The Realms of Water | ISC Class 11 Geography Notes

This note covers the realms of water: the submarine relief of the Atlantic, Pacific and Indian Oceans, sea floor deposits and marine resources, ocean pollution, the composition of seawater and the factors controlling salinity, temperature and density, and the movements of ocean water, namely waves, tides and currents, including El Niño and La Niña and their effect on the Indian monsoon.

What is the hydrological cycle and why is the earth called the Blue Planet?

Water is a cyclic resource: it can be used and re-used, and it moves continuously between the ocean, the atmosphere, the land surface, the subsurface and living organisms.

The hydrological cycle is the circulation of water within the earth's hydrosphere in its liquid, solid and gaseous phases, and the continuous exchange of water between the oceans, atmosphere, land surface, subsurface and organisms.

It has been working for billions of years, and all life on earth depends on it. Next to air, water is the most important element required for the existence of life.

Why the earth is called the Blue Planet

Water is a rare commodity in the solar system. No other planet in the solar system is known to have oceans of liquid water at its surface; elsewhere water is found mostly as ice or vapour. The earth, fortunately, has an abundant supply on its surface.

Hence the earth is called the Blue Planet. The distribution of water on earth is quite uneven: many locations have plenty of water while others have very limited quantity.

About 97 per cent of the planetary water is found in the oceans. The remaining water is held as freshwater in glaciers and icecaps, groundwater sources, lakes, soil moisture, atmosphere, streams and within life.

How water moves through the cycle

Nearly 59 per cent 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.

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, and the pollution of river waters has further aggravated the crisis.

Draw a block diagram of a landscape from mountains to ocean, with clouds and the sun. Mark arrows for precipitation, condensation, evapotranspiration, evaporation, transport by wind (advection), runoff, penetration into soil and underground flow, ending at the ocean.

How is the ocean floor divided, and what relief features does it have?

The oceans are confined to the great depressions of the earth's outer layer. Unlike the continents, the oceans merge so naturally into one another that it is hard to demarcate them.

Geographers have divided the oceanic part of the earth into five oceans: the Pacific, the Atlantic, the Indian, the Southern Ocean and the Arctic. The various seas, bays, gulfs and other inlets are parts of these large oceans.

A major portion of the ocean floor is found between 3 and 6 km below sea level.

The ocean floor exhibits complex and varied features, as those observed over the land: 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 tectonic, volcanic and depositional processes.

The ocean floors can be divided into four major divisions: (i) the Continental Shelf; (ii) the Continental Slope; (iii) the Deep Sea Plain; (iv) the Oceanic Deeps. Besides these, there are major and minor relief features such as ridges, hills, seamounts, guyots, trenches and canyons.

The continental shelf

The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs.

It is the shallowest part of the ocean, showing an average gradient of 1° or even less, and it typically ends at a very steep slope called the shelf break.

The average width of continental shelves is about 80 km, but the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width, while shelves are almost absent or very narrow along some margins such as the coasts of Chile and the west coast of Sumatra.

Shelf depth may be as shallow as 30 m in some areas and as deep as 600 m in others.

The continental slope, deep sea plain and oceanic deeps

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.

The gradient of the slope region varies between 2 and 5°, and its depth varies between 200 and 3,000 m. The slope boundary indicates the end of the continents, and canyons and trenches are observed in this region.

Deep sea plains are gently sloping areas of the ocean basins, the flattest and smoothest regions of the world, with depths varying between 3,000 and 6,000 m. They are covered with fine-grained sediments like clay and silt.

Oceanic deeps or trenches are the deepest parts of the oceans: relatively steep-sided, narrow basins some 3 to 5 km deeper than the surrounding ocean floor.

They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes, which is why they are very significant in the study of plate movements.

Draw three stacked cross-sections labelled Continental Margin, Deep Ocean Basin and Mid-Ocean Ridge, each with a depth scale in km and a horizontal distance scale in km.

Mark the continental shelf, shelf break, submarine canyon, continental slope, continental rise, abyssal plain, deep-sea trench, abyssal hills, guyot, seamounts, mid-ocean ridge, fracture and rift.

Minor relief features

Apart from the major relief features, some minor but significant features predominate in different parts of the oceans.

Ridges, seamounts and canyons

A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean's surface. Iceland, a part of the mid-Atlantic Ridge, is an example.

A seamount is a mountain with pointed summits, rising from the seafloor, that does not reach the surface of the ocean. Seamounts are volcanic in origin and can be 3,000 to 4,500 m tall.

The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.

Submarine canyons are deep valleys, some comparable to the Grand Canyon of the Colorado river. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best known submarine canyon in the world.

