GEOGRAPHY: Interior of the Earth

INTERIOR OF THE EARTH

What do you imagine about the nature of the earth? Do you imagine it to be a solid ball like cricket ball

or a hollow ball with a thick cover of rocks i.e. lithosphere? Have you ever seen photographs or images

of a volcanic eruption on the television screen? Can you recollect the emergence of hot molten lava,

dust, smoke, fire and magma flowing out of the volcanic crater? The interior of the earth can be

understood  only by indirect evidences as neither any one has nor any one can reach the interior of the

earth. The configuration of the surface of the earth is largely a product of the processes operating

in the interior of the earth. Exogenic as well as endogenic processes are constantly shaping

the landscape. A proper understanding of the physiographic character of a region remains

incomplete if the effects of endogenic processes are ignored. Human life is largely influenced

by the physiography of the region. Therefore, it is necessary that one gets acquainted with

the forces that influence landscape development. To understand why the earth

shakes or how a tsunami wave is generated, it is necessary that we know certain details of the

interior of the earth. In the previous chapter, you have noted that the earth-forming

materials have been distributed in the form of layers from the crust to the core. It is interesting

to know how scientists have gathered information about these layers and what are

the characteristics of each of these layers. This is exactly what this chapter deals with.

INTERIOR OF THE EARTH

SOURCES OF INFORMATION ABOUT THE INTERIOR

The earth’s radius is 6,370 km. No one can reach the centre of the earth and make

observations or collect samples of the material. Under such conditions, you may wonder how

scientists tell us about the earth’s interior and the type of materials that exist at such depths.

Most of our knowledge about the interior of the earth is largely based on estimates and

inferences. Yet, a part of the information is obtained through direct observations and

analysis of materials. Direct Sources The most easily available solid earth material

is surface rock or the rocks we get from mining areas. Gold mines in South Africa are as deep

as 3 - 4 km. Going beyond this depth is not possible as it is very hot at this depth. Besides

mining, scientists have taken up a number of projects to penetrate deeper depths to explore

the conditions in the crustal portions. Scientists world over are working on two major projects

such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. The

deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km. This and many

deep drilling projects have provided large volume of information through the analysis of

materials collected at different depths. Volcanic eruption forms another source of

obtaining direct information. As and when the molten material (magma) is thrown onto the

surface of the earth, during volcanic eruption it becomes available for laboratory analysis.

However, it is difficult to ascertain the depth of the source of such magma.

Indirect Sources

Analysis of properties of matter indirectly provides information about the interior. We

know through the mining activity that temperature and pressure increase with the

increasing distance from the surface towards the interior in deeper depths. Moreover, it is

also known that the density of the material also increases with depth. It is possible to find the

rate of change of these characteristics. Knowing the total thickness of the earth, scientists have

estimated the values of temperature, pressure and the density of materials at different depths.

The details of these characteristics with reference to each layer of the interior are

discussed later in this chapter.  Another source of information are the

meteors that at times reach the earth. However, it may be noted that the material that becomes

available for analysis from meteors, is not from the interior of the earth. The material and the

structure observed in the meteors are similar to that of the earth. They are solid bodies

developed out of materials same as, or similar to, our planet. Hence, this becomes yet another

source of information about the interior of the earth.

The other indirect sources include gravitation, magnetic field, and seismic activity.

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. The reading of the gravity at different places is influenced by many other factors.

These readings differ from the expected values. 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 surveys also provide information about the distribution of

magnetic materials in the crustal portion, and thus, provide information about the

distribution of materials in this part. Seismic activity is one of the most important sources of

information about the interior of the earth.  Hence, we shall discuss it in some detail.

Earthquake

The study of seismic waves provides a complete picture of the layered interior. An earthquake

in simple words is shaking of the earth. It is a natural event. It is caused due to release of

energy, which generates waves that travel in all directions.

Why does the earth shake?

The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks.

Rocks along a fault tend to move in opposite directions. As the overlying rock strata press

them, the friction locks them together. However, their tendency to move apart at some point of

time overcomes the friction. As a result, the blocks get deformed and eventually, they slide

past one another abruptly. This causes a release of energy, and the energy waves travel

in all directions. The point where the energy is released is called the focusof an earthquake,

alternatively, it is called the hypocentre. The energy waves travelling in different directions

reach the surface. The point on the surface, nearest to the focus, is called epicentre. It is

the first one to experience the waves. It is a point directly above the focus.

