MAJOR LANDFORM FEATURES
Major landforms are the major relief features of the earth‟s surface. They include mountains, plateaus and plains etc.
Formation of Landforms
Formation of landform is brought about by two main forces namely, Internal forces (tectonic forces) and external
forces.
Internal Forces:
Internal forces are forces that lead to the movement of the tectonic plates either laterally or vertically. These movements
generate stress in the form of compression (contracting of land surface) and tension (expansion of land surfaces), which
lead to different landforms.
EXTERNAL FORCES:
Internal forces are also known as forces of denudation. They are the forces, which operate on the earth‟s surface to
modify the relief features produced initially by the internal forces. They include weathering, erosion, transportation and
deposition.
A. MOUNTAINS:
These are greatly elevated land surfaces resulting from the intense action of internal forces. Steep slopes and prominent
peaks or ridges characterize them. Based on their mode of formation, there are four types of mountains. They include:
Fold Mountain; Block Mountain; Volcanic Mountain; Residual Mountain.
FOLD MOUNTAINS:
Fold Mountains are formed as a result of stresses caused by large- scale horizontal earth movement. Such stress subjects
the rocks to compression. These compressional forces pushing horizontally from opposite ends cause sedimentary rock
layers within the earth crust to wrinkle or fold. The wrinkles form upward folds called anticlines and downward folds
called synclines.
In simple folding, the upward folds form mountains known as Anticline Mountains while the downward folds in form
valleys known as synclinal valleys. Jura Mountain in Europe is an example of young simple Fold Mountain while Udi
Hills in Nigeria is an example of unfolds.
When folding continues, simple up and down folds become pronounced to form asymmetrical fold. As the forces of
compression continue to act on the folding area, some folds begin to lean over to one side to form over fold. Further
folding of the over fold forms the recumbent fold. This complicated fold breaks off at base and slides over the adjacent
land. The sliding surface is known as the thrust plane while the sliding fold is
known as the nappe Examples are
Rockies in USA and the Atlas Mountain etc.
CHARACTERISTICS
i. They consist of ridges with intervening narrow depression
ii. They usually have a steep sides
iii. They consist of different types of folds.
iv. Examples of fold mountains are rock mountain USA, Andes (South America) and Atlas (North Africa)
BLOCK MOUNTAINS:
FORMATION
These are generally flat-topped and steep-sided when newly formed but their slope may be lost after long erosion when
rivers may have dissected them. Faulting accompanied by tension and compressional forces causes Block Mountains.
Faults refer to fractures along a line of weakness in rocks. There are two types of block Mountain; block Mountain and
rift valley or Graben. When there is tension in an area of the earth‟s crust with two parallel normal faults, the land
enclosed by the two parallel faults remain as it is. The land on either side of the parallel faults subsides. Then the
enclosed central block now standing becomes the horst or block mountain e.g. Block forest of Europe.
In the formation of Graben or Rift valley, there could be an area of the earth‟s crust with two parallel reverse faults,
compression could force the area of land outside the area enclosed between the faults to slide upward thus rising above
the land enclosed by the two parallel faults. The enclosed area forms a rift valley or graben.
e.g. Central valley of Scotland. Other e.g. of block Mountains are the Fouta Djallon Mountain in Guinea, Tarkwa hill
region between Konongo and Tarkwa (Ghana), the east Africa Rift Valley etc.
CHARATERISTICS
i. They consist of old hard rocks
ii. They are usually associated with Rift valley
iii. They have flat or slightly sloping surface
iv. Examples are Hum struck of Germany, Vosges and Black forest of Europe
VOLCANIC MOUNTAINS:
FORMATION
Volcanic Mountains are formed when materials from the earth‟s crust are ejected through the cracks to the earth‟s
surface. The materials include molten lava, volcanic bombs, caders, ashes, pyroclasts, dust and liquid mud. The
materials fall around the vent in successive layers building up a characteristic volcanic cone. Volcanic Mountains are
also called Mountains of accumulation because the materials that form them accumulate gradually to attain their
heights. E.g. Mountain Fuji (Japan), Mountain Mayon (Philippines), Mountain Kilimanjaro (Kenya), Mountain
Cameroon, Elgon Mountain, Ruwenzori, Mountain Merapi (Sumatra) etc.
