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  1. 1. Lecture #6 Continental Margins and Ocean Basins 22 August 2007
  2. 2. The ocean floor is mapped by bathymetry http://www.the-planet-mars.com/spacecraft/Mars-global-surveyor.html Mars Global Surveyor Mars
  3. 3. http://www.the-planet-mars.com/map-of-mars.html
  4. 4. <ul><li>n o oceans </li></ul><ul><li>few storms </li></ul>Mars http://www.modern.tsukuba.ac.jp/earth.jpg Earth <ul><li>oceans and clouds cover 75% of the surface </li></ul>
  5. 5. <ul><li>Posidonius </li></ul><ul><li>conducted the first bathymetric studies </li></ul><ul><li>85 B.C. </li></ul>http://www-groups.mcs.st-and.ac.uk/history/BigPictures/Posidonius.jpeg 2 km Bathymetry = study of ocean floor contours The early, simplest methods involved lowering a weight on a line.
  6. 6. http://www.nefsc.noaa.gov/history/ships/albatross1/sigsbee-sounding.jpg <ul><li>Sigbee sounding machine </li></ul><ul><li>developed around 18 80 </li></ul><ul><li>Tanner sounding machine </li></ul><ul><li>developed around 18 80 </li></ul>http://woodshole.er.usgs.gov/operations/sfmapping/images/theb0914_small.jpg Sometimes the weight was tipped with wax to retrieve a sample of bottom sediment.
  7. 7. <ul><li>HMS Challenger </li></ul><ul><li>(1872-1876) </li></ul><ul><li>made the first systematic attempt to chart the basins of the world ocean </li></ul><ul><li>made 492 bottom soundings </li></ul>confirmed the discovery of the Mid-Atlantic Ridge
  8. 8. Scientists now use beams of sound to measure depth.
  9. 9. Scientists now use beams of sound to measure depth.
  10. 10. Scientists now use beams of sound to measure depth. <ul><li>Titanic </li></ul><ul><li>sank on its maiden voyage in 1912 </li></ul>
  11. 11. http://en.wikipedia.org/wiki/Reginald_Fessenden <ul><li>Reginald Fessenden (1866-1932) </li></ul><ul><li>Canadian inventor </li></ul><ul><li>in 1914, developed a type of sonar system for locating icebergs </li></ul>“ Iceberg Detector and Echo Depth Sounder”
  12. 12. http://en.wikipedia.org/wiki/Reginald_Fessenden <ul><li>Reginald Fessenden (1866-1932) </li></ul><ul><li>Canadian inventor </li></ul><ul><li>in 1914, developed a type of sonar system for locating icebergs </li></ul>“ Iceberg Detector and Echo Depth Sounder”
  13. 13. http://en.wikipedia.org/wiki/Reginald_Fessenden <ul><li>Reginald Fessenden (1866-1932) </li></ul><ul><li>Canadian inventor </li></ul><ul><li>in 1914, developed a type of sonar system for locating icebergs </li></ul>“ Iceberg Detector and Echo Depth Sounder”
  14. 14. http://en.wikipedia.org/wiki/Reginald_Fessenden <ul><li>Reginald Fessenden (1866-1932) </li></ul><ul><li>Canadian inventor </li></ul><ul><li>in 1914, developed a type of sonar system for locating icebergs </li></ul>“ Iceberg Detector and Echo Depth Sounder”
  15. 15. V = speed of sound in water (about 1.5 km/sec) T = time Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship.
