1. From Continental Drift Hypothesis to Plate Tectonic Theory

2. Observation: Some of the boundaries of the continents look as though they had once fit together
Initial hypothesis, put forth by A. Wegener: In the geologic past, all the continents formed one supercontinent, which subsequently broke up and drifted apart.

3. Supporting evidence: fossils of the same type found at these now-separated continental boundaries

4. Supporting evidence: evidence of similar climates at the now-separated areas

5. Supporting evidence: matching rock types and orientation of mountain ranges at the now-separated areas

6. Continental Drift Hypothesis Rejected!
How did the continents move through the oceans?
Tidal influence of the moon? Theoretical calculations proved this to be false.
Continental crust plows through or slides over ocean crust? Observations proved this false.
The conclusion of the scientific community at the time was that the continental drift hypothesis, as presented by Wegener could not be true.

7. Sea-floor spreading –modifying the hypothesis
The sea-floor spreading hypothesis provided a new mechanism for the change in the position of the continents, based largely on post-WW2 observations:
mapping of underwater mountain ranges (oceanic ridges) that circle the globe, often parallel to continental boundaries
dredging of sea floor sediment and rocks indicated the age of the oldest ocean crust was much younger than that of continental crust.
Recurring patterns of earthquakes and volcanoes in places such as the Circum-Pacific Belt and along the oceanic ridges.

8. Sea Floor Spreading Hypothesis led to the present Plate Tectonic Theory

9. The Major Tectonic Plates

10. Plates move apart. The mechanism called Sea-Floor Spreading
Divergent Boundaries
Plates move apart. The mechanism called Sea-Floor Spreading

11. Sea-Floor Spreading
Two plates move apart, creating a rift valley between them
Magma (molten rock) from the asthenosphere pushes into the rift and creates new seafloor
Oceanic ridges (underwater mountain range) develop along well-developed divergent boundaries
Mid-Atlantic Ridge
East Pacific Rise

12. Figure 15.10

13. Sea Floor Spreading on Land?
Sea floor spreading adds thin, low-elevation ocean crust to landmass. In time, water fills in and an ocean basin will develop
Arabian peninsula split from African continent
Process continues in East Africa rift valleys (note lakes filling in low lying ocean crust).

14. Some geologists are calling this the divergent boundary of the Somali Plate

15. Forming a Divergent Boundary
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16. Convergent Boundaries
These are boundaries where plates come together and lithosphere is being deformed and/or destroyed
There are 3 types of convergent boundaries
In 2 of the 3 types the process that occurs is called subduction, where oceanic lithosphere on one tectonic plate is thrust into the mantle beneath another plate.

17. Oceanic-continental convergence
Denser oceanic slab sinks into the asthenosphere (subduction)
Pockets of magma develop and rise
Continental volcanic arcs form (e.g. Andes, Cascades).

18. Figure 15.14a

19. Motion at Plate Boundaries
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20. Oceanic-oceanic convergence
Two oceanic slabs converge and the older, denser one subducts beneath the younger, more buoyant one.
Trench and volcanic island arc forms as volcanoes emerge from the sea.
Examples include the Aleutian, Mariana, and Tonga islands, and those shown in the map.

22. Motion at Plate Boundaries
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23. Continental-continental convergence
When converging plates contain continental material at their respective boundaries, subduction cannot occur, as the plates are too buoyant to sink.
The continents collide, causing the crust to shorten and thicken
This process produces high-elevation, non-volcanic mountains over a large area, such as we see in the Himalayas today.

24. Figure 15.14c

25. Convergent Margins: India-Asia Collision
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26. The Major Tectonic Plates

27. Transform Fault Boundaries
Conservative boundary (no loss or gain of lithosphere)
Plates slide past one another
Most transform faults join two segments of sea-floor spreading
Significant non-oceanic transform fault boundaries include
San Andreas Fault,
Alpine Fault
Anatolian Fault (Turkey)

28. Figure 15.16

29. Figure 15.17

30. Breakup of Pangaea
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