3Bathymetry 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
3
Fig. 4-2a, p. 79
Echo sounding- not always accurate
3
Figure 3.1- An Echo Sounding of US East Coast
3
Figure 3.2- Multibeam SONAR (Sound Navigation And Ranging)
3
Figure 3.3- Side-scan SONAR (Sound Navigation and Ranging)
3 With 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
3
Figure 3.5
3
Figure 3.6
3
Figure 3.8
3
Fig. 4-5, p. 82
3
Figure 3.9
3Shape of the Ocean
Floor
• shallow extensions of the continents extended seaward underwater
3
Figure 3.8
3Shape of the Ocean
Floor
• Submerged outer edge of the continents are called continental margins
• Deep-sea floor beyond these is called the ocean basin
3
Fig. 4-7, p. 83
3Continental Margins
• Passive margins – – continental margins not located on
plate boundaries – Atlantic-type margins
3Continental Margins
• Active margins- – continental margins on the edge of
convergent or transform plate boundaries
– Pacific-type margins
3
Fig. 4-8, p. 84
3
Figure 3.7
3
3
3Continental Margins
• Continental Shelves– Shallow, submerged extension of a
continent– broad, gently sloping
3
Figure 3.8
3
Fig. 4-9, p. 85
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
3
Fig. 4-8, p. 84
3
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)
3
Figure 3.8
3Continental Margins
• Continental rises– base of continental slope covered by
a blanket of accumulated sediment– gradual slope– on Passive margins only
3
Fig. 4-9, p. 85
3
3Continental Margins
• Submarine Canyons– cut into the continental shelf and
slope – formed by turbidity currents
(avalanche-like sediment movements)
3
3
Figure 3.10a
3SubmarineCanyonOff ofThe coastOf NewJersey
3
3
Figure 3.11
3
Fig. 4-7, p. 83
3
Figure 3.6
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
3Ocean Basin
• Oceanic ridges– Underwater mountain chain – an active spreading center– Central rift valley– offset at regular intervals by transform
faults
3
Fig. 4-16a, p. 89
3
Figure 3.15
3
Figure 3.19
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
3
Figure 3.17
3
Fig. 4-19, p. 91
3
3
Figure 2.26
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
3Ocean Basin
• Abyssal Plains– Flat, featureless, sediment-covered ocean
floor
3
Figure 3.12
3
3Ocean Basin
• Trenches– Arc-shaped depression in the deep
seafloor– a converging oceanic plate is subducted
3
Figure 3.13
3
Figure 3.14
3
Fig. 4-25, p. 96
3Ocean Basin
• Island Arcs– Curving chains of volcanic islands and
seamounts found paralleling the edge of trenches
3
3
Figure 3.C