The Origin And Evolution Of The Earth Class 11 Notes Geography Chapter 2 - CBSE
Chapter : 2
What Are The Origin and Evolution of the Earth ?
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- These are multiple theories and beliefs explaining the origin and evolution of Earth.
- One of them is the Nebular Hypotheses. It was originally proposed by the German philosopher Immanuel Kant in 1755 and modified in 1796 by Pierre Laplace.
- The theories believes that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk and became a solar system consisting of a star with orbiting planets.
- During the 20th century many theories were proposed including the planetesimal theory (Binary Theory) of Thomas Chamberlin and Forest Moulton (1901). This theory could not attain much popularity because of its high criticism.
- Otto Schmidt in Russia and Carl Weizascar were the believers of nebular theory and in 1950 they revised it with more supporting arguments. They proposed that the current planets are formed because of accretion of hydrogen, helium, or may be dust particles. This process happened due to friction and collision of particles of solar nebula with the sun.
- In 1920, Big Bang Theory or Expanding Universe hypothesis tried to justify how the universe is expanding. With the universe expanding, the distance between galaxies and galaxies themselves are also expanding. This theory can be explained through following developmental stages:
- Stages of the Big Bang theory:
(i) In the beginning, all matter was in the form of a tiny ball (singular atom).
(ii) About 13.7 billion years ago the tiny ball exploded violently. The expansion continues even today.
(iii) As a result, some energy was converted into matter.
(iv) Within fraction of second there was rapid expansion.
(v) The expansion went slowdown, but within first three minutes from the Big Bang event, the first atom began to form.
(vi) After 300,000 years the temperature dropped down to 4,500K and gave rise to atomic matter.
(vii) The universe became transparent.
- A single galaxy contains numerous stars and it has its origin related to nebula. The diameter of galaxies is measured in light years. Further, these nebulas develop their own dense gaseous clumps called stars. Our solar system is a small part of the Milky Way Galaxy or Akash Ganga.
- Our solar system consists of eight planets. The formation of planets can be explained through following stages:
(i) The high gravitational force of a nebulas leads to the formation of dense gaseous core and the lighter gases and dust formed a rotational disc around it.
(ii) The denser core gradually condensed into small rounded objects called Planetesimals.
(iii) With the passage of time and the process of accretion, these small planetesimals turned into large planets.
- Apart from 8 planets, our solar system consists of the sun (the star), 63 moons, millions of smaller bodies such as asteroids and comets and huge quantity of dust grains and gases.
- All the eight planets are not similar, the first four care called inner planets or terrestrials and the last four are called as outer or Jovial.
Inner Planets Or Terrestrials
- Mercury, Venus, Earth, and Mars are called Inner Planets.
- They are found between belt of asteroids and the sun.
- They are also called Terrestrial Planets because they share similarities with Earth.
- Smaller in Size with high density.
Outer Planets Or Jovian
- Jupiter, Saturn, Uranus, Neptune, Pluto are called Outer Planets.
- They are found after the belt of asteroids.
- They are called Jovian Planet because they are similar to Jupiter in structure.
- They are larger in size but with low density.
- Related to the origin of Moon, there are multiple theories. One such was preponed by Sir George Darwin, which says Moon and Earth were part of a single body and Moon was separated from Earth.
- However, most trusted theory, ‘The Big splat’ says a large size body bigger than of Mars collided with the Earth and that portion was separated from the earth. The same portion became as a moon which revolves around the Earth. The Moon was formed about 4.4 billion years ago.
- The lithosphere of the Earth has evolved from volatile state to what it is currently now. The extreme density
and temperature enable the materials to settle at different layers. The heavier materials to stay closer to the core and the lighter material to go outwards. - During the Big Splat or at the time of formation of the moon the earth again became hot and due to differentiation different layers of the lithosphere were formed.
Evolution Of Atmosphere And Hydrosphere
It can be explained through the following three stages:
- First Stage: Like all other terrestrial planets, Earth also lost its primordial atmosphere due to solar winds. This primordial atmosphere was mainly consisting of hydrogen and helium.
- Second Stage: At this stage, the earth started cooling and the volcanic eruptions occurred. Due to the same, huge number of gases, water vapours were released. This led to the formation of atmosphere. Furthermore, the gases and water vapour got condense and rain collected in the depressions gave birth to hydrosphere.
- Third Stage: Due to the process of photosynthesis, the composition of atmosphere again changed. This time
the huge amount of released oxygen gave birth to life on this beautiful planet.
Origin Of Life
The changes in the earth and atmosphere gradually led to the origin of life on Earth. Due to lightning, the complex organic molecules were combined into a certain form which can duplicate themselves as first single-cell animals. They are able to convert inanimate things into animate things. The earliest form of life existed about 3,000 million years ago. The life began on the earth about 3,800 million years ago.
