Geography - Geomorphology - Earth’s Interior – Earthquake Waves – Shadow Zone October 26, 2017 INTERIOR OF THE EARTH The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth. Exogenic as well as endogenic processes are constantly shaping the landscape. Why know about earth’s interior Understanding of the earth’s interior is essential to understand the nature of changes that take place over and below the earth’s surface. To understand geophysical phenomenon like volcanism, earthquakes etc.. To understand the internal structure of various solar system objects To understand the evolution and present composition of atmosphere Future deep-sea mineral exploration etc. Sources of information about the interior Direct Sources Deep earth mining and drilling reveals the nature of rocks deep down the surface. [Mponeng gold mine and TauTona gold mine in South Africa are deepest mines reaching to a depth of 3.9 km. And the deepest drilling is about 12 km deep] Volcanic eruption forms another source of obtaining direct information. Mponeng mine South Africa Deepest mine Gold mine Deapth: 2.4 miles (3.9 km) Indirect Sources Depth: With depth, pressure and density increases and hence temperature. This is mainly due to gravitation. Meteors: Meteors and Earth are solar system objects that are born from the same nebular cloud. Thus they are likely to have a similar internal structure. Gravitation: The gravitation force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator. This is because of the distance from the center at the equator being greater than that at the poles. The gravity values also differ according to the mass of material. The uneven distribution of mass of material within the earth influences this value. Such a difference is called gravity anomaly. Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth. Magnetic field: The geodynamo effect helps scientists understand what’s happening inside the Earth’s core. Shifts in the magnetic field also provide clues to the inaccessible iron core. But their source remains a mystery. What causes the magnetic field of earth? Our planet’s magnetic field is believed to be generated deep down in the Earth’s core. Nobody has ever taken the mythical journey to the centre of the Earth, but by studying the way shockwaves from earthquakes travel through the planet, physicists have been able to work out its likely structure. Right at the heart of the Earth is a solid inner core, two thirds of the size of the Moon and composed primarily of iron. At a hellish 5,700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid. Surrounding this is the outer core, a 2,000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid. Differences in temperature, pressure and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises. The Coriolis force, resulting from the Earth’s spin, also causes swirling whirlpools. This flow of liquid iron generates electric currents, which in turn produce magnetic fields. Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo. The spiraling caused by the Coriolis force means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field engulfing the planet. Some sources explained in detail High Levels of Temperature and Pressure Downwards Volcanic eruptions and existence of hot springs, geysers etc. point to an interior which is very hot. The high temperatures are attributed to automatic disintegration of the radioactive substances. Gravitation and the diameter of the earth helps in estimating pressures deep inside. Evidence From The Meteorites When they fall to earth, their outer layer is burnt during their fall due to extreme friction and the inner core is exposed. The heavy material composition of their cores confirms the similar composition of the inner core of the earth, as both evolved from the same star system in the remote past. The most important indirect source is seismic activity. The major understanding of the earth’s internal structure is mainly from the study of seismic waves. Seismic waves The study of seismic waves provides a complete picture of the layered interior. What causes earthquakes? Abrupt release of energy along a fault causes earthquake waves. A fault is a sharp break in the crustal rock layer. Rocks along a fault tend to move in opposite directions. But the friction exerted by the overlying rock strata prevents the movement of rock layer. With time pressure builds up. Under intense pressure, the rock layer, at certain point, overcomes the friction offered by the overlying layer and undergoes an abrupt movement generating shockwaves. This causes a release of energy, and the energy waves travel in all directions. The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. The energy waves travelling in different directions reach the surface. The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus. Earthquake Waves All natural earthquakes take place in the lithosphere (depth up to 200 km from the surface of the earth). An instrument called ‘seismograph’ records the waves reaching the surface. Earthquake waves are basically of two types — body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The velocity of waves changes as they travel through materials with different elasticity (stiffness) (Generally density with few exceptions). The more elastic the material is, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities. There are two types of body waves. They are called P and S-waves. Behavior of Earthquake Waves The earthquake waves are measured with the help of a