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The Earth and its Interior

Introduction :- Before we deal with the Earth and its interior, a historical review of the

same by ancients is worth considering. The formation of the Earth which we considered

in the preceding chapter was from our present understanding regarding the spherical

shape, rotation, revolution, etc.

everything” have given an extensive account of the Earth and its history from the point

of view of man’s understanding regarding its shape, size and interior. An ancient Greek

astronomer, Thales of Miletus in the 5

rested on water. Anaximander, a student of Thales also considered Earth as flat

unsupported cylinder whose depth was one

was suspended in emptiness, but that it remained in its place because it was equidistant

from all other objects in the universe. He also believed that Earth was not supported by

water or by any other elemental material but by a spiritual force, which is the cause of

all things and into which all things pass away.

Another astronomer, Anaximenes also of Miletus believed that Earth was flat broad

and supported by air.

Xenophenes of Colophon gave a

more philosophical description.

He asserted that the upper limit of

the world was at our feet, where

Earth’s surface meets the air, and

that the lower portion extended

downwards endlessly.

The first suggestion of a

spherical Earth is attributed to the

famous 6th century BC Greek

philosopher and mathematician,

Fig. 2.1 Pythagoras

(Picture Credit : 2.1, P.159)

Chapter 2


Before we deal with the Earth and its interior, a historical review of the



ation, revolution, etc. 2.1John Langone, et al in their book on “Theories of



ronomer, Thales of Miletus in the 5th century BC claimed of Earth to be flat that


unsupported cylinder whose depth was one-third its breadth. He proposed that Earth

ed in emptiness, but that it remained in its place because it was equidistant



all things and into which all things pass away.


and supported by air.

Xenophenes of Colophon gave a

philosophical description.

He asserted that the upper limit of

the world was at our feet, where

Earth’s surface meets the air, and

that the lower portion extended

The first suggestion of a

spherical Earth is attributed to the

century BC Greek

philosopher and mathematician,

Fig. 2.1 Pythagoras

(Picture Credit : 2.1, P.159)


Before we deal with the Earth and its interior, a historical review of the



John Langone, et al in their book on “Theories of



century BC claimed of Earth to be flat that


third its breadth. He proposed that Earth

ed in emptiness, but that it remained in its place because it was equidistant




Fig. 2.2 Rene Descartes

Pythagoras (Fig. 2.1). He viewed the world as a series of shapes, patterns and rhythmic

circles. He noted that other celestial bodies such as the Moon and the Sun are round

and that as the shadow cast on the Moon and Sun during an eclipse are round. He also

watched the ships on the horizon. All

phenomenon lead to the conclusion that

Earth was a sphere. The idea of a

spherical Earth was carried forward later

by Aristotle in the 4th century BC. In the

next century, Eratosthenes calculated the

circumference of the Earth.

The idea regarding the nature of Earth

was firmly established in the 17th century

French mathematician and philosopher,

Rene’ Descartes (1596-1650) (Fig. 2.2)

who in 1644 in his Principia Philosophae

presented his theory that the Earth could have startred as a molten mass the crust

forming a part of the cooling process.

The ancients, however, had a different view regarding the core of the Earth. In Fig.

2.3 is shown a 1664 vision of interior of Earth. Subterranean lakes and rivers surround a

central fiery core. It was in 1798 that the English physicist, Henry Cavendish (1731-

1810) (Fig. 2.4) calculated for the first time the average density of the Earth and

concluded that the core of the Earth must contain very dense heavy metals.

Fig. 2.4 Henry Cavendish

Fig. 2.3 A 1664 vision of the Earth’s interior (Picture Credit : 2.1, P.315)

It was the Scottish geologist, Charles

Lyell (1797-1875) (Fig. 2.5) who during 1830-

33 published “Principles of Geology” In 3

volumes containing many ideas based on

earlier work by James Hutton. Lyell

advocated not only uniformitarianism but also

gradualism. In his own words, “Earth’s

history is the result of uninterrupted

succession of physical events, governed by

the laws now in operation”.