Guyots, atolls and the number of deeps

Guyots are flat-topped seamounts. They show evidence of gradual subsidence through stages to become flat-topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Atolls are low islands found in the tropical oceans, consisting of coral reefs surrounding a central depression. The depression may be a part of the sea (a lagoon), or may sometimes enclose a body of fresh, brackish or highly saline water.

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.

Of the five, the Pacific is the largest ocean, the Atlantic the second largest and the Indian the third largest.

How does the submarine relief differ in the Pacific, Atlantic and Indian Oceans?

All three oceans have the same building blocks (shelf, slope, deep sea plain, ridges and deeps), but each arranges them differently. The Pacific is the deepest of the three, with an average depth of about 4,000 m; the Atlantic and the Indian Oceans are a little shallower on average.

The Pacific Ocean

The Pacific covers about one-third of the earth's surface. Its continental shelves are very narrow along the coasts of the Americas, where the sea floor drops steeply into deep trenches. A chain of trenches rings the ocean along its island arcs: the Aleutian, Kuril, Japan, Mariana and Tonga trenches in the north and west, and the Peru-Chile (Atacama) Trench in the east. The Challenger Deep in the Mariana Trench is the deepest point on the earth, nearly 11,000 m below sea level.

The Pacific floor is also dotted with thousands of seamounts and guyots and with coral atolls. Its mid-ocean ridge, the East Pacific Rise, lies close to the Americas rather than in the middle of the ocean, and the Hawaiian Islands with the Emperor seamounts form a long chain of volcanoes across its floor.

The Atlantic Ocean

The Atlantic's main feature is the S-shaped Mid-Atlantic Ridge, which runs about 16,000 km from Iceland in the north to near Bouvet Island in the far south, parallel to the coasts. It divides the ocean floor into an eastern and a western trough. Islands such as Iceland, the Azores, Ascension, St Helena and Tristan da Cunha rise from the ridge.

The Atlantic has wide continental shelves, especially off north-west Europe (the North Sea and the Dogger Bank) and off Newfoundland (the Grand Banks), which are rich fishing grounds. It has few trenches; the deepest point is in the Puerto Rico Trench, about 8,400 m deep.

The Indian Ocean

The Indian Ocean is enclosed by land on three sides. Its mid-ocean ridge, the Mid-Indian Ridge, is shaped like an inverted Y; its northern arm in the Arabian Sea is the Carlsberg Ridge. The Ninety East Ridge runs in an almost straight line north to south along the 90° E meridian.

The shelf is wide along the west coast of India and at the head of the Bay of Bengal, and the Mumbai High oil field lies on it. The ocean has only one major trench, the Java (Sunda) Trench south of Indonesia, more than 7,000 m deep. Its islands are of several kinds: continental (Madagascar, Sri Lanka), volcanic (Mauritius, Reunion) and coral (Lakshadweep, the Maldives).

What are sea floor deposits and why are marine resources important?

The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers and wind from the land and distributed by waves and currents.

Massive sedimentary deposits received over a long time by the continental shelves become the source of fossil fuels. The deep sea plains are covered with fine-grained sediments such as clay and silt.

Kinds of sea floor deposits

Sea floor deposits are of two main kinds. Terrigenous deposits come from the land: gravel, sand and mud carried by rivers, glaciers and wind. They are coarse near the coast and finer farther out, and they cover the shelves and slopes. Pelagic deposits form in the open ocean, far from land. They are fine and build up very slowly, only a few millimetres in a thousand years.

Why the shelves are the most productive part of the sea

The shelves are the most productive parts of the ocean for human use. Their shallow, sunlit water allows sunlight to reach the sea bed, so plankton can grow and fish gather there; this is why the world's best fishing grounds lie in the shallow shelf seas and in zones where warm and cold currents mix.

The shelves also hold petroleum and natural gas in their thick sediments, and their sands yield minerals. Tides and currents over the shelves can be used to generate electricity. The deep sea floor holds polymetallic (manganese) nodules rich in manganese, nickel, copper and cobalt; India has rights to explore an area of the Central Indian Ocean Basin for them.

Note: The mixing of warm and cold currents helps to replenish the oxygen and favours the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.

What is ocean pollution and how can it be overcome?

Ocean pollution is the addition to sea water of substances or forms of energy that harm marine life, human health or the use of the sea. The main sources are:

1. Sewage and river pollution: polluted river water carries untreated waste into the sea, and the pollution of river waters aggravates the water crisis on land and at the coast.
2. Oil spills: oil from tankers and offshore wells floats on the surface, coats the feathers of sea birds and blocks oxygen from entering the water.
3. Plastics and solid waste: these are dumped from ships and coasts and are swallowed by fish and other animals.
4. Agricultural runoff: fertilisers and pesticides washed off fields cause excess growth of algae, which uses up oxygen in the water.
5. Industrial waste and heated water: factories discharge chemicals and warm cooling water that change the temperature and chemistry of the sea.

