Introduction to Drainage System

Introduction to Drainage System

This article deals with ‘Introduction to Drainage System ’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Terminology

Channel A defined zone which has a certain depth in which there is continuous flow of water under normal conditions.
Drainage The flow of water in well defined channel is known as Drainage .  
Drainage System The network of channels which drains a region is known as Drainage System.  
Drainage Basin The drainage basin is the area drained by the river and it’s tributaries.  
Catchment / Catchment Area The river drains the water collected from a specific area. This area is known as the Catchment Area or Catchment .  
Water Divide / Watershed Watershed divides the one catchment area from other catchment area. It is also known as Water divide.  
River Regime The seasonal flow of water in a river is known as River regime. Hence, if the water availability is uniform around the year, it is known as Uniform River Regime. Whereas rivers where the water is seasonal, it is known as Non-uniform River Regime. Such seasonal rivers are also known as Ephemeral rivers.  
Inland Drainage Rivers which don’t reach the ocean . In desert or arid regions, such rivers lead to the formation of Salt Lakes or Playa Lakes .

Types of drainage pattern

Geometric system of streams in a region is determined by.

1. Slope 3. Hydraulic Variability.
2. Difference in rock resistance to erosion 4. Structural  control of Landscape.

1 . Antecedent or Inconsequent

  • Antecedent drainage pattern is one in which a  part of a river slope and the surrounding area gets uplifted but the river sticks to its original slope, cutting through the uplifted portion forming deep gorges
  • Eg : Those rivers which existed before upheaval of Himalayas like Indus, Satluj, Ganga, Brahmaputra, Arun, Tista etc. . These rivers originates in Tibet and cut across Himalayas forming deep gorges.

2. Consequent

  • Rivers which flow in direction of slope.
  • Most peninsular rivers like Godavari, Krishna, Cauvery etc.

3. Superimposed /Superinduced

  • In this, drainage pattern exhibits discordance with underlying rock because it originally developed on a cover of  rocks that has now disappeared .
  • The river has enough erosive power that it can cut through any kind of bedrock, maintaining its former drainage pattern.
  • Damodar, Subarnarekha, Chambal, Banas etc.

4. Dendritic

  • Drainage in which branches give appearance of tree .
  • Dendritic pattern develops in a terrain which has uniform lithology, and where faulting and jointing are insignificant.
  • Most rivers of Indo-Gangetic plain show Dendritic drainage pattern.
Introduction to Drainage System

5. Trellis

  • Rectangular pattern where two sets of structural control occur at right angle.
  • Eg : Drainage pattern of Singhbhum(Chotanagpur).
Trellis Drainage Pattern

6. Obsequent

  • In Obsequent drainage pattern, tributaries intend to flow upstream instead of downstream.
  • Eg : Arun river which is tributary of Kosi & Suru of Indus.

7. Rectangular

  • The main stream bends at right angles and the tributaries join at right angles creating rectangular patterns.
  • It differs from trellis as it is more irregular.
  • It is found in Vindhyan mountains.
Rectangular Drainage Pattern

8. Radial

  • Outflowing rivers , away from central point.
  • Radial pattern tends to develop on flanks of a dome or volcanic cone.
  • Rivers originating from Amarkantak hills , Chotanagpur Plateau & Mikir hills .
Radial Drainage Pattern

9. Annular

  • Subsequent stream flows curving prior to joining the consequent stream.
  • It is not very common . In India, it is found in Nilgiri hills of Tamil Nadu & Kerala.

10 . Parallel

  • Rivers flow parallel to each other. In this, rivers will not meet but keep on running parallel.
  • Small rivers originating in western ghats & discharging water in Arabian sea.

11. Deranged

  • Uncoordinated pattern of drainage, characteristic of a region recently vacated by ice sheet & has  not adjusted according to solid rocks underlying.
  • Eg : Drainage pattern found in glaciated valleys of Karakoram.
Deranged Drainage Pattern

Indian rivers

There are various ways to classify Indian Rivers.

