1. Continental Drift

4. Evidence: Fossils

5. Evidence: Rock Sequences

6. Matching Mountain Ranges

7. Glacier Evidence

8. Evidence: Polar Wander??

9. “Uh-Oh!”: Problems with Continental Drift….

10. …an alternate explanation?
1928: Arthur Holmes suggested fluids rise toward the crust, spread, and sink again.
1930: Wegner dies, unable to adapt Holmes’ idea to his theory.

11. Mantle/Crust Structure

12. A new hope: Sea-Floor Spreading
MID-ATLANTIC RIDGE

13. Evidence for Plate Tectonics
The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading.

14. Polarity of the Ocean Crust

15. Sea-Floor Spreading Divergent Plate Boundary

18. PILLOW BASALTS FORMING AT THE EAST PACIFIC RIFT VALLEY

22. The earth’s magnetic field has reversed hundreds of times

26. EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT

27. In Afar, Ethiopia, a 40-mile magmatic rift that opened up 1 year ago

28. Spreading Center

29. Evidence for Plate Tectonics
 Ocean Drilling
The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted.
The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.

30. Which graph best represents the geologic age of the surface bedrock on the ocean bottom?

31. Mid-ocean ridges (rifts) normally form where tectonic plates are
(1) converging (3) stationary
(2) diverging (4) sliding past each other
The motion of the convection currents in the mantle beneath the Atlantic Ocean appears to be mainly making this ocean basin
(1) deeper (3) wider
(2) shallower (4) narrower

32. If new crust is created at divergent boundaries, is earth getting larger?
CONVERGENT BOUNDARIES
Oceanic crust - Oceanic Crust
older, denser crust subducts forming ocean trench
Oceanic Crust – Continental Crust
denser oceanic crust subducts forming ocean trench
Continental Crust – Continental Crust
resists subduction - folding, faulting, and mountain building

33. Oceanic Crust – Oceanic Crust
Older, denser oceanic crust is SUBDUCTED
VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS

35. Oceanic-Continental Convergent Boundary - SUBDUCTION
DENSER OCEANIC CRUST GOES DOWN INTO MANTLE

37. Continental Crust – Continental Crust Convergent Boundary NO SUBDUCTION - COLLISION ZONE

39. Collision of India and Asia

42. More Evidence for Plate Tectonics
 Earthquake Patterns
Scientists found a close link between deep-focus earthquakes and ocean trenches.
The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory.

43. Convergent plate (subduction) boundaries have deeper earthquakes Divergent plate boundaries and collision zones have shallow earthquakes

44. f07_58_pg196
f07_58_pg196.jpg
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45. The greatest number of earthquakes in the cross section occurred at:
Sea level
Between sea level and a depth of l00 kms.
3. at a depth between 100 – 300 kms.
4. At a depth between 300 – 600 kms.

46. Which cross section has arrows that best represent the relative motion of the crustal plates along the Wadati-Benioff zone beneath the Tonga Trench?

47. TRANSFORM PLATE BOUNDARIES
Transform Fault Boundaries
TRANSFORM PLATE BOUNDARIES
At a transform fault boundary, plates grind past each other without creating or destroying the lithosphere.
SHALLOW EARTHQUAKES
 Transform faults
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel the direction of plate movement.

48. Transform Fault Boundary AT MID-OCEAN RIDGES

49. SUMMARY 3 PLATE BOUNDARIES
DIVERGING
CONVERGING (SUBDUCTION ZONE)
Oceanic Crust – Continental Crust
Oceanic Crust - Oceanic Crust
CONVERGING (COLLISION ZONE)
Continental Crust – Continental Crust
TRANSFORM

50. Boundaries: Diverging

51. Boundaries: Converging SUBDUCTION

52. CONVERGING –- COLLISION ZONE
No subduction – continental crust less dense - buoyant

53. Boundaries: Transform

56. So, what causes plates to move?
Convection currents: drag and move the lithospheric plates above the asthenosphere
(three sources of heat produce the convection currents):
Leftover heat from earth’s formation
Decay of Radioactive elements
Plate friction

57. Convection Cells

58. Convection Currents in the mantle drag and pull the lithospheric plates above them

59. 3 types of plate boundaries caused by convection currents
CASCADES
MID-ATLANTIC RIDGE
TRENCH

60. The arrows shown in the asthenosphere represent the
inferred slow circulation of the plastic mantle by a process
called
(1) insolation (3) conduction
(2) convection (4) radiation

61. More Evidence for Plate Tectonics
 Hot Spots
A hot spot is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands.
Hot spot evidence supports that the plates move over the Earth’s surface.

62. HOT SPOTS
Major regions of volcanic activity in the interior of plates away from plate boundaries.
The cause of these hot spots is thought to be plumes of magma rising up from mantle producing sites of active volcanism.
As the plate moves over a hot spot, a chain of volcanic mountains forms, like Hawaiian Islands

64. Which cross section best represents the plate boundaries and mantle movement beneath line AB?

67. Hot Spot

68. This diagram provides evidence that the Pacific Crustal
Plate was moving toward the
(1) south (3) southwest
(2) east (4) northwest

69. SUMMARY OF P.T.: The plate tectonic model suggests:
(1) continents can move across the surface of the globe
(2) patterns of volcanism can change and shift across the globe as plates and their boundaries evolve and move
(3) new oceans may grow
(4) oceans basins close and are deformed to produce mountains