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Chapter 2 Plate Tectonics. A combobulation of much of what we’ve trudged through up to now If you even looked at this chapter, you (should) know why it.

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Presentation on theme: "Chapter 2 Plate Tectonics. A combobulation of much of what we’ve trudged through up to now If you even looked at this chapter, you (should) know why it."— Presentation transcript:

1 Chapter 2 Plate Tectonics

2 A combobulation of much of what we’ve trudged through up to now If you even looked at this chapter, you (should) know why it has been held back until now –You needed at least a little background info

3 We’ve gone through volcanoes, weathering, deposition of sediments, formation of rocks Well and good……with what you now know (at least, I hope)…. How does one go about forming, for example, ……

4 Figure 2.1 Mountains…..

5 Many explanations, most did not explain things well Plate tectonics explains many Your instructor took a similar course as this at a time when there were arguments about this theory We had to know which professor believed which theory/hypothesis to “get the grade” –Consider yourselves lucky!

6 Who ever did a jigsaw puzzle? Who has looked at a map of the world, and noticed at least one jigsaw fit? And that fit is?.......... To begin:

7 (ignore the details for now)

8 It appears certain continents were joined in the past Evidence? –The last slide shows a fossil critter (a Mesosaur), found in both parts of South America and South Africa –Also, a fossil plant called Glossopteris –Not to mention, rock types and structures (mountain ranges, etc.) match when the two continents are put together –Its not a perfect fit

9 Figure 2.3 Fit using the continental shelves

10 Figure 2.6 – rock type/structure evidence

11 Figure 2.7 – paleoclimate evidence White = glacial coverage at end of Paleozoic Blue arrows = direction of ice movement from glacial grooves in bedrock

12 It seems obvious now Not so when I was coming through “the system” The original hypothesis was called “continental drift”. –Conceived by one Alfred Wegener, in 1915 –Looking at South America and Africa, it appears that the two continents broke apart and drifted away from each other –Well and good….but, how does solid rock drift about? –The explanation took a while

13 The basic data on which the hypothesis was formed were good, valid data The data also were used to support other hypotheses Later data could be combined to test each hypothesis (that would be, the scientific method) The basic hypothesis (continental drift) was a bit flawed

14 Objections to continental drift –The mechanism – tidal forces (too small) –How the continents moved: kinda like icebergs in the ocean (would imply a very weak material forming the ocean floor) –Tried to measure actual movement of Greenland over a number of years (did not have accurate enough measurements back then) –Wegener died in 1930; his ideas did not

15 Most scientists ridiculed Wegener’s ideas; others thought some of them to be valid 1928 – first plausible explanation of the driving mechanism 1937 – book published which threw out the weaker parts of the hypothesis, and added new evidence in support of the stronger parts 1950’s – other evidence gathered; one was paleomagnetism

16 Earth’s magnetic field and paleomagnetism Earth has a magnetic field, with a north & south magnetic pole These poles align roughly with the geographic (rotational) pole The field is like that produced by a bar magnet We do not feel the magnetic force, but it is revealed by use of a compass Lastly, it’s a vector field (has a strength and a direction)

17 Figure 2.8

18 Iron-rich minerals (specifically magnetite) become magnetized as they crystallize –This happens mainly in igneous rocks (esp. basalts), although some sediments may contain this mineral As the rock cools below a certain temperature, they become magnetized in the direction of the existing magnetic lines of force –That is, the grains “point” toward the magnetic poles at the time of formation –We actually measure vector components of the grains’ magnetization

19 The magnetic grains not only indicate the direction to the pole –They also indicate the latitude where they formed (i.e. distance from the pole) How does they do that? –Compass needle – constructed to freely rotate in a horizontal plane –Can also be constructed so that it rotates in a vertical plane (measures magnetic inclination) Inclination – the angle between horizontal and the line pointed by the needle –Measurement of the inclination of magnetization in a rock indicates the latitude at the time the rock became magnetized

20 Figure 2.9

21 Polar wandering Investigations (in the 1950s) of magnetic alignment of magnetic minerals in lava flows of different ages indicated many positions for the ancient magnetic poles Data from rocks in Greenland and in Europe These data showed interesting patterns

