25.Geography Notes PDF - Print Friendly 22-06-2016.pdf

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01.Climatology.pdf

02.Climatic Regions.pdf

03.Oceanography.pdf

04.Indian Geography.pdf

05.Economic Geography.pdf

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INTERIOR OF THE EARTH

 The configuration of the surface of the

earth is largely a product of the processes operating in the interior of the earth.

 Exogenic as well as endogenic processes

are constantly shaping the landscape.

Why know about earth’s interior

Understanding of the earth's interior is essential to understand the nature of changes that take place over and below the earth's surface.

 To understand geophysical phenomenon

like volcanism, earthquakes etc..

 To understand the internal structure of

various solar system objects

 To understand the evolution and present

composition of atmosphere

 Future deep-sea mineral exploration etc.

Sources of information about the

interior

Direct Sources

 Deep earth mining and drilling reveals the

nature of rocks deep down the surface. [Mponeng gold mine and TauTona gold mine in South Africa are deepest mines reaching to a depth of 3.9 km. And the deepest drilling is about 12 km deep]

 Volcanic eruption forms another source

of obtaining direct information.

Indirect Sources

 Depth: With depth, pressure and density

increases and hence temperature. This is mainly due to gravitation.

Mponeng mine

 South Africa

 Deepest mine

 Gold mine

 Deapth: 2.4 miles (3.9 km)

 Meteors: Meteors and Earth are solar

system objects that are born from the same nebular cloud. Thus they are likely to have a similar internal structure.

 Gravitation: The gravitation force (g) is not

the same at different latitudes on the surface. It is greater near the poles and less at the equator. This is because of the distance from the center at the equator being greater than that at the poles.

 The gravity values also differ 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 of the material 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. But their source remains a mystery.

Not important for exam. But if you are a science enthusiast and if you want to know more…

What causes the magnetic field of earth?

 Our planet’s magnetic field is believed to

be generated deep down in the Earth’s core.

 Nobody has ever taken the mythical

journey to the centre of the Earth, but by studying the way shockwaves from earthquakes travel through the planet, physicists have been able to work out its likely structure.

 Right at the heart of the Earth is a solid

inner core, two thirds of the size of the Moon and composed primarily of iron. At a

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hellish 5,700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.

 Surrounding this is the outer core, a

2,000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid.

 Differences in temperature, pressure and

composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises. The Coriolis force, resulting from the Earth’s spin, also causes swirling whirlpools.

 This flow of liquid iron generates electric

currents, which in turn produce magnetic fields. Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo.

 The spiraling caused by the Coriolis force

means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field engulfing the planet.

Some sources explained in detail

High Levels of Temperature and Pressure Downwards

 Volcanic eruptions and existence of hot

springs, geysers etc. point to an interior which is very hot.

 The high temperatures are attributed to

automatic disintegration of the radioactive substances.

 Gravitation and the diameter of the earth

helps in estimating pressures deep inside.

Evidence From The Meteorites

 When 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, as both evolved from the same star system in the remote past.

 The most important indirect source is

seismic activity. The major understanding of the earth’s internal structure is mainly from the study of seismic waves.

Seismic waves

 The study of seismic waves provides a

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What causes earthquakes?

 Abrupt release of energy along a fault

causes earthquake waves.

 A fault is a sharp break in the crustal rock

layer.

 Rocks along a fault tend to move in

opposite directions. But the friction exerted by the overlying rock strata prevents the movement of rock layer. With time pressure builds up.

 Under intense pressure, the rock layer, at

certain point, overcomes the friction offered by the overlying layer and

undergoes an abrupt movement

generating shockwaves.

 This causes a release of energy, and the

energy waves travel in all directions.

 The point where the energy is released is

called the focus of an earthquake, alternatively, it is called the hypocentre.

 The energy waves travelling in different

directions reach the surface. The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.

Earthquake Waves

 All natural earthquakes take place in the

lithosphere (depth up to 200 km from the surface of the earth).

 An instrument called ‘seismograph’

records the waves reaching the surface.

 Earthquake waves are basically of two

types — body waves and surface waves.

 Body waves are generated due to the

release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.

 The body waves interact with the surface

rocks and generate new set of waves called surface waves. These waves move along the surface.

 The velocity of waves changes as they

travel through materials with different elasticity (stiffness) (Generally density with few exceptions). The more elastic the material is, the higher is the velocity. Their

direction also changes as they reflect or refract when coming across materials with different densities.

 There are two types of body waves. They

are called P and S-waves.

Behavior of Earthquake Waves

 The earthquake waves are measured with

the help of a seismograph and are of three types—

1. the 'P' waves or primary waves (longitudinal nature),

2. secondary waves or 'S' waves

(transverse in nature) while the 3. surface waves are long or ‘L’ waves.

 The velocity and direction of the

earthquake waves undergo changes when the medium through which they are travelling changes.

 When an earthquake or underground

nuclear test sends shock waves through the Earth, the cooler areas, which generally are rigid, transmit these waves at a higher velocity than the hotter areas.

Primary Waves (P waves)

 Also called as the longitudinal or

compressional waves.

 Particles of the medium vibrate along the

direction of propagation of the wave.

 P-waves move faster and are the first to

arrive at the surface.

 These waves are of high frequency.

 They can travel in all mediums.

