Zone of the ocean floor that separates the thin oceanic crust from thick continental crust
Profile illustrating the shelf, slope and rise
A
continental margin
is the outer edge of
continental crust
abutting
oceanic crust
under coastal waters. It is one of the three major zones of the ocean floor, the other two being
deep-ocean basins
and
mid-ocean ridges
. The continental margin consists of three different features: the
continental rise
, the
continental slope
, and the
continental shelf
.
[1]
The continental shelf is the relatively shallow water area found in proximity to continents. Continental margins constitute about 28% of the oceanic area.
[2]
Zones of the continental margin
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The continental shelf is the portion of the continental margin that transitions from the shore out towards to ocean. Continental shelves are believed to make up 7% of the sea floor.
[3]
The width of continental shelves worldwide varies in the range of 0.03?1500
km
.
[4]
The continental shelf is generally flat, and ends at the
shelf break
, where there is a drastic increase in slope angle: The mean angle of continental shelves worldwide is 0
°
07′, and typically steeper closer to the coastline than it is near the shelf break.
[5]
At the shelf break begins the continental slope, which can be 1?5 km above the deep-ocean floor. The continental slope often exhibits features called
submarine canyons
.
[4]
Submarine canyons often cut into the continental shelves deeply, with near vertical sides, and continue to cut the morphology to the
abyssal plain
.
[5]
These canyons are often V-shaped, and can sometime enlarge onto the continental shelf. At the base of the continental slope, there is a sudden decrease in slope angle, and the sea floor begins to level out towards the abyssal plain. This portion of the seafloor is called the
continental rise
, and marks the outermost zone of the continental margin.
[1]
Types
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There are two types of continental margins:
active
and
passive
margins.
[1]
Active margins are typically associated with
lithospheric plate
boundaries. These active margins can be
convergent
or
transform
margins, and are also places of high tectonic activity, including
volcanoes
and
earthquakes
. The West Coast of North America and South America are active margins.
[4]
Active continental margins are typically narrow from coast to shelf break, with steep descents into
trenches
.
[4]
Convergent active margins occur where oceanic plates meet continental plates. The denser oceanic crust of one plate
subducts
below the less dense continental crust of another plate. Convergent active margins are the most common type of active margin. Transform active margins are more rare, and occur when an oceanic plate and a continental plate are moving parallel to each other in opposite directions. These transform margins are often characterized by many offshore
faults
, which causes high degree of relief offshore, marked by
islands
, shallow banks, and
deep basins
. This is known as the
continental borderland
.
[1]
Passive margins are often located in the interior of lithospheric plates, away from the plate boundaries, and lack major tectonic activity. They often face
mid-ocean ridges
.
[3]
From this, comes a wide variety of features, such as low-relief land extending miles away from the beach, long river systems and piles of sediment accumulating on the continental shelf.
[6]
The East Coast of the United States is an example of a passive margin. These margins are much wider and less steep than active margins.
Sediment accumulation
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As continental crust weathers and erodes, it degrades into mainly
sands
and
clays
. Many of these particles end up in streams and rivers that then dump into the ocean. Of all the sediment in the stream load, 80% is then trapped and dispersed on continental margins.
[3]
While modern river sediment is often still preserved closer to shore, continental shelves show high levels of glacial and relict sediments, deposited when sea level was lower.
[3]
Often found on passive margins are several kilometres of sediment, consisting of
terrigenous
and carbonate (
biogenous
) deposits. These sediment reservoirs are often useful in the study of
paleoceanography
and the original formation of ocean basins.
[3]
These deposits are often not well preserved on active margin shelves due to tectonic activity.
[4]
Economic significance
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The continental shelf is the most economically valuable part of the ocean. It often is the most productive portion of the continental margin, as well as the most studied portion, due to its relatively shallow, accessible depths.
[4]
Due to the rise of
offshore drilling
, mining and the limitations of
fisheries
off the continental shelf, the
United Nations Convention on the Law of the Sea
(UNCLOS) was established. The edge of the continental margin is one criterion for the boundary of the internationally recognized claims to underwater resources by countries in the definition of the "
continental shelf
" by the UNCLOS (although in the UN definition the "legal continental shelf" may extend beyond the geomorphological continental shelf and vice versa).
[2]
Such resources include fishing grounds, oil and gas accumulations, sand, gravel, and some heavy minerals in the shallower areas of the margin. Metallic minerals resources are thought to also be associated with certain active margins, and of great value.
[3]
See also
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References
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External links
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