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· The earth is the only known planet where life is possible
· Earth is spherical in shape, hot water and molten lava eject out from earth’s interior
· World’s deepest mining is limited only to the depth of less than 5 km as the temperature below earth is very high.
· The interior of the earth –
· Because of huge size and changing nature of its internal composition, it’s not possible to make direct observation.
· We have limited information of earth’s interior, through mining and drilling operation, it’s so hot that it can even melt any tool for drilling.
· The innermost layer surrounding the earth surface is called core.
· Core is the densest layer of the earth with its density more than 11.0.
· It is composed mainly of iron and nickel thus known as Nife.
· Core consist of two sub layers, the inner one is solid (C2) and the outer one is semi liquid (C1)
· The layer surrounding the core is mantle, is composed of basic silicates.
· Major constituent of mantle are magnesium and silicon, hence it’s also known as Sima (Silica+Magnesium).
· Its density varies from 3.1 to 5.1 and surrounded by the outermost layer of the earth, known as lithosphere and its density varies from 2.75 to 2.90.
· Major constituent elements of lithosphere are silica (Si) and aluminium (Al), it is also known as Sial.
· The outermost part of lithosphere is known as crust.
The sources which provide knowledge about the interior of the earth may be classified into
· Artificial sources
· Natural sources
· Evidence from the theories of the origin of earth
Artificial sources to understand Interior of the earth
We can know about the interior of the earth with the help of following sources
· It increases with the depth, as observed in the mines and deep wells
· The molten lava erupted from the earth’s interior also support this fact.
· It’s not uniform from the surface towards the earth’s centre, in beginning its increases at an average rate of 1 degree Celsius for every 32 meters increase in depth.
· With such a speed it will be 300 degree celcius at a depth of 10 km, and 1200 degree celcius at 40 km.
· But its interior is not in molten state, because rocks buried under the pressure of several km thicknesses of overlying rocks melt at higher temperature
· Than similar rock on the surface, a basaltic lava rock which will melt at 1250 degree celcius at surface, will take 1400 degree celcius at 32 km of depth.
- · It also increases from the surface towards the centre of the earth due to huge weight of overlying rocks.
- · In deeper portion pressure is tremendously high, its 3 to 4 million times near centre as compared with the pressure of atmosphere at sea level.
- · Due to high pressure the molten material beneath earth acquires the property of a solid and is probably in plastic state.
- · It increases due to increase in pressure and presence of heavier materials towards the earth’s centre
- · The outermost layer is composed of sedimentary rock; thickness varies from (0.8km to 1.6 km)
- · The second layer is of crystalline rocks, the density of which ranges from 3.0 to 3.5 at different places.
- · The average density of earth is about 5.5, so without doubt we can say that it will be more than 5.5 at the core.
- Natural sources to understand Interior of the earth
- We can know about the interior of the earth with the help of following sources
- · Vulcanicity
- · Seismology (EQ waves)
- · On the basis of upwelling of and spread of hot and liquid lava on the earth surface during volcanic eruption, we can say that there is such a layer which is in liquid state.
- · Such molten chamber has been termed as magma chamber which supplies magma and lava during volcanic eruptions.
- Seismology (Earth quake waves)
- · The place of the occurrence of earth quake is called focus, and place which experiences the seismic evens first called epicentre.
- · It is located on the earth’s surface and is always perpendicular to the focus.
- · The focus is always inside the earth, the deepest focus has been measured at the depth of 700km from the earth’s surface.
- · There are three broad categories of seismic waves
- Primary waves –
- · Longitudinal or compressional waves or P waves, are analogous to sound waves, particles move both to and fro from the line of the propagation of the ray.
- · It travels with fastest speed through solid materials.
- Secondary waves –
- · Also called transverse waves or distortional wave or simply S waves.
- · These are analogous to water ripples or light waves, wherein the particles move at right angles to the rays.
- · It cannot pass through liquid materials
- Surface waves –
- · Also called long period waves or simply L waves. These waves generally affects surface of earth and die out at smaller depth.
