Earth Science, 6e

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Earth Science, 6e

• The Ocean FloorChapter - 9

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A-The vast world ocean (page 260)

• Earth is often referred to as the blue planet
• 71 of Earths surface is represented by oceans
  and marginal seas
• Continents and islands comprise the remaining 29?
  
• Oceans represent 97 of the Hydrosphere

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Land HemisphereWater Hemisphere
Figure 9.1
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The vast world ocean

• Four main ocean basins
• Pacific Ocean - the largest and deepest
• Atlantic Ocean about half the size of the
  Pacific Ocean
• Indian Ocean slightly smaller than the Atlantic
• Arctic Ocean about 7 percent the size of the
  Pacific

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The oceans of Earth
Figure 13.2 B
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Mapping the ocean floor (page 265)

• Bathymetry measurement of ocean depths and the
  shape or topography of the ocean floor
• Echo sounder (also referred to as sonar)
• Invented in the 1920s
• Primary instrument for measuring depth
• Reflects sound from ocean floor

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Echo sounder and multibeam sonar
Figure 9.11
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Three major topographic units of the Ocean floor

• Continental margins (Active and Passive)
• Active margins
• Passive margins
• Ocean basin floor
• 3. Mid-ocean ridge

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Major topographic divisions of the North Atlantic
Ocean
Figure 9.14
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I - Continental margins

• Passive continental margins
• Found along most coastal areas that surround the
  Atlantic Ocean
• Not associated with plate boundaries
• Experience little volcanism and
• Few earthquakes
• Wider sandy beaches

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Features of a passive continental margin
Figure 13.9
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Passive continental margins

• Comprises three features
• Continental Shelf
• Continental Slope
• Continental Rise

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Passive continental margins

• Features comprising a passive continental margin
• 1. Continental shelf
• Flooded extension of the continent
• Contains oil and important mineral deposits

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Passive continental margins

• Features comprising a passive continental margin
• 2. Continental slope
• Marks the seaward edge of the continental shelf
• Relatively steep structure
• Submarine canyons and turbidity currents

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Turbidity currents
Figure 13.10
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Continental margins

• Passive continental margins
• Features comprising a passive continental margin
• 3. Continental rise
• Found in regions where trenches are absent
• Continental slope merges into a more gradual
  incline the continental rise
• Thick accumulation of sediment

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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
• Some subduction zones have little or no
  accumulation of sediments (narrow beaches)

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An active continental margin
Figure 9.18
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Active continental margins

• 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
• Volcanic islands arcs (Japan)
• Continental volcanic arcs (Andes, Cascades mts)

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II - Ocean basin floor

• Abyssal plains
• Likely the most level places on Earth
• Sites of thick accumulations of sediment
• Found in all oceans
• Seamounts and guyots
• Isolated volcanic peaks
• Many form near oceanic ridges

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Ocean basin floor

• Seamounts and guyots
• May emerge as an island
• May sink and form flat-topped seamounts called
  guyots or tablemounts

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III - Mid-ocean ridge

• Mid-ocean ridge
• Characterized by
• An elevated position
• Extensive faulting
• Numerous volcanic structures that have developed
  on newly formed crust

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Mid-ocean ridge

• Interconnected ridge system is the longest
  topographic feature on Earths surface
• Over 70,000 kilometers (43,000 miles) in length
• Twenty-three percent of Earths surface
• Winds through all major oceans
• Along the axis of some segments are deep down
  faulted structures called rift valleys

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Mid-ocean ridge

• Consist of layer upon layer of basaltic rocks
  that have been faulted and uplifted
• Mid-Atlantic Ridge has been studied more
  thoroughly than any other ridge system

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Seafloor sediments

• Ocean floor is mantled with sediment
• Sources
• Turbidity currents
• Sediment that slowly settles to the bottom from
  above
• Thickness varies
• Thickest in trenches accumulations may approach
  10 kilometers

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Seafloor sediments

• Thickness varies
• Pacific Ocean about 600 meters or less
• Atlantic Ocean from 500 to 1000 meters thick
• Mud is the most common sediment on the deep-ocean
  floor

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Seafloor sediments

• Types of seafloor sediments
• 1. Terrigenous sediment
• Material weathered from continental rocks
• Virtually every part of the ocean receives some
• Fine particles remain suspended for a long time
• Oxidation often produces red and brown colored
  sediments

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Seafloor sediments

• Types of seafloor sediments
• 2. Biogenous sediments
• Shells and skeletons of marine animals and plants
  
• Most common are calcareous oozes produced from
  microscopic organisms that inhabit warm surface
  waters
• Siliceous oozes composed of skeletons of diatoms
  and radiolarians
• Phosphate rich materials derived from the bones,
  teeth, and scales of fish and other marine
  organisms

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Seafloor sediments

• Types of seafloor sediments
• 3. Hydrogenous sediment
• Minerals that crystallize directly from seawater
• Most common types include
• Manganese nodules
• Calcium carbonates
• Metal sulfides
• Evaporites

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Distribution of marine sediments
Figure 13.17
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Seafloor sediments

• Distribution
• Coarse terrigenous deposits dominate continental
  margin areas
• Fine-grained terrigenous material is common in
  deeper areas of the ocean basin
• Hydrogenous sediment comprises only a small
  portion of deposits in the ocean
• There are a few places where very little sediment
  accumulates (Mid-ocean ridges)

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Resources from the seafloor

• Energy resources
• Oil and gas
• Gas hydrates
• Other resources
• Sand and gravel
• Evaporative salts
• Manganese nodules

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End of Section 1
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B - Ocean Chemistry (Page 261)

• Salinity is the total amount of material
  dissolved in water. It is the ration of the mass
  of dissolved substances to the mass of the water.
  It is expressed in per cent or part per thousand.

