Turbidity current
Encyclopedia
A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid
. The current moves because it has a higher density
and turbidity
than the fluid through which it flows. The driving force of a turbidity current is obtained from the sediment, which renders the turbid water heavier than the clear water above.
Turbidity currents are an example of density or gravity current
s, which include: ocean
ic fronts, avalanches, lahars, pyroclastic flows, and lava
flows. Seafloor turbidity currents are commonly used to describe underwater currents in lakes and oceans, which are usually triggered by earthquake
s , slumping
and sediment laden rivers. They are characterized by a well-defined front, also known as head, followed by a layer known as the body of the current.
Turbidity currents are characteristic of areas where there is seismic instability and an underwater slope, especially submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins.
With increasing continental shelf slope current velocity increases, as the velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment increases the density of the current, and thus its velocity, even further. Turbidity currents can reach speeds up to half the speed of sound.
Turbidity currents are traditionally defined as those sediment-gravity flows in which sediment is suspended by fluid turbulence. However, the term 'turbidity current' was adopted to describe a natural phenomenon
whose exact nature is often unclear. The turbulence within a turbidity current isn’t always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by a incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the term turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended
) sediment
, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid
is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.
is so large that the density of river water is greater than the density of sea water a particular kind of turbidity current can form called a hyperpycnal plume. The average concentration of suspended sediment for most river water that enters the ocean
is much lower than the sediment concentration needed for entry as a hyperpycnal plume. Although some rivers can often have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg m³. The sediment concentration needed to produce a hyperpycnal plume in marine
water is 35 to 45 kg m³, depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms, floods, glacier
outbursts, dam breaks, and lahar
flows. In fresh water environments, such as lakes, the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg m³).
and deposition
of the sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg
of the lake to the deepest area near the dam
, where the sediments can affect the operation of the bottom outlet and the intake structures. Controlling this sedimentation
within the reservoir can be achieved by using solid and permeable obstacles with the right design.
of continental crust
in the form of fluidization and physical shaking both contribute to their formation. Earthquakes have been linked to turbidity current deposition in many settings, particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition. Since the famous case of breakage of submarine cables by a turbidity current following the 1929 Grand Banks earthquake, earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone, the Northern San Andreas Fault, a number of European, Chilean and North American lakes
, Japanese lacustrine and offshore regions and a variety of other settings.
and sliding.
continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor. Understanding where turbidity currents flow on the ocean floor can help to decrease the amount of damage to telecommunication cables by avoiding these areas or reinforcing the cables in vulnerable areas.
When turbidity currents interact with other currents, such as contour currents, they can change their direction. This ultimately shifts submarine canyons and sediment deposition locations. One example of this is located in the western part of the Gulf of Cadiz
, where the Mediterranean outflow water (MOW) current strongly influences turbidity currents, ultimately causing shifting of valleys and canyons in the direction of the MOW flow. This changes the erosion and depositional zones, ultimately changing the ocean floor topography.
size can give indication of current velocity, grain lithology
and the use of foraminifera
for determining origins, grain distribution shows flow dynamics over time and sediment thickness indicates sediment load and longevity.
Turbidites are commonly used in the understanding of past turbidity currents, for example, the Peru-Chile Trench off Southern Central Chile
(36°S-39°S) contains numerous turbidite layers that were cored and analysed. From these turbidites the predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents.
, South America. This sediment-wave field covers an area of at least 29 000 km2 at a water depth of 4400–4825 meters. These antidunes have wavelengths of 110-2600m and wave height
s of 1-15m. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela
, Guyana and Suriname continental margins. Simple numerical modelling has been enabled to determine turbidity current flow characteristics across the sediment waves to be estimated: internal Froude number
= 0.7-1.1, flow thickness = 24–645 m, and flow velocity = 31–82 cm s-1. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.
environment) or to the water surface, and spreads out. Sediment falling from the plume produces a widespread fall-out deposit, termed hemiturbidite.
of erosion
by turbidity currents, and of the distribution of turbidite
deposits, such as their extent, thickness and grain size distribution, requires an understanding of the mechanisms of sediment transport and deposition, which in turn depends on the fluid dynamics
of the currents.
The extreme complexity of most turbidite systems and beds has promoted the development of quantitative models of turbidity current behaviour inferred solely from their deposits. Small-scale laboratory experiments therefore offer one of the best means of studying their dynamics. Mathematical models can also provide significant insights into current dynamics. In the long term numerical techniques are most likely the best hope of understanding and predicting three-dimensional turbidity current processes and deposits.
