1. Plate Tectonics

2. Contents Earth’s Layers Continental Drift Seafloor Spreading
Plate Tectonics

3. Earth’s Layers

4. Earth Layers Crust: oceanic and continental,
Lithosphere: crust and upper mantle
Asthenosphere: upper mantle, plastic-like consistency, lithosphere floats on it
Mantle (mesophere): solid silicate rock, below asthenosphere and above the outer core
Outer core: liquid iron and nickel
Inner core: solid iron and nickel

5. CONTINENTAL DRIFT

6. OBJECTIVES
Describe one piece of early evidence that led people to suggest the Earth’s continents may have once been joined.
Discuss evidence of continental drift.
Explain why continental drift was not accepted when it was first proposed.

7. DRIFTING CONTINENTS Early observations
Map-makers (Abraham Ortelius) noticed how continents fit on either side of the Atlantic.
Thought continents had been separated by floods and earthquakes
Edward Suess proposed continents had once been joined together as supercontinent known as Gondwanaland.

8. PANGAEA

9. CONTINENTAL DRIFT
Wegener proposes continental drift, calling his supercontinent Pangaea, which means “all Earth” in Greek
Proposes supercontinent began breaking apart about 200 million year ago.
Begins collection of scientific evidence to support his theory

10. EVIDENCE FROM ROCK FORMATIONS
Wegener hypothesized that the same types of rock formations should exist on both sides of the Atlantic.
Noticed rocks in the Appalachian mountains shared features with rocks in Greenland and Europe.
All rocks were dated older than 200 million years; therefore found together before continental drift began.

11. EVIDENCE FROM FOSSIL RECORDS
Fossils of the same animals and plants were can be found on separate continents.
Kannemeyerids and Labyrinthodonts fossils can be found in Africa, North America, South America, Antarctica, Greenland, and Northern Europe.
Glossopteris plants can also be found in many continents, strengthening the hypothesis that these continents shared the same climate.

12. Lystrosaurus
Glossopteris
Mesosaurus
Cynognathus

13. FOSSIL EVIDENCE SUPPORTING PANGAEA

14. ANCIENT CLIMATIC EVIDENCE
Wegener studied sedimentary rocks and found evidence of widespread climate change.
Coal deposits in Antarctica suggest that plants once lived in Antarctica; therefore Antarctica was once closer to the equator.

15. GLACIAL EVIDENCE
Glacier deposits can be found in Africa, India, Australia, and South America. This suggests that these areas were once cold enough for glaciers to form.

16. OPPOSITION TO THE HYPOTHESIS
Wegener could not explain why the continents had moved throughout geologic time.
Scientists doubted that such forces could exist on Earth to move entire continents.
Wegener could explain how continents could move through the ocean floor to new locations on Earth.
Wegener continued to collect evidence until his death in

17. SEAFLOOR SPREADING

18. OBJECTIVES
Summarize the evidence that led to the discovery of seafloor spreading.
Explain the significance of magnetic patterns on the seafloor.
Explain the process of seafloor spreading.

19. TECHNOLOGICAL ADVANCE
People before the 1900’s assumed the seafloor was essentially flat.
Advances in technology in the 1940’s and ’50s led to new ideas.
Sonar can map surfaces by bouncing high frequency sound waves in order to calculate distances (elevation)

21. TECHNOLOGICAL ADVANCES
Sonar revealed ridges had corresponding trenches
Mariana Trench is over 11 km deep.

22. OCEAN FLOOR TOPOGRAPHY
Use of magnetometer to detect variations in magnetic fields allow scientists to map the seafloor.
Images revealed underwater mountain ranges, called ocean ridges.
Ridges form the largest continuous mountain range on Earth.

23. GULF OF MEXICO SEAFLOOR

24. MID-OCEAN RIDGE

25. MARIANA TRENCH DEEPEST POINT

26. QUESTIONS
What could have formed the ridges and trenches found on the seafloor?
What is the source of volcanism associated with these locations?
What forces could be at work to move the entire seafloor and create trenches 6 times as deep as the Grand Canyon?

