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26-583: 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

52-608: A former continental margin created by volcanism during the break-up. This Princess Astrid Coast location article is a stub . You can help Misplaced Pages by expanding it . Riiser-Larsen Sea The Riiser-Larsen Sea is one of the marginal seas located in the Southern Ocean off East Antarctica and south of the Indian Ocean . It is delimited Astrid Ridge in the west and the Gunnerus Ridge and

78-458: A hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within

104-477: A peak in sediment transport. As the ice sheet had retreated sediments got trapped on the shelf and the sediment supply to the canyons was cut off. The Riiser-Larsen Sea was one of the first Antarctic marginal basins to be affected by the expanding ice sheet at c. 34 Ma. The oldest, pre-glacial deposit sequence consists of turbidites and hemipelagic sediments . The expanding ice sheet initially triggered slumps and flows of debris that were deposited on

130-426: A sea with a bed significantly below sea level is cut off from the larger ocean to which it is usually connected. The sea which is normally repleted by contact and inflow from the ocean is now no longer replenished and hence dries up over a period of time, which can be very short if the local climate is arid. In this scenario, rivers that previously flowed into the sea at a sea level elevation now can cut far deeper into

156-501: 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 (3 mi), 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

182-567: Is covered with drifting ice almost year-round. It would stretch over an area of 1,138,000 km². The seafloor of the Riiser-Larsen Sea is relatively flat. The bedrock in the Riiser-Larsen Sea is one of the oldest around Antarctica (145 Ma) and erosion over this long timespan has produced the largest submarine canyons of the continent. The two dozens canyons in the Riiser-Larsen Sea can be divided into two groups: surface canyons and buried canyons. The buried canyons are twice

208-537: The abyssal plain , where the particles settle out. About 3% of submarine canyons include shelf valleys that have cut transversely across continental shelves, and which begin with their upstream ends in alignment with and sometimes within the mouths of large rivers , such as the Congo River and the Hudson Canyon . About 28.5% of submarine canyons cut back into the edge of the continental shelf, whereas

234-444: The water depths as great as 3,000 meters (9,800 ft) where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths. The major mechanism of canyon erosion is thought to be turbidity currents and underwater landslides . Turbidity currents are dense , sediment-laden currents which flow downslope when an unstable mass of sediment that has been rapidly deposited on

260-608: The 1953 IHO document (which does not contain the name) remains currently in force. Leading geographic authorities and atlases do not use the name, including the 2014 10th edition World Atlas from the United States' National Geographic Society and the 2014 12th edition of the British Times Atlas of the World . But Soviet and Russian-issued maps do. Depths exceed 3,000 meters in most of the waters here. The area

286-872: The Kainanmaru Bank in the east. It is bordered by the Lazarev Sea to the west and the Cosmonauts Sea to the east, or between 14°E and 30°E . Its northern border is defined to be the 65th parallel south . The name, proposed by the Soviet Union, was never officially approved by the International Hydrographic Organization (IHO). To the south of this area lies the Princess Astrid Coast and Princess Ragnhild Coast of Queen Maud Land . In

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312-663: The Karoo eruption ( 185 to 180 Ma ) centered on southern Africa. Rifting then spread towards Antarctica and seafloor spreading opened what is now the Mozambique Basin , Riiser-Larsen Sea, West Somali Basin , and Weddell Sea during the Late Jurassic. The oldest identified magnetic anomalies (M25–M24) are 154–152 Ma old, but the break-up coincides with the Jurassic Quiet Zone and

338-837: The abyssal plain. Ancient examples have been found in rocks dating back to the Neoproterozoic . Turbidites are deposited at the downstream mouths or ends of canyons, building an abyssal fan . Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins . They show erosion through all substrates, from unlithified sediment to crystalline rock . Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion , or slumping . There are an estimated 9,477 submarine canyons on Earth, covering about 11% of

364-597: The bottom of the bed now exposed. The Messinian salinity crisis is an example of this phenomenon; between five and six million years ago, the Mediterranean Sea became isolated from the Atlantic Ocean and evaporated away in roughly a thousand years. During this time, the Nile River delta, among other rivers, extended far beyond its present location, both in depth and length. In a cataclysmic event,

390-541: The continental slope. Different mechanisms have been proposed for the formation of submarine canyons. Their primary causes have been subject to debate since the early 1930s. An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about 125 meters (410 ft) below present sea level, and rivers flowed to the edge of the continental shelf. However, while many (but not all) canyons are found offshore from major rivers, subaerial river erosion cannot have been active to

416-427: The continental slope. While at first glance the erosion patterns of submarine canyons may appear to mimic those of river-canyons on land, several markedly different processes have been found to take place at the soil/water interface. Many canyons have been found at depths greater than 2 km (1 mi) below sea level . Some may extend seawards across continental shelves for hundreds of kilometres before reaching

442-581: 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

468-407: 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 (as much as 70 km/h (43 mph)), eroding the continental slope and finally depositing sediment onto

494-415: The majority (about 68.5%) of submarine canyons have not managed at all to cut significantly across their continental shelves, having their upstream beginnings or "heads" on the continental slope, below the edge of continental shelves. The formation of submarine canyons is believed to occur as the result of at least two main process: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of

520-458: 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

546-653: The oldest anomaly probably is M40 (166 Ma). As Madagascar had been transferred to the African Plate at M10 (130–120 Ma) spreading began between Madagascar and Antarctica — the birth of the Southwest Indian Ridge that still separates the Mozambique Basin from the Riiser-Larsen Sea. 68°00′S 22°00′E  /  68.000°S 22.000°E  / -68.000; 22.000 Submarine canyon A submarine canyon

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572-507: The same canyon, or at different times during a canyon's development. However, if a primary mechanism must be selected, the downslope lineal morphology of canyons and channels and the transportation of excavated or loose materials of the continental slope over extensive distances require that various kinds of turbidity or density currents act as major participants. In addition to the processes described above, submarine canyons that are especially deep may form by another method. In certain cases,

598-415: The size and much older (4–7 Ma) than the surface canyons (younger than 2.4 Ma). The continental shelf of the Riiser-Larsen Sea is relatively narrow and forms rugged terraces at a depth of 750 m (2,460 ft), probably the maximum extent of seafloor-reaching icebergs. The buried canyons formed during a period of maximum glaciation when grounded icebergs reached the shelf edge which resulted in

624-638: The upper continental rise . Because of progradation later deposits ended up on the lower rise and the abyssal plain . Then large channel- levee complexes developed on the upper rise resulting in an unconformity associated with the Middle Miocene intensification of the Antarctic Circumpolar Current . The break-up of Gondwana began in the Early Jurassic between West Antarctica, Africa, and Madagascar following

650-471: The upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from " muddy water" to massive mudflow, and evidence of both these end members can be observed in deposits associated with the deeper parts of submarine canyons and channels, such as lobate deposits (mudflow) and levees along channels. Mass wasting , slumping, and submarine landslides are forms of slope failures (the effect of gravity on

676-666: The western part is the Lazarev Ice Shelf , and further east are Erskine Iceport and Godel Iceport , and the former Belgian Roi-Baudouin Station . The Riiser-Larsen Sea was named in 1962 by the Soviet Antarctic Expedition in honor of Norwegian aviation pioneer and polar explorer Hjalmar Riiser-Larsen (1890–1965). The IHO 2002 draft was never approved by the IHO (or any other organization), and

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