CHARACTERISTICS
i. They consist of hot rocks, lava, ashes, dust and liquid mud
ii. Material are ejected through hole called vent or pipes
iii. Material are built up to form mountain which may be conical or dome in shape
iv. Examples are Kenya, Elgon and Cameroon in Africa
Original mountain surface
Downward cutting by rivers
RESIDUAL MOUNTAINS:
FORMATION
These are formed as a result of external forces of denudation acting on the landform after tectonic forces have
structurally formed them. They are also referred to as Mountain of denudation. Residual Mountains are generally steep
sided and rugged in shape. When forces of denudation act on fold Mountain, they attack the anticlines more easily; wear
them down into valleys leaving the synclines to stand up as rugged Mountains. The eroded anticlines now become the
valley called anticlinal valleys while the synclines form Residual Mountain called synclinal Mountain e.g. Matter horn
in Switzerland.
Residual Mountains may also evolve from plateaux, which have been dissected by rivers into hills and valleys. A
volcano may be eroded leaving behind the hard solidified volcanic lava (duridust) to stand out as Residual Mountain
known as volcanic plug. E.g. Wase rock behind Wase village in Benue valley, Tangase rock and Okene Plug in Nigeria.
CHARATERISTICS
i. They are hard and resistant rocks that are left standing after the surrounding areas have been reduced by agents of
denudation
ii. They are remains of old mountains that have been reduced by agents of denudation
iii. They are called dissected mountain
iv. Examples of residual mountains are Monadnock in USA
IMPORTANCE OF MOUNTAIN:
1. They are sources of rich mineral e.g. coal from Appalachian mountain, gold from cordillera in USA, tin and copper
from the Andes and Bolivia.
2. They are sources of rivers e.g. Fouta Djallon highland and Cameroon mountain
3. They serve as recreational or tourism centers.
4. They modify the climatic condition of a place by lowering the temperature and produce an orographic rainfall.
5. They provide constructional materials for road and building
6. In the past, they serve as defensive barriers during inter tribal war.
B. PLATEAUX:
These are elevated uplands with extensive level surface and usually descend steeply to the surrounding lowland. They
are also referred to as tablelands. They have narrow valleys and sometimes form hydrological centers. Just like other
highlands, they are subjected to erosional processes. As a result their original characteristics may be greatly altered.
Plateaux may be divided into three groups according to their mode of formation and Physical appearance.
TECTONIC PLATEAUX:
FORMATION
This is formed by internal forces (earth movement). These forces (tensional or compressional) cause uplift and fairly
uniform altitude. The uplifted areas of level landform tectonic plateaux and the depressed areas form basins. E.g. the
continental blocks such as Deccan plateau in India. Some of the plateau may also be lifted like the Mistral of Central
Iberia or faulted like the Harz of Germany.
VOLCANIC PLATEAUX:
They are also called lava plateaux. They are formed when basic lava are ejected or erupts from the earth‟s crust and
spread over the earth‟s surface to from successive sheets of basaltic lava. They spread out over a great distance thereby
forming a highland with almost leveled tops known as volcanic plateau. The best Nigeria example is the Biu plateau.
Other examples include Antrim plateau of Northern Ireland. The most remarkable plateau built by lava is the Columbia
snake plateau, which covers an area almost twice as big as Malaysia.
DISSECTED PLATEAUX:
These are sometimes referred to as residual plateaux. They are formed when high and extensive plateaux are gradually
attacked or worn down by weathering and erosion processes. This gives the plateaux an irregular surface. They can be
cut by deep river valleys which break them up into smaller units, such plateaux are called dissected plateau e.g. the
Scottish highlands, the edges of the Jos plateau and Fouta Djallon. Highlands of Guinea have been subjected to
downward cutting by the various streams originating from them. Flat-topped remnants of such hills often capped by
resistant rocks and are called Mesas. Smaller remnants of mesas are known as Buttes. The resistant rock is often
duricrust (laterite)
INTERMONTANE PLATEAUX:
These are plateaux enclosed by Fold Mountains e.g. Tibetan plateau between the Himalayas and the Kunlun.
