13 CONSTITUTION OF THE EARTH’S INTERIOR

Dr.Ramashray Prasad

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Introduction

Learning Objectives

Meaning of Composition and Structure

Studying the Earth’s Interior

a. Artificial Sources and
b. Natural Sources

a. Artificial Sources: Density Pressure and

Temperature
Density
Pressure
Temperature
Relationships of Density, Pressure and Temperature with Depths in the Interior

b. Natural Sources Volcanism and Seismology.

Volcanicity
Seismology

Primary, Secondary and Surface waves

Characteristics of Primary (P) waves:
Characteristics of Secondary (S) waves:
Characteristics of Surface (L) waves:
Interpretation of Propagating Different Waves
Seismology and Constitution of the Earth’s Interior

Chemical Composition of the Earth
SIAL:
SIMA:
NIFE

Earth’s Internal Structure
Crust
Mantle
Core

Conclusions

Multiple Choice Questions

Answers of MCQs

References

Web Links

   

Introduction

 

The interior of the earth is that portion which is, possibly, not reachable for us. The parts, from where the sample is not supposed to be taken in our hands, are difficult to understand very well directly. You must have seen the vendor selling watermelon on roadside who used to get a slice out of it. We can see the quality of watermelon by your own eyes, or even you can test the same before buying. Our earth is not like the watermelon from where we can get the sample slice to study in detail.Constitution of the earth’s interior is the study to understand the internal parts of the earth, in terms of the real conditions existing there. These conditions could be about the state of rocks, or temperature, or pressure, or the chemical composition of the rocksat various depths. The study is facilitated by different branches/ disciplines of modern knowledges like science in general and physics, chemistry, seismology in particular. Let us study them systematically in detail.

 

Learning Objectives

 

After studying this module, you will be able to:

  • understand the changing conditions of temperature and pressure in the earth’s interior, explain the density variations with increasing depth from the earth’s surface,
  • describe the propagation of earthquake waves,
  • infer the state of rock based on the waves propagated,
  • explain the reasons of varying conditions in the interior and comprehend different layers of the earth’s internal parts.

 

Meaning of Composition and Structure

 

In a very simple term, meaning of composition is what something is consist of. In another words, it is unit of different parts for making a total. When we are referring the composition of the earth, we are supposed to study the earth with respect to the material of which it is composed of. The materials of earth’s surface are different than the interior. By material, we generally denote about different kinds of rocks occupying different volume and mass it has. It includes the physical state of rock, density, temperature or even the pressure they are bearing at different depths.

 

Structure is something to do with the overall assemblage of an object. In another words, it is the unified total made up of different parts. When we are referring to the structure of the earth, we are supposed to study the earth with respect to the parts making the total. The arrangement of different parts of the earth give a clear picture of the whole of the earth. Therefore, the structure of the earth is the way different parts are put together to get the plan or design of the earth.

 

    Studying the Earth’s Interior

 

The real sample is not possible to get from the earth’s deeper internal parts. Even if the internal material is ejected or received provides a different condition at the earth’s surface. The material’s temperature or the pressure recorded in the interior is different in comparison when it come at the surface. Therefore, to study the earth’s interior, we have to take the help of two sources:

 

c. Artificial Sources and

 

d. Natural Sources

 

Artificial Sources: Artificial sources are those sources which are basically derived from the mathematical assumptions and calculations. Important among them are:

 

Density,

Pressure and

Temperature

 

Density

 

The earth is made up of various types of rocks. Rocks are composed of minerals. Rock may be composed of one mineral or many minerals. The characteristics and properties of the minerals determine the rocks. The rocks available under our feet while walking can be simply examined very well in our laboratory. The rocks of various places are not alike but are different. The rocks lying in our agricultural field is different from the mined rocks. It could be different types like igneous or sedimentary or metamorphic. The rocks of the earth’s surface is different from the interior. Tostudy the density is one way to know it. The density is the property of the rock’s compactness. When the molecules of the rocks are very close, it is denser. In another words, it is defined as the relationship between the mass of the rock and how much volume it occupies. Thus, a rock with greater mass and lesser volume will become denser but when the mass is less and volume is more, the density is lower. Therefore, density is equal to the mass of the rock divided by its volume.

 

D=m/v

 

Where

 

D=Density,

 

m=mass of the rock and

 

v=volume of the same rock

 

By this formula, the result received is expressed as that much of gram/cm3.

