Chapter 4 Continental Margins and Ocean Basins

Chapter 4
Continental Margins and Ocean Basins
Bathymetry: a map of the ocean floor
Early bathymetric studies were often performed using a weighted
line to measure the depth of the ocean floor.
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• Bathymetry of Sea Floor
• Continental Margins and Ocean Basins
• Submarine Canyons
• Hydrothermal Vents
• Trenches
Echo sounding
LIDAR
Multibeam Systems
Satellite Altimetry
Vw ~= 1500 m/s (Pres, Temp, Salinity)
Echo Sounders Bounce Sound off the Seabed
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LIDAR: Light Imaging Detection And Ranging
Vw ~= 1500 m/s (Pres, Temp, Salinity)
Va ~= 344 m/s (Pres, Temp, other)
h, ft
Shinnecock Inlet
Moriches Inlet
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Multi-beam echo sounder
Using satellite measurements to map
the ocean floor
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Gravitational Anomalies:
Bathymetry of ocean floorfloor-ridges, shelves
Topography of Ocean Floors
Cross section of the
Atlantic ocean basin
and the continental
United States,
showing the range of
elevations.
Ocean depth is
clearly greater than
the average height of
the continent, but
general range of
contours is similar.
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An active
margin
Peru–Chile
Plate
Trench
boundary
Continental
margin
Nazca Plate
Submarine canyon profile (cut
through continental shelf)
Sediment
Continental shelf
Continental slope Oceanic
ridge
Continental rise
Sediment
Continental
crust (granitic)
Continental
crust (granitic)
Oceanic
crust (basaltic)
Asthenosphere
Plate
movement
A
passive
margin
Plate boundary
Andes
Mountains
Broad
South America continental shelf
Atlantic
Ocean
South American
Deep basin
Plate
Afr
Pacific Ocean
Continental
margin
Deep-ocean basin
Narrow
continental
shelf
ican Plate
Plate
movement
Subduction zone
(deep and shallow
earthquakes)
Plate
movement
Mid-Atlantic Ridge
(spreading centers,
shallow earthquakes)
• Atlantic = Passive Margin
little/no geologic activity
Oceanic
crust (basaltic)
• Pacific = Active Margin
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geologic activity
The ocean floor can be classified as
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Continental Margins
(a)
Continental Margins – the submerged outer edge of a
continent
(b) Ocean Basin – the deep seafloor beyond the continental
margin
There are two types of continental Margins
(a) passive or trailing margins: margin of continent that
moves away from spreading center – Atlantic-style margins
(also Artic Ocean, Antarctica and Indian Ocean). Very little
volcanic or earthquake activity is associated with passive
margins.
(b) active or leading margins: plate boundary located along a
continental margin – ocean trenches where there is
subduction of oceanic lithosphere – narrow, steep, with
volcanic mountains (West Coast of the Americas). Active
margins are the site of volcanic and earthquake activity. 11
• Region where continental crust meets oceanic crust
• Continental Shelf
• Shelf Break
• Continental Slope
• Continental Rise
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Continental Shelves:
Continental Shelves
broad shallow extension of the continents (~75km wide)
Regions of deposition (rivers, glaciers, scrapped marine
deposits, calcium carbonate)
• Gently sloping (~0.5 degrees)
• Depositional environments
• Average width 65 km (40 miles)
• Average depth 130 m (430 feet)
• Narrow along Active margins
• Wide along Passive margins
Large bedform features, reworked by tides, storms,
waves
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Shoreline Retreat During The Flandrian Transgression
The Flandrian Transgression
•Current sea level rise which began approximately 18-19,000 years
ago (during latest Pleistocene time and continuing progressively
during Holocene time to the present).
•This rise in sea level is directly related to the melting of
continental polar and mountain piedmont glaciers.
•During the "climax" of the Wisconsin glacial advance (lowstand) sea
level was anywhere between 70 to 150 meters below its current
level
-50 m
-40 m
-30 m
•Shelf Break = the outer edge of the continental shelf
-20 m
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-10 m
0m
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•30 kilometer wide band of sand ridges on the middle continental
shelf represent a broad band of degraded and submerged barrier
islands formed between 14,000 and 8,000 years before present
(Stubblefield, et al. 1983)
•Shelf currents are actively reworking the barrier sands into ridges
•It has been in the last 4000-6000 years that the majority of
modern coastal barrier islands and tidal wetlands have developed.