Intermontane plateau are some of the highest and most extensive plateau in the world. Bolivian plateau and the Andes is
another good example.
USES OF PLATEAU
1. They form climatic barriers and thus shelter settlement on the leeward side from the harsh climatic conditions on the
windward side.
2. Plateaux are rich in minerals e.g. Jos Plateau has tin and columbite.
3. In tropical lands cool plateau tops are conducive to human settlement e.g. Jos.
4. Some plateaux are centre of tourist attraction like Obudu and Jos plateau.
5. Some plateaux are sources of many rivers.
6. Plateaux also support the growth of pasture and grasses and legumes, which support the rearing of animals.
7. They form fertile soil for agricultural cultivation of specialized crops e.g. apples, tea, coffee
C. PLAINS
A plain is an extensive or expanse of lowland area, which is either level or undulating and are usually few hundred
metres above the sea level. They are areas of lowland and are gently undulating. They have low elevation and hardly
rise above a few hundred meters above sea level. Plains can be low hills with rolling topography.
TYPES OF PLAIN:
Based on their mode of formation, there are three major types of plains; structural plains, depositional plain and
erosional plains.
1. STRUCTURAL PLAINS:
These are plains that have not been disturbed by earth movement (internal forces). They are formed by horizontally
bedded rocks and are structurally depressed areas of the world. They make up the most extensive natural lowlands on
the earth‟s surface e.g. Russian platform, the great plains of USA, the central lowlands of Australia.
2. DEPOSITIONAL PLAINS:
These are plains formed by the deposition of materials that have been carried over a long distance by various agents of
transportation such as wind, water (river), waves and glaciers. Depositional plains include;
1. Alluvial Plains, flood plains and deltaic plains that are formed by running water.
2. Outwash plain and till plain or drift plain formed by ice-sheets or glaciers.
3. Coastal plains formed by waves.
4. Loess plains formed by wind.
Alluvial Plains: This is common in areas where slopes decrease gently downstream. As a result of decrease in slope,
the velocity of the flowing water reduces; this makes the water to deposit its eroded materials. The deposited materials
form the alluvial plains. Alluvial plains are associated with the old stage of a river e.g. fertile alluvial plain of Hwang
Ho in North China.
Flood plains: These are vast quantities of sediment deposited by a river at its lower course. The deposited materials
gradually build up gently sloping surface called flood plain. Flood plains stretch widely on both sides of the river e.g.
lowers Niger where it overflows its bank, the vast flood plains of the Ganges River in India and the Yang-Tze River in
China.
Deltaic Plains: These are formed when eroded materials are deposited at the mouth of a river. The deposited materials
build up to form deltaic plains. Examples of such plains include the Niger Delta in Nigeria, the Nile delta in Egypt as
well as the Mississippi and Ganges deltas in USA and India respectively.
Outwash Plains: These are plains formed by wide spread deposits of unsorted sand and gravels by gradual melting of
ice. They are usually infertile.
Till or Drift Plains: Melting of ice also forms this but the materials deposited are made up of boulder clay a mixture of
various sizes of boulders and clay. Examples are found in Northern Germany, Netherlands, mid-west of USA and East
Anglia in England.
Coastal Plains: They are formed when sediments or eroded materials are deposited by waves on some parts of the
continental shelf or coast. The land is uplifted to form a gently sloping coastal plain. E.g. includes coastal lowlands of
Belgium, the Netherlands and the gulf coast of USA.
When coastal lowland is raised slightly by uplift it is called Emergent coastal plain e.g. the coastal margins from Florida
to Texas.
Loess Plain: These are made up of very fine particles deposited by winds. The deposits are eroded materials from the
desert loess help to level an undulating plain by filling up grooves and depressions e.g. pampas of Argentina.