Ways to overcome ocean pollution

Ways to overcome ocean pollution include treating sewage and industrial waste before it reaches rivers and the sea, enforcing laws against dumping waste and oil at sea, reducing the use of plastics and recycling them, using fertilisers carefully on farms, and protecting coastal wetlands and mangroves that filter pollutants. International agreements and coastal clean-up campaigns also help.

What controls the temperature of ocean water?

Ocean waters get heated up by solar energy just as land does, but the process of heating and cooling of oceanic water is slower than on land. The factors which affect the distribution of temperature of ocean water are:

1. Latitude: the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
2. Unequal distribution of land and water: the oceans in the northern hemisphere receive more heat due to their contact with a larger extent of land than the oceans in the southern hemisphere.
3. Prevailing wind: 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, which produces longitudinal variation in temperature. Onshore winds, on the contrary, pile up warm water near the coast and raise the temperature.
4. Ocean currents: warm ocean currents raise the temperature in cold areas while cold currents decrease the temperature in warm ocean areas. The Gulf Stream (a warm current) raises the temperature near the eastern coast of North America and the west coast of Europe, while the Labrador Current (a cold current) lowers the temperature near the north-east coast of North America.

Horizontal distribution of temperature

All these factors influence the temperature of the ocean locally. The enclosed seas in the low latitudes record relatively higher temperature than the open seas, whereas the enclosed seas in the high latitudes have lower temperature than the open seas.

The average temperature of surface water of the oceans 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 degree of latitude.

The average temperature is around 22°C at 20° latitudes, 14°C at 40° latitudes and 0°C near the 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 hemispheres are around 19°C and 16°C respectively, a variation due to the unequal distribution of land and water.

The highest temperature is not recorded at the equator but slightly towards north of it.

Vertical distribution of temperature

The maximum temperature of the oceans is always at their surfaces because they directly receive heat from the sun, and the heat is transmitted to the lower sections through the process of convection.

The temperature falls very rapidly up to the depth of 200 m, and thereafter the rate of decrease is slowed down.

Plot increasing depth (m) from 0 to 4,500 on the vertical axis against increasing temperature (°C) from 0° to 24° on the horizontal axis. Draw a curve that is high near the surface, drops sharply through a shaded thermocline band, and levels off at greater depth.

On a world outline, draw isotherms for 0, 5, 10, 15, 20, 25 and 28°C across the oceans, with latitude and longitude gridlines at 0°, 90° and 180°.

The thermocline: how temperature changes with depth

The temperature-depth profile for ocean water shows how temperature decreases with increasing depth. The profile shows a boundary region between the surface waters of the ocean and the deeper layers.

This boundary usually begins around 100 to 400 m below the sea surface and extends several hundred 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, where temperatures approach 0°C.

The three-layer system of middle and low latitudes

The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to bottom:

1. Top layer: warm oceanic water, about 500 m thick, with temperatures ranging between 20° and 25°C. Within the tropical region it is present throughout the year, but in mid latitudes it develops only during summer.
2. Thermocline layer: lies below the first layer and is characterised by a rapid decrease in temperature with increasing depth. It is 500 to 1,000 m thick.
3. Third layer: very cold and extends up to the deep ocean floor.

The single cold layer of polar waters

In the Arctic and Antarctic circles, the surface water temperatures are close to 0°C, so the temperature change with depth is very slight. Here only one layer of cold water exists, which extends from the surface to the deep ocean floor.

What is salinity and which factors control its distribution?

All waters in nature, whether rain water or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in sea water.

It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater, and is usually expressed as parts per thousand (o/oo) or ppt. Salinity is an important property of sea water.

A salinity of 24.7 o/oo has been considered as the upper limit to demarcate brackish water.

The composition of sea water

Sea water is a solution of many salts. Sodium chloride (common salt) makes up nearly four-fifths of the dissolved salts, followed by the chlorides and sulphates of magnesium. The table gives the usual composition of sea water with a salinity of 35 o/oo.

Factors affecting salinity

The factors affecting ocean salinity are:

1. The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.
2. Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
3. Wind also influences salinity of an area by transferring water to other areas.
4. The ocean currents contribute to salinity variations. Salinity, temperature and density of water are interrelated, so any change in temperature or density influences the salinity of water in an area.

Horizontal distribution of salinity

The salinity for normal open ocean ranges between 33 o/oo and 37 o/oo. In the land-locked Red Sea it is as high as 41 o/oo, while in estuaries and the Arctic it fluctuates from 0 to 35 o/oo seasonally.

In hot and dry regions, where evaporation is high, the salinity sometimes reaches 70 o/oo.

The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.

