Earth’s interior structure and composition are characterized by its thermal and physical state. Earth consists of various layers or a solid body and is thought to be assembled with an area/group of rocks and other materials.
1. Unit 3 Interior of the Earth: Structure and Composition UNIT 3 INTERIOROFTHEEARTH: STRUCTUREAND COMPOSITION 3.1 Introduction 3.4 Earth’s Internal Structure: Expected Learning Outcomes Theories 3.2 Basic Concepts Edward Suess Van der Gracht The Earth as a Solid Body and Earth’s Interior Arthur Holmes Rock Cycle Seismology Layers and Discontinuities Earth’s Interior 3.3 Thermal and Physical State of 3.5 Summary the Earth’s Interior 3.6 Terminal Questions Temperature 3.7 Answers Pressure 3.8 References/Further Reading Density 3.1 INTRODUCTION In Units 1 and 2, you have learnt about various theories of the origin of the Earth and Solar System and Earth as a living planet. As you learnt, that many theories gave different logic for the origin of the Earth. You have also learnt in Units 1 and 2 that Sun is the main source of energy which drives all forms of life and associated processes over the Earth’s surface and beneath it as well. In this Unit, we will focus on the interior of the Earth, its structure and composition. We begin the unit with basic concepts related to our planet Earth by explaining Earth as a solid body, Earth’s interior, the rock cycle along with layers and discontinuities (Sec. 3.2). You will learn that all these concepts are inter-related with each other that forms a complex network of Earth’s interior processes, structure and composition. 49
2. Block 1 GEO-Tectonics Earth’s interior structure and composition are characterised by its thermal and physical state. In order to understand these, you must learn about its thermal and physical state which we will discuss in Sec. 3.3. While studying Sec. 3.4, you will learn about few important theories propounded by different Earth scientists. You will realize that seismology plays a pivotal role and provides a scientific basis to study the Earth’s interior. You will further learn about the intricate relationship between these two in Sec. 3.5. By and large, in this unit, you have been introduced to the Earth’s interior, its structure and composition. In the next Unit, you will learn about the Concept of Isostacy. Expected Learning Outcomes After studying this unit, you should be able to: describe the basic concepts, namely, Earth as a solid body, Earth’s interior, rock cycle along with layers and discontinuities; illustrate the thermal and physical state of the Earth’s interior; explain the Earth’s internal structure as propounded by different Earth scientists; Define the concept of seismology and Earth’s interior. 3.2 BASIC CONCEPTS As we know that basic concepts form the backbone of any field of study across the disciplines of studies or branches of knowledge. You will agree that the learning of basic concepts not only provides a sound understanding of the phenomenon under study but also acts as a source for further learning. 3.2.1 The Earth as a Solid Body and Earth’s Interior Earth consists of various layers or a solid body and is thought to be You may refer and watch assembled with area/group of rocks and other materials. The Earth as a solid a small documentary body or layer collectively means an area of rocks. You may recollect the details film titled ‘Earth 101’ at of the Earth’s lithosphere which you studied from Sub-sec. 2.2.1 in Unit 2. Do https:// you know that approximately 16 kilometres beneath the Earth’s surface, more nationalgeographic.com/ than three fourth of Earth’s matter is composed of rock materials. Basically, science/space/ solar the study of rocks is the domain of petrology (which is part of geology). It deals system/ earth and with the rock system which is made up of unique or aggregates of mineral explore relevant online resources for further matters. It is this mineral matter that helps to build the Earth’s crust. information on plant Earth’s interior is arranged into three layers namely crust, mantle and core. In Earth and its interior parts. simplest words, you may visualize it with the peeling off process of a boiled egg. The outer hard and thin layer may resemble to the crust, middle layer to the mantle and inner layer to the core respectively. The most common and reliable source to study the Earth’s interior is Seismology which you will learn in Sec. 3.4. Let us study the rock cycle which is exclusively the domain of
