Four realms of the earth
Our earth is a unique planet in the whole solar system, It is the only planet which is habitable and which supports life, Though some times it is supposed that conditions on the Mars, another planet in the solar system, are also favourable to support some form of life, yet there is no evidence to sustain it. Land, water and air are the major domains of the earth which sustain life. The land mass or the solid crust of rock is called the Lithosphere, The vast water mass which includes oceans and seas is called the Hydrosphere. The domain of air that surrounds the earth is called the Atmosphere.


    'Lithos' means 'stone' and as such lithosphere means the solid crust of the earth, It includes all types of land masses, big or small, even or uneven, high mountains and deep valleys, vast plains and plateaus, etc.
    Major landmasses are called continents while smaller landmasses are called islands. They are surrounded by water on all sides
    Continents –
    There are seven continents on the earth,
    In order of their size they are-(1) Asia (2) Africa (3) North America (4) South America     (5) Antarctica (6) Europe (7) Australia. Their size and population is given as under.
    Asia is the largest continent on the earth and it occupies about one-third the land of the world, About 58% of the people of the world live in Asia, There are now more than 46 countries in Asia, Our country is one of them.
    In fact, Asia and Europe together belong to the same landmass. They are not separated by a large watermass. Rural mountains and the Rural river separate Europe from Asia. Together they are called Eurasia. But for all practical purposes, Europe is treated      as a separate continent. In size it is the sixth out of the seven continents of the world, only bigger than Australia.

    Africa is next to Asia in size. The Suez Canal separates it from Asia. It is the only continent of the world which has the Tropic of Cancer, the Equator and the Tropic of Capricorn passing through it.

    North America is the third largest continent of the world after Asia and Africa. It is about eight times the size of India. It was Columbus who discovered this continent in 1492 A.D.

    South America is the fourth largest continent of the world after Asia, Africa and North America. It is more than five times the size of India.
    North America and the South America also formed a single landmass before they were separated by the Panama Canal.

    Australia is the smallest continent on the earth. It is sometimes called an island continent. It is surrounded on all sides by seas and oceans. Can you name them?

    Antarctica is a big continent. It lies around the South Pole. It is always covered with snow and ice, and as such there is no permanent human settlement here. It was first discovered in 1921.

    All the water bodies on the earth's surface, namely oceans, seas, lakes, rivers together with snow and ice, are collectively called the hydrosphere.


       There are four oceans on the earth. They are all inter-connected. In order of size they are: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and the Arctic Ocean. The water body which surrounds the continent of Antarctica is only an extension of the         Pacific, Atlantic and Indian Oceans.
1.    The Pacific Ocean – It separates Asia and Australia from North America. It is the largest of all the oceans. It is estimated that it covers an area of about 4, 166,884, 380 sq km which is greater than the area of all the continents put together. It is not only the          largest but also the deepest of all the oceans. Mariana Trench in the Pacific Ocean (about 11,022 metres deep) is the deepest place in the world.
2.    The Atlantic Ocean – It separates Europe and Africa from the North and South Americas. It is the busiest ocean in the world as all the important sea routes pass through the Atlantic Ocean. There are many important harbours on its coastline.
3.    The Indian Ocean – It is the only ocean named after a country, i.e. India. It lies to the south of India, west of Indonesia an Australia, and east of Africa.
4.    The Arctic Ocean – It surrounds the North Pole and lies within the Arctic Circle. A narrow stretch of water called Being Straits connects it with the Pacific Ocean. It is bound by Russia, Scandinavia, Greenland, Canada and Alaska?

Importance of Oceans
    Oceans have got a great importance of their own from many points of view :
    (1)    The presence of large quantities of water in the oceans is responsible for moderating the temperature.
    (2)    The occur currents often keep the hot countries quite cool and cold countries quite warm. In this way, they have a moderating influence on the climate of various countries.
    (3)    The tides keep the sea-shores quite clean.
    (4)    Oceans provide abundant water vapour to the atmosphere which results in rains on the plains.
    (5)    Oceans help in maritime activities and promote trade.
    (6)    Oceans provide an abundant source of food for mankind.


    Earth is surrounded by a thick blanket of air. The layer of air that envelopes the earth is called the atmosphere. This blanket of air extends up to several kilometres above the earth's surface. Air is a mixture of gases. It consists of nitrogen and oxygen in the          ratio about four to one –nitrogen 78 per cent, oxygen 21 per cent. Oxygen is essential for the survival of most organisms. The other gases like argon, carbon dioxide, neon, helium, krypton, hydrogen and xenon are present in smaller quantities. Air also holds      water vapour in various amounts depending on the temperature.
    On the basis of its composition, temperature and density, the atmosphere is divided into five concentric layers –Troposphere, Stratosphere. Mesosphere, Thermosphere and Exosphere
    Troposphere is the layer of the atmosphere closest to the earth's surface, reaching up to a height of about 10–15 km. It contains 75 per cent of the total air of the atmosphere. The temperature and the pressure drops as one goes higher up the troposphere.
    It is the layer where all weather changes occur. The stratosphere lies directly above the troposphere. Strong winds called jet streams blow here: This helps aircraft to fly.
    The next layer is the mesosphere. Temperature drops as low as –110°C here. Meteors or rock fragment burn up here.

    The ionosphere or thermosphere is a hot layer where temperature rises steadily to 1400°C. This layer contains electrically charged particles called ions which help in transmitting radio-waves back to the earth. Satellites, which help in transmitting signals              back to earth are located here.

   The exosphere is the outermost layer of the atmosphere and extends from 80 km above the earth's surface to the outer space.


    The biosphere is the sphere of earth where living organisms exist penetrating the lithosphere, the hydrosphere and the atmosphere. This zone of life is vast and covers most life forms from microbes to huge mammals. This zone supports several varieties of        organisms, both plants and animals. There are about 10,00,000  varieties of animals and 3,00,000 varieties of plants. Some of them are so minute that they can be viewed only with the help of a microscope while others are quite big and huge. Some of them      live under the surface of the earth while others live deep under water and still others roam about on the earth or in the air.

Man and Biosphere
    Human beings–an essential part of the biosphere-have disturbed the natural balance between the different elements of the biosphere by their activities. This has led to serious environmental problems-threatening the very existence of many pieces.
    Man has excavated mines and extracted coal, petroleum and natural gas from the earth. The burning of coal oil and natural gas, and cutting down of trees and reckless in industrial practices are altering the composition of the gases in the atmosphere and         polluting it. Increase in the level of carbon dioxide and some hanuful gases has led to global warming, climate change, depletion of the ozone layer, changes in sea level, melting of ice caps and high levels of pollution. Our earth is the only planet with a               congenial environment for life.

                                                                 Key Words

LITHOSPHERE :      The solid domain of the earth forming the land which supports animals, plants and humans.
HYDROSPHERE :    The water that fills the depressions of the earth's surface comprising of water bodies like the oceans, seas, rivers and ponds.
ATMOSPHERE :       The vast expanse of air that surrounds the earth.
BIOSPHERE :           The zone which contains all forms of life penetrating the lithosphere, hydrosphere and the atmosphere.
ISTHMUS :                A narrow stretch of land joining two larger landmasses.
TROPOSPHERE :    The lowest layer of the atmosphere near the earth's surface where all the weather phenomena takes place.
STRATOSPHERE :  The layer above the troposphere where the ozone layer is found.
MESOSPHERE :    The third layer of the atmosphere where the temperature drops as low as –110°C.
IONOSPHERE :    The fourth layer of the atmosphere that contains electrically charged particles called ions.
EXOSPHERE :    The last layer of the atmosphere which finally merges into space.

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