This unit deals with Origin and evolution of the earth;
- Interior of the earth; Wegener’s continental drift theory and plate tectonics;
- Earthquakes and volcanoes.

Do you remember the nursery rhyme
“…Twinkle, twinkle little star…”? Starry nights have always attracted us since the childhood. You may also have thought of these stars and had numerous questions in your mind. Questions such as how many stars are there in the sky? How did they come into existence? Can one reach the end of the sky? Many more such questions are still there in your mind.

A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. One of the earlier and popular arguments was by German philosopher Immanuel Kant. Mathematician Laplace revised it in 1796. It is known as Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

In 1950, Otto Schmidt in Russia and Carl Weizsacker in Germany somewhat revised the ‘nebular hypothesis’, though differing in details. They considered that the sun was surrounded by solar nebula containing mostly hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to the formation of a disk-shaped cloud and the planets were formed through the process of accretion.

The most popular argument regarding the origin of the universe is the Big Bang Theory. Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart. You can experiment and find what does the expanding universe mean.

The Big Bang Theory considers the following stages in the development of the universe: (i) 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. (ii) 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 distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years.

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

The earth has a layered structure...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.

The early atmosphere...largely contained water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little of free oxygen. Continuous volcanic eruptions contributed water vapor and gases to the atmosphere.

The last phase in the evolution of the earth relates to the origin and evolution of life. It is undoubtedly clear that initially the earth or even the atmosphere of the earth was not conducive for the development of life.