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• Bathymetry


  Measurement of ocean depths and the charting of the shape or topography of the ocean floor
• Calculating depth using echo sounders

  1. Transmit sound wave (ping) into water

  2. Measure time for echo to return

  3. Calculate depth using speed of sound in water (1500 m/s)
• Sidescan sonar provides images of the seafloor but does not provide bathymetric data
• Multibeam sonar


  • Sends out sound in several directions and records reflections from the seafloor through narrowly focused receivers
  • Maps a swath of ocean floor tens of kilometers wide
  • Can distinguish depth differences of less than a meter
• Only about 5% of the seafloor has been mapped in detail using multibeam sonar
• Seismic reflection profile


  Produces an image of the rock structure beneath the seafloor sediments by recording reflections of strong, low-frequency sounds
• Satellites equipped with radar altimeters can measure subtle differences in sea level to infer seafloor topography
• Major topographic provinces of the ocean floor

  • Continental margins
  • Deep-ocean basins
  • Oceanic (mid-ocean) ridges
• Passive continental margins


  • Found along most of the Atlantic and Indian Ocean coasts
  • Consist of continental crust capped with weathered materials eroded from adjacent landmasses
• Active continental margins


  • Found around the Pacific Rim
  • Narrow and consist of highly deformed sediments scraped from the descending oceanic lithosphere
• Features of a passive continental margin

  • Continental shelf
  • Continental slope
  • Continental rise
• Continental shelf


  Gently sloping, submerged surface extending from the shoreline toward the deep-ocean basin, underlain by continental crust
• Continental shelves are on average 80 km wide and have an average depth of 130 m at the shelf break
• Continental shelves represent 7.5% of the total ocean area and contain important mineral deposits and fishing grounds
• Countries can claim extensions of their continental shelf beyond 200 nautical miles if they provide scientific evidence
• Continental slope


  Marks the seaward edge of the continental shelf, with an average slope of about 5 degrees
• Margin


  Shelf break
• Continental shelf


  Extension of the continent
• Continental slope


  Boundary between continent and ocean
• Continental rise


  Turbidity fan currents
• Abyssal plain


  Topography covered by sediments
• Trench


  Where subduction occurs
• Faced with a shortage of land along its heavily populated coast, Japan has constructed artificial islands on the adjacent continental shelf. One of these islands is located about 3 miles off the coast of Osaka, where the Kansai Airport was built.
• In 2001, Russia was the first country to submit a continental-shelf claim, based mainly on known reservoirs of oil and natural gas. Many other nations have followed suit.
• Continental slope


  Relatively steep structure that marks the boundary between continental crust and oceanic crust
• Continental rise


  Gradual incline that may extend seaward for hundreds of kilometers, consisting of a thick accumulation of sediment that has moved down the continental slope and onto deep-ocean floor
• Most of the sediments are delivered to the seafloor by turbidity currents that periodically flow down submarine canyons
• When these muddy slurries emerge from the mouth of a canyon onto the relatively flat ocean floor, they deposit sediment that forms a deep-sea fan
• As fans from adjacent submarine canyons grow, they merge laterally to produce a continuous wedge of sediment at the base of the continental slope forming the continental rise
• Active continental margin


  The continental shelf is very narrow, if it exists at all, and the continental slope descends abruptly into a deep-ocean trench
• In active continental margin settings, sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the overriding continent, forming an accretionary wedge
• Prolonged plate subduction can produce massive accumulations of sediment along active continental margins
• Some active margins have little or no sediment accumulation, indicating that material is being carried into the mantle with the subducting plate
• In these locations the continental margin is very narrow, as the trench may lie a mere 50 kilometers (31 miles) offshore
• Features of a passive continental margin

  • Shelf
  • Slope
  • Rise
• Continental shelf


  Flooded extension of the continent
• Continental slope


  Has the steepest slope
• Between the continental margin and the oceanic ridge lies the deep-ocean basin, which covers almost 30 percent of Earth's surface
• Deep-ocean trenches


  Long, relatively narrow creases in the seafloor that represent the deepest parts of the ocean floor
• Most trenches are located along the margins of the Pacific Ocean, where many exceed 10 kilometers (6 miles) in depth