Human life is largely influenced by the physiography of the region. Therefore, one must get acquainted with the forces that influence landscape development. Also to understand why the earth shakes or how a tsunami wave is generated, it is necessary that we know certain details of the interior of the earth.
➢ Importance of understanding the Interior of The Earth
- Understanding the structure of the earth’s interior (crust, mantle, core) and various forces (heat, seismic waves) emanating from it is essential to understand.
- The evolution of the earth’s surface, its current shape and its future.
- The geophysical phenomenon like volcanism, earthquakes, etc.
- Earth’s magnetic field.
- The internal structure of various solar system objects.
- The evolution and present composition of the atmosphere.
- For mineral exploration.
➢ Earth’s Surface
- Many different geological processes shape the Earth’s surface.
- The forces that cause these processes come from both above and beneath the Earth’s surface.
- Processes that are caused by forces from within the Earth are endogenous (Endo meaning “in”).
- By contrast, exogenous processes (Exo meaning “out”) come from forces on or above the Earth’s surface.
- The major geological features of the earth’s surface like mountains, plateaus, lakes are mostly a result of endogenous processes like folding, faulting that is driven by forces from inside the earth.
➢ A Geophysical Phenomenon Like Volcanism, Earthquakes
- The forces that cause catastrophic events like earthquakes, volcanic eruptions come from deep below the earth’s surface.
- For example, earthquakes occur due to the movement of the tectonic plates and the energy required for this movement is supplied by the conventional currents in the mantle.
- Similarly, volcanism occurs through the vents and fissures created by the tectonic movements.
➢ Earth’s Magnetic Field
- Earth’s magnetic field is a result of convection currents in the outer core of the earth.
- Life on earth would not have been possible if not for the earth’s magnetic field which protects the earth’s atmosphere from the harmful solar wind.
Question for Interior of Earth
Try yourself:Which one of the following is the dominant element of the earth crust?
The internal Structure of Various Solar System Objects
The entire solar system was formed from a single nebular cloud, and the process of the formation of every solar system object is believed to be similar to that of the earth.
➢ Evolution and present composition of the atmosphere
- For life to flourish on the surface of the earth, the atmosphere needs to have essential components like oxygen for respiration, CO2 and other greenhouse gases to maintain the temperature on the surface, ozone to protect life from ultraviolet radiation and the right atmospheric pressure.
- All these components of the earth’s atmosphere owe their existence to the volcanic eruptions that unlock them from the earth’s interior.
➢ Mineral exploration
- Understanding volcanic activity and the nature of rocks is essential for mineral exploration.
- Most of the minerals like diamonds (form at a depth of 150-800 km in the mantle) that occur on the earth’s surface are formed deep below the earth’s surface. They are brought to the surface by volcanic activity.
Sources of Information
Most of the information about the Earth’s interior is based on inferences drawn from different sources – both direct and indirect.
➢ Direct Sources
- Our knowledge about the structure and interior of the earth from direct observation is very limited.
- No instrument has been invented so far which can see through the interior of the earth directly.
- The deepest depth of an oil well drilled so far is 8 kilometres. The deepest mine of the world is Robinson Deep in South Africa.
- Its depth is less than 4 kilometre. Besides mining, scientists have taken up several projects to penetrate deeper depths to explore the conditions in the crustal portions.
- Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km.
- This and many deep drilling projects have provided a large volume of information through the analysis of materials collected at different depths.
- Volcanoes are yet another major source of direct information – they tell us about the composition and characteristics of the materials found inside the Earth. However, it is difficult to ascertain the depth of the source of such material.
➢ Indirect Sources
- The centre of the earth downward is 6,371 kilometres away from the surface of the earth. In comparison to this distance, the depth of a deep well or a mine is insignificant.
- It is, therefore, necessary to take help of indirect scientific pieces of evidence to know about the interior of the earth.
- These sources include temperature, pressure and density of the earth, the behaviour of seismic waves (the waves generated by Earthquakes), Meteors, the Moon etc. These sources may be classified into three groups
(a) Artificial sources such as temperature, pressure and density.
(b) Evidence from the theories of the origin of the earth
(c) Natural Sources e.g. volcanic eruption, earthquakes, meteors and seismology.
- Temperature goes on increasing with the increase in depth inside the earth. This is clearly proved while going down a mine or deep wells.
- The volcanic eruptions or hot water springs also confirm this fact that temperature increasing towards the interior of the earth.
