1. The Unifying Theory of Geology
Plate Tectonics
The Unifying Theory of Geology

2. Standards/Purpose Know that the Lithosphere is broken into plates
Describe Earth’s plates and how they interact with each other
Explain plate tectonic theory and the evidence that supports it

3. Plate Tectonics
The lithosphere is broken up into plates that move over the aesthenosphere.
The plates collide, spread apart & slide past each other.
There are 12 major plates and several minor ones.

4. Plate Tectonics

5. Unifying Theory of Geology
Plate tectonics explains the geologic processes that occur on Earth.
It provides a framework into which most aspects of geology can be placed.
It explains the distribution of large-scale geologic features such as volcanoes, earthquakes, mountain chains and seafloor structures.
Ex: Ring of Fire – the belt characterized by numerous volcanoes & earthquakes that follows the edge of the Pacific Plate.

6. Ring of Fire

7. Continental Drift: the start of the theory
Idea started in 1912 with Alfred Wegner’s concept of continental drift – the large-scale movement of continents over the globe.

8. Continental Drift: the start of the theory
Wegner came up with the idea of a supercontinent, which he called Pangaea, that broke up into continents that we know today.
It was surrounded by a single ocean called Panthalassa.
It first broke up into two continents, the northern continent of Laurasia and the southern continent of Gondwanaland.

9. Pangaea

10. Wegner’s Early Evidence for Continental Drift:
1. Jig-saw puzzle fit of the coasts on both sides of the Atlantic.

11. Wegner’s Early Evidence for Continental Drift:
2. Similarity of rocks, geologic structures and fossils on both sides of the Atlantic.
3. Paleoclimatology data – Glacial deposits from the Permian in S. America, Africa, India and Australia; some in places close to equator today.
Explained by Gondwanaland being located in the southern polar region.

12. Problem with theory of Continental Drift
No driving mechanism to explain how continents could move over Earth’s surface, so not accepted until 1960’s.

13. Seafloor Spreading
The evidence that led to the acceptance of the theory of plate tectonics.
Seafloor spreading – creation of new seafloor where oceanic crust is pulling apart.
Evidence of seafloor spreading came during WWII when mapping of the Mid-Atlantic Ridge showed a deep valley or rift running down the center.

14. Recent Evidence for Plate Tectonics
1. Post WWII mapping of Atlantic crust showed magnetic striping (changes in magnetic direction from north to south and back repeatedly) that is symmetrical about the mid-Atlantic Ridge.

15. Recent Evidence for Plate Tectonics
2. Deep sea drilling showed that sediments in oceanic crust become older symmetrically with increasing distance from the mid-ocean ridge.
Age of the seafloor

16. Recent Evidence for Plate Tectonics
Satellite measurements – use of satellites to map the ocean floors. Locates mid-ocean ridges, deep-sea trenches and transform boundaries.
Global Positioning System (GPS) – ground based detectors send signals to satellites to directly record plate motions.

17. Driving Mechanism
This is what causes the plates (and thus the continents) to move.
The mantle is heated from the early formation of the Earth and from radioactive decay.
This heat creates convection cells – hot rock is less dense & more buoyant so it rises from deep in the mantle. As it nears the top of the mantle, it cools, becomes more dense, moves laterally and sinks.
This is the same kind of thing that happens in a pot of soup that is boiling on the stove.

18. Convection The mechanism that drives plate tectonics.
As the cooled mantle rock moves laterally, it pulls the plate with it, causing a rift.
As the cooled rock sinks, it continues to drag the plate with it.
This process is called
plate tectonics.
Recall that plates are
made up of the
lithosphere.

19. Slab Pull
Until the 1990’s, scientist thought that seafloor spreading was pushing the plates apart.
It is now thought that slab pull (the gravitational sinking of plates in subduction zones) has a greater roll in moving the plates.

20. Plate Boundaries There are 3 types of plate boundaries.
Convergent boundary
Divergent boundary
Transform boundary

21. Convergent Boundaries
Where two plates are colliding.
The plates are under forces of compression.
There are 3 types of convergent boundaries:
Oceanic-continental
Oceanic-oceanic
Continental-continental

22. Oceanic-Continental Convergence
Subduction - the sinking of one tectonic plate beneath another, occurs at this type of convergent boundary.
Subduction causes the lithospheric plate to be destroyed.
Oceanic plate is subducted beneath the continental plate because oceanic crust is made of basalt, which is denser than the continental crust of granite.
Causes uplift on continental plate, volcanoes and earthquakes.
Ex: Cascade Volcanoes (Mt. St. Helens, Rainier…)

23. Oceanic-Continental Convergence
Oceanic plate is subducted beneath continental plate

24. Oceanic-Oceanic Convergence
The denser of the two oceanic crusts is subducted under the less dense crust.
The older the oceanic crust, the cooler and denser it is.
Causes volcanic island arcs.
Ex: Islands of the West Indies, such as Montserrat (volcano in “Forces of Nature” film).

25. Oceanic-Oceanic Convergence
Older oceanic plate subducted beneath younger oceanic plate.

26. Continental-Continental Convergence
Continental crust is too buoyant to sink far.
This type of collision causes crust to pile up and create mountain ranges.
Ex: Himalayas – highest mountain range in the world is formed by the collision of the Indian Plate with the Eurasian Plate.

27. Continental-Continental Convergence
Continental plates push together and build up mountain ranges.

28. Himalayas

29. Divergent Boundaries Where two plates are pulling apart.
The plates experience tensional forces.
New lithosphere is created at divergent boundaries.
Two types of divergent boundaries:
Mid-Ocean Ridges
Rift Valleys

30. Divergent Boundary
Oceanic plates pull apart and magma wells up to create new crust

31. Mid-Ocean Ridges
Divergent boundaries in oceanic crust that form an elevated ridge.
This is where seafloor spreading occurs.
Exs:
-Mid-Atlantic Ridge – divergent boundary between the north American Plate and the African & Eurasian Plates. Runs up the middle of the Atlantic Ocean and surfaces in Iceland.
-East Pacific Rise – divergent boundary between the Pacific Plate and the Nazca Plate.

32. Iceland
Surface expression of the Mid-Atlantic Ridge

33. Rift Valleys
Divergent boundaries in continental crust that form trough or valley-like depressions.
Exs: - Rio Grande Rift – divergent boundary in the North American Plate in which Albuquerque is located.
- African Rift Valley – divergent boundary in the African Plate
Ol Doinya Lenge Volcano: East African
Rift Valley

34. Transform Boundaries
Where two plates are sliding horizontally past each other.
The plates experience shearing forces.
Lithosphere is neither created nor destroyed.
Earthquakes are common at these boundaries.
Ex: San Andreas Fault – is formed where the Pacific Plate and North American Plate are sliding past each other.

35. Transform Boundary
At the San Andreas fault, the Pacific plate moves north & the North American plate moves south.

36. Hot Spots
Areas where plumes of hot material rise from deep in the mantle.
They produce hot spot volcanoes in the middle of plates (most volcanoes occur at plate edges).
The plates move over the hotspots, leaving a trail of volcanoes. These volcanoes show the direction of plate motion.
Exs:
- Hawaiian Islands – chain of volcanoes formed as the Pacific Plate moves over a hot spot.
- Yellowstone – formed by a hot spot in middle of North American Plate (unusual because continental crust usually too thick for hot spots to form volcanoes).

37. Hawaiian Islands Kauai is oldest island & the Big Island is youngest.
Direction of plate motion is to the NW.

38. Kilauea

39. Study these notes!!!
You must know this information to be able to do the labs and activities in this unit!!!