Continental margin

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]

The continental shelf is the portion of the continental margin that transitions from the shore out towards to ocean. They 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] It 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 slopes, and continue to cut the morphology to the abyssal plain.[5] The valleys 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 end of the continental margin.[2]

Continental shelf
Profile illustrating the shelf, slope and rise

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 ridges.[3] 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

Economically, 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

References

  1. ^ a b P. J. Cook, Chris Carleton (2000) "Continental Shelf Limits: The Scientific and Legal Interface", ISBN 0-19-511782-4
  2. ^ a b c d V., Thurman, Harold (2014-01-01). Essentials of oceanography. Pearson. ISBN 9780321668127. OCLC 815043823.
  3. ^ a b c d e f 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. ^ a b c d e f Grotzinger, Jordan (2007). Understanding Earth. W H Freeman. pp. 491–496. ISBN 978-0716766827.
  5. ^ a b Gulicher, Andre (1958). Coastal and Submarine Morphology. Great Britain: Butler & Tanner Ltd. pp. 205–215.
Astrid Ridge

Astrid Ridge is an undersea ridge on the continental margin of Dronning Maud Land, East Antarctica. It is present on the GEBCO 5th edition charts. The name was approved by the Advisory Committee for Undersea Features in June 1987.The ocean along the continental margin of Dronning Maud Land, the Riiser-Larsen and Lazarev seas, are dominated by three topographic features: Astrid Ridge, Maud Rise, a large volcanic plateau in the Lazarev Sea (3°E, 65°S), and Gunnerus Ridge (33.5°E), probably underlain by continental crust.Astrid Ridge, extending from 65°S to the Antarctic margin and located between 9°E and 17°E, forms the eastern border of the Lazarev Sea and the western border of the Riiser-Larsen Sea. It is divided into two parts by the Astrid Fracture Zone: south of 67°S Astrid Ridge stretches N-S while the northern part follows the SW—NE direction of the fracture zone.The geological evolution of the Astrid Ridge remains enigmatic. The Riiser-Larsen Sea is a conjugate basin to the Mozambique Basin (in the ocean east of southern Africa) and the two once formed the first ocean to open during the break-up of Gondwana.

Astrid Ridge is a large volcanic features closely related to this 160 Ma break-up during which oceanic crust began to form in the Riiser-Larsen Sea. West of Astrid Ridge, however, neither oceanic crust nor magnetic anomalies have been found and this area is therefore interpreted as stretched continental crust and Astrid Ridge as a former continental margin created by volcanism during the break-up.

Census of Marine Life

The Census of Marine Life was a 10-year, US $650 million scientific initiative, involving a global network of researchers in more than 80 nations, engaged to assess and explain the diversity, distribution, and abundance of life in the oceans. The world's first comprehensive Census of Marine Life — past, present, and future — was released in 2010 in London. Initially supported by funding from the Alfred P. Sloan Foundation, the project was successful in generating many times that initial investment in additional support and substantially increased the baselines of knowledge in often underexplored ocean realms, as well as engaging over 2,700 different researchers for the first time in a global collaborative community united in a common goal, and has been described as "one of the largest scientific collaborations ever conducted".

Chagos-Laccadive Ridge

The Chagos-Laccadive Ridge (CLR), also known as Chagos-Laccadive Plateau, is a prominent volcanic ridge and oceanic plateau extending between the Northern and the Central Indian Ocean.

Extending from c. 10°S to 15°N, the CLR includes the Laccadive, Maldives, and Chagos archipelagos and can be divided into three corresponding blocks, of which the first is continental and the two latter are oceanic. The CLR is asymmetrical with a steeper eastern slope and has an average depth of less than 1,000 m (3,300 ft). It formed south of or near the Equator together with the remaining western continental margin of India, when India separated first from Madagascar in the Mid-Cretaceous and then from the Seychelles in the Late Cretaceous.