 

Probably, for the first time British scientist Henry Cavendish (1798) attempted to calculate the density of the earth on the basis of Newton’s law of Gravitation. He found it to be 5.48 gm/cm3. Poynting (1878) calculated the earth’s density and found it to be 5.49 gm/cm3.

    The density of the rocks found at the surface rocks: 2.7 – 2.9 gm/cm3

The density of rocks found at the sea floor is around: 3.0 gm/cm3

It is quite obvious that the central part of the earth is more than: 5.5 gm/cm3

The average density of the earth:  5.52 gm/cm3

 

The density calculated at different depths in the interior of the earth is given in the following Table 1.

 

Table 1: Density of Rocks at Different Depths in the Interior

 

Depth in km Density in gram/cm3
(Surface/ Sea Level) 0 2.70 to 2.90
100 3.38
Ite 500 3.85
1000 4.58
2000 5.12
2890 5.56
2900 9.90
4000 11.32
5000 12.12
5500 12.92
6371 13.09

     It is also very clear throughthe Table 1 that the density of the inner most part of the earth is around 13 gm/cm3. The increase in the density in the interior of the earth is not a continuous affair but changes very abruptly at different depths. The changing density at different depths in the interior may very clearly be seen at a glance through Figure 1.

   

 Figure 1: Density in the Interior

Source: http://hs.umt.edu/geosciences/faculty/sheriff/courses/438-gravity- electromagnetics/images/Density%20Depth.gif

 

Pressure

 

It is quite obvious that the density of the rocks lying in the interior is greater. It is scientifically proven that the density of the matter is increased when it is compressed. Earlier it was assumed that the density of the interior rock is grater due to increasing pressure of the rocks lying above. To some extent, this assumption is true but it is also universally proven fact that the density of the rocks cannot be increased beyond a certain limit, simply by compression or more pressure. Hence, the inference that the increasing pressure is the reason for greater density is not true. The higher dense material of the interior can be explained through the constituent of the rocks. It is now confirmed that the core is composed of essentially heavy metallic materials which have higher density. The pressure in the interior keeps on increasing with depth (Figure 2).

 

 

Figure 2: Pressure in the Interior

 

    Temperature

 

With the advancement in technology and knowhow to use the minerals, the mining activities started. We have been digging the earth since many centuries. But the deeper mining and oil exploration has led drilling the crust to the much deeper level in recent times. Our own observations record a rate of increase in temperature and it is about 30C per100 meter depth or about 300C per km. If we calculate the temperature at this rate, the earth’s core would witness a temperature of more than 1,90,0000 C. it is unimaginable. Probably, this much of high temperature would melt the entire earth.

 

The above calculationhas not taken the increasing pressure into account. A recent experiment conducted and calculated by the scientists suggests that the inner part of the earth, core, has about 50000 C temperature with a variation of 5000 C plus or minus. Hence, these two temperatures are contradictory to each other. The increase in temperature observed in the top layer of the earth is not constant. The rate may be higher near the surface for a few km but it is not true for the much deeper part of the earth.

 

The above conditions could very well be explained by presence of radioactive minerals in the top layers of the crust. Granitic rocks are very well known for the abundance of radioactive minerals like uranium and thorium. The higher temperature in the granitic layer found is accounted due to chemical disintegration. The process of fission and fusion of these minerals generate huge amount of heat. This leads to greater temperature in this zone of the earth’s strata. With increasing depth, the availability of radioactive minerals is less and hence, lesser temperature. Therefore, there is decrease in rate oftemperature with depth. The observed temperature and temperature needed to melt the rock at different depths is shown by a graphical representation in Figure 3.

 

Figure 3: Observed and Melting Temperature in the Interior

 

  Relationships of Density, Pressure and Temperature with Depths

 

We have already deliberated above about the density, pressure and temperature in the earth’s interior. From these discussions, it is quite clear that all the three components are increasing with increasing depth. Their increase in not uniform continuously but with changing rates. The reasons for their increase is already explained while dealing with those components. Their relationships are very clearly be seen from Figure 4.

    Natural Sources

 

There are two components of natural sources to probe the earth’s interior. They are volcanicity and seismology.

 

Volcanicity

 

Vulcanicityis a process within which a fracture or fault is created by endogenic forces. Through these fracture or fault magmatic materials, gases, vapour or rock fragments are brought to the surface. The ejected material or lava is very hot and it is the part of rocks in a melted form. Appearance of melted material provided an idea that a permanent magma chamber is existing in the lower part. But this idea is also refuted. Scientists have also proved that the increasing pressure in the interior also increases the melting point of the rocks. Hence, there may not be a permanent magma chamber.

