GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 6: The Seafloor
POWERPOINT SLIDE SHOW NOTES
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Topic 6: The Seafloor
Take time to watch the learner.org videos for this and other topics
Measuring the depths
Key vocabulary
Bathymetry of the sea floor
Cross section of a typical ocean basin, in this case the Atlantic.
Note the differences between the Atlantic and Pacific sea floors. Ridge versus rise: Both
are part of the mid-ocean ridge system. The names are often used interchangeably, but
in general, a ridge spreads more slowly than a rise. Note how many more seamounts
there are in the Pacific.
The Atlantic profile, notice the features named in the profile at the bottom. The map at
the top is a physiographic map, drawn by an artist using shading and perspective.
The Pacific profile. What features are the same as the Atlantic? Different?
Vertical exaggeration is an important tool to allow us to see vertical relationships. Look
back at slides 7, 8, and 9, for example. Now would be a good time to read “An Account
of a Trek to the Crest of the Mid-Atlantic Ridge.”
Work through this exercise on vertical exaggeration. Make sure the units are the same
for both the vertical and horizontal axes. Then divide the horizontal scale by the vertical
scale. In this example, the profile at the bottom right of the slide has a VE of 5. That
means the vertical scale has been stretched up 5 times the true scale, making it easier to
see the overall shape of the hill.
Components of the continental margin
Plate tectonics plays a major role in controlling the features of a continental margin.
Note the broad shallow shelf of a passive margin and the adjacent coastal planes just like
we have in Georgia.
Active continental margins are narrower and steeper.
Can you calculate the vertical exaggeration of the top diagram? The true-scale profile at
the bottom illustrates that the margin isn’t as steep as often portrayed.
Continental margin exaggerated and at true scale.
The bottom profile is at true scale.
The top diagram is at true scale and looks just like a straight line. Where did the trench
go?
The deepest spot in the Gulf of Mexico is the Sigsbee Deep.
The continental margins are greatly affected by sea level changes.
Sea level has changed many times throughout geologic history.
Big picture of the passive margin which is the east coast of the US.
Passive margins often have large rivers flowing on them. In our case, the thick pile of
sediment is derived from erosion of the Appalachian Mountains. The sediments have
built the continental margin out and up through time. The buried reef deposits are proof
that the continent was in the tropics at one time. Salt is commonly deposit during
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continental rifting when a young sea is forming and the area periodically flooded and
dried to leave the salt behind.
The Gulf of Mexico (GOM) is also a passive margin. The Mississippi River has deposited
vast quantities of sediment. There is much more salt at the base of the GOM sediments.
When enough weight is piled on top of salt, it can begin to flow rise toward the surface
because of its low density compared to the surrounding sediment. See how the
sediment layers bow up over the salt diapirs (more commonly known as salt domes).
The high organic content of the sediment makes this a good location for the formation of
oil and natural gas. The bowing up of the sediments makes a good environment to trap
the oil and gas. That’s why there is so much drilling for oil in this area.
The Continental shelf
The broadest shelves in the world are off Siberia.
Continental shelf break. Often overlooked or omitted from diagrams (see slide 13), it is
where the steepness or gradient of the continental margin changes from flat to steep.
Continental slope
Continental rise
Submarine canyons
Submarine canyons are like steep mountain rivers on the continental margins.
Some submarine canyons are associated with rivers on land, like the Hudson Canyon, but
many are not.
The Monterey Canyon in California comes almost all the way to the beach.
Turbidity currents can flow through submarine canyons and maintain or enlarge them.
Turbidity current flow
Turbidite
Resultant deposit from a turbidity current. There may be centuries or millennia between
turbidity current events.
Photo of a turbidity current
The 1929 Grand Banks turbidity current proved the erosive capability of these flows. It
broke a series of telegraph cables in succession so we could measure its speed.
A sand fall differs from a turbidity current. A turbidity current is sand and mud mixed up
into the water column, making the entire mass much denser. A sand fall is simply sand
falling through water.
Features of the ocean basin floor
The North Atlantic Ocean basin.
The deepest ocean floor is also the oldest. It is the farthest from the spreading centers.
Simplified Atlantic Ocean profile or cross-section.
Sea floor spreading and tablemounts (guyots).
Volcanic islands erode relatively rapidly.
A volcanic island becomes an atoll and if it continues to sink to form
Atoll, again
Real atolls are usually not perfectly round
A sonar depth chart showing seamounts and a guyot.
Ridges, rises, and trenches we’ve seen in plate tectonics.
You should be able to identify the major features.
Mid-ocean ridge
Mariana Trench (often written as Marianas).
Japan and Mariana Trenches
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Oceanic trenches are found on convergent plate boundaries.