7th Class Social Science Drainage Notes - The Hydrosphere

 

The Hydrosphere

 

 

When ten year old Ranjit and his sister returned from school, they were in a thoughtful mood. Their Social Studies teacher had said that 70 per cent of the earth's surface is covered with water. Yet their mother asks them not to waste water. They asked their mother 'why??  Their mother explained that 97 per cent of the water is present in oceans and seas which is too salty to drink or to be used for farming. Only 3 per cent is fresh water in rivers, lakes and under the ground which is about a spoonful in a bucket full of water. Their mother then explained in detail about the Hydrosphere.

 

The earth is the only planet in our solar system that has the necessary elements to sustain life. These are air, water and soil. Each of these elements forms its own realm, in which it is found. The realm in which water is found is called the hydrosphere. It covers nearly 71 per cent of the earth's surface. It consists of seas, oceans and other water bodies. Thus, the hydrosphere is all the water on or near the earth's surface. Most of it is sea water which is salty. Most of the water required by plants and animals is supplied from the oceans. Do you know how? A vast quantity of water evaporates from the oceans into the atmosphere, then circulates over the land in the form of clouds, and falls on the ground in the form of rain, snow, hail or sleet. This never-ending circulation of the earth's water is called the water or hydrological cycle, which involves evaporation, transpiration, condensation, precipitation and surface runoff.

 

THE WATER CYCLE

 

The conversion of water from one form to another brings about circulation of water in nature. Along the way the water may become liquid, gas or even solid. This continuous movement of water from oceans, seas, rivers and other water bodies to the atmosphere, to the land and back to the oceans is known as the hydrological cycle or the water cycle.

 

HOW THE WATER CYCLE WORKS

 

Water in oceans, seas, lakes, ponds, rivers and soil gets heated up by the Sun and turns to water vapour through evaporation. The leaves in plants also lose water through the process of transpiration. Water vapour being lighter, on reaching the higher altitudes cools down and condenses to form clouds. The clouds when loaded with water vapour change to water droplets. In other words, condensation takes place and water comes down on the earth in various forms of precipitation like rain, snow, hail, fog, mist, dew, etc. When rain falls, water that reaches the earth's surface may seep into the ground replenishing groundwater. Some of it may run off the land filling up rivers and streams, thus bringing back the lost water from the water bodies. This is how circulation of water continues in nature forming the water cycle.

 

 

 

 

Geography Reveals

March 22 is celebrated as the World Water Day.

 

WATER DISTRIBUTION IN THE HYDROSPHERE

 

Seas and oceans are large bodies of water that make up about 97 per cent of the hydrosphere; the rest is fresh water in the form of continental ice (especially in the polar regions), lakes, rivers and ground water. A small amount of water is found in the form of hydrated minerals and in living organisms too. Water distribution in the hydrosphere can be understood with the data given here:

 

 

The Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean and the Antarctic Ocean are the five major oceans of the earth. The two remaining oceans, namely, Arctic and the Antarctic are known as lesser ocean. The Northern Hemisphere contains most of the land and the Southern Hemisphere is covered by water. That is why, climate in the Northern Hemisphere is Continental type with hot summers and cold winters, while in the Southern Hemisphere it is Oceanic type i.e., moderate and humid seasons.

 

THE COMPOSITION OF SEA WATER

 

Unlike fresh water, the ocean water is salty. This is due to the presence of dissolved minerals. Sea water contains sodium and chlorine that we find in table salt. Besides these, magnesium is found in abundance along with sulphur and other common elements. All these make sea water salty. Non-metallic elements like calcium and silicon are also present in sea water. They are extracted by marine animals to form shells and skeletons.

 

 

 

 

 

Geography Reveals

Salinity of the ocean refers to the amount of salt present in the sea water. It is measured in gm/1000 gm unit. The average salinity of the world oceans is 35 parts per thousand.

 

Q. can you find the answer why nobody can sink in the Dead Sea, Israel?

 

OCEAN CIRCULATION

 

Waters in the seas and oceans are constantly in motion due to various reasons such as-prevailing winds blowing over them, variation in the salinity of water at different points, differences in temperature and the gravitational forces of the Sun and the moon. The movements of sea water can be categorised as (i) Waves, (ii) Tides and (iii) Currents.

 

WAVES

 

Waves are the undulating rise and fall of water. They are created when the winds blow over the seas and oceans. The peak of the wave is known as the crest and the lowest point is known as the trough. The greater the wind speed, the higher and more powerful are the waves. Winds push the sea waters towards the land in the form of waves and the waves collide with one another producing surf. Waves break at the coast because water becomes shallow near the coast.

 

 

When there is an earthquake or volcanic eruption on the ocean floor, enormous waves known as tsunamis occur, which cause major destruction along coastal areas. Tsunami is a Japanese word meaning, 'harbour wave'.

 

 

It is not possible to stop a tsunami but its intensity can be reduced by adopting certain safety measures such as planting of dense mangrove forests along coastal areas. Japan has constructed high rising walls (5 to 10 metre high) to lessen the intensity of tsunami waves.

 

 

 

 

 

 

THE TIDES

 

Tides are rhythmic in nature. Tides are large-scale movement of ocean waters under the gravitational influence of the Sun and the moon. Tides occur twice a day near coastal areas. A rise in the level of ocean water is called the high tide while a fall in the water level of the ocean is called the low tide. This rise and fall in the level of sea water characterise tides as: (i) Spring Tides, and (ii) Neap Tides.