 

We even know very well that the food is cooked very quickly in the pressure cooker than in an open pot. In an open pot, the water gets boiled at a much lower temperature (1000 C) in comparison to pressure cooker (1210 C). The vapour generated in the pressure cooker is not easily allowed to escape. It keeps on accumulating inside. Accumulation of vapour increases the pressure inside. Increase in pressure forces the water to boil at a much higher temperature. This experiment suggests that the increasing pressure is the cause to boil water or melt rocks at a much higher temperature.

 

Scientists suggest that there is no permanent magma chamber. Magma is generated because of removal of the overlying rock pressure. Overlaying rocks are removed by shifting of rock mass due to faulting. Once magma is generated, it becomes larger in volume. Since there is no vacuum space inside, magma pushes the overlying rocks and breaks. The volcanism is the result and it appears on the surface. Therefore, vulcanicity is also not much of great help in understanding the interior.

   

Seismology

 

Seismology is a branch of scientific knowledge to study the earthquake and its propagated waves. An earthquake is a sudden vibration or shake of the ground of an area due to abrupt breaking of a part of a plate causing instability in the region. This leads to earthquake waves.

 

The earthquake waves are recorded by an instrument commonly known as seismograph. It is important to mention here that seismology is the only source by which entire earth could be probed. The probe provides authentic and complete information about all part of the earth.Earthquake gets its birth from a depth below the earth surface. This depth could be anything from a few meters to hundreds of km. According to United States Geological Survey (USGS), for scientific purposes, the originating depth of an earthquake may vary from 0 to 700 km.

 

The point from where the earthquake originates is known as focus. The shortest distance from the focus to the earth’s surface. It is perpendicular distance exactly above from focus. It is referred as epicenter. Epicenter, (Figure 5) being the closest place on the earth’s surface, experiences the earthquake first. It is recorded later to the distant places from the epicenter. Principally, there are three types of seismic waves – Primary, Secondary and Surface waves. They are recorded one after the other by the instrument.

 

Figure 5: Origin of the Earthquake

Primary Waves: Primary (P) waves are known as compressional waves. They are also termed as push and pull waves (Figure 6). It is like sound waves that we hear. The sound generated from its source pushes the air available nearby and it keeps on hitting the adjacent air molecules. In this way the sound reaches to our ear and we hear. Earthquake occurs due to plate movement and breaking of the plate edge. Thus,generated pressure pressurizes the surrounding rocks. This pressure keeps on moving further and are recorded by the seismographs installed at different places. It behaves as if we are piercing a nail in the wall with the help of a hammer. The force with which we hammer the nail, nail goes inside but with the same force hammer is returned towards us. This gives a back and forth or forward and backward movement of the quake waves.

 

Characteristics of Primary (P) waves:

 

It is the fastest wave, hence, it reaches first and gets recorded in the seismograph. It travels in all the three state of matter – solid, liquid and gas.

 

Its velocity is greater in solid, less in liquid and very slow in gas.

 

Its velocity increases if there is increase in the density of the rocks and vice versa.

 

Once the state of matter changes from solid to viscus or liquid, its velocity decreases even if the density is greater.

 

The velocity of P waves varies from 5.5 km per second at or near the surface to 13.0 km per second in the deep interior.

 

Figure 6: Propagation of P Waves

 

Source: http://www.geo.mtu.edu/UPSeis/images/P-wave_medium.jpg

 

Secondary Waves: Secondary (S) waves are known as transverse waves. These waves travel at right angle to direction of the wave propagation. These waves seem to be like the light waves. These type of waves are just like waves on calm water of pond when we throw a pebble (Figure 7). Since these waves are travelling with horizontal movement at the surface, they are more dangerous than P waves.

 

Characteristics of Secondary (S) waves:

  • Its velocity is less in comparison Primary (P) waves hence, it is recorded after P waves in the seismograph.
  • It travels only through the solid state of matter.
  • Its velocity increases if there is increase in the density of the rocks and vice versa.
  • Once the state of mater changes from solid to viscus, its velocity is reduced. But when the rocks are melted, it disappears completely.
  • The velocity of S waves varies from 3.25 km per second at or near the surface to 7.0 km per second in the interior.