William Thomson (Lord Kelvin) (1824-

1907) (Fig. 2.6) in 1862 estimated the age of

Earth to be 100 million years.

Fig. 2.7 Alfred Wegener

Fig. 2.5 Sir Charles Lyell

(Credit : 2.1, P.328)

Fig. 2.6 William Thomson (Lord Kelvin)

(Credit : 2.1, P.326)

The theory of continental drift was put forward in 1912 by German geophysicist

Alfred Wegener (1880-1930) (Fig. 2.7). Wegener was an interdisciplinary scientist,

bringing geology, geophysics, climatology and biology together into a comprehensive

theory of Earth’s inner workings. In 1910 he published “The Thermodynamics of the

Atmosphere”. In a letter to his fiancee Wegener remarked that the east coast of South

America looked as if it fit up against the West coast of Africa. In 1915, he published

“The Origin of Continents and Oceans”.

After dealing with the formation of Earth and its

present temperature in the last chapter, and with a

little bit of historical introduction, we are interested in

knowing the formation of continents and as to how

Earth looked in its various stages of evolution over a

period of time. Fig. 2.8 (a, b, c, d and e) shows the

continental drift since Pangaea 225 million years to

the one as it looked today. In order to include all the

continents, the shape is shown oval.

Fig. 2.8 (a) Earth some 225 million years ago

Fig. 2.8 (c) Earth some 135 million years ago

Earth is the largest of the inner planets of the solar system. In the year 1969, when

the Indian Nobel Prize winner, late Dr. C.V. Rama

by man on Moon and space exploration, he said:

Try to know more about the Earth on

which we live. What is inside the Earth

not fully known and one should explore

that first. Earth is the only planet in

which active tectonic development of

the surface appears to be going on at

present and so far as we know, Earth

has the most complex structure.

The lives of humans, animals and other organism on the Earth depend on the

behavior of Earth. By behavior is meant the effec

inside and outside the Earth as seen by humans, for example, the atmospheric air

which is the vital essence needed for breathing, the wind, the water of the sea, lakes

and rivers, the tides, the clouds and in general t

effects of various physical processes taking place some of them forming different

chapters that follow in this thesis.

Fig. 2.8 (e) Earth as it appears today

Fig. 2.8 (a) Earth some 225 million years ago Fig.2.8 (b) Earth between 180

some 135 million years ago Fig. 2.8 (d) Earth some 65 million years ago


the Indian Nobel Prize winner, late Dr. C.V. Raman was asked to comment on landing

by man on Moon and space exploration, he said:

Try to know more about the Earth on

which we live. What is inside the Earth

not fully known and one should explore

that first. Earth is the only planet in

tectonic development of

the surface appears to be going on at

present and so far as we know, Earth

has the most complex structure.


behavior of Earth. By behavior is meant the effects of various physical processes both



and rivers, the tides, the clouds and in general the climate which we feel are all the


chapters that follow in this thesis.

Fig. 2.8 (e) Earth as it appears today

Fig.2.8 (b) Earth between 180 -200 million years

Fig. 2.8 (d) Earth some 65 million years ago


n was asked to comment on landing

is


ts of various physical processes both



he climate which we feel are all the


Before we enter the interior of the Earth, we have to consider what is outside on the

surface and its surroundings such as clouds, tides and the atmosphere. Clouds and

tides form separate chapters in the thesis. We shall therefore deal little bit with the

atmosphere of the Earth. The study of Earth about its surface and surrounding may be

easier compared to the study of its interior. Speculations about the interior of Earth have

stimulated the imaginations of the humans for centuries but only after we learned of

seismic waves to obtain an x-ray picture of the Earth. Indications of the physical

processes that go on inside the Earth are earthquakes and volcanoes the former

creating seismic waves which are the probes for the study of Earth's interior.

The German-American seismologist, Beno Gutenberg (1889-1960) (Fig. 2.9)

discovered the core of the Earth. Seismic waves passing through the Earth are

refracted in ways that show distinct discontinuities within Earth’s interior and provide the

basis for the belief that Earth has a distinct core. Andrija Mohorovicic (1857-1936) (Fig.