3. Unit 3 Interior of the Earth: Structure and Composition geologists. They study the Earth’s history, structure and composition in detail. Geographers are required to know the basic properties of rocks also. 3.2.2 Rock Cycle You can think of any continuous activity. For example, four distinctive weather seasons in a year namely spring, summer, autumn and winter and many others such like nutrient cycle, carbon cycle, hydrological cycle or sowing and harvesting of wheat, rice or mustard etc. on every successive agricultural year. In similar ways, rock cycle refers to the constant organization and reorganization of various rock types found over the Earth’s Crust i.e. lithosphere. You can understand this with the following example as shown in Fig. 3.1. Soon after the process of solidification of magma and lava, igneous rocks are formed. You can visualise it in terms of a walls of the building soon after the process of cementing during construction. Rock materials comes from the Earth’s middle inner layer i.e. Mantle through volcanic eruptions. After the formation, igneous rocks become exposed to the agents of weathering and erosion on the Earth’s surface. These agents break the igneous rocks into small pieces. Further, the agents of denudation, for example wind (aeolian) and water (fluvial) transports these small rock particles and deposits into the seas and oceans. You will learn about these in Units 8 and 9, Block 2 of this course in detail. Have you ever wondered that what happens to the small rock particles after these gets accumulated? This process of continuous accumulation deep into the sea floor, in due course of time, transforms it into new sedimentary rocks as shown in Fig. 3.1. Further exposure to Earth processes keeps on repeating this Application of rock cycle Volcanic pressure and cruptions heat transforms provide the rock it into material in the metamorphic form of magma rocks and lava Solar Energy drives all the Earth Leads to processes, here Initially gradual accumulation of the process of rock magma cycle and lava solidifies rock particles and then cools to that turns into form Igneous sedimentary rocks rocks *DRP Igneous rocks *Disintegrated transported to lay exposed to Rock Particles the seas and the agents of oceans by natural weathering and agents of wind erosion subject to and water disintegration Fig. 3.1: Rock Cycle. 51
4. Block 1 GEO-Tectonics Have you ever thought that how metamorphic rocks are formed? Metamorphic rocks are formed by the application of pressure and heat. When metamorphic rocks lay exposed, it may also worn away to transform into sedimentary rocks and so on with the passage of time. The process of rock cycle is never ending. Most of the rock material is constantly recycled matching both formation and disintegration of rocks found over the Earth’s crust. 3.2.3 Layers and Discontinuities You may be amazed to know the way distinctive and complex system of layers and discontinuities do exist in the rock cycle. One of the most reliable sources of information about the Earth’s interior layers comes from the precise study of seismic waves. You are quite familiar with the same as these are nothing but simply waves that generates tremors resulting into Earthquakes. Seismic waves could be either natural or artificially stimulated one in a preset/ modulated environment. Based on the speed of seismic waves, three main layers along with discontinuities have commonly been recognized. These are crust, mantle and the core layers. Seismic waves could be of two kinds i.e. surface and body waves. Further, these three main layers are divided into sub- layers on the basis of changing speed of the seismic waves. You will learn about all these layers and their discontinuities in Sec. 3.4 of this unit in detail. SAQ 1 Explain the concept of rock cycle? Spend 5 mins 3.3 THERMAL AND PHYSICAL STATE OF THE EARTH’S INTERIOR You may wonder that what do we mean by the thermal and physical state of the Earth’s interior. It signifies the study of three vital parameters namely temperature, pressure and density etc. All of these parameters are responsible for the generation of present state of the affairs and may also shape the future development both at over the Earth’s surface and in the Earth’s interior parts too. Scientific analysis of all three parameters would enable you to comprehend the underlying details of the Earth’s interior. Afterwards, you would be able to study the dynamics of the Earth’s interior in a better way. 3.3.1 Temperature As understood, you must be familiar with the meaning of temperature. All of you will agree that to live and sustain in a given type of environment/place/ space or geographic region, all kinds of flora and fauna along with human beings will require a threshold temperature value. You will be surprised to know that temperature increases with increasing depth. Similarly, you will further uncover that temperature increases with increasing altitude on the Earth’s
