FUNDAMENTALS OF PHYSICAL GEOGRAPHY © NCERT › wp...and global warming on page 109); Social Science...

FUNDAMENTALSOF

PHYSICAL GEOGRAPHYTEXTBOOK FOR CLASS XI

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First EditionMarch 2006 Phalguna 1927

ReprintedOctober 2006 Kartika 1928

November 2007 Kartika 1929

December 2008 Pausa 1930

January 2010 Pausa 1931

January 2011 Magha 1932

March 2012 Phalguna 1933

January 2013 Magha 1934

November 2013 Kartika 1935

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FOREWORD

The National Curriculum Framework (NCF), 2005, recommends that children’slife at school must be linked to their life outside the school. This principle marksa departure from the legacy of bookish learning which continues to shape oursystem and causes a gap between the school, home and community. The syllabiand textbooks developed on the basis of NCF signify an attempt to implementthis basic idea. They also attempt to discourage rote learning and themaintenance of sharp boundaries between different subject areas. We hope thesemeasures will take us significantly further in the direction of a child-centredsystem of education outlined in the National Policy on Education (1986).

The success of this effort depends on the steps that school principals andteachers will take to encourage children to reflect on their own learning and topursue imaginative activities and questions. We must recognise that, given space,time and freedom, children generate new knowledge by engaging with theinformation passed on to them by adults. Treating the prescribed textbook asthe sole basis of examination is one of the key reasons why other resources andsites of learning are ignored. Inculcating creativity and initiative is possible if weperceive and treat children as participants in learning, not as receivers of a fixedbody of knowledge.

These aims imply considerable change in school routines and mode offunctioning. Flexibility in the daily time-table is as necessary as rigour inimplementing the annual calendar so that the required number of teachingdays are actually devoted to teaching. The methods used for teaching andevaluation will also determine how effective this textbook proves for makingchildren’s life at school a happy experience, rather than a source of stress orboredom. Syllabus designers have tried to address the problem of curricularburden by restructuring and reorienting knowledge at different stages withgreater consideration for child psychology and the time available for teaching.The textbook attempts to enhance this endeavour by giving higher priority andspace to opportunities for contemplation and wondering, discussion in smallgroups, and activities requiring hands-on experience.

The National Council of Educational Research and Training (NCERT)appreciates the hard work done by the textbook development committeeresponsible for this book. We wish to thank the Chairperson of the advisorycommittee for textbooks in Social Sciences, at the higher secondarylevel, Professor Hari Vasudevan and the Chief Advisor for this book,Professor M.H. Qureshi for guiding the work of this committee. Several teacherscontributed to the development of this textbook; we are grateful to their principalsfor making this possible. We are indebted to the institutions and organisations

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which have generously permitted us to draw upon their resources, material andpersonnel. We are especially grateful to the members of the National MonitoringCommittee, appointed by the Department of Secondary and Higher Education,Ministry of Human Resource Development under the Chairpersonship ofProfessor Mrinal Miri and Professor G.P. Deshpande, for their valuable time andcontribution. As an organisation committed to systemic reform and continuousimprovement in the quality of its products, NCERT welcomes comments andsuggestions which will enable us to undertake further revision and refinement.

Director

New Delhi National Council of Educational20 December 2005 Research and Training

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TEXTBOOK DEVELOPMENT COMMITTEE

CHAIRPERSON, ADVISORY COMMITTEE FOR TEXTBOOKS IN SOCIAL SCIENCES AT THE HIGHERSECONDARY LEVEL

Hari Vasudevan, Professor, Department of History, University of Calcutta,Kolkata

CHIEF ADVISOR

M. H. Qureshi, Professor, Centre for the Study of Regional Development,Jawaharlal Nehru University, New Delhi

MEMBERS

Indu Sharma, PGT, Geography, RIE Demonstration School, Ajmer

K. Kumaraswamy, Professor, Department of Geography, BharatidasanUniversity, Tiruchirapalli

K. N. Prudhvi Raju, Professor, Department of Geography, Banaras HinduUniversity, Varanasi

K. S. Sivasami, Professor (Retd.), Centre for the Study of Regional Development,Jawaharlal Nehru University, New Delhi

L. Cajee, Reader, Department of Geography, North-Eastern Hill University,Shillong

P. K. Malik, Lecturer, Geography, Govt. College, Tavru, Gurgaon

S. R. Jog, Professor (Retd.), Department of Geography, University of Pune, Pune

MEMBER-COORDINATOR

Aparna Pandey, Lecturer, Geography, DESSH, NCERT, New Delhi

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ACKNOWLEDGEMENTS

The National Council of Educational Research and Training acknowledges thecontribution of Ashok Diwakar, Lecturer, Geography, Govt. College, Sector-9,Gurgaon in the development of this textbook.