1 . On basis of Discharge of Water

  • All Indian rivers discharge water either in Arabian Sea or Bay of Bengal.
  • Nearly 77%  of the drainage area consisting of the Ganga, the Brahmaputra, the Mahanadi, the Krishna, etc. is oriented towards the Bay of Bengal while 23% comprising the Indus, the Narmada, the Tapi, the Mahi and the Periyar systems discharge their waters in the Arabian Sea.
  • They are separated from each other through the Delhi ridge, the Aravallis and the Sahyadri.
  • It should be mentioned here that the over 90% of the water carried by the Indian rivers is drained into the Bay of Bengal; the rest is drained into the Arabian Sea or forms inland drainage.

2. On Basis of Size of Watershed

Major River Basins Catchment area of more than 20,000 sq. km 14
Medium River Basins 2,000 to 20,000 sq. km 44
Minor River Basins Less than 2,000 sq. km 55

Ranking (catchment area)

Ganga Mahanadi
Indus Narmada
Godavari Kaveri
Krishna Tapi
Brahmaputra Pennar

3. On Basis of Mode of Origin

  1. Northern/Himalayan Rivers
  2. Peninsular Rivers.
    • Although it has the problem of including  Chambal,  Betwa, Son, etc. which are much older in age and origin. than other rivers that have their origin in the Himalayas, it is the most accepted basis of classification.
Rivers 
Himalayan 
Rivers 
Indus 
Ganga 
Brahmaputra 
Peninsular 
Rivers 
Mahanadi 
Godavari 
Krishna 
Cauvery 
Narmada 
Tapti

This marks the end of our article on Introduction to Drainage System. For other articles on geography, CLICK HERE.

Plateaus

Plateaus

This article deals with ‘Plateaus.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Plateau is an elevated tract of relatively flat land , limited on atleast one side by steep slope falling abruptly
  • Eg: Tibetan plateau.

Reasons for formation of Plateaus

There are many reasons for the formation of Plateaus

  • When two mountain ranges are forming, then landmass in between them rise too &  Plateaus are formed .
  • Deposition from lava – if lava is basaltic it will spread easily &  form flat elevated surface
  • Deposition from wind over long time => After compression & solidification of deposited material plateau will form.
  • When upland of any surface is eroded due to glaciers, plateau is formed.

Types of Plateaus

1 . Intermontane plateau

  • Intermontane plateaus are  highest, largest & most complex plateaus of world.
  • Intermontane plateaus are enclosed and surrounded by mountain ranges from different sides.

Examples include

a. Tibetan Plateau

  • Stretches 1000 km north to south &  2500 km east to west & average elevation is 4500m  => called Roof of the world .
  • Bounded by Kunlun Mountains in the north & Himalayas in the south. 
  • Many major rivers of Asia like Indus, Brahmaputra etc rise here & also holds constellation of salt & freshwater lakes .

b. Plateau of Bolivia

  • Lies largely  in Bolivia
  • It has average elevation of 1350 m.
  • Highland  was uplifted during tertiary period when the Andes were formed
  • Contrary to Tibetan Plateau, it is very dry and  has no exterior drainage

c. Plateau of Mexico

  • Plateau stands between the eastern and western Sierra Madre Mountains.
  • It has average elevation between 1800 meters to 2300 meters
  • Large  parts of this Plateau are very dry.

d. Colorado Plateau

  • Situated in USA between Cascade Range and Rocky Mountains
  • It is situated at elevation

e. Anatolia Plateau

  • Lies in Turkey between Taurus & Pontic Range
  • It’s elevation is cause of cool weather of Turkey 
Intermontane Plateaus

2. Piedmont or Border Plateau

  • These  Plateaus border mountain ranges and owe their present position to the same uplifts that raised the mountains. 
  • Examples include
    1. Piedmont Plateau on the border of Appalachian Mountains
    2. Patagonia Plateau in South America

3. Volcanic Plateau

  • Volcanoes form variety of plateaus. 
    • Larger : built by BASALTIC lava flow.
    • Small : formed by resistant lava caps that aren’t eroded & maintain its elevation after surrounding land has been worn away.
  • Examples are
    • Columbia Snake Plateau .
    • Deccan Plateau , India.
    • Shan Plateau , Myanmar
    • Katanga Plateau, Congo
    • North Island in New Zealand
Volcanic Plateaus of the world
Volcanic Plateaus of the world

