NCERT- Complete Geography

8/21/2019 NCERT- Complete Geography

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CONTENTS

FOREWORD iii

UNIT I : GEOGRAPHYASADISCIPLINE 1-12

1. Geography as a Discipline 2

UNIT II : THEEARTH 13-38

2. The Origin and Evolution of the Earth 14

3. Interior of the Earth 214. Distribution of Oceans and Continents 30

UNIT III : LANDFORMS 39-74

5. Minerals and Rocks 40

6. Geomorphic Processes 45

7. Landforms and their Evolution 58

UNIT IV : CLIMATE 75-110

8. Composition and Structure of Atmosphere 76

9. Solar Radiation, Heat Balance and Temperature 7910. Atmospheric Circulation and Weather Systems 88

11. Water in the Atmosphere 98

12. World Climate and Climate Change 103

UNIT V : WATER(OCEANS) 111-125

13. Water (Oceans) 112

14. Movements of Ocean Water 120

UNIT VI : LIFEONTHEEARTH 126-140

15. Life on the Earth 127

16. Biodiversity and Conservation 135

GLOSSARY 141-144
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GEOGRAPHYASADISCIPLINE

This unit deals with Geography as an integrating discipline; as a science of spatial

attributes

Branches of geography; importance of physical geography

UNITI


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Y

ou have studied geography as one of thecomponents of your social studies courseupto the secondary stage. You are

already aware of some of the phenomena ofgeographical nature in the world and itsdifferent parts. Now, you will study Geographyas an independent subject and learn about thephysical environment of the earth, humanactivities and their interactive relationships.

Therefore, a pertinent question you can ask atthis stage is Why should we studygeography? We live on the surface of the earth.Our lives are affected by our surroundings inmany ways. We depend on the resources tosustain ourselves in the surrounding areas.

Primitive societies subsisted on natural meansof subsistence, i.e. edible plants and animals.With the passage of time, we developedtechnologies and started producing our foodusing natural resources such as land, soil and

water. We adjusted our food habits andclothing according to the prevailing weatherconditions. There are variations in the naturalresource base, technological development,adaptation with and modification of physicalenvironment, social organisations and culturaldevelopment. As a student of geography, youshould be curious to know about all thephenomena which vary over space. You learnabout the diverse lands and people. Youshould also be interested in understanding thechanges which have taken place over time.Geography equips you to appreciate diversityand investigate into the causes responsible forcreating such variations over time and space.

You will develop skills to understand the globeconverted into maps and have a visual sense

of the earths surface. The understanding andthe skills obtained in modern scientifictechniques such as GIS and computer

cartography equip you to meaningfullycontribute to the national endeavour fordevelopment.

Now the next question which you may liketo ask is What is geography? You know thatearth is our home. It is also the home of manyother creatures, big and small, which live onthe earth and sustain. The earths surface isnot uniform. It has variations in its physicalfeatures. There are mountains, hills, valleys,plains, plateaus, oceans, lakes, deserts and

wilderness. There are variations in its socialand cultural features too. There are villages,cities, roads, railways, ports, markets andmany other elements created by human beingsacross the entire period of their culturaldevelopment.

This variation provides a clue to theunderstanding of the relationship between thephysical environment and social/culturalfeatures. The physical environment hasprovided the stage, on which human societiesenacted the drama of their creative skills withthe tools and techniques which they inventedand evolved in the process of their cultural

development. Now, you should be able toattempt the answer of the question posedearlier as to What is geography? In verysimple words, it can be said that geographyis the description of the earth. The termgeographywas first coined by Eratosthenese,a Greek scholar (276-194 BC.). The word has

been derived from two roots from Greeklanguage geo(earth) and graphos(description).

GEOGRAPHYASADISCIPLINE

C H A P T E R


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GEOGRAPHY AS A DISCIPLINE 3

Put together, they mean description of the earth.The earth has always been seen as the abodeof human beings and thus, scholars definedgeography as, the description of the earth as

the abode of human beings. You are aware ofthe fact that reality is always multifaceted andthe earth is also multi-dimensional, that is

why many disciplines from natural sciencessuch as geology, pedology, oceanography,

botany, zoology and meteorology and anumber of sister disciplines in social sciencessuch as economics, history, sociology, politicalscience, anthropology, etc. study differentaspects of the earths surface. Geography isdifferent from other sciences in its subjectmatter and methodology but at the same time,

it is closely related to other disciplines.Geography derives its data base from all thenatural and social sciences and attempts theirsynthesis.

We have noted that there exist variationsover the surface of the earth in its physical as

well as cultural environment. A number ofphenomena are similar and many are dissimilar.It was, therefore, logical to perceive geographyas the study of areal differentiation. Thus,geography was perceived to study all thosephenomena which vary over space.Geographers do not study only the variationsin the phenomena over the earths surface(space) but also study the associations withthe other factors which cause these variations.For example, cropping patterns differ fromregion to region but this variation in croppingpattern, as a phenomenon, is related to

variations in soils, climates, demands in themarket, capacity of the farmer to invest andtechnological inputs available to her/him.

Thus, the concern of geography is to find outthe causal relationship between any twophenomena or between more than one

phenomenon.A geographer explains the phenomena in

a frame of cause and effect relationship, as itdoes not only help in interpretation but alsoforesees the phenomena in future.

The geographical phenomena, both thephysical and human, are not static but highlydynamic. They change over time as a result ofthe interactive processes between ever

changing earth and untiring and ever-activehuman beings. Primitive human societies weredirectly dependent on their immediateenvironment. Geography, thus, is concerned

wi th the study of Nature and Humaninteractions as an integrated whole. Humanis an integral part of nature and nature hasthe imprints of human. Nature has influenceddifferent aspects of human life. Its imprints can

be noticed on food, clothing, shelter andoccupation. Human beings have come to terms

wi th nature through adapta tion andmodification. As you already know, the presentsociety has passed the stage of primitivesocieties, which were directly dependent ontheir immediate physical environment for

sustenance. Present societies have modifiedtheir natural environment by inventing andusing technology and thus, have expanded thehorizon of their operation by appropriating andutilising the resources provided by nature. Withthe gradual development of technology, human

beings were able to loosen the shackles of theirphysical environment. Technology helped inreducing the harshness of labour, increasedlabour efficiency and provided leisure tohuman beings to attend to the higher needs oflife. It also increased the scale of productionand the mobility of labour.

The interaction between the physicalenvironment and human beings has been verysuccinctly described by a poet in the followingdialogue between human and nature (God).You created the soil, I created the cup, youcreated night, I created the lamp. You createdwilderness, hilly terrains and deserts; Icreated flower beds and gardens. Human

beings have claimed their contribution usingnatural resources.With the help of technology,human beings moved from the stage ofnecessity to a stage of freedom. They have put

their imprints everywhere and created newpossibilities in collaboration with nature. Thus,

we now find humanised nature andnaturalised human beings and geographystudies this interactive relationship. The spacegot organised with the help of the means oftransportation and communication network.

The links (routes) and nodes (settlements of alltypes and hierarchies) integrated the space and


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FUNDAMENTALS OF PHYSICAL GEOGRAPHY4

gradually, it got organised. As a social sciencediscipline, geography studies the spatialorganisation and spatial integration.

Geography as a discipline is concerned with

three sets of questions:(i) Some questions are related to the

identification of the patterns of naturaland cultural features as found over thesurface of the earth. These are thequestions about what?

(ii) Some questions are related to thedistribution of the natural and human/cultural features over the surface of theearth. These are the questions aboutwhere?

Taken together, both these questions take

care of distributional and locational aspects ofthe natural and cultural features. Thesequestions provided inventorised information of

what features and where located. It was a verypopular approach during the colonial period.

These two questions did not make geographya scientific discipline till the third question wasadded. The third question is related to theexplanation or the causal relationships

between features and the processes andphenomena. This aspect of geography is relatedto the question, why?

