Abyssal fan

Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater version of alluvial fans and can vary dramatically in size, with widths from several kilometres to several thousands of kilometres[1] The largest is the Bengal Fan, followed by the Indus Fan, but major fans are also found at the outlet of the Amazon, Congo, Mississippi and elsewhere.[2][3][4]

Formation

Abyssal (or submarine) fans are formed from turbidity currents.

These currents begin when a geologic activity pushes sediments over the edge of a continental shelf and down the continental slope, creating an underwater landslide. A dense slurry of muds and sands speeds towards the foot of the slope, until the current slows. The decreasing current, having a reduced ability to transport sediments, deposits the grains it carries, thus creating a submarine fan. The slurry continues to slow as it is moved towards the continental rise until it reaches the ocean bed. Thus results a series of graded sediments of sand, silt and mud, which are known as turbidites, as described by the Bouma sequence.

See also

References

  1. ^ Gluyas, J. & Swarbrick, R. (2004) Petroleum Geoscience. Publ. Blackwell Publishing
  2. ^ Clift; Gaedicke; Edwards; Lee; Hildebrand; Amjad; White & Schlüter (2002). "The stratigraphic evolution of the Indus Fan and the history of sedimentation in the Arabian Sea". Marine Geophysical Researches. 23 (3): 223–245. doi:10.1023/A:1023627123093.
  3. ^ Covault, J.A. (2011). "Submarine Fans and Canyon-Channel Systems: A Review of Processes, Products, and Models". Nature Education Knowledge. 3 (10): 4.
  4. ^ Shanmugam, G. (2016). "Submarine fans: A critical retrospective (1950–2015)". Journal of Palaeogeography. 5 (2): 110–184. doi:10.1016/j.jop.2015.08.011.

Sources

Amazon Canyon

The Amazon Canyon is a submarine canyon within the Amazon Fan in the Atlantic Ocean, located approximately 200 mi (322 km) from the mouth of the Amazon River, near South America. It covers an area of 2,250 km2 (870 sq mi). It was formed in the mid to late Miocene period. The canyon is believed to have formed through mass failures, and subsequently evolved through underwater erosion. Because of its relatively small size, the canyon has been extensively mapped.

Astoria Fan

The Astoria Fan is a submarine fan. It has sediment, radiating asymmetrically southward from the mouth of the Astoria Canyon. From Astoria Canyon's mouth, the fan extends about 100 kilometres (62 mi) to its western end, which is the Cascadia Channel. The fan proper ends 160 kilometres (99 mi) south of the canyon mouth, although its depositional basin extends southward another 150 kilometres (93 mi) to the Blanco Fracture Zone.

Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Bay of Bengal

The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman Islands of India and Myanmar and the Nicobar Islands of India. Its southern limit is a line between Sri Lanka and the north western most point of Sumatra (Indonesia). It is the largest water region called a bay in the world. There are countries dependent on the Bay of Bengal in South Asia and Southeast Asia.

The Bay of Bengal occupies an area of 2,172,000 square kilometres (839,000 sq mi). A number of large rivers flow into the Bay of Bengal: the Ganges–Hooghly, the Padma, the Brahmaputra–Jamuna, the Barak–Surma–Meghna, the Irrawaddy, the Godavari, the Mahanadi, the Brahmani, the Baitarani, the Krishna and the Kaveri. Among the important ports are Chennai-Ennore, Chittagong, Colombo, Kolkata-Haldia, Mongla, Paradip, Port Blair, Tuticorin, Visakhapatnam and Dhamra. Among the smaller ports are Gopalpur Port, Kakinada and Payra.

Cascadia Channel

Cascadia Channel is the most extensive deep-sea channel currently known (as of 1969) of the Pacific Ocean. It extends across Cascadia Abyssal Plain, through the Blanco Fracture Zone, and into Tufts Abyssal Plain. Notably, Cascadia Channel has tributaries, akin to river tributaries.Cascadia Channel has two contributing tributaries—Juan de Fuca Channel from the north, and the outflow of Quinault and Willapa Channels in the south.

Frigg gas field

Frigg gas field is a natural gas field on Norwegian block 25/1 in the North Sea, on the boundary between the United Kingdom and Norway. The field is named after the goddess Frigg. King Olav V of Norway officially opened production on 8 May 1978. Production was closed on 26 October 2004. The field is situated 230 kilometres (140 mi) northwest of Stavanger. Operator for the field was the French oil company Elf Aquitaine, which merged and changed name to Total S.A.

Operations were regulated according to an agreement between the UK and Norwegian governments called the Frigg Treaty.

Infrastructural changes were made in three phases:

Phase I - 1977

Phase II - 1978

Phase III - 1981

Hydraulic jump

A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface. The rapidly flowing liquid is abruptly slowed and increases in height, converting some of the flow's initial kinetic energy into an increase in potential energy, with some energy irreversibly lost through turbulence to heat. In an open channel flow, this manifests as the fast flow rapidly slowing and piling up on top of itself similar to how a shockwave forms.