4. Erosional Plateau

  • Form in semiarid regions where streams have cut away portions of high lands.
  • Examples include
    1. Allegany Plateau near  New York 
    2. Cumberland Plateau near Appalachians in USA

5. Depositional Plateau

  • Formed due to depositional action of wind
  • Examples include Loess Plateau in China => It is formed due to process of deposition of sediments carried from the desert  by the Anticyclonic  winds which develop over Russia and come to China shedding their load in this area
Loess Plateau

6. Dome Plateau

  • These plateaus are uplifted by folding and faulting processes  into a broad dome.
  • Entrenched Meanders are feature of these plateaus
  • Examples include Ozark Plateau of USA

7. Glacial Plateau

  • These are formed due to  erosional action of glaciers.
  • Examples include
    1. Laurentian Plateau of Canada (North America)
    2. Garhwal Plateau of India

Importance of Plateaus

  • Plateaus have large amount of   mineral wealth like Gold, Iron, Copper, Diamond, Manganese, Mica , Granite etc. which forms industrial base of any economy. Eg : Katanga Plateau of Congo is very rich in copper and Deccan Plateau of India is very rich in resources.
  • Plateaus are have more plain regions as compared to pure mountainous regions which helps in development of means of transport. Rail and road transport is lesser costly in plateau regions as compared to mountainous regions.
  • Plateau regions have abrupt slopes which are beneficial for setting up hydroelectrical centres thus helping in overall development of the region
  • Plateaus greatly effect the climate of region . For example Tibet plateau divides western Jet Stream in two parts while in summer and helps to create low pressure over Indian subcontinent which results into attraction for Monsoons 
  • They are important agriculturally as well . Eg : Deccan Plateau in India has black soil which is unmatchable for production of Cotton and Sugarcane.
  • Plateaus especially Intermontane Plateaus are great source of water resources as well. Eg: Large number of rivers like Indus , Brahmaputra etc originates from Tibet plateau.

Continental Drift and Plate Tectonic Theory

Continental Drift and Plate Tectonic Theory

This article deals with ‘Continental Drift and Plate Tectonic Theory.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Continental Drift Theory

  • Given by Alfred Wegener (German Meteorologist)  in  1912   . It speaks about rifting and drifting of continents .
  • According to Wegener, about 250 million years ago all the continents formed a single continental mass and mega ocean surrounded the same. 
    • Super continent was named PANGAEA, which meant all earth.
    • Mega-ocean was called PANTHALASSA, meaning all water.
  • He argued that, around 200 million years ago, the super continent, Pangaea, began to split
    • Pangaea broke to Laurasia/ Angaraland (forming Northern Continents) and Gondwanaland (Southern Continents) with Tethys Sea between them. 
    • Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. (Note – India was part of Gondwanaland.)
  • Interesting theory but was scrapped saying it GEO-POETRY because he wasn’t able to explain forces of movement.

Continental Drift theory was based on following clues

a. Continental fit / Jig Saw Fit

  • Continental lands can be joined together like jig saw puzzle.

b. Rocks of same age across oceans

  • Belt of ancient rocks of 2,000 million years from Brazil coast matches with  western Africa. 
  • Geological Structure of Appalachian Mountains matches with Morocco and Algeria in North Africa.

c. Fossils

Mesosaurus Freshwater reptile found in Africa & South America.
Glassopteris Fern found on all southern continents.
Lemur Found in India, Africa and Madagascar.

d. Placer Deposits

  • The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region => gold bearing veins are in Brazil => Ghana & Brazil Plateau used to lay side by side.

e. Tillite

  • Tillite are the sedimentary rock formed out of deposits of glaciers.
  • Gondwana system of sediments from India has counter parts in six different landmasses of Southern Hemisphere.
Pr

Forces for Drifting

  • Wegener suggested that movement responsible for drifting of continents was caused by pole-fleeing force and tidal force.
  • Polar-fleeing force relates to the rotation of the earth.
  • Tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters.
  • Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be inadequate 

Post Drift Studies

  • It is interesting to note that for continental drift, most of the evidences were collected from the continental areas .
  • Number of discoveries during the post-war period added new information to geological literature. Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.