In most cases there are more variables than governing equations and the models rely upon simplifying assumptions in order to achieve a result. The accuracy of the individual models thus depends upon the validity and choice of the assumptions made. Experimental results provide a means of constraining some of these variables as well as providing a test for such models. Physical data from field observations, or more practical from experiments, are still required in order to test the simplifying assumptions necessary in mathematical model
s. Most of what is known about large natural turbidity currents (i.e. those significant in terms of sediment transfer to the deep sea) is inferred from indirect sources, such as submarine cable breaks and heights of deposits above submarine valley floors. Although during the 2003 Tokachi-oki earthquake a large turbidity current was observed by the cabled observatory which provided direct observations, which is rarely achieved
that over geologic time gets buried, compressed and transformed into hydrocarbons. The use of numerical modelling and flumes are commonly used to help understand these questions. Much of the modelling is used to reproduce the physical processes which govern turbidity current behaviour and deposits.
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
. The current moves because it has a higher density
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...
and turbidity
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
than the fluid through which it flows. The driving force of a turbidity current is obtained from the sediment, which renders the turbid water heavier than the clear water above.
Turbidity currents are an example of density or gravity current
Gravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...
s, which include: ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
ic fronts, avalanches, lahars, pyroclastic flows, and lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows. Seafloor turbidity currents are commonly used to describe underwater currents in lakes and oceans, which are usually triggered by 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...
s , slumping
Slumping
Slumping is one broad technique of warm glass working, for the forming of glass by applying heat to the point where the glass will soften. The increasing fluidity of the glass with temperature causes the glass to 'slump' into or onto the mold under the force of gravity.- Technique :Glass is most...
and sediment laden rivers. They are characterized by a well-defined front, also known as head, followed by a layer known as the body of the current.
Turbidity currents are characteristic of areas where there is seismic instability and an underwater slope, especially submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins.
With increasing continental shelf slope current velocity increases, as the velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment increases the density of the current, and thus its velocity, even further. Turbidity currents can reach speeds up to half the speed of sound.
Turbidity currents are traditionally defined as those sediment-gravity flows in which sediment is suspended by fluid turbulence. However, the term 'turbidity current' was adopted to describe a natural phenomenon
Natural phenomenon
A natural phenomenon is a non-artificial event in the physical sense, and therefore not produced by humans, although it may affect humans . Common examples of natural phenomena include volcanic eruptions, weather, decay, gravity and erosion...
whose exact nature is often unclear. The turbulence within a turbidity current isn’t always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by a incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the term turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended
Suspended
Suspended: A Cryogenic Nightmare is an interactive fiction computer game written by Michael Berlyn and published by Infocom in 1983. Like most Infocom titles, it was available on most popular personal computers of the day, such as the Apple II, PC, Atari ST and Commodore 64...
) 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....
, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...
is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.
Hyperpycnal Plume
When the concentration of suspended sediment at the mouth of a riverRiver
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...
is so large that the density of river water is greater than the density of sea water a particular kind of turbidity current can form called a hyperpycnal plume. The average concentration of suspended sediment for most river water that enters the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
is much lower than the sediment concentration needed for entry as a hyperpycnal plume. Although some rivers can often have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg m³. The sediment concentration needed to produce a hyperpycnal plume in marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
water is 35 to 45 kg m³, depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms, floods, glacier
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...
outbursts, dam breaks, and lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...
flows. In fresh water environments, such as lakes, the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg m³).
Sedimentation in Reservoirs
The transportTransport
Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...
and deposition
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
of the sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg
Thalweg
Thalweg in geography and fluvial geomorphology signifies the deepest continuous inline within a valley or watercourse system.-Hydrology:In hydrological and fluvial landforms, the thalweg is a line drawn to join the lowest points along the entire length of a stream bed or valley in its downward...
of the lake to the deepest area near the dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
, where the sediments can affect the operation of the bottom outlet and the intake structures. Controlling this sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
within the reservoir can be achieved by using solid and permeable obstacles with the right design.
Earthquake Triggering
Turbidity currents are most commonly triggered by tectonic disturbances of the sea floor. The displacementDisplacement (fluid)
In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
of continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
in the form of fluidization and physical shaking both contribute to their formation. Earthquakes have been linked to turbidity current deposition in many settings, particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition. Since the famous case of breakage of submarine cables by a turbidity current following the 1929 Grand Banks earthquake, earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone, the Northern San Andreas Fault, a number of European, Chilean and North American lakes
, Japanese lacustrine and offshore regions and a variety of other settings.
