The ocean surface has highs and lows, similar to the hills and valleys of Earth’s land surface, called as ocean surface topography or dynamic topography. These are mapped using measurements of sea surface height relative to Earth’s geoid. Ocean surface topography is specifically the distance between the height of the ocean surface from the geoid. Ocean surface topography is caused by ocean waves, tides, currents, and the loading of atmospheric pressure. The main purpose to measure ocean surface topography is to understand the large-scale circulation of the ocean.
The submarine relief may be classified into following categories:
1. Continental Shelf
2. Continental Slope
3. Continental Rise
4. Deep-Ocean Basin (Abyssal Plains)
5. Submarine Ridges
6. Oceanic Trenches
7. Submarine Canyons
8. Island Arcs
1. Continental Shelf
– The submerged margins of continental mass extending from the shore to the first prominent break are sloped usually at a depth of about 120 m.
– Continental shelf occupies about 7.6% of the ocean floor.
– The width of the continental shelf varies considerably. The largest shelf – the Siberian Shelf in the Arctic Ocean – stretches to 1,500 kilometers in width.
– Continental shelves are well known for oil and natural gas and mineral deposits. Coral reefs and bio-classic materials are also common on continental shelves.
– The coastal states exercise sovereign rights over the continental shelf for the purpose of exploring it & exploiting its natural resources.
2. Continental Slope
– The slope that extends from a continental shelf down to the ocean deep is the continental slope.
– Continental slopes occupy about 8.5% of the total area of the oceans.
– The gradient of the slope is lowest off stable coasts without major rivers and highest off coasts with young mountain ranges and narrow continental shelves.
– The boundary between the continental slope and the continental shelf is known as the shelf break.
– These are characterized by submarine canyons.
3. Continental Rise
– The continental rise is found between the continental slope and the abyssal plain. It represents the final stage in the boundary between continents and the abyssal plain.
– Accumulated sediments can be found at the bottom of the continental rise.
– The general slope of the continental rise is between 0.5 degrees and 1.0 degrees. Deposition of sediments at the mouth of submarine canyons may form enormous fan-shaped accumulations called submarine fans.
– Submarine fans form part of the continental rise. Beyond the continental rise stretches the abyssal plain, an extremely deep and flat area of the sea floor.
– Together, the continental shelf, continental slope, and continental rise are called the continental margin. Continental margins constitute about 28% of the oceanic area.
4. Abyssal Plains
– An abyssal plain is a deep ocean floor, usually found at depths between 3,000 metres and 6,000 metres. Abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest and least explored regions on Earth.
– The creation of the abyssal plain is the end result of spreading of the seafloor (plate tectonics) and melting of the lower oceanic crust.
– They result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt.
– In addition to their high biodiversity, abyssal plains are of great current and future commercial and strategic interest.
– Sediments of certain abyssal plains contain abundant mineral resources, notably polymetallic nodules. These potato-sized concretions of manganese, iron, nickel, cobalt, and copper, distributed on the seafloor at depths of greater than 4000 meters, are of significant commercial interest.
5. Submarine Ridges
– A mid-ocean ridge is an underwater mountain system formed by plate tectonics. The various mountains are linked in chains, typically having a valley known as a rift.
– This type of oceanic mountain ridge is characteristic of an oceanic spreading center, which is responsible for seafloor spreading.
– These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains. Running for a total length of 75,000 km, these ridges form the largest mountain systems on the earth.
– A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.
– The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. The continuous mountain range is 65,000 km long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km.
6. Oceanic Trenches
– The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor.
– They are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates varying from a few mm to over ten cm per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab.
– Trenches are generally parallel to a volcanic island arc.
– The bottom temperature of trenches is very cold (between 1° to 4°C).
– Great earthquakes and Tsunamis are born in the ocean trenches.
7. Submarine Canyons
– A submarine canyon is a steep-sided V-shaped valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls.
– These 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 finally deposit sediment onto the abyssal plain, where the particles settle out.
– The formation of submarine canyons occurs as the result of two main processes: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of the continental slope.
– Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins. Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins.
