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Topic 6The Seafloor

GEOL 2503Introduction to Oceanography

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• See the video “The Sea Floor” on Learner.org under the Earth Revealed series

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Measuring the Depths

• Posidonius made first measurements• Old methods used weighted ropes or cables

—used for 2000 years (refer back to Topic 1)

• Problems were stretch and drift of ropes and cables

• Echo sounder developed in 1920s

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Key Vocabulary

• Sounding—any depth measurement• Fathom—old length measure still in use,

equal to 6 feet• Bathymetry—study and measurement of

the sea floor and the variations of water depth; the topography of the sea floor

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Bathymetry of the Sea Floor

• Just as rugged and complex as the land• Erosion is very slow• Main areas:

– The Continental Margins– The Ocean Basin Floor– The Ridges, Rises, and Trenches

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Example of an ocean basin: the Atlantic profile

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Example of an ocean basin:the Pacific profile

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Vertical Exaggeration (VE)

• The distortion of the vertical scale of a topographic profile to emphasize the relief and slope of the land

• VE = horizontal scale (distance) divided by vertical scale (elevation or depth)

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Vertical Exaggeration Example

• The horizontal scale is 1:12,000 which means 1 inch equals 12,000 inches, or 1 inch equals 1,000 feet

• The vertical scale is 1 inch equals 200 feet

• 1,000 ft 200 ft = 5• VE = 5

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The Continental Margin

• Edges of the continents• Sea-level changes affect margins• Main components

– Continental shelf– Continental shelf break– Continental slope– Continental rise

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Submarine canyons

Coastal plain

Continental shelf

Continental slope

Continental rise

ContinentalcrustOceanic crust

Mantle

Deep-sea fanAbyssal plain

Passive Continental Margins

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Continental crust

Oceanic crust

Mantle Accretionarywedge

Offshoretrench

Continentalvolcanic belt

Active Continental Margins: Convergent

15The bottom diagram is at true scale. What is the vertical exaggeration of the top diagram?

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17Remember this graphic? The bottom diagram is at true scale. What is the vertical exaggeration of the top diagram?

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Marianas Trench

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Sigsbee Deep

Deepest spot in the Gulf of Mexico

3787 meters

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Recent sea-level changes have been the right magnitude to have alternately flooded and exposed the continental margins.

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There have been many changes in sea level throughout Earth’s history. Look at this curve for the last 30 million years.

The curve on the previous page shows only the last 40 thousand years. You can’t even see that little bit of time on the curve at left.

Eustatic refers to a uniform global sea-level change.

http://strata.geol.sc.edu/exerices/seismic/Sealevel.jpg

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Continental Shelf

• Part of the continents, underlain by granite• Covered with shallow water• Slope less than one degree• Widths average 65 km (40 mi)

– Narrower along active margins than passive• Water depth varies 20-500 m (65-1640 ft)

– Average about 130 m (427 ft)

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Continental Shelf Break

• Boundary between shallow shelf and beginning of deep ocean

• Zone of abrupt change in slope

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Continental Slope

• Steep slope extending to ocean basin floor• Water depth increases from less than 200 m

to average of 4,000 m• Occurs over very narrow width

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Continental Rise

• At base of continental slope• Accumulation of sediment on seafloor• Lessens the steepness of the slope• Similar to alluvial fans on land

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Submarine Canyons

• A dominant feature of continental slopes• Steep-sided with V-shaped cross section• Many associated with modern river systems

on land, probably cut into shelf during lower sea levels

• Evidence suggests most were formed and/or maintained by turbidity currents

31Submarine canyons are a dominant feature of continental margins

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Offshore Monterey Bay, California. Note all the submarine canyons.

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Turbidity Currents

• A fast-moving avalanche of mud, sand, and water

• Flow down slopes by gravity• May travel at 90 kph (56 mph)• Erode sea bottom and excavate canyons• Caused by earthquakes or overloading of

sediments on steep slopes

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Turbidite

• Deposit of sediment from a turbidity current• As current passes, largest grains settle out

first • Successively smaller grains settle• Called a “graded bed”• Have scoured bottoms from erosion by

passing current

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Image of a turbidity current moving down the continental slope off California 

39This is how we first learned of the erosive potential in the deep sea

40A sand fall in a submarine canyon off Baja California

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The Ocean Basin Floor

• Abyssal plain—flattest places on earth• Abyssal hill—small undersea volcano• Seamount—undersea volcano 1,000+ m• Guyot—flat-topped seamount (see plate

tectonics)• Atoll—circular reef around shallow lagoon

(Charles Darwin)

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43Deepest areas are mostly older parts of ocean and abyssal plains

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The Salt Water

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• As you go from an oceanic ridge outwards, the lithosphere cools and compacts, causing islands that were once above water to be below the water.

46Volcanic islands weather and erode rapidly in tropical areas. Eventual submergence will lead to formation of an atoll.

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Charles Darwin was the first to realize how atolls are formed

As the former volcanic island continues to sink, eventually a guyot will form

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• Fringing reef Barrier reef Atoll– Caused by sinking of volcanic island as it

moves away from oceanic ridge

49Each of these Pacific atolls began as a volcanic island

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Abyssal Plain

Seamounts Guyot

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The Ridges, Rises, and Trenches

• The highs and lows of the seafloor• Plate tectonics will explain their origin

52The Mid-Ocean Ridge system is an undersea volcanic mountain chain

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True scale

Trenches are linear features and are the deepest spots on Earth

55The Mariana Trench is the deepest spot on Earth

Mariana Trench

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Convergent boundaries are responsible for oceanic trenches