Physical Geography - Block 2: Lithosphere PDF

BGGCT-131 PHYSICAL GEOGRAPHY Indira Gandhi National Open University School of Sciences Block 2 LITHOSPHERE UNIT 5 Materials of the Earth’s Crust: Rocks and Minerals 7 UNIT 6 Continental Drift, Mountain Buil...

BGGCT-131 PHYSICAL GEOGRAPHY Indira Gandhi National Open University School of Sciences Block 2 LITHOSPHERE UNIT 5 Materials of the Earth’s Crust: Rocks and Minerals 7 UNIT 6 Continental Drift, Mountain Building and Plate Tectonics 19 UNIT 7 Endogenetic Forces 37 UNIT 8 Exogenetic Processes-I 62 UNIT 9 Exogenetic Processes-II 79 Glossary 98 Course Design Committee Prof. Vijayshri Prof. Milap Chand Sharma Dr. Satya Raj Former Director, Centre for the Study of Discipline of Geography, School of Sciences, Regional Development, School of Sciences, IGNOU, New Delhi Jawajarlal Nehru University, IGNOU, New Delhi New Delhi Prof. (Retd.) H. Ramachandran Prof. Mahendra Singh Nathawat Dr. Koppisetti Nageswara Rao Discipline of Geography, Discipline of Geography, Discipline of Geography, University of Delhi School of Sciences, School of Sciences, Delhi IGNOU, New Delhi IGNOU, New Delhi Prof. Sachidanand Sinha Dr. Vijay Kumar Baraik Dr. Vishal Warpa Centre for the Study of Discipline of Geography, Discipline of Geography, Regional Development, School of Sciences, School of Sciences, Jawajarlal Nehru University, IGNOU, New Delhi IGNOU, New Delhi New Delhi Prof. N.R. Dash Dr. Subhakanta Mohapatra Department of Geography, Department of Geography, The MS University, School of Sciences, Baroda IGNOU, New Delhi Block Preparation Team Prof. Biplab Biswas, (Unit 5 and 6) Dr. Koppisetti Nageswara Rao (Unit 7, 8 and 9) Dept. of Geography, Assistant Professor The University of Burdwan, Discipline of Geography, Golapbag, Burdwan School of Sciences, IGNOU, New Delhi Prof. H.S. Sharma,(Editor) Retd. Prof. Dept. Of Geography, University of Rajasthan Course Coordinators: Prof. Mahendra Singh Nathawat and Dr. Satya Raj Block Production Team Sh. Sunil Kumar AR (P), IGNOU Graphics Artists/Cartographers: Mr Ajit Kumar Mr. Sumit Mr Anil July 2019 © Indira Gandhi National Open University, 2019 Disclaimer: Any material adapted from web-based resources or any other sources in this module are being used for educational purposes only and not for commercial purposes. All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from the Indira Gandhi National Open University. Further information on the Indira Gandhi National Open University courses may be obtained from the University's office at Maidan Garhi, New Delhi-110 068 or the official website of IGNOU at www.ignou.ac.in. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by Director, SOS, IGNOU. Printed at:  CONTENTS Block and Unit Titles 1 Credit page 2 Contents 3 BLOCK 2: LITHOSPHERE 6 Unit 5 Materials of the Earth’s Crust: Rocks and Minerals 7-18 5.1 Introduction 7 Expected Learning Outcomes 8 5.2 Fundamentals 8 5.2.1 Basic Concept of the Earth's Crust 8 5.2.2 Concept of Continental Crust 9 5.2.3 Concept of Oceanic Crust 10 5.3 Composition of the Earth's Crust 10 5.4 Rocks and Minerals: Crust Forming Materials 11 5.5 Origin and Classification of Rocks 12 5.5.1 Igneous Rocks 12 5.5.2 Sedimentary Rocks 14 5.5.3 Metamorphic Rocks 15 5.6 Rock Cycle 16 5.7 Summary 17 5.8 Terminal Question 17 5.9 Answers 17 5.10 References/Further Reading 18 Unit 6 Continental Drift, Mountain Building and Plate Tectonics 19-36 6.1 Introduction 19 Expected Learning Outcomes 20 6.2 Continental Drift Theory of Wegener 20 6.2.1 Energy Responsible for Continental Drift 20 6.2.2 Mechanism of Continental Drift as Described by Holmes 22 6.3 Theories of Mountain Building 22 6.3.1 Geosynclinal Orogen Theory of Kober 22 6.3.2 Convection Current Theory of Holmes 24 6.3.3 Plate Tectonic Theory and Mountain Building 26 6.4 Plate Tectonic Theory 28 6.4.1 Mechanism of Plate Movement 29  6.4.2 Plate Tectonics Theory: Mountain Building, Volcanism, Earthquake, and Tsunamis 31 6.4.3 Evidences of Continental Drift and Underlying Plate Tectonics 33 6.5 Summary 34 6.6 Terminal Questions 35 6.7 Answers 35 6.8 References/Further Reading 36 Unit 7 Endogenetic Forces 37-61 7.1 Introduction 37 Expected Learning Outcomes 38 7.2 Endogenetic Forces: Basics and Classification 38 7.3 Diastrophic Forces 40 7.3.1 Epeirogenic Movements 40 7.3.2 Orogenic Movements 41 7.4 Volcanism 47 7.4.1 Types of Volcanoes 49 7.4.2 Distribution of Volcanoes 51 7.5 Earthquakes 52 7.5.1 How Earthquakes Occur? 52 7.5.2 Earthquakes Waves 54 7.5.3 Magnitude and Intensity of Earthquake 55 7.6 Summary 59 7.7 Terminal Questions 60 7.8 Answers 60 7.9 References/Further Reading 61 Unit 8 Exogenetic Processes-I 62-78 8.1 Introduction 62 Expected Learning Outcomes 63 8.2 Weathering and Mass Wasting 63 8.2.1 Physical or Mechanical Weathering 64 8.2.2 Chemical Weathering 66 8.2.3 Mass Wasting 69 8.3 Concept of Cycle of Erosion 71 8.3.1 Davis Model 72 8.3.2 Penck Model 73 8.4 Summary 76 8.5 Terminal Questions 77 8.6 Answers 77 8.7 References/Further Reading 77 4 Unit 9 Exogenetic Processes-II 79-97 9.1 Introduction 79 Expected Learning Outcomes 80 9.2 Work of River 80 9.3 Work of Glaciers 83 9.4 Work of Groundwater 86 9.5 Work of Sea Waves 89 9.6 Work of Wind 92 9.7 Summary 96 9.8 Terminal Questions 96 9.9 Answers 96 9.10 References/Further Reading 97 Glossary 98-99 5  BLOCK 2: LITHOSPHERE In the previous block of this course, you have learnt about earth as a living planet. So now you are aware of various theories and concepts related to the origin of earth. You have also studied about the interior of earth which still holds a lot of curiosity as interior of earth is the least explored part. You have also learnt about isostasy which is the stability between the upstanding features and low lying areas. In this block you will study about lithosphere. Lithosphere is the rocky outermost layer of earth consisting of the crust and the solid outermost layer of the upper mantle. This block is divided into 5 units. In unit 5 you will learn about the materials of the earth's crust which includes different types of rocks and minerals. In unit 6 you will learn that as lithosphere is broken into several rigid blocks or plates, there are plate movements which is responsible for the drift of continents and the formation of mountains etc. You will also get acquainted with different theories related to the continental drift and mountain building. Unit 7 explains to you different endogenetic forces which are occurring beneath the surface of earth and constantly changing the earth topography through creation, destruction, recreation and maintenance of geo-materials. Unit 8 and 9 discusses about exogenetic forces which occur on the earth's surface. You will get acquainted with the concept of cycle of erosion given by Davis and Penck and also study the landforms created by river, glacier, groundwater, sea waves, and wind. So after studying this block, you should be able to:  define lithosphere and identify various types of rocks;  explain the process of drifting of continents and formation of mountains and plate movements;  discuss endogenetic and exogenetic processes;  describe the landforms created by river, glacier, groundwater, sea waves, and wind. So we wish you good luck for studying this block. 