 Velocity of P waves in Solids > Liquids >

Gases

 Their velocity depends on shear strength

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[We usually say that the speed of sound waves depends on density. But there are few exceptions. For example: Mercury (liquid metal) has density greater than Iron but speed of sound in mercury is lesser compared to that in iron. This is because the shear strength of mercury is very low (this is why mercury is liquid) compared to that of iron.]

 The shadow zone for ‘P’ waves is an area

that corresponds to an angle between

1030_{and 142}0

 This gives clues about Solid inner core

Secondary Waves (S waves)

 Also called as transverse or distortional

waves.

 Analogous to water ripples or light waves.

 S-waves arrive at the surface with some

time lag.

 A secondary wave cannot pass through

liquids or gases.

 These waves are of high frequency waves.

 Travel at varying velocities (proportional to

shear strength) through the solid part of the Earth's crust, mantle.

 The shadow zone of 'S' waves extends

almost halfway around the globe from the earthquake's focus.

 The shadow zone for ‘S’ waves is an area

that corresponds to an angle between

1030_{and 180}0

 This observation led to the discovery of

liquid outer core. Since S waves cannot travel through liquid, they do not pass through the liquid outer core.

Surface Waves (L waves)

 Also called as long period waves.

 They are low frequency, long wavelength,

and transverse vibration.

 Generally affect the surface of the Earth

only and die out at smaller depth.

 Develop in the immediate neighborhood of

the epicenter.

 They cause displacement of rocks, and

hence, the collapse of structures occurs.

 These waves are responsible for most the

destructive force of earthquake.

 Recoded last on the seismograph.

Propagation of Earthquake Waves

 Different types of earthquake waves travel

in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.

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 P-waves vibrate parallel to the direction of

the wave. This exerts pressure on the

material in the direction of the

propagation.

 As a result, it creates density differences

in the material leading to stretching and squeezing of the material.

 Other two waves vibrate perpendicular to

the direction of propagation.

 The direction of vibrations of S-waves is

perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.

Emergence of Shadow Zone

 Earthquake waves get recorded in

seismographs located at far off locations.

 However, there exist some specific areas

where the waves are not reported. Such a zone is called the ‘shadow zone’.

 The study of different events reveals that

for each earthquake, there exists an altogether different shadow zone. Figure 3.2 (a) and (b) show the shadow zones of P and S-waves.

 It was observed that seismographs located

at any distance within 105 ° from the

epicenter, recorded the arrival of both P and S-waves.

 However, the seismographs located

beyond 145 ° from epicenter, record the arrival of P-waves, but not that of S-waves.

 Thus, a zone between 105 ° and 145 °

from epicenter was identified as the shadow zone for both the types of waves. The entire zone beyond 105 ° does not receive S-waves.

 The shadow zone of S-wave is much larger

than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105 ° and 145 ° away from the epicenter.

 The shadow zone of S-waves is not only

larger in extent but it is also a little over 40 per cent of the earth surface.

But how these properties of ‘P’

and ‘S’ waves help in determining

the earth’s interior?

 Reflection causes waves to rebound

whereas refraction makes waves move in different directions.

 The variations in the direction of waves

are inferred with the help of their record on seismograph.

 Change in densities greatly varies the

wave velocity.

 By observing the changes in velocity, the

density of the earth as a whole can be estimated.

 By the observing the changes in direction

of the waves (emergence of shadow zones), different layers can be identified.

Not important for exam. But if you are a science enthusiast and if you want to know more…

Why does sound wave travel faster in a denser medium whereas light travels slower?

 Sound is a mechanical wave and travels

by compression and rarefaction of the medium.

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 Its velocity in an elastic medium is

proportional to the square root of Tension in the medium.

 A higher density leads to more elasticity in

the medium and hence the ease by which compression and rarefaction can take place. This way the velocity of sound increases by increase in density.

 Light on the other hand is a transverse

electromagnetic wave.

 It does not depend on the elastic property

of the medium in which it travels.

 Its velocity in a medium is determined by

the electromagnetic (e.g. dielectric)

properties of the medium.

 Effective path length on the other hand is

increased by an increase in the density and hence it leads to higher refractive index and lower velocity.

Why S-waves cannot travel through liquids?

 S-waves are shear waves, which move particles perpendicularly to their direction of propagation.

 They can propagate through solid rocks

because these rocks have enough shear strength.

 The shear strength is one of the forces

that hold the rock together, and prevent it from falling into pieces.

 Liquids do not have the same shear

strength: that is why, if you take a glass of water and suddenly remove the glass, the water will not keep its glass shape and will just flow away.

 In fact, it is just a matter of rigidity:

S-waves need a medium rigid enough to propagate. Hence, S-waves do not propagate through liquids.

Earth’s Layers

 Earth’s layers are identified by studying

various direct and indirect sources [we

studied this in previous post: Earth’s

Interior – Earthquake Waves – Shadow Zone].

 The structure of the earth's interior is

made up of several concentric layers.

 Broadly three layers can be identified—

crust, mantle and the core.

Watch video for better and quick understanding

http://www.youtube.com/watch?v=dkdZnD5c k-I

Earth’s Layers based on chemical

properties

1. crust,

2. mantle, and 3. Core.

Earth’s Layers - The Crust

 Crust is the outer thin layer with a total

thickness normally between 30-50 km.

 The thickness of the crust varies under

the oceanic and continental areas.

 Oceanic crust is thinner (5-30 km

thick) as compared to the continental crust (50-70 km thick).

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 The continental crust is thicker in the

areas of major mountain systems. It is as much as 70 -100 km thick in the Himalayan region.

 It forms 5-1.0 per cent of the earth’s

volume.