It was first observed and documented by Leonardo da Vinci in 1500s. The mathematics were first described by Giorgio Bidone when he published a paper called Experiences sur le remou et sur la propagation des ondes.The phenomenon is dependent upon the initial fluid speed. If the initial speed of the fluid is below the critical speed, then no jump is possible. For initial flow speeds which are not significantly above the critical speed, the transition appears as an undulating wave. As the initial flow speed increases further, the transition becomes more abrupt, until at high enough speeds, the transition front will break and curl back upon itself. When this happens, the jump can be accompanied by violent turbulence, eddying, air entrainment, and surface undulations, or waves.

There are two main manifestations of hydraulic jumps and historically different terminology has been used for each. However, the mechanisms behind them are similar because they are simply variations of each other seen from different frames of reference, and so the physics and analysis techniques can be used for both types.

The different manifestations are:

The stationary hydraulic jump – rapidly flowing water transitions in a stationary jump to slowly moving water as shown in Figures 1 and 2.

The tidal bore – a wall or undulating wave of water moves upstream against water flowing downstream as shown in Figures 3 and 4. If considered from a frame of reference which moves with the wave front, you can see that this case is physically similar to a stationary jump.A related case is a cascade – a wall or undulating wave of water moves downstream overtaking a shallower downstream flow of water as shown in Figure 5. If considered from a frame of reference which moves with the wave front, this is amenable to the same analysis as a stationary jump.

These phenomena are addressed in an extensive literature from a number of technical viewpoints.

Juan de Fuca Channel

Juan de Fuca Channel is a submarine channel off the shore of Washington state, USA and the Strait of Juan de Fuca.

List of landforms

Landforms are categorised by characteristic physical attributes such as elevation, slope, orientation, rock exposure, and soil type.

List of submarine volcanoes

A list of active and extinct submarine volcanoes and seamounts located under the world's oceans. There are estimated to be 40,000 to 55,000 seamounts in the global oceans. Almost all are not well-mapped and many may not have been identified at all. Most are unnamed and unexplored. This list is therefore confined to seamounts that are notable enough to have been named and/or explored.

Northwest Atlantic Mid-Ocean Channel

The Northwest Atlantic Mid-Ocean Channel (NAMOC) is the main body of a turbidity current system of channels and canyons running on the sea bottom from the Hudson Strait, through the Labrador Sea and ending at the Sohm Abyssal Plain in the Atlantic Ocean. Contrary to most other such systems which fan away from the main channel, numerous tributaries run into the NAMOC and end there. The density of those tributaries is the highest near the Labrador Peninsula, but the longest tributary, called Imarssuak Mid-Ocean Channel (IMOC), originates in the Atlantic Ocean.

Most topography data on the NAMOC originate from wide-range sonar scans. With a total length of about 3,800 km (2,361 mi), NAMOC is one of the longest underwater channels in the world. It is 100–200 m deep and 2–5 km wide at the channel floor. The rising levees of the NAMOC (about 100 m above the sea bed) often hinder confluence of some tributaries, which instead run along NAMOC for hundreds of km. Its western (right-hand, max. height 250 m) levee rises some 100 m above the eastern one (max. height 150 m). This asymmetry is attributed to the Coriolis effect affecting the turbidity currents, which reach velocities of 6–8.5 m/s and deposit silt and clay over the channel. The levee is absent in some parts of the NAMOC, for example between 56°N and 57°N, due to the local side-flows of sand.The meandering of the NAMOC is relatively small compared to other underwater channels, such as Amazon Canyon. It is more developed in the northern part with a period increasing from 25 km between 59°45'N and 56°N to 50 km between 56°N and 54°30'N. The channel becomes on average more straight towards the south, but it still contains abrupt turns due to local seamounts sea bed fractures.

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Physical oceanography

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography.

Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models, which describe the fluid motions as precisely as possible.

Dynamical physical oceanography focuses primarily upon the processes that govern the motion of fluids with emphasis upon theoretical research and numerical models. These are part of the large field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of Fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.

Quileute Canyon

Quileute Canyon (also Quillayute Canyon) is a submarine canyon, off of Washington State, United States.

Quinault Canyon

The Quinault Canyon is a submarine canyon, off Washington State, in Olympic Coast National Marine Sanctuary.

Submarine canyon

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 (as much as 70 km/h), eroding the continental slope and finally depositing sediment onto 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 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 the continental slope. While at first glance, the erosion patterns of submarine canyons may appear to mimic those of river-canyons on land, due to the markedly different erosion processes that have been found to take place underwater at the soil/ water interface, several notably different erosion patterns have been observed in the formation of typical submarine canyons.Many canyons have been found at depths greater than 2 km below sea level. Some may extend seawards across continental shelves for hundreds of kilometres before reaching 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.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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