Earthquake Waves

All natural earthquakes take place in the lithosphere. You will learn about different

layers of the earth later in this chapter. It is sufficient to note here that the lithosphere refers

to the portion of depth up to 200 km from the surface of the earth. An instrument called

‘seismograph’ records the waves reaching the surface. A curve of earthquake waves recorded

on the seismograph is given in Figure 3.1. Note that the curve shows three distinct sections

each representing different types of wave patterns. Earthquake waves are basically of two

types — body waves and surface waves. Body waves are generated due to the release of energy

at the focus and move in all directions travelling through the body of the earth. Hence, the name

body waves. The body waves interact with the surface rocks and generate new set of waves

called surface waves. These waves move along the surface. The velocity of waves changes as

they travel through materials with different densities. The denser the material, the higher

is the velocity. Their direction also changes as they reflect or refract when coming across

materials with different densities. There are two types of body waves. They

are called P and S-waves. P-waves move faster and are the first to arrive at the surface. These

are also called ‘primary waves’. The P-waves are similar to sound waves. They travel

through gaseous, liquid and solid materials. S-waves arrive at the surface with some time

lag. These are called secondary waves. An important fact about S-waves is that they can

travel only through solid materials. This characteristic of the S-waves is quite

important. It has helped scientists to understand the structure of the interior of the

earth. Reflection causes waves to rebound whereas refraction makes waves move in

different directions. The variations in the direction of waves are inferred with the help of

their record on seismograph. The surface waves are the last to report on seismograph.

These waves are more destructive. They cause displacement of rocks, and hence, the collapse

of structures occurs.

Propagation of Earthquake Waves

Different types of earthquake waves travel in different manners. As they move or propagate,

they cause vibration in the body of the rocks through which they pass. P-waves vibrate

parallel to the direction of the wave. This exerts pressure on the material in the direction of the

propagation. As a result, it creates density differences in the material leading to stretching

and squeezing of the material. Other three waves vibrate perpendicular to the direction of

propagation. The direction of vibrations of S-waves is perpendicular to the wave direction

in the vertical plane. Hence, they create troughs and crests in the material through which they

pass. Surface waves are considered to be the most damaging waves.

Emergence of Shadow Zone

Earthquake waves get recorded in seismo-graphs located at far off locations. However,

there exist some specific areas where the waves are not reported. Such a zone is called the

‘shadow zone’. The study of different events reveals that for each earthquake, there exists

an altogether different shadow zone. Figure 3.2 (a) and (b) show the shadow zones of P and

S-waves. It was observed that seismographs located at any distance within 105°

from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located

beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves. Thus, a

zone between 105 ° and 145 ° from epicentre was identified as the shadow zone for both the types

of waves. The entire zone beyond 105 ° does not receive S-waves. The shadow zone of S-wave is

much larger than that of the P-waves. The shadow zone of P-waves appears as a band

around the earth between 105 ° and 145 ° away from the epicentre. The shadow zone of S-waves

is not only larger in extent but it is also a little over 40 per cent of the earth surface. You can

draw the shadow zone for any earthquake provided you know the location of the epicentre.

Types of Earthquakes

(i) The most common ones are the tectonic earthquakes. These are generated due to

sliding of rocks along a fault plane.

(ii) A special class of tectonic earthquake is sometimes recognised as volcanic

earthquake. However, these are confined to areas of active volcanoes.

(iii) In the areas of intense mining activity, sometimes the roofs of underground

mines collapse causing minor tremors. These are called collapse earthquakes.

(iv) Ground shaking may also occur due to the explosion of chemical or nuclear

devices. Such tremors are called explosion earthquakes.

(v) The earthquakes that occur in the areas of large reservoirs are referred to as

reservoir induced earthquakes.

Measuring Earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the

shock. The magnitude scale is known as the Richter scale. The magnitude relates to the

energy released during the quake. The magnitude is expressed in absolute numbers,

0-10. The intensity scale is named after Mercalli, an Italian seismologist. The intensity

scale takes into account the visible damage caused by the event. The range of intensity scale

is from 1-12.

