NCERT Solutions for Class 11 Geography Chapter 2 - Origin and Evolution of the Earth

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    48. Multiple choice questions.

    (i) Which one of the following figures represents the age of the earth?

    (a) 4.6 million years

    (b) 13.7 billion years

    (c) 4.6 billion years

    (d) 13.7 trillion years

    Ans. (c) 4.6 billion years

    (ii) Which one of the following has the longest duration?

    (a) Eons

    (b) Period

    (c) Era

    (d) Epoch

    Ans. (a) Eons

    (iii) Which one of the following is not related to the formation or modification of the present atmosphere?

    (a) Solar winds

    (b) Differentiation

    (c) Degassing

    (d) Photosynthesis

    Ans. (b) Differentiation

    (iv) Which one of the following represents the inner planets?

    (a) Planets between the sun and the earth

    (b) Planets between the sun and the belt of asteroids

    (c) Planets in gaseous state

    (d) Planets without satellite(s)

    Ans. (b) Planets between the sun and the belt of asteroids

    (v) Life on the earth appeared around how many years before the present?

    (a) 13.7 billion

    (b) 3.8 million

    (c) 4.6 billion

    (d) 3.8 billion

    Ans. (d) 3.8 billion

    49. Answer the following questions in about 30 words.

    (i) Why are the terrestrial planets rocky?

    Ans. Terrestrial planets or the inner planets are rocky because they are made up of mainly rocks and metals. Their structure is very similar to Earth’s structure.

    (ii) What is the basic difference in the arguments related to the origin of the earth given by :

    (a) Kant and Laplace

    (b) Chamberlain and Moulton

    Ans. (a) The hypothesis given by Immanuel Kant and revised by Laplace in 1796 was called as Nebular Hypothesis and states that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

    (b) 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.

    (iii) What is meant by the process of differentiation?

    Ans. 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. This process of the earth forming material got separated into different layers is called differentiation.

    (iv) What was the nature of the earth surface initially?

    Ans. Our planet earth initially was a barren, rocky and hot object.

    (v) What were the gases which initially formed the earth’s atmosphere?

    Ans. The gases which initially formed the earth’s atmosphere were helium and hydrogen. These two gases were present in abundance.

    50. Answer the following questions in about 150 words.

    (i) Write an explanatory note on the ‘Big Bang Theory’.

    Ans. In 1920, Big Bang Theory or Expanding Universe hypothesis tried to justify how the universe is expanding. With expanding universe, the distance between galaxies and galaxies themselves are also expanding. This theory can be explained through following developmental stages:

    Stages of Big Bang theory:

    (a) In the beginning, all matter was in the form of tiny ball (singular atom).

    (b) About 13.7 billion years ago the tiny ball exploded violently. The expansion continues  even today.

    (c) As a result, some energy was converted into matter.

    (d) Within fraction of a second there was rapid expansion.

    (e) The expansion went slowdown, but within first three minutes from the Big Bang event, the first atom began to form.

    (f) After 300000 years the temperature dropped down to 4,500 K and gave rise to atomic matter.

    (g) The universe became transparent.

    (ii) List the stages in the evolution of the earth and explain each stage in brief.

    Ans. Our planet earth was formed some 4.6 billion years ago. Like all other planets, earth’s formation took place as a result of a slow process mentioned below in three stages:

    There are three different stages in the evolution of the earth:

    Stage I: When planetesimals accretion happened and the earth originated. Earth was hot sphere, rocky, barren and thin atmosphere of hydrogen and helium.

    Stage II: Development of Lithosphere: The heavier materials like iron and nickel moved towards the center and the lighter one moved towards the surface. Over a period of time it cooled and solidified and condensed, later led to the formation of core, mental and crust.

    Stage III: Evolution of Atmosphere: During cooling process of earth, grass were out poured from the interior the process is called degassing. This started the evolution of the early atmosphere which contained nitrogen, carbondioxide, water vapours methane and little oxygen and finally the composition of the atmosphere was modify by the process of photosynthesis.

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