27. OCEAN ROCKS AND SEDIMENTS
Scientists found predictable patterns in the ages of rock on the seafloor.
Rock ages with distance from ocean ridges
Ocean floor is dated at its oldest around 180 million years.
Continental rocks are dated at 3.8 billion years.
Why is the seafloor so young in comparison?

28. OCEAN ROCK AND SEDIMENT

29. OCEAN ROCK AND SEDIMENT
Thickness of ocean sediments is much thinner than expected.
Typically a few hundred meters thick.
Continents may have sediments up to 20 km thick.
Why are the sediments so thin?
Why isn’t seafloor as thick as continental crust?
Sediment is found to be thicker the further it is from ridges.

30. PALEOMAGNETISM
Study of the Earth’s magnetic field in iron-bearing minerals is known as paleomagnetism.
Seafloor is composed of iron-rich basalt, in which the iron becomes aligned with Earth’s magnetic field as molten rock cools.
When rock hardens, the iron orients itself to Earth’s magnetic field at that time

31. PALEOMAGNETISM
Studies in the 1960’s revealed pattern of magnetic reversals over time.
Magnetic reversals are changes in the Earth’s magnetic field.
Data from these reversals allowed scientists to construct a geomagnetic time scale.

32. MAGNETIC REVERSAL

33. THE INVESTIGATION
Scientists towed magnetometers behind ships to measure the magnetic field of the ocean floor.
Scientists noticed alternating + and – values for the magnetic field readings.

34. MAGNETIC SYMMETRY
Scientists noticed that the + and – magnetic bands were symmetrical, originating from the ocean ridges.
Scientists also discovered basaltic lava flows on land matching these magnetic reversals.
Age of the ocean floor was ascertained from this data.

35. ISOCHRONIC MAPPING
Isochron is a line on a map that connects points that have the same age.
Young rock is found at mid-ocean ridges, while older rock is found further away near deep sea trenches.
Why?

36. ISOCHRON MAP

37. SEAFLOOR SPREADING
After all this data, Harry Hess proposes the theory of seafloor spreading.
Theory proposes that new ocean floor is created at the ridges and destroyed at the trenches.
Magma rises to the surface and hardens at the ocean ridges.
Consecutive bands of seafloor are created in this way.

38. SEAFLOOR SPREADING

39. SEAFLOOR SPREADING

40. SEAFLOOR SPREADING

41. WEGENER’S THEORY REVISITED
Wegener’s theory of continental drift did not account for the forces in action.
Seafloor spreading completes the picture and describes the forces that shape the drifting of the continents.
Continents are like groceries on the conveyer belt at the checkout line; they don’t push through the ocean floor, they ride on top of it as the ocean floor moves.

42. THEORY OF PLATE TECTONICS

43. OBJECTIVES Explain the theory of plate tectonics.
Compare and contrast the three types of plate boundaries and the features associated with each.

44. THE THEORY Why do earthquakes happen? Why do volcanoes erupt?
Why do mountains exist?
Plate Tectonics is a theory that states the Earth’s crust and rigid upper mantle are broken into enormous slabs called plates.
Each plate moves at a particular rate, and measured using a system of satellites and receivers

45. WORLD TECTONIC PLATES

46. PLATE COMPOSITION Oceanic plates: basalt
Dark (black) and dense rock type composed of silicates, iron and magnesium
Continental plates – granite and andesite
Light colored (pink, white and gray) and low density rock type composed almost entirely of silicates.

47. PLATE BOUNDARIES
Tectonic plates interact at places called plate boundaries.
Divergent
Convergent
Transform
Each boundary has certain geologic characteristics.

