NCERT Textbook - The Origin and Evolution of the Earth | Geography for UPSC CSE PDF Download

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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 12
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
2024-25
Page 2


FUNDAMENTALS OF PHYSICAL GEOGRAPHY 12
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
2024-25
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. 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
THE ORIGIN AND EVOLUTION
OF THE EARTH
CHAPTER
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
2024-25