26 February 2021 19:34

Geography
Combined Geo-scientist
Prelims
Lec-2

Geography
Lec-1

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26 February 2021 19:38

Geography For Combined Geo-

Lec_2
Scientist (Prelims)
1. Interior of the Earth
2. Earth's Magnetic Field
By: Swapnil Singh Bisht

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Earth Internal Structure
26 February 2021 20:50

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Interior of the Earth
26 February 2021 19:43

1. Understanding the structure of the earth’s interior (crust, mantle, core) and
various forces (heat, seismic waves) emanating from it is essential to
understand the evolution of the earth’s surface, its current shape and its
future.
2. We have to study the Earth's Interior
• to understand the earth’s surface
• to understand the geophysical phenomenon like volcanism,
earthquakes, etc.
• to understand the earth’s magnetic field
• to understand the internal structure of various solar system objects
• to understand the evolution and present composition of the atmosphere
• for mineral exploration

1. Earth’s surface
• Many different geological processes shape the
Earth's surface.
• The forces that cause these processes come
from both above and beneath the Earth's surface.
• Processes that are caused by forces from within
the Earth are endogenous processes (Endo
meaning "in").
• By contrast, exogenous processes (Exo meaning
"out") come from forces on or above the
Earth's surface.
• The major geological features of the earth’s surface
like mountains, plateaus, lakes are mostly a
result of endogenous processes like folding,
faulting that are driven by forces from inside
the earth.
2. Geophysical phenomenon like volcanism,
earthquakes
• The forces that cause catastrophic events like
earthquakes, volcanic eruptions come from
deep below the earth’s surface.
• For example, earthquakes occur due to the
movement of the tectonic plates and the energy
required for this movement is supplied by the
conventional currents in the mantle.
• Similarly, volcanism occurs through the vents
and fissures created by the tectonic movements

3. Earth’s magnetic field

• Earth’s magnetic field is a result of convection
currents in the outer core of the earth.
• Life on earth would not have been possible if
not for the earth’s magnetic field which protects

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 currents in the outer core of the earth.
• Life on earth would not have been possible if
not for the earth’s magnetic field which protects
the earth’s atmosphere from the harmful solar
wind.
4. The internal structure of various solar system
objects
• The entire solar system was formed from a single
nebular cloud, and the process of the formation
of every solar system object is believed
to be similar to that of the earth.

5. Evolution and present composition of the atmosphere

• For life to flourish on the surface of the earth,
the atmosphere needs to have essential components
like oxygen for respiration, CO2 and
other greenhouse gases to maintain the temperature
on the surface, ozone to protect life
from ultraviolet radiation and the right atmospheric
pressure.
• All these components of the earth’s atmosphere
owe their existence to the volcanic eruptions
that unlock them from the earth’s interior.
6. Mineral exploration
• Understanding volcanic activity and the nature
of rocks is essential for mineral exploration.
• Most of the minerals like diamonds (form at a
depth of 150-800 km in the mantle) that occur
on the earth’s surface are formed deep below
the earth’s surface. They are brought to the
surface by volcanic activity

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How?
26 February 2021 19:54

 Deepest Scientific Well has only reached about 12km, i.e. Kola
Super deep (Russia)
 Earth's Radius is 6371Km , we have only scratched its surface.
 Deep parts of the Earth is indirectly studied, largely through
branch of geology called Geophysics.
 Which is the application of physical laws and principles to the
study of earth.
 Geophysics include the study of Earth's magnetic field , seismic
waves, gravity and heat.

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Sources of Information about the Interior
26 February 2021 20:06

Direct Sources of information about the interior

• Deep earth mining and drilling reveal the nature of rocks deep
down the surface.
• But as mining and drilling are not practically possible beyond a
certain depth, they don’t reveal much information
about the earth’s interior.
• Mponeng gold mine (deepest mine in the world) and TauTona gold
mine (second deepest mine in the
world) in South Africa are deepest mines reaching to a depth of only
3.9 km.
• And the deepest drilling is only about 12 km deep hole bored by
the Soviet Union in the 1970s over the Kola Peninsula.