The Council also gratefully acknowledges the support of individuals andorganisations as listed below for providing various photographs, and other materialssuch as articles used in this textbook : R. Vaidyanadhan (Fig. 6.3 and 7.1);N. S. Saini (Fig. 6.4, 6.7 and 7.4); Y. Ramesh and Krishnam Raju, VSVG, (USA)(Fig. 7.11); K.N. Prudhvi Raju (Fig. 7.2, 7.5, 7.7, 7.9, 7.12 and 7.15); ITDC/Ministryof Tourism, Govt. of India, (Fig. 11.1 and 11.2); Ministry of Environment andForests, Govt. of India (Fig. 16.1, 16.2, 16.3 and 16.4); The Times of India, NewDelhi (Photograph on earthquake destruction, Collage on tsunami on page 25and global warming on page 109); Social Science Textbook for Class VIII, Part II(NCERT, 2005), (Photographs related to volcanoes on page 26-27).

Acknowledgements are due to Savita Sinha, Professor and Head, Departmentof Education in Social Sciences and Humanties for her support and finalisingthis textbook.

The Council also gratefully acknowledges the contributions of Ishwar Singhand Arvind Sharma, DTP Operators; Sameer Khatana and Amar Kumar Prusty,Copy Editors; Bharat Sanwaria, Proof Reader; Dinesh Kumar, Computer Incharge,who have helped in giving a final shape to this book. The contribution of thePublication Department, NCERT are also duly acknowledged.©

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CONTENTS

FOREWORD iii

UNIT I : GEOGRAPHY AS A DISCIPLINE 1-12

1. Geography as a Discipline 2

UNIT II : THE EARTH 13-38

2. The Origin and Evolution of the Earth 14

3. Interior of the Earth 21

4. 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 79

10. 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 : LIFE ON THE EARTH 126-140

15. Life on the Earth 127

16. Biodiversity and Conservation 135

GLOSSARY 141-144

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CONSTITUTION OF INDIAPart III (Articles 12 – 35)

(Subject to certain conditions, some exceptionsand reasonable restrictions)

guarantees these

Fundamental Rights

Right to Equality

• before law and equal protection of laws;

• irrespective of religion, race, caste, sex or place of birth;

• of opportunity in public employment;

• by abolition of untouchability and titles.

Right to Freedom

• of expression, assembly, association, movement, residence and profession;

• of certain protections in respect of conviction for offences;

• of protection of life and personal liberty;

• of free and compulsory education for children between the age of six and fourteen years;

• of protection against arrest and detention in certain cases.

Right against Exploitation

• for prohibition of traffic in human beings and forced labour;

• for prohibition of employment of children in hazardous jobs.

Right to Freedom of Religion

• freedom of conscience and free profession, practice and propagation of religion;

• freedom to manage religious affairs;

• freedom as to payment of taxes for promotion of any particular religion;

• freedom as to attendance at religious instruction or religious worship in educational

institutions wholly maintained by the State.

Cultural and Educational Rights

• for protection of interests of minorities to conserve their language, script and culture;

• for minorities to establish and administer educational institutions of their choice.

Right to Constitutional Remedies

• by issuance of directions or orders or writs by the Supreme Court and High

Courts for enforcement of these Fundamental Rights.

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GEOGRAPHY AS A DISCIPLINEThis unit deals with

• Geography as an integrating discipline; as a science of spatialattributes

• Branches of geography; importance of physical geography

UNITI

You have studied geography as one of thecomponents of your Social Science courseupto the secondary stage. You arealready aware of some of the phenomena ofgeographical nature in the world and itsdifferent parts. Now, you will study ‘Geography’as 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 andwater. 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 earth’s surface. The understanding andthe skills obtained in modern scientifictechniques such as GIS and computercartography 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 earth’s surface isnot uniform. It has variations in its physicalfeatures. There are mountains, hills, valleys,plains, plateaus, oceans, lakes, deserts andwilderness. 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 culturaldevelopment. 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 termgeography was first coined by Eratosthenese,a Greek scholar (276-194 BC.). The word hasbeen derived from two roots from Greeklanguage geo (earth) and graphos (description).