Coast Mountains

The Coast Mountains are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the Coast of British Columbia south to the Fraser River. The mountain range's name derives from its proximity to the sea coast, and it is often referred to as the Coast Range. The range includes volcanic and non-volcanic mountains and the extensive ice fields of the Pacific and Boundary Ranges, and the northern end of the volcanic system known as the Cascade Volcanoes. The Coast Mountains are part of a larger mountain system called the Pacific Coast Ranges or the Pacific Mountain System, which includes the Cascade Range, the Insular Mountains, the Olympic Mountains, the Oregon Coast Range, the California Coast Ranges, the Saint Elias Mountains and the Chugach Mountains. The Coast Mountains are also part of the American Cordillera—a Spanish term for an extensive chain of mountain ranges—that consists of an almost continuous sequence of mountain ranges that form the western backbone of North America, Central America, South America and Antarctica.

The Coast Mountains are approximately 1,600 kilometres (1,000 mi) long and average 300 kilometres (190 mi) in width. The range's southern and southeastern boundaries are surrounded by the Fraser River and the Interior Plateau while its far northwestern edge is delimited by the Kelsall and Tatshenshini Rivers at the north end of the Alaska Panhandle, beyond which are the Saint Elias Mountains, and by Champagne Pass in the Yukon Territory. Covered in dense temperate rainforest on its western exposures, the range rises to heavily glaciated peaks, including the largest temperate-latitude ice fields in the world. On its eastern flanks, the range tapers to the dry Interior Plateau and the subarctic boreal forests of the Skeena Mountains and Stikine Plateau.

The Coast Mountains are part of the Pacific Ring of Fire—the ring of volcanoes and associated mountains around the Pacific Ocean—and contain some of British Columbia's highest mountains. Mount Waddington is the highest mountain of the Coast Mountains and the highest that lies entirely within British Columbia, located northeast of the head of Knight Inlet with an elevation of 4,019 metres (13,186 ft).

Continental arc

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

Continental shelf

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 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 (310 mi) 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.

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.

Cow Head (town)

Cow Head is a town in the Canadian province of Newfoundland and Labrador. The town had a population of 475 in the Canada 2011 Census.

The Dr. Henry N. Payne Community Museum (c. 1941) in Cow Head, Newfoundland and Labrador is on the Canadian Register of Historic Places.An interesting geologic feature found at Cow Head is a section of the former continental margin of Laurentia which dipped into the Iapetus ocean

East Greenland Current

The East Greenland Current (EGC) is a cold, low salinity current that extends from Fram Strait (~80N) to Cape Farewell (~60N). The current is located off the eastern coast of Greenland along the Greenland continental margin. The current cuts through the Nordic Seas (the Greenland and Norwegian Seas) and through the Denmark Strait. The current is of major importance because it directly connects the Arctic to the Northern Atlantic, it is a major contributor to sea ice export out of the Arctic, and it is a major freshwater sink for the Arctic.

Eastern Andes Metamorphic Complex

The Eastern Andes Metamorphic Complex is a large coherent but varied group of metamorphic and sedimentary rocks –in other words a geologic complex– that crops out in the eastern Patagonian Andes in Chile and Argentina. The metamorphic grade of rocks varies but does not exceed greenschist facies, the only exception to this are rocks near plutons affected by contact metamorphism. The sedimentary protoliths sedimented in the Late Paleozoic. The pressures and temperatures of metamorphism of the Eastern Andes Metamorphic Complex are different those usually expected from accretionary complexes. The sedimentary protoliths of the Eastern Andes Metamorphic Complex were likely deposited in a passive continental margin.Cordillera Darwin Metamorphic Complex shows affinity with the Eastern Andes Metamorphic Complex albeit both complexes lie hundreds of kilometers from each other.

Helvetic nappes

The Helvetic nappes (German: Helvetische Decken) are a series of nappes in the Northern part of the Alps and part of the Helvetic zone. They consist of Mesozoic limestones, shales and marls that were originally deposited on the southern continental margin of the European continent. During the Alpine orogeny they were thrusted north over a décollement and at the same time were internally deformed by folding and thrusting.