Salinity decreases from 35 o/oo to 31 o/oo on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region, and after 15° to 20° south it decreases to 33 o/oo.

The average salinity of the Atlantic Ocean is around 36 o/oo. The highest salinity is recorded between 15° and 20° latitudes, and maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W to 60° W; it gradually decreases towards the north.

The average salinity of the Indian Ocean is 35 o/oo. Low salinity is observed in the Bay of Bengal due to the influx of river water, while the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

On a world outline, draw salinity contours for 30, 32, 33, 34, 35, 36 and 37 o/oo, marking the Equator, the Tropic of Cancer and the Tropic of Capricorn.

How do salinity and density change with depth?

Salinity changes with depth, but the way it changes depends upon the location of the sea.

Salinity at the surface increases by the loss of water to ice or evaporation, or decreases by the input of fresh waters such as from rivers.

Salinity at depth is very much fixed, because there is no way that water is lost or salt is added. There is a marked difference in salinity between the surface zones and the deep zones of the oceans.

The halocline and stratification by salinity

The lower salinity water rests above the higher salinity dense water. Salinity generally increases with depth, and there is a distinct zone called the halocline, where salinity increases sharply.

Other factors being constant, increasing salinity of seawater causes its density to increase. High salinity seawater generally sinks below the lower salinity water, and this leads to stratification by salinity.

What density depends on

Density is the mass of a given volume of sea water. It depends on temperature and salinity together: cold water is denser than warm water, and saline water is denser than fresher water.

Denser water tends to sink, while relatively lighter water tends to rise. This vertical movement is one of the ways in which the deep water of the oceans is renewed.

How are waves formed and what are their parts?

Waves are actually the energy, not the water as such, which moves across the ocean surface. Water particles only travel in a small circle as a wave passes.

Wind provides energy to the waves: wind causes waves to travel in the ocean and the energy is released on shorelines. The motion of the surface water seldom affects the stagnant deep bottom water of the oceans.

As a wave approaches the beach it slows down, due to the friction occurring between the dynamic water and the sea floor. When the depth of water becomes less than about half the wavelength, the wave begins to drag on the bottom: it slows, its crest steepens and it soon breaks.

How wind builds waves

The largest waves are found in the open oceans, and waves continue to grow larger as they move and absorb energy from the wind.

Most waves are caused by the wind driving against water. When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until white caps appear in the breaking waves.

Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf. A wave's size and shape reveal its origin: steep waves are fairly young ones and are probably formed by local wind, while slow and steady waves originate from far away places, possibly from another hemisphere.

The maximum wave height is determined by the strength of the wind, that is, how long it blows and the area over which it blows in a single direction.

Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward. The falling water pushes the former troughs upward, and the wave moves to a new position.

The actual motion of the water beneath the waves is circular: things are carried up and forward as the wave approaches, and down and back as it passes.

The parts and characteristics of a wave

1. Wave crest and trough: the highest and lowest points of a wave are called the crest and trough respectively.
2. Wave height: the vertical distance from the bottom of a trough to the top of a crest of a wave.
3. Wave amplitude: one-half of the wave height.
4. Wave period: the time interval between two successive wave crests or troughs as they pass a fixed point.
5. Wavelength: the horizontal distance between two successive crests.
6. Wave speed: the rate at which the wave moves through the water, measured in knots.
7. Wave frequency: the number of waves passing a given point during a one-second time interval.

Draw a cross-section of waves moving across a water surface, with crests and troughs and an arrow showing the wave travelling toward the right. Show the circular motion of water particles beneath the waves, with crests falling and troughs rising.

What causes tides and what are spring and neap tides?

The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.

Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges; surges are not regular like tides.

The study of tides is very complex, spatially and temporally, as tides have great variations in frequency, magnitude and height.

The origin of tides

The moon's gravitational pull to a great extent, and to a lesser extent the sun's gravitational pull, are the major causes for the occurrence of tides. Another factor is centrifugal force, the force that acts to counterbalance gravity.

Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth.

On the side of the earth facing the moon a tidal bulge occurs, while on the opposite side, though the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes a tidal bulge.

The tide-generating force is the difference between these two forces, that is, the gravitational attraction of the moon and the centrifugal force.

On the surface of the earth nearest the moon, the attractive force of the moon is greater than the centrifugal force, so there is a net force causing a bulge towards the moon.

On the opposite side, the attractive force is less as it is farther away from the moon and the centrifugal force is dominant, so there is a net force away from the moon, creating the second bulge.

Direct and indirect tides, and the time of tides

The high tide on the side of the earth facing the moon is the direct tide. The high tide at the same time on the opposite side of the earth is the indirect tide (also called the opposite tide). As the earth rotates, each place passes through both bulges, so most places have two high tides and two low tides a day.

