TEACHING PLATE TECTONICS FROM THE EVIDENCE. Stephen T. Allard Associate Professor of Geoscience Winona State University

Transcription

1 TEACHING PLATE TECTONICS FROM THE EVIDENCE Stephen T. Allard Associate Professor of Geoscience Winona State University 1

2 TEACHING PLATE TECTONICS FROM THE EVIDENCE Stephen T. Allard Associate Professor of Geoscience Winona State University So what does it mean to teach from the evidence? 2

3 Open the layered Earth Quiz: The password is melt Be sure to enter the number that goes with the letter 3

4 Parts of the lithosphere (OK my preferred model) Oceanic crust is thin and dense ρ=~3.0 g/cm 3 Continental crust is thick and light ρ=~2.7 g/cm 3 Lithospheric Mantle is peridotite ρ=~3.3 g/cm 3

5 Physiographic map of the world 5

6 Physiographic map of the world We all see this fit pretty quickly, 6

7 Geographer Antonio Snider-Pellegrini (1858) Two maps showing his version of how the American and African continents may once have fit together, then later separated. (Reproductions of the original maps courtesy of University of California, Berkeley.)

8 Antonio Snider-Pellegrini (1858) vs. Modern reconstruction

9 Antonio Snider-Pellegrini (1858) vs. Modern reconstruction But one good fit does not make a theory!

10 Alfred Wegener (1915) The Origin of Continents and Oceans proposed his Theory of continental drift Named single continent Pangea Based on more bits of evidence than just the fit of the continents

11 Reconstruction explains fit of The continents 11

12 Reconstruction explains locations of past glaciations 12

13 Glacial striations form as Glacier ice flows from high point at pole outward Reconstruction explains locations of past glaciations Also leave deposits that can be recognized and interpreted 13

14 Reconstruction explains locations of Paleoclimate belts Important: interpret past global climate and past continent locations 14

15 Reconstruction explains locations of fossils 15

16 Reconstruction explains locations of mountain belts 16

17 Wegner s one flaw? The mechanism! Continent at a pole would pull apart if spun. Continental crust plows through the oceanic crust. So it was not accepted, but research continued! 17

18 How did sea shells wind up on Mountain tops? Other ideas prevailed until the 1960 s -Noah s Flood -Expanding Earth -Shriveled apple Harry Hess lead this revolution with observations of the ocean floor. It wasn t until 1968 that Plate tectonics gained a hold and was fully accepted 18

19 Here s where this workshop can go in two directions Either, I can give suggestions as to how you can: get your students to make observations first and hopefully recognize correlations before introducing the theory (perhaps with a little introduction) Then see if they can understand better the process responsible for the geologic features present. Or, I can just lecture on everything I know about plate tectonics? (all my tectonics lecture slides are included for your use) 19

20 Physiographic map of the world We all see this fit pretty quickly, but what else does this map show? 20

21 What does the sea floor look like? First by soundings Then using sonar Why the interest in Bathymetry? (hint-wwii) 21

22 What does the sea floor look like? 3 types of areas in the ocean floor Continental Margin Abyssal plain Mid-ocean ridge (appears symmetrical) 22

23 Detailed features of the sea floor Fractures perpendicular to the ridge Offset segments of ridge Trenches often adjacent To Volcanic chains Abyssal plain features Often linear arrangements 23

24 Correlating geologic processes with topographic features USGS Google Earth Page 24

25 Other observations of the sea floor Earthquakes: Occur mostly along ridges and trenches More about this when we discuss earthquakes 25

26 Earthquakes mark plate boundaries 26

27 Three Boundary Types Red arrows depict plate movement Yellow arrows depict stresses (forces) 27

28 Let s fill in this table (we will return to these for earthquakes and volcanoes) 28

29 Lets take each environment separately Let s start with-divergent boundaries 29

30 Let s start at the break-up of South America and Africa 30

31 Divergent boundaries on the continents (how continents break up) note how the crust thins before it breaks Do we see this happening today on the planet? (Milky Way bar analogy) 31

32 In SW US, extension has not included new oceanic crust, (yet) 32

33 East African Rift Typically start as a triple junction Here it has begun to form a linear sea 33

34 Mid-Continent rift 1.1 Ga North American Continent Dike Swarms in canada (East Africa rift?) 34

35 Leaves a passive margin along the continents edges 35

36 Widening oceans 36

37 Other observations of the sea floor Heat flow varies 37

38 Other observations of the sea floor Sediment: -Not thick enough compared to continents -thickness increases from ridge outward Hint: thickness = time 38

