Bathymetry of the Ocean Floor

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Bathymetry of the Ocean Floor

• The ocean floor is mapped by SONAR.
• (Sound navigation and ranging)
• Depth (time x 1500 m/sec)/2 (round trip)
• At 25 degrees Celsius

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Relationship above and below the water
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Ocean Floor Topography

• Major features continental shelf, trenches,
  abyssal plains, mid-ocean ridges.
• After 1920s, these features could be mapped in
  detail by sonar (bouncing sound waves off the
  ocean floor).

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Continental Shelf

• Area of shallow water along passive continental
  margin (not an active plate boundary).
• Shelf drops off abruptly at outer edge.
• Much of shelf was above sea level during past
  glacial periods.
• Important processes at a passive margin
• Accumulation of sediment.
• Subsidence (sinking) of crust as sediment is
  added.
• Mass wasting at edge of continental shelf.
• Turbidity currents (large rapid flows of dense,
  sediment-laden water).

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Continental Rise

• gentle slope at the base of the continental slope
  caused by
• - turbidity currents - deposition of sediment by
  underwater landslides and other processes that
  carry mud, sand silt down the slope
• - occurs at the base of the slopes gently slope
  seaward to the deep sea floor

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Trenches

• Occur along active margins (present-day plate
  boundaries) where subduction is taking place
• deepest part of the ocean floor, typically 3 - 4
  km deeper than surrounding seafloor
• relatively narrow, few 10s of km wide and
  thousands of km long
• most occur in the Pacific, mostly western
  Pacific, but most of the Pacific is surrounded by
  trenches
• deepest spot in the oceans is the Challenger Deep
  in the Marianas Trench, 11,035 m
• trenches are associated with active volcanoes and
  earthquakes
• most are near chains of volcanic islands

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Abyssal Plane

• Flat areas making up much of the ocean floor
  apart from trenches and mid-ocean ridges

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Mid-Ocean Ridges

• Largest topographic features on earth.
• Broad ridges with a
• deep rift valley down
• the center.
• Offset by faults.
• Location where basaltic
• lava flows erupt.
• Age of the ocean floor
• Detailed studies in 1950s showed that there are
  bands of similar aged rocks parallel to mid-ocean
  ridges.
• Bands get older with greater distance from the
  ridges.

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Submarine Canyons

• Submarine Canyons - ripple marks observed on the
  floor of submerged canyons and sediments fanning
  out at the end suggest they were formed by moving
  sediments and water
• - cut by turbidity currents - caused by
  earthquakes or buildup of sediment on a steep
  slope
• - fast moving avalanches of mud, sand and water
  that flow down slope, erode walls and pick up
  sediment
• - as flow reaches bottom, it slows down, fans out
  and the sediment settles out
• Submarine canyons cut through the shelves and
  slopes, look like river valleys on land, cut
  during periods of low sea level by turbidity
  currents,  some associated with major rivers,
  e.g. Hudson Canyon

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Ocean Floor is seaward of the continental margin

• covers 30 of the earth's surface compared to 29
  covered by the continents - in most places,
  seafloor is a flat plain - Abyssal Plain
• covered by sediment deposits of turbidity
  currents covering an irregular seafloor plain
  interrupted by
• abyssal hills (lt 1 km above the seafloor) - cover
  -80 of the Pacific and -50 of the Atlantic sea
  mounts (rise steeply, sometimes above the surface
  to form islands)
• guyots (flat topped seamounts found most often in
  the Pacific)
• table mounts, usually I - 1.7 km below the
  surface
• - many have ancient coral reefs on top indicating
  that they were once at the surface
• - flat tops due to wind rain erosion
• - subsided due to their own weight crustal
  movement

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Basins

• Abyssal plains
• Abyssal hill
• Seamount
• Guyot
• Trenches
• Mid Ocean ridges
• Rift Valley

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