1. Plate Tectonics The Discovery of Plate Tectonics A Mosaic of Plates
Types of Plate Boundaries
“How” Plates Move

2. A. Continental Drift (Alfred Wegener)
I. The Discovery of Plate Tectonics
A. Continental Drift (Alfred Wegener)
Proposed large-scale movement of the continents

3. Evidence: 1. “Puzzle fit” of continents to form “Supercontinents”
a. Suess (1900)-Gondwana
b. Wegner (1915)-Pangea

4. More Evidence… 2. Similar rock ages
3. Similar geologic structures such as mountain ranges
4. Fossil Evidence from the Mesosaurus
5. Climate Evidence such as glacial deposits
**Wegener’s theory did not gain much support because he could not explain how the tectonic plates move

5. B. Seafloor Spreading- Henry Hess)
Convection currents move plates around
Mantle source of heat and convection
proposed new and recycled seafloor
4. Evidence from fathometers on submarines during WWII around the Mid-Atlantic Ridge showed the seafloor is not completely flat
**There was no evidence for this theory, until…

6. C. Magnetic “Tape Recording”
1. Magnetic Reversals
Switching strength to the south
Preserved in lava (iron points towards stronger pole)
Age can be dated and if the seafloor is spreading from a center, should be mirror images on each side

7. Magnetic “Tape Recording” (continued)
Magnetic anomaly
Normal-positive anomaly
Reverse-negative anomaly

8. Mid-ocean ridge
Million years ago (Ma)
4.0
3.0
2.0
Ocean
crust today
5.0
3.3
2.5
0.7
0.7
2.5
3.3
5.0 million
years old

9. Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
…alternating bands of high
and low magnetism.
Iceland
Mid-Atlantic
Ridge
Mid-Atlantic Ridge
High intensity
Symmetrical bands on both sides. Why?
Low intensity

10. Looking at pole reversals preserved in the ocean floor provides the evidence needed to support seafloor spreading.
Seafloor spreading provides the mechanism for “how” the continents move! (Wegener)

11. II. A Mosaic of Plates
Entire Earth’s surface is made up of lithospheric plates
Both oceanic crust & continental crust
C. Geologic activities occur at plate boundaries:
1. Earthquakes
2. Volcanoes
3. Rifts
4. Folding
5. Faulting

12. A. Divergent Boundaries
III. Types of Plate Boundaries
A. Divergent Boundaries
Continental plate separation
Oceanic plate separation-Mid Ocean Ridges “spreading centers”

13. Divergent Boundaries Continental Plate Separation East African
Rift Valley
Somali Subplate
African Plate

14. Parallel valleys; volcanoes and earthquakes.
Divergent Boundaries
Continental Plate Separation
Parallel valleys; volcanoes and earthquakes.
East African
Rift Valley
Somali Subplate
African Plate

15. Divergent Boundaries Oceanic Plate Separation Mid- Atlantic Ridge
North American
Plate
Eurasian
Plate

16. Volcanoes and earthquakes concentrate.
Divergent Boundaries
Oceanic Plate Separation
Volcanoes and earthquakes concentrate.
Mid-
Atlantic
Ridge
North American
Plate
Eurasian
Plate

17. B. Convergent Boundaries
III. Types of Plate Boundaries
B. Convergent Boundaries
- Conserve Earth’s surface area
1. Ocean-ocean convergence
2. Ocean-continent convergence
3. Continent-continent convergence

18. Convergent Boundaries
Ocean-Ocean Convergence
Mariana Islands
Marianas Trench
Philippine
Plate
Pacific Plate

19. Deep-sea trench; volcanic island arc.
Convergent Boundaries
Ocean-Ocean Convergence
Deep-sea trench; volcanic island arc.
Mariana Islands
Marianas Trench
Philippine
Plate
Pacific Plate

20. Convergent Boundaries
Ocean-Continent Convergence
Andes
Mountains
Peru-Chile Trench
South
American
Plate
Nazca Plate

21. Convergent Boundaries
Ocean-Continent Convergence
A volcanic belt of
mountains forms.
Andes
Mountains
Peru-Chile Trench
South
American
Plate
Nazca Plate

22. *when oceanic crust is involved in convergence the process of subduction occurs, where the more dense plate is pushed under the least dense plate

23. Convergent Boundaries
Continent-Continent Convergence
Tibetan
Plateau
Himalaya
Main
thrust
fault
Indian-Australian Plate
Eurasian
Plate

24. Convergent Boundaries
Continent-Continent Convergence
Crust crumbles, creating high
mountains and a wide plateau.
Tibetan
Plateau
Himalaya
Main
thrust
fault
Indian-Australian Plate
Eurasian
Plate

25. C. Transform-Fault Boundaries
III. Types of Plate Boundaries
C. Transform-Fault Boundaries
Plates slide past one another
Fracture with relative displacement

26. Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Eurasian Plate
North American Plate

27. Spreading centers offset.
Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Spreading centers offset.
Eurasian Plate
North American Plate

28. Transform-Fault Boundaries
Continental Transform Fault
Pacific Plate
North American Plate

29. Offset continental crust.
Transform-Fault Boundaries
Continental Transform Fault
Offset continental crust.
Pacific Plate
North American Plate

31. As plates
move past each other...

32. As plates
move past each other...
…creek beds are offset

33. As plates move past each other... …creek beds are offset San Francisco
San Andreas
fault
Los Angeles

34. IV. “How” Plates Move A. Driving Forces
Mantle convection (heating of a fluid)- warm matter rises, cool matter sinks
Gravitational pull

35. IV. “How” Plates Move B. Plate Recyling
New lithosphere occurs at ridges
Old lithosphere occurs at subduction zones
‘Recycling’ of heat energy within upper and lower mantle creates seismic waves

36. IV. “How” Plates Move C. Convection Currents
Explain the movement of lithospheric plates
Create Mantle plumes (upwelling and collection of magma) that fuel
Hot spots
Volcanoes

37. Whole-mantle convection
Upper
mantle
700 km
Lower
mantle
2900 km
Outer core

38. Whole-mantle convection
Upper
mantle
Outer core
Inner core
700 km
Lower
mantle
2900 km
Outer core

39. Stratified convection
Boundary near
700 km separates
the two convection
systems.