EROSIONAL PLAINS:
Peneplains formed as erosion of uniform layers by running water progresses
These are plains formed as a result of erosion or wearing down of the land to a level by agents of erosion (wind, rain,
rivers and ice). These agents help to smooth out the earth‟s surface and even high mountains can be reduced to low
undulating plains. Erosional plains include; pene plains, ice-scoured plains and pedi plains.
Peneplains: These are formed when highlands are reduced by erosion to form gentle undulating lowlands. Peneplains
are usually very large e.g. the great lake plain of the Hudson Bay area in North America. Rivers can deepen and widen
their valleys to form river plains e.g. the Amazon basin is a river eroded plain.
Inselbergs
Pediplains formed as weaker rocks are eroded by wind
Ice-scoured plains: These are the lowered land where continuous glaciations reduces the highlands to a very low level
e.g. the plains of Finland
Pediplains: These are plains formed as a result of winds erosion. They are common in semi-arid and arid regions where
wind deflation sweeps away much of the eroded desert materials lowering the level of the land and forming extrusive
plains. Weathering also wears back mountain slopes to leave gently sloping pediments or Pedi plains and some steep
hills (inselbergs) remain.
IMPORTANCE OF PLAINS
1. Plains provide good communication route ways such as roads, railways, airports etc.
2. They provide good pastures for roaming animals.
3. They provide good settlement and population concentration.
4. Plains provide rich fertile soils which favour intensive cultivation of crops e.g. flood plains.
5. They provide mineral resources such as coal, petroleum etc.
DEPRESSIONS OR BASINS:
These are depressions in the earth‟s crust. They may be formed by erosion when the removal of soil by running water
results in the formation of gullies or by tectonic activities such as convergence of plates. Where Tectonic Plates
converge, the earth crust is often folded downwards as a result of subduction and this forms depressions on the earth
crust E.g. Deep-sea trenches. Some depressions or basins may be filled with sediments deposited by wind or running
water and such depressions often develop an external drainage system like the Congo Basin. Other depressions or
basins have inland drainage e.g. the Chad Basin, El Djouf and Sudan Basins.
IMPORTANCE OF BASINS
1. Some of them serve as in-land drainages.
2. They can be very fertile and they are very valuable for agriculture e.g. Qattara depression in Egypt.
3. They are homes for submarine features e.g. Mariana trench etc.
4. Some of them serve as in-land drainages.
5. They can be very fertile and they are very valuable for agriculture e.g. Qattara depression in Egypt.
6. They are homes for submarine features e.g. Mariana trench etc.
7. Some of them serve as in-land drainages.
8. They can be very fertile and they are very valuable for agriculture e.g. Qattara depression in Egypt.
9. They are homes for submarine features e.g. Mariana trench etc
OCEANS
Ocean is the large masses of salt ware that occupies a considerable portion of the earth surface. The ocean of the world
occupies more than 71% of the earth total area of 510 million square kilometers. The ocean basins are the large depression on
the earth surface, which are filled with a great mass of salt water.
There are four major oceans in the world. They include the Pacific Ocean which is the largest ocean, the Atlantic Ocean the
second largest ocean, the Indian Ocean which is the third largest ocean, the Southern or Antarctic Ocean which is the fourth
largest ocean and the Arctic Ocean which is the smallest ocean. Besides oceans, there are the seas of the world which are
considerably large bodies of water. They include the Caspian Sea, Mediterranean Sea, Caribbean Sea, etc. The seas of the
world are said to cover a total of about 25 million Km2 of the earth surface. S/NO
OCEANS OF THE WORLD
AREA IN
MILLION KM2
1.
Pacific
155.557
2.
Atlantic
76.762
3.
Indian
68.556
4.
Arctic
14.056
5.
Southern (Antarctic) 20.327
Oceans are studied by oceanographers who find out the chemical composition of oceans water, its movement, its marine life,
landforms, the influence of oceans on the continents and the importance of oceans to man. In their studies, they make use of
various techniques such as echo sounding, satellite images, marine life observation and chemical analysis.
