GC Leong Summary: The Oceans | Geography for UPSC CSE PDF Download

Continental Shelf

• 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.

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.

• Beyond the continental slope is continental rise, an area of gentle slope with angle varying from 0.5 - 1*.
• With increasing depth, it virtually becomes flat & merges with 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.

• 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)
• 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 / 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.

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

• 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.

• 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.
  - 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 Nad > Mgcl2 > MgS04 > CaS04 > CaC03 > 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 (238) > Great Salt lake, US (220)

• 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

• A boundary region in oceans, from where, rapid decrease in temperature starts generally at 0*C.
• 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.

• Vertical Movement → Tides
• Horizontal Movement → Ocean waves & Ocean currents

• Periodic rise & fall in level of water in the oceans mainly caused by gravitational pull of sun & moon.
• Due to the large size of sun it should attract more but because of its greater distance from the earth (1.5 x 10A8 km), it is unable to exert much influential gravitational force.
• Whereas, moon is much smaller than the sun but attracts more due to its lesser distance from the earth (4.07 x 10A5 km).
• Hence, it produces a force approx. 2.17 times stronger than the sun.
• Centrifugal force caused by earth’s rotation is another major cause of tides formation.
• Together the gravitational pull & centrifugal forces are responsible for creating two major tidal bulges on earth.
• On side facing the moon, tidal bulge occurs due to gravitational pull of the moon whereas on opposite side, tidal bulge occurs due to centrifugal force of rotation of the earth.

• On full moon & new moon, moon & sun are almost in line with the earth.
• Hence exerts combine pull on the earth.
• On these 2 days tides are highest & called Spring tides.

• When moon is in the 1st quarter or last quarter, sun & moon makes right angle to each other, with earth as center.
• Hence attraction of sun & moon balances each other, which results in low amplitude tides known as Neap tides.

• Oscillatory movements in water manifested by alternate rise & fall of sea surface.
• Formed mainly by friction & pressure of wind on sea surface.
• Water in waves does not move, only wave train move.
• Means if we throw a cork in water, it will move up & down (to & fro) with approaching wave but will not travel unless blown away by the wind.

• Ocean currents are the large masses of surface water that circulates in regular pattern around the oceans.
• Only water in current moves forward while water on its sides & below remains almost stationary.
• Those that flow from equatorial region polewards have a higher surface temperature & are warm currents Those that flow from Polar Regions equator ward have a lower surface temperature & are cold currents.

• Prevailing winds, salinity differences of water, Density, Temp, difference, Evaporation, Rotation & revolution of earth, Shape of coasts, coriolis force, gravitational force.
• Prevailing winds are the major factors as they blow consistently in one direction & drag the surface water due to force of friction.
• There is much difference in the temp, of ocean water at the equator & at the poles.
• As warm water is lighter & rises, and cold water is denser & sinks; warm equatorial water move slowly along the surface polewards, while the heavier cold water of polar regions creeps slowly along the bottom of the sea equator wards.
• Waters of high salinity is denser than the waters of low salinity, hence the waters of low salinity flow on the surface of water of high salinity, while water of high salinity flow at the bottom towards the water of low salinity.

• The steady trade winds constantly drift two streams of water from east to west as North equatorial current & South equatorial current.
• Between the west flowing North & South Equatorial currents is the east flowing Equatorial counter current.

• In the North Atlantic Ocean, the Cayenne current is joined & reinforced by North equatorial current.
• Heads north-westward as a large mass of equatorial water into the Caribbean Sea.
• Part of the current enters the Gulf of Mexico & emerges from the Florida straight between Florida & Cuba as Florida current.
• The rest of the equatorial water flows northwards, east of Antilles to join the Gulf Stream off the SE USA.
• Gulf Stream hugs the coast of America as far as Cape Hartteras, where it get deflected eastwards under the combined effect of westerlies & rotation of the earth; and reaches Europe as North Atlantic drift.
• From the North Atlantic, it fans out in three directions viz. eastward to Britain, northward to Arctic & southward along the Iberian coast as the cool Canaries current.
• The Canaries current flowing southward eventually merges with North equatorial current, completing the clockwise circuit in the North Atlantic ocean
• Within this ring of currents, an area in the middle of Atlantic has no perceptible current.
• A large amount of floating seaweed gathers in here & the area is called Sargasso Sea.
• Apart from the clockwise circulation of currents, there are also cold currents that enter the North Atlantic from the Arctic regions, which are blown south by out flowing polar winds.
• The Irminger current or East Greenland current flows between Iceland & Greenland & cools the North Atlantic drift at the point of convergence.
• The cold Labrador current drifts south-eastwards between west Greenland & Baffin island to meet the warm Gulf stream off Newfoundland.

• At the shoulder of NE Brazil, the protruding land mass splits the South equatorial current into —
• Cayenne current which flows along the Guiana coast
• Brazilian current which flows southwards along the coast of Brazil
• The South Atlantic Ocean follows the same pattern but the major differences are the circuit is anti-clockwise here & collection of seaweed in mid-south Atlantic is not so distinctive
• Where the South equatorial current is split (at Cape Sao Roque), one branch turns south as the warm Brazilian current
• At about 40*S, the influence of westerlies & rotation of the earth, propels the current eastward to merge with the cold South Atlantic current.
• On reaching the west coast of Africa, the current is diverted northward as the cold Benguela current
• Driven by the regular SE trade winds, the Benguela current surges equator wards to join South equatorial current.
• This completes the circulation of the currents in South Atlantic.

• Deepest + Largest + Triangular in shape + l/3rd of Earth’s area
• The NE trade winds blow the North equatorial current off the coast of the Philippines & Formosa into the East China Sea as the Kuroshio or Japan current.
• It moves poleward as North Pacific drift, keeping the ports of Alaskan coast ice free in winters.
• The cold Bering current or Alaskan current creeps southwards from the narrow Bering strait & is joined by Okhotsk current, to meet the warm Japan current as cold Oyashio current.
• The cold water eventually sinks below the warm water of North Pacific drift & part of Oyashio moves eastward as the cold Californian current along the coast of western USA, which finally coalesces with the North Equatorial current to complete the clockwise circulation.
• The South equatorial current, driven by the SE trade winds, flows southward along the coast of Queensland as the East Australian current.
• The current turns eastward towards New Zealand under the full effect of westerlies in the Tasman Sea & merges with south pacific current.
• Obstructed by the tip of southern Chile, the current turns northward along the western coast of South America as the cold Humboldt Current or Peruvian current.
• The Peruvian current eventually links up with the South Equatorial current & completes the cycle of currents in South Pacific.

• The equatorial current, turning southward past Madagascar as the Agulhas or Mozambique current merges with the South Pacific current.
• Flowing eastward, it turns equatorward as cold West Australian current.
• In north Indian Ocean, there is complete reversion of direction of currents between summers & winters, due to change of monsoon winds.
• In summers, under the effect of SW monsoon, it flows clockwise, & in winters under the effect of NE monsoon, it flows anti-clockwise.
• Ideating by solar rays’ causes expansion of water that is why near the equator ocean water is 8 cm higher in level compared to mid latitudes.