NCERT Geography Class-VII -Water

CHAPTER 6-Water

When seen from space, our Earth appears as a blue ball. This is because 71% of the Earth's surface is covered with water. No wonder it is also known as the blue planet. Water found on the Earth in all its forms-running water in oceans, seas, rivers, and lakes, ice in glaciers, underground water, and water vapour (in air) -makes up the hydrosphere. Most of the water present on the Earth is found in large water bodies called oceans. Oceans account for 97% of the water on the Earth, the rest (3%) is found in rivers, ponds, streams, underground springs, glaciers, etc. Therefore, we can say that the oceans are the main constituents of the hydrosphere. In all, there are four major oceans in the world the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean. Besides them, there is mass of water surrounding the continent of Antarctica. It is formed by the meeting of the Indian Ocean, the Atlantic Ocean, and the Pacific Ocean and is called the Southern or Antarctic Ocean. The oceans are all interconnected, but their continuity is broken by different landmasses. In this chapter we will learn about water bodies especially ocean and associated features.

 

 

THE HYDROLOGICAL CYCLE

Life is possible on our planet because of the presence of water in all its forms- solid, liquid, and gas. Water is constantly changing its form on the Earth. Water keeps evaporating from land and water bodies because of the heat of the Sun A large amount of water evaporates from the oceans every day/In fact, four-fifths of the total water that evaporates from the Earth's surface is from the oceans. Warm vapour of water cools down as it rises up and condenses to form clouds. The clouds shed excess moisture in the form of precipitation, such as rain, snow, sleet, or hail. In this way, water is continuously replenished back to the Earth. This constant change in the form of water and its circulation between the lithosphere, atmosphere, and hydrosphere is called the hydrological cycle or the water cycle.

 

 

DISTRIBUTION OF WATER BODIES

Water is present in two forms on the Earth - fresh water and ocean water. Ocean water is salty, and therefore we cannot consume it. Only 3% of water found on the Earth is fresh water. It has very little quantity of dissolved salts and therefore, it is fit for use. This water is found in ground water, lakes, rivers, air, etc. Most of it is locked in ice sheets and glaciers in Polar and mountain regions. Therefore, fresh water that is actually available for use is in a very limited quantity (about 1% of total water on the Earth). Precipitation in the from of rain, snow sleet hail, etc., is a major source of fresh water on the Earth.

 

The distribution of water on the Earth is highly uneven. If you look at a globe, you will find that most of the oceans and seas lie in the Southern Hemisphere. Therefore, it is also referred to as the Water Hemisphere.

 

SALINITY OF OCEAN WATERS

Water in the seas and oceans is quite salty, or saline. In fact, sea water is more than 200 times saltier than fresh water. We know that rivers originate in the mountains and end in the seas, Rivers erode the rocks and landforms they pass through as they travel towards the sea. In the process, salts and minerals get dissolved in the water and are carried to the sea. Most of the salt found in sea water is sodium chloride or table salt, which we use in our food. Although all oceans are saline, the amount of salts in their waters varies.

 

Salinity of ocean water is defined as the amount of salts in 1000 grams of water. The average salinity of sea water is 35 parts per thousand. Of this, 26 parts is sodium chloride. The Dead Sea has a salinity of 340 parts per thousand. Because of the high amount of salt, swimmers can float in Dead Sea water without any effort.

 

 

The salinity of oceans or seas depends on the rate of evaporation of water and the amount of rainfall received. Tropical oceans have the most saline water as they receive the greater part of the heat from the Sun and receive little rainfall. In the equatorial regions, sea and ocean water is less saline in spite of the high rate of evaporation. This is because of high rainfall in the regions.

 

 

 

Circulation of water in the oceans

Like rivers, water in oceans is constantly moving. The movement of ocean water can be divided into waves, tides, and currents. The circulation of water in the oceans is a result of wind movements, uneven heating of oceans, differences in temperature, salinity, and density of oceans' water, and gravitational forces.

 

WAVES

The continuous and rhythmic rise and fall of ocean water is called a wave. The top part of a wave is called a crest. The hollow between two consecutive crests is called a trough. The horizontal distance between two consecutive crests or troughs is known as wavelength. The vertical distance between a crest and a trough is known as wave height.

