1. Plate Tectonics
Chapter 8

2. What Is Plate Tectonics?
Section 1
What Is Plate Tectonics?
Theory that describes the formation, movements, and interactions of Earth’s lithospheric plates.
Lithosphere is broken into large “plates” that move (“float”) on top of the asthenosphere.
3 ways
Away from each other (divergent plate boundaries)
Toward each other (convergent plate boundaries)
Slide past each other (transform plate boundaries)

3. Early Ideas About Plate Movements
Have you ever noticed that the continents look like they would fit together like the pieces of a puzzle??? If you have, you’re not alone….
1912 Alfred Wegener
hypothesis  continental drift
Continents have “drifted” over time
Observations to support hypothesis
Shape of continents
Fossils
~Mesosaurus
*South America & Africa
Distinctive rock formations
Climate change evidence
~coal deposits
Continental Drift Video

4. Evidence for Joined Continents Interaction

5. The Theory of Plate Tectonics
Continents (and ocean basins) are part of lithospheric plates.
Plate movement = continent movement
Evidence/Explanations of many geologic processes
Location of volcanoes & earthquakes
Formation of new crust (ocean floor)

6. Locations of Earthquakes & Volcanoes
Occur in concentrated areas
Mark locations of plate boundaries
Where plates move apart, together, or past each other
Strain builds up  releases causing EQs
Molten rock rises  volcanic activity
Pacific Ocean
Ring of Fire
Plates & Volcanoes Video

7. Locations of Earthquakes & Volcanoes
Ring of
Fire

8. Magnetism & the Age of the Ocean Floor
Magnetic properties & ages of igneous rocks on the ocean floor provide evidence for theory of plate tectonics.
Some igneous rocks contain magnetic minerals.
Provide record of direction of Earth’s magnetic field when rock formed

9. Magnetism & the Age of the Ocean Floor
Records of Earth’s magnetic field
Some rocks recorded reversals in magnetic field
normal polarity
what we know today as N & S magnetic poles
reversed polarity
present N magnetic pole became the S magnetic pole
present S magnetic pole became the N magnetic pole
A # of magnetic reversals have taken place at different times over millions of years

11. Magnetism on the Ocean Floor

12. Magnetism & the Age of the Ocean Floor
Mid-ocean ridge: long chain of volcanic mountains on the ocean floor with a deep central valley
Magnetic reversals are recorded in bands of rock on opposite sides of the ridge
Mirror image
Center of ridge shows current orientation of Earth’s magnetic field (normal or reversed)
Rocks at center of ridge are youngest (most newly formed)
As move further from center rocks get older
Magnetic Polarity Recorded in Rocks Animation

13. Magnetism & the Age of the Ocean Floor
Mid-ocean ridges  boundaries between lithospheric plates
New rock along ridge is formed by hot, molten rock which rises up between the plates (because it is less dense)
As new rock forms plates spread
Older rocks pushed away from ridge (both sides)
Rock at center of ridge hottest
Temperature decreases as move out from center

14. HW
Read pgs
Answer #s 1, 2, 3

15. Types of Plate Boundaries
Section 2
Types of Plate Boundaries
Earth’s lithosphere is broken in to large plates
move in 3 ways
Divergent plate boundaries
Convergent plate boundaries
Transform plate boundaries

16. Divergent Boundaries Plates move apart Most are along ocean floor
Sometimes called spreading centers
Most are along ocean floor
Creates mid-ocean ridge

17. Divergent Boundaries Rift valley Center of mid-ocean ridge
Border between 2 diverging plates
Sea-floor spreading  molten rock forces through cracks (rifts) in valley
Oceanic crust forms as rock cools
older crust moves away from mid-ocean ridge as the new crust is formed

18. Divergent Boundaries Rift valley (continued) Broken into sections
offset from each other by breaks (fracture zones)
Perpendicular (right angle) to ridge
Source of earthquakes at mid-ocean ridges
Divergent Plate Boundary Animation

19. Mid-Ocean Ridges
Mid-Atlantic Ridge
East Pacific Rise

20. Convergent Boundaries
2 plates move towards each other
2 broad classifications
Subduction boundaries
Collision boundaries

21. Subduction Boundaries
Oceanic plate subducts (plunges) below another plate
Boundary between the 2 plates is called a subduction boundary
Features of subduction boundaries
Deep-sea (or ocean) trenches
Deepest part of the ocean
Volcanic activity
Island arcs
Along the coast of continents
Earthquakes