Geography as a discipline is related tospace and takes note of spatial characteristicsand attributes. It studies the patterns ofdistribution, location and concentration ofphenomena over space and interprets themproviding explanations for these patterns. Ittakes note of the associations and inter-relationships between the phenomena overspace and interprets them providingexplanations for these patterns. It also takesnote of the associations and inter-relationships

between the phenomena resulting from the

dynamic interaction between human beingsand their physical environment.

GEOGRAPHYASANINTEGRATINGDISCIPLINE

Geography is a discipline of synthesis. Itattempts spatial synthesis, and historyattempts temporal synthesis. Its approach isholistic in nature. It recognises the fact thatthe world is a system of interdependencies. The

present world is being perceived as a globalvillage. The distances have been reduced bybetter means of transportation increasingaccessibility. The audio-visual media and

information technology have enriched the database. Technology has provided better chancesof monitoring natural phenomena as well asthe economic and social parameters.Geography as an integrating discipline hasinterface with numerous natural and socialsciences. All the sciences, whether natural orsocial, have one basic objective, ofunderstanding the reality. Geographyattempts to comprehend the associations ofphenomena as related in sections of reality.Figure 1.1 shows the relationship of geography

with other sciences. Every discipline, concernedwith scientific knowledge is linked withgeography as many of their elements vary overspace. Geography helps in understanding thereality in totality in its spatial perspective.Geography, thus, not only takes note of thedifferences in the phenomena from place toplace but integrates them holistically whichmay be different at other places. A geographeris required to have a broad understanding ofall the related fields, to be able to logicallyintegrate them. This integration can beunderstood with some examples. Geographyinfluences historical events. Spatial distanceitself has been a very potent factor to alter thecourse of history of the world. Spatial depthprovided defence to many countries,particularly in the last century. In traditional

warfare, countries with large size in area, gaintime at the cost of space. The defence provided

by oceanic expanse around the countries ofthe new world has protected them from wars

being imposed on their soil. If we look at thehistorical events world over, each one of themcan be interpreted geographically.

In India, Himalayas have acted as greatbarriers and provided protection but thepasses provided routes to the migrants andinvaders from Central Asia. The sea coast hasencouraged contact with people from East andSoutheast Asia, Europe and Africa. Navigationtechnology helped European countries tocolonise a number of countries of Asia and

Africa, including India as they got accessibility


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through oceans. The geographical factors havemodified the course of history in different partsof the world.

Every geographical phenomenon undergoes

change through time and can be explainedtemporally. The changes in landforms, climate,vegetation, economic activities occupations andcultural developments have followed a definitehistorical course. Many geographical featuresresult from the decision making process bydifferent institutions at a particular point oftime. It is possible to convert time in terms ofspace and space in terms of time. For example,it can be said that place A is 1,500 km fromplace B or alternately, it can also be said thatplace A is two hours away (if one travels byplane) or seventeen hours away (if one travels

by a fast moving train). It is for this reason,time is an integral part of geographical studiesas the fourth dimension. Please mention otherthree dimensions?

Figure1.1 amply depicts the linkages ofgeography with different natural and socialsciences. This linkage can be put under twosegments.

Physical Geography and Natural Sciences

All the branches of physical geography, asshown in Figure 1.1, have interface with natural

sciences. The traditional physical geographyis linked with geology, meteorology, hydrologyand pedology, and thus, geomorphology,climatology, oceanography and soil geographyrespectively have very close link with thenatural sciences as these derive their data fromthese sciences. Bio-Geography is closely relatedto botany, zoology as well as ecology as human

beings are located in different locational niche.A geographer should have some proficiency

in mathematics and art, particularly in drawingmaps. Geography is very much linked with thestudy of astronomical locations and deals withlatitudes and longitudes. The shape of the earthis Geoidbut the basic tool of a geographer is amap which is two dimensional representationof the earth. The problem of converting geoidsinto two dimensions can be tackled byprojections constructed graphically ormathematically. The cartographic andquantitative techniques require sufficientproficiency in mathematics, statistics and

econometrics. Maps are prepared throughartistic imagination. Making sketches, mentalmaps and cartographic work requireproficiency in arts.

Geography and Social Sciences

Each social science sketched in Figure 1.1hasinterface with one branch of geography. Therelationships between geography and historyhave already been outlined in detail. Everydiscipline has a philosophy which is the raisondetrefor that discipline. Philosophy providesroots to a discipline and in the process of itsevolution, it also experiences distinct historicalprocesses. Thus, the history of geographicalthought as mother branch of geography is

included universally in its curricula. All thesocial science disciplines, viz. sociology,political science, economics and demographystudy different aspects of social reality. The

branches of geography, viz. social, political,economic and population and settlements areclosely linked with these disciplines as eachone of them has spatial attributes. The coreconcern of political science is territory, peopleand sovereignty while political geography isalso interested in the study of the state as aspatial unit as well as people and their political

behaviour. Economics deals with basicattributes of the economy such as production,distribution, exchange and consumption. Eachof these attributes also has spatial aspects andhere comes the role of economic geography tostudy the spatial aspects of production,distribution, exchange and consumption.Likewise, population geography is closelylinked with the discipline of demography.

The above discussi on shows thatgeography has strong interface with naturaland social sciences. It follows its own

methodology of study which makes it distinctfrom others. It has osmotic relationship withother disciplines. While all the disciplines havetheir own individual scope, this individualitydoes not obstruct the flow of information as incase of all cells in the body that have individualidentity separated by membranes but the flowof blood is not obstructed. Geographers usedata obtained from sister disciplines and


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Figure1.1:Geogra

phyanditsrelationwithothersubj

ects


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attempt synthesis over space. Maps are very

effective tools of geographers in which the

tabular data is converted into visual form to

bring out the spatial pattern.

BRANCHESOFGEOGRAPHY

Please study Figure 1.1 for recapitulation. It has

very clearly brought out that geography is an

interdisciplinary subject of study. The study of

every subject is done according to some

approach. The major approaches to study

geography have been (i) Systematic and

(ii) Regional. The systematic geography approach

is the same as that of general geography. This

approach was introduced by Alexander Von

Humboldt, a German geographer (1769-1859)while regional geography approach was

developed by another German geographer and a

contemporary of Humboldt, Karl Ritter

(1779-1859).

In systematic approach (Figure 1.2), a

phenomenon is studied world over as a whole,

and then the identification of typologies or

spatial patterns is done. For example, if one is

interested in studying natural vegetation, the

study will be done at the world level as a first

step. The typologies such as equatorial rain

forests or softwood conical forests or monsoon

forests, etc. will be identified, discussed and

delimited. In the regional approach, the world

is divided into regions at different hierarchical

levels and then all the geographical phenomena

in a particular region are studied. These

regions may be natural, political or designated

region. The phenomena in a region are studied

in a holistic manner searching for unity in

diversity.

Dualism is one of the main characteristics

of geography which got introduced from thevery beginning. This dualism depended on the

aspect emphasised in the study. Earlier scholars

laid emphasis on physical geography. But

human beings are an integral part of the earths

surface. They are part and parcel of nature. They

also have contributed through their cultural

development. Thus developed human

geography with emphasis on human activities.

BRANCHESOFGEOGRAPHY(BASEDONSYSTEMATIC APPROACH)

1. Physical Geography

(i) Geomorphology is devoted to the studyof landforms, their evolution and relatedprocesses.

(ii) Climatologyencompasses the study ofstructure of atmosphere and elementsof weather and climates and climatictypes and regions.

(iii) Hydrologystudies the realm of waterover the surface of the earth includingoceans, lakes, rivers and other water

bodies and its effect on different life

forms including human life and theiractivities.

(iv) Soil Geographyis devoted to study theprocesses of soil formation, soil types,their fertility status, distribution anduse.

2. Human Geography

(i) Social/Cultural Geography encom-passes the study of society and itsspatial dynamics as well as the culturalelements contributed by the society.

(ii) Population and Settlement Geography(Rural and Urban). It studies populationgrowth, distribution, density, sex ratio,migration and occupational structureetc. Settlement geography studies thecharacteristics of rural and urbansettlements.

(iii) Economic Geographystudies economicactivities of the people includingagriculture, industry, tourism, trade,and transport, infrastructure andservices, etc.