Indirect Sources of information about the interior

• Gravitation and the diameter of the earth help in estimating
pressure deep inside.
• Volcanic eruptions and existence of hot springs, geysers etc. point
to an interior which is very hot.
Seismic Waves :
• They are the most important source available to understand the
layered structure of the earth.
• The velocity of seismic waves changes as they travel through
materials with different elasticity and density.
• The more elastic and denser the material is, the higher is the
velocity.
• They also undergo refection or refraction when they come across
materials with different densities.
• Earth’s internal structure can be understood by analysing the
patterns of reflection, refraction and change in velocity of the
seismic waves when they travel through it.
Meteorites
• Meteorites and Earth are born from the same nebular cloud. Thus,
they are likely to have a similar internal
structure.
• When meteoroids they fall to earth, their outer layer is burnt
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• When meteoroids they fall to earth, their outer layer is burnt
during their fall due to extreme friction and
the inner core is exposed.
• The heavy material composition of their cores confirms the similar
composition of the inner core of the
earth.
Gravitation
• The gravitation force differs according to the mass of material. The
uneven distribution of mass of material
within the earth influences this value. Such a difference is called
gravity anomaly.
• Gravity anomalies give us information about the distribution of
mass in the crust of the earth.
Magnetic field
• The geodynamo effect helps scientists understand what's
happening inside the Earth's core. Shifts in the
magnetic field also provide clues to the inaccessible iron core.

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Earthquake And Seismic Waves
26 February 2021 20:20

Seismic waves
• Seismic: relating to earthquakes or other vibrations of the earth
and its crust.
• Seismic waves are waves of energy that travel through the Earth's
layers and are a result of earthquakes,
volcanic eruptions, magma movement, large landslides and large
human-made explosions.
• The refraction or reflection of seismic waves is used for research
into the structure of the Earth's interior.
• The terms seismic waves and earthquake waves are often used
interchangeably

How are earthquake waves produced?

• The abrupt release of energy along a fault (sharp break in the
crustal layer) causes earthquake waves.
• Rock layers along a fault tend to move in opposite directions due
to the force excreted on them but are
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to the force excreted on them but are
held in place by counteracting frictional force exerted by the
overlying rock strata.
• The pressure on the rock layers builds up over a period and
overcomes the frictional force resulting in a
sudden movement generating shockwaves (seismic waves) that
travel in all directions.
• The point where the energy is released is called the focus or the
hypocentre of an earthquake.
• The point on the surface directly above the focus is called
epicentre.
• An instrument called ‘seismograph’ records the waves reaching the
surface

1.1 Types of Seismic waves or earthquake waves

• The seismic waves or earthquake waves are basically of two
types — body waves and surface waves.

Body waves
• Body waves are generated due to the release of energy at the focus and
move in all directions travelling
through the interior of the earth. Hence, the name body waves.
• There are two types of body waves:
1) the P-waves or primary waves (longitudinal in nature ― wave propagation is
similar to sound waves),
and
2) the S-waves or secondary waves (transverse in nature ― wave propagation
is similar to ripples on the
surface of the water).
Primary Waves (P-waves)
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Primary Waves (P-waves)
• Primary waves are called so because they are the fastest among
the seismic waves and hence are recorded
first on the seismograph.
• P-waves are also called as the
➢ longitudinal waves because the displacement of the medium is in
the same direction as, or the opposite
direction to, (parallel to) the direction of propagation of the wave; or
➢ compressional waves because they produce compression and
rarefaction when travelling through a
medium; or
➢ pressure waves because they produce increases and decreases in
pressure in the medium.
• P-waves creates density differences in the material leading to
stretching (rarefaction) and squeezing (compression)
of the material.

