The
continental shelf is the extended perimeter of each
continentA continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criterion, with seven regions commonly regarded as continents – they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
and associated
coastal plainA coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in western South America. The southwestern coastal plain of North America is notable for its species diversity...
, and was part of the continent during the glacial periods, but is undersea during
interglacial periodsThe general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual...
such as the current epoch by relatively shallow
seaA sea is any large amount of water filled with animals such as crabs, whales, sharks, and fish, but there is inconsistency as to its precise definition and application. Most commonly, a sea may refer to a large expanse of saline water connected with an ocean, but it is also used sometimes for a...
s (known as
shelf seas) and
gulfsA bay is an area of water mostly surrounded or otherwise demarcated by land. Bays generally have calmer waters than the surrounding sea, due to the surrounding land blocking some waves and often reducing winds....
.
The continental rise is below the slope, but landward of the abyssal plains. Its gradient is intermediate between the slope and the shelf, on the order of 0.5-1°.
The
continental shelf is the extended perimeter of each
continentA continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criterion, with seven regions commonly regarded as continents – they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
and associated
coastal plainA coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in western South America. The southwestern coastal plain of North America is notable for its species diversity...
, and was part of the continent during the glacial periods, but is undersea during
interglacial periodsThe general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual...
such as the current epoch by relatively shallow
seaA sea is any large amount of water filled with animals such as crabs, whales, sharks, and fish, but there is inconsistency as to its precise definition and application. Most commonly, a sea may refer to a large expanse of saline water connected with an ocean, but it is also used sometimes for a...
s (known as
shelf seas) and
gulfsA bay is an area of water mostly surrounded or otherwise demarcated by land. Bays generally have calmer waters than the surrounding sea, due to the surrounding land blocking some waves and often reducing winds....
.
The continental rise is below the slope, but landward of the abyssal plains. Its gradient is intermediate between the slope and the shelf, on the order of 0.5-1°. Extending as far as 500 km from the slope, it consists of thick sediments deposited by
turbidity currentA turbidity current or density current is a current of rapidly moving, sediment-laden water moving down a slope through air, water, or another fluid...
s from the shelf and slope. Sediment cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise.
Under the
United Nations Convention on the Law of the SeaThe United Nations Convention on the Law of the Sea , also called the Law of the Sea Convention or the Law of the Sea treaty, is the international agreement that resulted from the third United Nations Conference on the Law of the Sea , which took place from 1973 through 1982...
, 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. See the
Territorial watersTerritorial waters, or a territorial sea, as defined by the 1982 United Nations Convention on the Law of the Sea, is a belt of coastal waters extending at most twelve nautical miles from the baseline of a coastal state...
page for more details.
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 plateOceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima...
dives beneath
continental crustThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
in an offshore subduction zone such as off the coast of
ChileChile, officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
or the west coast of
SumatraSumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world .-Etymology:Sumatra was known in ancient times by the Sanskrit...
. The largest shelf – the
Siberian ShelfThe Siberian Shelf, one of the Arctic shelves, is the largest continental shelf of the Earth, a part of the continental shelf of Russia. It extends from the continent of Eurasia in the general area of North Siberia into the Arctic Ocean. It stretches to 1500 kilometers offshore. It is relatively...
in the
Arctic OceanThe Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest, and shallowest of the world's five major oceanic divisions.
The International Hydrographic Organization recognizes it as an ocean, although some...
– stretches to 1500 kilometers (930 miles) in width. The
South China SeaThe South China Sea is a marginal sea*south of mainland China and Taiwan,*west of the Philippines,*north west of Sabah , Sarawak and Brunei,*north of Indonesia,*north east of the Malay peninsula and Singapore, and...
lies over another extensive area of continental shelf, the
Sunda ShelfGeologically, the Sunda Shelf is an extension of the continental shelf of Southeast Asia, covered during interglacials by the South China Sea, which isolates as islands Borneo, Sumatra Java and smaller islands. During glacial periods, the sea level falls, and great expanses of the Sunda Shelf are...
, which joins
BorneoBorneo is the third largest island in the world and is located at the centre of Maritime Southeast Asia. Administratively, this island is divided among Indonesia , Malaysia and Brunei . Indonesians refer to the island as Kalimantan...
, Sumatra, and Java to the Asian mainland. Other familiar bodies of water that overlie continental shelves are the
North SeaThe North Sea is a marginal, epeiric sea on the European continental shelf. The Dover Strait and the English Channel in the south and the Norwegian Sea in the north connect it to the Atlantic Ocean. It is more than long and wide, with an area of around...
and the
Persian GulfThe Persian Gulf, in the Southwest Asian region, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula. Historically and commonly known as the Persian Gulf, this body of water is sometimes controversially referred to as the Arabian Gulf by most Arab states or simply The...
. The average width of continental shelves is about . The depth of the shelf also varies, but is generally limited to water shallower than . The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief is also minimal, at less than .