2.10) is a Croatian seismologist who discovered Crust/mantle boundary. The inner core

was discovered by L Lehmann (1888-1993) (Fig. 2.11).

Fig. 2.9 Beno Gutenberg

Fig. 2.10 Andrija Mohorovicic

Fig. 2.11 L. Lehmann

REVIEW OF LITERATURE

The Atmosphere of Earth : 2.3Gilbert M. Master, et al have given in their book on

“Introduction to Environmental Engineering and Science”, a brief description on Earth’s

atmosphere. When the Earth was first formed some 4.6 billion years ago, the geologists

believed that it had an atmosphere of helium and compounds of hydrogen forming

gases such as molecular hydrogen, methane and ammonia. This early atmosphere is

thought to have escaped into space and the present atmosphere is formed through

volcanic activity, gases such as carbon dioxide, water vapour, various compounds of

nitrogen and sulfur were released over a period of time. Photodissociation of water

vapour and photosynthesis by plants created molecular oxygen (O2) the vital essence

Fig. 2.12 The US Standard Atmosphere

(Credit : 2.3, P.504)

needed for any life on Earth. The excess of oxygen created ozone (O3) which formed an

upper layer (ozone layer) absorbing incoming ultra violet radiation of the Sun thereby

protecting life on the Earth. This was probably a stage when actual life would have

started on Earth.

Composition of the atmosphere : Excluding the greenhouse gases such as CO, CO2,

CH4 and Nitrous Oxide (N2O), the composition of the atmosphere is given in Table 2.1

(Credit: 2.3). It is a data for clean dry air taken sometime in 2006. For the sake of

convenience, the atmosphere is being divided into various horizontal layers and a US

standard atmosphere in a graphical form is shown in Fig. 2.12. The division is based on

the temperature profile consisting of 4 major layers. The graph is self-explanatory. 2.4George Gamow and John Cleveland in their popular book titled, “Foundations and

Frontiers”, even though an old reference, has given lot of information regarding the

Physics of atmosphere. Table 2.1 (Credit : 2.3, P.503)

Constituent Formula Percentage By volume Parts per million

Nitrogen N2 78.08 780,800

Oxygen O2 20.95 209,500

Argon Ar 0.93 9,300

Carbon dioxide CO2 0.038 380

Neon Ne 0.0018 18

Helium He 0.0005 5.2

Methane CH4 0.00017 1.7

Krypton Kr 0.00011 1.1

Nitrous Oxide N2O 0.00003 0.3

Hydrogen H2 0.00005 0.5

Ozone O3 0.000004 0.04

There is a decrease of

temperature about 6°C for every

kilometer of altitude of atmosphere

which continues up to an altitude of

about 20 km or so and

temperature of about –60°C

(~210° K). At still higher altitudes the temperature starts rising and then falling to

freezing point of water and further dropping to –90°C (~180°K) at an altitude of 80 km

as shown in Fig. 2.13. Above 80 km, the temperature changes are reversed again

reaching room temperature at an altitude of about 130 km, Boiling Point of water at 160

km and temperature of molten lead at 250 km.

But, however, the effect of the heat is not felt because of the negligible density of air

and having absolutely no conductivity.

Fig. 2.13 Distribution of density and temperature i n terrestrial atmosphere

(Credit : 2.4, P.510)

At this higher altitudes, the ultraviolet radiation of the Sun is absorbed by nitrogen

and oxygen atoms, their outer electrons being knocked off, the atoms are in an ionized

state and this region of terrestrial atmosphere is called ‘ionosphere’ possessing high

degree of electrical conductivity because of the presence of free electrons and positive

ions. Ionosphere is a good reflector of radio waves bouncing between the surface of the

Earth and reflecting layers of the ionosphere as shown in Fig. 2.14.

