1. CHAPTER 2 Plate Tectonics and the Ocean Floor
Fig. 2-32

2. Thin, rigid blocks move horizontally
Plate tectonics
Thin, rigid blocks move horizontally
Interactions of plates build major features of Earth’s crust
Mountain ranges, trenches, volcanoes, etc.

3. Theory of Plate Tectonics explains:
Global distribution of
Volcanoes
Earthquakes
Faults
Mountain belts
Features of seafloor
Evolution of continents and oceans

4. Continental drift
Alfred Wegener proposed continental drift hypothesis in 1912 stating:
One large continent – Pangaea
Surrounded by single large ocean
Panthalassa
About 200 million years ago
As you can imagine, was NOT widely accepted at first

5. Evidence for continental drift
Puzzle-like fit of continents
Sir Edward Bullard fit continents at 2000m water depth (1965)
Used edge of continental shelves
Better fit than Wegener’s using coastlines
Fig. 2.3

6. Evidence for continental drift
Wegener matched sequences of rocks and mountain chains
Similar age, rock types, structures

8. Evidence for continental drift
Wegener noted glacial ages and other climate evidence
Ancient glaciation in modern tropical regions
If there is evidence of ancient glacial activity, that area had to be closer to poles at some point
Direction of glacial flow

9. Similar fossils on separate southern continents
Evidence for continental drift
Distribution of organisms
Same land plants and animals distributed in different continents (e.g., South America and Africa)
Similar fossils on separate southern continents

10. Similar fossils on separate southern continents

11. Objections to Wegener’s continental drift hypothesis:
Continents cannot “plow” through ocean crust
Celestial mechanism –gravitational forces associated with tides too small
Proposed a celestial mechanism for moving plates – incorrrect , lost favor in 1930’s
More data was needed, collected using new technologies
sonar for mapping ocean bottom
measurements of Earth’s magnetism

12. Evidence for plate tectonics
Echo-sounding
Showed mountain ranges in middle of oceans, deep trenches near continents
Radiometric dating of bottom cores
Ocean is 4 billion years old but ocean rocks are less than 180 million years old – why?

15. Evidence for plate tectonics
Earth’s magnetic field
Paleomagnetism – study of earth’s ancient magnetism
N or S magnetic alignment of magnetite particles when rock hardens
Magnetic inclination (magnetic dip)
Latitude
If rocks move on plates, the latitude of origin is reflected in the rocks magnetic dip

16. Apparent polar wandering
When rocks from different continents were aligned as Pangea, polar wandering curves matched, showing a single stationary pole, but moving plates
Fig. 2.8

17. Magnetic polarity reversals
Reversals aged by radiometric dating
About 1000 reversals over last 76 my (once per 200K years)
Fig. 2.9

18. Sea floor spreading
Harry Hess (1962)

19. Fig. 2.11
More recent

20. Seafloor Spreading and Magnetization
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21. Magnetic anomalies
Fig. 2.12

22. Evidence to support sea floor spreading
Heat flow is highest at crest of mid-ocean ridge
Most large earthquakes occur along plate margins

23. Global distribution of earthquakes
Fig. 2.13

24. Plate Boundary Features
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25. Plate tectonics theory
Lithospheric plates “float” on the asthenosphere
Large scale geologic features occur at plate boundaries
Two major tectonic forces
Slab pull
Generated by subducting plate that pulls rest behind it
Slab suction
Subducting plate drags across mantle and sucks it down toward subduction zone

26. Types of plate boundaries
Divergent
Convergent
Transform
Fig. 2.14

27. Divergent boundary features
Fig. 2.15
Plates move apart
Mid-ocean ridge
Rift valley
New ocean floor created
Shallow earthquakes
As opposed to large, deep earthquakes found at convergent boundaries

28. Divergent boundary features
Fig. 2.17
Divergent boundary features

29. Divergent Boundaries Types of spreading centers
Oceanic rise
Fast-spreading
Gentle slopes
Oceanic ridge
Slow-spreading
Steep slopes
Ultra-slow
Deep rift valley
Widely scattered volcanoes
For example,
not all of the mid-Atlantic ridge
moves at same rate, some areas are “rises”
same are “ridges”