5. Unit 3 Interior of the Earth: Structure and Composition surface in Block 3 on Climatology of this course. On an average, the temperature increases at a rate of 1° C for every 32 metres as one goes deep inside the Earth’s interior. At this rate of temperature rise, the core layer of the Earth is expected to witness temperature values of around 4000° C. You will be amazed to know that temperature is likely to vary Threshold value means from 1200-2000° C at this rate deep at 48 kilometres into the asthenosphere. the value at which a given You will agree that at such a high range of temperature values, the underlying substance or material rocks along with their mineral constituents may alter their shape, structure and attains the optimally properties altogether. You can witness this change from solid to varying required value. degrees of liquid state in various kinds of minerals along with their For this reason, you will wonder that the source of volcanic eruptions is considered to be at a depth of 48 kilometres inside the Earth’s interior. But, it is only true for first 8 kilometres of depth inside the Earth beyond which it is very difficult to gauge the temperature values. Temperature values keep on progressively declining at far greater depths beyond 100 kilometres. Reason for this is the gradual decline in the availability of two heat sensitive radioactive minerals of uranium and thorium respectively (Singh, S. 2012). 3.3.2 Pressure You might be getting curious about the cause for such high density of the Earth’s core layer. Traditional view stated it to be the heavy pressure of the rock strata succeeding each other. It is well understood fact that the overlying pressure from rock layers simply increases the density of rocks. It further increases with the increase in depth. However, this opinion is not entirely true since the density of a given rock types have threshold value beyond which the density ceases to increase. Thus, the factor of overlying pressure cannot alone increase the density of rocks to greater extent lying underneath the lithosphere. This inference leads towards the fact that the core layers of the Earth could be made up of intense metallic matter having inherently very high density. However, scientific studies have clearly set out the fact that the core layer of the Earth is constituted by two heavy metallic components of iron and nickel. It has also been confirmed based on the geocentric magnetic field of the Earth’s interior. A layer of crystalline rock strata encircles the topmost portions of the metallic core of the Earth. 3.3.3 Density On an average, the density of the Earth’s interior is 5.5. However, it is 2.7 in the continental shells i.e. upper surface layers which consist of granitic rocks and sial i.e. silica and aluminium. It is also believed by the Earth scientists, particularly geologists that the density of material increases directly in proportion to the depth. In other words, you can say that the density of materials increases as one goes down into the Earth’s interior parts. You will be amazed to know that the density of materials found in the core layers of the Earth are around dozen times denser compared to the water. Iron and nickel together known as ‘nife’ forms the core layer of the Earth’s interior. 53
6. Block 1 GEO-Tectonics The density is 4.3 in the middle layers composed of sima i.e. silica and magnesium. Apparently, heavier the material, the density will also be correspondingly high. SAQ 2 Briefly discuss the temperature, pressure and density conditions of the Earth’s Spend interior. 5 mins 3.4 EARTH’S INTERNAL STRUCTURE: THEORIES A number of Earth scientists propounded various theories regarding the Earth’s internal structure. Few important ones are discussed below for you to have a broader idea regarding the same. 3.4.1 Edward Suess His theory deals with the chemical properties of the Earth’s interior structure. He said that a thin sheet of sedimentary rocks having extremely small thickness envelops the Earth’s crust. It is made up of crystalline rocks with silicate as a mineral. Two most important minerals are mica and feldspar. Light silicate matter makes up its upper portion whereas heavy silicate matter determines its lower portion. This scholar categorised the Earth’s internal structure into three zones as discussed below. i) The Sial: Sial is made up of two words i.e. Si plus and Al which denote silica and aluminum. It lies beneath the outer sedimentary envelop of the Earth’s internal structure. On an average, its density is 2.9 and varies from 50 to 300 kilometers in its thickness. This layer is characterized by the surplus availability of acid materials along with silicates of potassium, sodium and aluminum etc. Major structural parts of the planet Earth i. e. continents are created on sial layer. ii) The Sima: A layer situated just below the sial is known as ‘sima’. It is constituted by the basalt material. Sima is the only provider of magma and lava resources blown up from volcanoes during active phases. It is composed of two words Si plus and Ma, whereby Si stands for silica and Ma for magnesium. Hence, silica and magnesium are two important minerals. Average density varies from 2.9 to 4.7. Its thickness varies from 1000 to 2000 kilometers respectively. This layer is characterized by the availability of plenty of base materials including those of calcium, iron and magnesium silicates etc. iii) The Nife: It is placed beneath the middle layer of sima. Nickel (Ni) and ferrium (Fe) are two dominant minerals. Nife has very high density because of the dominance of heavy metals. Its thickness is 6880 kilometers. Availability of iron i.e. ferrnium shows two properties of firmness and magnetism.