 

Mostly, waves are formed when winds blow over the surface of oceans. Winds transfer their energy to the molecules of water, which in turn pass on the energy in the form of waves. The molecules of water do not move forward. Instead, they move up and down at the same place and transmit their energy to adjacent molecules. The stronger the wind, the larger is the wave. During a storm or a cyclone, winds blow at a very high speed. Such winds create huge waves, which can cause a lot of destruction.

 

The size and height of the waves also depends on other phenomena such as earthquakes, landslides, and volcanic eruptions on the sea bed. These can shift huge amount of ocean water to form tsunamis, i.e., giant waves that travel at very high speeds. Such waves can reach heights of up to 15 m. Tsunami waves have even been known to reach heights of 150 m. These waves can travel at a speed of more than 700 km per hour. Tsunami means 'harbour waves' in Japanese, because these waves hit the shores with a great force and cause great damage to harbours. They can even travel inland and submerge large portions of land.

 

TIDES

Ocean water advances towards and recedes from the shore twice a day. Such a rhythmic rise and fall of water is called tide. When ocean water advances towards the shore and covers most of it by rising to its highest level, it is said to be high tide. When ocean water moves away from the shore and falls to its lowest level, it is said to be low tide. Tides are caused by the gravitational pull exerted by the Moon, and to some extent by the Sun, on the Earth's surface. All celestial bodies exert some gravitational pull on each other. The intensity of this force depends on the mass of the bodies as well as their distance from each other. The Moon exerts a strong gravitational pull on the Earth because it is very close to it. The Sun, although it is farther from the Earth than the Moon, exerts a gravitational pull on the Earth due to its mass. The effect of the pull of the Moon and the Sun is seen most in the oceans in the form of tides.

 

The gravitational pull of the Moon causes ocean water to bulge in the part of the Earth facing the Moon and the one facing away from it. These bulges are known as tidal bulges. The Moon's pull is the strongest at point as it is closest to it.each place on the Earth experiences two high tides and two low tides every 24 hours.

 

Spring tide and Neap tide

Spring tides form on Full Moon and New Moon days, i.e., when the Sun, the Moon, and the Earth fall in the same line. On these days, the combined gravitational pull of the Sun and the Moon produces tides which are higher than usual. In fact, high tides are the highest and low tides the lowest on such days.

 

Neap tides are formed when the Moon is in its first and last quarters, i.e., the Moon and the Sun are at right angles in relation to the Earth. On such days, the gravitational pull of the Sun and the Moon tend to neutralise each other, which results in lower height of the tides. On such days, low tides are not very low and high tides are not very high.

 

Importance of tides

Tides play an important role. Fishermen depend on tides. They sail out to the sea and return home with the tides. High tides also bring fish closer to the shore for fishermen to catch. The level of water near the shore gets raised during high tides. It helps ships to navigate their way easily into ports. Tidal waters are trapped in salt pans for making salt. Accumulation of sand or silt on the bottom of a harbour is known as silting. It usually happens where a river empties its load into a bay, where the port or harbour is located. Silting reduces the depth of harbours and makes it difficult for large ships to navigate them. Tides help to prevent silting of harbours as the constant rise and fall of water takes away any mud brought in by rivers. High and low tides can also be harnessed to generate electricity.

 

OCEAN CURRENTS

'Ocean currents are streams of hot or cold water which flow continuously on the ocean surface in a particular direction. They are caused due to unequal heating of water, and differences in salinity and density of ocean waters, prevailing winds, and rotation of the Earth. The direction in which currents flow is determined by the direction of the prevailing winds. Ocean currents usually flow clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere.

 

Warm currents originate near the Equator and move towards the Polar regions." The equatorial regions get more direct heat from the Sun. As a result, oceans' water in the Equatorial regions get heated up more than the water at the Poles. This unequal heating of water generates convectional currents with warm equatorial waters flowing to the colder Polar regions. The Gulf Stream current Cold currents is an example of the warm current. originate from Polar regions or higher latitudes and move towards tropical or lower latitudes. Water at the Poles or higher latitudes is colder and heavier than the warm equatorial waters. This unequal heating of water generates convectional currents. The cold water sinks to the sea floor and moves towards the Equator or lower latitudes. The Labrador Ocean current is an example of a cold current.

