Class 11 Fundamentals of Physical Geography NCERT Chapter - 2

The Origin and Evolution of the Earth 🌍 

Early Theories 

Origin of the earth 

• A Large number of hypothesis were put forth by different philosophers and scientist 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 planet were formed out of a proud of material associated with a youthful Sun, which was slowly rotating. 
•  In 1900, Chamberlain and Moulton considered that a wandering star aproached the sun .As a result, a cigar shaped extension of material or separated from the Solar surface. 
• Sir James Jeans and later sir Harold Jeffrey supported  this argument.
•   The argument considered of a companion to the sun to have been coexisting.
•  These argument are called binary theories. 
• In 1950, Otto Schmidt in Russia and Charl weizascar in Germany somewhat revised the " nebular hypothesis" through differing in details. 
• The friction and collision of particles lead to formation of a disk- shaped cloud and the planets were formed through the process of accretion. 
• Scientist in later period look up the problem 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 The Big Bang Theory. 
• It is also called expanding Universe hypothesis.
•  Edwin Hubble, in 1920 provided evidence that universe is expanding.

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 form of "tiny ball "with unimaginably small volume, infinite temperature and infinite density .

2.  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. 

3. Within 300,000 Years from the big  bang,  temperature dropped to4,500k and  give rise to atomic matter .

• The universe become transparent.
• The expension of Universe means increased in space between the galaxies.

The star formation 

• The distribution of matter and energy was not even in the early universe. 
• These initial density difference gave rise to differences in graviation forces it caused drawn together these formed the bases for development of galaxies. 
• A galaxies contain a large number of stars.
•  Galaxies spread over vast distance that are measured in thousands of light years. 
•  The diameters of individual galaxies range from 80000- 150,000 light years .
• A Galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called Nebula.
•  The formation of stars is believed to have taken place some 5-6 billion a year ago.

1.  A light year is measure of distance and not of time .
2. Light travel at a speed of 300,000 km per second.
3.  Considering this, the distance the light will travel in one year is taken to be one light year. this is equal to 9.461 into 10  power 12 km.
4.  The mean distance between the sun and the earth149,598,000 km .
5. In term of light year, it is 8.3 11 minutes.

 Formation of planets 

The following are considered to be the stage in the development of planets:

•  The star are localised lumps of gas within 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 developed around the gas core.
•  In the next stage, the gas cloud stars getting condensed and the matter around the core develops into small rounded objects. 
•  These small rounded object by the process of Cohesion develop into what is called planetesimals.
•  Large bodies start forming by collision, and gravitational attraction causes the material to stick together.
•  planetesimals are large number of smaller bodies.
•  In the final stage, these large number of small planetesimals accrete to form a few were 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 coar formation sometime 5 -5.6 billion a years ago and the planet were formed about 4.6 billion a year ago.
•  Our solar system consists of the sun eight planets, 63moon, million of smaller bodies like asteroid and comets and huge quantity of dust grains and gases.
•  Out of the 8th planet, mercury, Venus, Earth and mars called as a inner planet as they lie between the sun and the belt of asteroids The Other four planet are called outer planet .
• The first four are called Terrestrial meaning of earth like as they are made up of Rock and metals and have relativelying high densities.
•  The rest 4 are called jovian gas Giant planet Jupiter like most of them are much larger than the Terrestrial planet and have thick atmosphere, mostly of helium and hydrogen .
• All the planets were formed in the same period sometime about 4.6 billion a year ago.
•  Till recently August 2006, Pluto was also considered a planet. In a meeting of international astronomical Union, a decision was taken that Pluto like other celestial object (2003 UB313 )discovered in recent past may be called dwarf planet. 

The differences between Terrestrial and jovian Planet can be attributed to the following condition:

•  The Terrestrial planets were formed in the close vicinity of the parent start where it was too warm for gases to condense to solid particles. jovian planets were formed at quite a distance location.
• The Solar wind was most intense nearer the sun: so, it blew off lots of gas and dust from the traditional 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.
•  like the origin of the Earth there have been a time 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 become a dumb- bell -shaped body and eventually it broke.
•  It was also suggest that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.
•  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 mass 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 4.4 billion a year ago.

Evolution of the Earth 

• The planet earth initially was a bArden,   Rocky and hot object with a thin atmosphere of hydrogen and helium. 
• This is far from the present day picture of the earth .
• The earth has a layered structure. from the outermost end of the atmosphere to the centre of the earth , the material that exist is not uniform
•  The atmospheric matter has the least density.

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 .
• 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 part , we have layers like the crust,  mantlers outer Core and inner core. from the to the code that density of material increases.

Evolution of Atmosphere and Hydrosphere 

• The present composition of earth 's atmosphere is chiefly contributed by nitrogen and oxygen .
• There are three stage 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 wind.
•  During the cooling of the earth, gases and water vapour were released from the entire solid earth. 
•  This started the evolution of the present atmosphere .
• The earlier atmosphere largely contained water vapour, nitrogen, carbon dioxide, Methane, Ammonia and very litreof free oxygen.
•  The process through which the gases were out outpoured from  the interior is called degassing .
• Continuous volcanic eruption contributed water vapour and gases to the atmosphere. as the earth called the water vapour release start getting condensed .
• The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. 
• The rain water falling onto the surface got collected in the depression to give rise to Oceans.
•  The earth's Ocean was formed within 500 million Years from the formation of the earth .

Origin of the 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 generate Complex organic molecules and assembled them .
• The record of life that existed on this planet in different period is found in rocks in the form of fossils.
•  The microscopic structure closely related to the present form of Blue Algae have been found in  geological formation much older than some 3000 million years .
• It can be assumed that life began to evolve sometimes 3,800 million years ago.