8. Island Arcs
– An island arc is a type of archipelago, often composed of a chain of volcanoes, situated parallel and close to a boundary between two converging tectonic plates.
– Most island arcs consist of two parallel, arcuate rows of islands. The inner row of such a double arc is composed of a string of explosive volcanoes, while the outer row is made up of non-volcanic islands. In the case of single arcs, many of the constituent islands are volcanically active.
– An island arc typically has a land mass or a partially enclosed, unusually shallow sea on its concave side. Along the convex side there almost invariably exists a long, narrow deep-sea trench.
– Destructive earthquakes occur frequently at the site of island arcs.
Temperature of the oceans is not uniform. It is heated from the surface downward by sunlight, but at depth most of the ocean is very cold.
The temperature of the surface waters varies mainly with latitude. The polar seas (high latitude) can be as cold as -2°C (28.4°F) while the Persian Gulf (low latitude) can be as warm as 36°C (96.8°F). The average temperature of the ocean surface waters is about 17°C (62.6°F).
DISTRIBUTION OF OCEAN WATER TEMPERATURE
The horizontal distribution of the temperature of ocean water depends upon the following factors:
(1) Latitudinal Distance: Temperature of the ocean water decreases as we move away from the equator. The average temperature of ocean water is 26°C in open seas at equator but the temperature decreases to 23°C at 20° North and South latitudes. Temperature further decreases to 14°C at 40° latitudes and to 1°C at 60° latitudes.
(2) Change of Season: The effect of season is far more pronounced in air than in water. Ocean water records a seasonal range of only 1.2° between 20° and 30° latitudes. The range is still 1.2°C beyond 50° latitudes. The greatest range is found near New Foundland (4.5°C).
(3) Enclosed Seas: The highest temperature of ocean water is found in enclosed or partially enclosed seas in tropical areas. For example a temperature of 38°C has been recorded in Red Sea though the average temperature in summer is only 29°C.
(4) The Effect of Ocean Currents: The temperature of warm current is higher than that of the surrounding areas. The warm currents keep the coastal lands warmer. For example, the Gulf Stream does not allow the Norway Coast to freeze even in winter and thus helps the development of trade and commerce in that country. The temperature between Davis Strait and New Foundland drops down because of cold Labrador current washing the coasts.
(5) Prevailing Winds: The prevailing winds deflect the warm and cold currents and causes change in temperature of ocean water. For example, the currents on the east coast in the Trade Wind Belt shift away from the coast. Hence, the warm currents flowing along the coast moves away from it which leads to the upwelling of cold water from below near the coast. Hence the temperature remains low in spite of the passage of warm currents. This is why the temperature remains lower on the eastern than on western parts of the oceans.
(6) The effect of Land Masses: The small seas are affected by the adjacent land masses. The temperature rises in summer and falls in winter because of the influence of the land masses.
(7) Iceberg: Icebergs are found near polar areas and can be seen to be floating up to 50° latitudes. One part of iceberg is above sea and eight parts remain submerged under sea water. Many icebergs have a height of hundreds of metres above sea level. Thousands of icebergs can bee seen moving away from North Atlantic. The Falkland and Benguela currents carry them to far off places. It lowers the temperature of the water at great depth.
HORIZONTAL DISTRIBUTION OF TEMPERATURE
• Oceans temperature decreases from equator towards poles. In the equatorial region the oceans temperature remains around 26°C at 23° latitude, 20°C at 40° latitude and at 60° around 1°C.
• The annual range of temperature is low in the Oceans. It is, however, greater in the Atlantic Ocean than in the Pacific and Indian Oceans. The greatest annual range of temperature (about 25°C) is in the N.W. Atlantic (near New Foundland).
• The annual range of temperature is greater in the Northern Hemisphere. The highest surface water temperature is recorded in the Red sea.
VERTICAL DISTRIBUTION OF TEMPERATURE
The factors affecting vertical distribution of temperature are:
• Upwelling of cold water.
• Sinking of dense surface water.
• Cold and warm currents.
• Regional insolation.
• Submarine topography.
• Open and enclosed seas.