6 Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... UNIT 5 MATERIALSOFTHE EARTH'SCRUST:ROCKS ANDMINERALS Structure 5.1 Introduction 5.5 Origin and Classification of Expected Learning Outcomes Rocks Igneous Rocks 5.2 Fundamentals Sedimentary Rocks Basic Concept of the Earth's Crust Metamorphic Rocks Concept of Continental Crust Concept of Oceanic Crust 5.6 Rock Cycle 5.3 Composition of the Earth's Crust 5.7 Summary 5.4 Rocks and Minerals: Crust 5.8 Terminal Questions Forming Materials 5.9 Answers 5.10 References/Further Reading 5.1 INTRODUCTION In the First Block of this course, you have learnt about geosphere. In this Unit 5 of Block 2, you will study the materials of the earth's crust i.e. rocks and minerals. You will be amazed to know that not only the landmass parts but oceanic base also forms a part of the earth's crust. Literally even as small as every centimetre portion of solid mass of planet earth is composed and covered by rock particles across the variety of physical landscapes. As you all know that rocks are very hard substance and soon after formation they may last for millions of year's altogether. You will be surprised to know that the oldest known rocks are around 3.6 billion years old which roughly corresponds with the age of planet earth itself. Such types of crust forming materials are called rocks and minerals. In this unit, you will study about rocks and minerals. 7 Block 2 Lithosphere.................................................................................................................................................................................................................................... We would like to mention here that the study of rocks and minerals are not actually an essential or central part of 'Physical Geography'. It is the forte of a geologist. However, the rocks which lay exposed to various geomorphic agents like glacier, river, ocean, air or rainfall and wind etc. that helps to shape and develop different landform types are of particular interest in Physical Geography. The study of landforms is the central theme in physical geography and so it is useful to study the rocks and minerals. In this unit, we shall try to make an understanding of the basic ideas of the earth's crust, rocks and minerals as its constituents. Along with, we will also delve into the origin and classification of rocks, characteristics of the rocks and minerals and rock cycle spread over various sections and sub-sections of this unit. Expected Learning Outcomes After studying this unit, you should be able to:  explain the fundamentals regarding the materials of the earth's crust;  describe the origin and classification of different types of rocks and minerals; and  analyse the rock cycle process which is continuous and repetitive in nature on the planet earth. 5.2 FUNDAMENTALS You have already learnt in Unit 1 that earth's crust is the upper most part in the arrangement of layers or spheres spread over the earth's surface. It is a part of lithosphere which is made up of several rocks and minerals. Its average thickness is about 35-40 kilometres. In this section on fundamentals, you will learn about three concepts including concept of the earth's crust, continental crust and oceanic crust. This will provide a basis for the proper understanding and appreciation of origin and classification of rocks as well as the process of rock cycle. 5.2.1 Basic Concept of the Earth's Crust Do you have an idea about how the earth's crust has been formed? It was The cooling of earth can nearly thousands of years ago when the earth was in very hot state and be compared with the everything was in its molten state. Slowly the temperature started reducing and cooling of hot milk. As we a solid layer has formed in the outer part of Earth. It is known as 'Earth's crust', find a relative hard or where the ambient conditions exist for the continuous thriving of higher forms of thick layer part that forms on top when hot milk life including flora and fauna along with human beings. after boiling is left for Earth's crust is the top most part which is relatively rigid part like the shell of an cooling egg. The crust is formed by a combination of several rocks and minerals and there are several layers in it. All of these layers are together known as 'lithosphere'. Thickness of the crust may vary from few kilometres under the oceans to 75 kilometres under the mountainous regions like that of Himalayas. 8 Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... The crust is separated from the lower layer called Mantle. The process of separation of different layers depends upon the chemical composition, density, temperature of the constituting rocks and minerals. The line that separates crust from the mantle is known as 'Moho Discontinuity'. It was discovered in 1909 by A. Mohorovicic which marks the boundary between lower crust and upper mantle. The earth's crust is further classified into two layers i.e. 'continental crust' and 'oceanic crust. Continental crust is that part of the earth's crust where human beings live and conduct all of their activities. This part is not continuous and is separated by very large water bodies including seas and oceans, rivers, streams and lakes etc. As opposed to this, oceanic crust is continuous and lies beneath Mid-Oceanic Ridges are the volcanic mountain the continental crust and extends up to mid-oceanic ridges. It is at this point i.e. system under the oceans. mid-oceanic ridges, the earth's crust originates. These two layers are separated Here magma is coming by a line of discontinuity known as 'Conrad Discontinuity'. The Conrad out to form oceanic crust discontinuity corresponds to the sub-horizontal boundary between Continental and spreads towards the continents. and Oceanic Crust. This boundary is observed in various continental regions at a depth of 15 to 20 kilometres. However, it is not found in Oceanic regions. 5.2.2 Concept of Continental Crust It is observed that 7 billion cubic kilometres of continental crust lie above the mean sea level. It supports the life, settlement, agriculture and provide drinking water. We human beings are able to live here only because of the presence of continental crust. It is fragmented to form various major geographical features like continents, oceans and islands. About 3.7 billion years ago, due to the volcanic activities over the very thin heavier oceanic crust, the continental shields started to form. The processes of formation of continental crust from volcanic activities are visible in different parts of the World till today. You will further study in this unit that it is because of volcanic activities that gave birth to Igneous rocks which subsequently after erosion gets transported and deposited to form Sedimentary rocks. All the above rocks later on under tremenadous heat and pressure gets modified to become Metamorphic rocks. It is a continuous and repetitive process known as rock cycle. So you can understand and say that the continental crust is made up of all the three types of rocks. The continental crust is less dense than all the layers of the earth's interior. Its density is 2.7g/cm3 whereas the density of the oceanic crust is 3.0 g/cm 3 and that of the earth's mantle is 3.3 g/cm3. Main mineral composition of continental crust is Silica and Aluminium. This layer is also commonly