1. The Earth Inside and Out The Theory of Continental Drift
Choose to view chapter section with a click on the section heading.
The Earth Inside and Out
The Theory of Continental Drift
The Theory of Seafloor Spreading
The Unifying Theory: Plate Tectonics
Chapter Topic Menu

2. The Earth Inside and Out
Chapter 13 Page 13-4 to 13-9

3. Earth’s Internal Layers
The Earth’s interior consists of multiple layers: the inner core, the outer core, the mantle, and the crust.
Based on 2008 seismic data, the inner core is thought to be solid and composed mainly of iron crystals.
The inner core has an inner core 1,180 kilometers (733 miles) in diameter. The entire inner core is thought to be 2,400 kilometers (1,491 miles) in diameter.
The inner course is thought to have a temperature of 5,000˚C (9,032˚F) – nearly the same temperature as the surface of the sun.
The inner core is theorized to be solid due to intense pressure.
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

4. Earth’s Internal Layers
The outer core consists of the same elements at the same temperature.
Scientists theorize that with less pressure, it is liquid.
The thickness is 2,270 kilometers (1,411 miles).
In some places the outer core has thermal plumes, which are localized areas of high heat release that might possibly be related to volcanic activity.
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

5. Earth’s Internal Layers
The mantle is the layer above the core and is approximately 2,900 kilometers (1,802 miles) in thickness.
The mantle is thought to contain mostly silicon and oxygen with some iron and magnesium. Seismic wave studies have given scientists a picture of the mantle’s structure – it consists of the upper mantle and the lower mantle. The lower mantel is made of hot, dense magma (molten rock).
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

6. Earth’s Internal Layers
The upper mantle is composed of the asthenosphere and a portion of the lithosphere.
The asthenosphere is solid magma that flows slowly over time.
The lithosphere is the uppermost, rigid part of the upper mantle and the crust. It is the cool, solid rock portion of the outer Earth that rests on the warmer astehnosphere.
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

7. Earth’s Internal Layers
The outermost layer of the Earth is the crust.
The crust is composed mainly of oxygen, silicon, magnesium, and iron.
It varies in thickness and is the outer layer of the lithosphere. Low density magma from the asthenosphere rises and can eventually flow from a volcano or other opening in the crust. When molten rock exist the opening, it is called lava.
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

8. Earth’s Internal Layers
Cross-Section of the Earth
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

9. Earth’s Internal Layers
Scientists separate the uppermost mantle from the crust because they think the mantle’s elemental composition changes little.
The crust, however, consists of different rock types thought to undergo change over long periods.
Besides chemical properties, conditions such as temperature and pressure differentiate the crust from the mantle.
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6

10. The Rock Cycle The three rock types found in the crust are: Igneous
Sedimentary
Metamorphic.
Rocks form or change over long periods due to the processes of the rock cycle.
You can think of this as the Earth’s recycling machine, endlessly converting rock from one type to another.
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7

11. The Rock Cycle
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7

12. The Rock Cycle
Igneous rocks form when magma or lava cools and hardens. Granite is a common example of igneous rock.
Igneous and other rock types break apart into rock particles due to weathering and water flow.
Water flow carries away the rock particles, along with loose soil and particles of organic material. This process is called erosion.
Wind, glaciers, and gravity cause erosion.
Chemical processes break down and change rock through reactions.
When the water flow slows or stops, the particles may be deposited as sediment.
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7

13. The Rock Cycle
Scientists have concluded that, over time, sediments are compressed and cemented together to form sedimentary rock.
Because sediments contain organic matter, this is the only type of rock in which scientists find fossils, coal, petroleum, and other fossil fuels.
As layers build up on top of each other, the upper layers subject lower layers to increasing pressure and heat.
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7

14. The Rock Cycle
Metamorphic rock forms when pressure and heat become great enough to change the rock chemically.
Rock may return to the mantle as part of plate tectonic processes. Subjected to the heat of the Earth’s interior, rocks remelt and become magma, returning to the crust as igneous rock.
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7

15. Isostatic Equilibrium
It is currently thought that the crust doesn’t sit on anything rigid, but literally floats on the mantle.
The continental crust (the crust under the continents) consists primarily of granite, whereas the ocean crust (the crust under the ocean basins) consists primarily of basaltic rock.
With the crust floating on the mantle, there must be a balance between the weight of the crust and the upward force of buoyancy. This is the application of Archimedes’ Principle of buoyancy.
The Earth Inside and Out
Chapter 13 Pages to 13-9