THE OCEAN BASINS AND SEAS OF THE WORLD
There are four main types of ocean in the world: Pacific, Atlantic, Indian and Arctic Ocean
PACIFIC OCEAN:
This is by far the largest ocean in the world occupying one third of earth surface and is made up of some of the greatest
trenches, some of the trenches extend 5,000fathoms (900metres) deep. Pacific Ocean extends from the western coast of the
Americas to eastern Asia and Australia
ATLANTIC OCEAN:
This is by far the most important ocean of the world. It is located between the eastern shore of the Americas to the western
coast of Europe and Africa. It is the most important ocean of the world because most of the part which serves some of the
most heavily industrialized nation of the world are along it ridges. It contains trenches but not as notable as that of the Pacific
Ocean. Some of the trenches are Puerto Rico deep, the south sandwich trench, and the romanche deep. It also has an
elongated ridge called the mid Atlantic ridge
INDIAN OCEAN:
This is the third largest ocean in the world. It stretches more to the southern hemisphere than to the northern hemisphere. This
ocean stretches from the coast of Africa, India and Indo – China and the coast of Australia, about 60% of its floor form the
deep sea plain with a depth ranging from 2000 – 3000fathoms.
ARCTIC OCEAN
It is the smallest ocean in the world and surrounds the whole of the Arctic region. The Northern fringes of North America and
Europe bind it. The Arctic Ocean occupies the sea around the North of North America and Eurasia. It is a shallow basin with
a mean depth of 5,449metres and a total area of 14millin Sq km. The largest landmass in the Arctic Ocean is the Greenland.
THE SEA
The seas of the world cover an area of 25million Sq km and they include the Mediterranean, the Black, the Dead the
Caribbean Sea etc
THE NATURE OF THE OCEAN FLOOR (OCEAN PROFILE)
The ocean floor is covered with several features that appear similar to those on ground. They include the continental shelf,
continental slope, deep-sea plains, marine ridges and plateau, ocean deeps, trenches and ocean islands.
Relief profile of an ocean floor
THE CONTINENTAL SHELF:
This is considered to be a continuation of the landmass into the ocean .It is the shallowest part of the ocean. It forms a gentle
sloping edge extending under the sea to an average depth of about 180m. The continental shelf varies from one shelf to
another. Some shelves stretch for a few meters while others stretch for up to 1200km.
GEOGRAPHICAL SIGNIFICANCE/IMPORTANCE OF CONTINENTAL SHELF
1. One of the major importance of continental shelves to man is that they provide good fishing ground. The shallow nature of
the water allows sunlight to penetrate through it and aid the development of plankton an essential fish food. Some of the
fishing grounds include Grand Banks off Newfound land (USA), the shelf North Sea and the continental shelf extending from
Mauritania to Senegal in Africa.
2. The shallowness of the continental shelf makes easy exploration of crude oil in the area.
3. The continental shelf brings about development of natural harbour as this has been reflected in the seaport of Southampton,
London, Hamburg, Singapore, and Hong Kong etc.
4. They also bring about development of tourist centers. A common feature of a continental shelf is sand which sometime
drifts out to from beaches.
THE CONTINENTAL SLOPE:
This marks the end of continental shelf. It links the continental shelf with the deep-sea plains or ocean floor. It extends to a
depth of about 3000m where the deep sea plain begins. Along the continental slope, mud and red clay can be found.
THE DEEP SEA PLAIN:
This is made up of fairly undulating or level floor of about 2-3km below sea level and covering 2\3 of the ocean floor. It is
also called abyssal plain.
OCEAN RIDGE:
This takes the form of either a ridge or a plateau, which rises from the deep sea plain. Examples include Mid-Atlantic ridge,
South Pacific plateau etc. Some of the ridges and the plateaux rise above the ocean surface to form oceanic islands example
the Falkland and Canary Islands, etc. Others are the peaks of ocean floor volcanoes example Hawaiian and Fiji.