- · This wave covers longest distances of all the seismic waves.
- · Their speed is slower than P and S waves but these are most violent and destructive.
- How seismic waves are helpful in determining the earth’s interior
- · So we can say that the nature and properties of the composition of the interior of the earth may be successfully obtained on the basis of the study of these waves.
- · The main aspects such as velocity and travel paths of these waves passing through a homogeneous solid body.
- · But these waves reflect or refract while passing through a body having heterogeneous composition
- · In reality the recorded seismic waves denote the fact that these waves seldom follow the straight line paths.
- · Thus it become clear that earth is not made of homogenous materials rather there is variation of density inside the earth
- Evidence from the theories of the origin of earth
- Several attempts to explain the origin and evolution of the continent and ocean basin have been registered, the scientifically recognized are
- 1. Planetesimal hypothesis
- 2. Tidal hypothesis
- 3. Nebular hypothesis
- Planetesimal hypothesis
- · The earth was originated due to accretion and aggregation of solid dust particles.
- · Based on this corollary the core of the earth should in a solid state
- Tidal hypothesis
- · The core of the earth should be in liquid state.
- · According to this the earth have been formed from the tidal material ejected from the primitive sun
- Nebular hypothesis
- · It was coined by Laplace, and according to this theory the core of the earth should be in the gaseous state.
- But from the above discussion we have already proved that these hypotheses have no place in current scenario, and now we have solid evidence to prove earth’s origin.
- Important geographical phenomenon
- · In order to understand important geographical phenomenon of geography, as per the syllabus of GS it is necessary to understand the origin of continents and ocean basins.
- · Continent and ocean basins are fundamental relief feature of the globe.
- · They are considered as relief features of the first order, and different theories regarding the origin of the continents and oceans are being put forth by the scientists.
- · About 70.8 percent of the total surface area of the globe is represented by the oceans and 29.2 are represented by the continents.
- Distributional pattern of the continents and ocean basins
- · There is overwhelming dominance of land areas in the northern hemisphere; more than 75% of total globe is situated to the north of the equator.
- · Continents are arranged in roughly triangular shape; most have their bases (of triangle) in the north while their apices are pointed towards south.
- · The oceans are also triangular in shape, contrary to the continents the base of oceans is in the south while their apices are in the north.
- · The North Pole is surrounded by oceanic water, while South Pole is surrounded by land area (of the Antarctic Ocean).
- · There is antipodal situation of continents and oceans, only 44.6 percent oceans are situated opposite to oceans.
- · 1.4 percent of the total land area of the globe is opposite to land area; more than 95% of the total land area is situated opposite to water bodies.
- · The great Pacific Ocean basin occupies almost one – third of the entire surface area of the globe.
- The above mentioned characteristic would help us to validate the authenticity of any hypothesis or theory dealing with the origin and evolution of the continents and ocean basins.
- There are various hypotheses regarding the origin of the earth, but we would discuss the following one, they are considered to be much more scientific.
- 1. Tetrahedral hypothesis postulated by Lowthian Green
- 2. Continental drift theory of Taylor
- 3. Plate tectonic theory
- 4. Sea floor spreading
- Tetrahedral hypothesis
- · It is based on the fundamental principles of geometry
- · Elie de Beaumont is considered to be first the attempted in this field
- · But the theory of Lowthain (in 1875) is most significant of all hypotheses based on gromatric principles.
- · He based his hypothesis on the two basic principle of geometry
- (i) A sphere is that which contains the largest volume with respect to its surface area
- (ii) A tetrahedron is that body which contains the largest volume with respect to its surface area.
- Basic assumption
- · It is based on the characteristics of a tetrahedron which is a solid body having four equal plane surfaces.
- · Each of which is an equilateral triangle. He postulated it after considering the characteristics of the distributional pattern of the land and water over the globe.
- · There is dominance of land area in the northern hemisphere and water areas in the southern hemisphere
- · Triangular shape of continent and oceans,
- · Situation of continuous ring of land around north polar sea and location of south pole in land area (Antarctica) surrounded by water from all sides
- · Antipodal arrangement of continent and oceans
- · Largest extent of Pacific Ocean covering one third area of the globe
- · Location of chain folded mountains around Pacific Ocean.