 

Spring Tides?when the Sun, the moon and the earth are in a line on a full moon or new moon, the tides rise higher. These tides are known as Spring Tides.

 

 

THE TSUNAMI?26TH DECEMBER, 2004                 

 

The tsunami that occurred on 26th December, 2004 was caused by an earthquake of 9.0 magnitude on the Richte Scale. After this Tsunami, the Government of India had announced a comprehensive plan for setting up an Advanced Tsunami Warning System in the Indian Ocean with the help of the United States. Some facts about this event are:

 

 

Epicenter?Epicentre of the tsunami was close to the west coast of Sumatra, Indonesia.

 

Cause?the Indian plate went below the Burma plate. Sudden movement caused rising of sea waves by several meters. The water withdrew from coastal areas in great quantity and later returned as massive waves to fill in the gaps, towards coastal areas around the Indian Ocean.                                                                                                                                                                                                    

 

Areas effected-Indonesia, Thailand, Andaman and        

Nicobar Islands (Indra point, southern most land part of India is below water now), the east coast of India, Sri Lanka and Africa were badly affected. Speed?Speed of tsunami waves was 800 km per hour.

 

Financial losses?financial losses in India over Rupees 5322 crores, lives lost 20,000. Losses in other countries were difficult to estimate.

 

Neap Tides-at a quarter or three-quarter moon, the Sun and the moon are at right angles with the earth. So, the gravitational pull of the moon is partially cancelled by the gravitational pull of the Sun, and the tides are not so high then. These tides are called Neap Tides. On such days low tides are not very low and high.

 

Position of the sun, the Moon and the Earth during Neap Tide

 

Advantages of Waves and Tides

Sea coasts are a contact zone between the land and sea. So, coastal areas receive most of the advantages provided by waves and tides. Waves and tides bring in and take back sediments which help in forming various coastal landforms. Tides are helpful in navigation and fishing. Their energies have been tapped to generate electricity in some advanced countries such as; USA, UK, France, Canada and Japan. Tides help in clearing mouths of rivers like the River Hooghly in India, and River Thames in England.

 

Geography Reveals

The oceans supply 72 per cent of oxygen to the atmosphere which is produced by the marine plants like-phytoplanktons, kelp, algae and planktons. Rainforests produce 28 per cent of oxygen and the remaining 2 per cent of the oxygen comes from other sources.

 

 

 

OCEAN CURRENTS

 

Ocean currents are the regular streams of sea water that flow in a constant direction. These currents are caused due to different reasons, such as:

·         the shape of the continent

·         the direction of the prevailing winds

·         the rotation of the earth (It makes ocean currents move clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere as per Ferrell?s  Law of Deflection).

·         the varying temperatures of oceans that result in warm and cold ocean currents

·         the differences in the density of ocean waters

 

Effects of Ocean Currents

 

Different phenomena take place in the seas and oceans and thereby, on the coastal areas due to oceanic currents. They are as follows:

·         Winds that blow on-shore pass over ocean currents and are influenced by the temperature of these currents. Thus, hot and cold currents affect the climate of coastal region.

·         When warm ocean currents move from the middle latitudes (which are warmer) to higher latitudes (which are cooler), the warmth or heat is carried from the middle latitudes of the equatorial and tropical regions to the higher latitudes of the temperate and polar regions.

·         When cold and warm ocean currents meet, thick fog is formed over the seas and oceans. It is very dangerous for ship navigation but good for fishing purpose. The Grand Banks of Canada is a good example.

·         Ships move much faster and save time and fuel if they sail along with the ocean current.

 

Activity

Using the table below list some of the influence that ocean currents have on the climate of our earth. One has been done for you.

 

	Warm Current	Cold Current
Temperature
Rainfall_
Fog	Fog Uncommon	Fog occurs offshore
Icebergs

 

 

Water pollution

Over the past 200 years, oceans are absorbing carbon dioxide (CO2) and other harmful gases emitted from automobiles, industries and burning of fossil fuels. These harmful gases mix up in the atmosphere and with the sea water. The atmospheric pollution traps more solar radiation than in normal, thus warming the sea water.

 

Effects of water pollution: The temperature of the sea water is rising due to pollution since 1901. Other harmful effects are:

 

·      As the sea ice melts, the algae living in water disappears. It affects the food web of marine animals. Some animals like walrus, penguin, polar bear and whale; loose their habitat because of polar ice breaking.

·      The rain-bearing monsoon needs a pronounced    temperature    difference (between the hot lands and the cool oceans) to start. Without which, the monsoon never materialises. Pollution of sea water prevents such temperature differences to take place.

·      Corals (formed in the sea) are getting bleached due to acidic nature of water and they loose their beautiful colours as well.

·      Changing weather causes droughts and floods. Mountains receive less snow and more rainfall.

·      The acid rain caused by atmospheric pollution contains high quantities of sulphuric and nitric acids, thus polluting the sea water and killing aquatic animals.

·      Effluents from urban areas, farms, industries and households are polluting underground water as well.

·      Major oil spills in oceans from tankers and drilling operations are affecting aquatic life.

·      Global warming has raised sea levels by 10 to 25 centimetres over the last 100 years. If this continues, more and more land will be submerged under the sea.