Because the moon moves forward in its orbit while the earth rotates, the earth takes about 24 hours 52 minutes to bring a place under the moon again. High tides therefore come about 12 hours 26 minutes apart, and the tides arrive about 50 minutes later each day.

On the surface of the earth, the horizontal tide-generating forces are more important than the vertical forces in generating the tidal bulges.

The tidal bulges on wide continental shelves have greater height. When tidal bulges hit the mid-oceanic islands they become low.

The shape of bays and estuaries along a coastline can also magnify the intensity of tides, and funnel-shaped bays greatly change tidal magnitudes. When the tide is channelled between islands or into bays and estuaries they are called tidal currents.

Draw three diagrams of the earth with the moon and sun to one side. In the first show gravitational force, in the second centrifugal force, and in the third the two resultant tidal bulges on the earth.

Spring tides and neap tides

The height of rising water (high tide) varies appreciably depending upon the position of the sun and moon with respect to the earth.

Spring tides occur when the sun, the moon and the earth are in a straight line, so the height of the tide will be higher.

They occur twice a month, one on full moon period and another during new moon period.

Neap tides occur when the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another.

Normally there is a seven day interval between the spring tides and neap tides. The moon's attraction, though more than twice as strong as the sun's, is diminished by the counteracting force of the sun's gravitational pull.

Once in a month, when the moon's orbit is closest to the earth (perigee), unusually high and low tides occur and the tidal range is greater than normal.

Two weeks later, when the moon is farthest from earth (apogee), the moon's gravitational force is limited and the tidal ranges are less than their average heights.

When the earth is closest to the sun (perihelion), around 3rd January each year, tidal ranges are also much greater, with unusually high and unusually low tides.

When the earth is farthest from the sun (aphelion), around 4th July each year, tidal ranges are much less than average.

The time between the high tide and low tide, when the water level is falling, is called the ebb. The time between the low tide and high tide, when the tide is rising, is called the flow or flood.

Types of tides

Case study: Tides of the Bay of Fundy, Canada

The highest tides in the world occur in the Bay of Fundy, between New Brunswick and Nova Scotia in Canada. The tidal bulge is 15 to 16 m.

Because there are two high tides and two low tides every day (roughly a 24 hour period), a tide must come in within about a six hour period.

As a rough estimate, taking the range as 15 m, the tide rises about 2.5 m an hour (1,500 cm divided by 6 hours).

A person walking down a beach with a steep cliff alongside, which is common there, must watch the tides: after walking for about an hour, if the tide is coming in, the water will be over your head before you get back to where you started.

Worked example 1. The tidal bulge at the Bay of Fundy is 15 to 16 m.

There are two high tides and two low tides every day, roughly a 24 hour period, so a tide comes in within about a six hour period. Estimate the rise of the tide per hour.

Answer: Take the tidal range as 15 m, that is 1,500 cm. Divide by 6 hours: 1,500 ÷ 6 = 250 cm an hour. So the tide rises about 250 cm, or 2.5 m, every hour.

Why are tides important?

Since tides are caused by the earth-moon-sun positions, which are known accurately, tides can be predicted well in advance. This helps navigators and fishermen plan their activities.

Tidal flows are of great importance in navigation, and tidal heights are very important, especially for harbours near rivers and within estuaries having shallow bars at the entrance, which prevent ships and boats from entering the harbour.

Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.

Tides are used to generate electrical power in Canada, France, Russia, China and South Korea. In India a 3 MW tidal power project was planned at Durgaduani in the Sundarbans of West Bengal, and the Gulf of Khambhat and the Gulf of Kutch in Gujarat have the country's greatest tidal power potential.

What are ocean currents and which factors affect them?

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: primary forces that initiate the movement of water, and secondary forces that influence the currents to flow.

The primary forces

The primary forces that influence the currents are: (i) heating by solar energy; (ii) wind; (iii) gravity; (iv) coriolis force.

Heating by solar energy causes the water to expand, which 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. Wind blowing on the surface of the ocean pushes the water to move, and friction between the wind and the water surface affects the movement of the water body in its course.

Gravity tends to pull the water down the pile and create gradient variation. 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, and they produce large circular currents in all the ocean basins.

Density and the vertical movement of water

Differences in water density affect the vertical mobility of ocean currents. Water with high salinity is denser than water with low salinity, and in the same way cold water is denser than warm water.

Denser water tends to sink, while relatively lighter water tends to rise. Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator.

Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water.

Characteristics and types of currents

Currents are referred to by their drift, the speed of the current, which is measured in knots.

Usually the currents are strongest near the surface and may attain speeds over five knots; at depths, currents are generally slow with speeds less than 0.5 knots.

A current is usually strongest at the surface and decreases in strength with depth, and most currents have speeds less than or equal to 5 knots.

