Oceans – Bottom Relief
Four major divisions can easily be identified on the ocean floor:
- The continental shelf,
- the continental slope,
- the continental rise,
- the abyssal plain.
Besides these, there are many associated features—ridges, hills, seamounts, guyots, trenches, canyons, sleeps, fracture zones, island arcs, atolls, coral reefs, submerged volcanoes and sea-scarps.
This great variety of relief is largely due to interaction of tectonic, volcanic, erosional and depositional processes. At greater depths, the tectonic and volcanic phenomena are more significant processes.
Continental Shelf: This is a gentle seaward sloping surface extending from the coasts toward s the open sea. In all, about 7.5% of the total area of the oceans is covered by the continental shelves. The shelf is formed by the drowning of a part of a continent with a relative rise in sea level or marine deposition beneath the water.
The average width of the continental shelf is about 70 km and mean slope is less than one degree, but the width shows great variety from location to location. For instance, it is almost absent in the eastern Pacific, especially off South America and is upto 120 km wide along the eastern coast of USA. The seaward edge of the shelf is usually 150-200 metres deep.
The continental shelves are mostly covered by sediments of terrestrial origin. There are various types of shelves—glaciated shelf, coral reef shelf, shelf of a large river, shelf with dendritic valleys and the shelf along young mountain ranges.
As the continental shelf nears its seaward edge, the gradient becomes steeper—two to five degrees. This is the site of the continental slope which descends to a depth of 3,500 metres and joins the shelf to the deep ocean floor. The site of the slope also indicates the end of the continental block. The slopes may be furrowed by canyons and trenches. Continental Rise
The continental slope gradually loses its steepness with depth. When the slope reaches a level of between 0.5° and 1°, it is referred to as the continental rise. With increasing depth the rise becomes virtually flat and merges with the abyssal plain.
Beyond the continental rise, at depths from 3,000 m to 6,000 m, lie the deep sea plains, called abyssal plains or abyssal floors. Covering nearly 40% of the ocean floor, the abyssal plains are present in all major oceans and several seas of the world. They are uniquely flat with a gradient of less than 10,000. The large supply of terrigenous and shallow water sediments buries the irregular topography to form a generally flat relief.
Submarine ridges are mountain ranges, a few hundred kilometres wide and hundreds and often thousands of kilometres in length on the floors of oceans. Running for a total length of 75,000 km, these ridges form the largest mountain systems on earth.
These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains. These oceanic ridge systems are of tectonic origin and provide evidence in support of the theory of Plate Tectonics.
These are elevated features of volcanic origin. A submarine mountain or peak rising more than 1,000 metres above the ocean floor is known as a seamount. The flat topped mountains are known as guyots.
Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around 10,000.
Submarine Trenches or Deeps: These are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors. A trench is a long, narrow and steep-sided depression on the ocean bottom, which is usually 5,500 metres in depth. The trenches lie along the fringes of the deep-sea plain and run parallel to the bordering fold mountains or the island chains.
They are believed to have resulted from down faulting or down folding of the earth’s crust and are, therefore, of tectonic origin. The trenches are very common in the Pacific Ocean and form an almost continuous ring along the western and eastern margins of the Pacific. The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with a depth of more than 11 kilometres.
These are steep valleys, forming deep gorges on the ocean floor. They are mainly restricted to the continental shelf, slope and rise. Broadly, there are three types of submarine canyons:
- Small gorges which begin at the edge of the continental shelf and extend down the slope to very great depths, e.g., Oceanographer Canyons near New England.
- Those which begin at the mouth of a river and extend over the shelf, such as the Zaire, the Mississippi and the Indus canyons.
- Those which have a dendritic appearance and are deeply cut into the edge of the shelf and the slope, like the canyons off the coast of southern California. The Hudson Canyon is the best known canyon in the world. The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.
Bank, Shoal and Reef
These marine features are formed as a result of erosional, depositional and biological activity. Also, these are produced upon features of diastrophic origin. Therefore, they are located on upper parts of elevations.
A bank is a flat topped elevation located in the continental margins. The depth of water here is shallow but enough for navigational purposes. The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic off Newfoundland are famous examples. The banks are sites of some of the most productive fisheries of the world.