(iv) Historical Geography studies thehistorical processes through which thespace gets organised. Every region hasundergone some historical experiences

before attaining the present day status.The geographica l fea tures alsoexperience temporal changes and theseform the concerns of historicalgeography.


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Figure 1.2 : Branches of geography based on systematic approach

(v) Political Geography looks at the space

from the angle of political events and

studies boundaries, space relations

between neighbouring political units,

delimitation of constituencies, election

scenario and develops theoretical

frameworkto understand the politicalbehaviour of the population.

3. Biogeography

The interface between physical geographyand human geography has lead to thedevelopment of Biogeography whichincludes:(i) Plant Geography which studies the

spatial pattern of natural vegetation intheir habitats.


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(ii) Zoo Geography which studies thespatial patterns and geographiccharacteristics of animals and theirhabitats.

(iii) Ecology /Ecosystem deals with thescientific study of the habitatscharacteristic of species.

(iv) Environmental Geography concernsworld over leading to the realisation ofenvironmental problems such as landgradation, pollution and concerns forconservation has resulted in theintroduction of this new branch ingeography.

BRANCHESOFGEOGRAPHYBASEDONREGIONALAPPROACH (FIGURE1.3)

1. Regional Studies/Area StudiesComprising Macro, Meso and MicroRegional Studies

2. Regional PlanningComprising Country/Rural and Town/Urban Planning

3. Regional Development4. Regional Analysis

There are two aspects which are commonto every discipline, these are:(i) Philosophy

(a) Geographical Thought(b) Land and Human Interaction/

Human Ecology(ii) Methods and Techniques

(a) Cartography including ComputerCartography

(b) Quantitative Techniques/Statistical

Techniques

(c) Field Survey Methods

(d) Geo-informatics comprising

techniques such as Remote

Sensing, GIS, GPS, etc.

The above classification gives acomprehensive format of the branches of

geography. Generally geography curricula is

taught and learnt in this format but this

format is not static. Any discipline is bound

to grow with new ideas, problems, methods

and techniques. For example, what was once

manual cartography has now been

transformed into computer cartography.

Technology has enabled scholars to handle

large quantum of data. The internet provides

extensive information. Thus, the capacity to

attempt analysis has increased tremendously.

GIS has further opened vistas of knowledge.

GPS has become a handy tool to find out exact

locations. Technologies have enhanced the

capacity of attempting synthesis with sound

theoretical understanding.

You will learn some preliminary aspects of

these techniques in your book,Practical work

in Geography Part I (NCERT, 2006).You will

continue to improve upon your skills andlearn about their application.

PHYSICALGEOGRAPHYAND ITSIMPORTANCE

This chapter appears in the book entitledFundamentals of Physical Geography. Thecontents of the book clearly reflect its scope.It is therefore, appropriate to know theimportance of this branch of geography.

Figure 1.3 : Branches of geography based on regional approach


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Physical geography includes the study oflithosphere (landforms, drainage, relief andphysiography), atmosphere (its composition,structure, elements and controls of weather

and climate; temperature, pressure, winds,precipitation, climatic types, etc.), hydrosphere(oceans, seas, lakes and associated features

with water realm) and biosphere ( life formsincluding human being and macro-organismand their sustaining mechanism, viz. foodchain, ecological parameters and ecological

balance). Soils are formed through the processofpedogenesisand depend upon the parentrocks, climate, biological activity and time.

Time provides maturity to soils and helps inthe development of soil profiles. Each element

is important for human beings. Landformsprovide the base on which human activities arelocated. The plains are utilised for agriculture.Plateaus provide forests and minerals.Mountains provide pastures, forests, touristspots and are sources of rivers providing waterto lowlands. Climate influences our housetypes, clothing and food habits. The climatehas a profound effect on vegetation, croppingpattern, livestock farming and someindustries, etc. Human beings have developedtechnologies which modify climatic elements

in a restricted space such as air conditionersand coolers. Temperature and precipitationensure the density of forests and quality ofgrassland. In India, monsoonal rainfall sets theagriculture rhythm in motion. Precipitationrecharges the ground water aquiferswhichlater provides water for agriculture anddomestic use. We study oceans which are thestore house of resources. Besides fish and other

sea-food, oceans are rich in mineral resources.India has developed the technology forcollecting manganese nodules from oceanic

bed. Soils are renewable resources, which

influence a number of economic activities suchas agriculture. The fertility of the soil is bothnaturally determined and culturally induced.Soils also provide the basis for the biosphereaccommodating plants, animals and microorganisms.

The study of phys ical geography isemerging as a discipline of evaluating andmanaging natural resources. In order toachieve this objective, it is essential tounderstand the intricate relationship betweenphysical environment and human beings.Physical environment provides resources, andhuman beings utilise these resources andensure their economic and culturaldevelopment. Accelerated pace of resourceutilisation with the help of modern technologyhas created ecological imbalance in the world.Hence, a better understanding of physicalenvironment is absolutely essential forsustainable development.

What is Geography?

Geography is concerned with the descriptionand explanation of the areal differentiation ofthe earths surface.

Richard Hartshorne

Geography studies the differences ofphenomena usually related in different partsof the earths surface.

Hettner

EXERCISES

1. Multiple choice questions.

(i) Which one of the following scholars coined the term Geography?

(a) Herodotus (c) Galileo

(b) Erathosthenese (d) Aristotle

(ii) Which one of the following features can be termed as physical feature?

(a) Port (c) Plain

(b) Road (d) Water park


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(iii) Make correct pairs from the following two columns and mark the correctoption.

1. Meteorology A. Population Geography

2. Demography B. Soil Geography

3. Sociology C. Climatology

4. Pedology D. Social Geography

(a) 1B,2C,3A,4D (c) 1D,2B,3C,4A

(b) 1A,2D,3B,4C (d) 1C,2A,3D,4B

(iv) Which one of the following questions is related to cause-effect relationship?

(a) Why (c) What

(b) Where (d) When

(v) Which one of the following disciplines attempts temporal synthesis?

(a) Sociology (c) Anthropology

(b) Geography (d) History

2. Answer the following questions in about 30 words.

(i) What important cultural features do you observe while going to school?Are they similar or dissimilar? Should they be included in the study ofgeography or not? If yes, why?

(ii) You have seen a tennis ball, a cricket ball, an orange and a pumpkin.Which one amongst these resembles the shape of the earth? Why haveyou chosen this particular item to describe the shape of the earth?

(iii) Do you celebrate Van Mahotsava in your school? Why do we plant so manytrees? How do the trees maintain ecological balance?

(iv) You have seen elephants, deer, earthworms, trees and grasses. Where dothey live or grow? What is the name given to this sphere? Can you describesome of the important features of this sphere?

(v) How much time do you take to reach your school from your house? Hadthe school been located across the road from your house, how much timewould you have taken to reach school? What is the effect of the distance

between your residence and the school on the time taken in commuting?

Can you convert time into space and vice versa?


(i) You observe every day in your surroundings that there is variation in

natural as well as cultural phenomena. All the trees are not of the same

variety. All the birds and animals you see, are different. All these different

elements are found on the earth. Can you now argue that geography is

the study of areal differentiation?

(ii) You have already studied geography, history, civics and economics as

parts of social studies. Attempt an integration of these disciplineshighlighting their interface.


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Project Work

Select forest as a natural resource.

(i) Prepare a map of India showing the distribution of different types of forests.

(ii) Write about the economic importance of forests for the country.

(iii) Prepare a historical account of conservation of forests in India with focuson Chipko movements in Rajasthan and Uttaranchal.


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THEEARTH

This unit deals with Origin and evolution of the earth; Interior of the earth; Wegeners

continental drift theory and plate tectonics; earthquakes andvolcanoes

UNITII


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D

o you remember the nursery rhymeTwinkle, Twinkle little star?

Starry nights have always attracted us sincethe childhood. You may also have thought ofthese stars and had numerous questions in

your mind. Questions such as how many starsare there in the sky? How did they come intoexistence? Can one reach the end of the sky?May be many more such questions are stillthere in your mind. In this chapter, you willlearn how these twinkling little stars wereformed. With that you will eventually also readthe story of origin and evolution of the earth.