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Origin of Sea salt

• Sources of sea salts
• Chemical weathering of rocks
• Outgassing gases from volcanic eruptions
• Processes affecting seawater salinity
• Variations in salinity are a consequence of
  changes in the water content of the solution

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Composition of seawater

• 80 of the ocean depth is salty, cold and dark
• Seawater consists of about 33 to 38
• (by weight) dissolved minerals
• Salinity is the total amount of solid material
  dissolved in water
• Typically expressed in parts-per-thousand ()
• Average salinity is 35
• Major constituent is sodium chloride

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Relative proportions of water and dissolved
components in seawater
Figure 9.3
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Composition of seawater

• Processes affecting seawater salinity
• Processes that decrease salinity (add water)
• Precipitation
• Runoff from land
• Icebergs melting
• Sea ice melting
• Processes that increase salinity (remove water)
• Evaporation
• Formation of sea ice

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Ocean temperature

• Surface water temperature varies with the amount
  of solar radiation received
• Lower surface temperatures are found in
  high-latitude regions (Polar regions)
• Higher temperatures found in low-latitude regions
  (Equatorial regions)
• Flashback Latitude and Longitude

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Ocean temperature

• Temperature variation with depth
• Low-latitudes (Equatorial regions)
• High temperature at the surface
• Rapid decrease in temperature with depth
  (thermocline)
• High-latitudes (Polar regions)
• Cooler surface temperatures
• No rapid change in temperature with depth
  (thermocline absent)

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Variations in ocean water temperature with depth
Figure 14.4
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Ocean density

• Density is mass per unit volume - how heavy
  something is for its size
• Determines the waters vertical position in the
  ocean
• Factors affecting seawater density
• Salinity (function of temperature)
• Temperature - the greatest influence

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Variations in the oceans surface temperature and
salinity with latitude
Figure 14.3
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Ocean density

• Ocean Density Variations with depth
• Low-latitudes (Equatorial regions)
• Low density at the surface
• Density increases rapidly with depth (pycnocline)
  because of colder water
• High-latitudes (Polar regions)
• High-density (cold) water at the surface
• Little change in density with depth (pycnocline
  absent)

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Variations in ocean water density with depth
Figure 14.5
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Ocean density

• Ocean layering
• Layered according to density
• Three-layered structure
• 1. Surface mixed zone
• Sun-warmed zone
• Zone of mixing
• Shallow (300 meters)

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Ocean density

• Ocean layering
• Three-layered structure
• 2. Transition zone
• Between surface layer and deep zone
• Zone of Thermocline and pycnocline
• 3. Deep zone
• Sunlight never reaches this zone
• Temperatures are just a few degrees above
  freezing
• Constant high-density water

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Layering in the ocean
Figure 14.6
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PRS Review Test

• No talking please.

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Which one is NOT part of a passive continental
margin?

• a. Continental shelf
• b. Continental slope
• c. Continental rise
• d. Oceanic trench

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Which one is NOT true of deep ocean trenches?

• a. They are long and narrow depressions at
  subduction zones
• b. They are sites where plates plunge back into
  the mantle
• c. They are associated with mid ocean ridges
• d. They are part of active continental margins

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Calcareous ooze and siliceous ooze are an example
of

• Terrigenous sediment
• Biogenous sediment
• Hydrogenous sediment
• Both terrigenous sediment and biogenous sediment

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The _?_ represents a rapid __?_change with depth

• a. thermocline temperature
• b. pycnocline temperature
• c. halocline pressure
• d. thermocline salinity

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Deep-ocean circulation is referred to as
__?_circulation and is driven by __?_differences.

• a. Thermocline, temperature
• b. Pycnocline, density
• c. Thermohaline, density
• d. Coriolis, latitude

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This instrument greatly enhanced our knowledge of
the ocean floor.

• a. Pulsar
• b. Echo sounder
• c. Submarine transit
• d. Tuzometer

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The oldest ocean floor rock is about _?_ years of
age

• a. 2 billion
• b. 200 million
• c. 4.5 billion
• d. 15 billion

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The eastern coast of the US is a (n)

• a. Active continental margin
• b. Passive continental margin
• c. None of the above

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Which is the correct ordering of the passive
margin's subdivisions?

• a. Continental shelf-Continental
  slope- Continental rise-Abyssal plain
• b. Continental shelf-Abyssal Plain- Continental
  rise-Continental slope
• c. Abyssal plain- Continental rise- Continental
  slope-Continental shelf

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Where would you expect to find the thickest
accumulation of seafloor sediment?

• a. Abyssal plain
• b. Continental slope
• c. Atop seamounts
• d. Deep-ocean trench
• e. Mid-ocean ridge

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Which one is NOT one of the three broad
subdivisions of the ocean floor?

• a. coastal plain
• b. oceanic ridge
• c. deep ocean basin
• d. continental margin