16. Isostatic Equilibrium
Archimedes’ Principle of buoyancy states: an object immersed in a fluid (gas or liquid) is buoyed up by a force equal to the weight of the fluid displaced. An object that weighs less than the fluid it displaces will float.
If the weight of the crust changes, according to Archimedes’ Principle the landmass must rise or subside (sink) to compensate.
The Earth Inside and Out
Chapter 13 Pages to 13-9

17. Isostatic Equilibrium
The balance between the weight of the crust and the buoyancy provided by the mantle is called isostatic equilibrium.
As material adds to the oceanic crust (from deposition by sedimentation, glaciers, and volcanic activity) or leaves the continental crust (from erosion), this balance becomes disrupted.
The Earth Inside and Out
Chapter 13 Pages to 13-9

18. Isostatic Equilibrium
The additional weight will cause the crust to deflect downwards, while the removal of material causes the crust to deflect upwards.
This is called isostatic rebound.
The Earth Inside and Out
Chapter 13 Pages to 13-9

19. Isostatic Equilibrium
This is one theorized cause of earthquakes. To restore equilibrium, landmasses will sink or rise slightly along a weak area called a fault.
During an earthquake, the landmasses (continental or ocean basin) on either side of the involved fault do not move together.
The Earth Inside and Out
Chapter 13 Pages to 13-9

20. Isostatic Equilibrium
Until the last half of the 20th century, the prevailing view in science was that earthquakes and continental movement only involved vertical rises and fall.
This changed with the acceptance of the theory of plate tectonics, which suggests that the continents move in horizontal directions and that earthquakes also result from that movement.
The Earth Inside and Out
Chapter 13 Pages to 13-9

21. The Theory of Continental Drift
Chapter 13 Page to 13-13

22. Alfred Wegener and Pangaea
In 1912, a German meteorologist and polar explorer named Alfred Wegener proposed what was at the time a startling idea.
He proposed that in the distant past all the Earth’s continents had been a single giant continent. Wegener called this continent Pangaea.
Surrounding Pangaea, he said, was a single large ocean he called Panthalassa.
The Theory of Continental Drift
Chapter 13 Page 13-10

23. Alfred Wegener and Pangaea
Over the years, since the first accurate maps, others (including Leonardo da Vinci and Sir Francis Bacon) had observed how well the continents could be pieced together like a puzzle.
The Theory of Continental Drift
Chapter 13 Page 13-10

24. Alfred Wegener and Pangaea
Wegener was the first to formally propose a process that explained the fit and present placement of the continents.
Wegener theorized that because the less dense continents floated on the molten rock of the mantle, Pangaea broke by floating apart into separate pieces.
The separate continents reached their present locations by drifting apart for more than 200 million years.
The theory that the continents were once a single landmass that drifted apart (and are still doing so) is called the theory of continental drift.
The Theory of Continental Drift
Chapter 13 Page 13-10

25. Evidence for Continental Drift
In the 1600s, the first accurate world maps became available.
People noticed that the continents apparently fit together like jigsaw-puzzle pieces.
This was the first evidence of continental drift. It was hundreds of years later that Wegener began to see further evidence from other sources.
The Theory of Continental Drift
Chapter 13 Page to 13-13

26. Evidence for Continental Drift
In 1855, the German scientist Edward Suess found fossils of the Glossopteris fern in South America, Africa, Australia, India, and Antarctica.
The seeds of this fern are too heavy to travel by wind and too fragile to survive significant sea crossings.
To Wegener and other early advocates of continental drift, this suggested that these landmasses must have once been much closer together for the fern to have spread so widely.
The Theory of Continental Drift
Chapter 13 Page to 13-13

27. Evidence for Continental Drift
Continental drift proponents also studied the distribution of animals and fossils.
Two examples were the extinct aquatic reptile Mesosaurus and an extinct bear-like animal called Lytrosaurus.
Based on the distribution of fossils for these animals, Wegener and others who supported the theory of continental drift hypothesized that Pangaea had split into two continents about 200 million years ago.
They called the hypothetical northern continent Laurasia. It included today’s North America, Greenland, and Eurasia.
They named the southern continent Gondwanaland which included the remaining continents.
The Theory of Continental Drift
Chapter 13 Page to 13-13

