Chapter 18 The Sea Floor

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Chapter 18The Sea Floor
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Method of studying the sea floor

• Depth was originally measured by lowering
  weighted lines overboard
• Echo sounder (also referred to as sonar)
• Invented in the 1920s
• Primary instrument for measuring depth
• Reflects sound from ocean floor
• Multibeam sonar
• Employs an array of sound sources and listening
  devices
• Obtains a narrow profile of seafloor

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Mapping the Ocean Floor
Echo Sounder
Record
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A seismic profile
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Features of the sea floor

• Three major topographic units of the ocean floor
  
• Continental margins
• Passive
• Active
• Deep-ocean basins
• Mid-ocean ridges
• Seamounts

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North Atlantic Ocean (major topographic
divisions)
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Passive Continental Margins

• Found along most coastal area that surround the
  Atlantic ocean
• Not associated with plate boundaries
• Experience little volcanism and few earthquakes
• Features of passive continental margin
• Continental shelf
• Continental slope
• Continental rise

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Passive vs. active Margins
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Passive Continental Margin
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Passive Continental Margins

• Continental shelf
• Flooded extension of the continent
• Varies greatly in width
• Gently sloping
• Continental slope
• Marks the seaward edge of the continental shelf
• Relatively steep structure
• Boundary between continental crust and oceanic
  crust

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Passive Continental Margins

• Continental rise
• Continental slope merges into a more gradual
  incline the continental rise
• Thick accumulation of sediment
• At the base of the continental slope turbidity
  currents deposit sediment that forms deep-sea
  fans

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Active Continental Margins

• Continental slope descends abruptly into a
  deep-ocean trench
• Located primarily around the Pacific Ocean
• Accumulations of deformed sediment and scraps of
  ocean crust form accretionary wedges

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Active Continental Margin
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Deep-Ocean Trenches

• Long, relatively narrow features
• Deepest parts of ocean
• Most are located in the Pacific Ocean
• Sites where moving lithospheric plates plunge
  into the mantle
• Associated with volcanic activity

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Worlds Major Deep-Oceanic Trenches
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Submarine Canyons

• Deep, steep-sided valleys cut into the
  continental slope
• Some are extensions of river valleys
• Most appear to have been eroded by turbidity
  currents

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Submarine Canyon(eroded by turbidity current)
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Turbidity Currents

• Downslope movements of dense, sediment-laden
  water
• Deposits are called turbidites
• Turbidites are layered and exhibit graded bedding
  (decrease in sediment grain size from bottom to
  top)

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TurbiditesPacifica, CA
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Mid-Ocean Ridges

• Characterized by
• An elevated position
• Extensive faulting
• Numerous volcanic structures that have developed
  on newly formed ocean crust
• Interconnected ridge system is the longest
  topographic feature on Earths surface
• Over 70,000 km (43,000 mi) in length
• Twenty percent of Earths surface
• Winds through all major oceans

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Mid-Ocean Ridges

• Along the axis of some segments are deep down
  faulted structures called rift valleys
• Consist of layer upon layer of basaltic rocks
  that have been faulted and uplifted
• Formed at divergent plate boundaries
• Mid-Atlantic Ridge has been studied more than any
  other ridge system

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Mid Ocean Ridge

• Concept formulated in the early 1960s by Harry
  Hess
• Seafloor spreading occurs along relatively narrow
  rift zones, located at the crests of ocean ridges
  
• As plates move apart, magma wells up into the
  newly created fractures and generates new slivers
  of oceanic lithosphere

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Deep-Ocean Basin

• Abyssal plains
• Likely the most level places on Earth
• Sites of thick accumulations of sediment
• Found in all oceans
• Seamounts
• Isolated volcanic peaks
• Many form near oceanic ridges
• May emerge as an island
• May sink and form flat-topped seamounts called
  guyots

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Coral Reefs and Atolls

• Coral Reefs
• Calcite-rich skeletons of coral and algae
• Includes skeletons of animals/plants
• Found in warm, clear water
• Pacific, Indian oceans, Bahamas
• Coral Atolls
• Coral islands a continuous ring of coral reef
  surrounding a central lagoon
• Form on flanks of sinking volcanic island
  (hypothesis proposed by Charles Darwin)

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Formation of Coral Atoll and Guyot
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Tetiaroa Atoll, Pacific Ocean
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Seafloor Sediment

• Types of seafloor sediment
• Terrigenous sediment derived from land
• Pelagic sediment settle slow in open ocean water

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Oceanic crust and ophiolites

• 3 layers
• Marine sediments
• Pillow basalts and sheeted dikes
• Gabbro intrusions

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Hydrothermal activity
At the ridge
Black smoke hot plume of metal sulfide