Geography: EARTH

Chapter 2 Class 11th NCERT Geography book. 

THE ORIGIN AND EVOLUTION

OF THE EARTH

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?

May be 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 origin and evolution of the earth.

EARLYTHEORIES

Origin of the Earth

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. 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. Sir James Jeans and later Sir Harold Jeffrey supported this

argument. At a later date, the arguments considered of a companion to the sun to have

been coexisting. These arguments are called binary theories. In 1950, Otto Schmidt in

Russia and Carl Weizascar in Germany somewhat revised the ‘nebular hypothesis’,

though differing in details. 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. However, scientists in later period took up

the problems of origin of universe rather than that of just the earth or the planets.

MODERN THEORIES

Origin of the Universe

The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also

called expanding universe hypothesis. 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. Take a balloon and mark some

points on it to represent the galaxies. Now, if you start inflating the balloon, the points marked

on the balloon will appear to be moving away from each other as the balloon expands.

Similarly, the distance between the galaxies is also found to be increasing and thereby, the

universe is considered to be expanding. However, you will find that besides the increase

in the distances between the points on the balloon, the points themselves are expanding.

This is not in accordance with the fact. Scientists  believe that though the space between the

galaxies is increasing, observations do not support the expansion of galaxies. So, the

balloon example is only partially correct.

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 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.

(iii) 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. An alternative

to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same

at any point of time. However, with greater evidence becoming available about the

expanding universe, scientific community at present favours argument of expanding

universe.

The Star Formation

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 development of galaxies. A galaxycontains

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 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.

A light year is a measure of distance and not of time. Light travels at a speed of

300,000 km/second. 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 following are considered to be the stages in the development of planets :

(i) 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.

(ii) 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.

(iii) In the final stage, 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 eight planets. The nebula from which our Solar system is supposed

to have been formed, started its collapse and core formation some time 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 asteroidsand cometsand 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. Jovian means jupiter-like. Most

of them are much larger than the terrestrial planets and have thick atmosphere, mostly of

helium and hydrogen. All the planets were formed in the same period sometime about 4.6 billion

years ago. Till recently (August 2006), Pluto was also considered a planet. However, in a meeting

of the International Astronomical Union, a decision was taken that Pluto like other celestial

objects (2003 UB 313) discovered in recent past may be called ‘dwarf planet’. Some data regarding

our solar system are given in the box below.

The difference between terrestrial and jovian planets can be attributed to the following

conditions:

(i) 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.

(ii) 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.

(iii) 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. Like the origin of the earth, there have

been attempts to explain how the moon was formed. 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. However, the present scientists do not

accept either of the explanations. It is now generally believed that the formation of moon,

as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.

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 THEEARTH

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

conducive atmosphere favouring 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. 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. The atmospheric matter has the least density. From the surface to deeper

depths, the earth’s interior has different zones and each of these contains materials with

different characteristics.

Evolution of Lithosphere

The earth was mostly in a volatile state during its primordial stage. 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

layers. 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. We

shall discuss in detail the properties of each of this layer in the next chapter.

Evolution of Atmosphere and Hydrosphere The present composition of earth’s atmosphere

is chiefly contributed by nitrogen and oxygen. You will be dealing with the composition and

structure of the earth’s atmosphere in Chapter 8. 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. This happened not only in case of the earth, but also in all the

terrestrial planets, which were supposed to have lost their primordial atmosphere through

the impact of solar winds.

During the cooling of the earth, gases and water vapour were released from the interior

solid earth. This started the evolution of the present atmosphere. The early atmosphere

largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very

little of free oxygen. The process through which the gases were outpoured from the interior is

called degassing. Continuous volcanic eruptions contributed water vapour and gases to the atmosphere.

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. The earth’s

oceans were formed within 500 million years from the formation of the earth. This tells us

that the oceans are as old as 4,000 million years. 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.

Oceans began to have the contribution of oxygen through the process of photosynthesis.

Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to

flood the atmosphere.

Origin of Life

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. Modern scientists refer to the origin of life as a kind of

chemical reaction, which first generated complex organic molecules and assembled

them. This assemblage was such that they could duplicate themselves converting

inanimate matter into living substance. 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 some 3,000 million years. It can be assumed that life began to evolve sometime

3,800 million years ago.