EFFECTS OFEARTHQUAKE

Earthquake is a natural hazard. The following are the immediate hazardous effects of

earthquake:

(i) Ground Shaking

(ii) Differential ground settlement

(iii) Land and mud slides

(iv) Soil liquefaction

(v) Ground lurching

(vi) Avalanches

(vii) Ground displacement

(viii) Floods from dam and levee failures

(ix) Fires

(x) Structural collapse

(xi) Falling objects

(xii) Tsunami

The first six listed above have some bearings upon landforms, while others may be

considered the effects causing immediate concern to the life and properties of people in

the region. The effect of tsunami would occur only if the epicentre of the tremor is below

oceanic waters and the magnitude is sufficiently high. Tsunamis are waves

generated by the tremors and not an earthquake in itself. Though the actual quake

activity lasts for a few seconds, its effects are devastating provided the magnitude of the

quake is more than 5 on the Richter scale.

Frequency of Earthquake Occurrences

The earthquake is a natural hazard. If a tremor of high magnitude takes place, it can cause

heavy damage to the life and property of people. However, not all the parts of the globe

necessarily experience major shocks. We shall be discussing the distribution of earthquakes

and volcanoes with some details in the next chapter. Note that the quakes of high

magnitude, i.e. 8+ are quite rare; they occur once in 1-2 years whereas those of ‘tiny’ types

occur almost every minute.

STRUCTURE OF THEEARTH

The Crust

It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust

varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the

continental crust. The mean thickness of oceanic crust is 5 km whereas that of the

continental is around 30 km. The continental crust is thicker in the areas of major mountain

systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks having

density of 3 g/cm³. This type of rock found in the oceanic crust is basalt. The mean density

of material in oceanic crust is 2.7 g/cm³.

The Mantle

The portion of the interior beyond the crust is called the mantle. The mantle extends from

Moho’s discontinuity to a depth of 2,900 km. The upper portion of the mantle is called

asthenosphere. The word astheno means weak. It is considered to be extending upto 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 (3.4 g/cm³). The crust and the

uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.

The lower mantle extends beyond the asthenosphere. It is in solid state.

The Core

As indicated earlier, the earthquake wave velocities helped in understanding the

existence of the core of the earth. The core-mantle boundary is located at the depth of

2,900 km. The outer core is in liquid state while the inner core is in solid state. The density of

material at the mantle core boundary is around 5 g/cm³ and at the centre of the earth at 6,300

km, the density value is around 13g/cm³. The core is made up of very heavy material mostly

constituted by nickel and iron. It is sometimes referred to as the nife layer.

VOLCANOES AND VOLCANIC LANDFORMS

You may have seen photographs or pictures of volcanoes on a number of occasions. A volcano

is a place where gases, ashes and/or molten rock material – lava – escape to the ground. A

volcano is called an active volcano if the materials mentioned are being released or have

been released out in the recent past. The layer below the solid crust is mantle. It has higher

density than that of the crust. The mantle contains a weaker zone called asthenosphere.

It is from this that the molten rock materials find their way to the surface. The material in

the upper mantle portion is called magma. Once it starts moving towards the crust or it

reaches the surface, it is referred to as lava The material that reaches the ground includes

lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen

compounds, sulphur compounds and minor amounts of chlorene, hydrogen and argon.

Volcanoes

Volcanoes are classified on the basis of nature of eruption and the form developed at the

surface. Major types of volcanoes are as follows:

Shield Volcanoes

Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.

The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made

up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes

are not steep. They become explosive if somehow water gets into the vent; otherwise,

they are characterised by low-explosivity. The  upcoming lava moves in the form of a fountain

and throws out the cone at the top of the vent and develops into cinder cone.

Composite Volcanoes

These volcanoes are characterised by eruptions of cooler and more viscous lavas

than basalt. These volcanoes often result in explosive eruptions. Along with lava, large

quantities of pyroclastic material and ashes find their way to the ground. This material

accumulates in the vicinity of the vent openings leading to formation of layers, and this makes

the mounts appear as composite volcanoes.

Caldera

These are the most explosive of the earth’s volcanoes. They are usually so explosive that

when they erupt they tend to collapse on themselves rather than building any tall

structure. The collapsed depressions are called calderas. Their explosiveness indicates that

the magma chamber supplying the lava is not only huge but is also in close vicinity.

Flood Basalt Provinces

These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the

world are covered by thousands of sq. km of thick basalt lava flows. There can be a series of

flows with some flows attaining thickness of more than 50 m. Individual flows may extend

for hundreds of km. The Deccan Trapsfrom India, presently covering most of the

Maharashtra plateau, are a much larger flood basalt province. It is believed that initially the

trap formations covered a much larger area than the present.