- On average, there is a rise of 1oC temperature for every 32 meters of depth. This rapid increase in temperature continues to great depth thereafter the temperature increases slowly.
The main reasons for the increase in heat and temperature in the interior of the earth are the following:
- Radioactive disintegration within rocks which liberates heat
- Internal and external forces (gravitational pull, the weight of overlying rocks etc.)
- Chemical reactions
- It is tempting to think that under the conditions of this enormous temperature in the interior of the earth nothing can be found in a solid-state.
- Under such conditions, all existing rocks should be either in a liquid or gaseous state. But it is not so. Along with the increase in temperature with depth, pressure increases in the interior of the earth.
- This pressure is lacs of times more than the pressure exercised by atmospheric layers on the surface of the water in oceans. For this reason, due to enormous pressure, liquid state rocks of the core have the properties of solids.
- These rocks might be in the plastic state. It is why these rocks have elasticity.
- Due to the pressure of overlying layers on the earth’s interior, these rocks do look solid up to 2900 kilometres’ depth.
- Sometimes due to lessening of overlying pressure, the rocks in the interior meltdown and the fluid comes to the surface or is in the process of finding its way to the surface of the earth. A volcanic eruption is one such example.
Question for Interior of Earth
Try yourself:Which of the following elements exhibit the following properties?
1. It is the third most abundant element in the earth crust.
2. It exists in a stable combination with other materials mainly silicates and oxides.
3. It has a high strength-to-weight ratio.
Choose the answer using the code given below:
The properties of aluminium include: low density and therefore low weight,
high strength, superior malleability, easy machining, excellent corrosion
resistance and good thermal and electrical conductivity are amongst
aluminium’s most important properties. Aluminium is also very easy to recycle.
- By Newton’s laws of gravity, the earth’s density has been calculated to be 5.5 (gms per cubic centimetre).
- However, it is surprising that the rocks near the surface of the earth have an average density of 2.7 only (gms per cubic centimetre).
- This density is less than half the average density of the earth as a whole. From this, it is clear that the density also increases with the increase in depth.
- The earth’s internal part is composed of very dense rocks; their density must be in the range of 8-10 (gms per cubic centimetre).
- The density of the central part of the core is still more. Higher density could be due to heavy metals like Nickel and Iron at the centre as well as due to pressure of overlying layers
➢ Gravitation force
- 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 centre 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. The reading of gravity in different places is influenced by many other factors. These readings differ from the expected values. 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.
- Gravity anomalies also inform us about the distribution of molten material in the crust of the earth.
➢ Magnetic surveys
- The earth also acts like a huge magnet. The rapid spinning of the earth creates electric currents in its centre (molten outer core) that creates a magnetic field around the earth.
- The magnetic field is strongest at the magnetic north and south poles. The magnetic north and south poles do not coincide with geographic north and south poles.
- In fact, the earth’s magnetic field keeps on changing. Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
- The space debris, while entering the atmospheric layers of earth are burnt due to the friction of air.
- Only the heavier objects whose outer layers have been burnt fall to the earth. Man has discovered many such meteorites and after examining them obtained evidence about the interior of the earth.
The meteorites which have been examined are of two types:
(i) Rock; and (ii) Metals.
The metallic meteorites chiefly contain heavy materials like iron and nickel. The meteorites too have originated during the formation of the solar system. It is, therefore, very much to believe that both the meteorites and the earth are made of similar materials.
➢ The Moon
- The first information about the earth’s interior had been obtained through the study of the moon. There are several ways of determining the moon’s orbit around earth.
- Among these one of the important factors is earth’s mass. Remember, there is a close relationship between the mass and the earth’s gravitation. The movements of the moon and its distance from the earth provide the basis for determining the mass of the earth by earth scientists.
Question for Interior of Earth
Try yourself:Consider the following statements about the interior of the Earth:
1. The continental mass of the crust is made up of silica and magnesium.
2. Crust is thicker at the ocean floors.
3. The innermost core of the Earth is made only of molten iron.
Choose the correct answer using the codes below:
- Just like an onion, the Earth is made up of several concentric layers with one inside another. The Earth’s uppermost layer is the crust, which is the thinnest of all the layers. On the continental masses, it is about 35km and on the ocean floors only 5 km.
- Silica and alumina arc the main minerals that constitute the continental mass, and are called sial (si, silica; al, alumina). Silica and magnesium are the main minerals that constitute the oceanic crust and are called sima (si, silica; ma, magnesium).