Though the continental shelf is treated as a physiographic province of the
oceanAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
, 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
AtlanticThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
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 decreasing slope (called the
shelf break). 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 plainAbyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among the Earth's flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths between...
. The continental shelf and the slope are part of the
continental marginThe continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
.
The shelf area is commonly subdivided into the
inner continental shelf,
mid continental shelf, and
outer continental shelf, each with their specific
geomorphologyGeomorphology is the scientific study of landforms and the processes that shape them...
and
marine biologyMarine biology is the scientific study of living organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment...
.
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 ; 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 canyonA submarine canyon is a steep-sided valley on the sea floor of the continental slope. Many submarine canyons are found as extensions to large rivers; however there are many that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea...
s. The physical mechanisms involved in forming these canyons was not well understood until the 1960s.
Sediments
The continental shelves are covered by
terrigenousIn oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, that are derived from terrestrial environments. Consisting of sand, mud, and silt carried to sea by rivers, their composition is usually related to their source rocks; deposition of these sediments...
sediments; that is, those derived from erosion of the continents. However, little of the sediment is from current
riverA river is a natural watercourse, usually freshwater, flowing toward an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water...
s; 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 sedimentsPelagic sediments, also known as marine sediments, are those that accumulate in the abyssal plain of the deep ocean, far away from terrestrial sources that provide terrigenous sediments; the latter are primarily limited to the continental shelf, and at some point were probably deposited by rivers...
.
Biota
Combined with the sunlight available in shallow waters, the continental shelves teem with life, compared to the biotic desert of the oceans'
abyssal plainAbyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among the Earth's flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths between...
. The pelagic (water column) environment of the continental shelf constitutes the
neritic zoneThe neritic zone, also called the sublittoral zone, is the part of the ocean extending from the low tide mark to the edge of the continental shelf, with a relatively shallow depth extending to about 200 meters . The neritic zone has generally well-oxygenated water, low water pressure, and...
, and the benthic (sea floor) province of the shelf is the sublittoral zone.
Though the shelves are usually fertile, if
anoxicOceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...
conditions in the sedimentary deposits prevail, the shelves may in geologic time become sources of
fossil fuelFossil fuels or mineral fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
s.
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
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon and hydrogen are referred to as "pure"...
extraction, takes place on the continental shelf. Sovereign rights over their continental shelves up to 350
nautical mileThe nautical mile is a unit of length corresponding approximately to one minute of arc of latitude along any meridian....
s from the coast were claimed by the marine nations that signed the
Convention on the Continental ShelfThe Convention on the Continental Shelf was an international treaty created to codify the rules of international law relating to continental shelves. The treaty, after entering into force 10 June 1964, established the rights of a sovereign state over the continental shelf surrounding it, if there...
drawn up by the UN's
International Law CommissionThe International Law Commission was established by the United Nations General Assembly in 1948 for the "promotion of the progressive development of international law and its codification."It holds an annual session at the United Nations Office at Geneva....
in 1958 partly superseded by the 1982
United Nations Convention on the Law of the SeaThe United Nations Convention on the Law of the Sea , also called the Law of the Sea Convention or the Law of the Sea treaty, is the international agreement that resulted from the third United Nations Conference on the Law of the Sea , which took place from 1973 through 1982...
.
See also
- Baseline
A baseline is the line from which the seaward limits of a State's territorial sea and certain other maritime zones of jurisdiction are measured. Normally, a sea baseline follows the low-water line of a coastal State...
- Continental Island
- Continental shelf pump
In oceanic biogeochemistry, the continental shelf pump is proposed to operate in the shallow waters of the continental shelves, acting as a mechanism to transport carbon from surface waters to the interior of the adjacent deep ocean.-Overview:Originally formulated by Tsunogai et al. , the pump is...
- Continental shelf of Russia
The terms continental shelf of Russia or Russian continental shelf have two related meanings. Geologically, it is the total of the continental shelves adjacent to Russia...
- Exclusive economic zone
Under the law of the sea, an Exclusive Economic Zone is a seazone over which a state has special rights over the exploration and use of marine resources. It stretches from the edge of the state's territorial sea out to 200 nautical miles from its coast...
- International waters
The terms international waters or trans-boundary waters apply where any of the following types of bodies of water transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regional seas and estuaries, rivers, lakes, groundwater systems , and wetlands...
- Land bridge
A land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, which allows terrestrial animals and plants to cross over and colonise new lands...
- Outer Continental Shelf
The Outer Continental Shelf is a peculiarity of the political geography of the United States and is the part of the internationally recognized continental shelf of the United States which does not fall under the jurisdictions of the individual U.S...
- Territorial waters
Territorial waters, or a territorial sea, as defined by the 1982 United Nations Convention on the Law of the Sea, is a belt of coastal waters extending at most twelve nautical miles from the baseline of a coastal state...
- Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
External links