The continental shelf is an underwater landmass which extends from a continent, resulting
in an area of relatively shallow water known as a shelf sea. Much of the shelves were exposed
during glacial periods and interglacial periods. The shelf surrounding an island is known as
an insular shelf. The continental margin, between the continental
shelf and the abyssal plain, comprises a steep continental slope followed by the flatter
continental rise. Sediment from the continent above cascades down the slope and accumulates
as a pile of sediment at the base of the slope, called the continental rise. Extending as
far as 500 km from the slope, it consists of thick sediments deposited by turbidity
currents from the shelf and slope. The continental rise's gradient is intermediate between the
slope and the shelf, on the order of 0.5–1°. Under the United Nations Convention on the
Law of the Sea, the name continental shelf was given a legal definition as the stretch
of the seabed adjacent to the shores of a particular country to which it belongs.
Geographical distribution
The width of the continental shelf varies considerably – it is not uncommon for an
area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic
plate dives beneath continental crust in an offshore subduction zone such as off the coast
of Chile or the west coast of Sumatra. The largest shelf – the Siberian Shelf in the
Arctic Ocean – stretches to 1,500 kilometers in width. The South China Sea lies over another
extensive area of continental shelf, the Sunda Shelf, which joins Borneo, Sumatra, and Java
to the Asian mainland. Other familiar bodies of water that overlie continental shelves
are the North Sea and the Persian Gulf. The average width of continental shelves is about
80 km. The depth of the shelf also varies, but is generally limited to water shallower
than 150 m. The slope of the shelf is usually quite low, on the order of 0.5°; vertical
relief is also minimal, at less than 20 m. Though the continental shelf is treated as
a physiographic province of the ocean, it is not part of the deep ocean basin proper,
but the flooded margins of the continent. Passive continental margins such as most of
the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges
derived from long erosion of a neighboring continent. Active continental margins have
narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep
sea. Topography
The shelf usually ends at a point of increasing slope. The sea floor below the break is the
continental slope. Below the slope is the continental rise, which finally merges into
the deep ocean floor, the abyssal plain. The continental shelf and the slope are part of
the continental margin. The shelf area is commonly subdivided into
the inner continental shelf, mid continental shelf, and outer continental shelf, each with
their specific geomorphology and marine biology. The character of the shelf changes dramatically
at the shelf break, where the continental slope begins. With a few exceptions, the shelf
break is located at a remarkably uniform depth of roughly 140 m; this is likely a hallmark
of past ice ages, when sea level was lower than it is now.
The continental slope is much steeper than the shelf; the average angle is 3°, but it
can be as low as 1° or as high as 10°. The slope is often cut with submarine canyons.
The physical mechanisms involved in forming these canyons were not well understood until
the 1960s. Sediments
The continental shelves are covered by terrigenous sediments; that is, those derived from erosion
of the continents. However, little of the sediment is from current rivers; some 60-70%
of the sediment on the world's shelves is relict sediment, deposited during the last
ice age, when sea level was 100–120 m lower than it is now.
Sediments usually become increasingly fine with distance from the coast; sand is limited
to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water
far offshore. These shelf sediments accumulate at an average rate of 30 cm/1000 years, with
a range from 15–40 cm. Though slow by human standards, this rate is much faster than that
for deep-sea pelagic sediments. Biota
Continental shelves teem with life, because of the sunlight available in shallow waters,
in contrast to the biotic desert of the oceans' abyssal plain. The pelagic environment of
the continental shelf constitutes the neritic zone, and the benthic province of the shelf
is the sublittoral zone. Though the shelves are usually fertile, if
anoxic conditions prevail during sedimentation, the deposits may over geologic time become
sources for fossil fuels. Economic significance
The relatively accessible continental shelf is the best understood part of the ocean floor.
Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, and
hydrocarbon extraction, takes place on the continental shelf. Sovereign rights over their
continental shelves up to a depth of 200 metres or to a distance where the depth of waters
admitted of resource exploitation were claimed by the marine nations that signed the Convention
on the Continental Shelf drawn up by the UN's International Law Commission in 1958. This
was partly superseded by the 1982 United Nations Convention on the Law of the Sea. which created
the 200 nautical mile exclusive economic zone and extended continental shelf rights for
states with physical continental shelves that extend beyond that distance.
The legal definition of a continental shelf differs significantly from the geological
definition. UNCLOS states that the shelf extends to the limit of the continental margin, but
no less than 200 nautical miles from the baseline. Thus inhabited volcanic islands such as the
Canaries, which have no actual continental shelf, nonetheless have a legal continental
shelf, whereas uninhabitable islands have no shelf.
See also
Baseline Continental Island
Continental shelf pump Continental shelf of Russia
Exclusive economic zone International waters
Land bridge Outer Continental Shelf
Region of Freshwater Influence Territorial waters
Passive margin Notes
References Gross, Grant M. Oceanography: A View of the
Earth. Englewood Cliffs: Prentice-Hall, Inc., 1972. ISBN 0-13-629659-9
Pinet, Paul R. Invitation to Oceanography. St. Paul, MN: West Publishing Co., 1996. ISBN
0-7637-2136-0 External links
Office of Naval Research: Ocean Regions: Continental Margin & Rise
UNEP Shelf Programme Anna Cavnar, Accountability and the Commission
on the Limits of the Continental Shelf: Deciding Who Owns the Ocean Floor