Earth-internal structure and construction, Physics tutorial


Earth is a terrestrial body whose plentiful water, oxygen and solid surface - that is, rich atmosphere - have joined to form the appropriate environment for the growth of life on it. There are   mainly three states of matter, that is, the solid on that we rest, being the solid, ocean is the LIQUID and atmosphere illustrates the gas. Comprehending the arrangement and behavior of the solid and origin of earth is very important to a meaningful study of the solid portion of the earth.

Early Theories on the Origin of the Earth:

The hypothesis depicts the construction of the planet consisting of the earth.

Cold Origin Theory:                         

Nebula explained that a primeval rotating gas and cloud of dust filled the universe primarily. Kant explains more that the sun is formed by denser centre of the rotating dust in the Nebula's hypothesis. Few dense portion of the rotating dust divided into pieces to construct the planets from which one of them is earth. Though, this theory failed to describe angular momentum observation in the solar system whereby sun alone comprises 99.9% mass of the solar system however 99% of the angular momentum of the solar system is mainly concentrated in the large farter planets which are farther from the sun

Hot Origin Theory:

   The hypothesis says that gravitational attraction between the sun and from the colliding stars the preexisting passing stars brings violent collisions and materials were closed. The turn off particles connected in parts and created the planets in which the earth is a part.

The drawbacks of this theory are as follows:

  • In this theory, the observation of the angular momentum regarding the solar system was not illustrated.
  • They failed to explain why the collisions occurred once, is vague.
  • Why collisions are always perfectly elastic? The materials could have dispersed.

Peculiar Features of the Earth: 

The planet earth has few distinct characteristics that differentiated it from other planets. Some features are suited for the development of lives in it.

Some of the properties are:   

  • objects on its surface is kept by high gravitational field
  • Uniform motion such as rotation, revolution and wobbling which creates seasonal changes day and night.
  • Occurrence of enough water for actions such as agriculture.
  • From the sun balanced distance is not too short nor too far
  • In gaseous form there is Presence of rich atmosphere and contains ozone layer.

Sources of Earth's Zoning:

Approximately 4.7 billion years ago there was an accretion of conglomeration of unordered particles spread in the universe form the earth as one of the planets.  As a result of the following processes the earth became warmed up.

  • Particle acceleration: To heat energy can be formed by the kinetic energy of the particles coming together.
  • Gravitational pull between the particles:  the heat energy is increased by high pressure the result of pulls
  • Radioactivity: During disintegrations of radioactive elemental  evolve heat

Components of the earth's particles

The temperature of the earth increased by the heat from the above source to the melting point of iron that melted the whole earth - the event called IRON CATASTROPHE.

Zoning of the Solid Earth:

Iron catastrophe results in molten earth settled such that smaller particles like silicon etc float, cool and form of outer crust and mantle. The heavier particles such as iron etc established under the crust form the core. Therefore, iron  produced of the earth mass (Figure shown below) Water contains the particle in the earth goes through chemical changes and free water which settled on the surface to figure the ocean. Gas contains particles also went through chemical reaction release gasses to shape the lightest atmosphere that is positioned on top of the liquid and part of the earth.    

1884_Model of the Solid Earth.jpg

Fig: Simplified Model of the Solid Earth

Introduction to Motions of the earth:

The non-  stability  property of the planet earth is exemplified in various events such as seasonal variation ,happening of day and night as well as annual irregular length of days and night are some of the procedures that exemplify the progress of the earth. This entity examines the various modes of motion like wobbling, rotation and revolution and their effect are such as oblate nature of the earth and centrifugal sorting.

Types of motion of the earth:

In order to maintain celestial dynamic equilibrium with other planets the earth and other heavenly bodies, is undergo sequence of complex motions. The three main types of the motion whose impact are visible in our daily life. 

Revolutionary motion:

1873_Revolutionary motion.jpg

Fig: Revolutionary motion

The earth rotates along an elliptical orbit whose plane is practically perpendicular to the polar axis of the earth. F1 and F2 are the foci of the orbit; one of the foci is at sun. The periodicity of this motion is 31557600 seconds (i.e. a year) and determines the global season. The seasonal change is bringing regarding as a result of earth distance from the sun at every location on its orbit.  At location B for instance, the earth is distant from the sun, so for much rain the sun's heat may not be much to cause enough evaporation. The earth experiences much heat of the sun rays at location A. This cause extreme evaporation, leads to rain possibility.