Canyon-flushing
When large turbidity currents flow into canyons they may become self-sustaining, 1979), and may entrain sediment that has previously been introduced into the canyon by littoral drift, storms or smaller turbidity currents. Canyon-flushing associated with surge-type currents initiated by slope failures may produce currents whose final volume may be several times that of the portion of the slope that has failed (e.g. Grand Banks).Slumping
Sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons can create turbidity current due to overloading, thus consequent slumpingSlumping
Slumping is one broad technique of warm glass working, for the forming of glass by applying heat to the point where the glass will soften. The increasing fluidity of the glass with temperature causes the glass to 'slump' into or onto the mold under the force of gravity.- Technique :Glass is most...
and sliding.
Effect on Ocean Floor
Large and fast-moving turbidity currents can incise and erodeErode
Erode is a city, a municipal corporation and the headquarters of Erode district in the South Indian state of Tamil Nadu.It is situated at the center of the South Indian Peninsula, about southwest from the state capital Chennai and on the banks of the rivers Cauvery and Bhavani, between 11° 19.5"...
continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor. Understanding where turbidity currents flow on the ocean floor can help to decrease the amount of damage to telecommunication cables by avoiding these areas or reinforcing the cables in vulnerable areas.
When turbidity currents interact with other currents, such as contour currents, they can change their direction. This ultimately shifts submarine canyons and sediment deposition locations. One example of this is located in the western part of the Gulf of Cadiz
Gulf of Cadiz
The Gulf of Cádiz is the arm of the Atlantic Ocean between Cape St. Vincent in Portugal and Cape Trafalgar at the western end of the Strait of Gibraltar...
, where the Mediterranean outflow water (MOW) current strongly influences turbidity currents, ultimately causing shifting of valleys and canyons in the direction of the MOW flow. This changes the erosion and depositional zones, ultimately changing the ocean floor topography.
Deposits
When the energy of a Turbidity current lowers, its ability to keep suspended sediment decreases, thus sediment deposition occurs. These deposits are called turbidites. Turbidity currents are rarely seen in nature, thus turbidites can be used to determine turbidity current characteristics. Some examples: grainGRAIN
GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and...
size can give indication of current velocity, grain lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
and the use of foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
for determining origins, grain distribution shows flow dynamics over time and sediment thickness indicates sediment load and longevity.
Turbidites are commonly used in the understanding of past turbidity currents, for example, the Peru-Chile Trench off Southern Central Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
(36°S-39°S) contains numerous turbidite layers that were cored and analysed. From these turbidites the predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents.
Antidune Deposits
Some of the largest Antidunes on Earth are formed by turbidity currents. One observed sediment-wave field is located on the lower continental slope off GuyanaGuyana
Guyana , officially the Co-operative Republic of Guyana, previously the colony of British Guiana, is a sovereign state on the northern coast of South America that is culturally part of the Anglophone Caribbean. Guyana was a former colony of the Dutch and of the British...
, South America. This sediment-wave field covers an area of at least 29 000 km2 at a water depth of 4400–4825 meters. These antidunes have wavelengths of 110-2600m and wave height
Wave height
In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighbouring trough. Wave height is a term used by mariners, as well as in coastal, ocean and naval engineering....
s of 1-15m. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
, Guyana and Suriname continental margins. Simple numerical modelling has been enabled to determine turbidity current flow characteristics across the sediment waves to be estimated: internal Froude number
Froude number
The Froude number is a dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics, the Froude number is used to determine the resistance of an...
= 0.7-1.1, flow thickness = 24–645 m, and flow velocity = 31–82 cm s-1. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.
Reversing Buoyancy
The behaviour of turbidity currents with buoyant fluid (such as currents with warm, fresh or brackish interstitial water entering the sea) has been investigated to find that the front speed decreases more rapidly than that of currents with the same density as the ambient fluid. These turbidity currents ultimately come to a halt as sedimentation results in a reversal of buoyancy, and the current lifts off, the point of lift-off remaining constant for a constant discharge. The lofted fluid carries fine sediment with it, forming a plume that rises to a level of neutral buoyancy (if in a stratifiedStratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
environment) or to the water surface, and spreads out. Sediment falling from the plume produces a widespread fall-out deposit, termed hemiturbidite.
Prediction
PredictionPrediction
A prediction or forecast is a statement about the way things will happen in the future, often but not always based on experience or knowledge...
of erosion
Erosion
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...
by turbidity currents, and of the distribution of turbidite
Turbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
deposits, such as their extent, thickness and grain size distribution, requires an understanding of the mechanisms of sediment transport and deposition, which in turn depends on the fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
of the currents.