22 Figure 2.11A

23 Interpretation of these data –Based on the assumption that the magnetic poles correspond with the geographic poles –If North America moved relative to Europe, there is a good match in the direction to the poles – another kudo for the old idea of continental drift

24 Figure 2.11B

25 In addition to the direction to the poles, the inclination data suggested climatic data for the past –300 mya: about the Pennsylvanian Period –Inclination data indicate Europe & N. America were near the equator –The Pennsylvanian (or Carboniferous) was a time of coal formation for several areas – suggestive of tropical climates

26 And a bit before all of this magnetic stuff was noted, we had WW II Has to do with beasts called U-boats (submarines) –In the 1950s, US Navy funded much oceanographic exploration –Wanted to know depths to the bottom, so they could differentiate subs from the bottom –Sonar –Discovered oceanic ridge systems winding through the major oceans (like the seams on a baseball)

27 Investigations of the Mid-Atlantic Ridge revealed a central rift valley –Suggested tensional forces – the crust is being pulled apart here –High heat flow and volcanism also noted

28 Earthquake studies in the western Pacific indicated activity at depth below deep-ocean trenches (not well explained by previous theories) Samples of oceanic crust were to dated to be less than 200 my Sediment accumulations in the deep ocean were found to be thin, not the thousands of meters (or feet) predicted

29 These discoveries did not fit the existing models –Existing models stated that cooling and contraction of the earth produced compressional forces that deform the surface –The thin sediment layers in the oceans indicated either that the rate of sedimentation was much less than thought, or that the ocean floor was actually young

30 A new hypothesis Early 1960s, Harry Hess presented a paper called “an essay in geopoetry” –Oceanic ridges located above zones of convective upwelling in the mantle –Rising material spreads laterally, carrying seafloor away from the ridge crest –The convective currents descend at the location of deep-sea trenches

31 Figure 2.12

32 This hypothesis was open to ridicule, yet provided testable ideas One test came a few years later –The data came from magnetic measurements of rock samples –Over a period of time, the Earth’s magnetic field reverses polarity (that is, sometimes the magnetite compass needle points to the north, and sometimes to the south) –Rocks showing the same magnetism as the present field are normal; those opposite are reverse

33 Figure 2.13 Rock magnetization in northern hemisphere

34 Investigation of numerous lava flows (magnetically and radiometrically) led to a time scale based on magnetic reversals –Major divisions of about 1 my, called chrons –Also found several, shorter-lived reversals (less than 200,000 yrs) during a given chron

35 Figure 2.14

36 At the same time, oceanographers were conducting magnetic surveys of the ocean floor as part of investigations related to seafloor topography These surveys showed alternating stripes of high- and low-intensity magnetism

37 Figure 2.15 High intensity Low intensity

38 An explanation came in 1963, by a grad student & his advisor –The high-intensity strips are areas where the paleomagnetism shows normal polarity (the rocks reinforce the present field strength) –The low-intensity strips are areas showing reverse polarity (the rocks weaken the observed field strength) –Has to do with vector fields –Also a quick jump into plate tectonics

39 Figure 2.16

40 Figure 2.17

41 For us geologists, things were still in turmoil Some believed seafloor spreading and “continental drift” could account for observed data Others preferred an expanding Earth to explain how continents drifted apart, with new seafloor filling in between –Not a good explanation, since most materials shrink as they cool –Also did not explain how mountains were formed

42 Figure 2.18 The expanding Earth

43 The basis of plate tectonics The year…1965 The man….one J. Tuzo Wilson (don’t worry about the name, just a bit of science history) Originally a physicist, he went even more wacko and became a geologist His idea: –large faults (the ridges) connect global mobile belts into a continuous network –Earth’s outer shell divided into several “rigid” plates –Also described how the plates moved relative to one another

44 The plate movements are: –Divergent – two plates move apart, with upwelling of material from the mantle to create new crust –Convergent – two plates move together One plate moves beneath an overriding plate Collision creates mountains –Transform fault – two plates move past each other without production or destruction of crust In his published paper, he also referred to the idea as “geopoetry” – meaning, he knew the ideas would draw criticism

45 “Geopoetry becomes geofact” The title of the main science story for a Time issue in January 1970 (it was deemed that important) Plate tectonics accepted by many The theory provided a unified explanation for numerous seemingly unrelated observations among the various geoscience fields Opposition to the theory continued at least through the early 1980s (I got my BS in 1978, so I know that well)