• These waves are of relatively high frequency and are the least
destructive among the earthquake waves.
• The trembling on the earth’s surface caused due to these waves is
in the up-down direction (vertical).
• They can travel in all mediums, and their velocity depends on shear
strength (elasticity) of the medium.
• Hence, the velocity of the P-waves in Solids > Liquids > Gases.
• These waves take the form of sound waves when they enter the
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• These waves take the form of sound waves when they enter the
atmosphere.
• P-wave velocity in earthquakes is in the range 5 to 8 km/s.
• The precise speed varies according to the region of the Earth's
interior, from less than 6 km/s in the Earth's
crust to 13.5 km/s in the lower mantle, and 11 km/s through the
inner core

Why do P-waves travel faster than S-waves?

• P-waves are about 1.7 times faster than the S-waves.
• P-waves are compression waves that apply a force in the direction
of propagation and hence transmit their
energy quite easily through the medium and thus travel quickly.
• On the other hand, S-waves are transverse waves or shear waves
(motion of the medium is perpendicular
to the direction of propagation of the wave) and are hence less
easily transmitted through the medium.
P-waves as an earthquake warning
• Advance earthquake warning is possible by detecting the non-
destructive primary waves that travel more
quickly through the Earth's crust than do the destructive secondary
and surface waves.
• Depending on the depth of focus of the earthquake, the delay
between the arrival of the P-wave and other
destructive waves could be up to about 60 to 90 seconds (depends
of the depth of the focus).

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S-Waves and Surface waves
26 February 2021 20:26

Secondary Waves (S-waves)

• Secondary waves (secondary ➔ they are recorded second on the
seismograph) or S-waves are also called
as transverse waves or shear waves or distortional waves.
• They are analogous to water ripples or light waves.
• Transverse waves or shear waves mean that the direction of
vibrations of the particles in the medium is
perpendicular to the direction of propagation of the wave. Hence,
they create troughs and crests in the
material through which they pass (they distort the medium).
• S-waves arrive at the surface after the P-waves.
• These waves are of high frequency and possess slightly higher
destructive power compared to P-waves.
• The trembling on the earth’s surface caused due to these waves is
from side to side (horizontal).
• S-waves cannot pass through fluids (liquids and gases) as fluids do
not support shear stresses.
• They travel at varying velocities (proportional to shear strength)
through the solid part of the Earth.

Surface waves (L-Waves)

• The body waves interact with the surface rocks and generate new
set of waves called surface waves
(long or L-waves). These waves move only along the surface.
• Surface Waves are also called long period waves because of their
long wavelength.
• They are low-frequency transverse waves (shear waves).
• They develop in the immediate neighbourhood of the epicentre
and affect only the surface of the earth
and die out at smaller depth.
• They lose energy more slowly with distance than the body waves
because they travel only across the
surface unlike the body waves which travel in all directions.
• Particle motion of surface waves (amplitude) is larger than that of

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• Particle motion of surface waves (amplitude) is larger than that of
body waves, so surface waves are
the most destructive among the earthquake waves.
• They are slowest among the earthquake waves and are recorded
last on the seismograph.
Love waves
• It's the fastest surface wave and moves the ground from side-to-
side.
Rayleigh waves
• A Rayleigh wave rolls along the ground just like a wave rolls across
a lake or an ocean.
• Because it rolls, it moves the ground up and down and side-to-side
in the same direction that the wave is
moving.
• Most of the shaking and damage from an earthquake is due to the
Rayleigh wave.

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Application of seismic Waves
26 February 2021 20:29

How do seismic waves help in understanding the earth’s interior?

• Seismic waves get recorded in seismographs located at far off locations.

• Differences in arrival times, waves taking different paths than expected (due
to refraction) and absence of
the seismic waves in certain regions called as shadow zones, allow mapping of
the Earth's interior.
• Discontinuities in velocity as a function of depth are indicative of changes in
composition and density.
• That’s is, by observing the changes in velocity, the density and composition of
the earth’s interior can be
estimated (change in densities greatly varies the wave velocity).
• Discontinuities in wave motion as a function of depth are indicative of
changes in phase.
• That is, by observing the changes in the direction of the waves, the emergence
of shadow zones, different
layers can be identified.