Which currents flow in the Indian, Pacific and Atlantic Oceans?

Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and the Coriolis force. 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, more pronounced in the southern hemisphere than in the northern hemisphere, and the oceanic circulation pattern corresponds with the same.

At higher latitudes, where the wind flow is mostly cyclonic, the oceanic circulation follows this pattern. In regions of pronounced monsoonal flow, the monsoon winds influence the current movements.

Due to the Coriolis force, the warm currents from low latitudes tend to move to the right in the northern hemisphere and to their left in the southern hemisphere.

The oceanic circulation transports heat from one latitude belt to another in a manner similar to the heat transported by the general circulation of the atmosphere.

The cold waters of the Arctic and Antarctic circles move towards warmer water in tropical and equatorial regions, while the warm waters of the lower latitudes move polewards.

In the northern Indian Ocean the currents reverse with the monsoon winds: the south-west monsoon drift flows from west to east in summer, and the north-east monsoon drift flows from east to west in winter.

On a world outline, draw arrows for the major warm and cold currents of the three oceans, with a legend distinguishing cold and warm currents, the Equator marked, and a scale bar.

How do ocean currents modify the climate of coastal areas?

Ocean currents have a number of direct and indirect influences on human activities. West coasts of the continents in tropical and subtropical latitudes, except close to the equator, are bordered by cool waters.

Their average temperatures are relatively low with a narrow diurnal and annual range. There is fog, but generally the areas are arid.

West coasts of the continents in the middle and higher latitudes are bordered by warm waters which cause a distinct marine climate, characterised by cool summers and relatively mild winters with a narrow annual range of temperatures.

Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes, which results in warm and rainy climates; these areas lie in the western margins of the subtropical anticyclones.

The mixing of warm and cold currents helps to replenish the oxygen and favours the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.

Case study: The Gulf Stream and the climate of north-west Europe

The Gulf Stream is a warm current that raises the temperature near the eastern coast of North America and the west coast of Europe, while the Labrador Current, a cold current, lowers the temperature near the north-east coast of North America.

Because the warm current flows along the west coasts of Europe in middle and higher latitudes, these coasts have a distinct marine climate with cool summers and relatively mild winters and a narrow annual range of temperature.

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, a continuation of the same warm current system.

Worked example 2. Two stations lie at the same latitude in Europe (illustrative figures). Station A is on the west coast, washed by a warm current, and has a January mean of 6°C and a July mean of 16°C.

Station B, far inland at the same latitude, has a January mean of −4°C and a July mean of 20°C. Compare their annual ranges and explain the difference.

Answer: Annual range of A = 16 − 6 = 10°C. Annual range of B = 20 − (−4) = 24°C.

Station A has a narrow annual range and mild winters because the warm current raises the temperature of the coastal water and the onshore winds carry its warmth over the land, giving a marine climate.

Station B is far from the moderating sea, so it has colder winters, warmer summers and a much wider range.

What are El Niño and La Niña and how do they affect the Indian monsoon?

El Niño is the abnormal warming of the surface waters of the eastern equatorial Pacific Ocean, off the coast of Peru and Ecuador, which occurs every two to seven years and lasts for many months.

La Niña is the opposite condition, an abnormal cooling of the same eastern equatorial Pacific waters.

The two are the warm and cold phases of a see-saw in the Pacific Ocean that also shifts the pressure and wind patterns over the tropics.

How the two conditions affect the monsoon

These conditions affect the intensity of the monsoons over India. In an El Niño year the monsoon over India is often weak, with below normal rainfall and a greater chance of drought.

In a La Niña year the monsoon is often strong, with above normal rainfall and a greater chance of floods.

The effect is not the same in every year, because other factors such as the pressure over the Indian Ocean and the movement of the monsoon trough also influence the monsoon.

The mechanism in steps

1. The trade winds over the Pacific normally blow from east to west, piling warm surface water in the west and allowing cold, nutrient-rich water to upwell off the Peruvian coast.
2. In an El Niño year these trade winds weaken or reverse, so the warm surface water spreads eastward across the equatorial Pacific.
3. The upwelling off Peru stops, which harms the fishing there, and the pattern of rainfall shifts eastward, bringing heavy rain to the normally dry Peruvian coast.
4. The changed pattern of pressure and rising air over the Pacific alters the circulation over the Indian Ocean, and the monsoon over India often weakens.
5. In a La Niña year the trade winds strengthen, the eastern Pacific cools, and the monsoon over India is often stronger than normal.

Note: El Niño and La Niña are conditions that affect the intensity of the monsoons over India. El Niño is associated with weak monsoons and La Niña with strong monsoons, though the link is not the same in every year.