Fig. 2.14 Ionosphere reflects radio waves back into the Earth’s surface

(Credit : 2.4, P.511)

Earth’s Crust and the interior of Earth: - 2.2A cross section of Earth from surface to

centre with temperature variation is shown in Fig. 2.15. Earth is divided into a number of

layers. Starting from the surface and going towards the centre, we have the ‘crust’,

‘mantle’, ‘outer core’ and ‘central core’. The crust is important for humans and animal

life in the Earth. Crust is separately shown in Fig. 2.16. It is composed of separate

pieces of two rather different types of rocks (granite and basalt) strongly welded

together and floating on the underlying layer of plastic basalt material. The crust

constitutes only 0.6% of the Earth’s volume and it varies from 5 km to 60 km from the

surface of the Earth. The adjustment of the Earth’s crust under the shifts of mass on its

Fig. 2.16 The structure of Earth’s crust (Credit : 2.4, P.505)

Fig. 2.17 Possible temperatures within the Earth (Credit : 2.5, P.40)

surface has played a very important role in the evolution of the face of our planet. For

example, considerable basaltic adjustment took place during the glacial periods when

thick sheets of ice covered much of North America and Europe. The weight of the ice

caused the northern regions of these continents to sink deeper into the plastic layer of

basalt underneath.

After the crust, we have the mantle which consists about 80% of the volume of

Earth. The boundary between the crust and mantle was discovered by the Croatian

seismologist, Andrija Mohorovicic (Fig. 2.10) in 1909. The boundary is called

Mohorovicic discontinuity or simply ‘moho’.

Fig. 2.15 Illustration of interior of

Earth

Below moho is the mantle up to

a depth of about 2900 km. The

composition of mantle is

oxygen, iron, silicon and

magnesium. A majority of the

mantle is solid with the upper

part called asthenosphere is

partially liquid. The German-

American seismologist, Beno

Gutenberg (Fig. 2.9) discovered the core-mantle boundary known as Gutenberg

discontinuity.

It separates the mantle from the core. This part consists mainly nickel and iron and

constitutes about 17% of the volume of Earth. From the base of the mantle extending to

a depth of over 5000 km is the outer core. From the bottom of the outer core to the

centre of Earth is the inner core lying at a depth of 6371 km from the surface of the

Earth. The temperature of the inner core is estimated to be about 4000 °C even though

some books on geology quotes the temperature as high as 5000 °C.

A possible variation of temperature Fig. 2.17 Possible temperatures within the Earth

within the Earth is given in Fig. 2.17. As far as the scientific study and utility of minerals,

the crust is of importance. The crust forms what is known as the lithosphere and its

density, composition, thickness, etc. are given in Table 2.2.

Tale 2.2

Crust Density kg/m 3 Composition Thickness Age

Continental 2800 Felsic Thick 20 ~ 100km ~4b Yrs.

Oceanic 3200 Mafic Thin 2 ~ 10 km < 200 M Yrs.

Variation of density in the interior of Earth is given in Table 2.3 and the

corresponding graph in Fig. 2.18.

Table 2.3 (Credit : 2.5, P.30)

Depth km Density kg/m 3

– 2840

33 3320

413 3640

984 4550

2000 5110

2898 5560

*2898 9980

4000 11420

4980 12170

5120 12250

5120 –

6371 1251

Fig. 2.18 Reduced density within the Earth

From the table it is seen that the density is

above 12000 kg/m3 depth of 5000 km and more

indicating that the density of inner core is

*As in original.

In 1961, Princeton geologist, Harry

Hammond Hess (Fig. 2.19) reasoned that if

Earth’s crust spread along oceanic ridges, it

must collide elsewhere. He suggested that the

Atlantic Ocean was expanding along the mid

Atlantic Ridge. If that were true, then, to

compensate, the Pacific Ocean must be

contracting. Hess theorized that the Pacific

crust was descending into deep, narrow canyons along the rim of the ocean basin.

Seismic data with whatever instruments available in 1960s revealed earthquake zones

in the same area where Hess predicted spreading and shrinking.