826_Flip-flop Motion.jpg

Fig: Flip-flop Motion of the Earth

About its equator the earth exhibit Flip-flop (wobble) motion as it is revolve around the sun. It always noted that northern hemisphere is close to the sun than southern hemisphere. Therefore, at this time northern hemisphere experience longer day and shorter night. But, In case of location B the situation is vice-versa.


This is a rotation regarding polar axis from west to east in an anticlockwise direction, viewing from the tip of the northern pole. The earth's rotation average period is 86400 seconds. (That is a day). Thus, rotation of the earth: 

  • Determine the duration of the day and night
  • Cause its ellipsoidal/oblate shape that is equatorial radius being larger than the Polar radius
  • The surface to the centre of the earth and density variation is result.

Hypotheses about the Rotation of the Earth:

Cassini's hypothesis:

He proposed that the earth is rotating around the equator with the polar axis perpendicular to the axis to rotation. If this is true then the earth will suppose Prolate ellipsoid shape and one face of it would be experienced permanent day light whereas the other side would be in darkness permanently.

567_Cassini’s Prolate Ellipsoidal earth.jpg

Fig: Cassini's Prolate Ellipsoidal earth

Newtonian Hypothesis:

Newton proved that the earth is rotating around the polar axis which leads to the oblate ellipsoidal shape of the earth. 

2011_Newtonian Oblate Ellipsoidal Earth.jpg

Fig: Newtonian Oblate Ellipsoidal Earth

The geophysical evidence  as longer equatorial radius than the polar radius, centrifugal sorting of the earth into its density regimes in which the density raised to the surface towards the centre and happening of day and night hold Newtonian hypothesis.

Effect of rotation on the internal structure:

A state attained during Iron Catastrophe, given the fact that the earth was initially melted, as the earth moves it acts like a centrifuge, send the lighter materials of crust to the flanks while the denser and homogeneous mantle and core materials remain close to the centre of the earth. Geophysical studies have confirmed the in homogeneous crust, denser and homogenous mantle (Figure shown below).

2128_Centrifugal Sorting.jpg

Fig: Centrifugal Sorting of the Earth Interior

Earth Shaping due to its Rotation:  


491_Deviation of the Earth Shape.jpg

Fig: Deviation of the Earth Shape from a Perfect Sphere

The earth rotates around its pole with angular velocity of 7.292 x 10-5.As the earth rotates centrifugal and centripetal forces (CF and CP) cancelled out and net force is 0. Thus, no decomposition is caused along the equator. The centrifugal force reduced alone the pole from the equator. Thus there is net force across the centre of the earth that raises the magnitude as we move further from the equator. CF reduced to 0 .At the equators and a maximum centripetal that is, net force, mounted to the centre is achieved at the poles. As a result the Polar Regions are compressed and the equatorial Region bulges outwards. That's why; the earth assumes the oblate spheroid nature. Geophysical studies have confirmed this when the equatorial radius is greater than polar radius.  Earth flattening f = (Re - Rp)/Re Factors Responsible for the Sculpture of the Earth Surface  The earth surface solid part is not smooth; it exists with Wrinkles of hills, valleys, plateaus etc. Some reasons of the surface shapes are as follows:

1)  Gravitational pulls: After iron catastrophe side particles holds gravitational pulls from the interior of the earth. As the pressure is from all the directions around the solid earth. So, the earth seems fairly spherical (Figure shown below). However the in homogeneity of the crustal layer varies the magnitude of the gravitational pulls from place to place depending on the density of the area concerned. Hence, areas with denser crust appear lower (i.e. low land like valley) than those areas that have light crusts beneath. Earth's surface sculpture by the gravitational pull.

1950_Earth’s surface sculpture.jpg

Fig: Earth's surface sculpture by the gravitational pull

2) Volcanic eruption: Sporadic volcanic eruption from the interior of the earth sometimes protrudes to the surface and form high-rise land as example continental platform and mountain.