Intermontane Belt

The Intermontane Belt is a physiogeological region in the Pacific Northwest of North America, stretching from northern Washington into British Columbia, Yukon, and Alaska. It comprises rolling hills, high plateaus and deeply cut valleys. The rocks in the belt have very little similarities with the North American continent.

Intra-arc basin

In geology an intra-arc basin is a sedimentary basin that exists amidst a volcanic arc. Being located next to volcanoes intra-arc basins tend to host Volcano-sedimentary sequences. Cura-Mallín at the border of Chile and Argentina is an example of an intra-arc basin.

Some Neoproterozoic clastic metasedimentary rocks in the Central Eastern Desert of Egypt (CED) derived from bimodal volcanic sources appear to have been deposited in arc-related basins, including interarc or back-arc basins, intra-arc basins, and retro-arc basin of active continental margin.

Mélange

In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies. Both tectonic and sedimentary processes can form mélange.

Mélange occurrences are associated with thrust faulted terranes in orogenic belts. A mélange is formed in the accretionary wedge above a subduction zone. The ultramafic ophiolite sequences which have been obducted onto continental crust are typically underlain by a mélange. Smaller-scale localized mélanges may also occur in shear or fault zones, where coherent rock has been disrupted and mixed by shearing forces.

Large-scale melanges formed in active continental margin settings generally consist of altered oceanic crustal material and blocks of continental slope sediments in a sheared mudstone matrix. The mixing mechanisms in such settings may include tectonic shearing forces, ductile flow of a water-charged or deformable matrix (such as serpentinite), sedimentary action (such as slumping, gravity-flow, and olistostromal action), or some combination of these. Some larger blocks of rock may be as much as 1 kilometre (0.62 mi) across.

Before the advent of plate tectonics in the early 1970s, it was difficult to explain mélanges in terms of known geological mechanisms. A particularly troubling paradox was the occurrence of blueschist blocks (low temperature and high pressure metamorphic rocks) in direct contact with graywacke (a coarse sandstone with lithic fragments) that was deposited in a sedimentary environment.

Nazca Ridge

The Nazca Ridge is a submarine ridge, located on the Nazca Plate off the west coast of South America. This plate and ridge are currently subducting under the South American Plate at a convergent boundary known as the Peru-Chile Trench at approximately 7.7 cm/yr. The Nazca Ridge began subducting obliquely to the collision margin at 11°S, approximately 11.2 Ma, and the current subduction location is 15°S. The ridge is composed of abnormally thick basaltic ocean crust, averaging 18 ±3 km thick. This crust is buoyant, resulting in flat slab subduction under Peru. This flat slab subduction has been associated with the uplift of Pisco Basin and the cessation of Andes volcanism and the uplift of the Fitzcarrald Arch on the South American continent approximately 4 Ma.

Niger Delta Basin (geology)

The Niger Delta Basin, also referred to as the Niger Delta province, is an extensional rift basin located in the Niger Delta and the Gulf of Guinea on the passive continental margin near the western coast of Nigeria with suspected or proven access to Cameroon, Equatorial Guinea and São Tomé and Príncipe. This basin is very complex, and it carries high economic value as it contains a very productive petroleum system. The Niger delta basin is one of the largest subaerial basins in Africa. It has a subaerial area of about 75,000 km2, a total area of 300,000 km2, and a sediment fill of 500,000 km3. The sediment fill has a depth between 9–12 km. It is composed of several different geologic formations that indicate how this basin could have formed, as well as the regional and large scale tectonics of the area. The Niger Delta Basin is an extensional basin surrounded by many other basins in the area that all formed from similar processes. The Niger Delta Basin lies in the south westernmost part of a larger tectonic structure, the Benue Trough. The other side of the basin is bounded by the Cameroon Volcanic Line and the transform passive continental margin.

Obduction

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Subsequently, this definition has been broadened to mean the emplacement of continental lithosphere by oceanic lithosphere at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.

Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction may occur where a small tectonic plate is caught between two larger plates, with the lithosphere (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic lithosphere between them is often a part of the resulting orogeny.Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.