known as Sial layer. Do you have any idea that how coastal landforms are created? The continental crusts are drifting over the heavier oceanic crust and the collision between two continental crusts or continent and oceanic crust give rise to several landforms and associated features. The oceanic crust being heavier subducts back into mantle and melts down. It is the collision zone of two continents that gives rise to taller and heavier mountain chain systems like the Himalayas. Furthermore the greatest thickness of the continental crusts is also found there. Since the 9 Block 2 Lithosphere.................................................................................................................................................................................................................................... height of the mountain is always determined by the thickness of the continental crust. On account of high temperature, pressure conditions and distortion leads to the modification of the continental crust to that of metamorphic rocks. You will be in a better position to examine the origin and destruction of the continental crust more aptly with the help of Plate Tectonics theory which you will study in the next unit of this block. 5.2.3 Concept of Oceanic Crust You will be astonished to discover that about 70% of the earth's crust is covered by oceans. The oceanic crust originates from the Mid-oceanic Ridges and spreads towards the continents and then it goes down to magma again and it melts. This could be the reason that we always find the younger oceanic crust and it is not more than 200 million years old. All of the oceanic crusts are made up of basalt rock which is a volcanic rock. Some deposits of continental sediments near the continents and organic depositions in the deep sea zone are also found there. Oceanic crusts are heavier and its density is 3.0 g/cm3. Primary wave (P) travels through this layer at a speed of 7 km/s. Mineral compositions of the rocks are mainly silica and magnesium. This layer is known as Sima. According to the Plate Tectonic Theory, the oceanic crust has its origin at the mid oceanic ridges and subducts at the convergent boundaries of two plates. Coastal features like Island arcs, volcanoes, oceanic trenches are formed at the subduction zone of two oceanic crusts. When an oceanic crust converges with a continental crust, it always subducts below the continental crust as the oceanic crust is more dense and heavy and it melts and gets subsumed into the mantle after subduction. Before you proceed to next sub-section, it will be good to recapitulate few concepts and complete the given exercise below to make a better understanding. SAQ 1 a) Define earth's crust. Spend b) Why is continental crust known as sial? 5 mins 5.3 COMPOSITION OF THE EARTH'S CRUST You are well aware that the earth's crust is an integral part of the lithosphere. The word 'litho' means rocks. Rocks are made up of minerals which are composed by its elements. So the earth's crust is composed of minerals. You will be surprised to know that there are only 8 elements in varying proportions which make up the crust of the earth's surface as shown in Table 5.1. Oxygen is the most common chemical element of the crust accounting 46% of 10 the rocks. Next important mineral is Silicon (28%). Aluminium, iron, calcium, Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... sodium, potassium and magnesium constitute about 24% whereas others constitute 1% only. The minerals vary in their structure, shape, stability, solubility, hardness, crystal structure and colour. Minerals are seldom found in isolation but exist in stable compound form. Table 5.1: Elements of the Earth's Crust ELEMENT CRUST (symbol) Percent by mass Percent by volume Oxygen (O) 46.10 94.04 Silicon (Si) 28.20 0.88 Aluminum (AL) 8.23 0.48 Iron (Fe) 5.63 0.49 Calcium (Ca) 4.15 1.18 Sodium (Ma) 2.36 1.11 Magnesium (Mg) 2.33 0.33 Potassium (K) 2.09 1.42 Nitrogen (N) Traces Traces Hydrogem (H) Traces Traces Other 0.91 0.07 It is interesting to know that different ratio of mixing of mineral elements generates different rock types over the earth's crust. If you are keen to know about the multifarious properties of minerals then you can visit the website of following governmental institutions for further exploration like 'Geological Survey of India' and 'Wadia Institute of Himalayan Geology' etc. 5.4 ROCKS AND MINERALS: CRUST FORMING MATERIALS Have you ever wondered about the ingredients which make up the earth's crust? Basically it is formed by different rock types and their constituent minerals. All rock types are formed with particular composition of different minerals. The size of the minerals may vary from microscope to few inches. You may be wondering that how it is possible to form rocks from such tiny mineral particles. The minerals are held together with particular strength and density and they determine the nature of any rock. The most common rock types found over the continental surface is:  Shale: it is composed mainly by clay minerals and mixed with fine quartz grains,  Sandstone: it is composed mainly by quartz sand and mixed with feldspars,  Granitic rocks: these are composed together from quartz and potash feldspars mixed with amphibole or biotite mica, 11 Block 2 Lithosphere....................................................................................................................................................................................................................................  Limestone and dolomite: these two are composed mainly from calcite or dolomite,  Basaltic rocks: it is composed of plagioclase feldspars, biotite mica, pyroxenes and olivine etc. You have learnt that together these above-mentioned five rock types constitute about 92% of the total rocks found over the earth's surface. Let's attempt to recapitulate and memorize the key points studied so far before moving to the next section. SAQ 2 State the composition of the earth's crust. Spend 5 mins 5.5 ORIGIN AND CLASSIFICATION OF ROCKS You may be wondering that how we can classify rocks into distinct types. Well rocks can be classified on the basis of several criteria. Such criteria includes chemical and physical properties, mineral content, mineral size grains, types of elements and origin of rocks etc. However, rocks are most commonly classified on the basis of their origin. Three major well known identified rock groups are Igneous, Sedimentary and Metamorphic rocks. You will now learn and explore more about these three major rock types along with their sub- types in the subsequent sections. 5.5.1 Igneous Rocks Igneous rocks are originated in the initial stage of earth formation by the cooling of hot magma. Approximately, more than 80 per cent of earth's crust is made up of igneous rocks. The oldest known igneous rocks are thought to be as old as nearly 3.6 billion years. And the formation of youngest igneous rocks is still in progress as you are reading this unit since rock cycle is a continuous and repetitive natural process. Igneous rocks are also known as primary or parent rocks. All rock types are developed either directly or indirectly from the igneous rock. Magma is a very hot mixture of minerals and gases. Often, it comes out to the earth's surface through some weak points or some time gets stored under the surface. Classification of the igneous rocks depends on several factors. These factors encompass location of cooling of the hot magma, the mineral content, mineral size and acidity etc. The major characteristics are as follows:  the rock forming elements are very compact which increases its hardness;  the igneous rocks are very hard and heavy and therefore resistant to erosion;  water cannot percolate through the rocks as there is no porosity;  there is no layer as it is seen in sedimentary rocks;  Igneous rocks do not contain any fossil but contains crystals and may be 12 visible to the bare eyes. Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... You have studied that igneous rocks originate initially from the cooling of the magma inside the earth's crust or on the earth's surface. The cooling rate gives rise and helps to shape the chemical composition and mineral size etc. So the igneous rocks are classified primarily based on two parameters origin and chemical composition. Now you will learn about the classification as well as variety of same rock type on the basis of a) place of origin and b) chemical composition. a) Based on Place of Origin When the rocks originate deep inside the earth's crust, it is known as intrusive igneous rocks. Granite is an example of intrusive rock. Intrusive rocks are characterized by larger mineral size and may be visible with bare eyes to a keen observer. Some igneous rocks originate in very deep portions of the crust and are having even larger minerals. This type of intrusive rock is called plutonic igneous rock. When it originates immediate below the earth's surface, it is known as hypabyssal igneous rock having smaller mineral size. The example is Porphyry. Magma reaches the earth's surface through the weak points like joints, fractures and faults etc. Subsequently it cools very fast when it comes into contact with air and generates very finer microscopic minerals. This type of igneous rock is known as extrusive rock. Basalt is an example of extrusive rock. Magma may come out at the earth's surface from the mantle through volcanoes and generates lava rocks and the ash and other Pyroclastic rock. Tuff is an example of pyroclastic rock. b) Based on Chemical Composition Silica is a very important mineral of igneous rock and determines the pH or acidity of the rocks. Based on the acidic condition of the igneous rock, rocks can be classified into four categories. i) Acidic Igneous rock: The content of silica is more than 65%. Granite is an acidic igneous rock. ii) Basic Igneous Rock: Basalt is an example where the content of silica lies between 45-60%. Iron content of this rock is very high. iii) Intermediate Igneous Rock: The pH of the rock is neutral and silica content is such that neither the rock falls in acidic nor in basic category. Diorite is an example of this type of rock. iv) Ultra-basic: Igneous rock is peridotite. In this rock silica content is less than 45%. Before you start reading the next section let us remember the crux of this section with the help of following activity in the form of check your progress. SAQ 3 Classify igneous rocks based on their origin and chemical composition. Spend 5 mins 13 Block 2 Lithosphere.................................................................................................................................................................................................................................... 5.5.2 Sedimentary Rocks It will be interesting for you to know that nearly 70 per cent of landmass of the planet earth is covered under thin layers of sediments or debris. Such huge piles of sediments eventually gets settled on the beds of major and minor water bodies like oceans and seas, lakes and rivers etc. With the passage of time, rock materials becomes compacted over millions of years and thereafter erosion takes place and material is carried together with older rocks by agents of erosion such as water, ice, wind or wave etc. under the influence of gravity. Such kind of eroded and transported sediments/debris materials are known as sedimentary rocks. Later on the materials come to rest and gradually gets compacted, consolidated and cemented together. In this way you will notice that sand becomes sedimentary sandstone rock and similarly clay becomes sedimentary shale rock etc. Although sedimentary rocks cover extensive areas but account only 8% of the total area of the earth's surface. Sedimentary rocks exist as layers of different rock beds. These are further separated by bedding planes. It is interesting to know that sedimentary rocks are very important source of coal, natural oil, In 1802, a farmer of drinking water and ores. Such types of natural resources play an important role Connecticut Valley of New in the spatial and economic activities. England found a block of The main characteristics of sedimentary rocks are as follows: red sandstone in his field showing footprint of  sedimentary rocks are secondary formation and they have layers of beds dinosaur of 200 million separated by bedding plane; years ago. It was a Fossil! We can say that  it is characterized by pore spaces and is lighter than the igneous rocks; the remnants of plants,  it is the source of fossil fuels. They do not have any crystalline structure. animals or their impressions may get Now we will discuss the classification scheme on the basis of a) place of origin converted as fossils. and b) chemical composition as discussed earlier in the case of igneous rocks. a) Based on Place of Origin Sedimentary rocks are secondary rocks as it originates from the primary igneous rocks. The weathered and eroded materials are transported by air, water, sea or glaciers. Such type of material gets deposited in different depositional environment like river bed, lakes or in seas. The sediment characteristics vary significantly on the basis of weathering processes, natural agents of erosion along with transportation. Furthermore, the characteristics also differ on account of the particle size, mixing and sorting of particles, chemical alteration, organic character, cementing and compaction etc. When the parent rocks are broken up into small pieces, it starts accumulating and compacting to form new rocks known as clastic sedimentary rocks. Based on the depositional agency, environment and grain size, this group of sedimentary rocks can further be classified as conglomerate, sandstone, loess and clay etc. b) Based on Chemical Composition Rock particles may be chemically altered during the process of chemical 14 weathering and gets deposited to make chemically formed sedimentary rocks. Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... This rock is commonly found in the arid and semi arid regions of the world. Gypsum is the best example of this type of rock. Decomposition and disintegration of organic residue sometimes leads to the cementing and compaction of the rock particles to form a type of sedimentary rocks. Coal, dolomite and limestone are this type of rock. Most of the sedimentary rocks originate from the marine, sea, river or lake environment. They can be grouped into aqueous sedimentary rocks. Loess sedimentary rock originated mainly due to aeolian process.It is known as aeolian sedimentary rock. Another type like till and moraine types of sedimentary rocks originated mainly due to the glacial processes. They can be grouped into glacial sedimentary rocks. You are required to recapitulate the main points through the exercise before reading the next section on metamorphic rocks. SAQ 4 What are the major characteristics of sedimentary rocks? Spend 5.5.3 Metamorphic Rocks 5 mins The primary igneous and secondary sedimentary rocks may change their physical and chemical characteristics and form due to the huge pressure and heat. The transformed tertiary rocks are called metamorphic rocks. This word has its origin in the Greek word metamorphosis. Metamorphism makes the rocks to go through the process of re-crystallization which results into the transformation of the Protolith into harder and denser rock. Re-crystallization destroys bedding planes and joints systems. As a result, the marble or quartz may show no stratifications in the rocks. The metamorphic rocks cover a significant part of the earth's crust. It can be classified on the basis of types of transformed rocks, transforming agents and place of transformation. The major characteristics of metamorphic rocks are as follows:  metamorphic rocks are denser and harder than the original Protolith.  the re-crystallization process makes bigger crystals which are prominent.  the ores are segregated that makes it easy to mine. These rocks do not contain fossils.  they may contain well developed foliated or lineated structure. Metamorphism of the original igneous or sedimentary rocks may happen due to intense pressure and heat by agents of metamorphism in many places or localities. The original rocks may undergo intense pressure due to convergence of two plates and the rocks may get folded and transformed. The magma may come to the earth's surface and the heat may transform the rocks. Sometimes water penetrates deep into the crust and its heat, pressure and chemical reaction can alter the original rocks. In all the cases the effect of transformation may be very localized or regional. The metamorphism of rocks generates new texture, composition and foliation of rocks. These are classified based on the foliation and original rocks. 15 Block 2 Lithosphere.................................................................................................................................................................................................................................... Foliated Metamorphic Rocks: Foliated Metamorphic rocks are very complex in its composition. The texture is layered, foliated, lineated, banded and the minerals are oriented. Slates and schist are examples of foliated metamorphic rocks. Non-Foliated Metamorphic Rocks: These rocks are simple in its composition. The texture is granular and equi-dimensional and has no definite orientation of the minerals. Quartzite and Marble are non-foliated metamorphic rocks. Metamorphic Rocks with an Igneous Protolith: Metamorphic rocks are considered to be derived from an original igneous protolith either because of the lithological characteristics (i.e. preservation of igneous textures and in some cases composition or mineralogy) or the lithological associations of the rock. Such rock types are classified as igneous metamorphic rocks. Pyroxene and hornblende are examples of this type of rocks. Metamorphic Rocks with a Sedimentary Protolith: If the rocks are known to be derived from original sedimentary rock i.e. protolith, either because of the lithological characteristics or the lithological associations of the rock it is classified as metamorphic rocks with sedimentary protolith. Phyllite and quartzite come under this type of rocks. Before going to the last section, you should carry out the following exercise just to ensure that you have memorized all the key points of this section well. SAQ 5 Describe the classification of the metamorphic rocks. Spend 5 mins 5.6 ROCK CYCLE You are already familiar with and learnt about the rock cycle in Unit 3 of Block 1 of this course. You can refer back this section to learn about the same in detail. By now, you are already familiar that it is one of the several cycles continually happening over the earth's surface being driven by the Sun's energy as like all other systems on the planet earth. Just to let you understand one of the other parallel systems as like that of rock cycle is hydrological cycle. You will study about it in Unit 15 of same course. Some other prominent ones could be ecosystem, lithosphere, atmosphere, hydrosphere, cryosphere, biosphere etc. which you have already studied in Unit 1 of this course and further study in block 3 of same course. To recapitulate, it refers to the constant cycling and recycling of rock particles over the planet earth's lithosphere. After doing so, you can see, notice and understand the intricate and complex relationship between one system to another as well as across all the systems through the seamless flow and exchange of energy and matter. In other words, it means the change of one rock type into another rock type and vice-versa. The rock particles continuously alter its shape as well as structure under the conditions of pressure and heat. It is a gradual and long-term 16 process which operates under the influence of natural agents. These include Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... weathering and mass wasting processes, wind and water etc. In turn the rock cycle facilitates our understanding about the myriad inter-relationships that exists between different rock types on one hand as well as amongst different components of the planet earth as a system on the other hand. Besides it also helps to explore the genesis and interrelationships of three major rock types encompassing igneous, sedimentary and metamorphic. 5.7 SUMMARY In this unit, you have learnt the following issues which characterize the materials of the earth's crust:  Crust is the upper part of planet earth and is made up of many rocks and minerals.  The crust is forming at the mid-oceanic ridges, in volcanoes and upliftments of sedimentary deposits through earth's movements.  The crust is destroying at the converging plate margins.  You have also learned that the earth's crust is having two parts- continental part and oceanic part.  The crust is made up of different kinds of rocks including three major groups' being igneous, sedimentary and metamorphic rocks.  The distinction and classification of the rocks are made by their constituent minerals and structure.  Further, you have got the idea that each of the rocks is classified into diverse forms based on their nature of origin and mineral composition and other factors.  And, you have also learnt that mostly all inter and intra -categories of rocks have distinct characteristics and rock cycle which happens to be a continuous and repetitive process on the planet earth. 5.8 TERMINAL QUESTIONS 1. Briefly describe the composition of the earth's crust. 2. Highlight the salient features regarding the origin and classification of igneous rocks. 3. Discuss in detail the chief characteristics of metamorphic rocks. 