 

Figure 7: Propagation of S Waves

Source: http://www.geo.mtu.edu/UPSeis/images/S-wave_medium.jpg

 

Surface waves: Surface waves or Longitudinal (L) waves travel through the earth’s surface. Surface waves are manifestations of the P and S waves which finally reach at the surface from the interior. These waves travel in the same way as if you are giving jerk to the wet towel before spreading on a rope. They are categorized into two – Love (named after A E H Love) waves and Rayleigh (named after J W S Rayleigh) waves. Love waves (Figure 8) travel in both horizontal and perpendicular directions of the wave propagation. The particle motion in Rayleigh waves (Figure 9) is in elliptical motion generally retrograde in the vertical plane and parallel to the direction of wave propagation.

 

Characteristics of Surface (L) waves:

 

Its velocity is less in comparison Primary (P) and Secondary (S) waves. Hence, it is recorded after P and S waves on the seismograph.

It travels only through the solid state of matter.

The velocity of L waves varies from 2.0 to 4.4 km per second while the velocity of R waves is slightly lesser (from 2.0 to 4.2 km per second).

The velocity of L and R waves is dependent on the frequency of wave as well as their penetration in the upper layer of the earth.

 

Figure 8: Propagation of L Waves

Source: http://www.geo.mtu.edu/UPSeis/images/Love_medium.jpg

 

Figure 9: Propagation of R Waves

 

Source: http://www.ngdir.ir/sitelinks/kids/image/geology-farsi/rayleigh%20wave.jpg

 

Interpretation of Propagating Different Waves

 

As mentioned before, all seismic waves start propagating with the occurrence of earthquake simultaneously. But they are recorded at different times on the seismograph. Its reason is different velocity of different waves. The faster wave reaches quickly but slower wave reaches after a time lag. Look at the Figure 10 given below. It is showing the concept written above. In the initial stage, the strain and stress operating on the rock produces very-very faint noise and probably experienced by some sensitive animals whose body or nose is near to the surface. They get frustrated and behaves very differently. After this P wave arrives as it is the fastest. S waves takes a little more time to reach the same place because of slower velocity. The difference between the arrival of P and S is known as time lag (Figure 10). The duration of time lag depends on the distance between the epicenter of the earthquake and the referred place or measuring station. Nearer place would observe smaller time lag whereas the far distant places would observe bigger time lag. At the epicenter, probably it is difficult to identify the time lag because of less gap, or practically speaking, no time lag between P and S waves.

 

Figure 10: Recorded Earthquake on Seismograph

 

Source: http://earthguide.ucsd.edu/mystery_detectives/teach/epicenter/images/seismogram_2.jpg

 

Seismology and Constitution of the Earth’s Interior

 

The waves generated at the time of earthquake occurrence radiates in all directions from the focus. The radiated waves are not passing in straight lines but follow curved paths. Curvature of the paths is due to changing density from the earth’s surface to the core. Due to refraction of the waves, S waves are not found beyond an angular distance of 1050 from the epicenter of the earthquake. In the same way, P waves are not traceable from 1050 to 1400 from the epicenter. These are known as shadow zones (Figure 11).

Figure 11: Propagation of P and S Waves in the Interior

 

Source: http://www.cyberphysics.co.uk/graphics/diagrams/Earth/pand%20s%20shadow.png

 

But more important about the interior of the earth is revealed by the nature of the propagation of waves particularly P and S. From the surface of the earth towards the interior, both waves P and S are propagating with increasing velocity. Approximately at an average depth of 40 km, there is increase in the velocity of both waves. It suggests that there is sudden increase in density of rocks at that depth.

 

At a depth of around 100 to 250 km, the velocity of both waves starts declining and after around 700 km velocity again become greater. Decreasing velocity in this belt indicate that the matter of rocks is semi-solid. Because of this the velocity of both waves declines. This low velocity zone is known as asthenosphere. It is also named as transition zone. Further beyond, the velocity of both waves increases continuously until a depths of 2900km. Increasing velocity shows that the density is higher and the state of the rocks is solid. From 2890 km to 2900 km, the rocks are again almost in a plastic state, i.e., neither solid nor liquid. This narrow belt is called as D layer which in a condition of a transition zone (Figure 11).

 

Beyond the depth of 2900 km, there is no trace of S waves and the velocity of P waves declines very drastically. Remember the characteristics of both waves, it suggests that the rocks at this depth is melted and S waves does not travel in liquid. Reduction in the velocity of P waves is also due to the changing state of the matter orrocks. At around 5150 km depth, the velocity of the P waves increases. This is a proof that the rocks becomes solid again.