Glossary

• Hydrological cycle: the circulation of water within the earth's hydrosphere in liquid, solid and gaseous phases, and its continuous exchange between oceans, atmosphere, land and organisms.
• Continental shelf: the shallow, gently sloping extended margin of a continent under the sea, ending at the shelf break.
• Shelf break: the steep slope at the outer edge of the continental shelf where it drops to the continental slope.
• Thermocline: the boundary zone, usually 100 to 400 m below the surface, where temperature falls rapidly with depth.
• Halocline: a distinct zone in the ocean where salinity increases sharply with depth.
• Salinity: the total content of dissolved salts in sea water, measured as grams of salt per 1,000 gm of seawater.
• Brackish water: water with salinity up to about 24.7 o/oo, between fresh water and normal sea water.
• Guyot: a flat-topped seamount formed by gradual subsidence of a submerged volcanic mountain.
• Atoll: a low tropical island of coral reef surrounding a central lagoon or enclosed body of water.
• Tide: the periodical rise and fall of sea level, once or twice a day, mainly due to the attraction of the sun and the moon.
• Spring tide: a higher tide occurring when the sun, moon and earth are in a straight line, at full moon and new moon.
• Neap tide: a lower tide occurring when the sun and moon are at right angles and their forces counteract.
• Perigee: the position of the moon when its orbit is closest to the earth, giving unusually high and low tides.
• Gyres: large accumulations of water and the circular flow around them, produced by the Coriolis force in ocean basins.
• Drift: the speed of an ocean current, measured in knots.
• Direct tide: the high tide on the side of the earth facing the moon; the high tide on the opposite side is the indirect tide.
• Pelagic deposit: fine sediment of the open ocean, such as ooze or red clay, that builds up very slowly.

Common errors and misconceptions

• Misconception: In a wave the water itself travels across the ocean. Correct: It is the energy that moves; water particles only travel in a small circle as a wave passes.
• Misconception: The highest ocean temperature is at the equator. Correct: The highest temperature is recorded slightly north of the equator, because of the unequal distribution of land and water.
• Misconception: Spring tides occur in spring season. Correct: Spring tides occur twice a month, at full moon and new moon, when the sun, moon and earth are in a straight line.
• Misconception: Salinity is highest in the open ocean. Correct: Normal open ocean salinity is 33 to 37 o/oo; land-locked water bodies such as Lake Van (330 o/oo) and the Dead Sea (238 o/oo) are far more saline.
• Misconception: The continental shelf is the steepest part of the ocean floor. Correct: The shelf has an average gradient of 1° or less; the continental slope has a gradient of 2 to 5°.
• Misconception: Tides and surges are the same. Correct: Tides are regular and caused by the sun and moon; surges are caused by meteorological effects and are not regular.
• Misconception: El Niño always brings drought to India. Correct: El Niño is often associated with a weak monsoon, but other factors also influence the monsoon, so the effect is not the same every year.

Exam-style questions with model answers

Q1. State what is meant by the thermocline and give its usual depth range. [2 marks]

1. The thermocline is the boundary region between the surface waters of the ocean and the deeper layers, where temperature decreases rapidly with depth.
2. It usually begins around 100 to 400 m below the sea surface and extends several hundred metres downward.

Q2. Distinguish between spring tides and neap tides. [3 marks]

1. Spring tides occur when the sun, the moon and the earth are in a straight line, so the gravitational pulls of the sun and moon act together and the tide is higher.
2. Neap tides occur when the sun and moon are at right angles to each other, so their forces tend to counteract one another and the tide is lower.
3. Spring tides occur twice a month, at full moon and new moon, while neap tides occur about seven days after each spring tide.

Q3. Explain any three factors that control the distribution of salinity in the oceans. [3 marks]

1. Evaporation and precipitation: high evaporation raises surface salinity, as in the Red Sea (41 o/oo), while heavy rain or river inflow lowers it.
2. Fresh water flow from rivers: the Bay of Bengal has low salinity because of river influx, and the Baltic Sea is low for the same reason.
3. Freezing and thawing of ice in polar regions, and ocean currents: the North Sea has higher salinity because more saline water is brought by the North Atlantic Drift.

Q4. Describe the four major divisions of the ocean floor, giving the depth range of each. [4 marks]

1. The continental shelf is the shallowest part, with a gradient of 1° or less and a depth of 30 m to 600 m, ending at the shelf break.
2. The continental slope connects the shelf and the ocean basins, with a gradient of 2 to 5° and a depth of 200 to 3,000 m.
3. The deep sea plain is the flattest and smoothest region, with depths of 3,000 to 6,000 m, covered with clay and silt.
4. The oceanic deeps or trenches are the deepest parts, 3 to 5 km deeper than the surrounding floor, occurring along island arcs and associated with volcanoes and earthquakes.