5.9 ANSWERS Self-Assessment Questions 1. a) Surface layer of the earth is called earth's crust. Its average depth is about 35-40 kilometres. The crust is formed by several rocks and minerals and there are several layers in it. All these together are called lithosphere. b) Main composition of minerals of continental crust is Silica and Aluminium. That is why this layer is known as Sial layer. 17 Block 2 Lithosphere.................................................................................................................................................................................................................................... 2. Oxygen is the most common chemical elements of the crust accounting 46% followed by Sillicon (28%). Aluminium, iron, calcium, sodium, potasium and magnesium constitute about 24% whereas others constitute 1% only. 3. Based on origin- intrusive and extrusive Based on Chemical composition- acidic, basic, intermediate and ultra- basic. 4. Characteristics of sedimentary rocks are: (i) secondary formation and they have layers of beds separated by bedding plane; (ii) pore space and is lighter than the igneous rocks; (iii) source of fossil fuel; and (iv) do not have any crystallise structure. 5. Based on the foliation - folliated and non-folliated Based on original rocks - Metamorphic rocks with igneous protolith and metamorphic rocks with sedimentary protolith. Terminal Questions 1. As you know that the earth's crust is composed of various rocks and their constituent minerals. Your answer should focus and cover on such rocks and minerals. For answering this question you can refer sub-section 5.3 in detail. Just try to make us understand that you have got it right by highlighting the salient features. 2. You have studied three major rock groups and their constituent minerals in detail. Your answer should highlight the salient features of rock forming materials. For answering this question you can refer section 5.5.1 in detail. 3. You know that metamorphic rocks are the third major rock types. Your answer should highlight importance of this rock type. For answering this question you can refer section no. 5.5.3 in detail. 5.10 REFERENCES/FURTHER READING 1. Butzer, K. W. (1976). Geomorphology From The Earth. Harper & Row Publishers, New York. 2. Lake, P. (1966). Physical Geography. Macmillan and Co Ltd., London. 3. Majid, H. (2007). Fundamentals of Physical Geography. Rawat Publication, New Delhi. 4. Monkhouse, F.J. (1984). Principles of Physical Geography. Hodder and Stoughton, London. 5. Singh, S. (2012). Physical Geography. Prayag Pustk Bhawan, Allahabad. 6. Sparks, B.W. (1971). Rocks and Relief. London, Longmans. 18 Unit 5 Materials of the Earth's Crust: Rocks and Minerals.................................................................................................................................................................................................................................... UNIT 6 CONTINENTALDRIFT, MOUNTAINBUILDING ANDPLATETECTONICS Structure 6.1 Introduction 6.4 Plates Tectonic Theory Expected Learning Outcomes Mechanism of Plate Movement 6.2 Continental Drift Theory of Plate Tectonic Theory: Mountain Wegner Building, Volcanism, Earthquake Energy Responsible for Continental and Tsunamis Drift Evidences of Continental Drift Mechanism of Continental Drift as and Underlaying Plate Tectonics described by Holmes 6. 5 Summary 6.3 Theories of Mountain Building 6. 6 Terminal Questions Geosynclinal Orogen Theory of Kober 6. 7 Answers Convection Current Theory of Holmes 6. 8 References/Further Reading Plate Tectonics Theory and Mountain Building 6.1 INTRODUCTION You have studied materials of the Earth's crust in Unit 5 of this Block. In this unit you will learn about continental drift theory of Wegener, mountain building theories of Kober and Homes, and theory of plate tectonics. The continents are in motion relative to each other. After going through the Section 6.2, you will be able to define the 'Continental Drift' which is the movement of the Earth's continents relative to each other by appearing to drift across the ocean bed. This concept was independently and more precisely proposed by Alfred Wegener. The theories related to mountain building are propounded by various scientists explained in Section 6.3. Wegener's idea eventually helped to form the theory of plate tectonics. The theory of continental drift was supported by the theory of plate tectonics, which has been explained elaborately in Section 6.4. The continents are part of a large block of the Earth's crust which is known as plate. 19 Block 2 Lithosphere.................................................................................................................................................................................................................................... Expected Learning Outcomes After studying this unit, you should be able to:  explain the basic idea of continental drift;  explain the relationship between the theories of continental drift and plate tectonics;  describe the origin and relationship of mountain building, Earthquake, volcanicity, tsunamis with plate tectonics. 6.2 CONTINENTAL DRIFT THEORY OF WEGENER It is established fact that the Earth crust is made up of different parts, called plates, which move horizontally over the asthenosphere. This idea was later fully adopted and developed by Alfred Wegener in 1912 written in a book "The Density of the continental Origin of Continents and Oceans". This whole idea of the movements of crust is lighter than all continent crustal plates was termed as continental drift. The continents might the layers of the Earth's have drifted which was first put forwarded by Abraham Ortelius in 1596. Let us interior and its density is now discuss about the continental drift theory. The 'Continental Drift Theory' of 2.7g/cm3. Main Alfred Wegener suggests that there might have been horizontal displacement composition of minerals of the continental masses on a global scale. Wegener assumed that there was of continental crust is Silica and Aluminum. only one super-continent named Pangaea meaning 'all lands'. It was That is why this layer is surrounded by another super-ocean called Panthalassa. The northern part of known as SIAL layer. Pangaea was called Angaraland and the southern part was called Gondwanaland. The northern and southern parts were separated by geosyncline, a long narrow shallow sea called Tethys, in which sediments were deposited. The minerals constituents silica and aluminum covering Oceanic crusts are continental crust moved over the oceanic crust, which consists of silica and heavier and its density is 3.0 g/cm3. The mineral magnesium minerals. As you have already studied in Unit 5, there are two compositions of the types of crusts i) continental crust also known as SIAL and ii) oceanic crust rocks are mainly Silica called as SIMA. The deep sea floor formed as the upper surface of SIMA is and Magnesium and so largely composed of basalt. During the Carboniferous period, the super the layer is known as continent Pangaea got broken and started drifting over SIMA. SIMA. Although, it was not well defined what may be the condition of the continents during pre-Carboniferous period but the postulation of a Carboniferous Pangaea does not mean that Wegner disbelieved in pre-Carboniferous drift. Major events before this time are known with much less certainty whereas the distribution of plants and animals can largely be explained by movements, which have taken place since the Carboniferous period (Steers, 1961). The Pangaea was disrupted during subsequent periods and broken landmasses drifted away from each other and thus the present position of the continents and ocean basins became possible. 