A shoal is a detached elevation with shallow depths, since they project out of water with moderate heights, they are dangerous for navigation.
A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge. Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots. The largest reef in the world is found off the Queensland coast of Australia . Since the reefs may extend above the surface, they are generality dangerous for navigation.
Salinity of oceans
Salinity is defined as the ratio between the weight of the dissolved materials and the weight of the sample sea water. Generally, salinity is defined as ‘the total amount of solid material in grams contained in one kilogram of sea water and is expressed as part per thousand (%o) e.g., 30%o (means 30 grams of salt in 1000 grams of sea water).
The oceanic salinity not only affects the marine organisms and plant community but it also affects the physical properties of the oceans such as temperature, density, pressure, waves and currents etc. The freezing point of ocean water also depends on salinity e.g., more saline water freezes slowly in comparison to less saline water.
Controlling Factors of Salinity
There is a wide range of variation in the spatial distribution of salinity within the oceans and the seas. The factors affecting the amount of salt in different oceans and seas are called as controlling factors of oceanic salinity.
There is direct positive relationship between the rate of evaporation and salinity e.g., greater the evaporation, higher the salinity and vice versa. In fact, salt concentration increases with rapid rate of evaporation. Evaporation due to high temperature with low humidity (dry condition) causes more concentration of salt and overall salinity becomes higher. For example, salinity is higher near the tropics than at the equator because both the areas record high rate of evaporation but with dry air over the tropics of Cancer and Capricorn.
Precipitation is inversely related to salinity e.g., higher the precipitation, lower the salinity and vice versa. This is why the regions of high rainfall (equatorial zone) record comparatively lower salinity than the regions of low rainfall (sub-tropical high pressure belts).
The extra water in the temperate regions supplied by melt-water of ice coming from the polar areas increases the volume of water and therefore reduces salinity. It may be simply stated that the volume of water in the oceans is increased due to heavy rainfall and thus the ratio of salt to the total volume of water is reduced.
Influx of river water
Though the rivers bring salt from the land to the oceans but big and voluminous rivers pour down immense volume of water into the oceans and thus salinity is reduced at their mouths. For example, comparatively low salinity is found near the mouths of the Ganga, the Congo, the Nizer, the Amazon, the St. Lawrence etc.
The effect of influx of river water is more pronounced in the enclosed seas e.g. the Danube, the Dneister, the Dneiper etc. reduce the salinity in the Black Sea (180/00). Salinity is reduced to 50/00 in the Gulf of Bothnia due to influx of immense volume of water brought by the rivers. On the other hand, where evaporation exceeds the influx of fresh river waters, there is increase in salinity (Mediterranean Sea records 400/00).
Atmospheric pressure and wind direction
Anticyclonic conditions with stable air and high temperature increase salinity of the surface water of the oceans. Sub-tropical high pressure belts represent such conditions to cause high salinity. Winds also help in the redistribution of salt in the oceans and the seas as winds drive away saline water to less saline areas resulting into decrease of salinity in the former and increase in the latter.
Circulation of oceanic water
Ocean currents affect the spatial distribution of salinity by mixing seawaters. Equatorial warm currents drive away salts from the western coastal areas of the continents and accumulate them along the eastern coastal areas. The high salinity of the Mexican Gulf is partly due to this factor. The North Atlantic Drift, the extension of the Gulf Stream increases salinity along the north-western coasts of Europe. Similarly, salinity is reduced along the north-eastern coasts of N. America due to cool Labrador Current.
Distribution of Salinity
The average salinity in the oceans and the seas is 35%o but it spatially and temporally varies in different oceans, seas, and lakes. The variation in salinity is both horizontal and vertical (with depth). Salinity also varies from enclosed seas through partially closed seas to open seas.
Horizontal Distribution: Horizontal distribution of oceanic salinity is studied in relation to latitudes but regional distribution is also considered wherein each ocean is separately described. Similarly, the pattern of spatial distribution of salinity in enclosed seas, partially enclosed seas and open seas is also considered.
On an average, salinity decreases from equator towards the poles. It may be mentioned that the highest salinity is seldom recorded near the equator though this zone records high temperature and evaporation but high rainfall reduces the relative proportion of salt. Thus, the equator accounts for only 350/00 salinity.
