Interior of the Earth PDF

18 FUNDAMENTALS OF PHYSICAL GEOGRAPHY CHAPTER INTERIOR OF THE EARTH W hat do you imagine about the nature...

18 FUNDAMENTALS OF PHYSICAL GEOGRAPHY CHAPTER INTERIOR OF THE EARTH W hat do you imagine about the nature SOURCES OF INFORMATION ABOUT THE INTERIOR of the earth? Do you imagine it to be The earth’s radius is about 6,378 km. No one a solid ball like cricket ball or a can reach the centre of the earth and make hollow ball with a thick cover of rocks i.e. observations or collect samples of the material. lithosphere? Have you ever seen photographs Under such conditions, you may wonder how or images of a volcanic eruption on the scientists tell us about the earth’s interior and television screen? Can you recollect the the type of materials that exist at such depths. emergence of hot molten lava, dust, smoke, fire Most of our knowledge about the interior of and magma flowing out of the volcanic crater? the earth is largely based on estimates and The interior of the earth can be understood only inferences. Yet, a part of the information is obtained through direct observations and by indirect evidences as neither any one has nor analysis of materials. any one can reach the interior of the earth. The configuration of the surface of the earth Direct Sources is largely a product of the processes operating The most easily available solid earth material in the interior of the earth. Exogenic as well as is surface rock or the rocks we get from mining endogenic processes are constantly shaping areas. Gold mines in South Africa are as deep the landscape. A proper understanding of the as 3 - 4 km. Going beyond this depth is not physiographic character of a region remains possible as it is very hot at this depth. Besides mining, scientists have taken up a number of incomplete if the effects of endogenic processes projects to penetrate deeper depths to explore are ignored. Human life is largely influenced the conditions in the crustal portions. Scientists by the physiography of the region. Therefore, world over are working on two major projects it is necessary that one gets acquainted with such as “Deep Ocean Drilling Project” and the forces that influence landscape “Integrated Ocean Drilling Project”. The development. To understand why the earth deepest drill at Kola, in Arctic Ocean, has so shakes or how a tsunami wave is generated, it far reached a depth of 12 km. This and many is necessary that we know certain details of the deep drilling projects have provided large interior of the earth. In the previous chapter, volume of information through the analysis of you have noted that the earth-forming materials collected at different depths. materials have been distributed in the form of Volcanic eruption forms another source of obtaining direct information. As and when the layers from the crust to the core. It is interesting molten material (magma) is thrown onto the to know how scientists have gathered surface of the earth, during volcanic eruption information about these layers and what are it becomes available for laboratory analysis. the characteristics of each of these layers. This However, it is difficult to ascertain the depth of is exactly what this chapter deals with. the source of such magma. 2024-25 INTERIOR OF THE EARTH 19 Indirect Sources information about the interior of the earth. Hence, we shall discuss it in some detail. Analysis of properties of matter indirectly provides information about the interior. We Earthquake know through the mining activity that temperature and pressure increase with the The study of seismic waves provides a complete increasing distance from the surface towards picture of the layered interior. An earthquake the interior in deeper depths. Moreover, it is in simple words is shaking of the earth. It is a also known that the density of the material also natural event. It is caused due to release of increases with depth. It is possible to find the energy, which generates waves that travel in rate of change of these characteristics. Knowing all directions. the total thickness of the earth, scientists have estimated the values of temperature, pressure Why does the earth shake? and the density of materials at different depths. The release of energy occurs along a fault. A The details of these characteristics with fault is a sharp break in the crustal rocks. reference to each layer of the interior are Rocks along a fault tend to move in opposite discussed later in this chapter. directions. As the overlying rock strata press Another source of information are the them, the friction locks them together. However, meteors that at times reach the earth. However, their tendency to move apart at some point of it may be noted that the material that becomes time overcomes the friction. As a result, the available for analysis from meteors, is not from blocks get deformed and eventually, they slide the interior of the earth. The material and the past one another abruptly. This causes a structure observed in the meteors