3) Interaction between lithosphere and the two fluids envelops (i.e. atmosphere and hydrosphere). The fluids wear down the volcanic (igneous) rocks. Wind and water erosions transport the loose rock materials to another place to form sedimentary and metamorphic rocks.

4) Plate tectonics: This refers to the significant movements noticeable on earth's layer considered as Plates. A few numbers of the processes of plate tectonics are: Sea-floor Spreading, earth quake, continental drift, fault etc.

Internal and external structures of the earth:

By using some geophysical methods the internal structures of the solid earth are probed .The information obtained are utilized to illustrate model of the pictures regarding the Interiors of the solid   section earth. Further, most of the properties existing on the earth surface are Result of the events which either took place at the time of construction of the earth or after mat effects of other event take place alone with time. This unit concern with the models of the interior structure of the earth based on the scientific facts. The external features of the solid earth are treated the causes and structure.

Evidences of the Inhomogeneity of the Solid Earth:   

The closest layer of the surface of the solid earth is crust. This layer within the rock layers shows high level of structural variations. The following shows the in homogeneity of the earth.            

(a)  Density variation:  The surface density of the earth has been initiated to be 2.67g/cm3, but the average density of the earth is 5.5g/cm3. This means that denser is the deep below the earth surface, that is density increases with depth.  

(b) Moment of inertia (I) is the Rotational Analogous mass. That is, since I is a measure of resistance of a rotating body to changing its angular velocity (ω ) change its initial states,  the measure of resistance of a body is mass. So, I depend on mass and the division of mass in a body. thus the sphere with uniform density, I = bma2, where b = 2/5, Now, for earth model I = b = ma2, If b is greater than 2/5, density reduce to depth and if b is less than 2/5, density increases with depth. But for planet earth b has been found to be 0.3308 which is less than 2/5; hence density increases with depth in the earth.

Internal Models of the Solid Earth:

On the available scientific information the various criteria for obtaining nature of solid surface of the earth's interior. Discussed below are the two models.

Earth's Model based on chemical composition:

With equatorial and polar radii approximately equal to 637.8388 km and 6356.912 km respectively the solid earth is oblate.  The following subdivision is given by seismological evidence based on chemical compositions:

i) Crust: This is a solid layer and there is continental and oceanic crust.   Initially 30 - 40 km thick, is continental crust mostly granite rocks and primary wave velocity of 6 - 7 km/s gabbros seismic on the geologic history of formation. Over the last 100 million years.  It is based on the three forms basically. It mainly consists of three regions: Stable region (cartons), Semi-mobile region and Mobile belts.

Oceanic crusts are mostly basalts and Gabbros, 5 - 11 km thick and they mostly exist in three layers namely: Layer 1, Layer 2 and Layer 3.

ii) Mohorovicic Boundary: From underlying Ultramafic rocks of mantle it is chemical boundary separating silicic crustal rock with high Feldspar content.    

iii) Mantle:  It is 2900 km thick and solid layer. The largest of all the subdivisions, and 69% mass and 84% by amount of the entire earth and distributed into 3 zones namely: Upper mantle, Transition layer and Lower Mantle.

iv) Gutenberg Boundary: This is a discontinuity among the mantle and the core.

v) Core:  This is 16% and 13% by volume and mass of the earth. It is approx 3470 km thick extended to the centre of the earth and is in 3 zones that is, Outer core, Transition zone and Inner core.

Earth's Model based on Ability to flow :

On the basis of ability of its layer to flow the earth model can be drained. The layers are:

  • Lithosphere- cold and brittle.
  • Asthonosphere } capable to flow
  • Rheonosphere } capable to flow

As a result of pressure it becomes hardened again towards the center it was reported. 

Surface Structure of the Solid Earth:

The surface of the solid earth witnessed sequence of properties as example oceans and Continents which are consequences of geological events that took place over a large Duration of time; some of the features are illustrated below.

Formation of Continents, Ocean and Atmosphere:

(a)  Continents:  As a result of volcanism lava flows from the interior to the outer layer became continent and cooled.

(b)  Oceans: as a product of the  Process of heating up and differentiation from the interior of the solid earth  elements  is beard  by some water free out  their water molecules that  accumulate on the surface to form the oceanic water. 

(c)  Atmosphere: Out-gassing, resulting from differentiation and some gas releasing chemical reactions brought about much gas that accumulated beyond the solid earth.

