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the sea floor

the sea floor. seafloor. • water covers 70% of Earth’s surface. seafloor map from Dana (1894). seafloor. • deep seafloor largely unknown prior to 1950’s. seafloor. • oceans originated mostly from volcanic de-gassing of water vapor from Earth’s interior.

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the sea floor

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  1. the sea floor

  2. seafloor • water covers 70% of Earth’s surface

  3. seafloor map from Dana (1894) seafloor • deep seafloor largely unknown prior to 1950’s

  4. seafloor • oceans originated mostly from volcanic de-gassing of water vapor from Earth’s interior additional small amount may have come from late comet impacts after the Earth reached close to its current mass

  5. studying the seafloor direct methods • rock dredges • sea floor drilling • submersibles indirect methods • sonar • seismic reflection profiling

  6. direct methods rock dredges

  7. direct methods sediment corer

  8. direct methods sea floor sediment core

  9. JOIDES Resolution (1990’s-being overhauled) DSDP (Deep Sea Drilling Project); ODP (Ocean Drilling Project) …IODP (Integrated Ocean Drilling Project)…

  10. Chikyu Japanese drill ship

  11. direct methods submersibles • manned or unmanned

  12. indirect methods sonar (sound navigation and ranging) • sound sent from ship, bounced off sea floor, and recorded at ship distance to seafloor is calculated from speed of sound in water multiplied by time to get return signal divided by two (wave goes down and up)

  13. indirect methods known for a long time that sound travels through water 1822 attempt to determine speed of sound in water

  14. indirect methods seismic reflection • penetration of sediments by sound waves • hydrophones record signals echo sounding, swath bathymetry, sidescan

  15. indirect methods sea floor profile

  16. indirect methods South Pacific sea floor

  17. seafloor, continents, and plate boundaries yellow lines are plate boundaries sea floor was critical in development of plate tectonics

  18. features of the seafloor general profile through ocean from left to right shelf, slope, abyssal plain, mid-oceanic ridge seamounts, trench, slope shelf passive continental margin (no plate boundary) active continental margin (plate boundary) mid-oceanic ridge (plate boundary)

  19. continental shelf and slope • broad, shallow shelf (100-200 m water depth) • steeper slope dives to abyssal plain topographic profile has 25x vertical exaggeration (vertical and horizontal scales are not the same) slope angle is only 4-5°

  20. passive margin NO plate boundary at edge of continent • shelf and slope • continental rise (less steep than slope) • abyssal plain (smooth, deep seafloor)

  21. submarine canyons and abyssal fans • start on shelf and end at base of slope • allow for transport of sediment from shelf to sea floor

  22. sand falls offshore Baja, California

  23. submarine canyons and abyssal fans (California) “turbidity currents” flow down canyons and deposit on fans

  24. offshore southern California

  25. submarine canyons landslide triggered by earthquake cable breaks in different locations at different times as landslide arrives

  26. continental rises and abyssal plains continental rise: gently sloping wedge of sediment of sediment at base of slope sediments deposited by turbidity currents and contour currents move along elevation contours abyssal plain: flattest region on Earth; form where turbidity currents bury features

  27. active margin plate boundary at edge of continent • shelf and slope • oceanic trench (deepest features in ocean) • volcanoes (on-land) Wadati-Benioff zone --dipping zone of earthquakes that begin at trench and extend landward (red stars)

  28. active margins (trenches-plates converge)

  29. mid-ocean ridge (plate boundary-plates diverge)

  30. mid-ocean ridge • 80,000 km long; 1,500-2,500 km wide • elevations of 2,000-3,000 m above sea floor • rift valley ~1,000 m deep at crest of ridge axial valley NORTH AMERICA AFRICA sea floor spreading (divergence) from: http://www.geo.duke.edu/geo41/sfs.htm

  31. mid-ocean ridge • basalt flows and volcanism • high heat flow and small, shallow earthquakes • hot springs supporting biological communities

  32. black smoker (first ever seen) 1979

  33. life at oceanic ridge tube worms spider crab giant clams

  34. explore using submersibles ALVIN was first one; 3 passenger

  35. mid-ocean ridge • exposed on-land in Iceland both from: http://pubs.usgs.gov/publications/text

  36. mid-ocean ridge transform faults • offset of mid-ocean ridge between adjacent ridges --earthquakes occur along them (red stars)-- fracture zones • continuation of transform fault beyond ridge --no eq’s--

  37. transform fault--fracture zone animation greenare ridge segments;redis transform fault

  38. from: http://www.whoi.edu/page.do?pid=7545&tid=441&cid=49514&ct=61&article=29566

  39. other sea floor features seamount conical mountain that rises > 1,000 m above sea floor; basaltic volcanoes; chains of seamounts occur (aseismic ridges) (Emperor seamounts) guyot flat-topped seamount; erosion from waves; reefs common around them

  40. seamount chains and ages of seamounts in one (hot spot track -- more later) Emperor seamounts

  41. sea floor sediments terrigenous: derived from land and brought to sea floor …sands/silts that make up continental rise… pelagic: accumulate by settling through water column …clays from wind; skeletons of microsopic organisms… sea floor spreading leads to greater thickness of pelagic sediments away from ridge crest (no sediment at mid-ocean ridge)

  42. composition of the oceanic crust seismic surveys suggest ~ 7 km thick with 3 layers 1) marine sediments (sampled) 2) pillow basalts (sampled) 3) gabbros (not sampled) (intrusive equivalent to basalt)

  43. pillow basalts

  44. resources of the ocean offshore drilling

  45. mining the ocean floor? manganese nodules

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