Continental margin

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Profile illustrating the shelf, slope and rise Continental shelf.png
Profile illustrating the shelf, slope and rise

The continental margin 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 is the shallow water area found in proximity to continent. [1] The continental margin consists of three different features: the continental rise, the continental slope, and the continental shelf. [2] Continental margins constitute about 28% of the oceanic area.[1]

Contents

Zones of the continental margin

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 percent of the sea floor. [3] The width of continental shelves worldwide varies from a 30 meters to 1500 kilometers. [4] The continental shelf is generally flat, and ends at the shelf break, where there is a drastic increase in slope angle. The mean slope of continental shelves worldwide is 0° 07’ degrees, 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 one to five kilometers 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, 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. [2]

Types

There are two types of continental margins: active and passive margins. [2]

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 plate subducts below the less dense continental 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. [2]

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 ridge s. [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 sloped than active margins.

Sediment accumulation

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 kilometers 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

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 "Law of the Sea" 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 United Nations Convention on the Law of the Sea (although in the UN definition the "legal continental shelf" may extend beyond the geomorphological continental shelf and vice versa). [1] 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

Related Research Articles

Oceanic trench Long and narrow depressions of the sea floor

Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr.

Sedimentary basin Regions of long-term subsidence creating space for infilling by sediments

Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime, the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.

Convergent boundary Region of active deformation between colliding lithospheric plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Continental shelf A portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea

A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea. Much of these shelves has been exposed during glacial periods and interglacial periods. The shelf surrounding an island is known as an insular shelf.

Kaikoura Peninsula peninsular mountain on New Zealands South Island

The Kaikoura Peninsula is located in the northeast of New Zealand's South Island. It protrudes five kilometres into the Pacific Ocean. The town of Kaikoura is located on the north shore of the peninsula. The peninsula has been settled by Maori for approximately 1000 years, and by Europeans since the 1800s, when whaling operations began off the Kaikoura Coast. Since the end of whaling in 1922 whales have been allowed to thrive and the region is now a popular whale watching destination.

Seabed The bottom of the ocean

The seabed is the bottom of the ocean.

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The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the United States. As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present-day Appalachians and midcontinental North America.

Oceanic basin Large geologic basins that are below sea level

In hydrology, an oceanic basin may be anywhere on Earth that is covered by seawater but geologically ocean basins are large geologic basins that are below sea level. Geologically, there are other undersea geomorphological features such as the continental shelves, the deep ocean trenches, and the undersea mountain ranges which are not considered to be part of the ocean basins; while hydrologically, oceanic basins include the flanking continental shelves and shallow, epeiric seas.

Submarine canyon A steep-sided valley cut into the seabed of the continental slope

A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from canyon floor to canyon rim, as with the Great Bahama Canyon. Just as above-sea-level canyons serve as channels for the flow of water across land, submarine canyons serve as channels for the flow of turbidity currents across the seafloor. Turbidity currents are flows of dense, sediment laden waters that are supplied by rivers, or generated on the seabed by storms, submarine landslides, earthquakes, and other soil disturbances. Turbidity currents travel down slope at great speed, eroding the continental slope and finally depositing sediment onto the abyssal plain, where the particles settle out.

Passive margin The transition between oceanic and continental lithosphere that is not an active plate margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Continental rise An underwater feature connecting the continental slope and the abyssal plain

The continental rise is an underwater feature found between the continental slope and the abyssal plain. This feature can be found all around the world, and it represents the final stage in the boundary between continents and the deepest part of the ocean. The environment in the continental rise is quite unique, and many oceanographers study it extensively in the hopes of learning more about the ocean and geologic history.

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An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

This is a list of articles related to plate tectonics and tectonic plates.

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Nias Basin

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Offshore Indus Basin

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The Tyrrhenian Basin is a sedimentary basin located in the western Mediterranean Sea under the Tyrrhenian Sea. It covers a 231,000 km² area that is bounded by Sardinia to the west, Corsica to the northwest, Sicily to the southeast, and peninsular Italy to the northeast. The Tyrrhenian basin displays an irregular seafloor marked by several seamounts and two distinct sub-basins - the Vavilov and Marsili basins. The Vavilov deep plain contains the deepest point of the Tyrrhenian basin at approximately 3785 meters. The basin trends roughly northwest-southeast with the spreading axis trending northeast-southwest.

Congo Canyon

Congo Canyon is a submarine canyon found at the end of the Congo River in Africa. It is one of the largest submarine canyons in the world.

The geology of the Norwegian Sea began to form 60 million years ago in the early Cenozoic, as rifting led to the eruption of mafic oceanic crust, separating Scandinavia and Greenland. Together with the North Sea the Norwegian Sea has become highly researched since the 1960s with the discovery of oil and natural gas in thick offshore sediments on top of the Norwegian continental shelf.

References

  1. 1 2 P. J. Cook, Chris Carleton (2000) "Continental Shelf Limits: The Scientific and Legal Interface", ISBN   0-19-511782-4
  2. 1 2 3 4 V., Thurman, Harold (2014-01-01). Essentials of Oceanography. Pearson. ISBN   9780321668127. OCLC   815043823.
  3. 1 2 3 4 5 6 Board., National Research Council (U.S.). Ocean Sciences (1979-01-01). Continental margins : geological and geophysical research needs and problems. National Academy of Sciences. ISBN   0309027934.
  4. 1 2 3 4 5 6 Grotzinger, Jordan (2007). Understanding Earth. W H Freeman. pp. 491–496. ISBN   978-0716766827.
  5. 1 2 Gulicher, Andre (1958). Coastal and Submarine Morphology. Great Britain: Butler & Tanner Ltd. pp. 205–215.
  6. http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry26.html