28. Evidence for Continental Drift Map of Pangaea
The Theory of Continental Drift
Chapter 13 Page to 13-13

29. Evidence for Continental Drift
Wegener also saw the distribution of coal as evidence.
In 1908, the famed polar explorer Ernest Shackleton discovered coal in the Antarctic. Scientists theorize that coal originates when geological processes bury vegetation in warm, swampy climates faster than it can decompose. Pressure and heat alter the vegetation, eventually turning it into coal.
Since Antarctica doesn’t presently have the appropriate climate for this kind of vegetation, Wegener reasoned, it must have in the past been in a different place with a different climate.
The Theory of Continental Drift
Chapter 13 Page to 13-13

30. Evidence for Continental Drift
Despite these examples and other evidence, the theory of continental drift wasn’t widely accepted while Wegener was still alive.
This may have partly been because Wegener was a meteorologist, not a geologist.
The Theory of Continental Drift
Chapter 13 Page to 13-13

31. The Theory of Seafloor Spreading
Chapter 13 Pages to 13-20

32. New Technology and Seafloor Knowledge
The invention of Sonar (an acronym for Sound Navigating And Ranging) made it possible to “see” through long distances under water.
Sonar detects objects under water by transmitting a sound and receiving an echo. Based on the echo’s angle, how long it took to return, and changes in frequency, sonar operators can determine where an object is, its distance, and whether it’s moving.
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-15

33. New Technology and Seafloor Knowledge
Sonar made it possible to detect an otherwise invisible iceberg 5 kilometers (3.1 miles) away.
Sonar underwent improvements and played a pivotal role in World War II.
Scientists began using an echo sounder - essentially modified sonar specifically for mapping bottom terrain.
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-15

34. New Technology and Seafloor Knowledge
The ability to map the seafloor in greater detail revealed important new features to scientists.
One of the most important discoveries was a 70,000-kilometer (43,497-mile) mountain range that extends through the Atlantic, Indian, and Pacific Oceans.
Scientists also discovered trenches, which are deep ravines in the seafloor, and rift valleys, which are deep valleys running through the center of the Mid-Atlantic Ridge and other mid-ocean ridges.
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-15

35. New Technology and Seafloor Knowledge
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-15
Major Earthquake and Volcano Zone
Patterns of Ridges and Trenches

36. The Creation and Destruction of Seafloor
In 1960, geologists Harry Hess and Robert S. Dietz proposed an explanation of seafloor features. They hypothesized that the seafloor is in a constant state of creation and destruction through a process called seafloor spreading.
In the theory of seafloor spreading, new crust emerges from the rift valley in a mid-ocean ridge.
Magma from the asthenosphere pushes up through the rift and solidifies into new crust.
Through this process, new seafloor near the ridge continuously pushes old seafloor away from the ridge.
The Theory of Seafloor Spreading
Chapter 13 Page 13-16

37. The Creation and Destruction of Seafloor
Other scientists proposed that old seafloor subsides (sinks) at the trenches. The old seafloor is drawn downward by gravity and inertia, eventually reaching the asthenosphere and melting into magma again.
Therefore, the theory of seafloor spreading says that new seafloor forms at the rift valleys and mid-ocean ridges, spreading away from the ridges until it returns as part of the rock cycle at subduction zones (trenches).
The Theory of Seafloor Spreading
Chapter 13 Page 13-16

38. The Creation and Destruction of Seafloor
The Theory of Seafloor Spreading
Chapter 13 Page 13-16

39. Evidence of Seafloor Spreading
There are several forms of evidence that support the theory of seafloor spreading.
Radiometric dating.
Ocean-bottom sediment samples.
Evidence is supplied by rheology.
Magnetometer data.
Since about 1990, geodesists have been measuring plate movements directly.
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-20

40. Evidence of Seafloor Spreading
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-20
Radiometric Dating of the Seafloor

41. Evidence of Seafloor Spreading
The Theory of Seafloor Spreading
Chapter 13 Pages to 13-20
Alternating Polarity of the Mid-Ocean Ridge

42. The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-29

43. Seafloor Spreading and Continental Drift Combine
In 1965, Canadian geophysicist John Tuzo Wilson introduced a new theory that united the theories of continental drift and seafloor spreading.
This was the birth of today’s theory of plate tectonics. This theory combined ideas from both theories, along with some of the original isostatic equilibrium concepts.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

44. Seafloor Spreading and Continental Drift Combine
According to the theory of plate tectonics, the Earth’s lithosphere consists of more than a dozen separate plates.
Some plates have entirely oceanic crust, some have continental crust, and some have both types.
The plates are rigid and float on the asthenosphere.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

45. Seafloor Spreading and Continental Drift Combine
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25
Earth’s Crustal Plates