GEOGRAPHY AS A DISCIPLINE

C H A P T E R

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 asthe abode of human beings”. You are aware ofthe fact that reality is always multifaceted andthe ‘earth’ is also multi-dimensional, that iswhy 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 earth’s 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 aswell 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 earth’s 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 tovariations 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 onephenomenon.

A geographer explains the phenomena ina 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 concernedwith the study of Nature and Humaninteractions as an integrated whole. ‘Human’is an integral part of ‘nature’ and ‘nature’ hasthe imprints of ‘human’. ‘Nature’ has influenceddifferent aspects of human life. Its imprints canbe noticed on food, clothing, shelter andoccupation. Human beings have come to termswith nature through adaptation andmodification. As you already know, the presentsociety has passed the stage of primitivesocieties, which were directly dependent ontheir immediate physical environment forsustenance. 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, humanbeings 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. Humanbeings 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 puttheir 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

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 withthree sets of questions:

(i) Some questions are related to theidentification 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 takecare of distributional and locational aspects ofthe natural and cultural features. Thesequestions provided inventorised information ofwhat 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.

(iii) The third question is related to theexplanation or the causal relationshipsbetween features and the processes andphenomena. This aspect of geographyis related to 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-relationshipsbetween the phenomena resulting from thedynamic interaction between human beingsand their physical environment.

GEOGRAPHY AS AN INTEGRATING DISCIPLINE

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 andinformation 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 geographywith 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 traditionalwarfare, countries with large size in area, gaintime at the cost of space. The defence providedby oceanic expanse around the countries ofthe new world has protected them from warsbeing 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 andAfrica, including India as they got accessibility


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

Every geographical phenomenon undergoeschange 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 travelsby 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 naturalsciences. 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 humanbeings are located in different locational niche.

A geographer should have some proficiencyin 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 Geoid but 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 raisond’etre for 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 isincluded universally in its curricula. All thesocial science disciplines, viz. sociology,political science, economics and demographystudy different aspects of social reality. Thebranches 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 politicalbehaviour. 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 discussion shows thatgeography has strong interface with naturaland social sciences. It follows its ownmethodology 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


Fig

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1.1

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attempt synthesis over space. Maps are veryeffective tools of geographers in which thetabular data is converted into visual form tobring out the spatial pattern.

BRANCHES OF GEOGRAPHY

Please study Figure 1.1 for recapitulation. It hasvery clearly brought out that geography is aninterdisciplinary subject of study. The study ofevery subject is done according to someapproach. The major approaches to studygeography have been (i) Systematic and(ii) Regional. The systematic geography approachis the same as that of general geography. Thisapproach was introduced by Alexander VonHumboldt, a German geographer (1769-1859)while regional geography approach wasdeveloped by another German geographer and acontemporary of Humboldt, Karl Ritter(1779-1859).

In systematic approach (Figure 1.2), aphenomenon is studied world over as a whole,and then the identification of typologies orspatial patterns is done. For example, if one isinterested in studying natural vegetation, thestudy will be done at the world level as a firststep. The typologies such as equatorial rainforests or softwood conical forests or monsoonforests, etc. will be identified, discussed anddelimited. In the regional approach, the worldis divided into regions at different hierarchicallevels and then all the geographical phenomenain a particular region are studied. Theseregions may be natural, political or designatedregion. The phenomena in a region are studiedin a holistic manner searching for unity indiversity.

Dualism is one of the main characteristicsof geography which got introduced from thevery beginning. This dualism depended on theaspect emphasised in the study. Earlier scholarslaid emphasis on physical geography. Buthuman beings are an integral part of the earth’ssurface. They are part and parcel of nature. Theyalso have contributed through their culturaldevelopment. Thus developed humangeography with emphasis on human activities.

BRANCHES OF GEOGRAPHY (BASED ONSYSTEMATIC APPROACH)

1. Physical Geography

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

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

(iii) Hydrology studies the realm of waterover the surface of the earth includingoceans, lakes, rivers and other waterbodies and its effect on different lifeforms including human life and theiractivities.