Mid-Ocean Ridge Volcanoes

These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more

than 70,000 km long that stretches through all the ocean basins. The central portion of this

ridge experiences frequent eruptions. We shall be discussing this in detail in the next chapter.

VOLCANIC LANDFORMS

Intrusive Forms

The lava that is released during volcanic eruptions on cooling develops into igneous

rocks. The cooling may take place either on reaching the surface or also while the lava is

still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks

are classified as volcanic rocks(cooling at the surface) and plutonic rocks(cooling in the

crust). The lava that cools within the crustal portions assumes different forms. These forms

are called intrusive forms. Some of the forms are shown in Figure 3.5.

Batholiths

A large body of magmatic material that cools in the deeper depth of the crust develops in the

form of large domes. They appear on the surface only after the denudational processes remove

the overlying materials. They cover large areas, and at times, assume depth that may be several

km. These are granitic bodies. Batholiths are the cooled portion of magma chambers.

Lacoliths

These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like

conduit from below. It resembles the surface volcanic domes of composite volcano, only

these are located at deeper depths. It can be regarded as the localised source of lava that

finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite

rocks. Most of these, now exfoliated, are examples of lacoliths or batholiths.

Lapolith, Phacolith and Sills As and when the lava moves upwards, a

portion of the same may tend to move in a horizontal direction wherever it finds a weak

plane. It may get rested in different forms. In case it develops into a saucer shape, concave

to the sky body, it is called lapolith. A wavy mass of intrusive rocks, at times, is found at

the base of synclines or at the top of anticline in folded igneous country. Such wavy materials

have a definite conduit to source beneath in the form of magma chambers (subsequently

developed as batholiths). These are called the phacoliths. The near horizontal bodies of the

intrusive igneous rocks are called sillor sheet, depending on the thickness of the

material. The thinner ones are called sheet while the thick horizontal deposits are

called sills.

Dykes

When the lava makes its way through cracks and the fissures developed in the land, it

solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a

wall-like structure. Such structures are called dykes. These are the most commonly found

intrusive forms in the western Maharashtra area. These are considered the feeders for the eruptions

that led to the development of the Deccan traps.

Geography: EARTH

Chapter 2 Class 11th NCERT Geography book. 

THE ORIGIN AND EVOLUTION

OF THE EARTH

Do you remember the nursery rhyme “…Twinkle, twinkle little star…”?

Starry nights have always attracted us since the childhood. You may also have thought of

these stars and had numerous questions in your mind. Questions such as how many stars

are there in the sky? How did they come into existence? Can one reach the end of the sky?

May be many more such questions are still there in your mind. In this chapter, you will

learn how these “twinkling little stars” were formed. With that you will eventually also read

the story of origin and evolution of the earth.

EARLYTHEORIES

Origin of the Earth

A large number of hypotheses were put forth by different philosophers and scientists

regarding the origin of the earth. One of the earlier and popular arguments was by German

philosopher Immanuel Kant. Mathematician Laplace revised it in 1796. It is known as

Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of

material associated with a youthful sun, which was slowly rotating. Later in 1900, Chamberlain

and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped

extension of material was separated from the solar surface. As the passing star moved away,

the material separated from the solar surface continued to revolve around the sun and it

slowly condensed into planets. Sir James Jeans and later Sir Harold Jeffrey supported this

argument. At a later date, the arguments considered of a companion to the sun to have

been coexisting. These arguments are called binary theories. In 1950, Otto Schmidt in

Russia and Carl Weizascar in Germany somewhat revised the ‘nebular hypothesis’,

though differing in details. They considered that the sun was surrounded by solar nebula

containing mostly the hydrogen and helium along with what may be termed as dust. The

friction and collision of particles led to formation of a disk-shaped cloud and the planets were

formed through the process of accretion. However, scientists in later period took up

the problems of origin of universe rather than that of just the earth or the planets.

MODERN THEORIES

Origin of the Universe

The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also

called expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the

universe is expanding. As time passes, galaxies move further and further apart. You can

experiment and find what does the expanding universe mean. Take a balloon and mark some

points on it to represent the galaxies. Now, if you start inflating the balloon, the points marked

on the balloon will appear to be moving away from each other as the balloon expands.

Similarly, the distance between the galaxies is also found to be increasing and thereby, the

universe is considered to be expanding. However, you will find that besides the increase

in the distances between the points on the balloon, the points themselves are expanding.

This is not in accordance with the fact. Scientists  believe that though the space between the

galaxies is increasing, observations do not support the expansion of galaxies. So, the

balloon example is only partially correct.