7. Unit 3 Interior of the Earth: Structure and Composition Suess has divided the Earth’s internal structure into three layers of sial, sima and nife with one or more than one common characteristics. 3.4.2 Van der Gracht He has categorized the Earth’s interior into four layers. Diverse properties of the same have been described in Table 3.1. Table 3.1. Earth’s Interior Layers According to Van der Gracht Sl. No. Layers Thickness Density 60 kilometres below Continents 2.75 – 2.9 (i) Outer sialic 20 kilometres below Atlantic Ocean Not present below Pacific Ocean (ii) Inner silicate 60 – 1140 kilometres mantle 3.1 – 4.75 (iii) Zone of mixed 1140 – 2900 kilometres 4.75 – 5.0 metals and silicates (iv) Metallic 2900 – 6371 kilometres 11.0 nucleus Source: Singh, S. 2012, Physical geography. His scheme indicates the Earth’s internal layers with varying figures regarding the number, thickness and density properties etc. As opposed to this scheme, most of the Earth scientists distinguished a universal pattern that deals the Earth’s internal structure into three layers as discussed below: i) Lithosphere: It is mostly made up of granitic rocks with a thickness of 100 kilometers. Silica and aluminum are the major minerals. Average density of lithosphere is 3.5. ii) Pyrosphere: Basalt is the main constituent mineral. Thickness of this layer is 2780 kilometres and average density is 5.6. iii) Barysphere: Iron and nickel are the two important minerals. This layer spans for nearly 200 kilometers to the core. Average density varies from 8 to 11. 3.4.3 Arther Holmes A. Holmes categorised the Earth’s internal structure into two major layers i.e. upper and lower layers. Upper layer has been named as crust. Sialic layer propounded by E. Suess and top parts of sima forms this layer. Substratum is the name given to the lower layer. It is made up of the lower portions of E. Suess’s sima layer. Thickness of sial lies underneath the continental shell. He outlined four characteristics of the Earth’s internal structure as below: i) Thermal conditions – 20 kilometers or less 55
8. Block 1 GEO-Tectonics ii) Surface seismic waves (l waves) – 15 kilometers or more iii) Longitudinal waves – 20 to 30 kilometers iv) Subsidence of the deepest geosynclines – 20 kilometers or even more. As like other scholars, he has also confirmed to the earlier theories. But, he gave rather incomplete theory of the Earth’s internal structure. Since, it has been recognized and categorized into three distinctive layering systems known as crust, mantle and core respectively. SAQ 3 Highlight the major key points of various theories. Spend 5 mins 3.3.4 Seismology Seismology is a kind of scientific endeavour akin to medical profession. It studies the Earth’s internal structure with the help of tremors including both the Earth tremors and nuclear outburst etc. You will be surprised to know that it is not a new idea as science of ‘Seismology’ was prevalent since ages. It was practiced in ‘Asian giant land of China’ around 2000 years before to determine the direction and source of seismic waves. A Chinese scholar named ‘Chang Heng’ was the first to devise an instrument used to detect the unfelt and guess the direction to the epicentre of Earthquakes during 132 A.D. (Lutgens et. al, 2011). Seismographs used in contemporary times are testimony, which resembles to this old Chinese instrument. You can see that how this instrument looks like and functions in Fig. 3.2. Did you notice the freely suspended hanging weight from a flat support base? The Fig. 3.2: Illustration of Seismograph.
9. Unit 3 Interior of the Earth: Structure and Composition moment seismic or Earthquake tremors hit the seismograph; it remains static owing to the factor of inertia of weight. It is relative to the moving objects of support and the Earth’s surface. In other words, you can say that factor of inertia of weight keeps the dormant objects at dormant state whereas dynamic objects at dynamic state. A seismograph that collects the evidence regarding the nature and types of ‘Tectonic plates’ refers Earthquake tremors are known as seismograms. It tells us about the main to the division of Earth’s crust or lithosphere into types of tremors produced due to the movement of rock strata. For instance, seven major and almost the continuous movement of tectonic plates (i.e. concept of plate tectonics dozen minor plates dealt in detail in Unit 5, Block 2 of this course) of the world in relation to each including ‘continental’ other which you will learn. and ‘oceanic plates’. There are two categories of seismic waves known as body and surface waves. Body waves moves along the internal division of the Earth and further divided into two sub-types. These are primary and secondary waves designated by the capital letter P and S. Surface waves as the name itself indicates moves at the external division of the Earth’s surface with two sub- types namely Rayleigh and Love waves. 1. Body waves: Medium of motion through dominant objects is the main characteristic feature of body waves at greater depths. Body waves are of two types viz; P and S waves. Among this, P or primary waves are the speediest moving at a speed of 5 kilometres per second. It is basically push and pull type waves moving towards the direction of wave motion as shown in Figure 3.3 a and b. This resembles to the sound produced by the strings of a musical instrument like sitar as it puts the air into motion to produce such sounds. All three medium of atmospheric elements including solid, gas and liquid shows resistance to change in volume at the moment compression is applied. Thus, these medium would eventually come back to their original state once the compression forces are withdrawn. Thus, P waves are able to move through all three mediums. Next type is S waves, which causes vibrations at right angles in the direction of wave movement. It is less speedy than P waves as it moves at nearly 3 kilometres per second. You can understand it by taking a 2.5 metre long piece of soft plastic garden pipe. Now, if you tie one side of a pipe over a vertical pole and vibrate it from the free side. You will notice the action in a pipe coming from the wave direction as S waves transform the form of a conveying substance altogether as opposed to P waves which momentarily does so. In other words, fluid materials such as liquid and gas are not stress tolerant which readily transforms the form of a matter. It means that fluids will not let the S waves pass through. 