  16. 16. <ul><li>During World War I (1914-1918) </li></ul><ul><li>used to detect enemy submarines </li></ul>http://www.eastlanddisaster.org/uc97.jpg <ul><li>Meteor expedition (1925-1927) </li></ul><ul><li>used to study the seabed </li></ul>http://oceanworld.tamu.edu/resources/ocng_textbook/chapter02/Images/Fig2-2s.jpg
  17. 17. World Ocean Floor
  18. 18. http://www.panorama-map.com/Europeans/Berann/berannpacificocean500.html Pacific Ocean
  19. 19. http://www.berann.com/panorama/Everest.jpg Mt. Everest (1962)
  20. 20. Two new techniques improved studies of the seafloor: 1) multibeam echo sounders 2) satellite altimetry
  21. 21. Multibeam systems combine many echo sounders. <ul><li>up to 121 beams </li></ul><ul><li>signal sent every 10 secs </li></ul><200 research vessels are equipped with multibeam systems
  22. 22.   Seabed contours can be mapped using satellites . <ul><li>Satellites cannot measure ocean depths directly </li></ul><ul><li>but, they can measure sea surface height </li></ul>
  23. 23. Sea surface Seafloor
  24. 24. ? Sea surface Seafloor
  25. 25. Gravitational attraction “pulls” water Over a 2000 m seamount, water rises about 2 m Seafloor Sea surface
  26. 26. Geosat satellite
  27. 27. http://www.ngdc.noaa.gov/mgg/image/global_grav_large.gif Mapped by: Geosat , TOPEX/Poseidon , and Jason-1 Seafloor topography inferred from sea surface height measurements
  28. 28. Ocean-floor topography varies with location
  29. 29. >50% of Earth’s surface is >3,000 m below sea level
  30. 30. Oceans can be divided into two major provinces: 1) continental margin 2) ocean basin
  31. 31. <ul><li>earthquakes </li></ul><ul><li>volcanic activity </li></ul>Continental margins are “ active ” or “ passive ”. <ul><li>no earthquakes </li></ul><ul><li>no volcanic activity </li></ul>Face the edges of diverging plates Near converging plates
  32. 32. Three main parts of the continental margin:
  33. 33. Continental shelf: shallow submerged extension of a continent
  34. 34. http://media.allrefer.com/s1/l/c0601400-continental-shelf.jpg <ul><li>up to 350 km </li></ul><ul><li>most material comes from erosion of continent </li></ul>
  35. 35. http://www.cryingvoice.com/Evolution/gifs/hydroNA.jpg Atlantic
  36. 36. active margin – often very narrow passive margin – broad The shelf width is usually determined by its proximity to a plate boundary.
  37. 37. Continental shelves are greatly influenced by changes in sea level Sea level rise
  38. 38. http://en.wikipedia.org/wiki/Image:Sea_level_temp_140ky.gif Sea level is now high and rising as the ocean warms. Wisconsin glaciation
  39. 39. ice age http://en.wikipedia.org/wiki/Image:Northern_icesheet_hg.png <ul><li>Wisconsin glaciation </li></ul><ul><li>70,000 to 10,000 years ago </li></ul><ul><li>Ice sheets </li></ul><ul><li>3-4 m thick </li></ul><ul><li>caused sea level to drop 125 m </li></ul>
  40. 40. http://whyfiles.org/shorties/202mass_extinct/images/land_bridge.gif Bering Strait
  41. 41. http://whyfiles.org/shorties/202mass_extinct/images/land_bridge.gif Bering Strait
  42. 42. http://whyfiles.org/shorties/202mass_extinct/images/land_bridge.gif Allowed human migration 12,000 years ago Bering Strait
  43. 43. http://www.morien-institute.org/images/korea_2_sing.jpg Ice age sea level in Asia
  44. 44. http://cocos.arecaceae.com/indo19.gif Ice age sea level in Asia
  45. 45. Continental slopes connect continental shelves to the deep-ocean floor shelf break
  46. 46. Submarine canyons form at the junction between continental shelf and continental slope.
  47. 47. http://www.tahoemaps.com/files/Monterey_large.jpg Monterey Bay canyon
  48. 48. http://www.marine-geo.org/gallery/images/MontereyBay3D.jpg 2000 m Monterey Bay canyon
  49. 49. Suruga Bay
  50. 50. How do submarine canyons form? Submarine canyons cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.
  51. 51. Submarine cables near Nova Scotia, Canada http://www.geol.lsu.edu/Faculty/Juan/PhysicalGeology_F2004/images/Turbidity2.gif
  52. 53. turbidity current an underwater “avalanche” of sediment http://unit.aist.go.jp/igg/rg/igi-rg/beta/sl-support/R-formation/TurbidityCurrent.jpg Most geologists believe that submarine canyons have been formed by abrasive turbidity currents plunging down the canyons.