seismograph and are of three types— the ‘P’ waves or primary waves (longitudinal nature), secondary waves or ‘S’ waves (transverse in nature) while the surface waves are long or ‘L’ waves. The velocity and direction of the earthquake waves undergo changes when the medium through which they are travelling changes. When an earthquake or underground nuclear test sends shock waves through the Earth, the cooler areas, which generally are rigid, transmit these waves at a higher velocity than the hotter areas. Primary Waves (P waves) Also called as the longitudinal or compressional waves. Particles of the medium vibrate along the direction of propagation of the wave. P-waves move faster and are the first to arrive at the surface. These waves are of high frequency. They can travel in all mediums. Velocity of P waves in Solids > Liquids > Gases Their velocity depends on shear strength or elasticity of the material. [We usually say that the speed of sound waves depends on density. But there are few exceptions. For example: Mercury (liquid metal) has density greater than Iron but speed of sound in mercury is lesser compared to that in iron. This is because the shear strength of mercury is very low (this is why mercury is liquid) compared to that of iron.] The shadow zone for ‘P’ waves is an area that corresponds to an angle between 103 Degree and 142 Degree. This gives clues about Solid inner core Secondary Waves (S waves) Also called as transverse or distortional waves. Analogous to water ripples or light waves. S-waves arrive at the surface with some time lag. A secondary wave cannot pass through liquids or gases. These waves are of high frequency waves. Travel at varying velocities (proportional to shear strength) through the solid part of the Earth’s crust, mantle. The shadow zone of ‘S’ waves extends almost halfway around the globe from the earthquake’s focus. The shadow zone for ‘S’ waves is an area that corresponds to an angle between 103 degree and 180 degree This observation led to the discovery of liquid outer core. Since S waves cannot travel through liquid, they do not pass through the liquid outer core. Surface Waves (L waves) Also called as long period waves. They are low frequency, long wavelength, and transverse vibration. Generally affect the surface of the Earth only and die out at smaller depth. Develop in the immediate neighborhood of the epicenter. They cause displacement of rocks, and hence, the collapse of structures occurs. These waves are responsible for most the destructive force of earthquake. Recoded last on the seismograph. Propagation of Earthquake Waves Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. Other two waves vibrate perpendicular to the direction of propagation. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Emergence of Shadow Zone Earthquake waves get recorded in seismographs 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 an altogether different shadow zone. Figure 3.2 (a) and (b) show the shadow zones of P and S-waves. It was observed that seismographs located at any distance within 105 ° from the epicenter, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145 ° from epicenter, record the arrival of P-waves, but not that of S-waves. Thus, a zone between 105 ° and 145 ° from epicenter was identified as the shadow zone for both the types of waves. The entire zone beyond 105 ° does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105 ° and 145 ° away from the epicenter. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface. But how these properties of ‘P’ and ‘S’ waves help in determining the earth’s interior? Reflection causes waves to rebound whereas refraction makes waves move in different directions. The variations in the direction of waves are inferred with the help of their record on seismograph. Change in densities greatly varies the wave velocity. By observing the changes in velocity, the density of the earth as a whole can be estimated. By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified. Why does sound wave travel faster in a denser medium whereas light travels slower? Sound is a mechanical wave and travels by compression and rarefaction of the medium. Its velocity in an elastic medium is proportional to the square root of Tension in the medium. A higher density leads to more elasticity in the medium and hence the ease by which compression and rarefaction can take place. This way the velocity of sound increases by increase in density. Light on the other hand is a transverse electromagnetic wave. It does not depend on the elastic property of the medium in which it travels. Its velocity in a medium is determined by the electromagnetic (e.g. dielectric) properties of the medium. Effective path length on the other hand is increased by an increase in the density and hence it leads to higher refractive index and lower velocity. Why S-waves cannot travel through liquids? S-waves are shear waves, which move particles perpendicularly to their direction of propagation. They can propagate through solid rocks because these rocks have enough shear strength. The shear strength is one of the forces that hold the rock together, and prevent it from falling into pieces. Liquids do not have the same shear strength: that is why, if you take a glass of water and suddenly remove the glass, the water will not keep its glass shape and will just flow away. In fact, it is just a matter of rigidity: S-waves need a medium rigid enough to propagate. Hence, S-waves do not propagate through liquids.