46 The new “geogospel” (a dangerous term) Theory of plate tectonics A composite of a number of ideas It explains many things that previously were difficult to explain Some of the basis provided in Chapter 12; more details in Chaps. 13 & 14

47 The lithosphere (crust & uppermost mantle) is broken into fairly rigid pieces called plates These overlie a weaker region called the asthenosphere (and, the two are mechanically disconnected) The plates are in motion relative to each other, and are continually changing in shape and size Seven major plates, with a few smaller Average plate movement is 5 cm (2 in) per year (about how fast your fingernails grow) And remember, all this motion is on a sphere, not a 2-D map

48 Figure 2.19Left

49 Figure 2.19Right

50 A quick overview of the boundaries Divergent boundaries (Ch. 13) –Where two crustal plates move apart –Two types: spreading centers, and rifts

51 Spreading centers –Usually, along the crests of oceanic ridges –New crust is generated here –Source of the new crust is a rift valley

52 Figure 2.20

53 Continental rifting –Splitting that occurs within a continent –Modern example – East African Rift Appears to be the initial stage of a breakup Vigorous volcanic activity along the edges We don’t know whether this process will continue

54 Figure 2.21

55 Convergent boundaries (Ch. 14) –If some things are moving apart on a constrained volume such as a sphere, some things must be moving together –Generally, two bodies meet, one moves up (relatively), the other moves down (relatively) –Thus, two plates collide, one tends to slide beneath the other The surface expression is a deep-sea trench Part of a subduction zone, since something is descending into the earth

56 Two types of crust, three possible encounters –Oceanic crust (average density 3.0 g/cm 3 ) –Continental crust (average density 2.7 g/cm 3 ) –The densities are important Water is most dense at 4º C – 1.000 g/cm 3 “Normal” water’s density is 0.9999 g/cm 3 Density of ice is 0.917 g/cm 3

57 Oceanic-continental convergence –Continental crust, being less dense, overrides –Oceanic crust dives down Can take with it water-saturated sediment, as well as water in the crust itself Leads to partial melting at a depth of about 100 km Resulting magma MAY reach the surface; often does not

58 Figure 2.22A

59 Oceanic-oceanic convergence –Similar to above, but the two plates are oceanic crust –Volcanism may occur; tends to build chains of volcanic islands –Ultimately builds arc-shaped chains of volcanic islands called (simply) island arcs Young examples: Tonga, Aleutian Islands, Lesser Antilles Older examples: Japan & Indonesia arcs

60 Figure 2.22B

61 Continental-continental convergence –Can you guess? Two continental masses collide –Happened in the past, the best modern example is India butting heads with Asia (creating the Himalayas) –Generally, involves closing of an ocean basin

62 Figure 2.23BC

63 Transform fault boundaries (no Chap.) –Here, parts of plate slide past one another without production or destruction of crust –Most associated with midocean ridge systems –May form part of a plate boundary (an example is the San Andreas fault)

64 Figure 2.24

65 Figure 2.25

66 Tests of the theory –Outlined in the text –Data from oceanic drilling –Hot spots –Paleomagnetism data –Recent measurements from spacecraft

67 So how does this all happen? Overall, the concept of convection Most models cannot account for all the details we observe What is convection?

68 Convection – basically, warm, buoyant rock rises, forcing cooler, less dense material downward Occurs in the mantle –Three main ideas

69 Ideas regarding the mantle –Three main ideas –First – convection layers –Two layers in which convection occurs; material between the two layers does not mix

70 Figure 2.31A

71 Whole-mantle convection –Overall convection by hot plumes deep in the mantle driving shallower processes –Hot up, cold down –The core being hot, releases heat to the bottom of the mantle

72 Figure 2.31B

73 Deep-layer model (the lava-lamp model) –Similar to the last model, but more complicated as to how processes occur –More along the lines that the real world does not follow our perfect circles that we draw –Convection within a spherical, constrained body is not as simple as in a 2-dimensional model

74 More details on boundary types are given at the end of Chapter 2 Incorporated in chapters 13 and 14

75 End of chapter 2

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