theory which describes the movement of Earth’s outer layer is called plate tectonics.

14. Because the theory of plate tectonics was the outgrowth of an earlier hypothesis known as continental drift, we will begin there.

15. A revolution in the Earth sciences began in the early 1900s when a radical proposal was made that the continents drifted about the face of Earth.

16. The development of this revolutionary hypothesis, known as continental drift, is credited to Alfred Wegener, a German scientist.

17. Wegener proposed that a supercontinent, that he named Pangea (all land), existed over 200 million years ago.

18. In addition, Wegener suggested that the supercontinent began breaking into smaller landmasses, which then “drifted” to their present positions, over millions of years.

19. Let’s take a look at some of the evidence Wegener cited to support his continental drift hypothesis:
1. Fit of the Continents
2. Fossil Evidence
3. Evidence from Rocks
4. Climatic Evidence

20. Fit of the Continents
Wegener first suspected that the continents might have been joined when he noticed the similarities between the coastlines on the opposite sides of the South Atlantic.

21. Wegener eventually made a crude jigsaw fit of all the continents.

22. Fossil Evidence
Fossils of the same ancient plants and animals are found today on widely separated continents.

23. For example, fossils of Cynognathus, a reptile that existed around 240 million years ago, have been found in parts of both South America and Africa.

24. Evidence of Glossopteris, a plant which existed around 260 million years ago, appears across all of these continents.

25. Furthermore, fossils of Mesosaurus, a reptile that swam in fresh water lakes and rivers about 260 million years ago, have been discovered in South America and Africa.

26. Why are the fossils of the Mesosaurus found on both sides of the South Atlantic but nowhere else in the world? If Mesosaurus had been able to swim well enough to cross the vast South Atlantic, why aren’t its remains found in other locations around the globe?

27. How did scientists, in the early 1900s, explain the existence of identical fossil organisms in places separated by thousands of kilometers of open ocean?

28. Evidence from Rocks
When working a jigsaw puzzle, everyone knows that in addition to the pieces fitting together, the picture must be continuous as well.

29. The picture that must be matched in the “Continental Drift Puzzle” is represented by rocks of a similar type and age…

30. …and mountain belts.

31. The Appalachian Mountains are found in eastern North America and disappear off the coast of Newfoundland.

32. Mountains of about the same age and structure are found in the British Isles and Scandinavia.

33. When these landmasses are placed in the pre- drift locations, these ancient mountain chains form a nearly continuous belt.

34. Climatic Evidence
Earth scientists have learned that about 250 million years ago, vast ice sheets covered portions of Earth’s continents.

35. Note that some of the areas that were once covered in ice sheets are found near the equator.

36. Scientists quickly rejected this idea, because at that time, large tropical swamps existed in the Northern Hemisphere.

37. Today, scientists realize that these ancient ice covered regions were joined together 250 million years ago and were located near the South Pole.

38. The arrangement of the continents provided ideal conditions for the development of vast ice covered areas and placed today’s northern landmasses near the equator.

39. The Great Debate
In 1924, when Wegener’s hypothesis was translated into English, French, Spanish and Russian, Earth scientists began to question his ideas.
Although correct in principle, Wegener’s hypothesis contained some incorrect details.

40. For example, Wegener proposed that as the continents drifted they broke through the ocean floor much like an ice breaker cuts through the ice.

41. Scientists at the time were quick to point out that the ocean floor was too strong to permit passage of the continents.

42. Although Wegener provided evidence to support his hypothesis in 1915, most scientists did not begin to accept continental drift until the late 1960s.

43. One of the reasons Wegener’s hypothesis had difficulty being accepted was Wegener incorrectly assumed the continents broke through the ocean floor.

44. If all of the continents were formed together as one supercontinent on one side of the globe, what was the nature of the hemisphere that lies opposite of Pangea?
One hundred million years ago, India was a continent that was positioned on the southern border of Africa, now it is part of Asia. What path did India take during the last 100 million years of Earth’s history?
135 million years ago, Australia was a part of Antarctica.
During the breakup of Pangea, North America drifted further north than it is today before settling with South America at its current location.