 

Figure 11: Assimilation of Study of Earth’s Interior

Source:http://geopick.uncc.edu/geologyWeb/physicalGeology/Topics/910earthquakes/imagesEarthqua kes/FIG18_020.JPG

 

 

Chemical Composition of the Earth

 

There are three major and almost concentric layers in the earth. These are explained by Swess and they are SIAL, SIMA and NIFE.

 

SIAL: It is the topmost layer of the earth found just below the sedimentary thin cover of the crust. Silicon (Si) and Aluminum (Al) are two very important elements found in abundance in this layer. They are named as SIAL. The average density of this layer is 2.75 to 2.90 g/cm3 and its average depth is 40 km. It is very thin below oceanic water (5 to 10 km) but below the mountains, it is very thick (upto 70 km). The main rock in this zone is granite.

 

SIMA: It is the second layer after SIAL from the surface towards the interior. It is named after Silicon (SI) and Magnesium (MA) as both of these elements are very much abundant in this layer. It is a very thick layer which goes almost upto a depth of 2900 km. It starts from a depth of 70 km below continents while below the oceans, it is found at 5 to 10 km depth. Its average density varies from 2.90 g/cm3 to 4.75 g/cm3. The main rocks in this layer are silicates of magnesiumand iron. This layer is largely composed of basalt.

 

    NIFE: It is the innermost layer of the earth. This layer is made up of Nickle (NI) and Ferrous (FE) and so it is named as NIFE. It is just below the SIMA from a depth of 2900 km to the earth’s centreencircling from all directions. Nickle and Ferrous are found here. They are very heavy and denser elements. Therefore, this layer have higher density. Its density is about 11 to 12 g/cm3. It is believed that the existence of nickel and iron in the NIFE is the main cause for earth’s magnetic properties.

 

Earth’s Internal Structure

 

The study of the propagation of seismic waves in the interior enabled the scientists to theorize about its structure. Based on the abrupt changes in the paths of seismic waves, the structure of the earth has clearly been demarcated into three zones. They are Crust, outer and a very thin layer of the earth; Mantle, an intermediary thick layer with large volume of rocks below the crust and Core, the innermost layer which is spread all-around the center of the earth.

 

The Earth’s Crust

 

The earth’s crust is the outermost and the thinnest layer with an average depth of 5km below oceans and 40km below the continents. Its depth reaches about 70 km below the mountains. Apart from a very thin sedimentary layer on the continental crust and adjoining ocean floors, it is primarily composed of igneous and metamorphic rocks. Its lower limit is very clearly marked as both P and S earthquake waves increases due to abrupt change in density. At the lower limit of the crust, the velocity of P waves is 7 km/second which increases to 8 km/second immediately entering the mantle lying below. The same thing is happening with S waves which is 3.7 km/second in the crust but it increases to 4.5 km/second after its boundary. The density of the crust at the surface is2.7 g/cm3 and at the bottom limit, it is 2.9 g/cm3. The demarcating limit is known as Mohorovicic or Moho discontinuity from where mantle starts.

 

The Mantle

 

Just after the upper layer, the density of the mantle at the boundary increases to 3.0 g/cm3 from where the velocity of both P and S waves increase very significantly. As mentioned in the above paragraph, the increase in velocity of seismic waves is due to increased density existing there. It extends from the base of the crust to about 2900 km below the surface (Figure 12). It occupies over 80% of the earth’s volume and 65% of the total mass. Mantle is composed of ultramafic rocks. It is igneous in nature and very rich in minerals. It is composed of magnesium and iron with very low silica content. The chemical composition of mantle is almost similar throughout. But there is change in temperature and pressure inside. With increasing depth, the physical properties changes and, therefore, the behavior of the rocks also changes accordingly. Roughly upto a depth of 100 to 250 km from the surface, rocks are firm, solid and rigid. Below this depth, the state of the matter is partially molten and plastic in behavior. This plastic and semi-solid belt extends about a depth of 700 km.

 

Figure 12: Earth’s Internal Structure

    Lithosphere: It is a top upper solid and rigid layer of the earth. Its depths varies from 100 km to 250 km below which the matter is in semi-solid state. Therefore, lithosphere consists both upper solid part of mantle and entire crust. Complete solid upper part of the earth is divided into several plates of which the surface is made up of, including both continental and oceanic.