- Continental crust is less dense than ocean crust as the latter is made of basaltic rocks.
- Core constitutes the innermost layer. Its radius is about 3500 km. Nickel and iron constitute the core, and it is called as nife (ni, nickel; fe, ferrous, i.e. iron). The temperature and pressure at the central core are very high. It is the molten iron in the core which gave rise to the magnetic field of Earth.
➢ Earthquake Waves
- Earthquakes are caused by the movements in the interior of the earth. These movements cause waves inside the earth just as waves are generated on the surface of the water in a lake when a stone is thrown into it.
- Incidentally, the majority of the earthquakes originate in the upper mantle. The earthquake waves are measured on the seismograph.
- The study of earthquake waves helps earth-scientists to get a lot of information about the types of rocks and layered composition in the interior of the earth.
- 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(origin of the earthquake) 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 a 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 densities. The denser the material, 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: P-waves and S-waves. Important surface waves are Rayleigh waves and L-waves (named after A. E. H. Love).
➢ Body waves
- Body waves are generated due to the release of energy at the focus and move in all directions travelling through the interior of the earth. Hence, the name body waves.
- There are two types of body waves:
- the P-waves or primary waves (longitudinal in nature ― wave propagation is similar to sound waves), and
- the S-waves or secondary waves (transverse in nature ― wave propagation is similar to ripples on the surface of the water).
➢ Primary waves (P-waves)
- Primary waves are called so because they are the fastest among the seismic waves and hence are recorded first on the seismograph.
- P-waves are also called as the longitudinal waves because the displacement of the medium is in the same direction as, or the opposite direction to, (parallel to) the direction of propagation of the wave; or compressional waves because they produce compression and rarefaction when travelling through a medium; or pressure waves because they produce increases and decreases in pressure in the medium.
- P-waves creates density differences in the material leading to stretching (rarefaction) and squeezing (compression) of the material.
- The vibration of particles in Longitudinal wave and Transverse wave (Source)
- These waves are of relatively high frequency and are the least destructive among the earthquake waves.
- The trembling on the earth’s surface caused due to these waves is in the up-down direction (vertical).
- They can travel in all mediums, and their velocity depends on shear strength (elasticity) of the medium.
- Hence, the velocity of the P-waves in Solids > Liquids > Gases.
- These waves take the form of sound waves when they enter the atmosphere.
- P-wave velocity in earthquakes is in the range 5 to 8 km/s.
- The precise speed varies according to the region of the Earth’s interior, from less than 6 km/s in the Earth’s crust to 13.5 km/s in the lower mantle, and 11 km/s through the inner core.
- We usually say that the speed of sound waves depends on density. But there are few exceptions ― mercury is denser than iron, but it is less elastic; hence the speed of sound in iron is greater than that in mercury
➢ Why do P-waves travel faster than S-waves?
- P-waves are about 1.7 times faster than the S-waves.
- P-waves are compression waves that apply a force in the direction of propagation and hence transmit their energy quite easily through the medium and thus travel quickly.
- On the other hand, S-waves are transverse waves or shear waves (motion of the medium is perpendicular to the direction of propagation of the wave) and are hence less easily transmitted through the medium.
➢ P-waves as an earthquake warning
- Advance earthquake warning is possible by detecting the non-destructive primary waves that travel more quickly through the Earth’s crust than do the destructive secondary and surface waves.
- Depending on the depth of focus of the earthquake, the delay between the arrival of the P-wave and other destructive waves could be up to about 60 to 90 seconds (depends on the depth of the focus).
➢ Secondary waves (S-waves)
- Secondary waves (secondary they are recorded second on the seismograph) or S-waves are also called as transverse waves or shear waves or distortional waves.
- They are analogous to water ripples or light waves.
- Transverse waves or shear waves mean that the direction of vibrations of the particles in the medium is perpendicular to the direction of propagation of the wave. Hence, they create troughs and crests in the material through which they pass (they distort the medium).
- S-waves arrive at the surface after the P-waves.
- These waves are of high frequency and possess slightly higher destructive power compared to P-waves.
- The trembling on the earth’s surface caused due to these waves is from side to side (horizontal).
- S-waves cannot pass through fluids (liquids and gases) as fluids do not support shear stresses.
- They travel at varying velocities (proportional to shear strength) through the solid part of the Earth.
➢ Surface waves (L-Waves)
- The body waves interact with the surface rocks and generate a new set of waves called surface waves (long or L-waves). These waves move only along the surface.
