🌊WATER (Oceans)
Hydrological Cycle
i). Water is a cyclic resource. It can be used and re-used.
ii). The hydrological cycle, is the circulation of water within the earth’s hydrosphere in different forms i.e. the liquid, solid and the gaseous phases. It also refers to the continuous exchange of water between the oceans, atmosphere, land surface and subsurface and the organisms.
iii). The remaining is held as freshwater in glaciers and icecaps, groundwater sources, lakes, soil, moisture, atmosphere, streams and within life.
iv). Nearly 59 percent of the water that falls on land returns to the atmosphere through evaporation from over the oceans as well as from other places. The remainder runs-off on the surface, infiltrates into the ground or a part of it becomes glacier.
v). It is to be noted that the renewable water on the earth is constant while the demand is increasing tremendously. This leads to water crisis in different parts of the world - spatially and temporally. The pollution of river waters has further aggravated the crisis.
vi). Water on the earth surface in terms of percentage are: Oceans - 97.25%, Icecaps and glaciers-2.05%, Ground water - 0.68%, Lakes -0.01%, Soil moisture - 0.005%, Atmosphere - 0. 001%, Streams and rivers - 0.0001% and Biosphere - 0.00004%.
vii). Components and processes of the water cycle are:
Components || Processes
Water storage in oceans || Evaporation
Evapotranspiration
Sublimation
Water in the atmosphere || Condensation
Precipitation
Water storage in ice and snow || Snowmelt runoff to streams
Surface runoff || Stream flow freshwater storage infiltration
Groundwater storage || Groundwater discharge springs
Relief Of The Ocean Floor
i). The oceans are confined to the great depressions of the earth’s outer layer.
ii). 4 oceans - Pacific, Atlantic, Indian, and Arctic. The various seas, bays, gulfs and other inlets are parts of these four large oceans. A major portion of the ocean floor is found between 3-6 km below the sea level.
iii). The ‘land’ under the waters of the oceans i.e, the ocean floor exhibits complex and varied features as those observed over the land.
iv). The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These features are formed, like those of the continents, by the factors of tectonic, volcanic and depositional processes.
Division of the Ocean Floor
The ocean floors can be divided into four major divisions: Continental shelf, Continental slope, The deep sea plain, The oceanic deeps. Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, sea mounts, guyots, trenches, canyons, etc.
The Continental Shelf
(a). It is the extended margin of each continent occupied by relatively shallow seas and gulfs.
(b). It is the shallowest part of the ocean showing an average gradient of 1° or even less.
(c). The shelf typically ends at a very steep slope, called the shelf break.
(d). The width varies with average width of 80 km.
(e). Siberian Shelf in the Arctic Ocean are largest with 1500 km.
(f). Depth can vary from 30 to 600m. Sedimentary deposits received become a source of fossil fuels.
Continent Slope
(a). The continental slope connects the continental shelf and the ocean basins. It begins where the bottom of the continental shelf sharply drops off into a steep slope.
(b). The gradient of the slope region varies between 2-5°.
(c). The depth of the slope region varies between 200 and 3,000 m.
(d). The slope boundary indicates the end of the continents.
(e). Canyons and trenches are observed in this region.
Deep Sea Plain
(a). Deep sea plains are gently sloping areas of the ocean basins.
(b). These are the flattest and smoothest regions of the world.
(c). The depths vary between 3,000 and 6,000m.
(d). These plains are covered with fine-grained sediments like clay and silt.
Ocean Deeps or Trenches
(a). These areas are the deepest parts of the oceans.
(b). The trenches are relatively steep sided, narrow basins.
(c). They are some 3-5 km deeper than the surrounding ocean floor.
(d). They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes, due to which they are significant in the study of plate movements.
(e). As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.
Minor Relief Features
Mid-oceanic Ridges :
(a). A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
(b). The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
(c). Iceland, a part of the mid Atlantic Ridge, is an example.
Submarine Canyons
(a). These are deep valleys, some comparable to the Grand Canyon of the Colorado river.
(b). They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.
(c). The Hudson Canyon is the best known submarine canyon in the world.
Guyots
(a). It is a flat topped seamount.