 

Oceans' water contain enough salt to cover all the continents with a layer of salt almost as high as a 40-storey building.

 

 

Importance of ocean currents

Warm and cold currents help in moderating the temperature of the Earth by transferring heat between high and low latitudes. They also modify the climates of coastal areas. Warm currents affect the temperature of land surface. Areas where warm currents flow are warmer. The warm waters of the North Atlantic drift keep the coast of Norway free of ice all through the year. An ocean current which is slower, shallower, and broader than a normal current is called a drift. Areas where cold currents flow are colder. The Labrador Coast of North America, which lies on almost the same latitude as Norway, remains frozen throughout the year. This is because of the cold Labrador Ocean current flowing along its coast.

 

Winds blowing over warm currents become warm and take in more moisture. Such moisture laden winds bring rainfall to regions where these currents flow. For example, western Europe receives rainfall all through the year due to the North Atlantic Drift, a warm current, flowing along its coast. When the Westerlies blow over the North Atlantic Drift, they pick up moisture and cause rainfall in the region. Winds blowing over cold currents become cold and dry. Any moisture carried by such winds condenses into fog. For example, the cold Peru current makes the Atacama Desert of Chile, South America, the driest desert in the world.

 

Some major ocean currents

·        The Indian Ocean: Agulhas current (warm), West Australian current (cold).

·        The Pacific Ocean: Alaska current (warm), California current (cold), East Australian current (warm), West Wind drift (cold).

·        The Atlantic Ocean: Canary current (cold), Cold Benguela current (warm), Gulf stream (warm), Brazil current (warm), Labrador current (cold).

 

Ships can make use of currents to move faster and save fuel. Ships which move in the direction of currents move quicker. There are some places, such as the seas around Japan, where warm and cold currents meet. Waters where warm and cold currents meet experience dense fog, which makes navigation difficult. Areas where warm and cold currents mix, such as the Grand Bank off the coast of Newfoundland, are considered ideal for fishing.

 

WATER POLLUTION

We have seen how little water on the Earth is available for human use and how this water is continuously replenished back to the Earth through the water cycle.

However, human activity is putting a huge strain on this already scarce resource. Discharge of domestic and industrial waste into water bodies makes the water contaminated and unfit for human consumption. It also affects plant and animal life. These days, many toxic wastes such as pesticides, metals, etc., are also being dumped in water bodies. Thus, drinking water is becoming harder to find. It is our duty to help conserve water and take steps to prevent its pollution. To value the conservation of water, March 22nd has been declared as World Water Day. This day seeks to remind us that we must all do our bit to protect resources of water, for our own sake.

 

New Terms

Hydrological cycle: Constant change in the form of water and its circulation between land, air, and water bodies.

Wave: Continuous and alternate rise and fall of ocean waters.

Crest: Top part of a wave.

Trough: Hollow between two crests.

Wave height: Vertical distance between a crest and a trough.

Tsunamis: Giant waves that travel at very high speeds; formed by earthquakes, landslides, and volcanic eruptions on the sea bed, which shift huge amount of oceans' water.

Tide: Rhythmic rise and fall of water towards and away from the shore twice a day.

High tide: Ocean water advances towards the shore and covers most of it by rising to its highest level.

Low tide: Ocean water moves away from the shore and falls to its lowest level.

Spring tide: Formed on Full Moon and New Moon days when the Sun, the Moon, and the Earth fall in the same line; tides are the highest on such days.

Neap tide: Formed when the Moon is in its first and last quarters; oceans' water get pulled in opposite directions by the gravitational force of the Sun and the Earth to form low tides.

Currents: Streams of hot or cold water which flow continuously on the ocean surface in a particular direction.

Warm currents: Originate at near the Equator and move towards the Polar regions.

Cold currents: Originate from Polar regions or higher latitudes and move towards tropical or lower latitudes.

Drift: An ocean current which is slower, shallower, and broader than a normal current.