NCERT Textbook - The Origin and Evolution of the Earth Humanities/Arts Notes | EduRev

 Page 1


THE EARTH
This unit deals with
• Origin and evolution of the earth; Interior of the earth; Wegener’s
continental drift theory and plate tectonics; earthquakes and
volcanoes
UNIT
II
© NCERT
not to be republished
Page 2


THE EARTH
This unit deals with
• Origin and evolution of the earth; Interior of the earth; Wegener’s
continental drift theory and plate tectonics; earthquakes and
volcanoes
UNIT
II
© NCERT
not to be republished
D
o 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.
EARLY THEORIES
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
THE ORIGIN AND EVOLUTION
OF THE EARTH
CHAPTER
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
© NCERT
not to be republished
Page 3


THE EARTH
This unit deals with
• Origin and evolution of the earth; Interior of the earth; Wegener’s
continental drift theory and plate tectonics; earthquakes and
volcanoes
UNIT
II
© NCERT
not to be republished
D
o 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.
EARLY THEORIES
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
THE ORIGIN AND EVOLUTION
OF THE EARTH
CHAPTER
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
© NCERT
not to be republished
THE ORIGIN AND EVOLUTION OF THE EARTH 15
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.
Figure 2.1 : The Big Bang
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 galaxy contains
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. Considering this,
the distances the light will travel in one
year is taken to be one light year. This
equals to 9.46110
12
 km. 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.
© NCERT
not to be republished
Page 4


THE EARTH
This unit deals with
• Origin and evolution of the earth; Interior of the earth; Wegener’s
continental drift theory and plate tectonics; earthquakes and
volcanoes
UNIT
II
© NCERT
not to be republished
D
o 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.
EARLY THEORIES
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
THE ORIGIN AND EVOLUTION
OF THE EARTH
CHAPTER
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
© NCERT
not to be republished
THE ORIGIN AND EVOLUTION OF THE EARTH 15
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.
Figure 2.1 : The Big Bang
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 galaxy contains
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. Considering this,
the distances the light will travel in one
year is taken to be one light year. This
equals to 9.46110
12
 km. 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.
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 16
(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 asteroids and comets and 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.
Why are the inner planets rocky while
others are mostly in gaseous form?
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
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058
Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66
Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88
Satellites 0 0 1 2 16   about 18 about 17 8
* Distance from the sun in astronomical unit i.e. average mean distance of the earth is 149,598,000
km = 1 @ Density in gm/cm
3
# Radius: Equatorial radius 6378.137 km = 1
The Solar System
© NCERT
not to be republished
Page 5


THE EARTH
This unit deals with
• Origin and evolution of the earth; Interior of the earth; Wegener’s
continental drift theory and plate tectonics; earthquakes and
volcanoes
UNIT
II
© NCERT
not to be republished
D
o 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.
EARLY THEORIES
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
THE ORIGIN AND EVOLUTION
OF THE EARTH
CHAPTER
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
© NCERT
not to be republished
THE ORIGIN AND EVOLUTION OF THE EARTH 15
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.
Figure 2.1 : The Big Bang
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 galaxy contains
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. Considering this,
the distances the light will travel in one
year is taken to be one light year. This
equals to 9.46110
12
 km. 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.
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 16
(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 asteroids and comets and 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.
Why are the inner planets rocky while
others are mostly in gaseous form?
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
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058
Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66
Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88
Satellites 0 0 1 2 16   about 18 about 17 8
* Distance from the sun in astronomical unit i.e. average mean distance of the earth is 149,598,000
km = 1 @ Density in gm/cm
3
# Radius: Equatorial radius 6378.137 km = 1
The Solar System
© NCERT
not to be republished
THE ORIGIN AND EVOLUTION OF THE EARTH 17
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 THE EARTH
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.
How was the layered structure of the
earth developed?
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
© NCERT
not to be republished