- Surface Waves are also called long-period waves because of their long wavelength.
- They are low–frequency transverse waves (shear waves).
- They develop in the immediate neighbourhood of the epicentre and affect only the surface of the earth and die out at smaller depth.
- They lose energy more slowly with distance than the body waves because they travel only across the surface, unlike the body waves which travel in all directions.
- Particle motion of surface waves (amplitude) is larger than that of body waves, so surface waves are the most destructive among the earthquake waves.
- They are slowest among the earthquake waves and are recorded last on the seismograph.
➢ Love waves
- It’s the fastest surface wave and moves the ground from side-to-side.
➢ Rayleigh waves
- A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
- Because it rolls, it moves the ground up and down and side-to-side in the same direction that the wave is moving.
- Most of the shaking and damage from an earthquake is due to the Rayleigh wave.
- Types of earthquake waves
➢ How do seismic waves help in understanding the earth’s interior?
- Seismic waves get recorded in seismographs located at far off locations.
- Differences in arrival times, waves taking different paths than expected (due to refraction) and absence of the seismic waves in certain regions called as shadow zones, allow mapping of the Earth’s interior.
- Discontinuities in velocity as a function of depth are indicative of changes in composition and density.
- That’s is, by observing the changes in velocity, the density and composition of the earth’s interior can be estimated (change in densities greatly varies the wave velocity).
- Discontinuities in wave motion as a function of depth are indicative of phase changes.
- That is, by observing the changes in the direction of the waves, the emergence of shadow zones, different layers can be identified.
Question for Interior of Earth
Try yourself:With reference to seismic waves, consider the following statements:
1. P wave or primary wave is the fastest kind of seismic wave and, consequently, the first to ‘arrive’ at a seismic station.
2. S wave or secondary wave can only move through solid rock, not through any liquid medium.
Which of the above statements is/are correct?
- The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the Earth.
- The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium.
- It is this property of S waves that led seismologists to conclude that the Earth’s outer core is a liquid. S waves move rock particles up and down, or side-to-side - perpendicular to the direction that the wave is travelling in (the direction of wave propagation).
- 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 emergence of Shadow Zone of P-waves and S-waves
- S-waves do not travel through liquids (they are attenuated).
- The entire zone beyond 103° does not receive S-waves, and hence this zone is identified as the shadow zone of S-waves. This observation led to the discovery of the liquid outer core.
- The shadow zone of P-waves appears as a band around the earth between 103° and 142° away from the epicentre.
- This is because P-waves are refracted when they pass through the transition between the semisolid mantle and the liquid outer core.
- However, the seismographs located beyond 142° from the epicentre, record the arrival of P-waves, but not that of S-waves. This gives clues about the solid inner core.
- Thus, a zone between 103° and 142° from epicentre was identified as the shadow zone for both the types of waves.
➢ Shadow Zone of P-waves and S-waves
- The seismographs located at any distance within 103° from the epicentre, recorded the arrival of both P and S-waves.
➢ Why do sound waves travel faster in a denser medium whereas light travels slower?
- The sound is a mechanical wave and travels by compression and rarefaction of 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 with an increase in density.
- Light, on the other hand, is a transverse electromagnetic wave.
- An increase in the density increases effective path length, and hence it leads to higher refractive index and lower velocity.
- The span of the shadow zone of the P-Waves = 78° [2 x (142° – 103°)]
- The span of the shadow zone of the S-Waves = 154° [360° – (103° + 103°)]
- The span of the shadow zone common for both the waves = 78°
➢ Structure of the Earth’s Interior
- The earthquake waves change at definite intervals during their propagation through the interior of the earth.
- They also undergo the action of reflection and refraction.
- Places on earth where seismic waves are not recorded are called “shadow zones”. S-waves are not recorded beyond 103o angular distance from the focus which indicates that the outer core of the earth is in molten or semi-molten in which S-waves cannot propagate.
- As P-waves are not recorded between angular distances of 103o to 142o, it indicates that the core has a different density, state and composition.
- From the analysis of the behaviour of these waves, it is clear that the interior of the earth has a layered structure of different densities.
- With the help of earthquake waves, we can get the information about the exact location of the layers, their depth, thickness and other physical and chemical properties.
- Based on the passage of these waves through different types of rocks and their behaviour we can conclude that the earth’s interior has three main layers.
These three layers are (i) Crust, (ii) Mantle and (iii) Core.
This arrangement can be compared to that of a boiled egg.