The Big Bang Theory considers the following stages in the development of the universe.

(i) In the beginning, all matter forming the universe existed in one place in the form

of a “tiny ball” (singular atom) with an unimaginably small volume, infinite

temperature and infinite density.

(ii) At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion.

It is now generally accepted that the event of big bang took place 13.7 billion

years before the present. The expansion continues even to the present day. As it

grew, some energy was converted into matter. There was particularly rapid

expansion within fractions of a second after the bang. Thereafter, the

expansion has slowed down. Within first three minutes from the Big Bang event,

the first atom began to form.

(iii) Within 300,000 years from the Big Bang, temperature dropped to 4,500 K

(Kelvin) and gave rise to atomic matter. The universe became transparent.

The expansion of universe means increase in space between the galaxies. An alternative

to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same

at any point of time. However, with greater evidence becoming available about the

expanding universe, scientific community at present favours argument of expanding

universe.

The Star Formation

The distribution of matter and energy was not even in the early universe. These initial density

differences gave rise to differences in gravitational forces and it caused the matter

to get drawn together. These formed the bases for development of galaxies. A galaxycontains

a large number of stars. Galaxies spread over vast distances that are measured in thousands

of light-years. The diameters of individual galaxies range from 80,000-150,000 light

years. A galaxy starts to form by accumulation of hydrogen gas in the form of a very large

cloud called nebula. Eventually, growing nebula develops localised clumps of gas. These

clumps continue to grow into even denser gaseous bodies, giving rise to formation of

stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

A light year is a measure of distance and not of time. Light travels at a speed of

300,000 km/second. The mean distance between the sun and the earth

is 149,598,000 km. In terms of light years, it is 8.311minutes

Formation of Planets

The following are considered to be the stages in the development of planets :

(i) The stars are localised lumps of gas within a nebula. The gravitational force

within the lumps leads to the formation of a core to the gas cloud and a huge

rotating disc of gas and dust develops around the gas core.

(ii) In the next stage, the gas cloud starts getting condensed and the matter

around the core develops into small-rounded objects. These small-rounded objects by the process of

cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and

gravitational attraction causes the material to stick together. Planetesimals are a large number of

smaller bodies.

(iii) In the final stage, these large number of small planetesimals accrete to form a

fewer large bodies in the form of planets.

OUR SOLAR SYSTEM

Our Solar system consists of eight planets. The nebula from which our Solar system is supposed

to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and

the planets were formed about 4.6 billion years ago. Our solar system consists of the sun (the

star), 8 planets, 63 moons, millions of smaller bodies like asteroidsand cometsand huge

quantity of dust-grains and gases. Out of the eight planets, mercury, venus,

earth and mars are called as the inner planets as they lie between the sun and the belt of

asteroids the other four planets are called the outer planets. Alternatively, the first four are called

Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high

densities. The rest four are called Jovian or Gas Giant planets. Jovian means jupiter-like. Most

of them are much larger than the terrestrial planets and have thick atmosphere, mostly of

helium and hydrogen. All the planets were formed in the same period sometime about 4.6 billion

years ago. Till recently (August 2006), Pluto was also considered a planet. However, in a meeting

of the International Astronomical Union, a decision was taken that Pluto like other celestial

objects (2003 UB 313) discovered in recent past may be called ‘dwarf planet’. Some data regarding

our solar system are given in the box below.

The difference between terrestrial and jovian planets can be attributed to the following

conditions:

(i) The terrestrial planets were formed in the close vicinity of the parent star where it

was too warm for gases to condense to solid particles. Jovian planets were

formed at quite a distant location.

(ii) The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust

from the terrestrial planets. The solar winds were not all that intense to cause similar

removal of gases from the Jovian planets.

(iii) The terrestrial planets are smaller and their lower gravity could not hold the

escaping gases.

The Moon

The moon is the only natural satellite of the earth. Like the origin of the earth, there have

been attempts to explain how the moon was formed. In 1838, Sir George Darwin suggested

that initially, the earth and the moon formed a single rapidly rotating body. The whole mass

became a dumb-bell-shaped body and eventually it broke. It was also suggested that

the material forming the moon was separated from what we have at present the depression

occupied by the Pacific Ocean. However, the present scientists do not

accept either of the explanations. It is now generally believed that the formation of moon,

as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.

