The Origin and Evolution of the Earth – Class 11 Geography notes, important questions and MCQs

Chapter overview

Scientists explain the origin of the universe mainly through the Big Bang theory, according to which the universe began about 13.7 billion years ago from a tiny, extremely hot and dense point and has been expanding since. Stars and galaxies formed from clouds of gas, and the solar system formed about 4.6 billion years ago from a rotating cloud of gas and dust called the solar nebula. The Earth evolved through stages that produced its layered structure, atmosphere, oceans and finally life.

Early theories and the Big Bang

• Early theories included the nebular hypothesis of Kant and Laplace, which suggested that planets formed from a cloud of material around a young sun.
• The Big Bang theory holds that the universe started with a huge expansion about 13.7 billion years ago; Edwin Hubble's evidence of expanding galaxies supports it.
• Stars form in nebulae; galaxies such as the Milky Way contain billions of stars.

Evolution of the Earth

• The planets formed by accretion of planetesimals; the inner planets are rocky (terrestrial) and the outer planets are gaseous (Jovian).
• The Earth was initially hot; heavier materials sank to form the core and lighter materials rose, a process called differentiation, producing the crust, mantle and core.
• The early atmosphere was lost; a new atmosphere formed through degassing from the interior and volcanic eruptions; water vapour condensed to form oceans about 4 billion years ago, and photosynthesis later added oxygen; life began about 3.8 billion years ago.

Key terms

Big Bang theory

the theory that the universe began with an expansion from a hot, dense point

Nebula

a cloud of gas and dust in space

Planetesimals

small bodies that combined to form planets

Differentiation

separation of Earth materials into layers by density

Degassing

release of gases from the Earth's interior

Important questions with answers

Quick revision

• Nebular hypothesis: Kant and Laplace.
• Big Bang: about 13.7 billion years ago; expanding universe.
• Solar system about 4.6 billion years ago; terrestrial and Jovian planets.
• Differentiation; atmosphere by degassing; oceans; life.

Exam tips

• Remember the ages: universe 13.7 billion years, Earth 4.6 billion years.
• Make a flowchart of the stages of the Earth's evolution.

Practice MCQs

1. The theory that the universe began from an expansion of a hot dense point is the

   1. Big Bang theory
   2. nebular hypothesis
   3. continental drift theory
   4. plate tectonics
   Show answer

   Answer: (a) Big Bang theory. It is the most accepted theory.

2. The age of the Earth is about

   1. 4.6 billion years
   2. 13.7 billion years
   3. 1 million years
   4. 4600 years
   Show answer

   Answer: (a) 4.6 billion years. The solar system formed then.

3. A cloud of gas and dust in space is called a

   1. planet
   2. comet
   3. satellite
   4. nebula
   Show answer

   Answer: (d) nebula. Stars form inside nebulae.

4. The nebular hypothesis is associated with

   1. Hubble and Hoyle
   2. Kant and Laplace
   3. Wegener and Holmes
   4. Darwin and Wallace
   Show answer

   Answer: (b) Kant and Laplace. It explained planet formation.

5. The inner planets are called terrestrial because they are

   1. made of gas
   2. far from the Sun
   3. very large
   4. rocky like the Earth
   Show answer

   Answer: (d) rocky like the Earth. Mercury, Venus, Earth and Mars are rocky.

6. The separation of Earth materials into layers by density is called

   1. degassing
   2. accretion
   3. differentiation
   4. erosion
   Show answer

   Answer: (c) differentiation. Heavy materials sank to the core.

7. Which evidence supports the Big Bang theory

   1. stars do not change
   2. the Earth is flat
   3. galaxies are moving apart, showing an expanding universe
   4. the Sun is the centre of the universe
   Show answer

   Answer: (c) galaxies are moving apart, showing an expanding universe. Hubble observed this.

8. How did the Earth's present atmosphere mainly form

   1. it was the original solar gas
   2. from the oceans only
   3. by degassing and volcanic activity, later modified by photosynthesis
   4. from meteorites alone
   Show answer

   Answer: (c) by degassing and volcanic activity, later modified by photosynthesis. Oxygen increased through photosynthesis.