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Class 11 Geography Notes Chapter 2 The Origin and Evolution Of The Earth

The Origin and Evolution Of The Earth

Starry nights have always attracted us since 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? Maybe many more such questions are still there in your mind. In this chapter, you will learn how these “twinkling little stars” were formed. With that, you will eventually also read the story of the origin and evolution of the earth.

There were many hypotheses put forth by philosophers and scientists regarding the origin of the earth.

Class 11 Geography Notes Chapter 2 The Origin and Evolution Of The Earth

Nebular Hypothesis (Early Hypothesis)

One of the earlier and more popular arguments was by German philosopher Immanuel Kant which was revised by mathematician Laplace in 1796. It is known as Nebular Hypothesis. According to this hypothesis, the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.


Binary Theories

In 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets. Later on, the arguments considered a companion to the sun to have coexisting. These arguments are called binary theories.

Big Bang Theory (Modern Hypothesis)

Big bang The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. The Big Bang Theory considers the following stages in the development of the universe.

  • 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.
  • At the Big Bang, the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of the big bang took place 13.7 billion years before the present.
  • Within 300,000 years from the Big Bang, the temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent

Formation Of Stars

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. These formed the bases for the development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to form by the accumulation of hydrogen gas in the form of a very large cloud called a nebula. Eventually, the growing nebula develops localized clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to the formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

What is a Light Year?

A light year is a measure of distance. The distance traveled by light in a span of 1 year is called 1 light year. 

1 Light year = 9.461×10^12 km

The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311minutes.

Formation Of Planets

  • The stars are localized lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.
  • In the next stage, the gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
  • In the final stage, these large number of small planetesimals accrete to form fewer large bodies in the form of planets.
  • Class 11 Geography Notes Chapter 2 The Origin and Evolution Of The Earth

Solar System

Our Solar system consists of eight planets. The nebula from which our Solar system is supposed to have been formed started its collapse and core formation sometime 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago. Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets, and huge quantities of dust grains and gases.Class 11 Geography Notes Chapter 2 The Origin and Evolution Of The Earth

class 11 geography notes ch 2 the origin and evolution of earth

The Moon

The moon is the only natural satellite of the earth. It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of a ‘giant impact’ or what is described as “the big splat”. A body the size of one to three times that of mars collided with the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

Evolution Of Earth

Do you know that 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 a conducive atmosphere favoring the existence of life. In the following section, you will find out how the period, between the 4,600 million years and the present, led to the evolution of life on the surface of the planet.

There are three stages in the evolution of the present atmosphere. The first stage is marked by the loss of primordial atmosphere. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere. Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis. Sometime around 3,800 million years ago, life began to evolve. However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved. Life was confined to the oceans for a long time.

Origin Of Life

The record of life that existed on this planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of blue algae have been found in geological formations much older than 3,000 million years. It can be assumed that life began to evolve sometime 3,800 million years ago.

Important Terms

  • Planetesimals: The gas cloud starts getting condensed and the matter around the core develops into small-rounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals.
  • Universe: All matter, energy, heavenly bodies, and all that is there in space is as a group called the universe.
  • Galaxy: A galaxy is a cluster of millions of stars and solar systems.
  • Outer Planets: Jupiter Saturn Uranus Neptune and Pluto are called Outer Planets.
  • Inner Planets: Mercury, Venus, Earth, and Mars are called Inner Planets.
  • Nebula: Slow circular moving gaseous clouds are called a nebula.
  • The big splat: The origin of the moon as a satellite of the earth is the result of a big collision which is called “the big splat”
  • Differentiation: The process through which the earth-forming material got separated into different layers is called differentiation.
  • Dwarf Planet: According to International Astronomical Union (IAU) on August 24, 2006, a planet is a celestial body that:- 1. Orbits around the sun. 2. Has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape. The non-satellites bodies fulfilling these two rules are called dwarf planets. Pluto is now considered a dwarf planet. Ceres, Eris, Makemake, and Haumea are some other dwarf planets.
  • Jovian: Jovian means Jupiter-like.
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