Convectional Current Theory

  • Wegener wasn’t able to explain the  force  behind Continental Drift.
  • Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion.
  • These convection currents are nothing but molten rocks
    • Rising limb :  it will pressurise crust in such a way that crust will break .
    • Diverging limbs :  take crust away from each other .
    • Descending limbs : make two crusts to collide (like Indian & Eurasian plate ).
  • According to Holmes , these Convection Currents are the Driving Force . This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
Convectional Current Theory

Ocean  Seafloor Spreading Theory

Post war studies using which Ocean Map was prepared showed that ocean floor is not just a vast plain but it is full of relief.

Mapping of the ocean floor and palaeo-magnetic studies of rocks from oceanic regions revealed the following facts :

  • It was realised that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava
  • Ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
  • Age  of the rocks increases as one moves away from  crest.
  • Sediments  on the ocean floor are unexpectedly very thin => nowhere was  sediment column found be older than 200 million years.
  • Deep trenches have deep earthquake occurrences while in mid-oceanic ridge areas, earthquake foci have shallow depths
  • Concept of zebra strip / Magnetostratigraphy : rocks equidistant on either sides of mid-oceanic ridges show  similar magnetic properties

This led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”

  • Constant eruptions at  crest of oceanic ridges cause  rupture of the oceanic crust and  new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads.
  • Ocean floor that gets pushed due to volcanic eruptions at crest, sinks down at the oceanic trenches & gets consumed (Seafloor Spreading Theory).
Ocean  Seafloor Spreading Theory

Plate Tectonic Theory

  • Given in 1967 by McKenzie & Parker
  • Tectonic plate ( lithospheric plate) is a massive slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere . Its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
  • A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion 
    • Pacific plate is largely an oceanic plate
    • Eurasian plate may be called a continental plate.
  • Theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
  • Young Fold Mountain ridges, trenches etc are formed due to movement and interaction of these plates

The major plates are

  • Antarctic and the surrounding oceanic plate
  • North American
  • South American
  • Pacific plate.
  • India-Australia-New Zealand plate .
  • Africa with the eastern Atlantic floor plate .
  • Eurasia and the adjacent oceanic plate.

Some important minor plates are

  • Cocos plate : Between Central America and Pacific plate .
  • Nazca plate :Between South America and Pacific plate .
  • Arabian plate : Mostly the Saudi Arabian landmass.
  • Philippine plate : Between the Asiatic and Pacific Plate .
  • Caroline plate : Between the Philippine and Indian plate (North of New Guinea) .
  • Fuji plate : North-east of Australia.
  • 2017 update – Zealandia is now considered separate Continent/Plate

These plates have been constantly moving over the globe throughout the history of the earth.

  • All the plates, without exception, have moved in the geological past, and shall continue to move in the future as well.
  • Pangaea of Wegner was also result of convergence of continental masses
Movement of Continents in history

Plate Boundaries

There are three types of plate boundaries according to Plate Tectonic Theory (these three types of plate boundaries are discussed in detail below)

Type of Boundaries

Rate of Plate Movement

  • Strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help the scientists to determine the rates of plate movement.
  • These rates vary considerably.
    • Arctic Ridge: slowest rate (less than 2.5 cm/yr),
    • East Pacific Rise in the South Pacific : fastest rate (more than 15 cm/yr).

Force for the Plate Movement

  • Convectional Currents which were first explained by Hess in his Convectional Current Theory was the main force behind plate movement

Side Topic : Palaeomagnetism & how it prove Plate Tectonics

Palaeomagnetism is the study of the record of the Earth’s magnetic field in rocks and sediments

How it proves Plate Tectonic Theory

  • Magnetostratigraphy, with rocks equidistant on either sides of mid-oceanic ridges show  similar magnetic properties
  • Polar wandering – Magnetic minerals formed at same time but on different continents points have different orientation .  So, there were either multiple north poles during the same time period or that the continents moved in relation to a single north pole. Geophysicists concluded that the magnetic poles remained stationary, and the continents moved
  • Palaeomagnetism is also used to match once joined landmasses that are now separated. For example, the orientation of magnetic minerals along the eastern coast of South America very closely matches that of similar minerals on the western coast of Africa. 