(iv) Soil Geography is 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 Geography studies 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 experiencesbefore attaining the present day status.The geographical features alsoexperience temporal changes and theseform the concerns of historicalgeography.


Figure 1.2 : Branches of geography based on systematic approach

(v) Political Geography looks at the spacefrom the angle of political events andstudies boundaries, space relationsbetween neighbouring political units,delimitation of constituencies, electionscenario and develops theoreticalframework to 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.


(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.

BRANCHES OF GEOGRAPHY BASED ON REGIONALAPPROACH (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/StatisticalTechniques

(c) Field Survey Methods(d) Geo-informatics comprising

techniques such as RemoteSensing, GIS, GPS, etc.

The above classification gives acomprehensive format of the branches ofgeography. Generally geography curricula istaught and learnt in this format but thisformat is not static. Any discipline is boundto grow with new ideas, problems, methodsand techniques. For example, what was oncemanual cartography has now beentransformed into computer cartography.Technology has enabled scholars to handlelarge quantum of data. The internet providesextensive information. Thus, the capacity toattempt analysis has increased tremendously.GIS has further opened vistas of knowledge.GPS has become a handy tool to find out exactlocations. Technologies have enhanced thecapacity of attempting synthesis with soundtheoretical understanding.

You will learn some preliminary aspects ofthese techniques in your book, Practical workin Geography – Part I (NCERT, 2006). You willcontinue to improve upon your skills andlearn about their application.

PHYSICAL GEOGRAPHY AND ITS IMPORTANCE

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


Physical geography includes the study oflithosphere (landforms, drainage, relief andphysiography), atmosphere (its composition,structure, elements and controls of weatherand climate; temperature, pressure, winds,precipitation, climatic types, etc.), hydrosphere(oceans, seas, lakes and associated featureswith water realm) and biosphere ( life formsincluding human being and macro-organismand their sustaining mechanism, viz. foodchain, ecological parameters and ecologicalbalance). Soils are formed through the processof pedogenesis and depend upon the parentrocks, climate, biological activity and time.Time provides maturity to soils and helps inthe development of soil profiles. Each elementis 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 elementsin 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 aquifers whichlater 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 oceanicbed. Soils are renewable resources, whichinfluence 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 physical 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 earth’s surface.

Richard Hartshorne

Geography studies the differences ofphenomena usually related in different partsof the earth’s 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


(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 distancebetween 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 innatural as well as cultural phenomena. All the trees are not of the samevariety. All the birds and animals you see, are different. All these differentelements are found on the earth. Can you now argue that geography isthe study of “areal differentiation”?

(ii) You have already studied geography, history, civics and economics asparts of social studies. Attempt an integration of these disciplineshighlighting their interface.


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.

THE EARTHThis unit deals with

• Origin and evolution of the earth; Interior of the earth; Wegener’scontinental drift theory and plate tectonics; earthquakes andvolcanoes

UNITII

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Do you remember the nursery rhyme“…Twinkle, twinkle little star…”?Starry nights have always attracted us since

the childhood. You may also have thought ofthese stars and had numerous questions inyour 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.

EARLY THEORIES

Origin of the Earth

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, whichwas 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

THE ORIGIN AND EVOLUTIONOF THE EARTH

C H A P T E R

argument. At a later date, the argumentsconsidered of a companion to the sun to havebeen coexisting. These arguments are calledbinary 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 to formationof a disk-shaped cloud and the planets wereformed through the process of accretion.

However, scientists in later period took upthe problems of origin of universe rather thanthat of just the earth or the planets.

MODERN THEORIES

Origin of the Universe

The most popular argument regarding the originof the universe is the Big Bang Theory. It is alsocalled expanding 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, ifyou start inflating the balloon, the points markedon the balloon will appear to be moving awayfrom 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. Scientistsbelieve that though the space between thegalaxies is increasing, observations do notsupport the expansion of galaxies. So, theballoon 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 theuniverse existed in one place in the formof a “tiny ball” (singular atom) with anunimaginably small volume, infinitetemperature and infinite density.