6.2.1 Energy Responsible for Continental Drift According to Wegener the continents after breaking away from the Pangaea moved in two directions: (i) Equator ward movement; and (ii) Westward movement. Refer to the Fig. 6.1 which explains how the present continents 20 were once united in a single super continent as Pangaea. Unit 6 Continental Drift, Mountain Building and Plate Tectonics.................................................................................................................................................................................................................................... PERMIAN TRIASSIC 250 million years ago 200 million years ago JURASSIC CRETACEOUS 145 million years ago 65 million years ago PRESENT DAY Fig. 6.1: Disruption of Pangaea and drifting of continents. (Source: Illustration from USGS, 2012, URL: http:// pubs.usgs.gov) The equator ward movement of southern part of broken Pangaea was mainly caused by gravitational differential force and force of buoyancy. The continental blocks are formed of lighter materials (SIAL) and are floating with friction on relatively denser oceanic crust (SIMA). Thus the equator ward movement of the continental blocks would depend on the relation of the centre of gravity and the centre of boundary of the floating continental mass. Generally, these two types of forces were operating on opposite directions. But because of the ellipsoidal form of the Earth, the resultant movements were directed toward the equator. The westward movement of the continents was caused by the tidal force of the sun and the moon. According to Wegener, the maximum gravitational force of the sun and the moon was there when the moon was nearest to the Earth. The force dragged the SIAL over the interior of the Earth, SIMA towards the west. This theory lacks the strength for explaining the potential force responsible for the movement of the continents. Such forces were extraordinarily small, but it is claimed that even these very small forces may be responsible to cause continental movement. 21 Block 2 Lithosphere.................................................................................................................................................................................................................................... SAQ 1 What are Pangaea and Panthalassa? Spend 5 mins 6.2.2 Mechanism of Continental Drift as Described by Holmes It was Arthur Holmes, in 1919, who suggested the mechanism of continental drift in more scientific way. The theory says- (i) that the continents are drifting/ carried away by the horizontal flow of hot magma on which they sit, and (ii) the mantle is flowing because of its convection current. Holmes suggested that rocks in the interior of the Earth would buoyantly rise toward the surface from deep within the Earth when heated by radioactivity and then sink back down as they cool and become denser. He theorized that convection currents move through the mantle the same way heated air circulates through a room, and radically reshape the Earth's surface in the process. He proposed that upward convection might lift or even rupture the crust, that lateral movement could propel the crust sideways like a conveyor belt, and that is where convection turned downwards, the buoyant continents would crumple up and form mountains. Holmes also understood the importance of convection as a mechanism for loss of heat from the Earth and of cooling its deep interior. Not until after World War II, could scientists produce the hard evidence to support Holmes's fundamental concept. Holmes' theories have continued to be reinforced by new data from seismologists, mineral physicists and geochemists (Earth: Inside and out, 2000). 6.3 THEORIES OF MOUNTAIN BUILDING Wegener also attempted to solve the problem of the origin of folded mountains of Tertiary period on the basis of his Continental Drift Theory. The frontal edges of westward drifting continental blocks of North and South America were crumpled and folded against the resistance of the rocks of the sea floor and thus the western cordilleras of the North and South America (Rockies, Andes and other mountains) were formed. Similarly, the Alpine ranges of Eurasia were folded due to equatorward movement of Eurasia and Africa together with Peninsular India. Here, Wegener postulated contradicting view points. According to Wegener, SIAL was floating upon SIMA without any friction and resistance but during the later part of his theory he pointed out that mountains were formed at the frontal edge of floating and drifting continental blocks due to friction and résistance offered by SIMA. 6.3.1 Geosynclinal Orogen Theory of Kober Kober not only attempted to explain the origin of mountains on the basis of his Geosynclinal Theory but he also attempted to elaborate the various aspects of mountain building, e.g. formation of mountains, their geological history and evolution and development. Kober's Geosynclinal Theory is based on the forces of the contraction 22 produced by the cooling of the Earth. The force of contraction generated due to Unit 6 Continental Drift, Mountain Building and Plate Tectonics.................................................................................................................................................................................................................................... cooling of the rigid masses or forelands which squeeze buckle and fold the sediments into mountain ranges. Base of the Theory According to Kober, there were mobile zones of water in the places of present day mountains. He called those mobile zones of water as geosynclines or Orogen. The mobile zones of geocyncline were surrounded by rigid masses which were termed by Kober as 'Kratogen'. Kober identified six major periods of mountain building. Three mountain building periods, about which very little is known, are reported to have occurred during Pre-Cambrian period. Palaeozoic era saw two major mountain building periods-the Caledonian orogenesis was completed by the end of Silurian period and the Variscan orogeny was culminated in Permo-Carboniferous period. The last (6th) orogenic activity known as Alpine orogeny was completed during Tertiary epoch. Mechanism of the Theory According to him, the whole process of mountain building passes through three closely linked stages of lithogenesis, orogenesis and gliptogenesis. The geosynclines are long and wide mobile zones of water which are bordered by rigid masses which have been named by Kober as forelands or kratogen. These upstanding land masses or forelands are subjected to continuous erosion by fluvial processes and eroded materials are deposited in the geosynclines. This process of sediment deposition is called sedimentation. The ever-increasing weight of the deposited sediments due to gradual sedimentation put enormous pressure on the beds of geosynclines, resulting gradual subsidence of the beds of the geosynclines. The second stage of mountain formation is called the stage of