Convergent Plate Boundaries

Convergent plate boundary is the margin where two plates collide with one another.

Convergent plate boundary

Convergent Plate Boundaries can be of three types :-

1 . Ocean Ocean Convergent Boundary

  • Denser of the two oceanic plates is subducted   . It goes to Asthenosphere & generate new Magma .
  • Andesitic Magma will from in this case . Andesitic Magma is less  mobile and solidifies quickly. As a result,  underwater Volcano or Volcanic island arc will form in this case.
  • Characterised by  Trenches , Underwater Volcanoes , Volcanic Island Arc and Earthquakes .
  • Island arc  (and not single island) will be formed because they will be formed on whole boundary where Ocean-Ocean plate is converging. All these islands will be volcanic islands .
Ocean Ocean Convergent Boundary

2. Ocean – Continental Convergence Boundary

  • Oceanic Plate is denser than Continental Plate . Hence, Ocean plate will be subducted into Asthenosphere &  melt down there. But at the same time, due to the great compressional force between two converging plates, folding will happen on the Continental Plate , resulting in formation of Marginal Fold Mountains . As we know, folding happens along the zones of  weakness , hence, when Magma of the subducted  Ocean plate will rise , it will  come out of the crust through these weak zones  resulting in formation of Volcanic Peaks .  
  • Hence, Volcanic mountains ranges, Trench and earthquakes  are common on boundaries
  • Examples of such volcanic mountain ranges are
Andes  mountains South America
Rockies North America
Atlas Africa
Ocean - Continental Convergence Boundary

3. Continental – Continental Convergent Boundary

  • When continental plate converges into continental plate , crust at  both sides is tool light  & buoyant to be subducted. Both are  compressed against each other and folding happens. Hence, in this case Fold mountain Belt forms.
  • No Volcanism but powerful earthquakes are created in this region .
  • Himalayas & Urals  are formed in this way due to convergence of Indo-Australian and Eurasian plates.
  • Pressure between plates is so high that metamorphic rocks form there.

Divergent plate Boundaries

  • Divergent plate boundary is the margin where two plates move apart. For instance, African plate and South American plate
  • Divergent plate boundary is termed as the constructive plate boundary as it leads to the formation of new lithosphere .
Divergent plate boundary

Divergent plate boundaries are of two types :-

1 . Ocean – Ocean Divergent Boundary

  • Creation of new crust takes place at submarine mountain ridge . Ocean crust is rifted apart & basaltic magma wells up to fill the opening.
  • Basaltic Magma will come out from the Mantle. Since basaltic magma  is very mobile, hence it will spread out. This magma hardens & forms igneous rock . Since magma is basaltic, hence ocean crust is basaltic in nature .
  • Mid-Atlantic Ridge is an ideal example of a submarine mountain ridge in the Atlantic Ocean. It is the longest mountain ridge in the world. It extends for about 16,000 km, in a ‘S’ shaped path, between Iceland in the north and Bouvet Island in the south
  • Water from hydrothermal vents (along the submarine ridges) is rich in dissolved minerals and supports organisms like chemo-autotrophic bacteria.

2 . Continental – Continental Divergent Plate

  • Rift  valley along with block mountains are formed when two continental  plates move apart.
  • Initially it leads to the development of a small body of water . But if rifting continues , body of water becomes bigger to juvenile ocean and consequently to Large Ocean.
  • Example : The Great Rift of Africa

Side Topic : Great Rift of Africa

  • In Great African Rift Valley, Continental Continental Divergence is observed leading to formation of Rift Valley.
  • Almost all the lakes in Africa are in the Rift Valley generated by diverging of  continental Plates except Lake Victoria.
Great Rift of Africa

Transform Plate Boundary

  • Where two plates are sliding past each other.
  • They are  under shear stress.
  • The lithosphere is neither destroyed nor created by the transform plate boundary. Hence , it is called Conservative or passive plate boundary.
  • Earthquakes are common & Volcanoes are not formed at Transform boundaries. Whenever plate boundary is active, Earthquakes are also experienced in that region.