22. Subduction Boundaries
Ocean-Ocean subduction
2 Oceanic plates collide
Deep-sea trench formed
accompanied by chain of volcanic islands (volcanic island arc) on the overriding plate
Example:
Pacific Plate subducts under Phillippine Plate
Pacific Plate pulled down
Forms Mariana Trench
Phillippine Plate overrides Pacific Plate
Forms Mariana Islands (volcanic island chain)
Subduction and Collision Boundary Animations

23. Ocean-Ocean Subduction Boundary

24. Ocean-Continent Subduction Boundary
The denser oceanic plate subducts below the less-dense continental plate
Ocean trenches form.
Volcanoes & mountains are formed on the continental plate.
Example:
Nazca Plate (off west coast of S. America) subducts under the South American Plate
Nazca Plate pulled down
Forms Peru-Chile Trench
South American Plate overrides Nazca Plate
Forms Andes Mountains and volcanoes (along western edge of S. America)
Subduction and Collision Boundary Animations

25. Ocean-Continent Subduction Boundary

26. Collision Boundaries Continent-continent collision Example:
Neither plate subducts, so the crust piles up (because the continents join to form a single larger continent)
Mountains are formed.
Example:
Indian subcontinent colliding into Eurasian Plate
Formed Himalaya Mountains
Subduction and Collision Boundary Animations

27. Continent-Continent Collision

28. Transform Plate Boundary
2 plates slide past each other at a fault.
Stress is released as an earthquake.
Example
California—San Andreas Fault
Pacific Plate (Southwestern Cali.) is moving NW
N. American Plate (rest of N. Amer.) moving SE

29. Transform Plate Boundary
Transform Boundary--San Andreas Fault Animation N

30. HW
Read pgs
Answer #s 1, 2, 3, 4, 6

31. Causes Of Plate Movement Ch. 8.3
Convection cells (caused by density differences) in the asthenosphere (mantle) cause hot material to rise, move outward (pushing the plates/ridge push), & sink as the material cools (pulling plates down/slab pull).
Convection in the Mantle Animation

32. The Big Picture

33. HW
Read pgs
Answer # 1

34. Plate Movements & Continental Growth Sec 4
Reconstructing the Past
Many different kinds of evidence give clues as to what Earth looked like in the past
Rocks in Ural & Appalachian Mountain
evidence of past subduction  formed at convergent boundary, but neither mountain range is located near a plate boundary today
Ages of rocks that form in ocean basins
Magnetic record of igneous rocks
Can reveal latitude of formation
Fossils
Organisms that lived in shallow seas  fossils found on high mountaintops
Rocks that show evidence of having been covered by glaciers, but now are in tropical areas

35. Plate Movements & Continental Growth
Plate Tectonics & Pangaea
Evidence suggests 250 million years ago (mya) that all the continents were welded together into one land mass  Pangaea

36. Plate Movements & Continental Growth
Formation of Pangaea
Use data from continents to make models of what Earth may have looked like before Pangaea
A large continental mass stretched between the south pole & the equator  Gondwana
made of smaller landmasses (S. America, southern Europe, Africa, the Near East, India, Australia, New Zealand, & Antarctica)
Other small landmasses ranged across the globe
Gondwana moved northward and converged w/ the other landmasses to form Pangaea

37. Plate Movements & Continental Growth
Break up of Pangaea
2 separate landmasses
Gondwana
Laurasia
Over time both broke into smaller landmasses which began to resemble the continents today
Pangaea is still breaking up…
The formation & break up may have happened many times in the past
Break up of Pangaea animation

39. Plate Movements & Continental Growth
Plate Tectonics & Continental Growth
P.T. affects shape of continents as well as position
Added rock materials to the margins of ancient continent cores (cratons)
~2.5 billion years ago (bya) core continental material stabilized
Ex. North American craton
at surface in most of eastern Canada  Canadian Shield
Some of the oldest rock material (~4 billion years old)
Rest of the craton is buried under a platform of sediments
Remainder of the continent is material added to the craton

41. Plate Movements & Continental Growth
Sources of Growth Material
Deep-sea sediments
Added when an oceanic plate plunges under a continental plate at a subduction boundary
Sediments from ocean floor scraped off & left behind on edge of continent
Igneous Rock
Magma rises beneath surface & cools  pluton
Volcanoes at subduction boundaries add to edges

42. Plate Movements & Continental Growth
River Sediments
Weathered & eroded materials deposited at edges
Terrane
large block of lithospheric plate that has been moved & attached to continent
3 characteristics to ID a terrane
1. bounded on all sides by faults
2. rocks & fossils found in terrane do not match those of neighboring terranes
3. magnetic record of terrane does not match surround terranes
Growth of a Continent Animation

43. HW
Read pgs
Answer # 3