(ii) At the Big Bang the “tiny ball” explodedviolently. This led to a huge expansion.It is now generally accepted that theevent of big bang took place 13.7 billionyears before the present. The expansioncontinues even to the present day. As itgrew, some energy was converted intomatter. There was particularly rapidexpansion within fractions of a secondafter the bang. Thereafter, theexpansion has slowed down. Within firstthree minutes from the Big Bang event,the first atom began to form.

(iii) Within 300,000 years from the BigBang, temperature dropped to 4,500 K(Kelvin) and gave rise to atomic matter.The universe became transparent.

The expansion of universe means increasein space between the galaxies. An alternativeto this was Hoyle’s concept of steady state. Itconsidered the universe to be roughly the sameat 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 basesfor development of galaxies. A galaxy containsa large number of stars. Galaxies spread overvast distances that are measured in thousandsof light-years. The diameters of individualgalaxies range from 80,000-150,000 lightyears. 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 havetaken 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 9.461 1012 km. The meandistance between the sun and the earthis 149,598,000 km. In terms of lightyears, it is 8.311minutes.

Formation of Planets

The following are considered to be the stagesin the development of planets :

(i) The stars are localised lumps of gaswithin a nebula. The gravitational forcewithin the lumps leads to the formationof a core to the gas cloud and a hugerotating disc of gas and dust developsaround 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-roundedobjects 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 number ofsmall planetesimals accrete to form afewer large bodies in the form of planets.

OUR SOLAR SYSTEM

Our Solar system consists of eight planets. Thenebula from which our Solar system is supposedto have been formed, started its collapse and coreformation some time 5-5.6 billion years ago andthe planets were formed about 4.6 billion yearsago. Our solar system consists of the sun (thestar), 8 planets, 63 moons, millions of smallerbodies like asteroids and comets and hugequantity of dust-grains and gases.

Out of the eight planets, mercury, venus,earth and mars are called as the inner planetsas they lie between the sun and the belt ofasteroids the other four planets are called the outerplanets. Alternatively, the first four are calledTerrestrial, meaning earth-like as they are madeup of rock and metals, and have relatively highdensities. The rest four are called Jovian or GasGiant planets. Jovian means jupiter-like. Mostof them are much larger than the terrestrialplanets and have thick atmosphere, mostly ofhelium and hydrogen. All the planets were formed

in the same period sometime about 4.6 billionyears ago. Till recently (August 2006), Pluto wasalso considered a planet. However, in a meetingof the International Astronomical Union, adecision was taken that Pluto like other celestialobjects (2003 UB

313)discovered in recent past

may be called ‘dwarf planet’. Some data regardingour solar system are 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 in theclose vicinity of the parent star where itwas too warm for gases to condense tosolid particles. Jovian planets wereformed at quite a distant location.

(ii) The solar wind was most intense nearerthe sun; so, it blew off lots of gas and dustfrom the terrestrial planets. The solar windswere not all that intense to cause similarremoval of gases from the Jovian planets.

(iii) The terrestrial planets are smaller andtheir 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 havebeen 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

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058

Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66

Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88

Satellites 0 0 1 2 16 about 18 about 17 8

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

# Radius: Equatorial radius 6378.137 km = 1

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became a dumb-bell-shaped body andeventually it broke. It was also suggested thatthe material forming the moon was separatedfrom what we have at present the depressionoccupied by the Pacific Ocean.

However, the present scientists do notaccept 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 ofblasted material then continued to orbit theearth and eventually formed into the presentmoon about 4.44 billion years ago.

EVOLUTION OF THE EARTH

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 events–processes, which may have caused this changefrom rocky, barren and hot earth to a beautifulplanet with ample amount of water andconducive atmosphere favouring the existenceof life. In the following section, you will findout how the period, between the 4,600 millionyears 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 earth’s interior has different zonesand each of these contains materials withdifferent characteristics.

How was the layered structure of theearth developed?

Evolution 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 thesurface. 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. Weshall discuss in detail the properties of each ofthis layer in the next chapter.

Evolution of Atmosphere and Hydrosphere

The present composition of earth’s atmosphereis chiefly contributed by nitrogen and oxygen.You will be dealing with the composition andstructure of the earth’s 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 earthcontributed to the evolution of the atmosphere.Finally, the composition of the atmosphere wasmodified by the living world through theprocess of photosynthesis.