Q5. Explain how ocean currents modify the climate of coastal areas, with examples. [5 marks]

1. West coasts of continents in tropical and subtropical latitudes are bordered by cool waters, so their average temperatures are relatively low with a narrow diurnal and annual range; there is fog, but the areas are generally arid.
2. West coasts in middle and higher latitudes are bordered by warm waters, which cause a distinct marine climate with cool summers and relatively mild winters and a narrow annual range of temperature.
3. Warm currents flow parallel to the east coasts of continents in tropical and subtropical latitudes, giving warm and rainy climates; these areas lie in the western margins of the subtropical anticyclones.
4. The Gulf Stream raises the temperature near the eastern coast of North America and the west coast of Europe, while the Labrador Current lowers the temperature near the north-east coast of North America.
5. The mixing of warm and cold currents replenishes oxygen and favours the growth of planktons, so the best fishing grounds of the world lie mainly in these mixing zones.

Q6. Explain the origin of tides and state their importance. [5 marks]

1. Tides are the periodical rise and fall of sea level, once or twice a day, mainly due to the attraction of the sun and the moon.
2. The moon's gravitational pull, to a great extent, and the sun's, to a lesser extent, together with centrifugal force, create two tidal bulges, one facing the moon and one on the opposite side.
3. The tide-generating force is the difference between the gravitational attraction of the moon and the centrifugal force; horizontal forces are more important than vertical forces in generating the bulges.
4. Tides can be predicted well in advance, which helps navigators and fishermen; tidal heights matter for harbours with shallow bars, and tides help desilt sediments and remove polluted water from estuaries.
5. Tides are used to generate electrical power in Canada, France, Russia, China and South Korea, and the Gulf of Khambhat and the Gulf of Kutch have India's greatest tidal power potential.

Q7. Explain how El Niño and La Niña affect the intensity of the monsoon over India. [4 marks]

1. El Niño is the abnormal warming of the surface waters of the eastern equatorial Pacific, and La Niña is the abnormal cooling of the same waters.
2. In an El Niño year the monsoon over India is often weak, with below normal rainfall and a greater chance of drought.
3. In a La Niña year the monsoon is often strong, with above normal rainfall and a greater chance of floods.
4. The effect is not the same in every year, because other factors such as pressure over the Indian Ocean also influence the monsoon.

Key takeaways

• About 97 per cent of the planetary water is in the oceans, and the earth is called the Blue Planet because of its abundant surface water.
• The ocean floor has four major divisions: continental shelf, continental slope, deep sea plain and oceanic deeps, each with its own gradient and depth range.
• Minor relief features include mid-oceanic ridges, seamounts, submarine canyons, guyots and atolls.
• Ocean temperature decreases from the equator poleward at about 0.5°C per degree of latitude, and falls rapidly with depth through the thermocline.
• Salinity is the salt content in grams per 1,000 gm of seawater; normal open ocean salinity is 33 to 37 o/oo.
• Waves carry energy, not water; tides are caused by the sun and moon, and spring and neap tides depend on their positions.
• Ocean currents are driven by solar heating, wind, gravity and the Coriolis force, and they modify the climate of coastal areas.
• El Niño and La Niña affect the intensity of the Indian monsoon, though the link is not the same in every year.

Test yourself

Why is the earth called the Blue Planet?

Water is a rare commodity in the solar system, but the earth has an abundant supply of water on its surface, so it is called the Blue Planet.

What is a seamount?

A seamount is a mountain with pointed summits, rising from the seafloor, that does not reach the surface of the ocean. Seamounts are volcanic in origin and can be 3,000 to 4,500 m tall.

What is the difference between wave height and wave amplitude?

Wave height is the vertical distance from the bottom of a trough to the top of a crest. Wave amplitude is one-half of the wave height.

What is the Coriolis force?

The Coriolis force is the force that causes water to move to the right in the northern hemisphere and to the left in the southern hemisphere, producing gyres in the ocean basins.

What is the average salinity of the Indian Ocean?

The average salinity of the Indian Ocean is 35 o/oo. The Bay of Bengal has lower salinity due to river influx, while the Arabian Sea has higher salinity due to high evaporation.

What is a halocline?

A halocline is a distinct zone in the ocean where salinity increases sharply with depth, leading to stratification by salinity.

What is the difference between ebb and flow?

Ebb is the time between high tide and low tide, when the water level is falling. Flow or flood is the time between low tide and high tide, when the tide is rising.

Name the deepest point of the Pacific, the Atlantic and the Indian Ocean.

The Challenger Deep in the Mariana Trench, nearly 11,000 m, is the deepest point of the Pacific and of the earth. The Atlantic is deepest in the Puerto Rico Trench (about 8,400 m) and the Indian Ocean in the Java (Sunda) Trench (over 7,000 m).