- It is the earth’s uppermost layer. The crust is solid, rigid and very thin compared with the other two.
- Like the shell of an egg, the Earth's crust is brittle and can break. The thickness of the crust is not the same everywhere.
- Oceanic crust is thinner as compared to the continental crust. The mean thickness of the 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.
➢ Its main parts are
- The uppermost thin layer– It is composed of such rocks which contain a large proportion of silica and aluminium. It is called SIAL (SI = Silica, AL = Aluminum). The continents are mostly composed of sial. Its average density is 2.7 and thickness is of about 28 kilometres.
- The lower layer of the crust is made of comparatively heavier rocks. Silica and magnesium are the major constituents in it. This part is, therefore, known as SIMA (SI – Silicon, MA = Magnesium). The oceanic floor is also made of this rock strata. Its average thickness is 6-7 kilometres and a density of about 3.0. The thickness of SIAL and SIMA put together does not exceed 70 kilometres.
- Its volume is 1% of the total volume of the earth. In comparison to 6378 km radius of the earth, the thickness of 70 kilometres is insignificant. However, this cannot be overlooked. This shallow crust is the ground of nature’s wonderful activities.
Question for Interior of Earth
Try yourself:Which of these rocks make up for large portions of the crust of Earth?
1. Granitic rocks
2. Basaltic rocks
3. Pumice rocks
4. Obsidian rocks
Choose the correct answer from the following codes:
Sial and sima share the major proportions of Earth’s crust which occupy the continental and ocean crust. Granite and basaltic rocks differ in silica content.
Its thickness is about 2900 Km. Its volume is 83% of the whole earth. Near the lower limit of the crust, the velocity of P-waves increases from about 6.4 kilometres per second to 8 km per second.
- This change in velocity of P-waves indicates the surface discontinuity between the crust and the mantle. It is popularly known as Moho or the Mohorovicic discontinuity (after the name of its discoverer).
- The mantle is made up of dense and heavy materials such as oxygen, iron and magnesium. The average density of the materials in the mantle varies between 3.5 g per cubic cm and 5.5 g per cubic cm.
- The temperature of this layer ranges between 900°C and 2200° C. The temperature is quite high and the hot rocks from magma in this layer.
- The pressure of the overlying layers keeps the lower part of the crust and the upper part of the mantle in an almost solid state.
- If cracks appear in the crust, the pressure is released and the molten matter from inside the Earth tries to reach the surface through volcanic eruptions.
The upper portion of the mantle is called the asthenosphere. The word Astheno means weak.
- It is considered to be extending up to 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions.
The mantle plays an important role in all the happenings in the interior of the earth.
- It also gives rise to Convection Currents. These currents supply energy for happenings like continental drift, earthquake, volcanoes, etc.
- It extends from 2900 Km depth up to the centre of the earth (6378 km). It is the interior most of the earth. It begins from Gutenberg Discontinuity. The mantle is demarcated from the core by Gutenberg Discontinuity.
- The core is divided into two parts: (i) The Outer Core, (ii) The Inner Core.
The outer core is possibly in a wholly liquid or semi-liquid state.
- The transverse or S-waves of earthquakes, seem to disappear at the Gutenberg Discontinuity.
- The outer core extends from the depth of 2900 km, up to 5150 km. It has an average density of 10.
- The inner core is believed to be solid. It extends from the depth of 5150 km up to the centre of the earth (6378 km).
- The velocity of P waves increases at the boundary of the outer and inner core. Its density is between 12-13. To the volume of the entire core is 16% of the earth as a whole.
- The mass of the core is 32% of the earth’s mass. The major part of the core is made up of heavy metals like iron and nickel.
- This zone is therefore known as Nife (Ni = Nickel, Fe = Ferrous). It is also known as Barysphere (which means heavy metallic rocks).
Crust and Mantle vs. Lithosphere and Asthenosphere
- Lithosphere, asthenosphere, and mesosphere represent changes in the mechanical properties of the Earth.
- Crust, Mantle and Core refer to changes in the chemical composition of the Earth. The lithosphere (litho: rock; sphere: layer) is the strong, upper 100 km of the Earth.
- The lithosphere is the tectonic plate (we talk about it in plate tectonics). The asthenosphere (asthenic: weak) is the weak and easily deformed layer of the Earth that acts as a “lubricant” for the tectonic plates to slide over.
- The asthenosphere extends from 100 km depth to 660 km beneath the Earth's surface. Beneath the asthenosphere is the mesosphere, another strong layer.