  53. 54. Continental rises form as sediments accumulate at the base of the continental slope continental rise <ul><li>much sediment </li></ul><ul><li>most of the sediment that forms the continental rise is transported to the area by turbidity currents </li></ul>
  54. 55. The topology of deep-ocean basins differs from that of the continental margin <ul><li>Deep-ocean basins comprise mainly: </li></ul><ul><li>oceanic ridge systems </li></ul><ul><li>sediment-covered plains </li></ul>
  55. 56. Oceanic ridges circle the world <ul><li>underwater mountain ranges </li></ul><ul><li>stretch 65,000 km </li></ul><ul><li>often covered with little sediment </li></ul>
  56. 57. http://www.berann.com/panorama/archive/image/PN_W_10.jpg Mid-Atlantic Ridge
  57. 58. Iceland fracture 15 cm/year http://library.thinkquest.org/C003124/images/diverg.jpg
  58. 59. http://www.berann.com/panorama/archive/image/PN_W_10.jpg Mid-Atlantic Ridge
  59. 60. http://www.berann.com/panorama/archive/image/PN_W_10.jpg Mid-Atlantic Ridge transform faults fracture zones
  60. 61. Hydrothermal vents are hot springs on active oceanic ridges <ul><li>discovered in 1977 by Robert Ballard and J. F. Grassle </li></ul>Alvin http://www.mbari.org/molecular/images/EPR%20mussel-map.jpg
  61. 62. Alvin <ul><li>can carry 3 people </li></ul><ul><li>can dive to 4000 m </li></ul><ul><li>1964 – 2007 </li></ul><ul><li>>4000 dives </li></ul><ul><li>manned submersible </li></ul><ul><ul><li>6,500 m </li></ul></ul><ul><li>unmanned submersible </li></ul><ul><ul><li>11,000 m </li></ul></ul>
  62. 63. http://www.csa.com/discoveryguides/vent/images/smoker.jpg http://whyfiles.org/coolimages/images/csi/nur04506.jpg “ black smokers” 20 m 350 o C 2,800 m depth <ul><li>solutions exiting vents are acidic (pH = ~3.5) and contain up to 300 ppm hydrogen sulfide (H 2 S) </li></ul><ul><li>a highly reduced molecule, so much energy can be obtained when it is oxidized </li></ul>
  63. 64. http://www.divediscover.whoi.edu/images/vent_chemistry.jpg “ black smokers” seawater is heated and reacts chemically with the surrounding basalt
  64. 65. hydrothermal vent community http://www.sciencedaily.com/images/2005/02/050223124700.jpg <ul><li>includes snails, shrimps, crabs, tube worms , fishes and octopuses </li></ul><ul><li>depends on chemosynthetic bacteria for food </li></ul>chemosynthesis Tube worms deep-sea vent mussels
  65. 66. Hydrothermal vents are common on oceanic ridges http://www.nature.com/nature/journal/v439/n7079/images/hydro_vents/index.html AND in freshwater (Lake Baikal)
  66. 67. Abyssal plains and abyssal hills cover most of Earth’s surface. <ul><li>Abyssal hills </li></ul><ul><li>small sediment-covered extinct volcanos or rock </li></ul><ul><li>Abyssal plains </li></ul><ul><li>40% of the ocean floor </li></ul><ul><li>common in the Atlantic </li></ul><ul><li>rare in the Pacific </li></ul><ul><li>covered by sediment </li></ul>Flat
  67. 68.   Volcanic seamounts and guyots project above the seabed <ul><li>about 30,000 </li></ul><ul><li>about 10,000 in the Pacific </li></ul>http://upload.wikimedia.org/wikipedia/en/thumb/2/25/Seamount_Locations.png/350px-Seamount_Locations.png <ul><li>>1 km in height </li></ul><ul><li>important fishing areas </li></ul>Emperor Seamounts seamount
  68. 69. blobfish orange roughy southern spineback southern whiptail
  69. 70. Guyot : f lat-topped seamount that once reached the surface http://en.wikipedia.org/wiki/Image:Guyot.jpg
  70. 71. Trench : arc-shaped depression on the deep-ocean floor <ul><li>occur near subduction zones </li></ul><ul><li>deepest places in the ocean </li></ul><ul><li>most in the Pacific </li></ul>http://geology.com/records/ocean-trench.gif
  71. 72. Peru-Chile trench http://en.wikipedia.org/wiki/Image:Peru-Chile_trench.jpg Puerto Rico trench
  72. 73. Japan Trench 10,595 m Mariana Trench 11,022 m
  73. 74. Trieste <ul><li>reached the bottom of the Mariana Trench in 1960 </li></ul>
  74. 75. Kaiko <ul><li>Japanese deep-sea submarine </li></ul><ul><li>sampled bacteria from the bottom (10,897 m) of the Mariana Trench in 1996 </li></ul>
  75. 76. Sampling of the world's deepest sea sediment by &quot; Kaiko &quot; at the Mariana Trench, Challenger Deep Bacteria collected from the Mariana Trench
  76. 77. <ul><li>lost at sea in typhoon in 2003 </li></ul>
  77. 78. Key Points 1. The ocean floor is mapped by bathymetry . 2. Ocean-floor topography varies with location . 3. Continental margins are “active” or “passive”. 4. The topology of deep-ocean basins differs from that of the continental margin.