48. DIVERGENT BOUNDARIES
Places where two tectonic plates are moving apart.
Most are found on the seafloor as ocean ridges (seafloor spreading, ridge push)
Example: Mid-Atlantic Ridge)
Formation of new ocean crust accounts for volcanism along these locations.
A rift valley is formed when a divergent plate boundary is located on continental crust.
Example: East African Rift Valley

49. DIVERGENT PLATE BOUNDARIES AT MID-OCEAN RIDGES

50. CONTINENTAL DIVERGENT PLATE BOUNDARY: EAST AFRICAN RIFT

51. DIVERGENT BOUNDARY
Map showing the Mid- Atlantic Ridge splitting Iceland and separating the North American and Eurasian Plates.

52. CONVERGENT BOUNDARIES
Places where two tectonic plates are moving toward each other.
Crust is destroyed at these boundaries.
Three types:
Oceanic converging with oceanic crust
Oceanic converging with continental crust
Continental converging with continental crust

53. CONVERGENT PLATES

54. SUBDUCTION AND CONVERGENCE
Occurs when a cooler, denser plate descends below a less dense plate.
In the event of oceanic and oceanic plate convergence, a trench is formed.
Subducted plate melts and becomes magma, rises to the surface, and erupts as a volcano.
Ex. Mariana Trench and islands
Ex. Aleutian Trench and islands

55. OCEANIC-OCEANIC CONVERGENCE

56. OCEANIC-CONTINENTAL CONVERGENCE
Denser oceanic plate subducts under less dense continental plate.
Forms volcanic mountain ranges.
Ex. Peru-Chile Trench and Andes Mountains

57. OCEANIC-CONTINENTAL CONVERGENCE

58. NAZCA - SOUTH AMERICAN PLATE BOUNDARY

59. CONTINENTAL-CONTINENTAL PLATE CONVERGENCE
Occurs when two continental plates collide.
Produces large folded mountain ranges with little volcanism.
Since continental crust is buoyant, crust crumples and folds rather than subducts.
Ex. Himalayas

60. CONTINENTAL-CONTINENTAL CONVERGENCE

61. HIMALAYAN MOUNTAINS

62. TRANSFORM BOUNDARIES
Place where two plates slide horizontally past each other.
Crust is deformed or fractured at these boundaries, but not created or destroyed.
Characterized by long faults, hundreds of km in length.
Earthquakes.
Most transform boundaries are not found on continents.
Exception: San Andreas Fault

63. TRANSFORM BOUNDARIES

64. SAN ANDREAS FAULT

65. MANTLE HOT SPOTS Areas of rising magma from deep in the mantle
High temperature basaltic lava
Occurs intraplate and not at plate boundaries
Stationary while the overlying plate passes over
Volcanic activity that can form islands and thus island chains
Affects primarily oceanic lithosphere and to a lesser degree continental lithosphere
Examples: Hawaiian Islands, Yellowstone National Park in Wyoming

66. HAWAIIAN MID-PLATE HOTSPOT

67. HAWAIIAN HOT SPOT TRAIL

68. MECHANISMS OF PLATE MOVEMENT
Ridge Push
Ridge Push
Slab Pull

69. WHY DO THE PLATES MOVE?
No single idea explains everything but we can identify several forces that contribute to the movement of the plates.
Slab pull
The sinking of the cooled, denser oceanic plates pulls on the rest of the plate as it sinks into the mantle
Ridge push
The hot molten rock rises through fissure in the ocean floor, cools and hardens to form a ridge, slides down from the rise and thus pushing the plate away from the fissure
Convection Currents
In the mantle, the hotter (less dense) molten rock rises, cools (becomes denser), and then sinks back into mantle where it is reheated

70. EARTH’S INTERNAL HEAT SOURCES
And what drives the convection currents?
The immense amount of heat in the Earth's mantle and core which comes from:
heat left over from Earth's formation out of hot gas and dust
gravitation
friction
radioactive decay

71. THE BIG PICTURE

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