1. Ch – 15 Plate Tectonics

2. Earth’s layers by physical properties
Crust and upper mantle:
Lithosphere – rigid solid which make up the tectonic plates, includes both crust and upper mantle
Asthenosphere – partially molten “weak” layer
Lower mantle (mesosphere) mostly solid
Core
outer core (molten)
inner core (solid)

3. Ocean and Continental Crust
Oceanic Crust
primarily basalt
4-7 km thickness (thin relative to continental crust)
denser (heavier) than continental crust
Continental Crust
primarily granite
20-70 km thickness
less dense (will not undergo subduction)

4. Age of Sea Floor Rocks (red-young, blue-old)

5. FIGURE 6.10 Earth’s lithosphere is broken into seven large tectonic plates, called the African, Eurasian, Indian-Australian, Antarctic, Pacific, North American, and South American plates.
A few of the smaller plates are also shown. White arrows show that the plates move in different directions. The three different types of plate boundaries are shown below the map: At a
transform plate boundary, rocks on opposite sides of the fracture slide horizontally past each other. Two plates come together at a convergent boundary. Two plates separate at a divergent
boundary.
Fig. 6.10, p.139

6. Plate Tectonics: the new paradigm
From left to right:
Transform boundary (conservative)
Convergent boundary (destructive)
Divergent boundary (constructive)

7. What happens at a divergent plate boundary
Sea Floor Spreading
Two plates move apart
Mantle material upwells to create new seafloor
Mid-Oceanic ridges (underwater mountain range) develop along well-developed divergent boundaries
Mid-Atlantic Ridge
East Pacific Rise

8. Geologic features found at divergent boundaries
volcanic activity (often underwater)
mid-ocean ridge (underwater mountain chain)
very young volcanic rock
“linear” seas (e.g. Red Sea, Sea of Cortez)
rift valley – long narrow valley, such as found in East Africa

9. Figure 15.10

10. Sea Floor Spreading on Land
Sea floor spreading adds thin, low-elevation ocean crust to landmass. Eventually water fills in
Arabian peninsula split from African continent
Process continues in East Africa rift valleys (note lakes filling in low lying ocean crust)
Somali Plate?

12. Geologic features found at divergent boundaries
volcanic activity (often underwater)
mid-ocean ridge (underwater mountain chain)
very young volcanic rock
“linear” seas (e.g. Red Sea, Sea of Cortez)
rift valley – long narrow valley, such as found in East Africa
shallow focus earthquakes

13. What happens at a convergent plate boundary
1. Oceanic-continental subduction
Denser oceanic lithosphere sinks into the asthenosphere under more buoyant continental lithosphere
Pockets of magma develop and rise
Continental volcanic arcs – chain of volcanoes a short distance from plate boundary (e.g. Andes, Cascades)
Deep focus earthquakes

14. Figure 15.14a

15. What happens at a convergent plate boundary
2. Oceanic-oceanic subduction
Two oceanic plates converge and the older, denser one descends beneath the younger, more buoyant one.
Pockets of magma develop and rise
Volcanic Island Arcs forms as volcanoes emerge from the sea
Examples include Japan, Philippines, and the Aleutian Island,
Deep focus earthquakes 15

16. What happens at a convergent plate boundary
Subduction (Cont’d)
Oceanic-oceanic convergence
Two oceanic slabs converge and the older, denser one descends beneath the younger, more buoyant one.
Forms volcanoes on the ocean floor
Volcanic Island Arcs forms as volcanoes emerge from the sea
Examples include the Aleutian, Mariana, and Tonga islands

17. Figure 15.14b

18. What happens at a convergent boundary
Continental Collision (no subduction)
Continental-continental convergence
When subducting plates contain continental material, two continents collide
Can produce non-volcanic mountain ranges such as the Himalayas

19. Figure 15.14c

20. What happens at 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 tranform fault boundaries include
San Andreas Fault,
Alpine Fault
Anatolian Fault (Turkey)

21. Figure 15.16

22. Figure 15.17

23. Modern discoveries supporting Plate Tectonic Theory
Mid-ocean ridges – underwater mountain chains that circle the globe and often mimic the shape of the coastline
Distribution and depths of earthquakes and volcanoes
Relatively young age of the oceanic crust (less than 180 million years)
Lack of deep-ocean sediment

27. Testing the plate tectonics model
Evidence for the plate tectonics model
Paleomagnetism
Probably the most persuasive evidence for sea floor spreading
Ancient magnetism preserved in rocks
Paleomagnetic records show Earth's magnetic field reversals recorded in rocks as they form at oceanic ridges

28. Figure 15.19

29. Paleomagnetic reversals recorded by basalt flows at mid-ocean ridges

30. Figure 15.24

31. Testing the plate tectonics model
Evidence from ocean drilling
Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment
Age of deepest sediments
Thickness of ocean-floor sediments verifies seafloor spreading

33. Testing the plate tectonics model
Hot spots and mantle plumes
Caused by rising plumes of mantle material
Volcanoes can form over them (Hawaiian Island chain)
Originate at great depth, perhaps at the mantle-core boundary

34. Figure 15.18

35. Testing the Plate Tectonics Model
Earthquake depths
Definite patterns exist
Shallow focus occur along the oceanic ridge system
Almost all deep-focus earthquakes occur in the circum-Pacific belt, particularly in regions situated landward of deep-ocean trenches

37. 37

38. What drives plate motion
Driving mechanism of plate tectonics
No one model explains all facets of plate tectonics
Earth's heat is the driving force
Several models have been proposed

39. Figure 6.13 Soup convects when it is heated from the bottom of the pot.
Fig. 6-13, p.136

40. What drives plate motion
Slab-pull and slab-push model
Descending oceanic crust pulls the plate
Elevated ridge system pushes the plate
Plate-mantle convection
Mantle plumes extend from mantle-core boundary and cause convection within the mantle

41. Several mechanisms contribute to plate motion
Figure 15.26

42. Whole-mantle convection
Figure B