The extreme complexity of most turbidite systems and beds has promoted the development of quantitative models of turbidity current behaviour inferred solely from their deposits. Small-scale laboratory experiments therefore offer one of the best means of studying their dynamics. Mathematical models can also provide significant insights into current dynamics. In the long term numerical techniques are most likely the best hope of understanding and predicting three-dimensional turbidity current processes and deposits.
In most cases there are more variables than governing equations and the models rely upon simplifying assumptions in order to achieve a result. The accuracy of the individual models thus depends upon the validity and choice of the assumptions made. Experimental results provide a means of constraining some of these variables as well as providing a test for such models. Physical data from field observations, or more practical from experiments, are still required in order to test the simplifying assumptions necessary in mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
s. Most of what is known about large natural turbidity currents (i.e. those significant in terms of sediment transfer to the deep sea) is inferred from indirect sources, such as submarine cable breaks and heights of deposits above submarine valley floors. Although during the 2003 Tokachi-oki earthquake a large turbidity current was observed by the cabled observatory which provided direct observations, which is rarely achieved
Oil Exploration
Oil and gas companies are also interested in turbidity currents because the currents deposit organic matterOrganic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
that over geologic time gets buried, compressed and transformed into hydrocarbons. The use of numerical modelling and flumes are commonly used to help understand these questions. Much of the modelling is used to reproduce the physical processes which govern turbidity current behaviour and deposits.
Examples of Turbidity Currents
- 1929 Grand Banks earthquake1929 Grand Banks earthquakeThe 1929 Grand Banks earthquake, also called the Laurentian Slope earthquake and the South Shore Disaster, was a magnitude 7.2 earthquake that occurred on November 18, 1929 in the Atlantic Ocean off the south coast of Newfoundland in the Laurentian Slope Seismic Zone.The earthquake was centred on...
, off the coast of Newfoundland. Minutes later, transatlantic telephone cableTransatlantic telephone cableA transatlantic telecommunications cable is a submarine communications cable running under the Atlantic Ocean. All modern cables use fibre optic technology....
s began breaking sequentially, farther and farther downslope, away from the epicenterEpicenterThe epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...
. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour (100 km/h) submarine landslide or turbidity current of water saturated sediments swept 400 miles (600 km) down the continental slope from the earthquake’s epicenter, snapping the cables as it passed. Subsequent research of this event have shown that continental slope sediment failures mostly occurred below 650 meter water depth. The slumpingSlumpingSlumping is one broad technique of warm glass working, for the forming of glass by applying heat to the point where the glass will soften. The increasing fluidity of the glass with temperature causes the glass to 'slump' into or onto the mold under the force of gravity.- Technique :Glass is most...
that occurred in shallow waters (5–25 meters) passed down slope into turbidity currents that evolved ignitively. The turbidity currents had sustained flow for many hours due to the delayed retrogressive failure and transformation of debris flowDebris flowA debris flow is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. It is differentiated from a mudflow in terms of the viscosity and textural properties of the flow. Flows can carry material ranging in size from clay to boulders, and may...
s into turbidity currents through hydraulic jumps.
- The Cascadia subduction zoneCascadia subduction zoneThe Cascadia subduction zone is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping fault that separates the Juan de Fuca and North America plates.New ocean floor is being created offshore of...
, off the northwestern coast of North America, has a record of earthquake triggered turbidites that is well-correlated to other evidence of earthquakes recorded in coastal bays and lakes during the Holocene. 41 Holocene turbidity currents have been correlated along all or part of the ~ 1000 km long plate boundary stretching from northern California to mid-Vancouver island. The correlations are based on radiocarbon ages and subsurface stratigraphic methods. The inferred recurrence interval of Cascadia great earthquakes is ~ 500 years along the northern margin, and ~ 240 years along the souther margin
- TaiwanTaiwanTaiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
is a hot spot for submarine turbidity currents as there are large amounts of sediment suspended in rivers, and it is seismically active, thus large accumulation of seafloor sediments and earthquake triggering. During the 2006 Pingtung earthquake off SW Taiwan eleven submarine cables across the Kaoping canyon and Manila trench were broken in sequence from 1500 to 4000 m deep, as a consequence of the associated turbidity currents. From the timing of each cable break the velocity of the current was determined to have a positive relationship with bathymetric slope. Current velocities on steepest slopes were 20ms-1 and 3.7ms-1 on the shallowest slopes.