NCERT Solutions for Class 11 Geography Chapter 26 - The Origin and Evolution of the Earth

Question 1:

Which one of the following figures represents the age of the earth?

  1. 4.6 million years
  2. 13.7 billion years
  3. 4.6 billion years
  4. 13.7 trillion years

(a). 4.6 million years

Question 2:

Which one of the following has the longest duration?

  1. Eons
  2. Period
  3. Era
  4. Epoch

(a). Eons

Question 3:

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

  1. Solar winds
  2. Differentiation
  3. Degassing
  4. Photosynthesis

(a). Solar winds

Question 4:

Which one of the following represents the inner planets?

  1. Planets between the sun and the earth
  2. Planets between the sun and the belt of asteroids
  3. Planets in gaseous state
  4. Planets without satellite(s)

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

Question 5:

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

  1. 13.7 billion
  2. 3.8 million
  3. 4.6 billion
  4. 3.8 billion

(d). 3.8 million

Question 6:

Why are the terrestrial planets rocky?


(a) 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. (b) 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 Jovian planets. (c) The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.

Question 7:

What is the basic differences in the arguments related to origin of the earth given by
(a) Kant and Laplace
(b) Chamberlain and Moulten?


According to Kant and Laplace, the planets were formed out of a cloud of material associated with a youthful sun which was slowly rotating. According to Chamberlain and Moulten the planets were formed according to Binary Theory and The planets were formed by a collision of a passing star with the sun.

Question 8:

What is meant by process of differentiation?


Planetesimals moved together to form the planets. The collection of the mass forming the earth also is believed to have followed the same sequence. As the material was getting collected through gravitation, the bodies getting collected had impact on the existing materials. These impacts generated lots of heat. The process continued and the heat generated caused melting of the materials. This occurred during and soon after the formation of the earth. Due to immense heat the earth became partially molten. The mix of lighter and denser material on melting started getting separated depending on their densities. This allowed the heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. This process of separation of lighter and denser materials is called differentiation. During the formation of the moon, due to the giant impact, the earth was further heated up leading to next phase of differentation. It is through the process of differentiation 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.

Question 9:

What was the nature of earth surface initially?


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 eventsprocesses, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water, conducive atmosphere favouring the existence of life.

Question 10:

Which were the gases which initially formed the earth’s atmosphere?


During the differentiation and cooling of the earth, gases and water vapour were relased from the interior solid earth. This started the evolution of 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 out poured from the interior is called degassing.

Question 11:

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


‘The scientists in later period took up the problems of origin of universe rather than that of just the Earth or the planets. The most popular argument regarding the origin of the universe, in the present world, is the Big Bang Theory. This is also called ‘expanding universe’. Edwin Hubble, in 1920 provided evidence that the universe is expanding. As the time goes on, the galaxies move further and further apart.
SR_Geo_G11_2021_img_2 Experiment. Let us see what does expanding universe means. We take a balloon and mark some points on it to represent the galaxies. Now if we 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 galaxies is also found to be increasing and thereby the universe is considered to be expanding. However, alongwith 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 analogy (example) is only partially correct.
The Big Bang Theory considers the following stages in the development of the Universe. Outlines of the theory.
(i) At 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. Scientists believe that the big bang occurred sometime between 10 and 20 billion years ago.
(iii) 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 present day. As it grew some energy was converted into matter. There was particularly very rapid expansion within fractions of second after the bang. Thereafter, the expansion has slowed down. Within first 3 minutes from the big bang event, first atom began to form.
(iv) Within 3,00,000 years from the big bang temperature dropped to 4500 K 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 same at any point of time. However, with greater evidence becoming available about the expanding universe, the scientific community does not support this concept, at present.

Question 11:

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


Formation of Planets. Following are considered to be the stages in the development of planets.
(a) Formation of Disc. The stars are the 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 the huge rotating disc of gas and dust develops around the gas core.
(b) Formation of Planetesimals. In the next stage, gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small rounded objects by process of cohesion develop into what is called as planetesimals.
(c) Cohesion. Larger bodies start forming by collision and cause the material to stick together by gravitational attraction. These are large number of small bodies.
(d) Formation of Planets. In the final stage these large number of planetesimals accrete to form a fewer large bodies in the form of planets.