NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 2: The Origin and Evolution of the Earth

Chapter at a glance

Exercise Questions

Question 1 (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

Answer

3. 4.6 billion years

Question 1 (ii)

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

Answer

Solar winds stripped the primordial atmosphere, degassing supplied gases, and photosynthesis added oxygen. Differentiation is the separation of the earth’s materials into layers by density; it is not related to forming the atmosphere.

2. Differentiation

Question 1 (iii)

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

Answer

4. 3.8 billion

Question 2 (i)

What is meant by the process of differentiation?

Answer

Differentiation is the process by which, as the young earth heated up and became partly molten, heavier materials (like iron) sank towards the centre and lighter materials rose to the surface. This separated the earth into layers — crust, mantle, outer core and inner core — each with different density.

The separation of earth’s materials by density, heavier ones sinking to the centre and lighter ones rising, forming layers (crust, mantle, core).

Question 2 (ii)

What was the nature of the earth surface initially?

Answer

Initially the earth was a barren, rocky and hot body with a thin atmosphere of hydrogen and helium. It was largely in a volatile, molten state, with intense heat and volcanic activity, and no oceans or life.

A hot, barren, rocky and largely molten surface with a thin hydrogen-helium atmosphere and no water or life.

Question 2 (iii)

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

Answer

The earliest (primordial) atmosphere consisted mainly of hydrogen and helium. After it was stripped away, degassing from the interior released water vapour, nitrogen, carbon dioxide, methane, ammonia and very little free oxygen.

Initially hydrogen and helium; later, from degassing, water vapour, nitrogen, carbon dioxide, methane and ammonia.

Question 3 (i)

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

Answer

The Big Bang Theory (expanding universe hypothesis) is the most widely accepted explanation of the origin of the universe.

1. In the beginning, all matter forming the universe existed in one place as a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
2. About 13.7 billion years ago, this tiny ball exploded violently (the Big Bang), leading to a huge expansion that continues to this day.
3. As it expanded, some energy was converted into matter. Within fractions of a second after the explosion there was very rapid expansion; thereafter the expansion slowed.
4. Within the first three minutes, the first atoms began to form.
5. Within 300,000 years, temperature dropped to about 4,500 K, giving rise to atomic matter, and the universe became transparent.
6. Gas clouds later condensed to form galaxies and stars.

Evidence: Edwin Hubble (1920) found that galaxies are moving away from each other — the universe is expanding. (An alternative, the “steady state” theory, is not widely accepted.)

The universe began about 13.7 billion years ago from an infinitely dense, hot point that exploded and has been expanding since; atoms formed within minutes and galaxies later. Hubble’s evidence of expansion supports it.

Question 3 (ii)

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

Answer

1. Formation of the planet: the solar nebula condensed; small planetesimals formed and collided, growing by gravitational attraction into the protoplanet earth about 4.6 billion years ago.
2. Development of lithosphere: the earth was hot and partly molten; through differentiation, heavy materials sank to form the core and lighter ones rose, forming the mantle and crust. The Moon formed from a giant impact around 4.44 billion years ago. As it cooled, a solid crust formed.
3. Evolution of the atmosphere:
   • Stage 1: loss of the primordial hydrogen-helium atmosphere due to solar winds;
   • Stage 2: degassing — the hot interior released gases and water vapour;
   • Stage 3: modification by photosynthesis, adding oxygen about 2,500–3,000 million years ago.
4. Evolution of the hydrosphere: as the earth cooled, water vapour condensed; continuous rain filled depressions, forming oceans within about 500 million years of earth’s formation (~4,000 million years ago).
5. Origin of life: around 3.8 billion years ago simple life (blue-green algae) appeared in the oceans; life then evolved to multi-cellular organisms and diverse forms.

Planet formation from planetesimals; layering of lithosphere by differentiation and cooling; atmosphere evolving through solar-wind loss, degassing and photosynthesis; oceans from condensed vapour; origin of life about 3.8 billion years ago.

Topic checklist

• Early theories of origin of the earth
• Big Bang theory and formation of stars and galaxies
• Formation of the solar system and the Moon
• Evolution of the lithosphere, atmosphere and hydrosphere
• Origin of life

Total questions answered: 8 of 8