2. Surface waves: You will learn that every category of seismic wave has its own unique qualities of motion. This category is characterised by complexity in motion at ground level as evident from Fig. 3.3 c and d. It makes everything upon the surface to be in motion including buildings, bridges, transmission lines and numerous man-made objects. It has two types. 57
10. Block 1 GEO-Tectonics Fig. 3.3: Different Categories of Seismic Waves. (a) P Waves (b) S Waves (c) Love Waves (d) Rayleigh Waves. First one is ‘Rayleigh wave’ with up and down motions as like that of an electronically operated lift in a multi-storey building. Second type is known as ‘love wave’ characterised by side to side motion resembling to that of a wave generated by a swiftly flowing perennial river system. Former type is considered more fatal than later as it carries the potential to damage the very base of man-made infrastructural amenities. ‘Perennial river system’ You will be aware that it has always been difficult to probe deep into the Earth’s refers to a river system having permanent flow of interior parts since ages. One of the plausible medium is through the light water like most of the penetration. But, light cannot penetrate the solid and liquid intervening rock Glacier fed Himalayan layers found beneath the Earth’s surface. Another medium could be either rivers. through drilling (e.g. to ascertain the availability of petroleum resources as being done in the Western coast i.e. Off the coast in Bombay High, India) and digging (e.g. to obtain the iron ore mineral as being done in gold mines at Kudremukh Ore Mines, in Chikkamagaluru district, Karnataka, India) techniques into the deep interior parts of the Earth to locate various mineral resources.
11. Unit 3 Interior of the Earth: Structure and Composition You will be surprised to know that it is nearly impossible to drill beyond specific depths, roughly 12.5 kilometres, so far. The reason for this is primarily because of prevailing extremely high temperature and pressure conditions existing therein. You may wonder that every year, planet Earth experiences hundreds of tremors which are recorded by the seismographs. Thus, it is possible through the medium of recording of huge natural tremors that spans the Earth along with seismic waves. These helps us to unravel the mysteries of the Earth’s interior segment. A seismic wave takes images of the Earth’s interior. You can understand this phenomenon as like that of an image taken by the x-ray, city scan or ultrasound machines for the affected parts of a human body to ascertain the exact reasons of debility and sickness for You will learn that due to the seismic wave’s complex movement, the study of recorded wave data by seismograms becomes difficult. Not following a direct movement, seismic waves tend to behave differently. They reflect, refract and diffract in their course of movement during tremors as shown in Fig. 3.4. Fig. 3.4: Complex Movements of Seismic Waves in the Earth’s Interior. You will be amazed to know that the junction of two intervening rock layers provides the platform to reflect seismic waves. They also refract in their course from one to another rock layers. Besides, seismic waves also get diffracted while facing any kind of natural/physical barriers like rivers and mountains etc. You can say that it is because of this shifting behavioural characteristic of seismic waves which facilitates the Earth scientists to determine the boundaries found in the Earth’s interior parts. You will further learn that seismic waves follow a much curved movement on account of rising speed directly in proportion to the growing depth. Its velocity accelerates if a rock layer through which it travels is not easily compressible and hard as well. In turn, these two features of a rock layer namely compressibility and hardness allows the investigation of composition and temperature conditions too. 59
12. Block 1 GEO-Tectonics SAQ 4 Explain the term seismology. Spend 5 mins 3.4.5 Earth’s Interior Thus, based on the study of seismology, Earth’s interior can be divided into three main layering systems as shown in Fig. 3.5. 1. Crust Crust is the outermost layer. It is also known as ‘lithosphere’ mostly made up of basalt rocks. It has two parts namely upper and lower crust. Mean thickness is around 20 kilometres. Average density of the upper crust is 2.8 whereas it is 3.0 in case of the lower crust. Little variation between the two is on account of the pressure caused by the superincumbent weight. You will also learn that minerals of the upper crust were formed with lesser pressure compared to the lower crust. Seismic waves lose their speed gradually both in the upper and lower parts of the crust. 2. Mantle You will be surprised to know that Mantle is characterised by the abrupt rise in the intensity of Earthquake waves. It is known as ‘mesosphere’ located at the boundary between lower crust and upper parts of the mantle. Speed of seismic waves is 6.9 kilometres per second at the base of lower crust. It rises rapidly to 8.1 kilometres per second due to the discontinuity. It lies between the separation zone of lower crust and upper mantle. This zone of separation was discovered by Yugoslavian seismologist named Andrija Mohorovicic during 1909. Thereafter, it came to be known as ‘Mohorovicic Discontinuity’ or ‘Moho Discontinuity’. 60 Fig. 3.5: Earth’s Layering System.
