1. Plate tectonics
Geology 101
Fall 2013

2. Why does the Earth have mountains and basins?
Basic question asked by many: erosion is evident everywhere, so why haven’t all mountains eroded and filled in all basins?
Alfred Wegener proposed continental drift in 1912.

3. Evidence for continental drift
Continental “fit” – South America and Africa separated by the Atlantic
Similar rocks separated by the Atlantic
Similar climate markers (glacial striations) separated by the Atlantic
Similar fossils separated by the Atlantic

4. Unifying theory of geology
Extension of Wegener’s continental drift theory from the early 20th century
Needed not only information about rocks but also breakthroughs in geochronology and geophysics
Continental drift failed to provide a sufficient mechanism (tides and wind were not enough); plate tectonics does

5. Mechanism(s) of tectonics
First, and foremost, the second law of thermodynamics says that heat is transferred from hot bodies to cold bodies (e.g., Earth’s core to space)

6. Mechanism(s) of tectonics
Heat transfers in three ways: radiation, conduction and convection. It is the last of these that occurs within the mantle.
Convection (and internal heating) seems to keep the mantle hot and plastic enough for plates to sink into it -- may not be enough to move plates

8. Mechanism(s) of tectonics
Slab pull seems to be the major “drag”, ridge push and plate sliding are more minor forces

9. Tenets of plate tectonics
Plates exist

10. Plate materials Remember: lithosphere, not crust
Continental plates are made of granite
Oceanic plates are made of basalt
Granite and basalt are types of igneous rocks

11. Tenets of plate tectonics
Plates move

12. Hotspots
Focused convection plumes from mid-mantle

13. Hotspots show plate movement

14. Problems with motion
Hotspots may not be fixed in position and they are certainly not permanent
GPS does a better job of measuring annual rates, though over the short-term only

15. Tenets of plate tectonics
Plate boundaries are areas of geological phenomena such as earthquakes, volcanoes and mountain building

16. Plate interactions
Convergent

17. Types of convergent boundaries
Continental plate - continental plate convergence results in non-volcanic mountain ranges

19. Types of convergent boundaries
Oceanic plate - oceanic plate convergence results in a volcanic oceanic island arc
Subduction occurs

21. Types of convergent boundaries
Continental plate - oceanic plate convergence results in a volcanic continental arc

23. Plate interactions
Divergent

24. Types of divergent boundaries
Between two oceanic plates

26. Sea floor spreading
Oceanic divergent boundaries provided evidence for plate tectonics. Paleomagnetism (the measurement of “stripes” of different polarity minerals in ocean floor rock of different ages) allowed Hess and others (1950s) to postulate the movement of the sea floor itself.

27. Types of divergent boundaries
Between two continental plates

29. Plate interactions
Transform

31. Tenets of plate tectonics
Plate interiors are geologically quiet, except for hotspots

33. Hotspots are areas of magma upwelling from the mantle
They can occur in the middle of a tectonic plate (e.g., Hawaii) or at a plate boundary (e.g., Iceland)

34. As shown below for Hawaii, the plate onto which the hotspot magma erupts continues to move, leaving a “trail” of extinct volcanoes
Hotspots remain more or less stationary during their lifetime, though there is some controversy about this.

35. Grander synthesis: the Wilson (supercontinent) cycle
J. Tuzo Wilson (1970) suggested that plate tectonics allow supercontinents to rift apart and reform over and over on a roughly half billion year cycle
Evidence for this: evidence of pre-Pangea supercontinents, such as Rodinia

41. Summary of plate tectonic theory
Plates exist
Plates move
Plates interact and generate geologic phenomena
Interiors of plates are geologically quiet*

42. Synthesis
All three types
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