ORIGINOFTHEEARTH

Early Theories

A large number of hypotheses were put forthby different philosophers and scientistsregarding the origin of the earth. One of theearlier and popular arguments was by Germanphilosopher Immanuel Kant. MathematicianLaplace revised it in 1796. It is known asNebular Hypothesis. The hypothesis consideredthat the planets were formed out of a cloud ofmaterial associated with a youthful sun, which

was slowly rotating. Later in 1900, Chamberlainand Moulton considered that a wandering starapproached the sun. As a result, a cigar-shapedextension of material was separated from thesolar surface. As the passing star moved away,the material separated from the solar surfacecontinued to revolve around the sun and itslowly condensed into planets. Sir James Jeansand later Sir Harold Jeffrey supported this

THEORIGINANDEVOLUTIONOFTHEEARTH

C H A P T E R

argument. At a later date, the argumentsconsidered of a companion to the sun to have

been coexisting. These arguments are called

binary theories. In 1950, Otto Schmidt inRussia and Carl Weizascar in Germanysomewhat revised the nebular hypothesis,though differing in details. They considered thatthe sun was surrounded by solar nebulacontaining mostly the hydrogen and heliumalong with what may be termed as dust. Thefriction and collision of particles led toformation of a disk-shaped cloud and theplanets were formed through the process ofaccretion.

Modern Theories

However, scientists in later period took up theproblems of origin of universe rather than thatof just the earth or the planets. The mostpopular argument regarding the origin of theuniverse is the Big Bang Theory. It is also calledexpanding universe hypothesis. EdwinHubble, in 1920, provided evidence that theuniverse is expanding. As time passes, galaxiesmove further and further apart. You canexperiment and find what does the expandinguniverse mean. Take a balloon and mark somepoints on it to represent the galaxies. Now, if

you start inflating the balloon, the pointsmarked on the balloon will appear to be movingaway from each other as the balloon expands.Similarly, the distance between the galaxies isalso found to be increasing and thereby, theuniverse is considered to be expanding.However, you will find that besides the increasein the distances between the points on the


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THE ORIGIN AND EVOLUTION OF THE EARTH 15

balloon, the points themselves are expanding.This is not in accordance with the fact.Scientists believe that though the space

between the galaxies is increasing, observations

do not support the expansion of galaxies. So,the balloon example is only partially correct.

Figure 2.1 : The Big Bang

The Big Bang Theory considers thefollowing stages in the development of theuniverse.

(i) In the beginning, all matter forming the

universe existed in one place in the form

of a tiny ball (singular atom) with an

unimaginably small volume, infinite

temperature and infinite density.

(ii) At the Big Bang the tiny ball exploded

violently. This led to a huge expansion.

It is now generally accepted that the

event of big bang took place 13.7 billion

years before the present. The expansion

continues even to the present day. As it

grew, some energy was converted into

matter. There was particularly rapid

expansion within fractions of a secondafter the bang. Thereafter, the

expansion has slowed down. Within first

three minutes from the Big Bang event,

the first atom began to form.

(iii) Within 300,000 years from the Big

Bang, temperature dropped to 4,500K

and gave rise to atomic matter. Theuniverse became transparent.

The expansion of universe means increasein space between the galaxies. An alternativeto this was Hoyles concept of steady state. Itconsidered the universe to be roughly the same

at any point of time. However, with greaterevidence becoming available about theexpanding universe, scientific community atpresent favours argument of expandinguniverse.

The Star Formation

The distribution of matter and energy was noteven in the early universe. These initial densitydifferences gave rise to differences ingravitational forces and it caused the matterto get drawn together. These formed the bases

for development of galaxies. A galaxycontainsa large number of stars. Galaxies spread over

vast distances that are measured in thousandsof light-years. The diameters of individualgalaxies range from 80,000-150,000 light

years. A galaxy starts to form by accumulationof hydrogen gas in the form of a very largecloud called nebula. Eventually, growingnebula develops localised clumps of gas. Theseclumps continue to grow into even densergaseous bodies, giving rise to formation ofstars. The formation of stars is believed to have

taken place some 5-6 billion years ago.

A light year is a measure of distance andnot of time. Light travels at a speed of300,000 km/second. Considering this,the distances the light will travel in oneyear is taken to be one light year. Thisequals to 9.4611012 km. The meandistance between the sun and the earthis 149,598,000 km. In terms of lightyears, it is 8.311 minutes of a year.

Formation of Planets

The following are considered to be the stages

in the development of planets :

(i) The stars are localised lumps of gas

within a nebula. The gravitational force

within the lumps leads to the formation

of a core to the gas cloud and a huge

rotating disc of gas and dust develops

around the gas core.


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(ii) In the next stage, the gas cloud startsgetting condensed and the matteraround the core develops into small-rounded objects. These small-rounded

objects by the process of cohesion developinto what is called planetesimals.Larger bodies start forming by collision,and gravitational attraction causes thematerial to stick together. Planetesimalsare a large number of smaller bodies.

(iii) In the final stage, these large numberof small planetesimals accrete to forma fewer large bodies in the form ofplanets.

OURSOLARSYSTEM

Our Solar system consists of nine planets. Thetenth planet 2003 UB

313has also been recently

sighted. The nebula from which our Solarsystem is supposed to have been formed,started its collapse and core formation sometime 5-5.6 billion years ago and the planets

were formed about 4.6 billion years ago. Oursolar system consists of the sun (the star), 9planets, 63 moons, millions of smaller bodieslike asteroidsand cometsand huge quantityof dust-grains and gases.

Out of the nine planets, mercury, venus,

earth and mars are called as the inner planetsas they lie between the sun and the belt ofasteroids the other five planets are called theouterplanets. Alternatively, the first four are calledTerrestrial,meaning earth-like as they are madeup of rock and metals, and have relatively highdensities. The rest five are calledJovianor GasGiant planets. Jovian means jupiter-like. Most

of them are much larger than the terrestrialplanets and have thick atmosphere, mostly ofhelium and hydrogen. All the planets were formedin the same period sometime about 4.6 billion

years ago. Some data regarding our solar systemare given in the box below.

Why are the inner planets rocky whileothers are mostly in gaseous form?

The difference between terrestrial and jovianplanets can be attributed to the followingconditions:

(i) The terrestrial planets were formed inthe close vicinity of the parent star

where it was too warm for gases to

condense to solid particles. Jovianplanets were formed at quite a distantlocation.

(ii) The solar wind was most intense nearerthe sun; so, it blew off lots of gas anddust from the terrestrial planets. Thesolar winds were not all that intense tocause similar removal of gases from the

Jovian planets.(iii) The terrestrial planets are smaller and

their lower gravity could not hold theescaping gases.

The Moon

The moon is the only natural satellite of theearth. Like the origin of the earth, there have

been attempts to explain how the moon wasformed. In 1838, Sir George Darwin suggestedthat initially, the earth and the moon formed asingle rapidly rotating body. The whole mass

The Solar System

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058 39.785

Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66 0.5-0.9

Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88 -0.3

Satellites 0 0 1 2 16 about 18 about 17 8 1

* Distance from the sun in astronomical unit i.e. average mean distance of the earth is 149,598,000 km = 1@ Density in gm/cm3

# Radius: Equatorial radius 6378.137 km = 1


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became a dumb-bel l- shaped body andeventually it broke. It was also suggested thatthe material forming the moon was separatedfrom what we have at present the depression

occupied by the Pacific Ocean.However, the present scientists do not

accept either of the explanations. It is nowgenerally believed that the formation of moon,as a satellite of the earth, is an outcome of giantimpact or what is described as the big splat.

A body of the size of one to three times that ofmars collided into the earth sometime shortlyafter the earth was formed. It blasted a largepart of the earth into space. This portion of

blasted material then continued to orbit theearth and eventually formed into the present

moon about 4.44 billion years ago.

