1. CHAPTER 9
PLATE TECTONICS

2. Theory of
PLATE TECTONICS
Section 9.3

3. PLATE TECTONICS A FAR-REACHING THEORY
A better understanding of geologic processes
Mountain building
Paleontology
(Study of prehistoric life)
Volcanism
Earthquakes
Etc.

4. PLATE TECTONICS MODEL Earth’s surface - composed of rigid plates
Lithosphere - rigid outer shell includes:
Crust – Continental & Oceanic crust
Uppermost Mantle
Consists of numerous plates
Plates move slowly: 1 – 5 cm/yr.
Plates are mostly beneath the oceans
Pacific Plate is the largest
Asthenosphere - Weaker, amorphous layer in the mantle
Hotter & weaker than lithosphere
Allows for motion of lithosphere
(Soft & plastic)

5. LITHOSPHERE & ASTHENOSPHERE

6. LITHOSPHERIC PLATES & BOUNDARIES

7. N.A. & EURASIAN Plate boundary uncertain
PLATE & BOUNDARIES
7 major plates & a dozen smaller plates
N.A. & EURASIAN Plate boundary uncertain

8. Seismic activity (Earthquakes) Volcanism (Volcano eruptions)
PLATE & BOUNDARIES
The Outline of Plate Boundaries
Looks at geologic activity associated with the plates
Seismic activity (Earthquakes)
Volcanism (Volcano eruptions)
Mountain building

9. 400,000+ Earthquake Events:
As early as the 1920s, scientists noted that earthquakes are
concentrated in very specific narrow zones indicated by dots.

10. Actions at
Plate boundaries

11. 3 TYPES OF PLATE BOUNDARIES
DIVERGENT
CONVERGENT
TRANSFORM

12. Types of Plate Boundaries
Divergent Boundaries (constructive)
Two plates move apart
Molten mantle Rises to surface
Creates new seafloor
Convergent Boundaries (destructive)
Two plates move together
Ocean plate plunges into mantle beneath overriding plate
Transform Fault Boundaries
plates grind (slide) past each other
Crust is not destroyed or created

13. DIVERGENT PLATE BOUNDARY Divergent/Constructive Plate Boundary (spreading apart)
Most occur along ocean ridges
Two plates move apart
Causes:
Seafloor spreading
Rifts / Rift Valleys
Molten mantle material Rises to surface
Creates new seafloor

14. Mid-Atlantic Ridge

15. Oceanic spreading ridge E.g. Mid-atlantic ridge
Close-up: mid-ocean ridge & sea-floor spreading
Oceanic spreading ridge E.g. Mid-atlantic ridge

16. Ocean crust broken by “rift-type” faults
SEAFLOOR SPREADING
The Process where plate tectonics forms new oceanic lithosphere
Ocean crust broken by “rift-type” faults
Fast spreading
“Pacific” type
ridge
Slow spreading
“Atlantic” type
ridge

17. FORMATION OF AN OCEAN
Up-warping
Magma pushes up the crust

18. Divergent: Atlantic Ridge
LAVA FOUNTAINS
KRAFLA VOLCANO
ICELAND

19. RIFTS & RIFT VALLEYS

20. Ocean Rifts Rifts form in center of mid-ocean ridges
Magma from mantle flows up and creates new crust
older crust is pushed away from the rift

21. CONTINENTAL RIFTS Rift Valleys: (Divergent plate Boundary)
Can occur on land
Volcanoes often line rift valleys
Magma from Mantle pushes up under continent
Continent is stretched and broken
Valley increases in size until water fills in to form a sea or ocean
EAST AFRICAN RIFT (Triple Junction)
The Continent breaks in 3 places
At least 2 of these “arms” split apart and become an ocean.

22. Diagram of a rift valley E.g. West African rift

23. Ol Doinyo Lengai volcano erupts inside the Africa rift zone

24. A modern triple junction
East African Rift
The “Afar Triangle”
A modern triple junction

25. Convergent Plate Boundaries

26. CONVERGENT PLATE BOUNDARY Covergent/Destructive Plate Boundary
Two Plates come together
Ocean plate plunges into mantle beneath overriding plate
destruction (recycling) of crust takes place along convergent boundaries
Overhead view
Sometimes one plate sinks
(is subducted) beneath the
other plate
SUBDUCTION ZONE

27. Example: Andes Mountains
West margin of the South American continent
oceanic Nazca Plate is pushed toward and beneath the continental of South America on the
South American Plate
typical example of a convergent plate boundary

28. Continental-Continental
3 Types of Convergent boundaries
Oceanic-Continental
Oceanic-Oceanic
Continental-Continental

29. Continental volcanic arc
3 Types of Convergent plate boundaries
Oceanic - Oceanic convergence
E.g. Japan, New Zealand
Oceanic - Continental convergence
E.g. Andes & Sierra Nevada Mtns.
Continental - Continental convergence
E.g. Himalaya Mtns.
Volcanic island arc
Sea-floor
&
continental sediment
Continental volcanic arc

30. Oceanic-Oceanic Convergent Boundary
Subduction processes in oceanic-oceanic plate convergence form volcanic island arcs along side deep ocean trenches

31. Features formed by: Oceanic - Oceanic convergence
Ocean - Ocean Boundary
Features formed by: Oceanic - Oceanic convergence
Deep-Ocean Trench
Volcanic Island arc
Piled up sedimentary rock

32. example of a “ Volcanic Island Arc” system
Japan:
example of a “ Volcanic Island Arc” system
Deep-Ocean Trench
Volcanic Island arc
Examples:
Aleutian Islands
Mariana Islands
Lesser Antilles
Tonga Islands

33. Oceanic-Continental Convergent Boundary
Subduction zone forms w/deep ocean
trench
(sediment from the continent fills the trench)
Forms Continental Volcanic Mountain Arcs

34. Oceanic-Continental Feature formed by: convergence
Continental volcanic arcs
Examples:
Andes mtns.
Sierra Nevada mtns.
Cascades mtns.

35. Continental-Continental Convergent Boundary
Neither plate subducts completely, they
just pile up like two colliding icebergs
plates resist downward motion
the continental rocks are relatively light
Mountains formed
Examples:
Appalachians
Himalayas
Urals
Alps

36. Features formed by: Continental - Continental convergence
Old ocean sediments
mountains
Uplifted plateau

37. HIMALAYAS Collision between the Indian and Eurasian plates pushed
up the Himalayas and the
Tibetan Plateau

38. RING OF FIRE OCEANIC-CONTINENTAL COLLISONS SUSTAIN MOST
VOLCANIC ACTIVITY AROUND THE PACIFIC OCEAN

39. Transform Boundaries

40. TRANSFORM BOUNDARY Two Plates grind (slide) past one another
Two Plates grind (slide) past one another
No new crust created nor is crust destroyed
Transform Faults - (Strike-Slip Fault)
Most are in oceanic crust
Perpendicular to mid-ocean ridges
Parallel to the direction of plate movement
Aids movement of crustal material

42. North American plate
Pacific plate

43. Example: San Andreas fault in California Pacific Plate slides past the
North American Plate.

44. Volcanoes and Plate boundaries Continental Volcanoes
Oceanic Volcanoes
Continental Volcanoes
Subduction Zone
Subduction Zone
Hot Spot
Hot Spot
Rift
Rift

45. Tectonic Settings and Volcanic Activity