  • Example –San Andreas Fault in USA (Pacific Plate & North American) , Chile etc
San Andreas FAult

Ocean Salinity

Ocean Salinity

This article deals with ‘Oceanography: Ocean Salinity’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Salinity of Solution is defined as amount of salt in 1000 gram of water .
  • Salinity of ocean water is 35.5 ppt ( parts per thousand)  & maximum amount is of common salt.
Sodium chloride 78%
Magnesium Chloride 12%
Magnesium sulphate 3.5%
Calcium Sulphate 2.5%

Why Sodium Chloride (NaCl) is present in highest proportion ?

  • Every salt has cycle & they remain in sea water for specific time( called residual time) & then precipitated to bottom surface .
  • Sodium (Na) & Chlorine (Cl) has highest residual time in ocean water leading to very gradual removal => that is why they are present in highest proportion .

Salt Budget

  • Irrespective of absolute salinity,  proportion of above salts remain same in all parts of the world  .
  • Amount of addition or extraction of fresh water compared to salt content in ocean water decides absolute salinity of oceans .
  • Salt Budget  = Budget of addition of salt & removal of salt .

Sources of salts on ocean water.

  • Sediments carried by rivers (most important) .
  • Submarine volcanism at Mid Oceanic Ridge .
  • Chemical reaction between rocks of geothermal vent of volcano & cold water.
  • Erosion of oceanic rocks and wave erosion of coastal rocks  .

Removal of Salts in ocean water.

  • Physical Removal :  waves break at beaches ie salt spray .
  • Biological removal : marine life forms extract calcium from sea water for their bones & shells .

Factors effecting salinity

Evaporation Higher the rate of evaporation ,higher is salinity.
Temperature Warmer parts are more saline than frigid ones.
Precipitation Higher the precipitation, lower is the salinity.
Influx of Freshwater Influx of freshwater leads to lower salinity .
Atmospheric pressure  
Circulation of Ocean water Stagnant water has more salinity (Eg: Sargasso sea).
Windy situation Wind accelerate evaporation => windy situation = more saline

Salinity of Oceans

  • Standard  salinity of ocean water is 35.5 ppt ie salinity of Atlantic ocean .
Greater than 35.5 High saline.
Lower than 35.5 Less  saline.
  • Some highly saline lakes . Man  seldom drown in sea with high salinity because water is  highly dense .
Dead Sea 238 ppt West Asia
Lake Van 330 ppt Turkey
Great Salt Lake 220 ppt USA
Lake Urmia   Iran

Overall pattern of Salinity across world

Ocean Salinity

a. Latitudinal variation

  • Salinity is highest at tropics(not Equator)  & decreases on both sides.
  • This is due to Interplay of evaporation & precipitation & other complex interactions.

b. Hemispheric variation

  • Northern Hemisphere is warmer => high evaporation => more saline .
  • But southern Pacific => Roaring 40, furious 50 , Shreaking 60 , Screaming 70 => very fast winds . Hence in Pacific ocean , southern hemisphere has more salinity

c. Local Variation in Salinity

Warm ocean currents – Increases temperature of water => equivalent to High evaporation .
– Leads to more salinity.
Upwelling Cooler water from depth come to surface => low salinity.
Transport by currents 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.

d. Enclosed Seas

Tropical region – Warmer than open sea .
– Will lead to high salinity.
– Eg Mediterranean Sea , Persian Gulf etc. 
High Latitude – Cooler than open sea .
– Lower salinity than open seas .
– Eg Baltic Sea, Gulf of Bothnia etc .

e. Inflow of large rivers

  • Ganga – Brahmaputra => flow into Bay of Bengal => large freshwater .
  • Bay of Bengal is less saline than Arabian Sea.

f. Glaciers

  • Those oceans /seas which receive greater glacier water are less saline
  • Baltic Sea is very less saline because of this reason .

Ocean Temperature

Ocean Temperature

This article deals with ‘Oceanography: Ocean Temperature’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Temperature of ocean water is important because
    1. Life of Phytoplankton & Zooplankton depend on it .
    2. Affects climate of coastal lands .
  • Points to note :-
    1. Main source of Energy on Earth is sun ie insolation .
    2. Oceans play important role in energy & temperature regulation of earth. Because of its specific heat, water doesn’t  get heated &  does-not get cold very fast . Hence, water  can store energy for very long time .
    3. Average temperature of ocean from surface to bottom is 3-5  Celsius.
    4. But average surface temperature of ocean is 25 Celsius.

