Chapter
2
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,
ation, revolution, etc.
2.1
John Langone, et al in their book on “Theories of
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,
ronomer, Thales of Miletus in the 5th century BC claimed of Earth to be flat that
rested on water. Anaximander, a student of Thales also considered Earth as flat
unsupported cylinder whose depth was one-third
one third its breadth. He proposed that Earth
was suspended
ed 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)
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
Fig. 2.2 Rene Descartes
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 (17311810) (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. 2.3 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 183033 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) (18241907) (Fig. 2.6) in 1862 estimated the age of
Earth to be 100 million years.
Fig. 2.4 Henry Cavendish
Fig. 2.5 Sir Charles Lyell
Fig. 2.6 William Thomson (Lord Kelvin)
(Credit : 2.1, P.328)
(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. 2.7 Alfred Wegener
Fig. 2.8 (a) Earth some 225 million years ago
Fig.2.8 (b) Earth between 180
180-200 million years
Fig. 2.8 (c) Earth some 135 million years ago
Fig. 2.8 (d) Earth some 65 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
Raman
n 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
is
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
Fig. 2.8 (e) Earth as it appears today
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
effects
ts of various physical processes both
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 the
the climate which we feel are all the
effects of various physical processes taking place some of them forming different
chapters that follow in this thesis.
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:
2.3
Gilbert 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
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.
2.4
George 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
0.000004
0.04
Ozone
There is a
O3
decrease of
temperature about 6°C for every
kilometer of altitude of atmosphere
which continues up to an altitude
about
20
temperature
km
of
or
about
so
of
and
–60°C
Fig. 2.12 The US Standard Atmosphere
(Credit : 2.3, P.504)
(~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 in 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:-
2.2
A 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
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
Earth
Below moho is the mantle up
of
Fig. 2.16 The structure of Earth’s crust
(Credit : 2.4, P.505)
a depth of about 2900 km. The
composition
oxygen,
of
iron,
mantle
silicon
is
and
magnesium. A majority of the
mantle is solid with the upper
part called asthenosphere is
partially liquid. The GermanAmerican
seismologist,
Beno
Fig. 2.17 Possible temperatures within the
Earth (Credit : 2.5, P.40)
to
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/m3
–
2840
33
3320
413
3640
984
4550
2000
5110
2898
5560
*2898
9980
4000
11420
4980
12170
5120
12250
5120
–
above 12000 kg/m3 depth of 5000 km and more
6371
1251
indicating that the density of inner core is
Fig. 2.18 Reduced density within the Earth
From the table it is seen that the density is
*As in original.
In
1961,
Princeton
largest.
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
midAtlantic Ridge. If that were true, then, to
compensate, the Pacific Ocean must be
contracting. Hess theorized that the Pacific
Fig. 2.19 Harry Hammond Hess
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 1000
1
°C to 1300 °C.
2.6
Gupte 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 consists
cons
of about 20
minerals which are the rock
rock-forming
forming minerals. They belong to the families of feldspars,
felspathoids, micas, amphiboles, pyroxenes and olivine, crystalline and non-crystalline
non
calcium carbonate and quartz the rock-forming
rock forming minerals. Except quartz and calcium
carbonate, the rock-forming
forming minerals are silicates Formed by the combination of silica
(SiO2) with bases like K2O (potash), N, a2O (soda), CaO (lime), MgO (magnesia), Fe O,
Fe2O3, A12 O3 (alumina), etc. As a result, all common rocks are silicate rocks containing
large amounts of silica.
Table 2.4 (Credit : 2.7, P.86)
We go very much by our title of the thesis
Element
Percentage
for which the main physical process to be
Oxygen
47.0
considered are Earthquakes and Volcanoes.
Silicon
28.0
We come across a cascade of reasons for the
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
generation of these processes.
2 8
Prof. 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
2.9
Eric 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 observations
observations of seismic tomography.
‘Tomos’ is a Greek word meaning ‘Section’.
Like the medical CT Scan
where x-rays
rays
examine
body
are used to
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
ducted
slabs
pass
Fig. 2.20 Earth’s thermal structure and convection
(Credit : 2.9)
without pausing through the phase boundary at 600 km. In another model, the phase
boundary is a temporary barrier that is
is broken down when enough sub-ducted
sub
material
accumulates and then flushes rapidly through the lower mantle. The lower mantle may
create convection by generating thin plumes that rise off the core
core-mantle boundary.
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
Subducting slabs of oceanic lithosphere may be dense enough to pass unobstructed
through the boundary between the upper and lower mantle. Let us now study seismic
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 branch
branch of Geophysics
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
stored
red elastic energy is
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. 2.21 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 =
•
S – Waves (Secondary waves) VS =
4 

K + 3 µ


and
ρ
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.
Fig. 2.23 A seismograph record of the waves
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.
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
Fig. 2.24 Determination of distances of
seismographic stations (Credit : 2.8, P.4)
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.
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
Fig. 2.25 Final determination of epicenter
(Credit : 2.8, P.4)
of a S-wave
Shadow zone
(Fig. 2.28).
Magnitude
Richter
Scale
Energy Possible
in
effects
Joule
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
Fig. 2.27 Illustration of P-wave shadow
planet (Credit : 2.8, P.10)
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)
2.7
The 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 Pickering and Wernher von Braun, holding
a model of the first successfully launched US Satellite, 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
2.10
Free 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]
2.7
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2.6
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2.3
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2.2
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