 Mohorovicic (Moho) discontinuity forms

the boundary between crust and

asthenosphere [asthenosphere is a part of mantle].

 The outer covering of the crust is

of sedimentary material (granitic

rocks) 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’).

Earth’s Layers - Mantle

 The mantle extends from Moho’s

discontinuity (35 km) to a depth of 2,900 km (Moho-Discontinuity to the outer core).

 The crust and the uppermost part of the

mantle are called lithosphere. Its

thickness ranges from 10-200 km.

 The lower mantle extends beyond the

asthenosphere. It is in solid state.

 The density of mantle varies between 2.9

and 3.3.

 The density ranges from 3.3 to 5.7 in the

lower part.

 It is composed of solid rock and magma.

 It forms 83 per cent of the earth's volume.

 The outer layer of the mantle is

partly simatic while the inner layer is composed of wholly simatic ultra-basic rocks.

Earth’s Layers - Asthenosphere

 The upper portion of the mantle is called

asthenosphere.

 The word astheno means weak.

 It is considered to be extending up to 400

km.

 It is the main source of magma that finds

its way to the surface during volcanic eruptions. It has a density higher than the crust’s.

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 Lies between 2900 km and 6400

km below the earth's surface.

 Accounts for 16 per cent of the earth's

volume.

 Core has the heaviest mineral materials of

highest density.

 It is composed of nickel and iron [nife].

 The outer core is liquid while the inner

core is solid.

 A zone of mixed heavy metals + silicates

separates the core from outer layers.

Earth’s Layers - Seismic Discontinuities

1. Mohorovicic Discontinuity (Moho) -

separates the crust from the mantle, its average depth being about 35 km.

2. A soft asthenosphere (highly viscous, mechanically weak and ductile). It’s a part of mantle.

3. Gutenberg Discontinuity - lies between the mantle and the outer core. Below 2900 km from earth’s surface.

Earth’s Chemical Composition

Composition of Earth’s Crust

In this post we will study about the most basic concepts of geomorphology - Earth Movements - Endogenetic Movements. Next post will be on Exogenetic Movements

Watch video for better and quick understanding

http://www.youtube.com/watch?v=s2lVo9yLk Qs

Earth Movements

 Our earth is undergoing deformations

imperceptibly [so slight, gradual, or subtle as not to be perceived] but continuously.

 These deformations are caused by the

movements generated by various factors like

1. The heat generated by the radioactive elements in earth's interior.

2. Movement of the crustal plates due to tectogenesis.

3. Forces generated by rotation of the earth.

4. Climatic factors like winds,

precipitation, pressure belts etc.

 Isostacy ==> According to this concepts,

blocks of the earth's crust, because of variations in density would rise to different levels and appear on the surface as mountains, plateau, plains or ocean basins

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 Tectonic ==> relating to the structure of

the earth's crust and the large-scale processes which take place within it.

Geomorphic processes

 Geomorphic == relating to the form of the

landscape and other natural features of the earth's surface.

 The endogenic and exogenic forces

causing physical and chemical changes on earth surface are known as geomorphic processes.

 Diastrophism and volcanism are

endogenic geomorphic processes.

 Weathering, mass wasting, erosion and

deposition are exogenic geomorphic

processes.

 Geomorphic agent == mobile medium (like

running water, moving ice masses, wind, waves and currents etc.) which removes, transports and deposits earth materials.

Earth Movements - Endogenetic

Movements

 The interaction of matter and temperature

generates these forces or movements inside the earth's crust. The earth

movements are mainly of two

types: diastrophism and the sudden

movements.

 The energy emanating from within the

earth is the main force behind endogenic geomorphic processes.

 This energy is mostly generated

by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth. This energy due to geothermal gradients and heat flow from

within induces diastrophism and

volcanism in the lithosphere.

Diastrophism

 Diastrophism is the general term applied

to slow bending, folding, warping and fracturing.

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 Wrap == make or become bent or twisted

out of shape, typically from the action of heat or damp; make abnormal; distort.

 All processes that move, elevate or build

up portions of the earth’s crust come under diastrophism. They include:

1. orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust;

2. epeirogenic processes involving uplift or warping of large parts of the earth’s crust;

3. earthquakes involving local relatively

minor movements;

4. plate tectonics involving horizontal

movements of crustal plates.

 In the process of orogeny, the crust is

severely deformed into folds. Due to

epeirogeny, there may be simple

deformation. Orogeny is a mountain building process whereas epeirogeny is continental building process.

 Through the processes of orogeny,

epeirogeny, earthquakes and plate

tectonics, there can be faulting and fracturing of the crust. All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks.

Epeirogenic or continent forming movements

 In geology, Epeirogenic movement refers to

upheavals or depressions of land

exhibiting long

wavelengths [undulations] and little

folding.

 The broad central parts of continents are

called cratons, and are subject to epeirogeny.

 The movement is caused by a set of forces

acting along an Earth radius, such as

those contributing

to Isostacy and Faulting in the

lithosphere

 Epeirogenic or continent forming

movements act along the radius of the

earth; therefore, they are also

called radial movements. Their direction may be towards (subsidence) or away (uplift) from the center. The results of such movements may be clearly defined in the relief.

Uplift

 Raised beaches, elevated wave-cut

terraces, sea caves and fossiliferous beds above sea level are evidences of uplift.

 Raised beaches, some of them elevated as

much as 15 m to 30 m above the present sea level, occur at several places along

the Kathiawar, Nellore, and

Thirunelveli coasts.

