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Continental Drift, Sea-Floor Spreading, and Plate Tectonics

Continental Drift, Sea-Floor Spreading, and Plate Tectonics. Topics to be Covered - 2Hypothesis of Sea-Floor SpreadingConfirmation of Sea-Floor SpreadingPlate TectonicsMountain Building

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Continental Drift, Sea-Floor Spreading, and Plate Tectonics

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    1. Continental Drift, Sea-Floor Spreading, and Plate Tectonics Topics to be Covered - 1 Early Ideas about Continental Drift Lack of a Mechanism Paleomagnetism and the Revival of Continental Drift Evidence from the Ocean Basins

    2. Continental Drift, Sea-Floor Spreading, and Plate Tectonics Topics to be Covered - 2 Hypothesis of Sea-Floor Spreading Confirmation of Sea-Floor Spreading Plate Tectonics Mountain Building – Part of the Story

    3. From Continental Drift to Plate Tectonics – Geological Revolution Early Ideas about Continental Drift Pre-1900 idea: drifting of continents Breakup of a supercontinent Pangaea Drifting away from each other.

    6. Alfred Wegener The Courage to Be Right

    7. Continental Drift - 2 Ideas of Alfred Wegener -1 Fit of Africa and South America Features common to southern continents: rock types and ages Glaciation of late Paleozoic age Similar Permo-Triassic fossils Plants - Glodsopteris Reptiles - Mesosaurus

    11. Why they didn’t have to swim or float

    12. Continental Drift Ideas of Alfred Wegener -2 Paleoclimatic evidence - Either continents had a different orientation with respect to the poles in the past. - Or, the poles have wandered.

    13. Wegener’s Downfall: Lack of a Mechanism Support from the southern continents Some acceptance in Europe Almost total skepticism in N.A. Condemnation by American geophysicists. NO MECHANISM!!!

    14. Alfred Wegener The Courage to Be Right

    15. Paleomagnetism and the Revival of Continental Drift Nature of Paleomagnetism Magnetism a property of iron- bearing minerals. If permitted to do so, these grains align parallel to earth’s magnetic field. Records position of earth’s magnetic poles. Determined in laboratory from field samples.

    16. Paleomagnetism, etc. Results of Paleomagnetic Studies Plotting position of pole based on North American data shows an apparent wandering through time. 13,000 MILES TO PRESENT POSITION IN ARCTIC CIRCLE.

    18. Paleomagnetism, etc. Results - 2 However, the position of the pole from European data does not match that of the data from North America. Indeed, each continent appears to have its own path for polar wandering. Because this is impossible, continents must have been moving about during this time. Only recently have they moved to the present position.

    19. Evidence from the Oceans World War II brought a spectacular advance in our abilities to study the ocean. Among the discoveries were: Guyots Heat-flow measurements Thickness of the oceanic crust Thickness and age of sedimentary deposits on the ocean floor. Earthquakes along ridges, trenches. Very large fractures, most of them faults.

    21. Volcanoes and Guyots

    22. Significance of Guyots Flat-topped mountains believed to be eroded volcanoes. - Implies erosion at sea level. May occur in chains at right angles to mid-oceanic ridges. The farther they are from the ridges, the Deeper they are. Odder they are.

    23. Significance of Guyots Explanation: Volcanoes formed as oceanic ridge and tops eroded by waves. Sea floor spreads out from ridges, slowly carrying guyots into deeper water as new ones form.

    24. Heat-Flow Measurements Areas of high heat flow; - On continents: young mountain ranges. -On ocean floor: over oceanic ridges. Area of low heat flow: over oceanic trenches. Over oceanic trenches.

    25. Heat-Flow Measurements A Puzzle Most of radioactive minerals are in rocks of the continental crust. Yet, on the average, heat flow from the oceans and continents is the same. Needed: a mechanism to redistribute the heat beneath the continents and ocean floors.

    26. Additional Sea-Floor Discoveries Oceanic crust (lithosphere) ~ 5 km , much thinner than for continental crust. Furthermore, of uniform thickness. Sediments on sea floor no more than 1000 feet thick. No rocks on ocean floor older than Jurassic. Earthquakes along the ridges, trenches, large fractures. Faults with movements of over 1000 km.

    27. Hypothesis of Sea-Floor Spreading Convection currents in mantle rise under oceanic ridges and spread. Driving force is here transferred from core to mantle. Oceanic crust (basaltic) created at ridges. Crust plus upper mantle (lithosphere) move laterally away – going along for the ride.

    29. Sea-Floor Spreading - 2 Lithosphere plunges into oceanic trenches. Does this explain the anomalies of ocean floor heat distribution? Continents don’t drift through the mantle but are passengers. Oceanic crust has to be young because older rocks have been: Plastered onto the edge of continents. Thrust down into the mantle. Why are the sedimentary materials on the ocean floor so thin?

    30. Confirmation of Sea-Floor Spreading Earth’s magnetic field periodically reversed. The age of these reversals determined by radiometric dating. Magnetic anomalies found on both sides of oceanic ridges. Preserved in basalt formed in center of rise.

    33. Confirmation of Sea-Floor Spreading Through sea-floor spreading, sea floor has served as a tape recorder – working in stereo. Preserved record of reversals and their time of occurrence. Can also determine rate of spreading. ~ ˝ inch per year in North Atlantic ~ 2 inches per year in South Pacific

    34. Plate Tectonics Basics Earth’s lithosphere broken into large plates. Upwelling of magma creates new oceanic crust. Added to edge of plate. Plates diverge from ridge. Elsewhere, plates Pass into oceanic trenches (subduction). Collide with each other. Push past each other along great strike-slip faults.

    37. Alfred Wegener The Courage to Be Right

    38. Plate Tectonics Collisions can involve an oceanic plate and a continental plate, two continental plates, or two oceanic plates. Continents do not drift, but are rafted about. Some oceanic basins are steadily widening; others are closing. Driving mechanism believed to be convection currents of some type. Shallow or deep?

    41. Plate Tectonics Other Considerations Mantle plumes: doming, splitting, radial flow. Transform faults Strange movements: what movements would you predict?

    43. Plate Tectonics Benioff zones: long narrow earthquake zones dipping into the lithosphere. Created by rupture of subducting plate. Hot spots where plumes of magma rise from the asthenosphere. Remain fixed Plate migrates over, creating chain of volcanoes. Hawaii, Yellowstone

    46. Terminology Used in the Study of CD, SFS, and PT Apparent polar wandering Continental drift Guyot Magnetic anomaly Paleomagnetism Pangaea Plate tectonics Polar reversals Polar wandering Rifting Sea-floor spreading Transform fault

    47. Student responsibilities with regard to CM, SFS, and PT Know the two parallel stories, the scientific and the human-interest histories as continental drift eventually evolved to sea-floor spreading and then on to plate tectonics. Understand how paleomagnetism played not one, but two, major roles in this story. Know other lines of evidence used for and against these ideas. Be able to concisely state the basic aspects of modern plate tectonics.

    48. Alfred Wegener The Courage to Be Right

    49. Alfred Wegener The Courage to Be Right

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