Vertically the oceans are divided into surface zone, thermocline zone, & deep zone.
• Surface Zone – Consists of oceans least dense water and it accounts for about 2% of total ocean volume. Surface zone is about up to 200 m depth. The temperature slowly decreases in this zone.
• Thermocline Zone – A thermocline is the transition layer between warmer mixed water at the ocean’s surface and cooler deep water below. The temperature fall in this zone is steady. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude and local environmental conditions, such as tides and currents.
It is semi-permanent in the tropics, variable in temperate regions (often deepest during the summer), and shallow to nonexistent in the polar regions. Thermoclines also play a role in meteorological forecasting, e.g. hurricane forecast.
• Deep Zone – Almost 90% of the total volume of ocean is found below the thermocline in the deep ocean. The deep ocean is not well mixed. The deep ocean is made up of horizontal layers of equal density. The water in the deep zone is very cold but above 1°C.
The temperature in the enclosed seas remains generally uniform (in the Red Sea around 21°C, in the Mediterranean Sea around 13°C).
The salinity of water is usually expressed in parts per thousand by weight (%) and is due to the presence of compounds of sodium, potassium, magnesium, calcium and other elements including a high proportion of sodium chloride (common salt). Rivers derive minerals from rocks and carry them to the sea. The salinity varies with the amounts of salts contributed, addition of fresh water by rainfall, rivers, and melting ice, and also with the rate of evaporation: rapid evaporation can cause relatively high surface salinity in open oceans
The concentration of natural elements and compounds dissolved in solution, as solute, is known as salinity. It is measured by weight (in parts per thousand 0/00) in sea-water. Much of the ocean salt originates from land. The volcanoes and decomposed plants and animals are the others sources of salt.
Distribution of salinity
The salinity of the oceans varies according to temperature, supply of fresh water, rainfall, ocean currents, & degree of mixing of surface and sub-surface water.
Reasons for the Varying Salinity of the Sea Water are:
(1) Evaporation and Precipitation: Evaporation causes concentration of salt. Highest salinity is found near the tropics, because of active evaporation owing to clear sky, high temperature and steady trade winds. Salinity decreases towards the equator because of heavier rainfall. In the Atlantic Ocean the salinity near the tropics is 37% and near the equator it is only 35%.
(2) Stream run off: The areas which receive fresh water by rivers have low salinity, e.g., huge amount of fresh water brought by the Danube, the Dneiper and the Don into the Black Sea reduces its salinity to 17%.
(3) Freezing and Melting of Ice: In the polar areas, there is very little evaporation and this coupled with the melting of ice, yielding fresh water, leads to a decrease in salinity, usually between 20% and 32%.
(4) Atmospheric Pressure and Wind Direction: Salinity changes slightly due to winds resulting from differences in atmospheric pressure. Of the Californian coast, North East Trade winds carry the warm saline water far off the coast and consequently colder and less saline water start upwelling from below.
(5) Ocean Currents: The currents, stirred by wind, sweep away saline water from the eastern coast of the high latitudes to the western coasts, whereas cold water penetrates into the low latitudes. Thus there is a tendency for salinity to increase from east to west. Salinity is higher in enclosed seas as compared to open seas. The salinity of the Red Sea is 40% and that of the Dead Sea is 2.28%. This is because of high rate of evaporation, lack of supply of fresh water in enclosed seas.
• The highest salinity in the ocean is found between 20°N and 40°N as this zone has high temperature, high rate of evaporation, low rainfall and excessive arid conditions.
• The salinity for normal open oceans ranges between 330/00 and 370/00. In the land locked Red Sea it is as high as 410/00. In the estuaries (enclosed mouth of a river where fresh and saline water get mixed) and the Arctic, the salinity fluctuates from 0 – 350/00.
• Temperature and density share an inverse relationship; as temperature increases, the density decreases. Salinity and density share a positive relationship.
• Salinity can decrease from the melting of polar ice or increase from the freezing of polar ice. Evaporation increases salinity and density while the addition of freshwater decreases salinity and density.
• The difference in density of cold water versus density of warmer water is responsible for ocean currents and upwelling.