 Several places which were on the sea some

centuries ago are now a few miles inland. For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli, were all flourishing sea ports about 1,000 to 2,000 years ago.

Subsidence

 Submerged forests and valleys as well as

buildings are evidences of subsidence.

 In 1819, a part of the Rann of Kachchh

was submerged as a result of an earthquake.

 Presence of peat and lignite beds below

the sea level in Thirunelveli and the Sunderbans is an example of subsidence.

 The Andamans and Nicobars have been

isolated from the Arakan coast by submergence of the intervening land.

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 On the east side of Bombay island, trees

have been found embedded in mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Thirunelveli coast in Tamil Nadu.

 A large part of the Gulf of Mannar and

Palk Strait is very shallow and has been submerged in geologically recent times. A part of the former town of Mahabalipuram near Chennai (Madras) is submerged in the sea.

Orogenic or the mountain-forming movements

 Orogenic or the mountain-forming

movements act tangentially to the earth surface, as in plate tectonics.

 Tensions produces fissures (since this

type of force acts away from a point in two directions) and compression produces folds (because this type of force acts towards a point from two or more directions). In the landforms so produced,

the structurally identifiable units are difficult to recognise.

 In general, diastrophic forces which have

uplifted lands have predominated over forces which have lowered them.

Sudden Movements

 These movements cause considerable

deformation over a short span of time, and may be of two types.

Earthquake

 It occurs when the surplus accumulated

stress in rocks in the earth's interior is relieved through the weak zones over the earth's surface in form of kinetic energy of wave motion causing vibrations (at times devastating) on the earth's surface. Such movements may result in uplift in coastal areas.

 An earthquake in Chile (1822) caused a

one-metre uplift in coastal areas.

 An earthquake in New Zealand (1885)

caused an uplift of upto 3 metres in some areas while some areas in Japan (1891) subsided by 6 metres after an earthquake.

 Earthquakes may cause change in

contours, change in river courses, 'tsunamis' (seismic waves created in sea by an earthquake, as they are called in Japan) which may cause shoreline changes, spectacular glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.

Volcanoes

 Volcanism includes the movement of

molten rock (magma) onto or toward the earth’s surface and also formation of many intrusive and extrusive volcanic forms.

 A volcano is formed when the molten

magma in the earth's interior escapes through the crust by vents and fissures in the crust, accompanied by steam,

gases (hydrogen sulphide, sulphur

dioxide, hydrogen chloride, carbon

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Depending on chemical composition and viscosity of the lava, a volcano may take various forms.

 Pyroclastic ==> adjective of or denoting

rock fragments or ash erupted by a volcano, especially as a hot, dense, destructive flow.

In this post we will study about Exogenetic Movements - Weathering.

Endogenetic Movements explained in the previous post

Watch video for quick and better understanding

http://www.youtube.com/watch?v=0VUo_ AX4Qac

Exogenetic Movements

 Exogenic (Exogenetic) processes are a

direct result of stress induced in earth materials due to various forces that come into existence due to sun’s heat.

 Force applied per unit area is called

stress. Stress is produced in a solid by pushing or pulling.

 Forces acting along the faces of earth

materials are shear stresses (separating forces). It is this stress that breaks rocks and other earth materials.

 Earth materials become subjected

to molecular stresses caused due to temperature changes.

 Chemical processes normally lead to

loosening of bonds between grains.

 Thus, the basic reason that leads to

weathering, erosion and deposition is development of stresses in the body of the earth materials.

 Temperature and precipitation are the two

important climatic elements that control various processes by inducing stress in earth materials.

Denudation

 All the exogenic geomorphic processes are

covered under a general term, denudation.

 The word ‘denude’ means to strip off or to

uncover.

 Weathering, mass wasting/movements,

erosion and transportation are included in denudation.

 Denudation mainly depends on rock type

and its structure that includes folds, faults, orientation and inclination of beds, presence or absence of joints, bedding

planes, hardness or softness of

constituent minerals, chemical

susceptibility of mineral constituents; the permeability or impermeability etc.

 The effects of most of the exogenic

geomorphic processes are small and slow but will in the long run affect the rocks severely due to continued fatigue.

Weathering

 Weathering is defined as mechanical

disintegration and chemical

decomposition of rocks through the actions of various elements of weather and climate.

 As very little or no motion of materials

takes place in weathering, it is an in-situ or on-site process.

 There are three major groups of

weathering processes: (i) chemical; (ii) physical or mechanical; (iii) biological weathering processes.

Chemical Weathering Processes

 A group of weathering processes viz;

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and reduction act on the rocks to decompose, dissolve or reduce them to a fine state.

 Water and air (oxygen and carbon dioxide)

along with heat speed up all chemical reactions.

Solution

 When something is dissolved in water or

acids, the water or acid with dissolved contents is called solution.

 On coming in contact with water many

solids disintegrate. Soluble rock forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.

 So, these minerals are easily leached out

without leaving any residue in rainy climates and accumulate in dry regions.

Carbonation

 Carbonation is the reaction of carbonate

and bicarbonate with minerals.

 Carbon dioxide from the atmosphere and

soil air is absorbed by water, to form carbonic acid that acts as a weak acid on various minerals.

Hydration

 Hydration is the chemical addition of

water.

 Minerals take up water and expand; this

expansion causes an increase in the volume of the material itself or rock.

 This process is reversible and long,

continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.