EVOLUTIONOFTHEEARTH

Do you know that the planet earth initially wasa barren, rocky and hot object with a thinatmosphere of hydrogen and helium. This isfar from the present day picture of the earth.Hence, there must have been some eventsprocesses, which may have caused this changefrom rocky, barren and hot earth to a beautifulplanet with ample amount of water andconducive atmosphere favouring the existence

of life. In the following section, you will findout how the period, between the 4,600 million

years and the present, led to the evolution oflife on the surface of the planet.

The earth has a layered structure. Fromthe outermost end of the atmosphere to thecentre of the earth, the material that exists isnot uniform. The atmospheric matter has theleast density. From the surface to deeperdepths, the earths interior has different zonesand each of these contains materials withdifferent characteristics.

How was the layered structure of the

earth developed?

Development of Lithosphere

The earth was mostly in a volatile state duringits primordial stage. Due to gradual increasein density the temperature inside hasincreased. As a result the material inside

started getting separated depending on theirdensities. This allowed heavier materials (likeiron) to sink towards the centre of the earthand the lighter ones to move towards the

surface. With passage of time it cooled furtherand solidified and condensed into a smaller size.

This later led to the development of the outersurface in the form of a crust. During theformation of the moon, due to the giant impact,the earth was further heated up. It is throughthe process of differentiation that the earthforming material got separated into differentlayers. Starting from the surface to the centralparts, we have layers like the crust, mantle,outer core and inner core. From the crust to thecore, the density of the material increases. We

shall discuss in detail the properties of each ofthis layer in the next chapter.

Evolution of Atmosphere and Hydrosphere

The present composition of earths atmosphereis chiefly contributed by nitrogen and oxygen.

You will be dealing with the composition andstructure of the earths atmosphere in Chapter 8.

There are three stages in the evolution ofthe present atmosphere. The first stage ismarked by the loss of primordial atmosphere.In the second stage, the hot interior of the earth

contributed to the evolution of the atmosphere.Finally, the composition of the atmosphere wasmodified by the living world through theprocess ofphotosynthesis.

The early atmosphere, with hydrogen andhelium, is supposed to have been stripped offas a result of the solar winds. This happenednot only in case of the earth, but also in all theterrestrial planets, which were supposed tohave lost their primordial atmosphere throughthe impact of solar winds.

During the cooling of the earth, gases and

water vapour were released from the interiorsolid earth. This started the evolution of thepresent atmosphere. The early atmospherelargely contained water vapour, nitrogen,carbon dioxide, methane, ammonia and verylittle of free oxygen. The process through whichthe gases were outpoured from the interior iscalled degassing. Continuous volcaniceruptions contributed water vapour and gases


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to the atmosphere. As the earth cooled, the

wa ter vapour released started gett ing

condensed. The carbon dioxide in the

atmosphere got dissolved in rainwater and the

temperature further decreased causing more

condensation and more rains. The rainwater

falling onto the surface got collected in the

depressions to give rise to oceans. The earths

oceans were formed within 500 million years

from the formation of the earth. This tells us

that the oceans are as old as 4,000 millionyears. Sometime around 3,800 million years

ago, life began to evolve. However, around2,500-3,000 million years before the present,the process of photosynthesis got evolved. Life

was confined to the oceans for a long time.Oceans began to have the contribution ofoxygen through the process ofphotosynthesis.Eventually, oceans were saturated with oxygen,and 2,000 million years ago, oxygen began toflood the atmosphere.

Eons Era Period Epoch Age/ Years Life/ Major Events

Before Present

Quaternary Holocene 0 - 10,000 Modern ManPleistocene 10,000 - 2 million Homo Sapiens

Tertiary Pliocene 2 - 5 million Early Human Ancestor

Miocene 5 - 24 million Ape: Flowering Plantsand Trees

Oligocene 24 - 37 Ma Anthropoid Ape

Eocene 37 - 58 Million Rabbits and Hare

Palaeocene 57 - 65 Million Small Mammals :

Rats Mice

Cretaceous 65 - 144 Million Extinction of Dinosaurs

Jurassic 144 - 208 Million Age of Dinosaurs

Triassic 208 - 245 Million Frogs and turtles

Permian 245 - 286 Million Reptile dominate-replaceamphibians

Carboniferous 286 - 360 Million First Reptiles:

Vertebrates: Coal beds

Devonian 360 - 408 Million Amphibians

Silurian 408 - 438 Million First trace of life on land:Plants

Ordovician 438 - 505 Million First Fish

Cambrian 505 - 570 Million No terrestrial Life :

Marine Invertebrate

Proterozoic 570 - 2,500 Million Soft-bodied arthropods

Archean 2,500 - 3,800 Million Blue green Algae:

Unicellular bacteria

Hadean 3,800 - 4,800 Million Oceans and Continents

form Ocean and

Atmosphere are rich inCarbon dioxide

Origin of 5,000 Million Origin of the sun

Stars

Supernova 12,000 Million Origin of the universe

Big Bang 13,700 Million

Cainozoic(From 65

million yearsto the

presenttimes)

Mesozoic65 - 245Million

Mammals

Palaeozoic245 - 570

Million

Pre-

Cambrian570 Million

- 4,800Million

5,000 -13,700Million

Geological Time Scale


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Origin of Life

The last phase in the evolution of the earthrelates to the origin and evolution of life. It is

undoubtedly clear that the initial or even theatmosphere of the earth was not conducive forthe development of life. Modern scientists referto the origin of life as a kind of chemicalreaction, which first generated complex organicmolecules and assembled them. This

assemblage was such that they could duplicatethemselves converting inanimate matter into

living substance. The record of life that existedon this planet in different periods is found inrocks in the form of fossils. The microscopicstructures closely related to the present form

of blue algae have been found in geologicalformations that are much older than these weresome 3,000 million years ago. It can beassumed that life began to evolve sometime3,800 million years ago. The summary ofevolution of life from unicellular bacteria to themodern man is given in the Geological TimeScale on page 18.

EXERCISES

1. Multiple choice questions. (i) Which one of the following figures represents the age of the earth?

(a) 4.6 million years (c) 4.6 billion years

(b) 13.7 billion years (d) 13.7 trillion years

(ii) Which one of the following has the longest duration?

(a) Eons (c) Era

(b) Period (d) Epoch

(iii) Which one of the following is not related to the formation or modificationof the present atmosphere?

(a) Solar winds (c) Degassing

(b) Differentiation (d) Photosynthesis

(iv) Which one of the following represents the inner planets?

(a) Planets between the sun and the earth

(b) Planets between the sun and the belt of asteroids

(c) Planets in gaseous state

(d) Planets without satellite(s)

(v) Life on the earth appeared around how many years before the present?

(a) 13.7 billion (c) 4.6 billion

(b) 3.8 million (d) 3.8 billion


(i) Why are the terrestrial planets rocky?

(ii) What is the basic difference in the arguments related to the origin of theearth given by :

(a) Kant and Laplace

(b) Chamberlain and Moulton


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(iii) What is meant by the process of differentiation?

(iv) What was the nature of the earth surface initially?

(v) What were the gases which initially formed the earths atmosphere?


(i) Write an explanatory note on the Big Bang Theory.

(ii) List the stages in the evolution of the earth and explain each stage inbrief.

Project Work

Collect information about the project Stardust (website: www.sci.edu/public.htmlandwww.nasm.edu) along the following lines.

(i) Which is the agency that has launched this project?

(ii) Why are scientists interested in collecting Stardust?

(iii) Where from has the Stardust been collected?


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W

hat do you imagine about the nature

of the earth? Do you imagine it to be

a solid ball like cricket ball or a

hollow ball with a thick cover of rocks i.e.lithosphere? Have you ever seen photographs

or images of a volcanic eruption on the

television screen? Can you recollect the

emergence of hot molten lava, dust, smoke, fire

and magma flowing out of the volcanic crater?

The interior of the earth can be understood only

by indirect evidences as neither any one has nor

any one can reach the 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 asendogenic processes are constantly shaping

the landscape. A proper understanding of the

physiographic character of a region remains

incomplete if the effects of endogenic processes

are ignored. Human life is largely influenced

by the physiography of the region. Therefore,

it is necessary that one gets acquainted with

the forces that influence landscape

development. To understand why the earth

shakes or how a tsunami wave is generated, it

is necessary that we know certain details of the

interior of the earth. In the previous chapter,

you have noted that the earth-forming

materials have been distributed in the form of

layers from the crust to the core. It is interesting

to know how scientists have gathered

information about these layers and what are

the characteristics of each of these layers. This

is exactly what this chapter deals with.