The emergence of Shadow Zone of P-waves and S-waves

• S-waves do not travel through liquids (they are attenuated).
• The entire zone beyond 103° does not receive S-waves, and hence this zone is
identified as the shadow
zone of S-waves. This observation led to the discovery of the liquid outer core.
• The shadow zone of P-waves appears as a band around the earth between
103° and 142° away from the
epicentre.
• This is because P-waves are refracted when they pass through the transition
between the semisolid mantle
and the liquid outer core.
• However, the seismographs located beyond 142° from the epicentre, record
the arrival of P-waves, but not
that of S-waves. This gives clues about the solid inner core.
• Thus, a zone between 103° and 142° from epicentre was identified as the
shadow zone for both the
types of waves.
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types of waves.

The span of the shadow zone of the P-Waves = 78° [2 x (142° - 103°)]
• The span of the shadow zone of the S-Waves = 154° [360° – (103° +
103°)]
• The span of the shadow zone common for both the waves = 78°

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26 February 2021 19:51

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Division of Earth's Internal Structure
26 February 2021 20:13

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Interior of the earth
26 February 2021 20:36

The interior of the earth is made up of several concentric layers of which the
crust, the mantle, the outer core and the inner core are significant because of
their unique physical and chemical properties.
• The crust is a silicate solid, the mantle is a viscous molten rock, the outer core
is a viscous liquid, and the inner core is a dense solid
Mechanically, the earth’s layers can be divided into lithosphere, asthenosphere,
mesospheric mantle
(part of the Earth's mantle below the lithosphere and the asthenosphere), outer
core, and inner core.
• Chemically, Earth can be divided into the crust, upper mantle, lower mantle,
outer core, and inner core
The Crust
• The crust is the outermost layer of the earth making up 0.5-1.0 per cent of the
earth’s volume and less
than 1 per cent of Earth’s mass.
• Density increases with depth, and the average density is about 2.7 g/cm3
(average density of the earth is
5.51 g/cm³).
• The thickness of the crust varies in the range of range of 5-30 km in case of
the oceanic crust and as 50-
70 km in case of the continental crust.
• The continental crust can be thicker than 70 km in the areas of major
mountain systems. It is as much as
70-100 km thick in the Himalayan region.
• The temperature of the crust increases with depth, reaching values typically in
the range from about 200 °C
to 400 °C at the boundary with the underlying mantle.
• The temperature increases by as much as 30 °C for every kilometre in the
upper part of the crust.
The outer covering of the crust is of sedimentary material and below that lie
crystalline, igneous and metamorphic
rocks which are acidic in nature.
• The lower layer of the crust consists of basaltic and ultra-basic rocks.
• The continents are composed of lighter silicates — silica + aluminium (also
called sial) while the oceans
have the heavier silicates — silica + magnesium (also called sima)
[Suess,1831–1914 ― this classification is
now obsolete (out of date)].
• The continental crust is composed of lighter (felsic) sodium potassium
aluminium silicate rocks, like
granite.
• The oceanic crust, on the other hand, is composed of dense (mafic) iron
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• The oceanic crust, on the other hand, is composed of dense (mafic) iron
magnesium silicate igneous
rocks, like basalt.
In geology, felsic refers to igneous rocks that are relatively rich in elements that
form feldspar and quartz.
It is contrasted with mafic rocks, which are relatively richer in magnesium and
iron.
Felsic refers to rocks which are enriched in the lighter elements such as silicon,
oxygen, aluminium, sodium,
and potassium.

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Discontinuities
26 February 2021 20:39

Seismic Discontinuities
• Seismic discontinuities are the regions in the earth where seismic
waves behave a lot different compared to
the surrounding regions due to a marked change in physical or chemical
properties.