(b). They show evidences of gradual subsidence through stages to become flat topped submerged mountains.
(c). It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Atoll
(a). These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
(b). It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.
Seamount
(a). It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall.
(b). The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is an example.
Temperature Of Ocean Waters
i). Ocean waters get heated up by solar energy just as land.
ii). The process of heating and cooling of the oceanic water is slower than land.
Factors Affecting Temperature Distribution
The factors which affect the distribution of temperature of ocean water are:
i). Latitude: The temperature of the surface decreases from equator to poles (due to the amount of insolation).
ii). Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat (due to larger extent of land).
iii). Prevailing wind: The winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below.
iv). Ocean currents: Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. (Gulf stream is warm current while Labrador current is cold current).
v). Enclosed sea in low latitude record higher temperature than open sea while enclosed sea in high latitude have lower temperature
Horizontal And Vertical Distribution Of Temperature
i). The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth.
ii). The profile shows a boundary region between the surface waters of the ocean and the deeper layers.
iii). The boundary usually begins around 100 - 400 m below the sea surface and extends several hundred of metres downward This boundary region, from where there is a rapid decrease of temperature, is called the thermocline. About 90 per cent of the total volume of water is found below the thermocline in the deep ocean. In this zone, temperatures approach 0° C.
iv). The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
v). The first layer represents the top layer of warm oceanic water and it is about 500m thick with in temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid latitudes it develops only during summer.
vi). The second layer called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
vii). The third layer is very cold and extends upto the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight. Here, only one layer of cold water exists, which extends from surface to deep ocean floor.
viii). Average temperature of ocean surface water is 27 degree Celsius and decreases from poles to equator. Rate of decline is 0.5 degree Celsius with latitude.
ix). Oceans of North Hemisphere record higher temperature than south hemisphere (due to unequal distribution of land and sea).
x). Maximum temperature of the ocean near the surface as it directly receives heat from the sun and it is transmitted to lower sections by conduction. It results in decrease in temperature with depth.
Salinity Of Ocean Waters
i). Salinity is the term used to define the total content of dissolved salts in the sea.
ii). It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.
iii). It is usually expressed as parts per thousand or ppt.
iii). Salinity of 24.7 %has been considered as the upper limit to demarcate ‘brackish water’.
Factors Affecting Ocean Salinity
i). The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.
ii). Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in Polar Regions by the processes of freezing and thawing of ice.
iii). Wind, also influences salinity of an area by transferring water to other areas.
iv). The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Highest salinity in water bodies are lake Van in Turkey, Dead Sea, Great Salt Lake.
v). Dissolved salts in seawater (gm of salt per kg of water) are: Chlorine - 18.97, Sodium - 10.47, Sulphate - 2.65, Magnesium - 1.28, Calcium - 0.41, Potassium - 0.38, Bicarbonate - 0.14, Bromine - 0.06, Borate - 0.02, Strontium -0.01.
Distribution of Salinity
Horizontal Distribution Of Salinity
i). It ranges from 33 and 37 with Red sea with 41 and Arctic with 0 to 35.In hot areas it can reach 70.
ii). Salinity in Western Pacific decrease due to influx of melted water from Arctic north (decreases from 35 to 31).
iii). Average salinity of the Atlantic is 36 with maximum between 15 to 20 degree latitudes and decreases to the north.
iv). The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift.
v). Baltic Sea and Black Sea records low salinity due to influx of river water in large quantity.
vi). Mediterranean Sea has higher salinity due to high evaporation.
vii). Salinity in Indian Ocean is 35 – low due to influx of river Ganga and low salinity in the Bay of Bengal.
viii). Salinity in Arabian sea is high due to higher evaporation and low influx of water.
Vertical Distribution Of Salinity
i). Changes with depth, but the way it changes depends upon the location of the sea.
ii). Salinity at the surface salinity increases due to loss of water to ice and evaporation or decreases due to freshwater input. At depth it remains fixed. Low salinity water rests above the higher salinity dense waters.
iii). Salinity increases with depth and there is a distinct zone called the halocline, where salinity increases sharply. Increasing salinity of seawater causes its density to increase. High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.
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