are similar release of energy, and the energy waves travel to that of the earth. They are solid bodies in all directions. The point where the energy is developed out of materials same as, or similar released is called the focus of an earthquake, to, our planet. Hence, this becomes yet another alternatively, it is called the hypocentre. The source of information about the interior of the energy waves travelling in different directions earth. reach the surface. The point on the surface, The other indirect sources include nearest to the focus, is called epicentre. It is gravitation, magnetic field, and seismic activity. the first one to experience the waves. It is a point The gravitation force (g) is not the same at directly above the focus. different latitudes on the surface. It is greater near the poles and less at the equator. This is Earthquake Waves because of the distance from the centre at the equator being greater than that at the poles. All natural earthquakes take place in the The gravity values also differ according to the lithosphere. You will learn about different mass of material. The uneven distribution of layers of the earth later in this chapter. It is mass of material within the earth influences sufficient to note here that the lithosphere refers this value. The reading of the gravity at different to the portion of depth up to 200 km from the places is influenced by many other factors. surface of the earth. An instrument called These readings differ from the expected values. ‘seismograph’ records the waves reaching the Such a difference is called gravity anomaly. surface. A curve of earthquake waves recorded Gravity anomalies give us information about on the seismograph is given in Figure 3.1. Note the distribution of mass of the material in the that the curve shows three distinct sections crust of the earth. Magnetic surveys also each representing different types of wave provide information about the distribution of patterns. Earthquake waves are basically of two magnetic materials in the crustal portion, and types — body waves and surface waves. Body thus, provide information about the waves are generated due to the release of energy distribution of materials in this part. Seismic at the focus and move in all directions travelling activity is one of the most important sources of through the body of the earth. Hence, the name 2024-25 20 FUNDAMENTALS OF PHYSICAL GEOGRAPHY body waves. The body waves interact with the propagation. As a result, it creates density surface rocks and generate new set of waves differences in the material leading to stretching called surface waves. These waves move along and squeezing of the material. Other three the surface. The velocity of waves changes as waves vibrate perpendicular to the direction of they travel through materials with different propagation. The direction of vibrations of densities. The denser the material, the higher S-waves is perpendicular to the wave direction is the velocity. Their direction also changes as in the vertical plane. Hence, they create troughs they reflect or refract when coming across and crests in the material through which they materials with different densities. pass. Surface waves are considered to be the most damaging waves. Emergence of Shadow Zone Earthquake waves get recorded in seismo- graphs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists Figure 3.1 : Earthquake Waves an altogether different shadow zone. Figure 3.2 There are two types of body waves. They (a) and (b) show the shadow zones of P and are called P and S-waves. P-waves move faster S-waves. It was observed that seismographs and are the first to arrive at the surface. These located at any distance within 105° from the are also called ‘primary waves’. The P-waves epicentre, recorded the arrival of both P and are similar to sound waves. They travel S-waves. However, the seismographs located through gaseous, liquid and solid materials. beyond 145° from epicentre, record the arrival S-waves arrive at the surface with some time of P-waves, but not that of S-waves. Thus, a lag. These are called secondary waves. An zone between 105° and 145° from epicentre was important fact about S-waves is that they can identified as the shadow zone for both the types travel only through solid materials. This of waves. The entire zone beyond 105° does not characteristic of the S-waves is quite receive S-waves. The shadow zone of S-wave is important. It has helped scientists to much larger than that of the P-waves. The understand the structure of the interior of the shadow zone of P-waves appears as a band earth. Reflection causes waves to rebound around the earth between 105° and 145° away whereas refraction makes waves move in from the epicentre. The shadow zone of S-waves different directions. The variations in the is not only larger in extent but it is also a little direction of waves are inferred with the help of over 40 per cent of the earth surface. You can their record on seismograph. The surface draw the shadow zone