The Mohorovicic (Moho) discontinuity

• Mohorovicic (Moho) discontinuity forms the boundary between the
crust and the asthenosphere (upper
reaches of the mantle) where there is a discontinuity in the seismic
velocity.
18
• It occurs at an average depth of about 8 kilometres beneath the
ocean basins and 30 kilometres beneath
continental surfaces.
• The cause of the Moho is thought to be a change in rock composition
from rocks containing feldspar
(above) to rocks that contain no feldspars (below).
Lithosphere
• The lithosphere is the rigid outer part of the earth with thickness
varying between 10-200 km.
• It is includes the crust and the upper part of the mantle.
• The lithosphere is broken into tectonic plates (lithospheric plates),

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• The lithosphere is broken into tectonic plates (lithospheric plates),
and the movement of these tectonic
plates cause large-scale changes in the earth’s geological structure
(folding, faulting).
• The source of heat that drives plate tectonics is the primordial heat
left over from the planet’s formation
as well as the radioactive decay of uranium, thorium, and potassium in
Earth’s crust and mantle.

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Mantle and core
26 February 2021 20:44

The Mantle
• It forms about 83 per cent of the earth's volume and holds 67% of
the earth’s mass.
• It extends from Moho’s discontinuity to a depth of 2,900 km.
• The density of the upper mantle varies between 2.9 g/cm3 and 3.3
g/cm3.
• The lower mantle extends beyond the asthenosphere. It is in a
solid state.
• The density ranges from 3.3 g/cm3 to 5.7 g/cm3 in the lower
mantle.
• The mantle is composed of silicate rocks that are rich in iron and
magnesium relative to the overlying
crust.
• Regarding its constituent elements, the mantle is made up of 45%
oxygen, 21% silicon, and 23% magnesium
(OSM).
• In the mantle, temperatures range from approximately 200 °C at
the upper boundary with the crust to approximately
4,000 °C at the core-mantle boundary.
• Because of the temperature difference, there is a convective
material circulation in the mantle (although
solid, the high temperatures within the mantle cause the silicate
material to be sufficiently ductile).
• Convection of the mantle is expressed at the surface through the
motions of tectonic plates.
• High-pressure conditions ought to inhibit seismicity in the mantle.
However, in subduction zones, earthquakes
are observed down to 670 km (420 mi).
Asthenosphere
• The upper portion of the mantle is called as asthenosphere
(astheno means weak).

• It lies just below the lithosphere extending up to 80-200 km.

• It is highly viscous, mechanically weak and ductile and its density is
higher than that of the crust.
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higher than that of the crust.
• These properties of the asthenosphere aid in plate tectonic
movement and isostatic adjustments (the
elevated part at one part of the crust area is counterbalanced by a
depressed part at another).
• It is the main source of magma that finds its way to the surface
during volcanic eruptions.
The Outer Core
• The outer core, surrounding the inner core, lies between 2900 km
and 5100 km below the earth's surface.
• The outer core is composed of iron mixed with nickel (nife) and
trace amounts of lighter elements.
• The outer core is not under enough pressure to be solid, so it is
liquid even though it has a composition
similar to the inner core.
• The density of the outer core ranges from 9.9 g/cm3 to 12.2
g/cm3.
• The temperature of the outer core ranges from 4400 °C in the
outer regions to 6000 °C near the inner core.
• Dynamo theory suggests that convection in the outer core,
combined with the Coriolis effect, gives rise
to Earth's magnetic field.
The Inner Core
• The inner core extends from the centre of the earth to 5100 km
below the earth's surface.
• The inner core is generally believed to be composed primarily of
iron (80%) and some nickel (nife).
• Since this layer can transmit shear waves (transverse seismic
waves), it is solid. (When P-waves strike the
outer core – inner core boundary, they give rise to S-waves)
• Earth's inner core rotates slightly faster relative to the rotation of
the surface.
• The solid inner core is too hot to hold a permanent magnetic field.
• The density of the inner core ranges from 12.6 g/cm3 to 13 g/cm3.
• The core (inner core and the outer core) accounts for just about 16
per cent of the earth's volume but
33% of earth’s mass.

