Composition and structure of the Atmosphere

·         The air is a mixture of several gases and it encompasses the earth from all sides.

·         The air surrounding the earth is called the atmosphere.

·         Nitrogen and oxygen are the two main gases in the atmosphere and 99 percentage of the atmosphere  is made up of these two gases.

Chemical Constituents of the Atmosphere:

1. Nitrogen (N2) - 78% (by volume).

2. Oxygen (O2) - 21%.

3. Argon (Ar) - 1%.

4. Carbon Dioxide (CO2) - 0.038% (376 ppm) and rising.

Structure of the Atmosphere:

Troposphere:

·         It is the lowermost layer.

·         Average height is 13 km with 16-18 km at the equator and 6-8 km at the poles.

·         75-80% of total mass.

·         Temperature generally decreases with height; Average lapse rate is 6.5°/km.

·         Referred to as the "weather layer".

·         Climate and weather changes occur here.

·         Zone separating troposphere from stratosphere is called tropopause.

Stratosphere:

•        It extends up to a height of 50 km.

•        Temperature is constant with height initially, then it increases with height, known as an inversion (temperature increases with height).

•        One important feature of stratosphere is that it contains a layer of ozone gas.

•        This layer is almost free from clouds and associated weather phenomenon,

making conditions most ideal for flying aeroplanes.

·         So aeroplanes fly in lower stratosphere, sometimes in upper troposphere where weather is calm.

·         Sometimes, cirrus clouds are present at lower levels in this layer.

Ozonosphere:

•        It lies at an altitude between 30 km and 60 km from the earth’s surface.

•        This layer reflects the harmful ultraviolet radiation.

•        The ozonosphere is also called chemosphere.

Mesosphere (from the Greek meso meaning middle):

•        It extends up to a height of 80 km.

•        The temperature gradually falls to -100°C at 80 km altitude.

•        Meteorites burn up in this layer on entering from the space.

Thermosphere (from the Greek therme meaning heat):

•        In thermosphere temperature rises very rapidly with increasing height.

•        It extends between 80-400 km.

•        Ionosphere is a part of this layer.

•        This layer helps in radio transmission.

•        The International Space Station and satellites orbit in this layer.

•        Aurora’s are observed in lower parts of this layer(Aurora Borealis and Aurora Australis).

•        Topped by exosphere.

Ionosphere:

•        This layer is located between 80 km and 400 km and is an electrically charged layer.

•        This layer is characterized by ionization of atoms.

•        Because of the electric charge, radio waves transmitted from the earth are reflected back to the earth by this layer.

•        Temperature again starts increasing with height because of radiation from the sun.

Exosphere:

•        Light gases like helium and hydrogen float into the space from here.

•        It is the outermost layer.

•        This layer coincides with space.

Vertical Structure of Composition:

1.Homosphere:(homo means same):

•        Well mixed, the chemical composition is constant.

•        This layer extended up to 80 km from the surface.

•        This part of the atmosphere continually circulates, so that the principal atmospheric gases are well mixed.

2.Heterosphere:(hetero means different):

•        Settling of gases according to weight above 80 km.

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BHABAR, TARAI, BHANGAR, KHADAR

BHABAR:

·         It lies along the foot of the Shiwaliks from the Indus to the Tista.

·         It is 8 to 16 kms wide.

·         It comprises of pebble- studded rocks in the shape of porus beds.

·         Due to porosity of the rocks, the streams disappear and flow underground.

·         This region is not much suitable for agriculture.

·         It is made up of porous and rocky soils that get made of the debris washed down from the higher ranges.

·         The climate here is subtropical and vegetation is Himalayan subtropical pine forests and Himalayan subtropical broadleaf forests.

·         The Himalayan subtropical pine forests are dominated by Chir trees and Himalayan subtropical broadleaf forests  are dominated by the sal tree (Shorea robusta).

TARAI:

·         It lies to the south of the Bhabar and run parallel to it.

·         It is 20 to 30 kms wide.

·         It is composed of compara­tively finer alluvium and is covered by forests.

·         The underground streams of the Bhabar re-emerge on the surface and give birth to marshy area.

·         Most parts of the Tarai area are reclaimed for agriculture.

·         The speed of the Himalayan Rivers is slowed down in the Terai region and these rivers deposit fertile silt during the monsoons.

BHANGAR:

·         It is the largest part of Northern plain.

·         It is made up of alluvium.

·         The soil in this region is not renewed frequently, it is old alluvial soil.

