Continental Margins and Ocean Basins

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					Continental Margins
        and
   Ocean Basins
Continental Margins
 Three Main Divisions




    Continental shelf
    Continental slope
    Continental rise
Continental Shelf
        Submerged part of the continent


        Slopes gently toward ocean basin
         (<1°)


        Composed of continental crust


        Shelf ends at shelf break –
         boundary between shelf and
         steeper slope
                    Continental Slope
 Extends from shelf break to rise

 Boundary between continental
  and oceanic crust

 Steeply sloping compared to shelf
  (5-25°)

 Submarine canyons are major
  features
  Submarine Canyon




 Origin of submarine canyons:
      • river erosion
      • turbidity currents
                    Submarine Canyon
                       Turbidity Currents




 Downslope movement of dense mixture of clay, silt, sand and water
 Deposits are called turbidites
 Turbidites are layered and exhibit graded bedding (decrease in
  sediment grain size from bottom to top)
      Continental Rise




 At base of continental slope
 Slope angle decreases
 Caused by the accumulation of sediment
      Continental Margins
             Types




 Active Margin    Passive Margin
                       Continental Margin
                             Passive
 No plate boundary


 Wide continental margin


 Thick sediment
  accumulation

 Little tectonic activity
Continental Margin
      Active

                Convergent plate boundary

                Trenches are boundaries


                Narrow continental margin

                Thin accumulation of
                 sediments

                Tectonically active
Continental Margin
  Passive vs. Active

              Passive Margins
                • Major rivers drain into ocean

                • Sediment transported by
                  river builds out shelf


              Active Margins
                • Large rivers uncommon

                • Irregular shelves
 Active Continental Margin
            Southern California




3-D shaded relief map


                Santa Monica Bay
Active Continental Margin
     Southern California
Ocean Basin Floor
      Features

  Abyssal Plain
  Trenches
  Ridges and Rises
  Seamounts and Guyots
  Coral Reefs and Atolls
          Ocean Basin Floor




 Covers about 30% of Earth’s surface
 Begins at base of continental rise
 Sedimentation: Passive and turbidity currents
 Contain abyssal plains, deep sea trenches, and seamounts
          Ocean Basin Floor
                Abyssal Plain




 Flat, deep ocean floor
 Depth may be 2-3 miles or more
 Thick sediment accumulation covers oceanic crust
Ocean Basin Floor
 Deep Sea Trenches
                  Occur at subduction zones
                   where oceanic crust is
                   forced downward into
                   mantle

                 Associated with earthquakes
                  and volcanoes

                  Deepest is Mariana Trench
                  (11,020 m)

                  Longest is Peru-Chile trench
                  (5,900 km)
    Ocean Basin Floor
         Ridges and Rises
                                       Ridges = steep slopes



                                       Rises = gentle slopes




 Underwater volcanic mountain chain
   • Extends for 65,000 km
   • 1,000 km wide
   • 1,000-2,000 m high
 Contain central rift valleys
    • 15-50 km wide
    • 500-1,500 m deep
 Offset by fractures
            Ocean Basin Floor
            Seamounts and Guyots




 Seamounts are underwater volcanoes formed along ocean
  ridges or over hot spots
 May emerge as an island
 May be eroded flat on top and called Guyots
                   Ocean Basin Floor
                    Coral Reefs and Atolls




 Volcanic islands (from seamount) form in warm latitudes
 Fringing coral reefs form in shallow, sunlit waters
 Dormant volcano subsides and flattens (becomes a guyot)
 Actively-growing reef becomes a barrier reef and then an atoll

				
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posted:9/13/2012
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