INTERIOROFTHEEARTH

SOURCESOFINFORMATIONABOUTTHEINTERIOR

The earths radius is 6,370 km. No one canreach the centre of the earth and makeobservations or collect samples of the material.Under such conditions, you may wonder howscientists tell us about the earths interior andthe type of materials that exist at such depths.Most of our knowledge about the interior ofthe earth is largely based on estimates andinferences. Yet, a part of the information isobtained through direct observations andanalysis of materials.

Direct Sources

The most easily available solid earth material

is surface rock or the rocks we get from miningareas. Gold mines in South Africa are as deepas 3 - 4 km. Going beyond this depth is notpossible as it is very hot at this depth. Besidesmining, scientists have taken up a number ofprojects to penetrate deeper depths to explorethe conditions in the crustal portions. Scientists

world over are working on two major projectssuch as Deep Ocean Drilling Project andIntegrated Ocean Drilling Project. Thedeepest drill at Kola, in Arctic Ocean, has sofar reached a depth of 12 km. This and manydeep drilling projects have provided large

volume of information through the analysis ofmaterials collected at different depths.

Volcanic eruption forms another source ofobtaining direct information. As and when themolten material (magma) is thrown onto thesurface of the earth, during volcanic eruptionit becomes available for laboratory analysis.However, it is difficult to ascertain the depth ofthe source of such magma.

C H A P T E R


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Indirect Sources

Analysis of properties of matter indirectlyprovides information about the interior. Weknow through the mining activity thattemperature and pressure increase with theincreasing distance from the surface towardsthe interior in deeper depths. Moreover, it isalso known that the density of the material alsoincreases with depth. It is possible to find therate of change of these characteristics. Knowingthe total thickness of the earth, scientists haveestimated the values of temperature, pressureand the density of materials at different depths.

The details of these characteristics withreference to each layer of the interior arediscussed later in this chapter.

Another source of information are themeteors that at times reach the earth. However,it may be noted that the material that becomesavailable for analysis from meteors, is not fromthe interior of the earth. The material and thestructure observed in the meteors are similarto that of the earth. They are solid bodiesdeveloped out of materials same as, or similarto, our planet. Hence, this becomes yet anothersource of information about the interior of theearth.

The other ind irec t sources inc lude

gravitation, magnetic field, and seismic activity.The gravitation force (g) is not the same atdifferent latitudes on the surface. It is greaternear the poles and less at the equator. This is

because of the distance from the centre at theequator being greater than that at the poles.

The gravity values also differ according to themass of material. The uneven distribution ofmass of material within the earth influencesthis value. The reading of the gravity at differentplaces is influenced by many other factors.

These readings differ from the expected values.

Such a difference is called gravity anomaly.Gravity anomalies give us information aboutthe distribution of mass of the material in thecrust of the earth. Magnetic surveys alsoprovide information about the distribution ofmagnetic materials in the crustal portion, andthus, provide information about thedistribution of materials in this part. Seismicactivity is one of the most important sources of

information about the interior of the earth.Hence, we shall discuss it in some detail.

Earthquake

The study of seismic waves provides a completepicture of the layered interior. An earthquakein simple words is shaking of the earth. It is anatural event. It is caused due to release ofenergy, which generates waves that travel inall directions.

Why does the earth shake?

The release of energy occurs along a fault. Afault is a sharp break in the crustal rocks.Rocks along a fault tend to move in opposite

directions. As the overlying rock strata pressthem, the friction locks them together. However,their tendency to move apart at some point oftime overcomes the friction. As a result, the

blocks get deformed and eventually, they slidepast one another abruptly. This causes arelease of energy, and the energy waves travelin all directions. The point where the energy isreleased is called thefocusof an earthquake,alternatively, it is called the hypocentre. Theenergy waves travelling in different directionsreach the surface. The point on the surface,nearest to the focus, is called epicentre. It isthe first one to experience the waves. It is a pointdirectly above the focus.

Earthquake Waves

All natural earthquakes take place in thelithosphere. You will learn about differentlayers of the earth later in this chapter. It issufficient to note here that the lithosphere refersto the portion of depth up to 200 km from thesurface of the earth. An instrument calledseismograph records the waves reaching the

surface. A curve of earthquake waves recordedon the seismograph is given in Figure 3.1. Notethat the curve shows three distinct sectionseach representing different types of wavepatterns. Earthquake waves are basically of twotypes body waves and surface waves. Body

waves are generated due to the release of energyat the focus and move in all directions travellingthrough the body of the earth. Hence, the name


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INTERIOR OF THE EARTH 23

body waves. The body waves interact with thesurface rocks and generate new set of wavescalled surface waves. These waves move alongthe surface. The velocity of waves changes as

they travel through materials with differentdensities. The denser the material, the higheris the velocity. Their direction also changes asthey reflect or refract when coming acrossmaterials with different densities.

propagation. As a result, it creates densitydifferences in the material leading to stretchingand squeezing of the material. Other three

waves vibrate perpendicular to the direction of

propagation. The direction of vibrations ofS-waves is perpendicular to the wave directionin the vertical plane. Hence, they create troughsand crests in the material through which theypass. Surface waves are considered to be themost damaging waves.

Emergence of Shadow Zone

Earthquake waves get recorded in seismo-graphs located at far off locations. However,there exist some specific areas where the wavesare not reported. Such a zone is called the

shadow zone. The study of different eventsreveals that for each earthquake, there existsan altogether different shadow zone. Figure 3.2(a) and (b) show the shadow zones of P andS-waves. It was observed that seismographslocated at any distance within 105from theepicentre, recorded the arrival of both P andS-waves. However, the seismographs located

beyond 145from epicentre, record the arrivalof P-waves, but not that of S-waves. Thus, azone between 105and 145from epicentre wasidentified as the shadow zone for both the types

of waves. The entire zone beyond 105does notreceive S-waves. The shadow zone of S-wave ismuch larger than that of the P-waves. Theshadow zone of P-waves appears as a bandaround the earth between 105and 145awayfrom the epicentre. The shadow zone of S-wavesis not only larger in extent but it is also a littleover 40 per cent of the earth surface. You candraw the shadow zone for any earthquakeprovided you know the location of the epicentre.(See the activity box on page 28 to know how tolocate the epicentre of a quake event).

Types of Earthquakes

(i) The most common ones are the tectonicearthquakes. These are generated due tosliding of rocks along a fault plane.

(ii) A special class of tectonic earthquake issometimes recognised as volcanicearthquake. However, these are confinedto areas of active volcanoes.

Figure 3.1 : Earthquake Waves

There are two types of body waves. Theyare called P and S-waves. P-waves move fasterand are the first to arrive at the surface. Theseare also called primary waves. The P-wavesare similar to sound waves. They travelthrough gaseous, liquid and solid materials.S-waves arrive at the surface with some timelag. These are called secondary waves. Animportant fact about S-waves is that they can

travel only through solid materials. Thischaracteristic of the S-waves is quiteimportant. It has helped scientists tounderstand the structure of the interior of theearth. Reflection causes waves to rebound

whereas refraction makes waves move indifferent directions. The variations in thedirection of waves are inferred with the help oftheir record on seismograph. The surface

waves are the last to report on seismograph.These waves are more destructive. They causedisplacement of rocks, and hence, the collapse

of structures occurs.

Propagation of Earthquake Waves

Different types of earthquake waves travel indifferent manners. As they move or propagate,they cause vibration in the body of the rocksthrough which they pass. P-waves vibrateparallel to the direction of the wave. This exertspressure on the material in the direction of the


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(v) The earthquakes that occur in the areas

of large reservoirs are referred to as

reservoir inducedearthquakes.

