CHAPTER INTERIOR OF THE EARTH 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 6,370 km. No one can Wa solid ball like cricket ball or a 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. 2021-22 22 FUNDAMENTALS OF PHYSICAL GEOGRAPHY 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 2021-22 INTERIOR OF THE EARTH 23 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.