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33% of earth’s mass.
• Scientists have determined the temperature near the Earth's
centre to be 6000 C, 1000 C hotter than previously
thought.
• At 6000°C, this iron core is as hot as the Sun’s surface, but the
crushing pressure caused by gravity prevents
it from becoming liquid.

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Sources of Earth's heat
26 February 2021 20:15

Sources of earth’s heat

Radioactive decay
• The high temperature below the crust is attributed to the
disintegration of the radioactive substances.
• The nuclear decay happens primarily in the crust and the mantle.
• Scientists believe that uranium could become sufficiently
concentrated at the base of Earth’s mantle to
ignite self-sustained nuclear fission, as in a human-made reactor.
• The new measurements suggest radioactive decay provides more
than half of Earth's total heat.
Nuclear fusion doesn’t occur inside the earth. For nuclear fusion to
occur there must be far more pressure and
temperature inside the earth. The earth is not massive enough to
cause such conditions.
Primordial heat
• The rest is the heat left over from Earth's formation known as the
primordial heat.
• Primordial heat is the kinetic energy transferred to Earth by
external impacts of comets and meteorites and
the subsequent effects (friction caused by sinking of heavy elements
like Fe, rising light elements like Si)
and latent heat of crystallisation released as the core solidified
Tidal friction
• The ocean tides are not the only effect of tidal forces (gravitational
influence of the moon and the sun on
earth; tides are explained in oceanography). The solid body of the
Earth also bulges slightly in this way.
• The daily flexing of the Earth (both solid body and the oceans)
cause loss of energy of the Earth's rotation,
due to friction.
• This energy goes into heat, leading to miniscule increase in the
Earth's internal temperature.
• The loss of rotational energy means that the Earth is slowing down
in its rotation rate, currently by about
0.002 seconds per century
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0.002 seconds per century

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Magnetic Field
26 February 2021 20:56

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Van Allen radiation belt
• A Van Allen radiation belt is a zone of energetic charged particles, most of
which originate from the solar
wind, that are captured by and held around a planet by that planet's magnetic
field.
• There are two such concentric tire-shaped regions. The inner belt is 1–2
Earth radii out while the outer belt
is at 4–7 Earth radii.
• By trapping the solar wind, the belts deflect the energetic particles and
protect the atmosphere.
• The belts endanger satellites, which must have their sensitive components
protected with adequate

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protected with adequate
shielding if they spend significant time near that zone.
• Spacecraft travelling beyond low Earth orbit enter the zone of radiation of
the Van Allen belts. Beyond the
belts, they face additional hazards from cosmic rays and solar particle events

Mars
• Mars does not have an intrinsic global magnetic field, but the solar
wind directly interacts with
the atmosphere of Mars, leading to the formation of a magnetosphere.
• The lack of a significant magnetosphere is thought to be one reason
for Mars's thin atmosphere

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How magnetic field is generated ?
26 February 2021 20:58

Dynamo theory proposes a mechanism by which a celestial body such

as Earth or a star generates a magneticn field and sustains it over
astronomical time scales (millions of years).

• Dynamo theory suggests that convection in the outer core, combined

with the Coriolis effect (caused
due to the rotation of the earth), gives rise to self-sustaining
(geodynamo) Earth's magnetic field.

Magnetic poles
• A magnet's North pole is thought as the pole that is attracted by the
Earth's North Magnetic Pole when the
magnet is suspended so it can turn freely.
• Since opposite poles attract, the North Magnetic Pole of the Earth is
the south pole of its magnetic
field.
• Magnetic dipole field (simple north-south field like that of a simple

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• Magnetic dipole field (simple north-south field like that of a simple
bar magnet) is usually aligned fairly
closely with the Earth's rotation axis; in other words, the magnetic
poles are usually fairly close to the geographic
poles, which is why a compass works.
• However, the dipole part of the field reverses after a few thousand
years causing the locations of the
north and south magnetic poles to switch.

