1. Plate Tectonics

2. Let’s review what scientists found and then look at their conclusion.
As people began learning about the structure of the earth, there appeared to be a geologic mystery….
Fossils, rocks, glaciers, and the bottom of the ocean all revealed a past that did not reflect the current conditions on earth. But what did that mean?
Let’s review what scientists found and then look at their conclusion.

3. Fossils
Scientists know that a species appears in one area on earth, then spreads out. Some species can only spread across land.
Scientists found identical fossil organisms in S. America and Africa: they would have to be connected for this to occur.

4. Fossils
Mesosaurus, a freshwater reptile: it should be more widely spread out if it could swim across oceans
Glossopteris, large ferns: the seeds are too heavy to be blown across oceans
Lystrosaurus and Cynognathus, land reptiles: found on 3 continents, which are separated by oceans, and they cannot swim

7. Rocks
On S. America and Africa there are rock and landscape similarities, such as matching mountain ranges.
The rocks in the mountains are of the same age and type, and fit together like a puzzle.

9. Glaciers
Glaciers are huge ice masses that move slowly over land. Glaciers spread outward from the poles.
Evidence from glaciers 300 million years ago is found in S.America, Africa, India and Australia. For this to happen, these continents would have had to be closer to the South Pole than they are now.

10. Glaciers
The pattern of rocks can be explained if the continents were one landmass when the glacier formed.
In addition, there is fossil evidence that the modern day Northern Hemisphere was tropical swampland.
These swamps became the coal fields of the United States, Europe and Siberia.

14. So, what did all of this mean?
In 1915, one man, Alfred Wegener, proposed an idea that explained all of these unusual findings.

15. Wegener/Continental Drift
In 1915, Wegener described the idea of Continental Drift, the gradual movement of continents over time across the surface of the earth, which would explain all of the unusual findings.
He proposed the idea of Pangaea, a giant supercontinent that had once contained all the world’s land, that slowly had drifted apart into today’s continents.

16. Alfred Wegener/Continental Drift
Pangea existed 200 million years ago, then it became fragmented into smaller continents and started to drift.
The last landmass to move into current position was India, 40 million years ago.

18. Alfred Wegener/Continental Drift
This idea was great! Except he was missing one piece of the puzzle… HOW the giant continents could move around the earth.
His idea was not accepted during his lifetime because he did not have a mechanism, a reason, for the movement of the continents.
He spent the rest of his life taking trips to Greenland to test his ideas. In 1930 while returning to the ship he suffered a heart attack and died.
He was buried as he was found, under the snow, to become part of the ice sheet.

19. So, was Alfred Wegener correct?
How could scientists explain giant continents moving around?
The answer to the mechanism of the movement lay at the bottom of the ocean…

20. Sea Floor Spreading
After WWII, oceanographers were able to use new technology (SONAR) to map the ocean floor, which also revealed unexpected findings.
Deep trenches and steep ridges were found on the ocean floor, creating a system that covered the earth.

22. Sea Floor Spreading
The ocean floor was covered in Basalt, an igneous rock, the most abundant rock on the earth.
How do you explain volcanic rock covering the bottom of the ocean?

23. Sea Floor Spreading
The rocks that were at the center of the mountain ridges were younger than the rest of the ocean floor, then became older as they moved away from the center of the ridge.

26. Sea Floor Spreading
In the 1960’s, Harry Hess proposed seafloor spreading, the idea that the oceanic ridge was a crack in the earth’s crust where the hot mantle pushed upward. The pieces of crust on either side of the crack were moving away from each other and magma pushed up between them, creating new oceanic rocks.

29. Sea Floor Spreading
As rising material spreads laterally, seafloor is carried conveyor-belt fashion away from the ridge crust, generating new oceanic crust.
To compensate for the other side of the plate moving, there are deep ocean trenches where the crust is brought back down to the mantle.

31. This was the missing mechanism describing how land could move
This was the missing mechanism describing how land could move! But why does magma push toward the surface?
Convection!

32. Convection
Heat transfer from a lower temperature to higher temperature by motion of a fluid, such as water or magma.
Heat from the core moves toward the surface through the mantle causing the material of the crust to circulate.

33. Convection
Matter rises in parts and once cooled, it sinks. This creates a circle of heat and matter called convection.
This causes rocks to melt, magma to erupt and the crust to move.

34. Convection
The rising and sinking material moves in a circular motion, which moves plates because the plates stick to the moving mantle due to friction, which causes them to move as the mantle moves.

35. Convection Currents
Convection currents are the result of uneven heating inside the mantle resulting in the melting of rock; convection, eruption and flow of magma; and the movement of crustal plates.
Rock layers are affected by the folding, breaking, and uplifting of rock layers due to plate motion.
The movement of crustal plates can cause earthquakes and volcanic eruptions that can result in mountain building and trench formation.

36. Today, we call his idea Plate Tectonics.
With the mechanism of convection in place, Wegener’s idea of moving continents was finally accepted.
Today, we call his idea Plate Tectonics.

37. Plate Tectonics
A geological model in which the Earth's crust is divided into a number of rigid segments, plates, which constantly move atop of the mantle.

39. Rates of Motion
These average rates of plate separations can range widely.
The Arctic Ridge has the slowest rate (less than 2.5 cm/yr.
The East Pacific Rise near Easter Island, in the South Pacific, has the fastest rate (more than 15 cm/yr).

40. Boundaries If they are moving, what happens when plates meet?
All interactions of plates occur along boundaries, where one ends and another begins.
The location of the boundaries was established by researching locations of earthquakes.
These boundaries show where the plates are being subducted, building mountains or creating new oceanic crust.

41. Boundaries
Continental plates: 6 of the 7 major plates, carry mostly continental crust and some oceanic crust
Oceanic plates: 1 of the 7 major plates, and several smaller plates, only made of oceanic crust

42. Boundaries Convergent: two plates moving together.
Oceanic-oceanic: one oceanic plate subducts, sinks under the other plate.
Oceanic-Continental: oceanic plate, which is denser, subducts under the continental plate.
Continental-Continental: neither can move under the other because they are both thick, they compress and buckle, forming mountains.

44. Boundaries
Divergent: two plates move apart, upwelling of material from mantle to form new seafloor.

46. Boundaries
Transform Fault: two plates grind past each other without production or destruction of crust.