The highest salinity is observed between 200-400N (360/00) because this zone is characterized by hi0gh temperature, high evaporation but significantly low rainfall. The average salinity of 350/00 is recorded between 100-300 latitudes in the southern hemisphere. The zone between 400-600 latitudes in both the hemispheres records low salinity where it is 310/00 and 330/00 in the northern and the southern hemispheres respectively.
Salinity further decreases in the polar zones because of influx of melt-water. On an average, the northern and the southern hemispheres record average salinity of 340/00 and 350/00 respectively.
No definite trend of distribution of salinity with depth can be spelt out because both the trends of increase and decrease of salinity with increasing depths have been observed. For example, salinity at the southern boundary of the Atlantic is 330/00 at the surface but it increases to 34.50/00 at the depth of 200 fathoms (1200 feet).
It further increases to 34.75% at the depth of 600 fathoms. On the other hand, surface salinity is 370/00 at 20°S latitude but it decreases to 350/00 at greater depth.
The following characteristics of vertical distribution of salinity may be stated:
- Salinity increases with increasing depth in high latitudes i.e. there is positive relationship between the amount of salinity and depth because of denser water below.
- The trend of increase of salinity with increasing depths is confined to 200 fathoms from the surface in middle latitudes beyond which it decreases with increasing depths. Salinity is low at the surface at the equator due to high rainfall and transfer of water through equatorial currents but higher salinity is noted below the water surface. It again becomes low at the bottom. More studies and data of salinity distribution at regular depths in different oceans and seas are required so that definite characteristic features of vertical distribution of salinity may be determined.
- Maximum salinity is found in the upper layer of the oceanic water. Salinity decreases with increasing depth. Thus, the upper zone of maximum salinity and the lower zone of minimum salinity is separated by a transition zone which is called as thermocline zone, on an average above which high salinity is found while low salinity is found below this zone. It may be remembered that this should not be taken as a general rule because the vertical distribution of salinity is very complicated.
- It may be mentioned that the depth zone of oceans between 300m and 1000m is characterized by varying trends of vertical distribution of temperature, density of seawater, and salinity of ocean water.
Ocean current, stream made up of horizontal and vertical components of the circulation system of ocean waters that is produced by gravity, wind friction, and water density variation in different parts of the ocean. Ocean currents are similar to winds in the atmosphere in that they transfer significant amounts of heat from Earth’s equatorial areas to the poles and thus play important roles in determining the climates of coastal regions. In addition, ocean currents and atmospheric circulation influence one another.
The general circulation of the oceans defines the average movement of seawater, which, like the atmosphere, follows a specific pattern. Superimposed on this pattern are oscillations of tides and waves, which are not considered part of the general circulation. There also are meanders and eddies that represent temporal variations of the general circulation. The ocean circulation pattern exchanges water of varying characteristics, such as temperature and salinity, within the interconnected network of oceans and is an important part of the heat and freshwater fluxes of the global climate. Horizontal movements are called currents, which range in magnitude from a few centimetres per second to as much as 4 metres (about 13 feet) per second. A characteristic surface speed is about 5 to 50 cm (about 2 to 20 inches) per second. Currents generally diminish in intensity with increasing depth. Vertical movements, often referred to as upwelling and downwelling, exhibit much lower speeds, amounting to only a few metres per month. As seawater is nearly incompressible, vertical movements are associated with regions of convergence and divergence in the horizontal flow patterns.
Distribution Of Ocean Currents
Maps of the general circulation at the sea surface were originally constructed from a vast amount of data obtained from inspecting the residual drift of ships after course direction and speed are accounted for in a process called dead reckoning. This information is collected by satellite-tracked surface drifters at sea at present. The pattern is nearly entirely that of wind-driven circulation.
