NCERT Solutions for Class 11th: Ch 2 The Origin and Evolution of the Earth

NCERT Solutions for Class 11th: Ch 2 The Origin and Evolution of the Earth

Exercises

Page No: 19

1. 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
► (c) 4.6 billion years

(ii) Which one of the following has the longest duration?
(a) Eons
(b) Period 
(c) Era
(d) Epoch
► (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
► (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)
► (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
► (d) 3.8 billion

2. Answer the following questions in about 30 words.

(i) Why are the terrestrial planets rocky?

Answer

The terrestrial planets are rocky because:

• 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.

• 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.

• The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.

(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

Answer

The Nebular Hypothesis, proposed by Immanuel Kant and later revised by Laplace in 1796 considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating while, in 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped material separated from the solar surface. As the passing star moved away, the material separated from the sun, continued to revolve around the sun and it slowly condensed into planets.

Page No: 20

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

Answer

The process by which earth forming material got separated into different layers is called differentiation. 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.

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

Answer

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.

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

Answer

Hydrogen and helium were the gases which intially formed the earth's atmosphere. The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the 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. 

3. Answer the following questions in about 150 words. 

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

Answer

The 'Big Bang Theory' also known as expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the universe is expanding. The distance between the galaxies is also found to be increasing and thereby, the universe is considered to be expanding.

Various stages in the development of the universe according to Big Bang Theory are:

• 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.

• 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.

• Within 300,000 years from the Big Bang, temperature dropped to 4,500K (Kelvin) and gave rise to atomic matter. The universe became transparent.

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

Answer

Stage I: 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 which led to the formation of the 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, the crust, mantle, outer core and inner core formed.

Stage II: The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. During the cooling of the earth, gases and water vapour were released from the interior solid earth which 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.

• 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.

• Life was confined to the oceans for a long time. Oceans began to have the contribution of oxygen through the process of photosynthesis.

Stage III: The last phase in the evolution of the earth relates to the origin and evolution of life.

• 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.

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