 The volume changes in minerals due to

hydration will also help in physical weathering

through exfoliation and granular disintegration.

Oxidation and Reduction

 In weathering, oxidation means a

combination of a mineral with oxygen to form oxides (rusting in case of iron) or hydroxides. Red soils appear red due to the presence of iron oxides.

 Oxidation occurs where there is ready

access to the atmosphere and water.

 The minerals most commonly involved in

this process are iron, manganese, sulphur etc.

 When oxidized minerals are placed in an

environment where oxygen is absent, reduction takes place.

 Such conditions exist usually below the

water table, in areas of stagnant water and waterlogged ground.

 Red colour of iron upon reduction turns to

greenish or bluish grey.

These weathering processes are

interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.

Biological activity and weathering

 Biological weathering is removal of

minerals and ions from the weathering environment and physical changes due to growth or movement of organisms.

 Burrowing and wedging by organisms like

earthworms, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.

 Human beings by disturbing vegetation,

ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials.

 Decaying plant and animal matter help in

the production of humic, carbonic and other acids which enhance decay and solubility of some elements.

 Algae utilise mineral nutrients for growth

and help in concentration of iron and manganese oxides.

 Plant roots exert a tremendous pressure

on the earth materials mechanically breaking them apart.

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Physical Weathering Processes

 Physical or mechanical weathering

processes depend on some applied forces like (i) gravitational forces (ii) expansion forces due to temperature changes, crystal growth or animal activity; (iii) water pressures controlled by wetting and drying cycles.

Unloading and Expansion

 Removal of overlying rock load because of

continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.

 In areas of curved ground surface, arched

fractures tend to produce massive sheets or exfoliation slabs of rock.

Granular Disintegration

 Rocks composed of coarse mineral grains

commonly fall apart grain by grain or undergo granular disintegration.

Exfoliation - Temperature Changes and Expansion

 With rise in temperature, every mineral

expands and pushes against its neighbor and as temperature falls, a corresponding contraction takes place.

 Because of diurnal changes in the

temperatures, this internal movement

among the mineral grains takes place regularly.

 This process is most effective in dry

climates and high elevations where diurnal temperature changes are drastic.

 The surface layers of the rocks tend to

expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface.

 Exfoliation results in smooth rounded

surfaces in rocks.

Block Separation

 This type of disintegration takes place in

rocks with numerous joints acquired by

mountain-making pressures or by

shrinkage due to cooling.

 This type of disintegration in rocks can be

achieved by comparatively weaker forces.

Shattering

 A huge rock may undergo disintegration

along weak zones to produce highly angular pieces with sharp corners and edges through the process of shattering.

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Freezing, Thawing and Frost Wedging

 During the warm season, the water

penetrates the pore spaces or fractures in rocks.

 During the cold season, the water freezes

into ice and its volume expands as a result.

 This exerts tremendous pressure on rock

walls to tear apart even where the rocks are massive.

 Frost weathering occurs due to growth of

ice within pores and cracks of rocks during repeated cycles of freezing and melting.

Salt Weathering

 Salts in rocks expand due to thermal

action, hydration and crystallisation.

 Many salts like calcium, sodium,

magnesium, potassium and barium have a tendency to expand.

 High temperature ranges in deserts favour

such salt expansion.

 Salt crystals in near-surface pores cause

splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may result in granular disintegration or granular foliation.

Mass Wasting

 Since gravity exerts its force on all matter,

both bedrock and the products of weathering tend to slide, roll, flow or creep

down all slopes in different types of earth and rock movements grouped under the term ‘mass wasting’.

Effects of Weathering

 Weathering and erosion tend to level down

the irregularities of landforms and create a

 The strong wind erosion leaves behind

whale-back shaped rocks in arid

landscape. These are called inselberg or ruware.

 Sometimes a solid layer of chemical

residue covers a soft rock. Sometimes, differential weathering of soft strata exposes the domelike hard rock masses, called tors. Tors are a common feature of South Indian landscape.

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Tors

Weathering and Erosion

 Lead to simultaneous process of

‘degradation' and ‘aggradation'.

 Erosion is a mobile process while

weathering is a static process

[disintegrated material do not involve any motion except the falling down under force of gravity].

Significance of weathering

 Weathering is the first step in formation of

soils.

 Weathering of rocks and deposits helps in

the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc.

 Weathering helps in soil enrichment.

 Without weathering, the concentration of

the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is what is called enrichment.

Another important process in Exogenetic movements is erosion. We will study about erosion in Indian Geography.

In this post we will study

about Continental Drift Theory. Various theories were proposed to explain the present location of continents. Continental Drift Theory was the earliest. It was later overshadowed by successful theories like 'See Floor Spreading theory' and

'Plate Tectonics'. We will study

about 'See Floor Spreading theory' and 'Plate Tectonics' in the future posts.

Watch video for quick and better understanding

http://www.youtube.com/watch?v=ysGYLrkrxmo

Continental Drift Theory -

Tectonics

 Tectonics == Large scale movement of

lithospheric plates.

Introduction

 During WW II, scientists discovered that

the ocean floor was not a flat surface but had some unique relief features like ridges, trenches, seamounts, shoals etc.

 The most important discoveries were

ridges and trenches which gave insights

into natural boundaries between

various lithospheric plates (sometime

called as crustal plates or tectonic

plates)

 These important discoveries led to the

theory of Plate Tectonics.

Plate Tectonics

 Plate tectonics is the large scale movement

of lithospheric plates due to forces emanating from earth’s interior.