orogenesis. Both the forelands started moving towards each other because of horizontal movement cause by the force of contraction resulting from the cooling of the Earth. The compressive force generated by the movement of foreland causes contraction, squeezing and ultimately folding of sediments deposited in the geosyncline to form folded mountain ranges. If the compressive forces are normal and of the moderate intensity, only the marginal sediments of the Marginal Ranges Marginal Ranges Fig. 6.2: Kober's Geosynclinal Theory of Mountain Building. (Source: Savindra Singh, 2012, Physical Geography) 23 Block 2 Lithosphere.................................................................................................................................................................................................................................... geosynclines are folded to form two marginal ranges and middle portion of the geosynclines remains unaffected by folding activity. This unfolded middle portion is called Zwischengebirge or median mass (Fig. 6.2). According to Kober, the Alpine mountain chain of Europe can be explained on the basis of median masses. Third stage of mountain building is characterised by gradual rise of mountains and their denudation by fluvial and other processes. Continuous denudation result into gradually lowering of the height of mountains. 6.3.2 Convection Current Theory of Holmes Arthur Holmes postulated his thermal convection current theory during 1928-29 to explain the intricate problems of the origin of major relief features of the Earth's surface. The driving force of mountain building implied by Arthur Holmes is provided by thermal convection current originating from deep within the Earth. Base of the Theory The origin of convective current within the Earth depends on the presence of radioactive elements in the rocks. The disintegration of radioactive elements generates huge heat which melts the rock as magma and thus causes convection current. According to Holmes, concentration of radioactive elements in the crust is also there but the generated temperature is not so high because there is gradual loss of heat through conduction and radiation from the upper surface at the rate of 60 calories per square centimeter per year. Though there is very low concentration of radioactive elements in the substratum but the gradual accumulation of heat produced by the radioactive elements cause convective currents. Ascending convective current originates under the crust near the equator because of greater thickness of crust, whereas descending convective currents are originated under the polar crust because of its shallow depth. Mechanism of the Theory The convective currents are divided into two major groups on the basis of their location i) convective currents of rising columns; and ii) convective currents of falling columns. According to Holmes, the equatorial crust was stretched and ruptured due to divergence of rising convective currents which carried the ruptured crustal block towards the north and south and a syncline was created as Tethys Sea between two blocks. This phase is called Opening of Tethys. Again two sets of convergent or downward moving currents pull Laurasia and Gondowanaland towards each other and thus Tethys was compressed and folded into Alpine mountains including Himalyas. This phase is called Closing of Tethys. Geosynclines are formed due to subsidence of crustal blocks mainly continental slabs due to compressive force generated by convergent convective current moving laterally together under continental and oceanic crust. According to Holmes, the cyclic pattern of convective current and related mountain building pass through three phases or stages. 24 Unit 6 Continental Drift, Mountain Building and Plate Tectonics.................................................................................................................................................................................................................................... In the first stage, the rising convective current of two centers converge under the continental slabs and thus form geosynclines due to compression coming from the convergence of two sets of lateral currents. As the sediments are pressed downward into geosynclines, these go further downward and are intensely heated and metamorphosed. Metamorphism of sediment causes rise in their density which further cause downward movement of the metamorphosed materials. This stage is called Lithogenesis. The second stage is marked by phenomenal increase in the velocity of convective currents. The main cause for this convective current is the downward movement of cold materials in the falling column and upward movement of hot materials in the rising columns of convective currents. The high velocity convergent convective current buckle geosyncline sediments and thus initiate the process of mountain building. This stage, thus, is called the stage of Orogenesis (Fig. 6.3). Fig. 6.3: Successive stage of Thermal Convective Currents under the Crust and Mountain Building. (Source: Savindra Singh, Physical Geography) The third stage is characterised by waning phase of thermal convective currents due to incoming hot materials in the falling column and upward movement of colder materials in the rising columns. The termination of the mechanism of convective current yields several results, e.g. (i) the materials of 25 Block 2 Lithosphere.................................................................................................................................................................................................................................... the falling columns start rising because of decrease in the pressure at the top of the falling column due to the end of deposition of materials. This mechanism causes further rise in the mountains. (ii) the metamorphosed rock (Eclogite) with increased density being heavier depressed downward and gets melted due to immense heat and thus it expands. This expansion in the volume of molten Eclogite causes further rise in the mountains. This stage is known as the stage of Gliptogenesis. 6.3.3 Plate Tectonic Theory and Mountain Building Mountain building can be explained by the theory of Plate Tectonics. The theory tells that the result of plate convergence and its involvement in sedimentation, deformation, igneous activities, erosion and isostatic balance leads to formation of mountains. There are four different types of plate convergence (Fig 6.4): 1) Convergence of two oceanic plates, 2) Convergence of oceanic and continental plates, 3) Convergence of two continental plates and 4) Continent- Arc collision Convergence of two Oceanic Plates and Mountain Building Two oceanic plates may converge at the destructive plate boundary where one oceanic plate, which is heavier and denser compared to other oceanic plate, subducts in the trench beneath the lighter low density plate. The resultant compression leads to the formation of island festoons and Island-arcs. The best examples of this type are the Japanese Islands and Philippines Island. The ocean crust, along with its sediments, is thrust beneath and the rocks on the continental side of the trench are metamorphosed under high pressure condition. The molten magma rises up and