As the Earth cooled to temperatures 1300 °C from 1500 °C, the first elements to

condense were likely aluminum and titanium followed by iron, nickel. Silicon, cobalt and

magnesium at temperatures of 1 2.6Gupte R.B., “A Text Book of Engineering Geology” talks about minerals found in

the Earth’s Crust. Mineral is a natural substance having a definite chemical composition

and formed by the inorganic process of nature. Earth’s crust cons

minerals which are the rock

felspathoids, micas, amphiboles, pyroxenes and olivine, crystalline and non

calcium carbonate and quartz the rock

carbonate, the rock-forming minerals are silicates Formed by the combination of silica

(SiO2) with bases like K2O (potash), N, a

Fe2O3, A12 O3 (alumina), etc. As a result, all common rocks are

large amounts of silica.

Fig. 2.19 Harry Hammond Hess

largest.

In 1961, Princeton geologist, Harry

Hammond Hess (Fig. 2.19) reasoned that if

Earth’s crust spread along oceanic ridges, it

must collide elsewhere. He suggested that the

Atlantic Ocean was expanding along the mid-

Atlantic Ridge. If that were true, then, to

compensate, the Pacific Ocean must be

contracting. Hess theorized that the Pacific



same area where Hess predicted spreading and shrinking.



magnesium at temperatures of 1000 °C to 1300 °C.

Gupte R.B., “A Text Book of Engineering Geology” talks about minerals found in


and formed by the inorganic process of nature. Earth’s crust cons

minerals which are the rock-forming minerals. They belong to the families of feldspars,

felspathoids, micas, amphiboles, pyroxenes and olivine, crystalline and non

calcium carbonate and quartz the rock-forming minerals. Except

forming minerals are silicates Formed by the combination of silica

O (potash), N, a2O (soda), CaO (lime), MgO (magnesia), Fe O,

(alumina), etc. As a result, all common rocks are silicate rocks containing

Fig. 2.19 Harry Hammond Hess





Gupte R.B., “A Text Book of Engineering Geology” talks about minerals found in


and formed by the inorganic process of nature. Earth’s crust consists of about 20

forming minerals. They belong to the families of feldspars,

felspathoids, micas, amphiboles, pyroxenes and olivine, crystalline and non-crystalline

forming minerals. Except quartz and calcium

forming minerals are silicates Formed by the combination of silica

O (soda), CaO (lime), MgO (magnesia), Fe O,

silicate rocks containing

Table 2.4 (Credit : 2.7, P.86)

Element Percentage

Oxygen 47.0

Silicon 28.0

Aluminum 8.0

Iron 4.5

Calcium 3.5

Magnesium 2.5

Sodium 2.5

Potassium 2.5

Titanium 0.4

Hydrogen 0.2

Carbon 0.2

Phosphorus 0.1

Sulfur 0.1

We go very much by our title of the thesis

for which the main physical process to be

considered are Earthquakes and Volcanoes.

We come across a cascade of reasons for the

generation of these processes. 2 8Prof. Nelson

Stephen A. of Tulane University in his paper on

Earthquakes and Earth’s Interior has

extensively dealt with the subject descriptively

with lot of illustration and graphs. Most natural

earthquakes are caused by sudden slippage

along a fault zone. According to the elastic

rebound theory, that if a slippage along a fault

is abruptly hindered such that elastic strain

energy builds up in the deforming rocks on

either side of the fault, when the slippage does

occur, the energy released causes an

earthquake releasing elastic waves of tremendous energy called seismic waves

throughout the Earth. The seismic waves generated by earthquake are the best source

for studying the interior of the Earth. The primary cause for tectonic plate movement is

the so-called convection taking place inside the Earth. The cascade of reasons for

seismicity can be represented according to the following sequence.

Convection →→→→ Plate Tectonics →→→→ Earthquake →→→→ Seismicity

2.9Eric H. Christiansen, et al have dealt with convection inside the Earth. Convection

of the core and mantle is the most important mechanism of heat transfer in the Earth.

Convection in the iron core probably creates the magnetic field and the convection in

the mantle creates mantle plumes and plate tectonics. We shall deal with the convection

in the core later under the topic of terrestrial magnetism. We shall now deal with the

convection in the mantle.