OCEAN DEEPS OR TRENCHES:
These are long, narrow and steep-sided depressions that occur at the deep-sea plain and they could have a depth of about
10,000m or more. An example of ocean deeps is the Marianas Trench off the island of Guam in the Pacific Ocean with a
depth of about 11,035. They occur mostly at the edge of the continents boarded by fold mountains example Atacama trench
off the coast of Chile and Peru.
SEA MOUNT:
These are under water mounts that rise from the ocean floor. They could be about 3000m below the ocean surface.
MARINE DEPOSITS
The materials eroded from the earth, which has not been deposited by rivers, are eventually brought into the ocean bed; these
materials are termed marine deposit. This deposition can be classify as follows
1. Muds: These are ferruginous deposit derived from land and are usually deposited on the continental shelves.
2. Oozes: These are pelagic deposit derived from the ocean; they consist of the remains of shelly, skeletal marine micro
organism
3. Clay: These occur mainly as red clays on the deepest parts of the ocean basins and are mostly abundant in the pacific.
OCEAN ISLANDS
These are mountains that are deep seated in the ocean with their peaks extending above the sea level. They may be formed by
folding or volcanic activity. Some ocean islands are detached from the continental landmasses but others are considered to be
part of the continents where the coastlands have been partially submerged e.g. Madagascar is part of the African continental
landmass but Hawaii is not attached to any continental land mass. Marine organisms whose shells are made of calcium,
sometimes stick together to form solid land-like masses which are seen above the ocean surface. These features are known as
Coral Reefs.
SALINITY OF THE OCEAN
Salinity of the ocean can be defined as the degree of saltiness or concentration of salt solution in oceans. Seawater contains
dissolved mineral matter of which sodium chloride is common. They provide marine organisms with calcium carbonate
needed for the formation of shells and bones. Magnesium chloride and magnesium sulphate are also found. The degree of
saltiness varies from ocean to ocean and is often measured in parts per thousand grams. The average salinity of the ocean is
about 35grams for every 1000 grams of water. In maps, a line called isohaline often joins all oceans having the same degree
of salinity. In General, the salinity in semi-closed seas in the tropical areas is relatively high because of high evaporation.
Salinity is also high in sub-tropical areas where there is a relatively small amount of water discharged into the sea. Examples
include the Mediterranean Sea, the Great lakes of North America, the Red sea, and the Dead Sea of Jordan. The highest
salinity is that of Lake Van in Asia Minor about 330 per 1000grams.
FACTORS INFLUENCING OCEAN SALINITY
The variation of salinity in the various seas and oceans is affected by the following factors:
1. RATE OF EVAPORATION: In high temperature areas, there is a consequent high rate of evaporation and as salt does
not rise with evaporation water, it concentration increases, thus places with low rates of evaporation tends to have lower level
of salinity. The high rate of evaporation in the Middle East accounts for the high level of salinity in seas within the area. The
Dead Sea for instance, has the highest salinity in the world of about 250 parts of salt per 1000 parts of water. The water body
with the highest salinity is Lake Van, which has about 330 parts of salt per 1000 parts of water.
2. AMOUNT OF FRESH WATER ADDED BY PRECIPITATION, STREAMS AND ICE BERGS: Salinity is lower
than average of 35 per thousand in equatorial waters because of the heavy daily rainfall and high relative humidity, when
these oceans becomes diluted with fresh water, their salinity decreases.
3. THE DEGREE OF WATER MIXING BY CURRENTS: In wholly, or partially enclosed seas such as the Caspian sea,
the water does not mix freely with water from other oceans, thus the change in salinity is negligible. Where there is free
mixing of high salinity ocean water with low salinity ocean water, the waters are diluted to bring about variation in their
various levels of salinity. Salinity increases in the ocean with lower salinity and on the other hand, salinity becomes lower in
the ocean with a higher salt content
OCEAN DEEPS OR TRENCHES:
These are long, narrow and steep-sided depressions that occur at the deep-sea plain and they could have a depth of about
10,000m or more. An example of ocean deeps is the Marianas Trench off the island of Guam in the Pacific Ocean with a
depth of about 11,035. They occur mostly at the edge of the continents boarded by fold mountains example Atacama trench
off the coast of Chile and Peru.
