1. Continents Change Position Over Time
1.2 Notes
Continents Change Position Over Time

2. Think About…
When creating your map of Pangaea, what clues helped you fit the pieces together?

4. Continents join together and split apart
As far back as the 1500’s, mapmakers noticed that the coasts of Africa and South America fit together like puzzle pieces.

5. Continents join together and split apart
In 1912, Alfred Wegener proposed a hypothesis called continental drift.

6. Continents join together and split apart
This hypothesis states that Earth’s continents were once joined in a single landmass and gradually drifted apart.

7. Continents join together and drift apart
This hypothesis was not accepted until the mid-1900’s.

8. Evidence for Continental Drift
Extraordinary claims require extraordinary evidence!
There are 3 main sources of evidence for continental drift: fossils, climate, and geology.

9. Evidence for Continental Drift
Fossils
Mesosaurus is an ancient reptile, the remains of which have been found ONLY in South America and Western Africa.

10. Evidence for Continental Drift
Climate
Modern day Greenland is found near the Arctic Circle and is covered in ice, yet fossils of tropical plants are found on its shores.
This is clue that Greenland once existed near the equator.

11. Evidence for Continental Drift
Climate
South Africa today has a warm climate, yet its rocks have deep scratches clearly made by past glaciers.
This is a clue that South Africa once existed much closer to the south pole.

12. Evidence for Continental Drift
Geology
The type of rock found in Brazil perfectly matches rock found in Western Africa.

13. Evidence for Continental Drift
Geology
Limestone layers in the Appalachian Mountains match limestone layers found in Scotlands Highlands.

14. Pangaea and Continental Drift
The continents were once joined in a huge supercontinent called Pangaea.
Pangaea comes from the Greek for “all lands.”

15. Pangaea and Continental Drift
Because Wegener could not explain HOW the continents drifted, many people disregarded his ideas for a long time.

16. Pangaea and Continetal Drift
The theory of plate tectonics explains how plates and their continents move.

17. The theory of plate tectonics explains how plates and their continents move
Eventually, the theory of plate tectonics arose, which built on Wegener’s idea and served to explain HOW continental drift occurs.

19. Evidence from the Sea Floor
When scientists started mapping the sea floor in the 1950’s, they expected it to be smooth and level, like a flat underwater desert.

20. Evidence from the sea floor
What scientists found on the sea floor instead were huge underwater mountain ranges, called mid-ocean ridges.

22. Evidence from the sea floor
Sea-Floor Spreading
Mid-ocean ridges form along cracks in the crust.
Molten rock rises through these cracks, cools, and forms new oceanic crust.

24. Evidence from the sea floor
Age of the Sea Floor
We can verify that the sea floor is spreading apart by new rocks being created by testing the age of rocks.
The youngest rocks lie close to spreading centers, while older rocks lie further away.

25. Evidence from the sea floor
Even the oldest rocks on the ocean floor are only 160 to 180 million years old, while continental crust can be upwards of 4 billion years old!
This is evidence that the ocean floor is constantly changing.

26. Evidence from the sea floor
Ocean Trenches
If the sea floor is constantly spreading, why is Earth not constantly growing larger?

27. Evidence from the sea floor
The answer lies in ocean trenches-deep canyons in the sea floor where crust is being destroyed.

28. Evidence from the sea floor

29. Causes of Plate Movement
Remember, tectonic plates rest on the asthenosphere-a layer of hot, soft rock.

30. Causes of Plate Movement
Rock in the asthenosphere and mantle move by convection.

31. Causes of Plate Movement
Convection is the transfer of energy (usually heat) by the movement of a material.

32. Causes of Plate Movement
Convection is what happens when you boil a pot of water.
The water at the bottom heats up, becomes less dense and rises. At the surface, it cools, becomes denser, and sinks.

33. Causes of Plate Movement
Rock in the asthenosphere acts the same way as a boiling pot of water.
When this rising and sinking pattern repeats, it forms a circular motion called a convection current.

34. Causes of Plate Movement
The movement of the mantle is much slower than a pot of boiling water-only a few centimeters each year!
This results in the asthenosphere moving the plates like heavy boxes on rollers-but very, very slowly!

35. Causes of Plate Movement
Two other possible causes of plate movement are slab pull and ridge push.

36. Causes of Plate Movement
Slab pull occurs where gravity pulls the edge of a cool dense plate into the asthenosphere.

37. Causes of Plate Movement
Ridge push occurs when material from a mid-ocean ridge slides downhill from the ridge.

38. Putting the theory together
The theory of plate tectonics encompasses scientists’ knowledge of Earth’s plates, the sea floor, and the asthenosphere.
There are 9 major tectonic plates on Earth.
Fill in your lithospheric plates map as we go! After it is filled in, shade each plate with a different color.

48. Putting the theory together
One plate cannot shift without affecting the others nearby.

49. Review
Why did many scientists reject Wegener’s continental drift hypothesis?
He could not explain how the continents moved.
The geology of continents did not support his hypothesis.
Fossil evidence showed that the continents were never joined.
The climates of the continents have remained the same.

50. Review
2. What evidence from the sea floor shows that tectonic plates move?
The sea floor is much older than any of the continents.
The sea floor is youngest near a mid-ocean ridge and older farther away.
Mid-ocean ridges circle Earth like seams in a baseball.
The sea floor is thinner than continental crust.

51. Review 3. A mid-ocean ridge forms where plates Move apart
Push together
Scrape past each other
Subduct

52. Review 4. Plate motion is caused partly by Magnetic reversals
Convection currents
Continental drift
Volcanic hot spots