A body of the size of one to three times that of mars collided into the earth sometime shortly

after the earth was formed. It blasted a large part of the earth into space. This portion of

blasted material then continued to orbit the earth and eventually formed into the present

moon about 4.44 billion years ago.

EVOLUTION OF THEEARTH

Do you know that the planet earth initially was a barren, rocky and hot object with a thin

atmosphere of hydrogen and helium. This is far from the present day picture of the earth.

Hence, there must have been some events–processes, which may have caused this change

from rocky, barren and hot earth to a beautiful planet with ample amount of water and

conducive atmosphere favouring the existence of life. In the following section, you will find

out how the period, between the 4,600 million years and the present, led to the evolution of

life on the surface of the planet. The earth has a layered structure. From

the outermost end of the atmosphere to the centre of the earth, the material that exists is

not uniform. The atmospheric matter has the least density. From the surface to deeper

depths, the earth’s interior has different zones and each of these contains materials with

different characteristics.

Evolution of Lithosphere

The earth was mostly in a volatile state during its primordial stage. Due to gradual increase

in density the temperature inside has increased. As a result the material inside

started getting separated depending on their densities. This allowed heavier materials (like

iron) to sink towards the centre of the earth and the lighter ones to move towards the

surface. With passage of time it cooled further and solidified and condensed into a smaller size.

This later led to the development of the outer surface in the form of a crust. During the

formation of the moon, due to the giant impact, the earth was further heated up. It is through

the process of differentiation that the earth forming material got separated into different

layers. Starting from the surface to the central parts, we have layers like the crust, mantle,

outer core and inner core. From the crust to the core, the density of the material increases. We

shall discuss in detail the properties of each of this layer in the next chapter.

Evolution of Atmosphere and Hydrosphere The present composition of earth’s atmosphere

is chiefly contributed by nitrogen and oxygen. You will be dealing with the composition and

structure of the earth’s atmosphere in Chapter 8. There are three stages in the evolution of

the present atmosphere. The first stage is marked by the loss of primordial atmosphere.

In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.

Finally, the composition of the atmosphere was modified by the living world through the

process of photosynthesis.

The early atmosphere, with hydrogen and helium, is supposed to have been

stripped off as a result of the solar winds. This happened not only in case of the earth, but also in all the

terrestrial planets, which were supposed to have lost their primordial atmosphere through

the impact of solar winds.

During the cooling of the earth, gases and water vapour were released from the interior

solid earth. This started the evolution of the present atmosphere. The early atmosphere

largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very

little of free oxygen. The process through which the gases were outpoured from the interior is

called degassing. Continuous volcanic eruptions contributed water vapour and gases to the atmosphere.

As the earth cooled, the water vapour released started getting

condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the

temperature further decreased causing more condensation and more rains. The rainwater

falling onto the surface got collected in the depressions to give rise to oceans. The earth’s

oceans were formed within 500 million years from the formation of the earth. This tells us

that the oceans are as old as 4,000 million years. Sometime around 3,800 million years

ago, life began to evolve. However, around 2,500-3,000 million years before the present,

the process of photosynthesis got evolved. Life was confined to the oceans for a long time.

Oceans began to have the contribution of oxygen through the process of photosynthesis.

Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to

flood the atmosphere.

Origin of Life

The last phase in the evolution of the earth relates to the origin and evolution of life. It is

undoubtedly clear that initially the earth or even the atmosphere of the earth was not

conducive for the development of life. Modern scientists refer to the origin of life as a kind of

chemical reaction, which first generated complex organic molecules and assembled

them. This assemblage was such that they could duplicate themselves converting

inanimate matter into living substance. The record of life that existed on this planet in

different periods is found in rocks in the form of fossils. The microscopic structures closely

related to the present form of blue algae have been found in geological formations much older

than some 3,000 million years. It can be assumed that life began to evolve sometime

3,800 million years ago.

The Beginning

Hi Folks,
    The very reason I'm starting this blog is the NCERT books. Yeah, I know that the NCERT books are already there with the NCERT's official website ready to be downloaded or to be read online from a computer. But, the problem is that people who don't have a smartphone like me can't read these books on their phones. Yes, I want to read NCERT books with my cheap mobile and I think there's many other like me who want the same facility.
    So what I'm going to do is just copy-paste the content of NCERT books to this blog so that me & others can later read these without the need of going to the PC/Lappy for it. And yes, I will put the contents of NCERT without any modification by me.
    Ok then, thats all for today. Happy Reading.