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 andwater 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, thewater vapour released started gettingcondensed. The carbon dioxide in theatmosphere got dissolved in rainwater and thetemperature further decreased causing morecondensation and more rains. The rainwaterfalling onto the surface got collected in thedepressions to give rise to oceans. The earth’soceans were formed within 500 million yearsfrom the formation of the earth. This tells us

that the oceans are as old as 4,000 millionyears. Sometime around 3,800 million yearsago, life began to evolve. However, around2,500-3,000 million years before the present,the process of photosynthesis got evolved. Lifewas confined to the oceans for a long time.Oceans began to have the contribution ofoxygen through the process of photosynthesis.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 EventsBefore Present

Quaternary Holocene 0 - 10,000 Modern Man

Pleistocene 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 million 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-replace

amphibians

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-Cambrian

570 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 isundoubtedly clear that initially the earth oreven the atmosphere of the earth was notconducive for the development of life. Modernscientists refer to the origin of life as a kind ofchemical reaction, which first generatedcomplex organic molecules and assembledthem. This assemblage was such that theycould duplicate themselves converting

inanimate matter into living substance. Therecord of life that existed on this planet indifferent periods is found in rocks in the formof fossils. The microscopic structures closelyrelated to the present form of blue algae havebeen found in geological formations much olderthan some 3,000 million years. 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


(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 earth’s 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.htmland www.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 the Stardust is being collected?

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What do you imagine about the natureof the earth? Do you imagine it to bea solid ball like cricket ball or ahollow ball with a thick cover of rocks i.e.lithosphere? Have you ever seen photographsor images of a volcanic eruption on thetelevision screen? Can you recollect theemergence of hot molten lava, dust, smoke, fireand magma flowing out of the volcanic crater?The interior of the earth can be understood onlyby indirect evidences as neither any one has norany one can reach the interior of the earth.

The configuration of the surface of the earthis largely a product of the processes operatingin the interior of the earth. Exogenic as well asendogenic processes are constantly shapingthe landscape. A proper understanding of thephysiographic character of a region remainsincomplete if the effects of endogenic processesare ignored. Human life is largely influencedby the physiography of the region. Therefore,it is necessary that one gets acquainted withthe forces that influence landscapedevelopment. To understand why the earthshakes or how a tsunami wave is generated, itis necessary that we know certain details of theinterior of the earth. In the previous chapter,you have noted that the earth-formingmaterials have been distributed in the form oflayers from the crust to the core. It is interestingto know how scientists have gatheredinformation about these layers and what arethe characteristics of each of these layers. Thisis exactly what this chapter deals with.

INTERIOR OF THE EARTH

SOURCES OF INFORMATION ABOUT THE INTERIOR

The earth’s 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 earth’s 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 materialis 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. Scientistsworld over are working on two major projectssuch as “Deep Ocean Drilling Project” and“Integrated 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 largevolume 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


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 indirect sources includegravitation, 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 isbecause 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 oppositedirections. 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, theblocks 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 the focus of 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 called‘seismograph’ records the waves reaching thesurface. 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. Bodywaves are generated due to the release of energyat the focus and move in all directions travellingthrough the body of the earth. Hence, the name

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 asthey 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 threewaves vibrate perpendicular to the direction ofpropagation. 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 105° from theepicentre, recorded the arrival of both P andS-waves. However, the seismographs locatedbeyond 145° from epicentre, record the arrivalof P-waves, but not that of S-waves. Thus, azone between 105° and 145° from epicentre wasidentified as the shadow zone for both the typesof waves. The entire zone beyond 105° does 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 105° and 145° awayfrom 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 cantravel 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 reboundwhereas refraction makes waves move indifferent directions. The variations in thedirection of waves are inferred with the help oftheir record on seismograph. The surfacewaves are the last to report on seismograph.These waves are more destructive. They causedisplacement of rocks, and hence, the collapseof 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


(v) The earthquakes that occur in the areasof large reservoirs are referred to asreservoir induced earthquakes.

Measuring Earthquakes

The earthquake events are scaled eitheraccording to the magnitude or intensity of theshock. The magnitude scale is known as theRichter scale. The magnitude relates to theenergy released during the quake. Themagnitude is expressed in absolute numbers,0-10. The intensity scale is named afterMercalli, an Italian seismologist. The intensityscale takes into account the visible damagecaused by the event. The range of intensity scaleis from 1-12.