30. Convergent boundary features
Plates move toward each other
Oceanic crust destroyed
Ocean trench
Volcanic arc
Deep earthquakes
Types:
Oceanic-continental
Continental-continental

31. Motion at Plate Boundaries
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32. Fig. 2.20

33. Types of convergent boundaries
Oceanic-continental convergence
Ocean plate subducted under continental
Continental arc
Find volcanoes on continent
Oceanic trench
Deep earthquakes

34. Fig. 2.21a,b

35. Tectonic Settings and Volcanic Activity
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36. Types of convergent boundaries
Continental-continental convergence
Uplifted mountain ranges
Deep earthquakes

37. Transform boundary features
Offsets oriented perpendicular to mid-ocean ridge
Segments of plates slide past each other
Offsets permit mid-ocean ridge to move apart at different rates
Shallow but strong earthquakes

38. Types of transform faults
Oceanic—wholly in ocean floor
Continental—extends from mid-ocean ridge across continent
San Andreas Fault
Fig. 2-23

39. Motion at Plate Boundaries
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40. Applications of plate tectonics model to intraplate features
Mantle plumes and hotspots
Volcanic islands within a plate
Island chains
Systematic variation of age
Record ancient plate motions
As plate moves over hot spot,
new islands are formed,
notice how the island of Hawaii
is newer than Kauai

41. Fig. 2.24

42. Tectonic Settings and Volcanic Activity
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43. Applications of plate tectonics model to intraplate features
Seamounts and tablemounts
Seamounts – conical on top
Tablemounts – flat on top
Wave erosion may flatten seamount

44. Applications of plate tectonics model to intraplate features
Coral reefs associated with subsiding seafloor
Fringing
Barrier
Atoll

45. Movement from mid-ocean ridge, mountain submersed
Fig. 2.27

46. Seamounts/Tablemounts and Coral Reef Stages
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47. Measuring plate motion by satellites
Fig. 2.30

48. Paleoceanography Reconstructing paleogeography Continental accretion
Continental material added to edges of continents through plate motion
Bits and pieces of volcanoes, islands, etc added to landmasses
Continental separation or rifting
Continents move apart

49. Paleo-
reconstructions
Fig. 2.31

50. Global Geography Through Geologic Time
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51. Future predictions Future positions of continents and oceans
Assume same direction and rate of plate motions as now

52. World map 50 million years from now
Fig. 2.32

53. Misconceptions All rocks and planets were formed at the same time.
Rocks became polarized by the North Pole.
The magnetism of the Earth has always been the same.

54. Ocean Literacy Standards
1a - The ocean is the dominant physical feature on our planet Earth—covering approximately 70% of the planet’s surface. There is one ocean with many ocean basins, such as the North Pacific, South Pacific, North Atlantic, South Atlantic, Indian and Arctic.
1b - An ocean basin’s size, shape and features (islands, trenches, mid-ocean ridges, rift valleys) vary due to the movement of Earth’s lithospheric plates. Earth’s highest peaks, deepest valleys and flattest vast plains are all in the ocean.

55. Sunshine State Standards
SC.7.E.6.1 Describe the layers of the solid Earth, including the lithosphere, the hot convecting mantle, and the dense metallic liquid and solid cores.
SC.7.E.6.3 Identify current methods for measuring the age of Earth and its parts, including the law of superposition and radioactive dating.
SC.7.E.6.4 Explain and give examples of how physical evidence supports scientific theories that Earth has evolved over geologic time due to natural processes.
SC.7.E.6.5 Explore the scientific theory of plate tectonics by describing how the movement of Earth's crustal plates causes both slow and rapid changes in Earth's surface, including volcanic eruptions, earthquakes, and mountain building.
SC.7.E.6.7 Recognize that heat flow and movement of material within Earth causes earthquakes and volcanic eruptions, and creates mountains and ocean basins.
SC.912.E.6.1 Describe and differentiate the layers of Earth and the interactions among them.
SC.912.E.6.3 Analyze the scientific theory of plate tectonics and identify related major processes and features as a result of moving plates.
SC.912.E.6.5 Describe the geologic development of the present day oceans and identify commonly found features.