Ophiolite

An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophis is Greek for "snake", and lite (from Greek lithos) means "stone", after the green-color rocks that make up many ophiolites.

Their great significance relates to their occurrence within mountain belts such as the Alps and the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.

Penokean orogeny

The Penokean orogeny was a mountain-building episode that occurred in the early Proterozoic about 1.86 to 1.83 billion years ago, in the area of Lake Superior, North America. The core of this orogeny, the Churchill Craton, is composed of terranes derived from the 1.86–1.81 Ga collision between the Superior and North Atlantic cratons. The orogeny resulted in the formation of the Nena and Arctica continents, which later merged with other continents to form the Columbia supercontinent. The name was first proposed by Blackwelder 1914 in reference to what is known as the Penokee Range, sometimes incorrectly called the Gogebic Range, in northern Michigan and Wisconsin.The Paleoproterzoic Penokean orogeny developed in an embayment on the southern margin of the Superior Craton. It extends east from Minnesota to the Grenville orogen near Lake Huron and south to the Central Plain in Wisconsin. It is composed of two domains separated by the Niagara Fault Zone: the southern, internal domain, the Wisconsin Magmatic Terranes, consists of Paleoproterzoic tholeiitic and calc-alkaline island arc rocks and calc-alkaline plutonic rocks; the northern, external domain consists of a continental margin foreland basin overlying an Archaean basement and includes the supracrustal rocks of the Animikie Group and Marquette Range Supergroup. The collision between the two domains around 1.88–1.85 Ga resulted in northward-directed thrusting and folding of the northern domain.Before this episode the area was a passive continental margin occupied by a shallow sea, which created large sedimentary deposits including the banded iron formations of the Iron Ranges.

The orogeny happened in two phases. First an island arc called the Pembine-Wausau terrane collided with the ancient North American craton along with volcanoes formed in its back-arc basin. The second phase involved a microcontinent called the Marshfield terrane, which today forms parts of Wisconsin and Illinois. The episode lasted about 10 million years.

Hundreds of millions of years later, the Keweenawan Rift occurred in the same area creating the basin that would eventually become Lake Superior. The remains of this orogeny can be seen today as the Iron Ranges of Minnesota and Ontario, the Northern Highlands of Wisconsin and the Upper Peninsula of Michigan.

Volcanology of Canada

Volcanology of Canada includes lava flows, lava plateaus, lava domes, cinder cones, stratovolcanoes, shield volcanoes, submarine volcanoes, calderas, diatremes, and maars, along with examples of more less common volcanic forms such as tuyas and subglacial mounds. It has a very complex volcanological history spanning from the Precambrian eon at least 3.11 billion years ago when this part of the North American continent began to form.Although the country's volcanic activity dates back to the Precambrian eon, volcanism continues to occur in Western and Northern Canada where it forms part of an encircling chain of volcanoes and frequent earthquakes around the Pacific Ocean called the Pacific Ring of Fire. But because volcanoes in Western and Northern Canada are in remote rugged areas and the level of volcanic activity is less frequent than with other volcanoes around the Pacific Ocean, Canada is commonly thought to occupy a gap in the Pacific Ring of Fire between the volcanoes of western United States to the south and the Aleutian volcanoes of Alaska to the north. However, the mountainous landscape of Western and Northern Canada includes more than 100 volcanoes that have been active during the past two million years and whose eruptions have claimed many lives. Volcanic activity has been responsible for many of Canada's geological and geographical features and mineralization, including the nucleus of North America called the Canadian Shield.

Volcanism has led to the formation of hundreds of volcanic areas and extensive lava formations across Canada, indicating volcanism played a major role in shaping its surface. The country's different volcano and lava types originate from different tectonic settings and types of volcanic eruptions, ranging from passive lava eruptions to violent explosive eruptions. Canada has a rich record of very large volumes of magmatic rock called large igneous provinces. They are represented by deep-level plumbing systems consisting of giant dike swarms, sill provinces and layered intrusions. The most capable large igneous provinces in Canada are Archean (3,800–2,500 million years ago) age greenstone belts containing a rare volcanic rock called komatiite.

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