46. Seafloor Spreading and Continental Drift Combine
Scientists can measure plate movement. Studies show that the plates move a few centimeters a year.
As plates move, they collide in some places, separate in others, and slide side-by-side in yet others.
Where plates meet at plate boundaries, there are three possible motions relative to each other; spreading apart, pushing together, or passing side-by-side.
The interaction where plates meet each other explains how mid-ocean ridges, rift valleys and trenches relate to plate tectonic concepts.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

47. Seafloor Spreading and Continental Drift Combine
At a transform boundary or fault, two plates slide past each other.
In the United States, perhaps the most well-known of these is the San Andreas Fault in California.
The plate to the west of the fault is moving north, while the plate to the east is moving south.
Earthquakes result as rocks move when the plates slide next to each other.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

48. Seafloor Spreading and Continental Drift Combine
The San Andreas Fault
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

49. Seafloor Spreading and Continental Drift Combine
At a spreading or divergent boundary, two plates are moving apart.
The crust pulls apart and forms valleys. Mid-ocean ridges and rift valleys mark divergent boundaries.
Magma from the asthenosphere flows up through the rift valley where the plates separate, creating new crust and widening the seafloor.
Besides forming new seafloor, volcanic activity at a divergent boundary may build mountains higher than sea level.
This builds islands such as Iceland.
Because new crust forms there, divergent boundaries are also called constructive boundaries.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

50. Seafloor Spreading and Continental Drift Combine
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25
Divergent Boundary

51. Seafloor Spreading and Continental Drift Combine
At a colliding or convergent boundary, two plates push together.
Convergent boundaries are also called destructive boundaries because movements along these boundaries destroy crust.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

52. Seafloor Spreading and Continental Drift Combine
There are three types of plate collision:
The first occurs between two oceanic plates. In this collision, one plate is subducted under (sinks beneath) the other.
This can result in a chain of volcanic islands or an island arc.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

53. Seafloor Spreading and Continental Drift Combine
The second type of plate collision occurs between oceanic and continental plates. The more dense oceanic plate is subducted under the less dense continental plate.
This subduction occurs in trenches.
A range of volcanic mountains may form at the edge of the continental plate as molten rock from the oceanic plate rises.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

54. Seafloor Spreading and Continental Drift Combine
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25
Island Arcs - Trench Development

55. Seafloor Spreading and Continental Drift Combine
The third type of collision occurs between two continental plates.
According to the theory, when two continental plates collide, mountains form as the crust folds.
The Himalayas are thought to still be forming as the Indo-Australian Plate pushes into the Eurasian Plate.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-25

56. Hot Spots
What makes the hot spot theory significant is the concept that hot spots do not move with tectonic plates.
Hot spots originate in the mantle, so the volcanic areas change on the plate as it moves over the hot spot. This results in a line or row of volcanic formations.
Volcanic island chains, for example, result from the plate moving over a hot spot.
The Unifying Theory: Plate Tectonics
Chapter 13 Page 13-26

57. Hot Spots
Hot spot theory is important to oceanographers because it explains the nature of features forming away from plate boundaries.
Perhaps the most prominent of these is a hot spot under the Pacific Plate that scientists think created the Emperor Seamounts and the Hawaiian Islands.
They formed from a hot spot as the plate moved northwest over the last 35 million years.
The Unifying Theory: Plate Tectonics
Chapter 13 Page 13-26

58. Plate Movement
The theory of continental drift wasn’t accepted early in its conception because no one could explain how continents could move. The theory of plate tectonics provided the explanation.
Convection is a primary force driving seafloor spreading and continental drift.
Convection is a vertical circulation pattern in a gas or liquid caused by hot material rising and cold material sinking.
This occurs in the mantle when warm molten rock rises and cool magma sinks.
This creates a current that moves the plates away from each other at the divergent boundaries, toward each other at the convergent boundaries, and past each other at the transform boundaries.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-29

59. Plate Movement The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-29

60. Plate Movement
Based on present theorized plate movements, what will happen over the next several million years? Will the plates ever rejoin as a super continent?
According to current theory, the Atlantic Ocean and Indian Ocean will expand while the Pacific Ocean will shrink.
Australia will continue to drift toward Eurasia.
Southern California will pass San Francisco as it moves to the northwest, and a new ocean will form in the East African rift valley.
The Mediterranean Sea will close as Africa moves northward.
The Unifying Theory: Plate Tectonics
Chapter 13 Pages to 13-29