 Prior to the theory of ‘Plate Tectonics’,

there were other theories like ‘Continental Drift Theory’ and ‘See Floor Spreading Theory’ which tried to explain the large scale movements on earth’s surface.

 In this post, we will study about

‘Continental Drift Theory’.

Important theories

1. Polar wandering (Similar to Continental Drift Theory)

2. Continental DriftTheory (CDT) 3. Convectional Current Theory (CCT) 4. Sea Floor Spreading Theory (SFST)

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5. Plate Tectonics (PT)

 Polar wandering is the relative movement

of the earth's crust and upper mantle with respect to the rotational poles of the earth.

 Continental drift refers to the movement of

the continents relative to each other.

 Convectional current theory forms the

basis of SFST and PT.

 Sea floor spreading describes the

movement of oceanic plates relative to one another.

 Plate tectonics is simply the movement of

crustal plates relative to each other.

Continental Drift Theory (Alfred

Wegener, 1922)

 This theory was suggested by Alfred

Wegener in 1920’s.

 According to Wegener’s Continental Drift

Theory, there existed one big landmass which he called Pangaea which was

covered by one big ocean

called Panthalassa.

 A sea called Tethys divided

the Pangaea into two huge

landmasses: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys.

 Drift started around 200 million years

ago (Mesozoic Era), and the continents began to break up and drift away from one another.

 To get a rough idea about Geologic

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Force for Continental Drift

The drift was in two directions-

1. equator wards due to the interaction of

forces of gravity, pole-fleeing

force and buoyancy (ship floats in water due to buoyant force offered by water), and

2. westwards due to tidal currents because of the earth’s motion (earth rotates form west to east, so tidal currents act from east to west. Watch video for better understanding).

 Wegener suggested that tidal force also

played a major role.

 The polar-fleeing force relates to the

rotation of the earth. You are aware of the fact that the earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth. [Greater

Centrifugal force at the equator.

Centrifugal force increases as we move

from poles towards equator. This increase in centrifugal force has led to pole fleeing].

 Tidal force is due to the attraction of the

moon and the sun that develops tides in oceanic waters.

 Wegener believed that these forces would

become effective when applied over many million years.

 According to Wegener, the drift is still

continuing.

Evidence in support of

Continental Drift

Apparent Affinity of Physical Features

 South America and Africa seem to fit in

with each other, especially, the bulge of Brazil fits into the Gulf of Guinea.

 Greenland seems to fit in well with

Ellesmere and Baffin islands.

 The east coast of India, Madagascar and

Africa seem to have been joined.

 North and South America on one side and

Africa and Europe on the other fit along the mid-Atlantic ridge.

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 The Caledonian and Hercynian

mountains of Europe and

the Appalachians of USA seem to be one continuous series.

Caledonian mountains

Caledonian and Hercynian mountains Criticism

 Coastlines are a temporary feature and are

liable to change.

 Several other combinations of fitting in of

landforms could be attempted.

 Continental Drift Theory shifts India's

position too much to the south, distorting its relation with the Mediterranean Sea and the Alps.

 The mountains do not always exhibit

geological affinity.

Causes of Drift

 Gravity of the earth, buoyancy of the seas

and the tidal currents were given as the main factors causing the drift, by Wegener.

Criticism

 This is illogical because for these factors to

be able to cause a drift of such a magnitude, they will have to be millions of times stronger.

Polar wandering (Shifting of Poles)

 The poles drifted constantly.

Criticism

 Poles may have shifted, not necessarily the

continents (don’t think deep).

Distribution of Fossils

 The observations that Lemurs occur

in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.

 Mesosaurus was a small reptile adapted to

shallow brackish water. The skeletons of these are found only in South Africa and Iraver formations of Brazil. The two localities presently are 4,800 km apart with an ocean in between them.

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 Presence of glossopteris vegetation in

carboniferous rocks of India, Australia, South Africa, Falkland Islands (Overseas territory of UK), Antarctica, etc. can be explained on the basis of the fact that parts were linked in the past.

Criticism

 Such vegetation is also found in the

northern parts like Afghanistan, Iran and Siberia.

 Similar vegetation found in unrelated

parts of the world.

Rocks of Same Age Across the Oceans

 The belt of ancient rocks of 2,000 million

years from Brazil coast matches with those from western Africa.

Criticism

 Rocks of same age and similar

characteristics are found in other parts of the world too.

Tillite deposits

 It is the sedimentary rock formed out of

deposits of glaciers. The Gondwana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere.

 At the base the system has thick Tillite

indicating extensive and prolonged

glaciation. Counter parts of this

succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India.

 Overall resemblance of the Gondwana type

sediments clearly demonstrates that these landmasses had remarkably similar histories.

 The glacial Tillite provides unambiguous

evidence of palaeoclimates and also of drifting of continents.

Placer Deposits

 Rich placer deposits of gold are found on

the Ghana coast (West Africa) but the source (gold bearing veins) are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.

Drawbacks of Continental Drift

Theory

 Wegener failed to explain why the drift

began only in Mesozoic era and not before.

 The theory doesn’t take oceans into

consideration.

 Proofs heavily depend

on assumptions and are very general in nature.

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 Forces like buoyancy, tidal currents and

gravity are too weak to be able to move continents.

 Modern theories (PT) accept the existence

of Pangaea and related landmasses but give a very different explanation to the causes of drift.