Global distribution of continents and oceans:  

The earth surface has 45% of sea-sea antipodal and 1.4% has land-land antipodal. Southern hemisphere has 61% water and 39% land while Northern hemisphere has 60% land and 40% water. In general, earth's surface has 29% continent and 71% of the earth's surface is covered by ocean. The total world area is about to 105×106 km2

Significant Surface Features of Oceans:

 To see surface on the continent height relative is the elevation h. 0.88 km is the average elevation of continents.3.8 km is the ocean floor or average depth. Continental rocks are lighter than oceanic rocks. Across the world there is worldwide mountain range that crosses the ocean basin 1000 km wide and 65,000 km long.

Sequence of the Solid Earth's Surface:

There are three forms of viewing the earth's surface   

(A)  The top surface is the continental platform wrinkled in nature.

(B)  Geoids is the sea level, whereas sea water will flow to if the continent were to be tunneled. In other way, geoids are the equipotential surface to which direction of gravity field is perpendicular between the geoids and a point on the earth's surface is the elevation the vertical separation is elevation

(C) In its oblate form Ellipsoid is the ideal smooth surface of the earth. It sometimes coincide with geoids and sometimes not.

Variation of some physical properties within the solid earth:

Some parameters diverge with depth (that is radius of earth due to the structure of the underlying interior of the earth is given by measurement of these on the surface gives. The deals with e with depth within the earth's rocks deal with the variation of density pressure, gravitational field, Velocity and temperature

Variation of Density ρ with depth:

Take density dependent on pressure P and temperature T. That is:

ρ = ρ (P, T) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (1)

Pressure gradient towards the centre of the earth by assuming that the stress in the earth's interior is equal to hydrostatic pressure (hρg).  

(dp/dr) = (- ρg) _ _ _ _  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  _ _ (2)

The radius of the earth is r. The negative sign implies pressure increase with decrease in radius is denoted by the negative sign. Taking a homogeneous layer in which the t temperature variation is adiabatic then, 

dp/ (dv/ v)=dp/(dp/ p)=κ e. i

κ/ρ= dp/(dp/ p) _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _(3)


dp/dr=(dp/dρ) • (dp/dp) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (4)

By calculating (2),(3) and (4)

dρ/dr = (- ρg/φ) _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ __ _ _ _ __ _(5)

g = (- GM/κr2) _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ __ _ _ _ __ _(6)

Starting from mantle down Equation (6) is applicable to region of uniform composition within the earth. That is, it shows maximum uniform density.


M = Mass of the region of the earth with the earth less by the mass of the crust

G = Gravitational constant                                  

κ = adiabatic incompressibility (that is, bulk modulus)

r = radius of the earth's region with uniform composition 

Variation of Pressure P with Depth:

Due to gravity it has to do with the pressure distribution and variation of acceleration.

Remember that g = Gm/r2_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ (1)

And P = rρg _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _(2)

Therefore, pressure gradient is:

dr/dp= - Gmp/r2 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ _ __  __ _ _(3)

Equation 3 implies that pressure reduces with depth, i.e. as r decreases as well; it shows that pressure has directly related with the density.

Variation of Acceleration due to gravity with depth:

Recall that  g = Gm/r2 .So, using  the  above relation  variation of g with depth can be calculated until a depth of  2400 km is reached where it decreases to zero. From 990 km not more than 1% of the value is differ .

Variation of Pressure P and S waves with Depth:

For the study the interior of the solid earth P and S waves are used in seismology. Longitudinal wave that can pass through liquid, solid and gas is P wave. Wave is a transverse wave that can travel vertically and horizontally and cannot pass through fluids IS s wave. P and S- waves are called body waves due to passing through the earth's interior. They however have different speeds while crossing the earth   material. Generally every time P wave is faster than S wave. Let P and S waves Velocities is α and β. It has been proved that:

Bulk modulus κ=λ+2µ/3

α= √ (λ+2µ/ρ)

β=√ (µ/ρ)

Calculating the above three equation, we prove that:

α = √ (κ+4µ/3)/ρ                              


κ = α2 ρ - (4ρβ2/3)

Here, λ and μ are called Lamês consonants.

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