for any earthquake waves are the last to report on seismograph. provided you know the location of the epicentre. These waves are more destructive. They cause (See the activity box on page 28 to know how to displacement of rocks, and hence, the collapse locate the epicentre of a quake event). of structures occurs. Types of Earthquakes Propagation of Earthquake Waves (i) The most common ones are the tectonic Different types of earthquake waves travel in earthquakes. These are generated due to different manners. As they move or propagate, sliding of rocks along a fault plane. they cause vibration in the body of the rocks (ii) A special class of tectonic earthquake is through which they pass. P-waves vibrate sometimes recognised as volcanic parallel to the direction of the wave. This exerts earthquake. However, these are confined pressure on the material in the direction of the to areas of active volcanoes. 2024-25 INTERIOR OF THE EARTH 21 (v) The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes. 10 Measuring Earthquakes 5 The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The 105 105 magnitude is expressed in numbers, 0-10. The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused 145 145 by the event. The range of intensity scale is from 1-12. EFFECTS OF EARTHQUAKE Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquake: (i) Ground Shaking (ii) Differential ground settlement (iii) Land and mud slides (iv) Soil liquefaction (v) Ground lurching (vi) Avalanches (vii) Ground displacement 105 105 (viii) Floods from dam and levee failures (ix) Fires (x) Structural collapse (xi) Falling objects (xii) Tsunami The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate Figure 3.2 (a) and (b) : Earthquake Shadow Zones concern to the life and properties of people in the region. The effect of tsunami would occur (iii) In the areas of intense mining activity, only if the epicentre of the tremor is below sometimes the roofs of underground oceanic waters and the magnitude is mines collapse causing minor tremors. sufficiently high. Tsunamis are waves These are called collapse earthquakes. generated by the tremors and not an (iv) Ground shaking may also occur due to earthquake in itself. Though the actual quake the explosion of chemical or nuclear activity lasts for a few seconds, its effects are devices. Such tremors are called explosion devastating provided the magnitude of the earthquakes. quake is more than 5 on the Richter scale. 2024-25 22 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Frequency of Earthquake Occurrences once in 1-2 years whereas those of ‘tiny’ types occur almost every minute. The earthquake is a natural hazard. If a tremor of high magnitude takes place, it can cause STRUCTURE OF THE EARTH heavy damage to the life and property of people. However, not all the parts of the globe The Crust necessarily experience major shocks. We shall be discussing the distribution of earthquakes It is the outermost solid part of the earth. It is and volcanoes with some details in the next brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. The Mantle The portion of the interior beyond the crust is called the mantle. The mantle extends from A view of the damaged Aman Setu at the LOC Moho’s discontinuity to a depth of 2,900 km. in Uri, due to an earthquake The upper portion of the mantle is called asthenosphere. The word astheno means weak. chapter. Note that the quakes of high It is considered to be extending upto 400 magnitude, i.e. 8+ are quite rare; they occur km. It is the main source of magma that finds 2024-25 INTERIOR OF THE EARTH 23 higher density than that of the crust. The mantle contains a weaker zone called asthenosphere. It is from this that the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava. The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorene, hydrogen and argon. Volcanoes Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows: Figure 3.3 : The interior of the earth Shield Volcanoes Barring the basalt flows, the shield volcanoes its way to the surface during volcanic are the largest of all the volcanoes on the earth. eruptions. The crust and the uppermost part The Hawaiian volcanoes are the most famous of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state. The Core As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth. The core- mantle boundary is located at the depth of 2,900 km. The outer core is in liquid state while the inner core is in solid state. The core is made up of very heavy material mostly constituted Shield Volcano by nickel and iron. It is sometimes referred to as the nife layer. VOLCANOES AND VOLCANIC LANDFORMS You may have seen photographs or pictures of volcanoes on a number of occasions. A volcano is a place where gases, ashes and/or molten rock material – lava – escape to the ground. A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past. The layer below the solid crust is mantle. It has Cinder Cone 2024-25 24 FUNDAMENTALS OF PHYSICAL GEOGRAPHY examples. These volcanoes are mostly