3

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

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)

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

3

Figure 3.5

3

Figure 3.6

3

Figure 3.8

3

Fig. 4-5, p. 82

3

Figure 3.9

3

Shape of the Ocean Floor

• shallow extensions of the continents extended seaward underwater

3

Figure 3.8

3

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

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

3

SubmarineCanyonOff 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