Measuring Earthquakes

The earthquake events are scaled either

according to the magnitude or intensity of the

shock. The magnitude scale is known as the

Richter scale. The magnitude relates to the

energy released during the quake. The

magnitude is expressed in absolute numbers,

0-10. The intensity scale is named after

Mercalli, an Italian seismologist. The intensity

scale takes into account the visible damage

caused by the event. The range of intensity scale

is from 1-12.

EFFECTSOFEARTHQUAKE

Earthquake is a natural hazard. The following

are the immediate hazardous effects of

earthquake:

(i) Ground Shaking

(ii) Differential ground settlement

(iii) Land and mud slides

(iv) Soil liquefaction

(v) Ground lurching

(vi) Avalanches(vii) Ground displacement

(viii) Floods from dam and levee failures

(ix) Fires

(x) Structural collapse

(xi) Falling objects

(xii) Tsunami

The first six listed above have some bearings

upon landforms, while others may be

considered the effects causing immediate

concern to the life and properties of people in

the region. The effect of tsunami would occuronly if the epicentre of the tremor is below

oceanic waters and the magnitude is

sufficiently high. Tsunamis are waves

generated by the tremors and not an

earthquake in itself. Though the actual quake

activity lasts for a few seconds, its effects are

devastating provided the magnitude of the

quake is more than 5 on the Richter scale.

Figure 3.2 (a) and (b) : Earthquake Shadow Zones

(iii) In the areas of intense mining activity,sometimes the roofs of undergroundmines collapse causing minor tremors.

These are called collapseearthquakes.(iv) Ground shaking may also occur due to

the explosion of chemical or nucleardevices. Such tremors are called explosionearthquakes.


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Frequency of Earthquake Occurrences

The earthquake is a natural hazard. If a tremorof high magnitude takes place, it can causeheavy damage to the life and property of

people. However, not all the parts of the globenecessarily experience major shocks. We shall

be discussing the distribution of earthquakesand volcanoes with some details in the next

STRUCTUREOFTHEEARTH

The Crust

It is the outermost solid part of the earth. It is

brittle in nature. The thickness of the crustvaries under the oceanic and continental areas.Oceanic crust is thinner as compared to thecontinental crust. The mean thickness ofoceanic crust is 5 km whereas that of thecontinental is around 30 km. The continentalcrust is thicker in the areas of major mountainsystems. It is as much as 70 km thick in theHimalayan region.

It is made up of heavier rocks havingdensity of 3 g/cm3. This type of rock found inthe oceanic crust is basalt. The mean densityof material in oceanic crust is 2.7 g/cm3.

The Mantle

The portion of the interior beyond the crust iscalled the mantle. The mantle extends fromMohos discontinuity to a depth of 2,900 km.

The upper portion of the mantle is calledasthenosphere. The word astheno means

weak. It is considered to be extending upto 400km. It is the main source of magma that finds

chapter. Note that the quakes of highmagnitude, i.e. 8+ are quite rare; they occuronce in 1-2 years whereas those of tiny typesoccur almost every minute.

A view of the damaged Aman Setu at the LOCin Uri, due to an earthquake


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its way to the surface during volcaniceruptions. It has a density higher than thecrusts (3.4 g/cm3). The crust and theuppermost part of the mantle are calledlithosphere. Its thickness ranges from 10-200 km.

The lower mantle ex tends beyond theasthenosphere. It is in solid state.

The Core

As indicated earlier, the earthquake waveve loci ties helped in understanding theexistence of the core of the earth. The core-mantle boundary is located at the depth of2,900 km. The outer core is in liquid state whilethe inner core is in solid state. The density ofmaterial at the mantle core boundary is around5 g/cm3and at the centre of the earth at 6,300km, the density value is around 13g/cm3. Thecore is made up of very heavy material mostlyconstituted by nickel and iron. It is sometimes

referred to as the nifelayer.

VOLCANOESANDVOLCANICLANDFORMS

You may have seen photographs or pictures ofvolcanoes on a number of occasions. A volcanois a place where gases, ashes and/or moltenrock material lava escape to the ground. A

volcano is called an active volcano if thematerials mentioned are being released or have

been released out in the recent past. The layerbelow the solid crust is mantle. It has higherdensity than that of the crust. The mantlecontains a weaker zone called asthenosphere.

It is from this that the molten rock materialsfind their way to the surface. The material inthe upper mantle portion is called magma.Once it starts moving towards the crust or itreaches the surface, it is referred to as lava.

The material that reaches the ground includeslava flows, pyroclastic debris, volcanic bombs,ash and dust and gases such as nitrogencompounds, sulphur compounds and minoramounts of chlorene, hydrogen and argon.

Volcanoes

Volcanoes are classified on the basis of natureof eruption and the form developed at thesurface. Major types of volcanoes are as follows:

Shield Volcanoes

Barring the basalt flows, the shield volcanoesare the largest of all the volcanoes on the earth.

The Hawaiian volcanoes are the most famous

Figure 3.4 : The interior of the earth

Shield Volcano

Cinder Cone


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examples. These volcanoes are mostly madeup of basalt, a type of lava that is very fluid

when erupted. For this reason, these volcanoesare not steep. They become explosive if

somehow water gets into the vent; otherwise,they are characterised by low-explosivity. Theupcoming lava moves in the form of a fountainand throws out the cone at the top of the ventand develops into cinder cone.

Composite Volcanoes

These vo lcanoes are characterised byeruptions of cooler and more viscous lavasthan basalt. These volcanoes often result inexplosive eruptions. Along with lava, largequantities of pyroclastic material and ashes

find their way to the ground. This materialaccumulates in the vicinity of the vent openingsleading to formation of layers, and this makesthe mounts appear as composite volcanoes.

more than 50 m. Individual flows may extendfor hundreds of km. The Deccan TrapsfromIndia, presently covering most of theMaharashtra plateau, are a much larger flood

basalt province. It is believed that initially thetrap formations covered a much larger areathan the present.

Mid-Ocean Ridge Volcanoes

These volcanoes occur in the oceanic areas.There is a system of mid-ocean ridges morethan 70,000 km long that stretches throughall the ocean basins. The central portion of thisridge experiences frequent eruptions. We shall

be discussing this in detail in the next chapter.

VOLCANICLANDFORMS

Intrusive Forms

The lava that is released during volcanic

eruptions on cooling develops into igneous

rocks. The cooling may take place either on

reaching the surface or also while the lava isstill in the crustal portion. Depending on the

location of the cooling of the lava, igneous rocks

are classified as volcanic rocks(cooling at the

surface) and plutonic rocks (cooling in the

crust). The lava that cools within the crustalportions assumes different forms. These forms

are called intrusive forms. Some of the forms

are shown in Figure 3.5.Composite Volcano

Caldera

These are the most explosive of the earthsvolcanoes. They are usually so explosive thatwhen they erupt they tend to collapse onthemselves rather than building any tallstructure. The collapsed depressions are calledcalderas. Their explosiveness indicates that

the magma chamber supplying the lava is notonly huge but is also in close vicinity.

Flood Basalt Provinces

These volcanoes outpour highly fluid lava thatflows for long distances. Some parts of the

world are covered by thousands of sq. km ofthick basalt lava flows. There can be a series offlows with some flows attaining thickness of Figure 3.5 : Volcanic Landforms


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Activity : Locating an Epicentre

For this you will need

Data from 3 seismograph stations about the time of arrival of P-waves, S-waves.

Procedure1. Find the time of arrival of P and S-waves of the given quake for the three stations for which

you have the data.2. Compute the time lag between the arrival of P and S-waves for each station; it is called time

lag. (Note that it is directly related to the distance of the seismograph from the focus.)

A. Basic rule : For every second of time lag, the earthquake is roughly 8 km away from you.

3. Using the rule quoted above, convert the time lag into distance ( # seconds of time lag * 8)for each station.

4. On a map locate the seismograph stations.5. Draw circles, taking the seismograph stations as the centre, with the radius equal to the

distance you have calculated in the previous step. (Do not forget to convert distance as perthe map scale.)

6. These circles will intersect each other in a point. This point is the location of the epicentre.

In normal practice, the epicentres are located using computer models. They take into accountthe structure of the earths crust. The locations with accuracy within a few hundred metrescan be achieved. The procedure outlined here is a much simplified version of what is normallydone, although the principle is the same.