Convection in the Mantle:- The authors2.9 in the book say that the Earth INDEED IS

LIKE A large heat engine constantly churning by internal convection. Earth’s thermal

structure and convection is shown in Fig. 2.20. The structure and convection can be

modeled using computers to complement the obs

‘Tomos’ is a Greek word meaning ‘Section’.

Like the medical CT Scan

where x-rays are used to

examine body parts of a

human being, in the CAT

(Computer Aided Tomography)

Scan, seismic waves that pass

through Earth in different. In

the model shown in Fig. 2.20,

the sub-ducted slabs pass

without pausing through the phase boundary at 600 km. In another model, the phase

boundary is a temporary barrier that i

accumulates and then flushes rapidly through the lower mantle. The lower mantle may

create convection by generating thin plumes that rise off the core

Some of the plumes may be triggered by the

In another competing mode!, the whole mantle convects as a single unit. Sub

ducting slabs of oceanic lithosphere may be dense enough to pass unobstructed

through the boundary between the upper and lower mantle. Let

waves.

Seismic Waves:- Immediately after an earthquake or rather accompanied by it, elastic

energy is released and sends out vibrations throughout the Earth. These vibrations

constitute what is known as seismic waves. Seismology is a b

wherein we study in detail these seismic waves. Seismometer is an instrument to record

and study these vibrations and the resulting graph obtained is known as seismograph.

The source of an earthquake is called ‘focus’ from where the sto

suddenly released. Epicentre is a point on the surface of the Earth directly above the

focus (Fig. 2.21). Different types of seismic waves emanate from the focus in different

directions.

Fig. 2.20 Earth’s thermal structure and convection(Credit : 2.9)

modeled using computers to complement the observations of seismic tomography.

‘Tomos’ is a Greek word meaning ‘Section’.

Like the medical CT Scan

rays are used to

examine body parts of a

human being, in the CAT

Aided Tomography)

Scan, seismic waves that pass

through Earth in different. In

the model shown in Fig. 2.20,

ducted slabs pass


boundary is a temporary barrier that is broken down when enough sub


create convection by generating thin plumes that rise off the core

Some of the plumes may be triggered by the sinking of the dense overlying mantle.

In another competing mode!, the whole mantle convects as a single unit. Sub


through the boundary between the upper and lower mantle. Let us now study seismic

Immediately after an earthquake or rather accompanied by it, elastic


constitute what is known as seismic waves. Seismology is a branch of Geophysics



The source of an earthquake is called ‘focus’ from where the stored elastic energy is



Fig. 2.20 Earth’s thermal structure and convection

ervations of seismic tomography.


s broken down when enough sub-ducted material


create convection by generating thin plumes that rise off the core-mantle boundary.

sinking of the dense overlying mantle.

In another competing mode!, the whole mantle convects as a single unit. Sub-


us now study seismic

Immediately after an earthquake or rather accompanied by it, elastic


ranch of Geophysics



red elastic energy is



Fig. 2.23 A seismograph record of the waves

Fig. 2.21 Focus and Epicentre of an

Earthquake

Fig. 2.22 Types of body waves

(Credit : 2.8, P.2)

The waves that travel through the body of the Earth are known as body waves.

There are two types of body waves (Fig. 2.22).

• P – Waves (Primary waves) VP =

K +

43 µ

ρ and

• S – Waves (Secondary waves) VS = µ

ρ

Where VP and Vs are

respectively the speed of P

and S waves, K the bulk

modulus, µ. the shear

modulus and ρ the density of

the material. P-waves are

longitudinal similar to

sound waves having high

velocity and will reach the

seismometers first. S-

waves are like transverse waves and do not travel in liquids as liquids have no rigidity.

They travel slowly as compared to P waves.

In addition to the P and S waves, there exists the Surface waves which do not travel

within the Earth, but travel parallel to the surface of the Earth with velocity lower than

that of the S-waves. Fig. 2.23 shows the record of the three waves in a seismograph.