- · According to him when earth was originated it was in sphere form
- · In the beginning earth was very hot but it gradually began to cool down due to loss of heat.
- · First the quarter part of the earth cooled down and thus was formed the crust but the inner part was continued to cool down
- · Consequently the inner part of the earth was subjected to much more contraction, thus there was marked reduction in the inner part of the earth.
- · Upper part of the earth (crust) was already cooled down, and could not be subjected to further contraction.
- · This resulted into possible gap between the upper and inner parts of the earth.
- · Consequently the upper part collapsed on the inner part and ultimately the earth began to assume the shape of a tetrahedron.
- · He also clarified that due to structural variations; there may be some deviations from a true tetrahedron.
- · Four oceans (the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and the Arctic Ocean) were created on the four plane faces of the terrestrial tetrahedron.
- · These plane faces retain water because of the fact that these were lower than the level of the apices or coigns of the terrestrial tetrahedron.
- Fig- Distribution of land and water on a tetrahedron
- · To major extent it successfully explains the characteristics feature of the distributional pattern of the present day continents and ocean basin.
- · But because of certain basic defects and errors the theory is not acceptable to the modern scientific community.
- · It is argued that balance of the earth in the form of a tetrahedron while rotating on an apex cannot be maintained.
- · The earth rotating so rapidly on its axis that the spherical earth cannot be converted into a tetrahedron while contracting on cooling.
- · This hypothesis believes more or less in the permanency of continents and ocean basins, while plate tectonic theory has validated the concept of continental drift.
- Continental drift theory of Taylor
- · F.B .Taylor postulated his concept of horizontal displacement of the continents in 1908.
- · The main purpose of his hypothesis was to explain the problems of the origin of the folded mountains of tertiary period.
- · The concept of Taylor is considered to be the first attempt in the field of continental drift, though Antonio Snider presented his views about drift in the year 1858 in France.
- Basic Assumption
- · According to him there were two land masses during Cretaceous period.
- · Lauratia and Gondwanaland were located near the north and South Pole respectively.
- · The continent were made of sial which was practically absent in the oceanic crust.
- · Continent moved towards the equator, the main driving force of the continental drift was tidal force.
- · Continents were displaced in two ways:
- (i) Equator wards movement
- (ii) Westward movement
- · Lauratia started moving away from the north pole because of enormous tidal force of the moon towards the equator in a radial manner.
- · This movement of landmass resulted into tensional force near the North Pole which caused stretching, splitting and rupture in landmass.
- · The displacement of the Gondwana land from the South Pole towards the equator caused splitting and disruption.
- · Arctic sea was formed between Greenland and Serbia due to earthward movement of Lauratia.
- · Atlantic and Indian Ocean were supposed to have been formed because of filling of gaps between the drifting continents with water.
- · Taylor assumed that the landmasses began to move in the lobe form while drifting through the zones of lesser resistance.
- · Thus mountain and island arcs were formed in the frontal part of the moving lobes.
- · The Himalayas. Caucasus and Alps are considered to have been formed during equator ward movement of the Lauratia and Gondwanaland from the north and south poles respectively
- · The Rockies and Andes formed due to westward movement of the landmasses.
- · Taylor’s main aim was to explain the origin of the tertiary folded mountains and hence he made the continents to move at a very large scale.
- · He described displacement of the landmasses for thousands of kilometres, but displacement of landmasses up to 32-64 Km would have been sufficient enough for the purpose.
- · Tidal force was used as mode of displacement , if it was so enormous during cretaceous period that it could displace the landmasses for thousands of kilometres
- · Apart then it might have also put a break on the rotatory motion of the earth and thus rotation of the earth might have been stopped within a year.
- · Though the concept of F.B. Taylor is not acceptable but his hypothesis is considered to be significant on the ground that
raised his voice very forcefully through deductive postulation against the
prevalent concept of the permanency of the continents and ocean basins