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Ocean Basins, Bathymetry and
Sea Levels
Chapter 4
Please read chapter 5: sediments for next class
and start chapter 6 on seawater for Thursday
Basic concepts in Chapter 4
Bathymetry — the measurement of water depths and mapping of sea
floor features — uses several types of technology.
The sea floor has two distinct regions: continental margins and deep ocean
basins.
Continental margins are the relatively shallow areas of the ocean
floor near shore. Geologically they are part of the adjacent continent.
Passive vs. active continental margins are distinguished by their plate
tectonic setting.
Major features: continental shelf, shelf break, continental slope,
continental rise, submarine canyons, submarine fans.
Deep-ocean basins differ from the continental margins in tectonic
origin, history, and composition. Most important features of the deep ocean
basins were formed by plate tectonic processes.
Major features: mid-ocean ridge, transform faults & fracture
zones, hydrothermal vents, abyssal plain, abyssal hills, seamounts
& guyots, oceanic trench & volcanic arc.
Evolution of the oceans (and continents)
over the last 650 million years
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Present
Vail Curve of
Sea Level
History
Transgression
540 million years BP
Regression
Blue = No deposition
Interglacial
Glacial
What about changes in sea level?
glacial/interglacial timescale
Increase of ~120
meters
What about changes in sea level?
OverFigure
the last5.13
century
Increase of ~15
centimeters
Bathymetry: Measuring Ocean Depths
As late as 1870 bathymetric studies were often
performed using a weighted line dropped to
measure depth.
Advances in
Bathymetry
• Echo sounding (1922)
• Multibeam systems
• Satellite altimetry
Echo sounding is a method of measuring depth using powerful sound pulses. The
time it takes for the sound pulse to travel to the sea bed and bounce back is a
measure of the depth.
Distance = Rate x Time
Trace of an echo sounder is just a line
Multibeam systems can provide more accurate measurements
than echo sounders. Multibeam systems collect data from as many
as 121 beams to measure the contours of the ocean floor.
Fig. 4-3b, p. 79
Fig. 4-4 (a-b), p. 80
Satellite altimetry is an indirect way of measuring depth and
detecting sea floor features. 1000 pulses a second while moving
4 miles a second over head. Height accuracy to within 1 inch!
Size comparison of various volcanic features
Continental Margins
Components:
1. Continental shelf = the shallow, submerged edge
of the continent
2. Shelf break = the abrupt transition from continental
shelf to the continental slope
3. Continental slope = the transition between the
continental shelf and the deep-ocean floor
4. Continental rise = thick accumulations of sediment
found at the base of the continental slope
Continental margin = the submerged outer edge of
a continent; really just an extension of the continental
crust, which has an average composition of granite.
vs.
Deep-ocean basin = the deep sea floor beyond the
continental margin; made up of oceanic crust, which
is composed mostly of volcanic basalt.
Types of continental margins
Passive margins (= “Atlantic-type” margins)
" They face the edges of diverging tectonic plates
" Very little volcanic or earthquake activity
Active margins (= “Pacific-type” margins)
" Located near the edges of converging plates, where one plate subducts beneath another at an
oceanic trench
" Extensive volcanic and earthquake activity
Fig. 4-10, p. 85
Fig. 4-11, p. 86
Submarine Canyons
Underwater landslides or avalanches called turbidity currents commonly flow down
submarine canyons. The debris settles out to build up a submarine fan at the base of the
canyon.
Continental
slope
Continental
rise
An
underwater
debris flow,
i.e. a
turbidite.
Fig. 4-14, p. 87
Deep Ocean Basins
Major features:
• Mid-ocean ridges
• Fracture zones and transform faults
• Hydrothermal vents
• Shallow earthquakes
• Abyssal plains
• Abyssal hills
• Seamounts and guyots
• Oceanic trenches
• Volcanic arcs
• Deep earthquakes
Fig. 4-17a, p. 90
Ju an d eF uc aS mo k er. av i
Abyssal Plains
Abyssal plains = broad flat areas of sediment-covered
ocean floor found between the continental margins and
the mid-ocean ridges
• Typically 4-6 km below sea level
• The flattest surface on Earth
• Sedimentation rates are very slow -- millimeters/1000 years!
• Mostly very fine clay, windblown dust, and shells of
microscopic organisms
• Carbonate sediments are rare, as most of the plain is below
the Carbonate Compensation Depth (CCD)
Thick layers of sediment on
top of submerged mountains
Trenches
Oceanic trenches are the deepest parts of the sea floor. They are formed by subduction.
Notice that most trenches (and therefore most subduction) occur in the Pacific ocean basin.
Subduction at oceanic trenches causes the descending plate to generate melting of the
overlying mantle, forming volcanic island arcs = lines of volcanic islands and seamounts
running parallel to the edges of trenches. Subduction also causes many earthquakes.
Seismic activity in the Aleutian Islands
The Aleutian Island Chain
Formation of Guyots and
submerged mountain chains
Fig. 4-23a, p. 95
Oceanic-Oceanic
Subduction Zone
Oceanic-Continental
Subduction Zone
Oceanic-Oceanic and Oceanic-Continental Subduction Zones
Passive Continental Margin