·         It is not very fertile.

·         It contains calcareous deposits locally known as 'Kankar '.

·         The Bhangar is the older alluvium along the river beds forming terraces higher than the flood plain.

·         ‘The Barind plains’ in the deltaic region of Bengal and the ‘bhur formations’ in the middle Ganga and Yamuna doab are regional variations of Bhangar.

KHADAR:

·         It is not the largest part of Northern plain.

·         It is made up of new alluvium and fine granules.

·         The soil is frequently renewed due to frequent flooding.

·         It does not contain calcareous deposits of calcium.

·         Suitable foe extensive cultivation.

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Inside Our Earth

Source : NCERT & NIOS

·         The earth is made up of several concentric layers. Concentric means having a common centre.

·         The earth has one layer inside another.

Broadly, there are three layers of the earth:

1.   Crust

2.   Mantle

3.   Core

The Crust:

·         It is the outermost layer over the earth’s surface.

·         It is the thinnest of all the layers.

·         It forms 0.5-1.0 per cent of the earth’s volume.

·         About 35 km on the continental masses and only 5 km on the ocean floors.

·         Crust is the outer thin layer with a total thickness normally between 30-50 km.

·         The thickness of the crust varies under the oceanic and continental areas.

·         Oceanic crust is thinner (5-30 km thick) as compared to the continental crust (50- 70 km thick).

·         The continental crust is thicker in the areas of major mountain systems.

·         It is as much as 70 -100 km thick in the Himalayan region.

The main minerals constituting the continental mass are:

(granitic rocks , average density = 2.7g/cm^3)

1.   Silica

2.   Alumina

The continental mass is also called ‘sial’.

The name sial is derived from letters ‘si’ of silicon and ‘al’ of alumina.

The main mineral constituents of the oceanic crust are:

(Basalt, average density = 3 g/cm^3)

1.   Silicon

2.   Magnesium

The oceanic crust is called ‘sima’.

The root of the word sima is ‘si’ from silica and ‘ma’ from magnesium.

Mantle:

·         It is the layer just beneath the crust and extends up to a depth of 2900 km below the crust.

·         Average density = 3.4 g/cm^3.

·         It forms 83 per cent of the earth's volume.

·         The crust and the uppermost part of the mantle are called lithosphere.

·         Its thickness ranges from 10-200 km.

Asthenosphere:

·         The upper portion of the mantle is called asthenosphere.

·         The word astheno means weak.

·         It is considered to be extending up to 400 km.

·         It is the main source of magma that finds its way to the surface during volcanic eruptions.

·         It has a density higher than the crust’s.

Core :

·         It is the innermost layer i.e. it is below the mantle. Its radius is about 3500 km.

·         The core consists of outer core and inner core.

·         Accounts for 16 per cent of the earth's volume.

·         The outer core is liquid while the inner core is solid.

It is mainly made up of:

1.   Nickel

2.   Iron

The innermost layer is also called ‘nife’.

·         The name nife is derived from the word ‘ni’ from nickel and ‘fe’ from ferrous meaning iron.

·         The temperature and pressure of the central core is very high.

Sources of information about the Interior:

Direct Sources:

·         Deep earth mining and drilling reveals the nature of rocks deep down the surface.

·         Volcanic eruption forms another source of obtaining direct information.

Indirect Sources:

Depth:

·         With depth, pressure and density increases and hence temperature.

·         This is mainly due to gravitation.

Meteors:

·         Meteors and Earth are solar system objects that are born from the

same nebular cloud.

·         Thus they are likely to have a similar internal structure.

Gravitation:

·         The gravitation force (g) is not the same at different latitudes on the

surface.

·         It is greater near the poles and less at the equator.

·         This is because of the distance from the centre at the equator being greater than that at the poles.

·         The gravity values also differ according to the mass of material.

·         The uneven distribution of mass of material within the earth influences this value.

·         Such a difference is called gravity anomaly.

·         Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.

Magnetic field:

·         The geodynamo effect helps scientists understand what's happening inside the Earth's core.

·         Shifts in the magnetic field also provide clues to the inaccessible iron core.

Seismic activity:

Earth’s interior discontinuity:

1.   Conorod :  discontinuity between upper and lower crust.

2.   Mohorovicc: Discontinuity between lower crust and upper mantle.

3.   Repiti : discontinuity between upper and lower mantle.

4.   Guttenberg:  discontinuity between lower mantle and outer core.

5.   Lehmann:  discontinuity between outer core and inner core.

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