Deforestation, Features of Earth, Structure of the Earth, Change of Season

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It is the process of destruction of the forests.

Causes of Deforestation: A number of agents are responsible for removal of forests. These are fires. Droughts or animals. The principle agent of deforestation is man himself. Humans are cutting forests to colonize the forest areas or to prepare more agricultural lands or for getting food.

Effects of Deforestation

  • Deforestation has many bad effects on human life. With destruction of the forest the process of cleaning of air and production of oxygen is badly affected.
  • Forests are major agents of evaporation and rainfall. With deforestation rainfall is decreased.
  • With cutting of forests, pollution is increasing as the hazardous gases are not completely absorbed by plants.
  • With deforestation carbon dioxide is increasing which is causing global warming.
  • With reduction in area of forest, soil erosion is taking place resulting in floods which destroy crops and human life.
  • Deforestation has badly damaged wild life and reduced recreation sites.


Features of Earth

  • The earth is the fifth largest planet of the solar system, where life exists.
  • Its equatorial diameter is 7,927 miles.
  • Its polar diameter is 7900 miles.
  • The earth has average density of 5 g/cc.
  • The rotation period of the earth is 23 hours, 56 minutes and 4 seconds.
  • The revolution period of the earth is 365 days.
  • The total mass of the earth is tons.
  • The earth is composed of shells or layers, which are, the crust, mantle and core.
  • It is surrounded by a blanket of gases, which is known as the atmosphere, mainly composed of Nitrogen and Oxygen.
  • The surface of the earth is rich in oxygen, silicon, iron, magnesium, sodium etc.

Structure of the Earth: The Earth Comprises of the Following Layers

  • The crust
  • Moho discontinuity
  • The mantle
  • The Core
  • The Crust:
    • It is the outermost layer of the earth.
    • It extends to about 25 miles (40 kms) and comprises of rocks.
    • The crust is divided into the oceanic and the continental crust.
    • Out of these the oceanic crust is 808 meter thick and consists of sedimentary mud.
    • The continental crust is divided into upper continental and lower continental crust.

Moho discontinuity: The sharp boundary between the crust and mantle is called Moho Discontinuity.

The Mantle

  • The layer of the earth lying below the crust and above the core is known as the mantle.
  • It is almost 2900 kms (1800 miles) thick and comprises about 80 % of the volume of the earth.
  • The chemical composition of the entire mantle is fairly homogenous.
  • However, temperature and pressure increases with depth.
  • The behaviour of the earthquake waves as they travel through the mantle further tells us that it consists of several layers and they are:
  • Lithosphere
  • Asthenosphere
  • Mesosphere

The Core

  • The innermost part of the earth is known as the core.
  • It extends from the base of the mantle to the centre of the earth.
  • This portion consists of melted iron and nickel that is why it is known as Nife.
  • The density of this molten mass of the core is 345 pounds per cubic feet.

Types of Movements of Earth

  • There are two types of motions of the earth.
  • One is around its own axis which is called Rotation. One rotation completes in 23 hours, 56 minutes and 4 seconds.
  • The other motion of earth is round the sun, and is called revolution of the earth.
  • One revolution completes in 365 days.
  • Northern end of the earth՚s axis is called the North Pole and the Southern end is called South Pole.
  • An imaginary line drawn round the earth midway between the poles is called equator.
  • The path which the earth takes round the sun is called earth՚s orbit.

Effects of These Movements

  • Rotation of earth round its axis cause days and nights.
  • That portion of earth, which is within the circle of illumination caused by the rays of sun, has its day.
  • The other side earth, which is away from sun, remains dark and therefore has its night.

Longer and Shorter Days

  • The circular areas near the North and South Pole of the Earth are called Arctic and Antarctic circles, respectively.
  • These are situated at North and South of the equator, respectively and form limits of polar region.
  • On June, the earth is in position A. north pole is inclined towards the sun and South Pole is away from it.
  • Obviously any place in Northern Hemisphere will have longer days because it remains in light for more than half the time of earth՚s rotation.
  • Places on equator remains in light for half the time making days and nights equal in this region.
  • The southern hemisphere remains in light for less time than half the rotation of earth so the days are shorter here.
  • Positions of days and nights in the northern and southern hemisphere are reversed on 22nd December, when the earth completes its half revolution around the sun, so days are longer in southern hemisphere than those in the north.

Change of Season

  • The second type of motion of earth is round the sun and is called revolution of the earth.
  • This is the cause of change of seasons.
  • On June 21, the earth is in position A, when the North Pole is inclined towards sun while the South Pole is away from it.
  • As the sun rays fall vertically and for a longer period in the northern hemisphere, it is summer here but in the southern hemisphere it is winter.
  • On December 22, when the earth completes half revolution, the conditions become reversed; it is summer in the southern hemisphere and winter in the northern hemisphere.


The layer of the gases which surrounds the earth is known as the atmosphere. OR The atmosphere is a thin layer of the gases held to the earth by the gravitational attraction. OR Atmosphere is the huge blanket of gas that circles the entire earth.

Composition of the Atmosphere

The atmosphere consists of:

  • Nitrogen:
  • Oxygen:
  • Argon:
  • Hydrogen:
  • Neon:
  • Helium:
  • Krypton:
  • Ozone:

Layers of the Atmosphere

The atmosphere of the earth is divided into following layers.

  • Troposphere
  • Stratosphere
  • Ionosphere
  • Exosphere
  • Ionosphere is sub-divided into:
  • Mesosphere
  • Thermosphere


  • Troposphere is the lowest layer of the atmosphere.
  • It extends roughly to a height of 8 kms near the poles and 16 - 18 kms at the equator.
  • It is troposphere where the people, plants, animals and insects live.
  • It is the layer where all weather occurs that՚s why it is also referred as - The weather Zone.
  • In the Troposphere temperature gradually falls with increasing altitude.
  • There is a thin buffer zone between the troposphere and stratosphere is called tropopause.

Stratosphere and Ozone Layer

  • The second layer of the atmosphere is known as the stratosphere.
  • The stratosphere extends from the tropopause to about 30 - 31 mile above ground level.
  • The important ozone layer is found in this region where heat is generated by absorption of UV.
  • Here the temperature either remains constant or increases with altitude.

Stratopause: It is the upper boundary of the stratosphere which occurs beyond 52 kms. Here the temperature remains constant with increase in height.


  • The third major layer of the atmosphere is the ionosphere.
  • It lies above the stratosphere.
  • It lies between about 30 and 90 miles above the surface of the earth.
  • It is divided into mesosphere and thermosphere.
  • The ionosphere is bombarded by cosmic radiation and solar x-rays, which causes the gases in the ionosphere to ionize.
  • Brilliant displays of colored lights in the sky called Aurora Borealis in the northern hemisphere and the Aurora Aural is in the southern hemisphere occur when streams of electrically charged particles from the sun (solar wind) ionize the atmosphere gases.


  • The uppermost layer of the atmosphere extending beyond Ionosphere is called exosphere.
  • Beyond 300 miles is the very rare field exosphere which consists only of scattered atmosphere of O, H and He.

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