SEA MOUNT:
These are under water mounts that rise from the ocean floor. They could be about 3000m below the ocean surface.
MARINE DEPOSITS
The materials eroded from the earth, which has not been deposited by rivers, are eventually brought into the ocean bed; these
materials are termed marine deposit. This deposition can be classify as follows
1. Muds: These are ferruginous deposit derived from land and are usually deposited on the continental shelves.
2. Oozes: These are pelagic deposit derived from the ocean; they consist of the remains of shelly, skeletal marine micro
organism
3. Clay: These occur mainly as red clays on the deepest parts of the ocean basins and are mostly abundant in the pacific.
OCEAN ISLANDS
These are mountains that are deep seated in the ocean with their peaks extending above the sea level. They may be formed by
folding or volcanic activity. Some ocean islands are detached from the continental landmasses but others are considered to be
part of the continents where the coastlands have been partially submerged e.g. Madagascar is part of the African continental
landmass but Hawaii is not attached to any continental land mass. Marine organisms whose shells are made of calcium,
sometimes stick together to form solid land-like masses which are seen above the ocean surface. These features are known as
Coral Reefs.
SALINITY OF THE OCEAN
Salinity of the ocean can be defined as the degree of saltiness or concentration of salt solution in oceans. Seawater contains
dissolved mineral matter of which sodium chloride is common. They provide marine organisms with calcium carbonate
needed for the formation of shells and bones. Magnesium chloride and magnesium sulphate are also found. The degree of
saltiness varies from ocean to ocean and is often measured in parts per thousand grams. The average salinity of the ocean is
about 35grams for every 1000 grams of water. In maps, a line called isohaline often joins all oceans having the same degree
of salinity. In General, the salinity in semi-closed seas in the tropical areas is relatively high because of high evaporation.
Salinity is also high in sub-tropical areas where there is a relatively small amount of water discharged into the sea. Examples
include the Mediterranean Sea, the Great lakes of North America, the Red sea, and the Dead Sea of Jordan. The highest
salinity is that of Lake Van in Asia Minor about 330 per 1000grams.
FACTORS INFLUENCING OCEAN SALINITY
The variation of salinity in the various seas and oceans is affected by the following factors:
1. RATE OF EVAPORATION: In high temperature areas, there is a consequent high rate of evaporation and as salt does
not rise with evaporation water, it concentration increases, thus places with low rates of evaporation tends to have lower level
of salinity. The high rate of evaporation in the Middle East accounts for the high level of salinity in seas within the area. The
Dead Sea for instance, has the highest salinity in the world of about 250 parts of salt per 1000 parts of water. The water body
with the highest salinity is Lake Van, which has about 330 parts of salt per 1000 parts of water.
2. AMOUNT OF FRESH WATER ADDED BY PRECIPITATION, STREAMS AND ICE BERGS: Salinity is lower
than average of 35 per thousand in equatorial waters because of the heavy daily rainfall and high relative humidity, when
these oceans becomes diluted with fresh water, their salinity decreases.
3. THE DEGREE OF WATER MIXING BY CURRENTS: In wholly, or partially enclosed seas such as the Caspian sea,
the water does not mix freely with water from other oceans, thus the change in salinity is negligible. Where there is free
mixing of high salinity ocean water with low salinity ocean water, the waters are diluted to bring about variation in their
various levels of salinity. Salinity increases in the ocean with lower salinity and on the other hand, salinity becomes lower in
the ocean with a higher salt content
OCEAN
Atlantic Ocean
Pacific Ocean
OCEANS OF THE WORLD
WARM CURRENT
COLD CURRENT
North Atlantic Drift current, Florida
current, Gulf Stream current, cayenne
current, Guinea current.
Kuroshio current, North Pacific Drift
current, Oyashio current.
Labrador current, canaries current,
Irminger current
California current, Okhotsk current.