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Oceanography by Pavan K

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Oceanography By Pavan K. Tiwari 2/10/2017 Pragnya IAS Academy Bangalore [Type text] Page 1 Hydrosphere – Ocean Floor Division Ocean Floor Division Continental Self . Basically the seaward extension of the continent from the shoreline to the continental edge approx. 200 m deep . Continental shelf is thus a shallow platform, whose width varies greatly from a few miles in North Pacific off the continent of North America to over 100 miles off North West Europe . In some places, where the coasts are extremely mountainous, such as at Rocky Mountains, the continental shelf may be entirely absent . The angle of slope is also variable with most common of 1 in 500 & is normally the least, where the continental shelf is widest . Continental shelves may be formed by submersion of part of continent due to increase in sea level or by wave erosion or conversely by off-shore deposition . Continental shelves shallowness enables sunlight to penetrate through the water, which encourages the growth of minute plants & other microscopic organisms such as planktons, on which millions of fishes & polyps survive. The continental shelves are therefore the richest fishing grounds in the world. Pavan K Tiwari Page 2 Continental Slope . At the southern edge of continental self, seaward slope immediately becomes steep & gradient to about 1 in 20 with average depth 200 – 3000 m. Most of the canyons & trenches are observed in this region Continental Rise . Beyond the continental slope is continental rise, an area of gentle slope with angle varying from .5 – 1* . With increasing depth, it virtually becomes flat & merges with abysmal plains Pavan K Tiwari Page 3 Abysmal Plains . Where the continental rise end, deep sea plains begin covering 2/3rd of the ocean floor . Average depth is of 3000 – 6000 m . Flattest & smoothest surfaces of the world Some important terms Oceanic Deeps or Trenches . Deepest part of the oceans . Generally steep sided narrow basins, deeper then oceanic floors . Occurs at the base of continental slopes & associated with active volcanos & strong earthquakes . Hence, Contrary to our expectations, most of the deepest trenches are not located in the midst of the oceans but found close to the continents . Deepest trench of the world → Marina trench (11 km) of Guam islands (Pacific Ocean) Pavan K Tiwari Page 4 . Hence we can see that ocean trenches are greater in magnitude than the highest mountains on the land There are thousands of hills on ocean floors which are submerged under ocean water. A submarine peak rising > 1000 m above the ocean floor is called a seamount. Flat topped seamounts are known as Guyots (Volcanic in origin) Gorge vs Canyon . Gorge / Trench → Narrow & steep side valley formed by down cutting action of river . Canyon → Magnified form of Gorge. Ex. Grand canyon of river colarado (US) → Largest one Difference between Straits & Isthmus Isthmus . Opposite of strait i.e. narrow channel of land Straits connecting 2 large water bodies . Narrow channel of water connecting 2 large land bodies . Panama Canal crosses Isthmus of Panama, connecting the North Atlantic & Pacific . Strait of Gibraltar →Connects Atlantic ocean & Oceans Mediterranean sea and separates Spain (Europe) from Morocco (Africa) . Suez Canal crosses isthmus of Sinai . Palk Strait → Separates India & Srilanka Peninsula connecting Mediterranean Sea & Indian Ocean Oceanic Deposits on Ocean Floor . These are terrigenous deposits as they are derived from land Muds . Are mainly deposited on the continental shelves . These are pelagic deposits as they are derived from the oceans . They are made of shelly & skeletal remains of marine micro-organisms with calcareous or siliceous parts Oozes . Oozes have very fine, flour like texture which either occur as accumulated deposits or Pavan K Tiwari Page 5 float as suspension . These occur mainly as red clays in the deeper parts of ocean basins . Are particularly abundant in Pacific Ocean . Red clay is believed to be an accumulation of volcanic dust blown out from volcanoes Clays during volcanic eruptions Pavan K Tiwari Page 6 Hydrosphere – Albedo, Fresh Water, Salinity & Temperature Water on Earth . 71 % of earth surface constitutes water & only 29 % constitutes land area of earth . 97 % of water area constitutes Oceans & Seas & 2.5 % as fresh water . 75 % of fresh water in glaciers & ice caps . 