Fig. 2.24 Determination of distances of seismographic stations (Credit : 2.8, P.4)

Determination of location of an Earthquake:- In order to determine the location of an

earthquake, we need seismographs from at least 3 different seismographic stations

situated at three different distances from the epicenter. The travel time curves for P and

S waves collected over a period of time already exists in earthquake research stations.

The S-P interval at each station is

to be noted as shown Fig. 2.24.

With the help of the S-P

interval, we can determine the

distance dl, d2 and d3 from the

epicenter to the seismographic

stations. Draw circles with radii

d1, d2 and d3 (Fig. 2.25). The

common intersection of the

three circles determines the

epicenter of the earthquake.

Magnitude of an earthquake:-

The magnitude or size of an

earthquake is the amplitude of

the largest recorded wave at a

specific distance from the

earthquake. The magnitude is given in terms of Richter scale (1935) named after

Charles W. Richter (1900-1985). The energy released E and the magnitude M is given

by the following relation

Log E = 11.8 + 1.5 M

Log E is the logarithm to the base 10

In the following Table 2.5 is the magnitude starting from Richter scale 1 to 8, their

corresponding energy and the possible effects. From the Table it is seen that for each

increase in Richter Magnitude, there is about 30 fold increase of energy released.

Fig. 2.25 Final determination of epicenter

(Credit : 2.8, P.4)

As we have already seen that the velocities of the P and S waves depend on K, µ

and ρ, their values differ at different points in the Earth and hence the study of P and S

waves in particular and the seismicity in general will certainly throw more light on the

interior of the Earth. If the seismic wave velocities gradually increase with depth in the

Earth, the waves will be refracted continuously as shown in Fig. 2.26. Seismologists,

however, discovered a discontinuity at a depth of 2900 km, the velocity of P-waves

suddenly decreases. It is at the

boundary of mantle and the core and

was discovered because of a zone

on the opposite side of the Earth

called P-wave shadow zone (Fig.

2.27). This discovery was followed

by the discovery of a S-wave

Shadow zone (Fig. 2.28).

Magnitude Richter Scale

Energy in

Joule

Possible effects

1 2.0 × 106

Detectable only by instruments

2 6.3 × 107

Barely detectable even near the epicentre

3 2.0 × 109

Felt indoors

4 6.3 × 1010

Felt by most people. Slight damage

5 2.0 × 1012

Felt by all. Damage minor to

moderate

6 6.3 × 1013

Moderately destructive

7 2.0 × 1015

Major damage

8 6.3 × 1016

Total and major damage

This S-wave shadow zone is due to the S-wave not reaching the opposite side of the

Earth from the focus. Thus the S-wave is obstructed from reaching the core and hence

its velocity in the core is zero.

Fig. 2.26 Paths of seismic waves in the

planet (Credit : 2.8, P.10)

Fig. 2.27 Illustration of P-wave shadow

zone (Credit : 2.8, P.10)

As VS = 0 and u = 0, the conclusion is that the Ccore is in a liquid state A. Mohorovicic (Fig. 2.10) discovered a boundary between crust and mantle which is named after him as the Mohorovicic discontinuity or simply ‘Moho’. The composition of the crust can be studied by analyzing the seismic wave velocities in the crust.

The magnetic field of the Earth:- Earlier somewhere when we dealt with convection, we considered only convection in the mantle and postponed the convection of the core to a later stage. Now is the time to deal with it. The origin of the magnetic field of the Earth is sought in the dynamo action in the core of the Earth. The motion, rather the convection, in the electrically conducting core taking place in a magnetic field induce electrical currents generating a magnetic field which will be maintained by

electromagnetic induction. 2.9The magnetic field is caused by Earth’s rotation combined with the convection of the molten metal in a shell surrounding the inner core. (Fig. 2.29)

(Credit: 2.9)

Fig. 2.28 Illustration of S-wave shadow zone

(Credit : 2.8, P.11)

Fig. 2.29 A computer model of convection showing magnetic field of Earth (Credit :

