Garrison Oceanography 7e Chapter 4

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Oceanography An Invitation to Marine Science,
7th Tom Garrison
Chapter 4 Continental Margins and Ocean Basins
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Chapter 4 Study Plan

• The Ocean Floor Is Mapped by Bathymetry
• Ocean-Floor Topography Varies with Location
• Continental Margins May Be Active or Passive
• The Topology of Deep-Ocean Basins Differs from
  That of the Continental Margin
• The Grand Tour

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Chapter 4 Main Concepts

• Tectonics forces shape the seabed.
• The ocean floor is divided into continental
  margins and deep-ocean basins. The continental
  margins are seaward extensions of the adjacent
  continents and are usually underlain by granite
  the deep seabeds have different features and are
  usually underlain by basalt.
• Continental margins may be active (earthquakes,
  volcanoes) or passive, depending on the local
  sense of plate movement.
• The mid-ocean ridge system is perhaps Earths
  most prominent feature. Most of the water of the
  world ocean circulates through hot oceanic crust
  in the ridges about every
• 10 million years.
• Using remote sensing methods, oceanographers have
  mapped the world ocean floor in surprising detail.

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The Ocean Floor Is Mapped by Bathymetry

• The discovery and study of ocean floor contours
  is called Bathymetry.
• (left) An illustration from the Challenger Report
  (1880).
• Seamen are handing the steam winch used to lower
  a weight on the end of a line to the seabed to
  find ocean depth.

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The Ocean Floor Is Mapped by Bathymetry

• How did early scientists study the ocean floor?
• Early bathymetric studies were often performed
  using a weighted line to measure the depth of the
  ocean floor.
• Advances in Bathymetry
• Echo sounding
• Multi-beam Systems
• Satellite Altimetry

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Echo Sounders Bounce Sound off the Seabed

• Echo sounding is a method of measuring seafloor
  depth using powerful sound pulses. The accuracy
  of an echo sounder can be affected by water
  conditions and bottom contours. The pulses of
  sound energy, or pings, from the sounder spread
  out in a narrow cone as they travel from the
  ship. When depth is great, the sounds reflect
  from a large area of seabed. Because the first
  sound of the returning echo is used to sense
  depth, measurements over deep depressions are
  often inaccurate.

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Multi-beam Systems Combine Many Echo Sounders

• Multi-beam systems provide more accurate
  measurements than echo sounders. Multi-beam
  systems collect data from up to 121 beams to
  measure the contours of the ocean floor.

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Satellites Can Be Used to Map Seabed Contours

• Satellite altimetry measures the sea surface
  height from orbit. Satellites can bounce 1,000
  pulses of radar energy off the ocean surface
  every second.

• (right) Geosat, a U.S. Navy satellite operated
  from 1985 through 1990, provided measurements of
  sea surface height from orbit. Moving above the
  ocean surface at 7 kilometers (4 miles) a second,
  Geosat bounced 1,000 pulses of radar energy off
  the ocean every second. Height accuracy was
  within 0.03 meters (1 inch)!
• (below) With the use of satellite altimetry, sea
  surface levels can be measured more accurately,
  showing sea surface distortion. Distortion of the
  sea surface above a seabed feature occurs when
  the extra gravitational attraction of the feature
  pulls water toward it from the sides, forming a
  mound of water over itself.

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The Topography of Ocean Floors

• Cross section of the Atlantic ocean basin and the
  continental United States, showing the range of
  elevations. The vertical exaggeration is 1001.
• Although ocean depth is clearly greater than the
  average height of the continent, the general
  range of contours is similar.

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Ocean-Floor Topography Varies with Location

• A graph showing the distribution of elevations
  and depths on Earth.
• This graph is not a land-to-sea profile of Earth,
  but rather a plot of the area of Earths surface
  above any given elevation or depth below sea
  level.
• Note that more than half of Earths solid surface
  is at least 3,000 meters (10,000 feet) below sea
  level.
• The average depth of the ocean (3,790 meters or
  12,430 feet) is much greater than the average
  elevation of the continents (840 meters or 2,760
  feet).

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Ocean-Floor Topography Varies with Location

• What are the two classifications of ocean floor?
• Continental Margins the submerged outer edge of
  a continent
• Ocean Basin the deep seafloor beyond the
  continental margin
• What are the two types of continental margins?
• Passive margins, also called Atlantic-type
  margins, face the edges of diverging tectonic
  plates. Very little volcanic or earthquake
  activity is associated with passive margins.
• Active margins, known as Pacific-type margins,
  are located near the edges of converging plates.
  Active margins are the site of volcanic and
  earthquake activity.

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Ocean-Floor Topography Varies with Location
Continental margins have several distinct
components.

• (above) Cross section of a typical ocean basin
  flanked by passive continental margins.
• The submerged outer edge of a continent is called
  the continental margin.
• The deep-sea floor beyond the continental margin
  is properly called the ocean basin.

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Ocean-Floor Topography Varies with Location

• Features of Earths solid surface shown as
  percentages of the Planets total surface.

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Continental Margins May Be Active or Passive

• Continental margins have several components
• Continental shelf the shallow, submerged edge
  of the continent.
• Continental slope the transition between the
  continental shelf and the deep-ocean floor.
• Shelf break the abrupt transition from
  continental shelf to the continental slope.
• Continental rise accumulated sediment found at
  the base of the continental slope.