Range of temperature

Daily/ Diurnal range of temperature

  • Difference between maximum & minimum temperature of a day.
  • Daily range of temperature is almost insignificant in case of rivers & is less than 1 degree C at max
  • Tropical water has higher diurnal range than equatorial waters because cooling & heating of water is rapid under clear sky .

Annual range of temperature

Average is 12 degree C but lot of regional variation.

  • Higher in case of enclosed oceans than open seas
  • Bigger the size lower annual range of temp because of more mixing .

Distribution pattern of Temperature

Studied in two ways

1 . Horizontal  distribution of temperature in oceans

Factors impacting horizontal distribution of temperature in oceans

a. Latitudinal Variation

  • There is decrease in temperature while going from equator toward poles because of decrease in insolation
  • But highest temperature is found not at equator but at tropics
    • Reason : High rainfall & cloud cover at the Equator resulting in High Albedo / reflection of sun-rays by clouds

b. Prevailing Winds

Direction of the wind affects the distribution of temperature of ocean water.

  • Off shore winds blowing from the land towards ocean or sea raise the temperature of ocean water.
  • Winds blowing from snow covered regions in winter lower the surface temperature

c. Hemispheric Variation

  • Waters in Northern Hemisphere warmer than Southern Hemisphere . Reason is the oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
  • Isotherms in north aren’t regular while in Southern Hemisphere regular.

d. Enclosed Sea

  • Marginal seas of tropics ( like Mediterranean Sea)  are warmer than open Ocean and marginal seas of  temperate region (eg Gulf of Bothnia)  are cooler than open sea.
  • Reasons
    • less mixing of water.
    • Land heats up surrounding waters too.

e. Ocean Currents

  • Warm currents have warming effect on ocean water
  • Cold currents have cooling effect on ocean water

f. Upwelling & Downwelling

  • Upwelling – Brings cool water from depth.
  • Downwelling – opposite ie warmer .

g. Salinity

  • If the sea water is more saline, its temperature will be higher because highly saline water contains more energy on the other hand the temperature of less saline water is low.

Horizontal distribution of Ocean Temperature

  • The average temperature of surface water of the oceans at equator is about 27°C and it gradually decreases from the equator towards the poles.
  • The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude
  • The average temperature is around 22°C at 20° latitudes, 14° C at 40° latitudes and 0° C near poles.
  • The oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere. The average annual temperatures for the northern and southern hemisphere are around 19° C and 16° C respectively. This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.
  • The highest temperature is not recorded at the equator but slightly towards north of it.
800N— 
400N 
00 
800S 
32 
30 
28 
26 
24 
22 
20 
18 
16 
14 
12 
10 
500F 
V. 
1500F 
LONGITUDE 
Figure 5.12 Sea surface temperature in July 1997

2. Vertical Distribution of Temperature of Ocean Waters

  • Maximum temperature is on the surface  because it receives insolation.
  • Transmitted to lower sections of ocean by convection.
  • Solar rays can penetrate very effectively till 200m  but seldom go below 1000 m . As a result, temperature falls very rapidly from 200m  till 1000m & after that , rate of decrease of temperature is very slow (rate & not absolute temperature).

Can be divided into three zones

First layer

  • Top layer of warm oceanic water & is 200 m thick.
  • Temperature ranges from  20 to 25degree  & no large variation with depth ( almost constant )
  • Present in tropics throughout year , in mid latitudes in summer & in cold areas never present .

Second layer

  • Thermocline layer
  • Below first layer from 200m to 1000 m.
  • Rapid rate of decrease of temperature to 4 C  with increase in depth from 200 m to 1000 m .
  • Rate of decrease is rapid at equator & tropics than towards pole because temperature already is very low in Polar Region
  • In polar , waters  are already at around 4C . Hence, this layer is not present in polars .

Third layer

  • Very cold & extend upto deep ocean floor.
  • Decrease in temperature with depth is almost nill.
  • Polar areas have only this layer from above till ocean floor.
Vertical pattern of temperature of Oceans