In this post we will study about a very important concept called See Floor Spreading. Before jumping directly into the concept of See Floor Spreading, we must understand some basic concepts that form the corner stones for the concept of See Floor Spreading. These corner stones are Convectional Current Theory and Paleomagnetism.

Previous Post: Continental Drift Theory –

Tectonics [Must Read this post before trying to understand See Floor Spreading] These topics are somewhat technical and it can be hard to understand them just by reading the post. So I made a video with all the required explanation. I strong suggest you to first watch the video before trying to understand these complex concepts.

http://www.youtube.com/watch?v=w3xaX 6gkecg

Convectional Current Theory -

Tectonics

 Arthur Holmes in 1930s discussed the

possibility of convection currents in the mantle.

 These currents are generated due to

radioactive elements causing thermal differences in mantle.

Mapping of the Ocean Floor

 Detailed research during World Wars

revealed that the ocean floor is not just a vast plain but it is full of relief with mountain ranges, deep trenches.

 The mid-oceanic ridges were found to be

most active in terms of volcanic eruptions.

 The dating of the rocks from the oceanic

crust revealed the fact that the latter is much younger than the continental areas (Rocks on ocean floor are much younger than those on the continents).

 Rocks on either side of the crest of oceanic

ridges and having equidistant locations

from the crest were found to

have remarkable similarities both in

terms of their constituents and their age.

Distribution of Earthquakes and

Volcanoes

 Volcanism and associated earthquakes at

plate margins are a direct consequence of convection currents in the mantle.

 Dots in the central parts of the Atlantic

Ocean and other oceans are almost parallel to the coastlines.

 In general, the foci of the earthquake in

the areas of mid-oceanic ridges are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones (deep focus earthquakes are more destructive).

 The map of volcanoes also shows a similar

pattern. The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area.

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These observations (ocean floor and the distribution of earthquakes and volcanoes) led to the theory of See Floor Spreading. Convectional Current Theory is the soul of See Floor Spreading theory.

Convectional Current Theory

 According to this theory, the intense heat

generated by radioactive substances in the mantle (100-2900 km below the earth surface) seeks a path to escape, and gives rise to the formation of convention currents in the mantle.

 Wherever rising limbs of these currents

meet, oceanic ridges are formed on the sea floor and wherever the failing limbs meet, trenches are formed.

Paleomagnetism

 It is the study of the record of the Earth's

magnetic field in rocks, sediment etc.. Why do we need to study this concept?

 Paleomagnetic rocks on either side of the

submarine ridges provide the most

important evidence to the concept of Sea Floor Spreading.

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Paleo == Rocks; Paleomagnetism == magnetism in rocks.

 Certain minerals in rocks lock-in a record

of the direction and intensity of the magnetic field when they form.

 This record provides information on the

past behavior of Earth's magnetic field and the past location of tectonic plates.

 Paleomagnetists led the revival of the

continental drift hypothesis and its

transformation into plate tectonics.

 Paleomagnetic studies of rocks and ocean

sediment have demonstrated that the orientation of the earth's magnetic field has frequently alternated over geologic time.

 Periods of "normal" polarity (i.e., when

the north-seeking end of the compass needle points toward the present north magnetic pole, as it does today) have alternated with periods of "reversed" polarity (when the north-seeking end of the compass needle points southward)[I have explained this in detail in the video].

 As today's magnetic field is close to the

earth's rotational axis, continental drift could be tested by ascertaining the magnetic characteristics of ancient rocks.

Paleomagnetism: Strong evidence

of See Floor Spreading and Plate

Tectonics

 Some of the strongest evidence in support

of the theory of see floor spreading and plate tectonics comes from studying the magnetic fields surrounding oceanic ridges.

 Rocks formed from this underwater

volcanic activity were mainly basalt, which

is low silica, iron-rich, volcanic

rock that makes up most of the ocean floor.

 Basalt contains magnetic minerals and as

the rock is solidifying, these

minerals align themselves in the

direction of the magnetic field.

 This basically locks in a record of which

way the magnetic field was positioned at the time that part of the ocean floor was created.

 Paleomagnetists [scientists who study past

magnetic fields], took a look at the ocean floor going out away from oceanic ridges (either side of the oceanic ridges), they found magnetic stripes that were flipped so that one stripe would be normal polarity and the next reversed.

How could this be?

 These oceanic ridges were actually

boundaries with tectonic plates pulling apart.

 This movement of the plates allowed the

magma to rise up and harden into new rock.

 As the new rock was formed near the

ridge, older rock, which formed millions of years ago when the magnetic field was reversed, got pushed farther away, resulting in this magnetic striping.

 Rising magma assumes the polarity of

Earth’s geomagnetic field before it solidifies into oceanic crust.

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 At spreading centres, this crust is

separated into parallel bands of rock by successive waves of emergent magma.

 When Earth’s geomagnetic field undergoes

a reversal, the change in polarity is recorded in the magma, which contributes to the alternating pattern of magnetic striping on the seafloor.

Concept of Sea Floor Spreading

 The idea that the seafloor itself moves (and

carries the continents with it) as it expands from a central axis was proposed by Harry Hess.

 According to this theory, the intense heat

generated by radioactive substances in the mantle (100-2900 km below the earth surface) seeks a path to escape, and gives rise to the formation of convention currents in the mantle.

 Wherever rising limbs of these currents

meet, oceanic ridges are formed on the sea floor and wherever the failing limbs meet, trenches are formed.

 Seafloor spreading is a process that occurs

at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

 Seafloor spreading helps explain

continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.

