Geography: Class 11 (Part 2) — Chapter 2: The Origin and Evolution of the Earth

One of the earliest and most popular arguments was proposed by the German philosopher Immanuel Kant and revised by the mathematician Laplace in 1796.

• It considered that the planets were formed out of a cloud of material associated with a youthful, slowly rotating sun.
• In 1950, Otto Schmidt (Russia) and Carl Weizascar (Germany) revised the hypothesis, suggesting the sun was surrounded by a solar nebula containing mostly hydrogen, helium, and dust.
• The friction and collision of particles led to the formation of a disk-shaped cloud, and planets were formed through the process of accretion.

Also known as the expanding universe hypothesis, this is the most popular argument regarding the origin of the universe. Edwin Hubble provided evidence of this expansion in 1920.

The Big Bang Theory considers the following stages in the development of the universe:

1. 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.
2. At the Big Bang, this "tiny ball" exploded violently, leading to a huge expansion. This event took place approximately 13.7 billion years before the present.
3. Within 300,000 years from the Big Bang, the temperature dropped to $4,500\text{ K}$ (Kelvin), giving rise to atomic matter and making the universe transparent.

An alternative to the Big Bang was Hoyle's Steady State concept, which considered the universe to be roughly the same at any point in time. However, the scientific community currently favours the expanding universe argument.

The distribution of matter and energy was not even in the early universe, leading to density differences and gravitational forces that drew matter together to form galaxies.

• A galaxy starts to form by the accumulation of hydrogen gas in the form of a very large cloud called a nebula.
• Eventually, growing nebulae develop localized clumps of gas that grow into dense gaseous bodies, forming stars.
• Star formation is believed to have taken place some 5-6 billion years ago.

The development of planets occurs in distinct stages:

1. Stars are localized lumps of gas within a nebula. Gravitational forces lead to the formation of a core and a rotating disc of gas and dust around it.
2. The gas cloud condenses, and matter around the core develops into small-rounded objects through cohesion, forming planetesimals.
3. In the final stage, these planetesimals accrete to form fewer, larger bodies in the form of planets.

During its primordial stage, the Earth was mostly in a volatile state. Due to gradual increases in density, the temperature inside increased, causing materials to separate based on density.

• Heavier materials (like iron) sank towards the center, while lighter ones moved towards the surface. This process is called differentiation.
• With time, it cooled, solidified, and condensed to form the outer crust.
• The Earth's layers include the crust, mantle, outer core, and inner core.

The evolution of the Earth's atmosphere occurred in three distinct stages:

1. Loss of Primordial Atmosphere: The early atmosphere of hydrogen and helium was stripped off by solar winds.
2. Degassing: During the cooling of the Earth, gases and water vapour were released from the interior through volcanic eruptions. This process is called degassing.
3. Photosynthesis: The composition of the atmosphere was modified by living organisms. Photosynthesis contributed oxygen to the oceans and eventually flooded the atmosphere around 2,000 million years ago.

Life began as a chemical reaction that generated complex organic molecules capable of duplicating themselves.

• Life began to evolve around 3,800 million years ago.
• The earliest records of life are found in rocks as fossils of blue-green algae dating back over 3,000 million years.