2.9, P.531) 2.7The effect of Earth’s magnetic field is felt in a region surrounding the Earth called

magnetosphere. There are regions known as Van Allen belts, named in honour of

James Van Allen (b.1914) (Fig. 2.30), the American physicist who discovered them in

1958 and 1959 with the help of radiation counters carried aboard the artificial satellite,

Explorer I (1958) and Pioneer 3 (1959). He discovered two regions of highly charged

particles above Earth’s equator and trapped by the magnetic field of the Earth. The first

belt extends from few km to 3200 km above the surface of Earth and the second

between 14,500 km to 19,000 km. The particles mainly electrons and protons come

from the solar wind and cosmic rays.

Fig. 2.30 John Van Allen (centre) with William Pick ering and Wernher von Braun, holding

a model of the first successfully launched US Satel lite, Explorer

In May 1998, there were a series of large solar disturbances that caused a new Van

Allen belt to form in the so-called “Slot region” between the inner and outer Van Allen

belts. The new belt eventually disappeared once the solar activity subsided.

Electrical conductivity within the Earth:- Due to the magnetic field of the Earth, the

conductivity of the core must be high enough to allow electrical currents to flow. The

conductivity of the mantle is found to be less than that of the core.

At the end before I conclude, I would like to quote a news item reported from London

and appeared in the 2.10Free Press Journal, Mumbai dated 24th February 2011, titled:

“Accurate estimation of Earth’s rotation found”. The news item is reproduced below in

italics.

“A new research gives the first accurate estimate of how much faster Earth’s core is rotating compared to the rest of the planet.

Earlier research had shown the Earth’s core rotates faster than the rest of the planet.

However, estimates of one degree quicker each year were inaccurate as the core is

actually moving much slower - approximately one degree every million years, a

University of Cambridge study discovered.

Their findings have been published in the Journal Nature Geoscience, reports PTI.

The inner core grows slowly over time as material from the fluid outer core solidifies

into its surface. During this process, an east-west hemispherical difference in velocity is

frozen into the structure of the inner core, the university said in a statement”.

Conclusion:- With the completion of this chapter, we now know what the Earth is both

its inside and outside. Our study in the chapter has proved the “dynamism” of the planet

Earth enriched with a number of physical processes of various types taking place day in

day out. The effects of the processes are as important as the processes themselves. In

the chapter, we have not dealt with the cause of earthquakes, its prediction and

forecasting. In the following, they form separate chapters.

REFERENCES :

[1] 2.7Carnegie Library of Pittsburgh, “Handy Science Answer Book”, 3rd Ed.2005,

ISBN 1-57859-140-6, p.82, 86.

[2] 2.9Christiansen Eric H. and W. Kenneth Hamblin; “Earth’s Dynamic Systems”,

Ed.2008, p.526-535.

[3] 2.5Cook A.H, “Interiors of the planets”, Cambridge Univ. Press, 1980, ISBN 0

521 232147, Ch.2, p.16-50.

[4] 2.10Free Press Journal, Mumbai dated 24th Feb. 2011.

[5] 2.4Gamow George and John Cleveland, “Foundations and Frontiers”, Prentice Hall

of India, 1963, Ch.32, p.496-513.

[6] 2.6Gupte R.B., “A Text Book of Engineering Geology”, Pune Vidyarthi Griha

Prakashan, ISBN 81-85825-03-3, 3RD Ed. 2001.

[7] 2.1Langone John, Bruce Stutz and Andrea Gianopoulos, “Theories for Everything”,

National Geographic, ISBN 978-1-4351-3339-6, 2006. p-304-357.

[8] 2.3Master M. Gilbert and Wendell P, Ela, “Introduction to Environmental

Engineering and Science”, Pearson Prentice Hall, 3rd Ed.2008, p.503-504.

[9] 2.8Nelson Stephen A., Tulane Univ., “Earthquakes and Earth’s Interior”, Sep.2003,

p.1.13.

[10] 2.2Science Explorer, Earth Sc, Prentice Hall Inc.

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