EFFECTS OF EARTHQUAKE

Earthquake is a natural hazard. The followingare the immediate hazardous effects ofearthquake:

(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 bearingsupon landforms, while others may beconsidered the effects causing immediateconcern to the life and properties of people inthe region. The effect of tsunami would occuronly if the epicentre of the tremor is belowoceanic waters and the magnitude issufficiently high. Tsunamis are wavesgenerated by the tremors and not anearthquake in itself. Though the actual quakeactivity lasts for a few seconds, its effects aredevastating provided the magnitude of thequake 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 collapse earthquakes.

(iv) Ground shaking may also occur due tothe 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 ofpeople. However, not all the parts of the globenecessarily experience major shocks. We shallbe discussing the distribution of earthquakesand volcanoes with some details in the next

STRUCTURE OF THE EARTH

The Crust

It is the outermost solid part of the earth. It isbrittle 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 fromMoho’s discontinuity to a depth of 2,900 km.The upper portion of the mantle is calledasthenosphere. The word astheno meansweak. 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


its way to the surface during volcaniceruptions. It has a density higher than thecrust’s (3.4 g/cm3). The crust and theuppermost part of the mantle are calledlithosphere. Its thickness ranges from 10-200 km.The lower mantle extends beyond theasthenosphere. It is in solid state.

The Core

As indicated earlier, the earthquake wavevelocities 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/cm3 and 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 sometimesreferred to as the nife layer.

VOLCANOES AND VOLCANIC LANDFORMS

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. Avolcano 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


examples. These volcanoes are mostly madeup of basalt, a type of lava that is very fluidwhen erupted. For this reason, these volcanoesare not steep. They become explosive ifsomehow 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 volcanoes are characterised byeruptions of cooler and more viscous lavasthan basalt. These volcanoes often result inexplosive eruptions. Along with lava, largequantities of pyroclastic material and ashesfind 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 Traps fromIndia, presently covering most of theMaharashtra plateau, are a much larger floodbasalt 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 shallbe discussing this in detail in the next chapter.

VOLCANIC LANDFORMS

Intrusive Forms

The lava that is released during volcaniceruptions on cooling develops into igneousrocks. The cooling may take place either onreaching the surface or also while the lava isstill in the crustal portion. Depending on thelocation of the cooling of the lava, igneous rocksare classified as volcanic rocks (cooling at thesurface) and plutonic rocks (cooling in thecrust). The lava that cools within the crustalportions assumes different forms. These formsare called intrusive forms. Some of the formsare shown in Figure 3.5.Composite Volcano

Caldera

These are the most explosive of the earth’svolcanoes. 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 thatthe 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 theworld 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


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 earth’s 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 don’t 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 surfaceonly 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 thatfinds 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 ahorizontal 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


EXERCISES


(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. Incase it develops into a saucer shape, concaveto the sky body, it is called lapolith. A wavymass of intrusive rocks, at times, is found atthe 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 awall-like structure. Such structures are calleddykes. These are the most commonly foundintrusive forms in the western Maharashtra area.These are considered the feeders for the eruptionsthat led to the development of the Deccan traps.

In the previous chapter, you have studied theinterior of the earth. You are already familiarwith the world map. You know that continentscover 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 changingtheir 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.

CONTINENTAL DRIFT

Observe the shape of the coastline of the AtlanticOcean. You will be surprised by the symmetryof the coastlines on either side of the ocean. Nowonder, many scientists thought of thissimilarity and considered the possibility of thetwo Americas, Europe and Africa, to be oncejoined together. From the known records of thehistory of science, it was Abraham 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 Wegener—a Germanmeteorologist who put forth a comprehensiveargument in the form of “the continental drift

DISTRIBUTION OF OCEANSAND CONTINENTS

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 continentsformed a single continental mass and megaocean surrounded the same. The supercontinent was named PANGAEA, which meantall earth. The mega-ocean was calledPANTHALASSA, meaning all water. He arguedthat, around 200 million years ago, the supercontinent, Pangaea, began to split. Pangaea firstbroke into two large continental masses asLaurasia and Gondwanaland forming thenorthern and southern componentsrespectively. Subse-quently, Laurasia andGondwanaland continued to break into varioussmaller continents that exist today. A variety ofevidence was offered in support of thecontinental 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 andunmistakable match. It may be noted that amap produced using a computer programmeto 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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