The terms magnetic north and magnetic south are not to be confused
with geographic north and geographic
south, and geomagnetic north and geomagnetic south

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Normal and Reversed field
26 February 2021 21:09

• The Earth's field has alternated between periods of normal polarity, in which the
predominant direction of the field was the same as the present direction, and reverse polarity,
in which it was the opposite.
Normal and Reversed field (The bar magnet at the centre represents earth’s magnetic field)
In Normal Polarity, Earth’s North Magnetic Pole is the South Pole of its Magnetic Field.
In Reverse Polarity, Earth’s North Magnetic Pole is the North Pole of its Magnetic Field.

The current location of the Magnetic Poles

• The North and South Magnetic Poles wander (Polar Shift Theory) due to changes in Earth’s
magnetic field.
• The North Magnetic Pole (86 N, 172 W) lie to the north of Ellesmere Island in northern
Canada and is rapidly
drifting towards Siberia.
• The location of the South Magnetic Pole is currently off the coast of Antarctica and even
outside the Antarctic Circle.
• Scientists suggest that the north magnetic pole migrates about 10 kilometres per year.
• Lately, the speed has accelerated to about 40 kilometres per year and could reach Siberia in
a few decades.
• Since the Earth's magnetic field is not exactly symmetrical, the North and South Magnetic
Poles are not antipodal (a straight line drawn from one to the other does not pass through the
centre of the Earth).
• The Earth's North and South Magnetic Poles are also known as Magnetic Dip Poles because
of the vertical
"dip" of the magnetic field lines at those points.
• That is, if a magnetic compass needle is suspended freely at the magnetic poles then it will
point straight
down at the north magnetic pole (south pole of earth’s magnetic field) and straight up at the

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down at the north magnetic pole (south pole of earth’s magnetic field) and straight up at the
south magnetic pole (north pole of earth’s magnetic field).

Compass
• A compass point north because all magnets have two poles, a north pole and a south pole,
and the north
pole of one magnet is attracted to the south pole of another magnet.
• The Earth is a magnet that can interact with other magnets in this way, so the north end of a
compass
magnet is drawn to align with the Earth's magnetic field.
• Because the Earth's Magnetic North Pole attracts the "north" ends of other magnets, it is
technically the
"South Pole" of our planet's magnetic field.
• While a compass is a great tool for navigation, it doesn't always point exactly north. This is
because the
Earth's magnetic North Pole is not the same as "true north (Earth's Geographic North Pole)."
• Although the magnetic declination (deviation from true north) does shift with time, this
wandering is slow
enough that a simple compass remains useful for navigation.
Using magnetoreception various organisms, ranging from some types of bacteria, sea turtles,
some migratory birds, pigeons, etc. use the Earth's magnetic field for orientation and
navigation

Magnetic declination
• Magnetic declination is the angle between magnetic north and true
north.
• It is positive when the angle derived is east of the true north, and it is
considered negative when the angle
measured is west of the true north.
• In which direction would a compass needle point if you were standing
on the true North Pole?

Magnetic Inclination or Magnetic Dip

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Magnetic Inclination or Magnetic Dip
• Magnetic dip, dip angle, or magnetic inclination is the angle made
with the horizontal by the Earth's
magnetic field lines.
• In simple terms, magnetic inclination is the angle made by a compass
needle when the compass is held in a
vertical orientation.
• The magnetic equator is the irregular imaginary line, passing round
the earth near the equator, on which a
magnetic needle has no dip (because magnetic field lines are parallel to
the horizontal at the equator).
• Again, the magnetic equator, like the magnetic field and poles, is not
fixed

Magnetic dip at the magnetic equator is 0, and at the magnetic poles, it

is 90.
• Importance: The phenomenon of magnetic dip is important in
aviation, as it causes the aeroplane's compass
to give erroneous readings during banked turns and airspeed changes.
Necessary corrections need to
be made to the compass reading to stay in the right course

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