13. Unit 3 Interior of the Earth: Structure and Composition It has mean density of 4.6 g/cm3. Beneath the Earth, the mantle spans nearly for a depth of 2900 kilometres. As a whole, it comprises 83 percent of the entire dimension as well as 68 percent of the overall accumulation of the Earth’s surface. However, the mantle falls short in its thickness which is less than half of the Earth’s radius of 6371 kilometres. You will learn that earlier mantle was divided into two zones on the basis of varying speed and density of the Earthquake waves. These are upper mantle that extends from the ‘Moho Discontinuity’ further down to a depth of 1000 kilometres and lower mantle from 1000 to 2900 kilometres. Presently, the mantle has been divided into three zones based on the facts from the discovery of the ‘International Union of Geodesy and Geophysics’ IUGG. These zones are as under: i) Moho Discontinuity to a depth of 200 kilometres ii) To a depth of 200 to 700 kilometres iii) Depth of 700 to 2900 kilometres Top region of the upper mantle is characterized by gradual decrease in the speed of Earthquake waves. It extends for a depth of 100 to 200 kilometres i.e. 7.8 kilometres per second. This is known as the zone of ‘low velocity seismic waves’. You will learn that Mantle is rich in silicate minerals like iron and magnesium 3. CORE You will come to know that Core is the deepest and absolutely remote zone in the Earth’s interior. It is also known as ‘barysphere’. Extent of core is located at a depth of 2900 kilometres from lower parts of the mantle to the Earth’s centre at a depth of 6371 kilometres. It is marked by the boundary known as ‘Weichert-Gutenberg Discontinuity’ between lower mantle and upper parts of the core. This is located at a depth of 2900 kilometres. You will be surprised to know that it is this discontinuity that marks the rapid change in the density from 5.5 g/cm3 to 10.0 g/cm3. It is also supported and denoted by the rise in the speed of primary seismic waves at 13.6 kilometres per second. You will further learn that density of the core increases proportionately with increase in the depth. It becomes 12.3 to 13.3 and then 13.6 respectively. Core layer surpasses the mantle in its density by almost double amount. But, as far as the quantity and accumulation are concerned, it has relatively lower figures of the same having 16 and 32 per cent of the planet Earth. You will be further surprised to know that at a depth of 5150 kilometres, Core has two divisions of outer and inner core. Outer core denotes the loss of secondary seismic waves and thus tells us about its molten state. Boundary of inner core goes down from 5150 kilometres up to the Earth’s centre 61
14. Block 1 GEO-Tectonics situated at a depth of 6371 kilometres. It is in solid state having density figures of 13.3 to 13.6. Primary seismic waves pass at a velocity of 11.23 kilometres per second through the inner core layer. The Earth scientists particularly geophysicists and geochemists consider the core to be made of metallic materials such as iron and nickel. SAQ 5 Discuss briefly about the Earth’s internal structure. Spend 5 mins 3.5 SUMMARY Thus, in this unit, you have studied and learnt the following concepts, key points and issues as highlighted below: You have learnt the basic concepts like Earth as a solid body, Earth’s interior, rock cycle, seismology and distinctive layering systems along with discontinuities and characteristics as well. You came to know that the most prominent discontinuities of ‘Mohorovicic discontinuity’ lies between crust and mantle and that of ‘Weichert-Gutenberg discontinuity’ marks the boundary between lower mantle and upper parts of the core. You have learnt about the various theories like E. A. Suess, V. D. Gracht and A. Homes etc. dealing with the Earth’s internal structure. You have also learnt about atmospheric and other variables such as temperature, pressure and density etc. along with their characteristics. In nutshell, you have learned about the Earth’s interior structure and its composition. This information will definitely serve as a key and fundamental to further probe the same in more scientific ways. 3.6 TERMINAL QUESTIONS 1. Describe in short the basic concepts pertaining to the interior of the Earth? 2. What do you understand by the mantle layer of the Earth’s interior? Explain? 3. Write down a detailed account of any one theory of the Earth’s interior? 3.7 ANSWERS Self-Assessment Questions 1. Rock cycle refers to the constant organization and reorganization of various rock types. Soon after the solidification of magma and lava, igneous rocks are formed. Igneous rocks break into small pieces by the