7 % of fresh water as groundwater . Fresh Water → Glaciers > Ground water > Ice & Snow > Lakes > soil moisture > atmosphere > Rivers Albedo . The fraction of Sun’s radiation reflected from a surface . Originated from the Latin word albus, meaning “white” . It is quantified as the proportion, or percentage of solar radiation of all wavelengths reflected by a body or surface to the amount incident upon it . Albedo → Fresh Snow > Ocean Ice > New Concrete > Desert Sand > Green Grass > Bare Soil > Deciduous Trees > Coniferous Trees > Worn Asphalt > Fresh Asphalt Pavan K Tiwari Page 7 Salinity of the Ocean . All sea water contains large amounts of dissolved mineral matter of which NaCl constitutes more than 77 % . The other important minerals include magnesium, calcium & potassium along with others . Due to free movement of ocean water, the proportion of different salts, remain remarkably constant in all oceans & even to great depths . But the degree of concentration of salt solution does vary appreciably in different areas, expressed as salinity . The average salinity of the oceans is approx. 35 PPT with variations shown in salinity distribution maps as isohalines, lines joining places having equal degree of salinity. Degree of salinity in various oceans & seas is affected mainly by . the rate of evaporation . amount of fresh water added by precipitation . streams & icebergs Pavan K Tiwari Page 8 . degree of water mixing by currents . Average salinity of sea water is 35 gm in 1 kg of water with order of salts is as Nacl > Mgcl2 > MgSO4 > CaSO4 > CaCO3 > MgBr2 . Highest salinity is found at Tropic of cancer & Tropic of Capricorn due to active evaporation, owing to clear skies, high temp. & steady trade winds . From the Tropics, salinity decreases both towards poles & Equator . Salinity increases with depth of the sea & increases rapidly after Halocline . Top 3 Saline Water Bodies → Van Lake, Turkey (330) > Dead Sea, Turkey (240) > Great Salt lake, US (220) Temperature of Ocean Water . Like land masses, ocean water varies in temperature from place to place both at the surface & at great depths . Since water warms up & cools down much more slowly than the land, the annual range of temperature in any part of the ocean is very smaller Thermocline . A boundary region in oceans, from where, rapid decrease in temperature starts generally at 0*C Pavan K Tiwari Page 9 . 90 % of the water is below this region . Generally there are 3 layers in an ocean viz. 1st layer is until ~500 m deep . 2nd layer is from ~500 ~ 4500 m (Thermocline layer) . 3rd layer is till oceanic floor . At Arctic & Antarctic Circle, we have only 1 such layer as surface temp. itself is 0*C with very slight change in temp. Generally, the mean annual temperature of oceans decrease from equatorial areas towards the poles, but the reduction of temperature with latitude is never constant, because of the interference by warm & cold currents, winds and air masses Pavan K Tiwari Page 10 Ocean currents Ocean currents are the most important ocean movements because of their influence on climatology of various regions. Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction. Ocean currents are influenced by two types of forces namely: 1. primary forces that initiate the movement of water; 2. secondary forces that influence the currents to flow. The primary forces that influence the currents are: 1. heating by solar energy; 2. wind; 3. gravity; 4. Coriolis force. The secondary forces that influence the currents are: 1. Temperature difference; 2. Salinity difference Primary Forces Responsible For Ocean Currents Explain the factors responsible for the origin of ocean currents. How do they influence regional climates, fishing and navigation? [Mains 2015] Influence of insolation Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope. The flow is normally from east to west. Influence of wind (atmospheric circulation) Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course. Winds are responsible for both magnitude and direction [Coriolis force also affects direction] of the ocean currents. Example: Monsoon winds are Pavan K Tiwari Page 11 responsible for the seasonal reversal of ocean currents in the Indian ocean. The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern. The air circulation over the oceans in the middle latitudes is mainly anticyclonic [Sub-tropical High Pressure Belt] (more pronounced in the southern hemisphere than in the northern hemisphere due to differences in the extent of landmass). The oceanic circulation pattern also corresponds with the same. At higher latitudes, where the wind flow is mostly cyclonic [Sub-polar Low Pressure Belt], the oceanic circulation follows this pattern. In regions of pronounced monsoonal flow [Northern Indian Ocean], the monsoon winds influence the current movements which change directions according to seasons. Influence of gravity Gravity tends to pull the water down to pile and create gradient variation. Influence of Coriolis force The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere. These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins.
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