1. Plate Tectonics Chapter 19

2. Alfred Wegener Proposed hypothesis in 1915 Published The Origin of Continents and Oceans Continental drift hypothesis Supercontinent Pangaea began breaking apart about 200 million years ago Continental drift: An idea before its time

3. Pangaea approximately 200 million years ago

4. The great debate Objections to drift hypothesis Inability to provide a mechanism capable of moving continents across globe Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice

5. Matching of mtn ranges on continents

6. Paleoclimatic evidence for Continental Drift

7. The great debate Continental drift and the scientific method Wegner’s hypothesis was correct in principle, but contained incorrect details For any scientific viewpoint to gain wide acceptance, supporting evidence required

8. Continental drift and paleomagnetism Renewed interest in continental drift came from rock magnetism Magnetized minerals in rocks Show direction to Earth’s magnetic poles Provide a means of determining their original latitude

10. Continental drift and paleomagnetism Polar wandering Apparent movement of magnetic poles in volcanic rocks indicates continents move Shows Europe was closer to equator when coal-producing swamps existed

11. Apparent polar-wandering paths for Eurasia and North America

12. The scientific revolution begins During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s

13. The scientific revolution begins Geomagnetic reversals Earth's magnetic field periodically reverses polarity – north magnetic pole becomes south magnetic pole, vice versa Dates when polarity of Earth’s magnetism changed were determined from lava flows

15. Paleomagnetic reversals recorded by basalt at mid-ocean ridges

16. Age of Oceanic Crust

17. Geomagnetic reversal Paleomagnetism was the most convincing evidence to support concepts of continental drift and seafloor spreading

18. Plate tectonics: The new paradigm More encompassing theory than continental drift Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading

19. Plate tectonics: The new paradigm Earth’s major plates Associated with Earth's strong, rigid outer layer –Known as the lithosphere –Consists of uppermost mantle and overlying crust –Overlies a weaker region in the mantle called the asthenosphere

20. Basal tractions drive plate motions

21. Earth’s major plates Seven major lithospheric plates Plates are in motion and change in shape and size Largest plate is the Pacific plate Several plates include an entire continent plus a large area of seafloor

22. Earth’s major plates Plates move relative to each other at a very slow but continuous rate –Average about 5 centimeters (2 inches) per year –Cooler, denser slabs of oceanic lithosphere descend into the mantle –Motion defined by rotation around a pole

23. Plate boundaries Interactions among individual plates occur along their boundaries Types of plate boundaries –Divergent plate boundaries –Convergent plate boundaries –Transform fault boundaries

24. Types of Plate Margins

25. Divergent plate boundaries Most are located along the crests of oceanic ridges Oceanic ridges and seafloor spreading seafloor is elevated forming oceanic ridges

26. Oceanic ridges and seafloor spreading Seafloor spreading occurs along the oceanic ridge system Spreading rates and ridge topography Ridge systems exhibit topographic differences Topographic differences are controlled by spreading rates (see map of age of oceanic crust for width of ridges relative to their age)

27. Divergent boundaries are located mainly along oceanic ridges

28. Spreading rates and ridge topography Topographic differences are controlled by spreading rates –Slow rates (1-5 cm/year), rift valley develops on ridge crest (30 to 50 km wide, 1500-3000 m deep) –Intermediate spreading rates (5-9 cm/year), rift valleys are shallow with subdued topography –At rates > 9 cm/year no rift valley develops or are narrow and extensively faulted

29. Divergent boundaries in Continents Continental rifts Splits landmasses into two or more smaller segments

30. Divergent boundaries Continental rifts Example includes East African rifts Produced by extensional forces acting on the lithospheric plates Not all rift valleys develop into spreading centers Otherwise Nevada would be an ocean!

31. The East African Rift

32. Development of Continental Rift into Ocean Basin

33. Convergent plate boundaries Old portions of oceanic plates are returned to the mantle Surface expression of descending plate is an ocean trench Called subduction zones Average angle at which oceanic lithosphere descends into the mantle is about 45 

34. All have same basic characteristics, but can have highly variable features Types of convergent boundaries Oceanic-continental convergence –Denser oceanic slab sinks into the asthenosphere –Bathymetry marked by trench –As plate descends, partial melting of mantle rock makes basaltic or andesitic magmas –Volcanic mountains associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades )

35. Types of Arcs

36. Types of convergent boundaries Oceanic-oceanic convergence –When two oceanic slabs converge, one descends beneath the other –Often forms volcanoes on the ocean floor –If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

37. Swim through the Marianas Trench

38. Types of Arcs

39. Types of convergent boundaries Continental-continental convergence –Continued subduction brings continents together –Less dense, buoyant continental lithosphere does not subduct –Result is a collision between two continental blocks –Process produces mountains (Himalayas, Alps, Appalachians)

40. The collision of India and Asia produced the Himalayas

41. Transform fault boundaries Third type of plate boundary Plates slide past one another and no new lithosphere is created or destroyed Transform faults Most join two segments of a mid-ocean ridge as parts of linear breaks in the oceanic crust known as fracture zones Accommodate simultaneous movement of offset ridges

42. Transform faults accommodate movement on offset ridge segments

43. Testing the plate tectonics model Plate tectonics and earthquakes Plate tectonics model accounts for the global distribution of earthquakes –Absence of deep-focus earthquakes along the oceanic ridge is consistent with tectonic theory –Deep-focus earthquakes associated with subduction zones –The pattern of earthquakes along a trench provides method to track plate's descent

44. Deep-focus earthquakes occur along convergent boundaries

45. Earthquakes near Japan trench

46. Evidence from ocean drilling Most convincing evidence confirming seafloor spreading comes from drilling directly into ocean-floor sediment –Age of deepest sediments –Thickness of ocean-floor sediments verifies seafloor spreading

47. Hot spots Caused by rising plumes of mantle material Volcanoes form over them (Hawaiian Island chain) Mantle plumes are long-lived structures and originate at great depth, perhaps at core-mantle boundary

48. The Hawaiian Islands form over stationary hot spot

49. No one driving mechanism accounts for all major facets of plate tectonics Researchers agree that convective flow in 2,900 km-thick mantle is main driving force of plate tectonics (by basal traction) Other mechanisms generate forces that contribute to plate motion Slab-pull Ridge-push

50. Importance of plate tectonics Provides a unified explanation of Earth’s major surface processes, especially oceans Within framework of plate tectonics, we find explanations for the distribution of earthquakes, volcanoes, and mountains Plate tectonics provides explanations for distribution/evolution of plants and animals and climate record