At the surface, aspects of wind-driven circulation cause the gyres (large anticyclonic current cells that spiral about a central point) to displace their centres westward, forming strong western boundary currents against the eastern coasts of the continents, such as the Gulf Stream–North Atlantic–Norway Current in the Atlantic Ocean and the Kuroshio–North Pacific Current in the Pacific Ocean. In the Southern Hemisphere the counterclockwise circulation of the gyres creates strong eastern boundary currents against the western coasts of continents, such as the Peru (Humboldt) Current off South America, the Benguela Current off western Africa, and the Western Australia Current. The Southern Hemisphere currents are also influenced by the powerful, eastward-flowing, circumpolar Antarctic Current. It is a very deep, cold, and relatively slow current, but it carries a vast mass of water, about twice the volume of the Gulf Stream. The Peru and Benguela currents draw water from this Antarctic current and, hence, are cold. The Northern Hemisphere lacks continuous open water bordering the Arctic and so has no corresponding powerful circumpolar current, but there are small cold currents flowing south through the Bering Strait to form the Oya and Anadyr currents off eastern Russia and the California Current off western North America; others flow south around Greenland to form the cold Labrador and East Greenland currents. The Kuroshio–North Pacific and Gulf Stream–North Atlantic–Norway currents move warmer water into the Arctic Ocean via the Bering, Cape, and West Spitsbergen currents.
In the tropics the great clockwise and counterclockwise gyres flow westward as the Pacific North and South Equatorial currents, Atlantic North and South Equatorial currents, and the Indian South Equatorial Current. Because of the alternating monsoon climate of the northern Indian Ocean, the current in the northern Indian Ocean and the Arabian Sea alternates. Between these massive currents are narrow eastward-flowing countercurrents.
Other smaller current systems found in certain enclosed seas or ocean areas are less affected by wind-driven circulation and more influenced by the direction of water inflow. Such currents are found in the Tasmanian Sea, where the southward-flowing East Australian Current generates counterclockwise circulation, in the northwestern Pacific, where the eastward-flowing Kuroshio–North Pacific current causes counterclockwise circulation in the Alaska Current and Aleutian Current (or Subarctic Current), in the Bay of Bengal, and in the Arabian Sea.
Deep-ocean circulation consists mainly of thermohaline circulation. The currents are inferred from the distribution of seawater properties, which trace the spreading of specific water masses. The distribution of density is also used to estimate the deep currents. Direct observations of subsurface currents are made by deploying current meters from bottom-anchored moorings and by setting out neutral buoyant instruments whose drift at depth is tracked acoustically.
Causes Of Ocean Currents
Following are some of the important causes of ocean currents:
The hydrostatic pressure, p, at any depth below the sea surface is given by the equation p = gρz, where g is the acceleration of gravity, ρ is the density of seawater, which increases with depth, and z is the depth below the sea surface. This is called the hydrostatic equation, which is a good approximation for the equation of motion for forces acting along the vertical. Horizontal differences in density (due to variations of temperature and salinity) measured along a specific depth cause the hydrostatic pressure to vary along a horizontal plane or geopotential surface, a surface perpendicular to the direction of the gravity acceleration. Horizontal gradients of pressure, though much smaller than vertical changes in pressure, give rise to ocean currents.
Earth’s rotation about its axis causes moving particles to behave in a way that can only be understood by adding a rotational dependent force. To an observer in space, a moving body would continue to move in a straight line unless the motion were acted upon by some other force. To an Earth-bound observer, however, this motion cannot be along a straight line because the reference frame is the rotating Earth. This is similar to the effect that would be experienced by an observer standing on a large turntable if an object moved over the turntable in a straight line relative to the “outside” world. An apparent deflection of the path of the moving object would be seen. If the turntable rotated counterclockwise, the apparent deflection would be to the right of the direction of the moving object, relative to the observer fixed on the turntable.
Movement of water through the oceans is slowed by friction, with surrounding fluid moving at a different velocity. A faster-moving fluid layer tends to drag along a slower-moving layer, and a slower-moving layer will tend to reduce the speed of a faster-moving layer. This momentum transfer between the layers is referred to as frictional forces. The momentum transfer is a product of turbulence that moves kinetic energy to smaller scales until at the tens-of-microns scale (1 micron = 1/1,000 mm) it is dissipated as heat. The wind blowing over the sea surface transfers momentum to the water. This frictional force at the sea surface (i.e., the wind stress) produces the wind-driven circulation. Currents moving along the ocean floor and the sides of the ocean also are subject to the influence of boundary-layer friction. The motionless ocean floor removes momentum from the circulation of the ocean waters.
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