Notes of Chapter 2 The Origin and Evolution of the Earth Class 11th Geography

Early Theories

Origin of the Earth

Nebular hypothesis

• Proposed by German philosopher Immanuel Kant.

• Mathematician Laplace revised it in 1796.

• Nebular Hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

• Later 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.

Binary Theories

Later, the arguments considered of a companion to the sun to have been coexisting.

Revised 'Nebular hypothesis'

• Given by Otto Schmidt in Russia and Carl Weizascar in Germany in 1950.

• They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust.

• The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.

Modern Theories

Origin of the Universe

Big Bang Theory

• Also called expanding universe hypothesis.

• Edwin Hubble, in 1920, provided evidence that the universe is expanding.

• This 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 big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into
matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.

→ Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent.

• The expansion of universe means increase in space between the galaxies.

The Star Formation

• A galaxy contains a large number of stars.

• Galaxies spread over vast distances that are measured in thousands of light-years.

• A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
→ Eventually, growing nebula develops localised clumps of gas.

• These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars.

• The formation of stars is believed to have taken place some 5-6 billion years ago.

Formation of Planets

• The stars are localised 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.

• 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.

• These large number of small planetesimals accrete to form a fewer large bodies in the form of planets.

Our Solar System

• Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.

• Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets.

• Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.

• The rest four are called Jovian or Gas Giant planets which means jupiter-like.

• All the planets were formed in the same period sometime about 4.6 billion years ago.

The difference between terrestrial and jovian planets:

• The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.

• The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.

• The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.

The Moon

• The moon is the only natural satellite of the earth.

Origin Theory

• In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body.

• The whole mass became a dumb-bell-shaped body and eventually it broke.

• It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.

Giant Impact or The Big Splat Theory

• A body of the size of one to three times that of mars collided into 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

• The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium.

• The earth has a layered structure.

• From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform.

• From the surface to deeper depths, the earth’s interior has different zones.

Evolution of Lithosphere

• During its primordial stage, the earth was mostly in a volatile state.

• Due to gradual increase in density the temperature inside has increased.

• As a result the material inside started getting separated depending on their densities.

• This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface.

• With passage of time it cooled further and solidified and condensed into a smaller size.

• This later led to the development of the outer surface in the form of a crust.

• During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different

• 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.

Evolution of Atmosphere and Hydrosphere

• The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.

• 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.

• The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.

• During the cooling of the earth, gases and water vapour were released from the interior solid earth which started the evolution of the present atmosphere.

• Continuous volcanic eruptions contributed water vapour and gases.

• As the earth cooled, the water vapour released started getting condensed.

• The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains.

• The rainwater falling onto the surface got collected in the depressions to give rise to oceans.

NCERT Solutions of Chapter 2 The Origin and Evolution of the Earth

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