Submarine canyon
Encyclopedia
A submarine canyon is a steep-sided valley
on the sea floor of the continental slope. Many submarine canyon
s are found as extensions to large river
s; however there are some that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea level
. They are formed by powerful volcanic
and earthquake
activity. Many submarine canyons continue as submarine channels across continental rise areas and may extend for hundreds of kilometers.
through all substrates, from unlithified sediment
to crystal
line rock
. They are more densely spaced on steep slopes while being rare on gentle slopes. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion
, or slumping.
An early and obvious theory was that the canyons present today were carved during glacial
times, when sea level was about 125 meters 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 the water depths as great as 3000 meters 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 now thought to be turbidity currents and underwater landslide
s. Turbidity currents are dense
, sediment-laden current
s which flow downslope when an unstable mass of sediment that has been rapidly deposited on the upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from "mud
dy 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 levee
s along channels.
Mass wasting
, slumping, and submarine landslides are forms of slope failures (the effect of gravity on a hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill; and would commonly include the effects of bioerosion: the burrow
ing, ingestion
and defecation
of sediment performed by organism
s. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. All are observed; all are contributory processes.
It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within 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, 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 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, the Mediterranean sea basin was flooded. One relevant consequence is that the submarine canyons eroded are now far below the present sea level.
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...
on the sea floor of the continental slope. Many submarine canyon
Canyon
A canyon or gorge is a deep ravine between cliffs often carved from the landscape by a river. Rivers have a natural tendency to reach a baseline elevation, which is the same elevation as the body of water it will eventually drain into. This forms a canyon. Most canyons were formed by a process of...
s are found as extensions to large river
River
A river is a natural watercourse, usually freshwater, flowing towards 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. Small rivers may also be called by several other names, including...
s; however there are some that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
. They are formed by powerful volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
and earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
activity. Many submarine canyons continue as submarine channels across continental rise areas and may extend for hundreds of kilometers.
Characteristics
Submarine canyons are more common on steep slopes than on gentle slopes. They show erosionErosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
through all substrates, from unlithified sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
to crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
. They are more densely spaced on steep slopes while being rare on gentle slopes. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion
Bioerosion
Bioerosion describes the erosion of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and...
, or slumping.
Examples
- Congo Canyon, the largest river canyon, extending from the Congo RiverCongo RiverThe Congo River is a river in Africa, and is the deepest river in the world, with measured depths in excess of . It is the second largest river in the world by volume of water discharged, though it has only one-fifth the volume of the world's largest river, the Amazon...
, is 800 km (497 mi) long, and 1200 m (3,937 ft) deep. - Amazon CanyonAmazon CanyonThe Amazon Canyon is a submarine canyon within the Amazon Fan in the Atlantic Ocean, located approximately from the mouth of the Amazon River, near South America. It covers an area of . It was formed in the mid to late Miocene period. The canyon is believed to have formed through mass failures,...
, extending from the Amazon RiverAmazon RiverThe Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined... - Hudson CanyonHudson Canyon__notoc__The Hudson Canyon is a submarine canyon that begins from the shallow outlet of the estuary at the mouth of the Hudson River. It extends out over seaward across the continental shelf finally connecting to the deep ocean basin at a depth of 3 to 4 km below sea level...
, extending from the Hudson RiverHudson RiverThe Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York... - Ganges Canyon, extending from the Ganges
- Indus Canyon, extending from the Indus RiverIndus RiverThe Indus River is a major river which flows through Pakistan. It also has courses through China and India.Originating in the Tibetan plateau of western China in the vicinity of Lake Mansarovar in Tibet Autonomous Region, the river runs a course through the Ladakh district of Jammu and Kashmir and...
- Monterey CanyonMonterey CanyonMonterey Canyon, or Monterey Submarine Canyon, is a submarine canyon in Monterey Bay, California. It is the subject of ongoing study by the scientists at the Monterey Bay Aquarium Research Institute, the Moss Landing Marine Laboratories, and other oceanographic institutions.Monterey Canyon begins...
, off the coast of central CaliforniaCaliforniaCalifornia is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area... - La Jolla and Scripps CanyonScripps CanyonScripps Canyon is a narrow underwater gorge located in the Pacific Ocean off the coast of southern California, USA. The canyon is approximately one mile long, which joins to La Jolla Canyon offshore....
, off the coast of La JollaLa Jolla, San Diego, CaliforniaLa Jolla is an affluent, hilly seaside resort community, occupying of curving coastline along the Pacific Ocean in Southern California within the northern city limits of San Diego. La Jolla had the highest home prices in the nation in 2008 and 2009; the average price of a standardized...
, southern CaliforniaSouthern CaliforniaSouthern California is a megaregion, or megapolitan area, in the southern area of the U.S. state of California. Large urban areas include Greater Los Angeles and Greater San Diego. The urban area stretches along the coast from Ventura through the Southland and Inland Empire to San Diego... - Whittard Canyon, Atlantic OceanAtlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
off southwest IrelandIrelandIreland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth... - Bering CanyonBering CanyonThe Bering Canyon is the longest of the Bering Sea canyons; it extends about 400 km across the Bering shelf and slope. It is confined at its eastern edge by the Aleutian Islands. The width of the canyon at the shelf break is about 65 km, only about two-thirds that of the Zhemchug Canyon...
, in the Bering SeaBering SeaThe Bering Sea is a marginal sea of the Pacific Ocean. It comprises a deep water basin, which then rises through a narrow slope into the shallower water above the continental shelves.... - Zhemchug CanyonZhemchug CanyonZhemchug Canyon is a giant underwater canyon located in the middle of the Bering Sea. This submarine canyon is the largest canyon in the ocean. The canyon has a vertical relief of 2600 metres dropping from the shallow shelf the Bering Sea to the depths of the Aleutian Basin. Zhemchug Canyon is...
the largest submarine canyon in the world, also in the Bering sea
Formation
Many mechanisms have been proposed for the formation of submarine canyons, and during the 1940s and 1950s the primary causes of submarine canyons were subject to active debate.An early and obvious theory was that the canyons present today were carved during glacial
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
times, when sea level was about 125 meters 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 the water depths as great as 3000 meters 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 now thought to be turbidity currents and underwater landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
s. Turbidity currents are dense
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, sediment-laden current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s which flow downslope when an unstable mass of sediment that has been rapidly deposited on the upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from "mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
dy 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 levee
Levee
A levee, levée, dike , embankment, floodbank or stopbank is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels...
s along channels.
Mass wasting
Mass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...
, slumping, and submarine landslides are forms of slope failures (the effect of gravity on a hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill; and would commonly include the effects of bioerosion: the burrow
Burrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
ing, ingestion
Ingestion
Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking...
and defecation
Defecation
Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
of sediment performed by organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. All are observed; all are contributory processes.
It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within the same canyon, or at different times during a canyon's development. However, if a primary mechanism must be selected, the downslope lineal morphology
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
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, 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 the bottom of the bed now exposed. The Messinian Salinity Crisis
Messinian salinity crisis
The Messinian Salinity Crisis, also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene...
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, the Mediterranean sea basin was flooded. One relevant consequence is that the submarine canyons eroded are now far below the present sea level.