made more than 50 m. Individual flows may extend up of basalt, a type of lava that is very fluid for hundreds of km. The Deccan Traps from when erupted. For this reason, these volcanoes India, presently covering most of the are not steep. They become explosive if Maharashtra plateau, are a much larger flood somehow water gets into the vent; otherwise, basalt province. It is believed that initially the they are characterised by low-explosivity. The trap formations covered a much larger area upcoming lava moves in the form of a fountain than the present. and throws out the cone at the top of the vent and develops into cinder cone. Mid-Ocean Ridge Volcanoes Composite Volcanoes These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more These volcanoes are characterised by than 70,000 km long that stretches through eruptions of cooler and more viscous lavas all the ocean basins. The central portion of this than basalt. These volcanoes often result in ridge experiences frequent eruptions. We shall explosive eruptions. Along with lava, large be discussing this in detail in the next chapter. quantities of pyroclastic material and ashes find their way to the ground. This material VOLCANIC LANDFORMS accumulates in the vicinity of the vent openings leading to formation of layers, and this makes Intrusive Forms the mounts appear as composite volcanoes. The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust). The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms. Some of the forms Composite Volcano are shown in Figure 3.4. Caldera These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity. Flood Basalt Provinces These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows. There can be a series of flows with some flows attaining thickness of Figure 3.4 : Volcanic Landforms 2024-25 INTERIOR OF THE EARTH 25 Batholiths case it develops into a saucer shape, concave A large body of magmatic material that cools to the sky body, it is called lapolith. A wavy in the deeper depth of the crust develops in the mass of intrusive rocks, at times, is found at form of large domes. They appear on the surface the base of synclines or at the top of anticline only after the denudational processes remove in folded igneous country. Such wavy the overlying materials. They cover large areas, materials have a definite conduit to source and at times, assume depth that may be several beneath in the form of magma chambers km. These are granitic bodies. Batholiths are (subsequently developed as batholiths). These the cooled portion of magma chambers. are called the phacoliths. The near horizontal bodies of the Lacoliths intrusive igneous rocks are called sill or These are large dome-shaped intrusive bodies sheet, depending on the thickness of the with a level base and connected by a pipe-like material. The thinner ones are called sheets conduit from below. It resembles the surface while the thick horizontal deposits are volcanic domes of composite volcano, only called sills. these are located at deeper depths. It can be regarded as the localised source of lava that finds its way to the surface. The Karnataka Dykes plateau is spotted with domal hills of granite When the lava makes its way through cracks rocks. Most of these, now exfoliated, are and the fissures developed in the land, it examples of lacoliths or batholiths. solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a Lapolith, Phacolith and Sills wall-like structure. Such structures are called As and when the lava moves upwards, a dykes. These are the most commonly found portion of the same may tend to move in a intrusive forms in the western Maharashtra area. horizontal direction wherever it finds a weak These are considered the feeders for the eruptions plane. It may get rested in different forms. In that led to the development of the Deccan traps. EXERCISES 1. Multiple choice questions. (i) Which one of the following earthquake waves is more destructive? (a) P-waves (c) Surface waves (b) S-waves (d) None of the above (ii) Which one of the following is a direct source of information about the interior of the earth? (a) Earthquake waves (c) Gravitational force (b) Volcanoes (d) Earth magnetism (iii) Which type of volcanic eruptions have caused Deccan Trap formations? (a) Shield (c) Composite (b) Flood (d) Caldera (iv) Which one of the following describes the lithosphere: (a) upper and lower mantle (c) crust and core (b) crust and upper mantle (d) mantle and core 2024-25 26 FUNDAMENTALS OF PHYSICAL GEOGRAPHY 2. Answer the following questions in about 30 words. (i) What are body waves? (ii) Name the direct sources of information about the interior of the earth. (iii) Why do earthquake waves develop shadow zone? (iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity. 3. Answer the following questions in about 150 words. (i) What are the effects of propagation of earthquake waves on the rock mass through which they travel? (ii) What do you understand by intrusive forms? Briefly describe various intrusive forms. 2024-25