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Continental Margins May Be Active of Passive

• Typical continental margins bordering the
  tectonically active (Pacific-type) and passive
  (Atlantic-type) edges of a moving continent. The
  vertical scale has been exaggerated.
• Passive margins continental margins facing the
  edges of diverging plates
• Active margins continental margins near the
  edges of converging plates (or near places where
  plates are slipping past each other)

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Continental Shelves Are Seaward Extensions of the
Continents

• The features of a passive continental margin
• (a) Vertical exaggeration 501
• (b) No vertical exaggeration
• margin.

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Continental Shelves Are Seaward Extensions of the
Continents

• Changes in sea level over the last 250,000 years,
  as traced by data taken from ocean-floor cores.
  The rise and fall of sea level is due largely to
  the coming and going of ice ages periods of
  increased and decreased glaciation, respectively.
  Because water that formed the ice-age glaciers
  came from the ocean, sea level dropped. Point a
  indicates a low stand of -125 meters (-410 feet)
  at the climax of the last ice age some 18,000
  years ago. Point b indicates a high stand of 6
  meters (19.7 feet) during the last interglacial
  period about 120,000 years ago. Point c shows the
  present sea level. Sea level continues to rise as
  we emerge from the last ice age and enter an
  accelerating period of global warming.

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Submarine Canyons Form at the Junction between
Continental Shelf and Continental Slope

• Submarine canyons are a feature of some
  continental margins. They cut into the
  continental shelf and slope, often terminating on
  the deep-sea floor in a fan-shaped wedge of
  sediment.

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Submarine Canyons

• (right) A turbidity current flowing down a
  submerged slope off the island of Jamaica. The
  propeller of a submarine caused the turbidity
  current by disturbing sediment along the slope
• Avalanche-like sediment movement caused when
  turbulence mixes sediments into water above a
  sloping bottom are called turbidity currents.

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The Topology of Deep-Ocean Basins Differs from
That of the Continental Margin

• What are some features of the deep-ocean floor?
• Oceanic Ridges
• Hydrothermal Vents
• Abyssal Plains and Abyssal Hills
• Seamounts and Guyots
• Trenches and Island Arcs

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Oceanic Ridges Circle the World

• An oceanic ridge is a mountainous chain of young,
  basaltic rock at an active spreading center of an
  ocean.

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Oceanic Ridges Circle the World

• Transform faults and fracture zones along an
  oceanic ridge
• Transform faults are fractures along which
  lithospheric plates slide horizontally past one
  another. Transform faults are the active part of
  fracture zones.

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Hydrothermal Vents Are Hot Springs on Active
Oceanic Ridges

• Hydrothermal vents are sites where superheated
  water containing dissolved minerals and gases
  escapes through fissures, or vents. Cool water
  (blue arrows) is heated as it descends toward the
  hot magma chamber, leaching sulfur, iron, copper,
  zinc, and other materials from the surrounding
  rocks. The heated water (red arrows) returning to
  the surface carries these elements upward,
  discharging them at hydrothermal springs on the
  seafloor.

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Volcanic Seamounts and Guyots Project above the
Seabed

• Seamounts are volcanic projections from the ocean
  floor that do not rise above sea level.
  Flat-topped seamounts eroded by wave action are
  called guyots
• Abyssal hills are flat areas of sediment-covered
  ocean floor found between the continental margins
  and oceanic ridges. Abyssal hills are small,
  extinct volcanoes or rock intrusions near the
  oceanic ridges.

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Trenches and Island Arcs Form in Subduction Zones

• Trenches are arc-shaped depressions in the ocean
  floor caused by the subduction of a converging
  ocean plate.
• Most trenches are around the edges of the active
  Pacific. Trenches are the deepest places in
  Earths crust, 3 to 6 kilometers (1.9 to 3.7
  miles) deeper than the adjacent basin floor. The
  oceans greatest depth is the Mariana Trench
  where the depth reaches 11,022 meters (36,163
  miles) below sea level.

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Trenches and Island Arcs Form in Subduction Zones

• The Mariana Trench
• (a) Comparing the Challenger Deep and Mount
  Everest at the same scale shows that the deepest
  part of the Mariana Trench is about 20 deeper
  than the mountain is high.
• (b) The Mariana Trench shown without vertical
  exaggeration.

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Chapter 4 in Perspective

• In this chapter you learned how difficult it has
  been to discover the shape of the seabed. Even
  today, the surface contours of Mars are better
  known than those of our ocean floor.
• We now know that seafloor features result from a
  combination of tectonic activity and the
  processes of erosion and deposition. The ocean
  floor can be divided into two regions
  continental margins and deep-ocean basins. The
  continental margin, the relatively shallow ocean
  floor nearest the shore, consists of the
  continental shelf and the continental slope. The
  continental margin shares the structure of the
  adjacent continents, but the deep ocean floor
  away from land has a much different origin and
  history. Prominent features of the deep ocean
  basins include rugged oceanic ridges, flat
  abyssal plains, occasional deep trenches, and
  curving chains of volcanic islands. The processes
  of plate tectonics, erosion, and sediment
  deposition have shaped the continental margins
  and ocean basins.
• In the next chapter you will learn that nearly
  all the ocean floor is blanketed with sediment.
  With the exception of the spreading centers
  themselves, the broad shoulders of the oceanic
  ridge systems are buried according to their age
  the older the seabed, the greater the sediment
  burden. Some oceanic crust near the trailing
  edges of plates may be overlain by sediments more
  than 1,500 meters (5,000 feet) thick. Sediments
  have been called the memory of the ocean. The
  memory, however, is not a long one. Before
  continuing, can you imagine why that is so?