 

Asthenosphere: The term asthenosphere is derived from the Greek word ‘asthenos’ meaning ‘weak’. Therefore, the literal meaning is to denote the layer which is weak in terms of fluidity. Geologists believe that the asthenosphere is semi-solid. It is composed of silicates both iron and magnesium. The overall consistency is just like hot tar, the material used to make blacktop road. Its depth is from 100 km to 700 km. Both waves, P and S, slow down while propagating through this layer. This clearly suggests that this layer is not fully melted, as S waves is not stopped but passes with slower velocity. Both asthenosphere as well as the solid upper portion of the mantle together are known as upper mantle.

 

Lower Mantle: The concentric layer from a depth of 700 km to about 2900 km is solid. In this zone, the velocity of both waves, P and S, increases abruptly until it reaches a depth of 2900 km. Beyond this depth, the velocity of P waves decreases very drastically from 13.7 km/second to 8.4 km/second and S waves disappears completely. It clearly suggests that at this depth, the state of matter is liquid or molten form as S waves is not entering. This distinct characteristics is defined by a boundary between two dissimilar sections of the earth. It is known as Gutenberg discontinuity. It is referred as core-mantle boundary (CMB).

 

The Core

 

It is the innermost layer of the earth starting from the Gutenberg discontinuity to the center of the earth. It is completely spherical in shape. It has a volume of only 17% of the earth but it contains 34% of the mass. It is because of the very high density of the material existing there. The density of the core at the Gutenberg discontinuity is 5.5 g/cm3 whereas the density of the core near the boundary is about 10 g/cm3. The core is made up of iron and nickel. Both of them have lower melting temperature than the material lying above. As mentioned in the above paragraphs, the disappearance of S waves and abrupt reduction in the P wave’s propagation leads to conclude that the material is in liquid state. The liquid state of matter is recorded till a depth of 5150 km. After this depth, it is in solid state. Here, the velocity of P waves increases. It is solid because the pressure is excessive and the melting temperature is increased but the temperature needed to melt is lower than the required at that pressure (Figure 13). Therefore, upper liquid part is known as the outer core and innersolid is the inner core.

 

    Figure 13: Estimated and Melting Temperature in the Interior

Conclusions

 

The upper thin layer of the earth is accessible to the humankind and can be studied to know every details directly. The interior of the earth is thousands of km deep and directly inaccessible. Therefore, direct method of study is impossible. Based on the principles of the science and their derivations, interior is studied indirectly. In a very conclusive manner it can briefly be stated that the temperature, pressure and densityare increasing with increasing depths. The nature of the materials is changing. The surface is made up of such materials which are lesser in density.The minerals constituting the surface rocks are different than that of the interior. Based on the chemical constitution, interior is classified into three SIAL, SIMA and NIFE. Seismic waves are the only source to reveal the interior. Depending upon the propagation of P and S waves in the interior, three very distinct layers crust, mantle and core are identified with distinct separating boundaries.

you can view video on CONSTITUTION OF THE EARTH’S INTERIOR

 

References

  • Clark, S. P. (1971) Structure of the Earth, Prentice-Hall: Englewood Cliffs.
  • Jacobs, J. A. (1992) The Interior of the Earth, Chapman and Hall: New York.
  • Khullar, D. R. (2012) Physical Geography, Kalyani Publishers: New Delhi.
  • MaGeary, D. and C.C. Plummer (1994) Earth Revealed, Wm C. Brown Publishers: Dubuque.
  • McDonald, L. (2010) The Structure of Earth, Benchmark Education Company: Guangzhou.
  • Poirier, P. P. (2000) Introduction to the Physics of the Earth’s Interior, Cambridge University Press: Cambridge.
  • Siddhartha, K. (2016) Physical Geography, Kitab Mahal: New Delhi.
  • Singh, S. (1998) Geomorphology, PrayagPuskakBhawan: Allahabad.

 

Web Links

  • http://www.clearias.com/interior-of-the-earth/
  • https://pubs.usgs.gov/gip/interior/
  • https://www.livescience.com/topics/earth-s-interior
  • http://solarviews.com/eng/earthint.htm
  • http://www.nationalgeographic.com/science/earth/surface-of-the-earth/earths-interior/
  • http://geology.com/nsta/earth-internal-structure.shtml
  • http://www.columbia.edu/~vjd1/earth_int.htm
  • https://www.nasa.gov/mission_pages/sunearth/news/gallery/earths-dynamiccore.html
  • https://www.britannica.com/place/Earth/The-interior
  • http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.ht ml
  • http://www.if.ufrgs.br/ast/solar/earthint.htm
  • https://pubs.usgs.gov/circ/1966/0532/report.pdf