1. Plate Tectonics The History of a Theory

2. Continental drift: An idea before its time
Alfred Wegener
First proposed his continental drift hypothesis in 1915
Continental drift hypothesis
Supercontinent called Pangaea began breaking apart about 200 million years ago

3. Pangaea approximately 200 million years ago

4. Continental drift continued…
Continental drift hypothesis
Continents "drifted" to present positions
Evidence used in support of continental drift hypothesis
Fit of the continents
Fossil evidence
Rock type and structural similarities
Paleoclimatic evidence

6. An Idea Before Its Time ◆ Evidence • The Continental Puzzle
• Matching Fossils
- Fossil evidence for continental drift includes several fossil organisms found on different landmasses.

7. ◆ Evidence • Rock Types and Structures
Rock evidence for continental exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean.
• Ancient Climates

10. The great debate… Objections to the continental drift hypothesis
Inability to provide a mechanism capable of moving continents across the globe
Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice

11. The great debate…what is our job as scientists?
Continental drift and the scientific method
Wegner’s hypothesis was correct in principle, but contained incorrect details
For any scientific viewpoint to gain wide acceptance, supporting evidence from all realms of science must be found
A few scientists considered Wegner’s ideas plausible and continued the search
Our job is to disprove…

12. Plate Tectonics: A modern version of an old idea
Much more encompassing theory than continental drift
◆ According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere.

13. Plate Tectonics: Continental Drift with a Twist…
Earth’s major plates
Associated with Earth's strong, rigid outer layer
Known as the lithosphere
Consists of uppermost mantle and overlying crust
Overlies a weaker region in the mantle called the asthenosphere

14. Plate Tectonics
◆ Divergent boundaries (also called spreading centers) are the place where two plates move apart.
◆ Convergent boundaries form where two plates move together.
◆ Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere.

15. Plate Tectonics
Plates move relative to each other at a very slow but continuous rate
Average about 5 centimeters (2 inches) per year, some more – some less…
Cooler, denser slabs of oceanic lithosphere descend into the mantle

16. Plate Tectonics Plate boundaries
New plate boundaries can be created in response to changes in the forces acting on these rigid slabs

17. Divergent Plate Boundary
◆ Oceanic Ridges and Seafloor Spreading
• Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries.
• Rift valleys are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land.
• Seafloor spreading produces new oceanic lithosphere.

20. Rift Valley

21. Divergent Boundary ◆ Continental Rifts
• When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift.

22. East African Rift Valley

23. Convergent Plate Boundary
◆ A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate.
◆ Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.
• Examples include the Andes, Cascades, and the Sierra Nevadas.

27. Oceanic-Continental Convergent Boundary

28. Convergent Plate Boundary
◆ Oceanic-Oceanic
• Two oceanic slabs converge and one descends beneath the other.
• This kind of boundary often forms volcanoes on the ocean floor.
• Volcanic island arcs form as volcanoes emerge from the sea.
• Examples include the Aleutian, Mariana, and Tonga islands.

31. Oceanic-Oceanic Convergence

32. Convergent Boundary ◆ Continental-Continental
• When subducting plates contain continental material, two continents collide.
• This kind of boundary can produce new mountain ranges, such as the Himalayas.

34. Continental-Continental Convergent Boundary

36. Transform Boundary
◆ At a transform fault boundary, plates grind past each other without destroying the lithosphere.
◆ Transform faults
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel the direction of plate movement.
• They aid the movement of oceanic crustal material.

41. Testing the plate tectonics model
Paleomagnetism
Ancient magnetism preserved in rocks at the time of their formation
Magnetized minerals in rocks
Show the direction to Earth’s magnetic poles
Provide a means of determining their latitude of origin

42. Testing the plate tectonics model
Paleomagnetism
Polar wandering
The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved
Polar wandering curves for North America and Europe have similar paths but are separated by about 24 of longitude
Different paths can be reconciled if the continents are place next to one another

43. Apparent polar-wandering paths for Eurasia and North America

44. Testing the plate tectonics model
Magnetic reversals and seafloor spreading
Earth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa
Dates when the polarity of Earth’s magnetism changed were determined from lava flows

45. Testing the plate tectonics model
Magnetic reversals and seafloor spreading
Geomagnetic reversals are recorded in the ocean crust
In 1963 the discovery of magnetic stripes in the ocean crust near ridge crests was tied to the concept of seafloor spreading

46. Paleomagnetic reversals recorded by basalt at mid-ocean ridges

47. Testing the plate tectonics model
Magnetic reversals and seafloor spreading
Paleomagnetism (evidence of past magnetism recorded in the rocks) was the most convincing evidence set forth to support the concept of seafloor spreading
The Pacific has a faster spreading rate than the Atlantic

48. Testing the plate tectonics model
Plate tectonics and earthquakes
Plate tectonics model accounts for the global distribution of earthquakes
Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory
Deep-focus earthquakes are closely associated with subduction zones
The pattern of earthquakes along a trench provides a method for tracking the plate's descent

49. Deep-focus earthquakes occur along convergent boundaries

50. Earthquake foci in the vicinity of the Japan trench

51. Testing the plate tectonics model
Evidence from ocean drilling
Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment
Age of deepest sediments
Thickness of ocean-floor sediments verifies seafloor spreading
Evidence #3

52. Testing the plate tectonics model
Hot spots
Caused by rising plumes of mantle material
Volcanoes can form over them (Hawaiian Island chain)
Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary
Evidence #4

53. The Hawaiian Islands have formed over a stationary hot spot

54. Measuring plate motions
A number of methods have been employed to establish the direction and rate of plate motion
Volcanic chains
Paleomagnetism
Very Long Baseline Interferometry (VLBI)
Global Positioning System (GPS)

55. Measuring plate motions
Calculations show that …
Hawaii is moving in a northwesterly direction and approaching Japan at 8.3 centimeters per year
A site located in Maryland is retreating from one in England at a rate of about 1.7 centimeters per year

56. The driving mechanism
Plate Tectonics supplies the mechanism for Wegener’s continental drift idea
No one driving mechanism accounts for all major facets of plate tectonics
Researchers agree that convective flow in the rocky 2,900 kilometer-thick mantle is the basic driving force of plate tectonics
Several mechanisms generate forces that contribute to plate motion
Slab-pull
Ridge-push

57. Mechanism of Plate Motion
◆ Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement.
Convective flow is the motion of matter resulting from changes in temperature.

58. ◆ Slab-Pull and Ridge-Push
Slab-pull is a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along. It is thought to be the primary downward arm of convective flow in the mantle.
Ridge-push causes oceanic lithosphere to slide down the sides of the oceanic ridge under the pull of gravity. It may contribute to plate motion.

59. ◆ Mantle Convection
Mantle plumes are masses of hotter-than-normal mantle material that ascend toward the surface, where they may lead to igneous activity.
The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion.

60. Mantle Convection Models