39 Other observations of the sea floor Magnetic anomalies 39

40 Why would we see changes in field strength? Why would rocks have an affect on the magnetic field? Why would anyone drag a magnetometer across the oceans? 40

41 Flow in Earth s liquid outer core leads to the magnetic field for the planet Declination: points north = latitude Inclination: angle down=longitude 41

42 Basics of magnetism Spinning electron generates a tiny magnetic field If random, non-magnetic (electrons normally paired) If all aligned, add to form a small magnetic field 42

43 Some minerals have lots of Fe and a lot of aligned spinning (unpaired) electrons These minerals act as magnets Magnetite Fe 3 O 4 43

44 Minerals like Amphibole and Pyroxene have a some aligned spinning electrons They have a very small magnetic field Amphibole Pyroxene Ca 2 (Fe Mg) 5 Si 8 O 22 (OH) 2 (CaFeMg) 2 Si 2 O 6 (Note their dark color) 44

45 Paleomagnetism: The alignment of magnetic minerals to Earth s Magnetic field at the time of crystallization 45

46 So the atoms in a mineral/rock with magnetism will align themselves to the magnetic field at the time of formation telling us where the north pole was at that time 46

47 So, What does this have to do with the ocean floor? Magnetite Amphibole Pyroxene All of the above minerals -Are Fe-rich -Are abundant in Basalt -So are abundant in the ocean floor Basalt/ocean crust 47

48 How does this explain the Magnetic anomalies? 48

49 Positive anomaly: Rock s magnetism points same as current: It add to today s field Negative anomaly: Rock s magnetism points opposite as current: It subtracts from today s field 49

50 Why does some of the crust add, and other parts subtract magnetism? Pole reversals! 50

51 But why are the magnetic stripes symmetric About the mid-ocean ridge? 51

52 But why are the magnetic stripes symmetric About the mid-ocean ridge? If reversal is short then stripe is narrow If reversal is long then stripe is wide Also explains why more sediment away from ridges, less near the ridges 52

53 53

54 Compare these anomalies to terrestrial environments where ages can be determined Old Young 54

55 Modern interpretation of ocean floor ages Young old Red- orange- yellow- green---lt blue---dk blue 55

56 In summary: New crust being added at the ridge and moving away towards continents is consistent with: Symmetrical magnetic stripes parallel to the ridge and thicker sediments away from ridge because the crust is older and therefore had more time to accumulate 56

57 Spreading rate problem: 57

58 Details of mid-ocean ridge And ocean crustal structure four layers of oceanic crust Hydrothermal vent Black Smoker 58

59 Why is the ridge high? The lithospheric mantle thickens as the crust moves away from the ridge and cools (the way ice thickens from below on a lake) 59

60 Driving Mechanism Rising hot mantle=convection currents diverging Also explains the high heat flow at ridges 60

61 Why does the ridge spread? 61

62 While all this adds to support ocean floor spreading wouldn t this cause the Earth keep expanding? Hess realized this and proposed subduction at trenches 62

63 Ocean-Ocean convergence 63

64 Development of a volcanic island arc on the overriding plate Arc tends to curve-convex toward trench 64

65 Development of a volcanic island arc on the overriding plate Arc tends to curve-convex toward trench This explains the pattern in island volcanoes we saw on the map earlier 65

66 Arcs concave to overriding plate 66

67 Back-Arc Spreading 67

68 ocean-continent convergence Active continental margin 68

69 The Andes This explains the pattern in continental volcanoes we saw on the map earlier 69

70 What happens when all the ocean crust is subducted? Continent-continent convergence BIG MOUNTAINS! Where do we 70

71 The Himalayas This explains why there are no volcanoes here as we saw on the map earlier 71

72 Earthquakes associated with subduction 72

73 Earthquakes associated with subduction This explains the pattern in earthquake depth we saw on the map earlier 73

74 74

75 Transform boundaries When plates slide by each other Clarification: fault v. boundary? 75

76 Transform boundaries in the oceans 76

77 Transform boundaries on land 77

78 Hot spots and volcanic chains As plate moves by Old volcanoes die New volcanoes grow 78

79 Hot spots and volcanic chains In the ocean Readout? 79

80 Hot spots and volcanic chains on land 80

81 Hot-spot problem: Hawaii Hot-spot problem: Yellowstone 81

82 Hot spots and volcanic chains World wide 82

83 Overview of plate tectonics 83