15. Unit 3 Interior of the Earth: Structure and Composition agents of weathering and erosion. Wind and water carries rock particles into the sea and oceans. It eventually transforms into new sedimentary and metamorphic rocks (application of pressure and heat) as well after accumulation. Rock cycle keeps on repeating. 2. Heavy pressure of the rock strata succeeding each other leads to high pressure which further increases with the increasing depth. Heavy pressure cannot alone increase the density of rocks to greater extent. It could be due to the presence of intense metallic matter having inherently very high density. Core layer is constituted by two heavy metallic components of iron and nickel. It has also been confirmed on the basis of geocentric magnetic field of the Earth’s interior. 3. A. Holmes has categorised the Earth’s internal structure into two major layers i.e. upper and lower layers. Upper layer has been named as crust. E. Suess’s sialic layer and top parts of Sima forms this layer. Substratum is the name given to the lower layer. It is made up of the lower portions of E. Seuss’s sima layer. He defined thickness of sial into four categories below the continental shell. He gave rather incomplete theory of the Earth’s internal structure. Since, it has been arranged into three distinctive layering systems. 4. Seismology is a kind of scientific endeavour to study the Earth’s internal structure. It does so with the help of tremors including both the Earth tremors and nuclear outburst etc. Seismographs gathering the evidence regarding the nature and types of Earthquake tremors are known as ‘seismograms’. It tells us about the main types of tremors produced due to the movement of rock strata. 5. Crust is the outermost layer which is also known as ‘lithosphere’ mostly made up of basalt rocks. It has two parts of upper and lower crust. Mean thickness is around 20 kilometres. Average density of the upper crust is 2.8 whereas it is 3.0 in case of lower crust. Seismic waves lose their speed gradually both in the upper and lower parts of the crust. Terminal Questions 1. In your answer, you should be able to not only describe the basic concepts pertaining to interior of the Earth, but also should cover the main tenets behind the same. You can refer to section 3.2. 2. Your answer should cover the mantle and its main features. You can refer to section 3.4. 3. While answering this question, you should be able to define the main doctrine behind the chosen theory to discuss the interior of the Earth. Your answer should cover and highlight the key points, strengths as well as weaknesses compared to other theories. You can refer to section 3.6. 3.8 REFERENCES/FURTHER READING 1. https://www.nationalgeographic.com/science/space/solarsystem/earth 63
16. Block 1 GEO-Tectonics 2. Anderson, D.L. (1989). Theory of the Earth. Boston: Blackwell Publications. 3. Grotzinger, J., & Jordan, T. H. (2010). Understanding Earth. Ed6, New York: W.H. Freeman and Company. 4. Hussain, M. (2001). Fundamentals of Physical Geography, New Delhi: Rawat publications. 5. Lutgens, F.K., & Tarbuck, E.J. (2011). Foundations of Earth Science. New Jersey: Pearson. 6. Robertson, E.C. (1966). The Interior of the Earth: An Elementary Description. U.S: Geological Survey Circular, 532, 10 pages. 7. Sharma, H.S., Sharma, M.L., & Mishra, R.N. (2010). Bhautik Bhoogol (Physical Geography in Hindi). Jaipur: Panchsheel Prakashan. 8. Singh, S. (2012). Physical Geography, Allahabad: Prayag Pustak Bhawan. 9. Siddhartha, K. (2000). The Earth’s Dynamic Surface. New Delhi: Kisalaya publications.
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