Big Bang Event

The Origin and Evolution of the Earth

Class 11 | Geography | Chapter 2 | Questions and Answers | NCERT

1. Who proposed the Nebular hypothesis?

German Philosopher Immanuel Kant

2. Who modified the Nebular hypothesis?

Pierre Laplace modified the Nebular hypothesis in 1796.

3. Who discovered the Big Bang theory?

Edwin Hubble discovered the Big Bang theory.

4. What is a planetesimal?

These are minute bodies of gas and dust. They are assumed to have orbited the sun during the initial stages of planet formation.

5. When did Planets start to form?

The Planets were formed 4.6 billion years ago.

6. What are inner planets and outer planets?

The planets that lie between the Sun and the asteroid belt are called inner planets. Among the 8 planets, Mercury, Venus, Earth and Mars are called as inner planets. On the other hand, those planets that lie away from the asteroid belt are called outer planets.

7. What is degassing?

The process of effluxion of gases from the core of the earth is called degassing.

8. Why are the terrestrial planets rocky?

Due to their close proximity with the Sun, the terrestrial planets become very hot. Due to this, the gases cannot condense easily. Moreover, the strength of solar winds near the Sun is very high due to which most of the gas and dust are blown away. So, the terrestrial planets have a rocky surface.

9. 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

(a) Kant and Laplace

According to this hypothesis, the planets are assumed to be formed out a cloud of material associated with a slowly rotating youthful sun. This principle also suggests that the interior of the earth must be gaseous as it has originated from gas form.

(b) Chamberlain and Moulton

According to this concept, a wandering star approached the sun. This resulted in the separation of a cigar-shaped extension of material from the solar surface. The star moved away but the separated material continued to revolve around the sun which slowly condensed to form the planets.

10. What is differentiation?

The density of the earth started to increase after its formation. As a result, the temperature also increased. And depending upon densities, the various earth forming materials started to separate from each other. Heavy materials like iron settled at the core while the lighter ones moved towards the top surface. This process is called differentiation.

11. What was the nature of the earth’s surface initially?

The nature of the earth’s surface initially was not the same as it is today. During the formation of the earth, the whole planet was empty and naked. Its surface was rocky and hot covered with a thin layer of hydrogen and helium.

12. What were the gases in the Earth’s early atmosphere?

The Earth’s early atmosphere consisted largely of water vapour, nitrogen, carbon-di-oxide, methane and a very little amount of free oxygen.

13. Explain Big Bang theory

Big Bang theory is the most accepted theory for the birth of the universe.

Initially, all the materials forming the universe were concentrated in one place in the form of a ‘tiny ball’. Though the size of this ball was small but its density and temperature was too high.

Secondly, this tiny ball exploded violently and its size increased considerably. During this expansion, some energy got transformed into matter. The first atom began to form within first three minutes from the Big Bang event.

Thirdly, within 300000 years from the Big Bang event, the temperature dropped to 4500k. At that time, atomic matter formed and the universe became transparent.

There was rapid expansion within fraction of a second from the Big Bang event. In this way, the universe is assumed to be formed. A large amount of data has been collected at present regarding the expansion of the universe. That’s why scientific community has favoured the concept of Big Bang.

Stages of Big Bang

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

We can divide the stages in the evolution of the earth into four categories –

  1. Evolution of lithosphere – Initially, the earth was mostly in volatile state. Gradually the density increased and the temperature of the core became very high. As a consequence and depending upon the densities, heavy materials like iron settled at the core and light materials remained at the top surface. By formation of the earth through differentiation, the materials got divided into various segments which, now we call as the crust, mantle, inner core and outer core, etc. Gradually the earth began to cool and decrease in size as well.
  2. Evolution of atmosphere – After the gradual cooling of the earth, the gas and water vapour from its interior oozed out. As a consequence, the atmosphere formed. In the beginning, this atmosphere consisted of water vapour, carbon-di-oxide, methane, ammonia and free oxygen to some extent. There was numerous volcanic eruption at that time and the water vapour and gases stuck to the atmosphere. The water vapour condensed to form rain which made the carbon-di-oxide to reduce to a great extent.
  3. Evolution of hydrosphere – After the formation of the earth, a lot of volcanic eruption took place and has led to the formation of some broad and deep depressions. The rain water got in those depressions forming seas and oceans. Thus, 500 million years after the formation of the earth, the oceans were formed.
  4. Origin of Life – Almost 3800 million years ago, life began to evolve on the earth. For millions of years, this life was confined to the oceans. Modern day scientists assume origin of life as a kind of chemical reaction.

After passing out of all the above stages, the present day green and life-giving earth was formed.

15. What are the stages of planet formation?

  1. The lumps of gas present within the nebula are actually stars. The gravitational force within the gaseous lumps leads to the development of a core around which the disc of gas and dust begin to rotate.
  2. In the second stage, the gaseous lumps start to condense thereby taking shape of some round-like objects. By a process called cohesion, these objects give rise to the formation of planetesimals. They collide and come together to form large bodies. As a result, the gravitational force increase and causes the bodies to stick together and increase in dimension.
  3. In the last stage, these small planetesimals aggregate in huge number and take the shape of a planet.

16. Write the differences between terrestrial planets and Jovian planets.

The differences between Terrestrial planets and Jovian planets are as follows –

Terrestrial Jovian
1. These planets lie in close proximity to the sun. As a result the temperature increases and the gases do not condense. 1. These planets lie away from the sun. So, the temperature remains low and the gases condense easily.
2. The solar winds are very powerful near the sun and so, the gases are blown away from these planets. 2. These phenomenon doesn’t take place due to the distance of these planets from the sun.
3. Due to small size of these planets, the force of gravity is low. As a result, the gases blown away by solar winds cannot be hold back. 3. They are of large size.

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