In the following diagram, the epicentre is located using this procedure. It also contains atable giving necessary data. Why dont you try for yourself?

Batholiths

A large body of magmatic material that coolsin the deeper depth of the crust develops in theform of large domes. They appear on the surface

only after the denudational processes removethe overlying materials. They cover large areas,and at times, assume depth that may be severalkm. These are granitic bodies. Batholiths arethe cooled portion of magma chambers.

Lacoliths

These are large dome-shaped intrusive bodieswith a level base and connected by a pipe-like

conduit from below. It resembles the surfacevolcanic domes of composite volcano, onlythese are located at deeper depths. It can beregarded as the localised source of lava that

finds its way to the surface. The Karnatakaplateau is spotted with domal hills of graniterocks. Most of these, now exfoliated, areexamples of lacoliths or batholiths.

Lapolith, Phacolith and Sills

As and when the lava moves upwards, aportion of the same may tend to move in a

horizontal direction wherever it finds a weak

Data

Arrival time ofStation P-waves S-waves Hour Min. Sec. Hour Min. Sec.

S1 03 23 20 03 24 45

S2 03 22 17 03 23 57

S3 03 22 00 03 23 55

Scale of the map 1cm = 40km


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EXERCISES

1. Multiple choice questions.

(i) Which one of the following earthquake waves is more destructive?

(a) P-waves (c) Surface waves

(b) S-waves (d) None of the above

(ii) Which one of the following is a direct source of information about theinterior of the earth?

(a) Earthquake waves (c) Gravitational force

(b) Volcanoes (d) Earth magnetism

(iii) Which type of volcanic eruptions have caused Deccan Trap formations?(a) Shield (c) Composite

(b) Flood (d) Caldera

(iv) Which one of the following describes the lithosphere:

(a) upper and lower mantle (c) crust and core

(b) crust and upper mantle (d) mantle and core


(i) What are body waves?

(ii) Name the direct sources of information about the interior of the earth.

(iii) Why do earthquake waves develop shadow zone?

(iv) Briefly explain the indirect sources of information of the interior of theearth other than those of seismic activity.


(i) What are the effects of propagation of earthquake waves on the rock massthrough which they travel?

(ii) What do you understand by intrusive forms? Briefly describe variousintrusive forms.

plane. It may get rested in different forms. In

case it develops into a saucer shape, concave

to the sky body, it is called lapolith. A wavy

mass of intrusive rocks, at times, is found at

the base of synclines or at the top of anticlinein folded igneous country. Such wavy materialshave a definite conduit to source beneath inthe form of magma chambers (subsequentlydeveloped as batholiths). These are called thephacoliths.

The near horizontal bodies of theintrusive igneous rocks are called sill orsheet, depending on the thickness of thematerial. The thinner ones are called sheets

while the thick horizontal deposits arecalled sills.

Dykes

When the lava makes its way through cracksand the fissures developed in the land, itsolidifies almost perpendicular to the ground.It gets cooled in the same position to develop a

wall-like structure. Such structures are calleddykes. These are the most commonly found

intrusive forms in the western Maharashtra area.These are considered the feeders for the eruptions

that led to the development of the Deccan traps.


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In the previous chapter, you have studied theinterior of the earth. You are already familiar

with the world map. You know that continents

cover 29 per cent of the surface of the earthand the remainder is under oceanic waters.

The positions of the continents and the oceanbodies, as we see them in the map, have notbeen the same in the past. Moreover, it is nowa well-accepted fact that oceans andcontinents will not continue to enjoy theirpresent positions in times to come. If this isso, the question arises what were their positionsin the past? Why and how do they change theirpositions? Even if it is true that the continentsand oceans have changed and are changing

their positions, you may wonder as to howscientists know this. How have they determinedtheir earlier positions? You will find the answersto some of these and related questions in thischapter.

CONTINENTALDRIFT

Observe the shape of the coastline of the AtlanticOcean. You will be surprised by the symmetryof the coastlines on either side of the ocean. No

wonder, many scientists thought of thissimilarity and considered the possibility of the

two Americas, Europe and Africa, to be oncejoined together. From the known records of thehistory of science, it wasAbraham Ortelius, aDutch map maker, who first proposed such apossibility as early as 1596.Antonio Pellegrinidrew a map showing the three continents together.However, it was Alfred Wegenera Germanmeteorologist who put forth a comprehensiveargument in the form of the continental drift

DISTRIBUTIONOFOCEANSANDCONTINENTS

C H A P T E R

theory in 1912. This was regarding thedistribution of the oceans and the continents.

According to Wegener, all the continents

formed a single continental mass, a mega oceansurrounded by the same. The super continent

was named PANGAEA, which meant all earth.The mega-ocean was called PANTHALASSA,meaning all water. He argued that, around 200million years ago, the super continent,Pangaea, began to split. Pangaea first broke

into two large continental masses as Laurasiaand Gondwanaland forming the northern andsouthern components respectively. Subse-quently, Laurasia and Gondwanalandcontinued to break into various smaller

continents that exist today. A variety of evidencewas offered in support of the continental drift.Some of these are given below.

Evidence in Support of the Continental Drift

The Matching of Continents (Jig-Saw-Fit)

The shorelines of Africa and South Americafacing each other have a remarkable and

unmistakable match. It may be noted that amap produced using a computer programme

to find the best fit of the Atlantic margin waspresented by Bullard in 1964. It proved to bequite perfect. The match was tried at 1,000-fathom line instead of the present shoreline.

Rocks of Same Age Across the Oceans

The radiometric dating methods developed inthe recent period have facilitated correlating therock formation from different continents across


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DISTRIBUTION OF OCEANS AND CONTINENTS 31

the vast ocean. The belt of ancient rocks of2,000 million years from Brazil coast matches

with those from western Africa. The earliestmarine deposits along the coastline of South

America and Africa are of the Jurassic age.This suggests that the ocean did not exist priorto that time.

Tillite

It is the sedimentary rock formed out ofdeposits of glaciers. The Gondawana systemof sediments from India is known to have itscounter parts in six different landmasses of theSouthern Hemisphere. At the base the systemhas thick tillite indicating extensive andprolonged glaciation. Counter parts of this

succession are found in Africa, Falkland Island,Madagascar, Antarctica and Australia besidesIndia. Overall resemblance of the Gondawanatype sediments clearly demonstrates that theselandmasses had remarkably similar histories.

The glacial tillite provides unambiguousevidence of palaeoclimates and also of driftingof continents.

Placer Deposits

The occurrence of rich placer deposits of goldin the Ghana coast and the absolute absence

of source rock in the region is an amazing fact.The gold bearing veins are in Brazil and it isobvious that the gold deposits of the Ghanaare derived from the Brazil plateau when thetwo continents lay side by side.

Distribution of Fossils

When identical species of plants and animalsadapted to living on land or in fresh water arefound on either side of the marine barriers, aproblem arises regarding accounting for suchdistribution. The observations that Lemursoccur in India, Madagascar and Africa led someto consider a contiguous landmass Lemurialinking these three landmasses. Mesosaurus

was a small reptile adapted to shallow brackishwater. The skeletons of these are found onlyin two localities : the Southern Cape provinceof South Africa and Iraver formations of Brazil.

The two localities presently are 4,800 km apartwith an ocean in between them.

Force for Drifting

Wegener suggested that the movementresponsible for the drifting of the continents

was caused by pole-fleeing force and tidal force.

The polar-fleeing force relates to the rotationof the earth. You are aware of the fact that theearth is not a perfect sphere; it has a bulge atthe equator. This bulge is due to the rotationof the earth. The second force that wassuggested by Wegenerthe tidal forceis dueto the attraction of the moon and the sun thatdevelops tides in oceanic waters. Wegener

believed that these forces would becomeeffective when applied over many million years.However, most of scholars considered theseforces to be totally inadequate.

Post-Drift Studies

It is interesting to note that for continental drift,most of the evidence was collecte

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NCERT- Complete Geography