 Basaltic magma rises up the fractures and

cools on the ocean floor to form new sea floor.

 Older rocks will be found farther away

from the spreading zone while younger rocks will be found nearer to the spreading zone.

Evidences

 The mapping of the ocean floor and

Paleomagnetic studies of rocks from oceanic regions revealed the following facts :

1. Volcanic eruptions are common all along the midoceanic ridges and they bring huge amounts of lava to the surface in this area.

2. The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities

3. Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest.

4. The age of the rocks increases as one moves away from the crest.

5. The deep trenches have deep-seated earthquake occurrences while in the midoceanic ridge areas, the quake foci have shallow depths.

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 It was on the basis of the continental drift

theory, theory of sea floor spreading, that

the theory of Plate Tectonics was

formulated—first outlined by Morgan in 1968.

 So, next post will be a detailed explanation

on Plate Tectonics.

 In this post we will study about one of the

most important concept of geomorphology called Plate Tectonics. This is the third post in ‘Tectonics’ after ‘Continental Drift Theory’ and ‘See Floor Spreading Theory’.

Plate Tectonics

http://www.youtube.com/watch?v=8hMD 52kUhRk

 In 1967, McKenzie and Parker suggested

the theory of plate tectonics. The theory was later outlined by Morgan in 1968.

 By then, the ‘continental drift theory’ was

completely discarded with the emergence of ‘convectional current theory’ and ‘see floor spreading theory’.

 Both ‘convectional current theory’ and ‘see

floor spreading’ paved the way for the Theory of Plate Tectonics.

Theory

 According to the theory of plate tectonics,

the

earth’s lithosphere is broken into distinc t plates which are floating on a ductile layer called asthenosphere (upper

mantle). Plates move horizontally over the asthenosphere as rigid units.

 The lithosphere includes

the crust and top mantle with its

thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.

 The oceanic plates contain mainly

the Simatic crust and are relatively thinner, while the continental plates contain Sialic material and are relatively thicker.

 Lithospheric plates (sometimes

called crustal plates, tectonic plates)

vary from minor plates to major

plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), sometime a combination of both continental and oceanic plates (Indo-Australian plate).

 The movement of these crustal plates

causes the formation of various landforms and is the principal cause of all earth movements.

Rates of Plate Movement

 The Arctic Ridge has the slowest rate (less

than 2.5 cm/yr), and the East Pacific Rise in the South Pacific [about 3,400 km west of Chile], has the fastest rate (more than 15 cm/yr).

 Indian plate’s movement during its

journey from south to equator was one of the fastest plate movements.

Major tectonic plates

1. Antarctica and the surrounding oceanic plate

2. North American plate 3. South American plate 4. Pacific plate

5. India-Australia-New Zealand plate

6. Africa with the eastern Atlantic floor plate 7. Eurasia and the adjacent oceanic plate

Minor tectonic plates

1. Cocos plate: Between Central America and Pacific plate

2. Nazca plate: Between South America and Pacific plate

3. Arabian plate: Mostly the Saudi Arabian landmass

4. Philippine plate: Between the Asiatic and Pacific plate

5. Caroline plate: Between the Philippine and Indian plate (North of New Guinea)

6. Fuji plate: North-east of Australia. 7. Turkish plate,

8. Aegean plate (Mediterranean region), 9. Caribbean plate,

10. Juan de Fuca plate (between Pacific

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11. Iranian plate.

 There are many more minor plates other

than the above mentioned plates. Most of the these minor plates were formed due to stress created by converging major plates. Example: the Mediterranean Sea is divided

into numerous minor plates due to the compressive force exerted by Eurasian and African plates.

 The figure below shows the changes in

landform with time due to the interaction of various plates.

25.Geography Notes PDF - Print Friendly 22-06-2016.pdf

01.Climatology.pdf

02.Climatic Regions.pdf

03.Oceanography.pdf

04.Indian Geography.pdf

05.Economic Geography.pdf

Table of Contents

INTERIOR OF THE EARTH

Why know about earth’s interior

Sources of information about the

interior

Some sources explained in detail

Seismic waves

What causes earthquakes?

Earthquake Waves

Behavior of Earthquake Waves

Propagation of Earthquake Waves

Emergence of Shadow Zone

But how these properties of ‘P’

and ‘S’ waves help in determining

the earth’s interior?

Earth’s Layers

Earth’s Layers based on chemical

properties

Earth’s Chemical Composition

Composition of Earth’s Crust

Earth Movements

Geomorphic processes

Earth Movements - Endogenetic

Movements

Diastrophism

Sudden Movements

Exogenetic Movements

Denudation

Weathering

Chemical Weathering Processes

Biological activity and weathering

Physical Weathering Processes

Effects of Weathering

Weathering and Erosion

Significance of weathering

Continental Drift Theory -

Tectonics

Introduction

Plate Tectonics

Important theories

Continental Drift Theory (Alfred

Wegener, 1922)

Force for Continental Drift

Evidence in support of

Continental Drift

Drawbacks of Continental Drift

Theory

Convectional Current Theory -

Tectonics

Mapping of the Ocean Floor

Distribution of Earthquakes and

Volcanoes

Convectional Current Theory

Paleomagnetism

Paleomagnetism: Strong evidence

of See Floor Spreading and Plate

Tectonics

Concept of Sea Floor Spreading

Evidences

Plate Tectonics

Theory

Rates of Plate Movement

Major tectonic plates

Minor tectonic plates