109,868 to 517,948 yd3/yr of sediment may be coming from offshore,
however the exact mechanism for the material transport into the
littoral zone has not been determined (Schwab et al., 1999)
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Anatomy of a passive margin
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Continental Margins
• Shelf Break
Edge of the continental shelf
Change in slope
(~ 140 m)
• Continental Slope
deep
sediments accumulate here, thickness varies
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Extends from break to ocean basin
Steep (3 – 6 degrees)
As high as 25 degrees
Little/no deposition
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Submarine Canyons Form at the Junction between
Continental Shelf and Continental Slope
Continental Slopes:
continental crust thins into oceanic crust
steep (~20km, 1-25 degrees), 5deg Pacific, 3deg Atlantic
extend to depths between 1500-4000 m
These are features of some continental margins. They cut
into the continental shelf and slope, often terminating on
the deep-sea floor in a fan-shaped wedge of sediment.
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Submarine Canyon
Turbidity Currents
• Fast moving avalanches of mud and sand
• Steep V shape channel, incised in the
scour slopes
continental slope (and shelf)
• Created by
• Form turbidite deposits
• 90 km/hr (56 mi/hr)
™Rivers during the last low stand (some)
™Turbidity currents
Turbidite bed
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ancient deposit, exposed
to erosion, graded
deposits: largest particles
at bottom
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Continental Margins
• Continental Rise
Base of the continental slope
slope 0.5 – 1 degree
Depositional environment
Formed by:
Turbidity currents
Underwater landslides
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Continental Rise:
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Features of the Sea floor
Fan like deposit where the continental slope intersects
the abyssal plains
Oceanic Ridges
Hydrothermal Vents
Abyssal Plains and Abyssal Hills
Seamounts and Guyots
Trenches and Island Arcs
Formed by turbidity currents
Seafloor:
Seafloor: 4000 – 6000 m water depth, 30% of the Earth’
Earth’s
surface
Abyssal Plain: vast, flat plain extending from the base of
the continental slope.
Ocean Basins: sections of the abyssal plain separated by
continental margins, ridges, and rises.
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Sea floor features
Basins
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Seamounts are volcanic projections from the ocean floor that do not
rise above sea level. FlatFlat-topped seamounts eroded by wave action are
called guyots
Abyssal hills are flat areas of sedimentsediment-covered ocean floor found
between the continental margins and oceanic ridges. Abyssal hills are
small, extinct volcanoes or rock intrusions near the oceanic ridges.
ridges. 33
MidMid-Ocean Ridges and Rises
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Hydrothermal vents are sites where superheated water containing
dissolved minerals and gases escapes through fissures, or vents. Cool
water (blue arrows) is heated as it descends toward the hot magma
magma
chamber, leaching sulfur, iron, copper, zinc, and other materials
materials from
the surrounding rocks. The heated water (red arrows) returning to
to
the surface carries these elements upward, discharging them at
hydrothermal springs on the seafloor.
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Major ocean trenches
An oceanic ridge is a mountainous chain of young,
basaltic rock at an active spreading center of an ocean.
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Trenches are arcarc-shaped depressions in the ocean floor caused by the
subduction of a converging ocean plate.
Most trenches are around the edges of the active Pacific. Trenches
Trenches are the
deepest places in Earth’
Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than
the adjacent basin floor. The ocean’
ocean’s greatest depth is the Mariana Trench
where the depth reaches 11,022 meters (36,163 miles) below sea level.
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level.
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Chapter 4 - Summary
• Bathymetric devices used to study seabed features
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include multibeam echo sounder systems and satellites
that use sensitive radar for altimetry.
Seafloor features result from a combination of tectonic
activity and the processes of erosion and deposition.
Near shore, the features of the ocean floor are similar
to those of the adjacent continents because they share
the same granitic basement. The transition to basalt
marks the edge of the continent and divides ocean
floors into two major provinces, The submerged outer
edge of a continent is called the continental margin.
The deepdeep-sea floor beyond the continental margin is
called the ocean basin
Features of the continental margins include continental
shelves, continental slopes, submarine canyons, and
continental rises.
Features of the deepdeep-ocean basins include oceanic
ridges, hydrothermal vents, abyssal plains and hills,
seamounts, guyots,
guyots, trenches, and island arcs.
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Good Stuff
Construct a transect (cross-section) across the Atlantic Ocean starting from the
east coast of the United States. Be sure to label all of the bathymetric
features and type of margin.
Construct a transect (cross-section) across the Pacific Ocean starting from the
west coast of the United States. Be sure to label all of the bathymetric
features and type of margin.
What are the noticeable differences between the two types of margins?
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