3 bathymetry of the ocean floor bathymetry= measuring ocean depths and charting the shape or...
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Bathymetry of the Ocean Floor
• Bathymetry= measuring ocean depths and charting the shape or topography of the ocean floor
• “Sounding” lines (started in 85 B.C.)
• echo soundings (1920’s)
• ocean was not deepest in the center
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Fig. 4-2a, p. 79
Echo sounding- not always accurate
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Figure 3.1- An Echo Sounding of US East Coast
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Figure 3.2- Multibeam SONAR (Sound Navigation And Ranging)
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Figure 3.3- Side-scan SONAR (Sound Navigation and Ranging)
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3With the use of satellite altimetry, sea surface levels can be measured more accurately, showing sea surface distortion.
(left) 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.
Satellites Can Be Used to Map Seabed Contours
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Figure 3.5
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Figure 3.6
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Figure 3.8
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Fig. 4-5, p. 82
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Figure 3.9
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Shape of the Ocean Floor
• shallow extensions of the continents extended seaward underwater
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Figure 3.8
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Shape of the Ocean Floor
• Submerged outer edge of the continents are called continental margins
• Deep-sea floor beyond these is called the ocean basin
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Fig. 4-7, p. 83
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3Continental Margins
• Passive margins –
– continental margins not located on plate boundaries
– Atlantic-type margins
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3Continental Margins
• Active margins- – continental margins on the edge of
convergent or transform plate boundaries
– Pacific-type margins
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Fig. 4-8, p. 84
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Figure 3.7
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3Continental Margins
• Continental Shelves– Shallow, submerged extension of a
continent– broad, gently sloping
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Figure 3.8
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Fig. 4-9, p. 85
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3Continental Margins
• Average width of Continental Shelf is about 70 km (43 miles)– Varies from 10s of m to 1500 km
• Width of Continental Shelf is determined by :
• proximity to a plate boundary• current speed in the region• sea-level
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Fig. 4-8, p. 84
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3Continental Margins
• Shelf break –– transition between the continental
shelf and the continental slope– Occurs at about 140m (360 ft)
• Continental Slopes– Steeper than the shelf – Average slope= about 4 degrees (but
varies from 1 to 25 degrees)
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Figure 3.8
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3Continental Margins
• Continental rises
– base of continental slope covered by a blanket of accumulated sediment
– gradual slope– on Passive margins only
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Fig. 4-9, p. 85
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3Continental Margins
• Submarine Canyons
– cut into the continental shelf and slope
– formed by turbidity currents (avalanche-like sediment movements)
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Figure 3.10a
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SubmarineCanyonOff ofThe coastOf NewJersey
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Figure 3.11
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Fig. 4-7, p. 83
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Figure 3.6
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3Ocean Basin
• thick layer of sediment (up to 5 km or 3mi thick) covering basaltic rocks
• Make up more than ½ of the earth’s surface
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3Ocean Basin
• Oceanic ridges– Underwater mountain chain – an active spreading center– Central rift valley– offset at regular intervals by transform
faults
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Fig. 4-16a, p. 89
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Figure 3.15
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Figure 3.19
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3Ocean Basin
• Hydrothermal vents– 1977 Robert Ballard & J F Grassle– average temp is about 8-16oC (46-61oF)
much warmer than the typical 3-4oC (37-39oF)
– support a unique community of organisms that depend on chemosynthetic bacteria
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Figure 3.17
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Fig. 4-19, p. 91
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Figure 2.26
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3Ocean Basin
• Seamounts
– Volcanic peaks that do not rise above the surface of the ocean
– They are tall (1 km or 0.6 mi) with steep slopes
• Guyots or Tablemounts
– Flat-topped seamounts that were eroded by wave action
• Abyssal Hills
– abundant, small (200m or 650 ft tall) sediment-covered extinct volcanoes
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3Ocean Basin
• Abyssal Plains
– Flat, featureless, sediment-covered ocean floor
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Figure 3.12
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3Ocean Basin
• Trenches
– Arc-shaped depression in the deep seafloor
– a converging oceanic plate is subducted
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Figure 3.13
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Figure 3.14
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Fig. 4-25, p. 96
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3Ocean Basin
• Island Arcs
– Curving chains of volcanic islands and seamounts found paralleling the edge of trenches
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Figure 3.C