Transcript Marine Provinces and the Ocean Floor

M. Anderson, 2009
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3 major provinces
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Continental margins
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Deep-ocean basins
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Mid-ocean ridge
◦ Shallow-water areas close to shore
◦ Deep-water areas farther from land
◦ Submarine mountain range
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Zones separating the part of a continent above sea
level from the deep-sea floor.
The true geologic margin of a continent - where
continental crust changes to oceanic crust - is
somewhere beneath the continental slope.
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Passive Continental
Margin - Trailing
end of a continental
plate.
Posses broad
continental shelves,
a continental slope
and rise.
Flat abyssal plains
are adjacent to the
rise. They lack the
seismic and
volcanic activity.
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Active
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Convergent active margin
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◦ Associated with convergent or transform plate
boundaries
◦ Much tectonic activity
Oceanic-continental convergence
Where oceanic lithosphere is subducted.
Seismicity, a young Mt. range and andesitic volcanism.
The shelf is narrow and descends directly into a trench.
Example: western South America
Continental
shelf
 Shelf break
 Continental
slope
 Continental
rise
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Continental Shelf - where the sea floor slopes very gently
seaward.
It ends at a steep drop, the shelf - slope break at @ 135m.
In the Pleistocene, sea level was much lower, @ 130m.
Much of the sediment on continental shelves accumulated in
stream channels and floodplains.
Some glaciers extended onto the exposed shelves.
Affected by waves and tidal currents.
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Change in gradient from shelf
Average gradient 4o
Submarine canyons cut into slope by turbidity
currents
◦ Mixture of seawater and sediments
◦ Move under influence of gravity
◦ Erode canyons
◦ Deposit sediments at base of slope
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Turbidity Currents ;
sediment- water mixture
denser than normal
seawater
Flow down-slope to the
deep-sea floor.
Coarsest particles are
deposited first - forming
a graded bed.
Deposits accumulate as
a series of overlapping
submarine fans, forming
a large part of the
continental rise.
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Continental rise - gently sloping area from the slope
to a trench.
Unaffected by surface processes and transport is
controlled by gravity. Where most of the sediments
are eventually deposited. Much of the sediment is
transported by turbidity currents.
Deep ocean Basin - seaward of the continental
margin.
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Deep, steep sided submarine canyons occur on
continental shelves, but are best developed on the
continental slopes.
Some Canyons can be traced across the shelf to
associated streams on land. Some can not.
Strong currents move through these canyons and are
probably responsible for their erosion.
Monterey Submarine Canyon
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Transition between continental crust and
oceanic crust
Turbidite deposits
◦ Graded bedding
◦ Submarine fans
Distal end of submarine fans becomes flat
abyssal plains
The product of turbidity currents in km water depth
 Present/future targets of the oil industry once
shallower resources are exhausted
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Flat abyssal plains are adjacent to the rise. They lack
the seismic and volcanic activity.
Abyssal Plains are the flattest, most featureless
areas of Earth A result of sediment deposition.
Found adjacent to Continental rises.
Common in the Atlantic, rare in the Pacific.
Along active margins, sediments are trapped in an
ocean trench so abyssal fans fail to develop.
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Very flat depositional surfaces from base of
continental rise
Suspension settling of very fine particles
Sediments cover ocean crust irregularities
Well-developed in Atlantic and Indian oceans
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Abyssal hills - average
@ 250m high.
They are common on
the sea floor and
underlie thick
sediments on the
abyssal plains.
Abyssal plains
Fig. 3.11
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Oceanic Trenches
Where lithospheric plates are consumed by subduction.
Long, narrow features restricted to active continental
margins.
Oceanic trenches are the sites of greatest oceanic
depth.
Marianas Trench : 11,000 m deep.
The crust here is cooler and slightly denser than
elsewhere.
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Trenches show a huge
negative gravity anomaly;
The crust is held down
and is not in equilibrium.
Seismic activity occurs at
or near the trenches.
They have Benioff zones
in which earthquake foci
become progressively
deeper in a landward
direction.
Most intermediate and
deep
earthquakes occur in
such zones.
They are associated with
volcanoes. (W. So.
America)
Emperor, Marshall and Hawaii seamounts
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Seamounts are isolated volcanic mountains scattered
across the ocean floor.
Most common in the Pacific Ocean, seamounts generally
rise more than 1,000 meters above the sea floor, often
forming islands.
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When the action of plate
tectonics moves a
seamount-formed island
away from the mid-ocean
ridge, the ocean crust
sinks, pulling the island
beneath the surface.
These submerged, often
flat-topped, seamounts
are called guyots.
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Poke through sediment cover
Below sea level:
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Above sea level:
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◦ Seamounts, tablemounts, or
guyots at least 1 km (0.6 m) above
sea floor
◦ Abyssal hills or seaknolls are less
than 1 km
◦ Volcanic islands
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Landward side of ocean trench
Island arc
◦ Chain of islands, e.g., Japan,
Hawaii, Aleutians
Continental arc
◦ Volcanic mountain range,
e.g., Andes Mountains
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As the South American
continent moved west, in
the Cretaceous Period
(140mya) the Nazca plate
subducted forming a trench.
By 130mya igneous activity
began and a Volcanic arc
was formed.
By 90 mya the trench had
migrated west and a new
volcanic arc formed along
the west coast, while
mountains to, now in the
interior, ceased activity.
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Longest mountain chain
On average, 2.5 km (1.5 miles)
above surrounding sea floor
Wholly volcanic
Basaltic lava
Divergent plate boundary
Central rift valley, faults, and
fissures
Seamounts
Pillow basalts
Hydrothermal vents
◦ Deposits of metal sulfides
◦ Unusual life forms
Fracture zones and transform
faults
 Oceanic
ridge
◦ Prominent rift valley
◦ Steep, rugged slopes
◦ Example: Mid-Atlantic Ridge
 Oceanic
rise
◦ Gentler, less rugged slopes
◦ Example: East Pacific Rise
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When lava flows enter the ocean, or when lava
outpourings actually originate within an ocean basin, the
flows outer zones quickly congeal.
The lava within the flow is able to move forward by
breaking through the hardened surface, when this occurs
over and over.
The lava flow resembles large bed pillows stacked one
upon another.
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1970'S @ 2,500m in the
Galapagos Rift in E. Pacific
Ocean Basin hydro-thermal
vents were first observed.
ALVIN: Woods Hole Submersible
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Hydrothermal vents
◦ Heated subsurface seawater migrates
through cracks in ocean crust
 Warm-water vents <30oC or 86oF
 White smokers >30oC <350oC or 662oF
 Black smokers > 350oC