1.2 Earth Interior Structhbjure

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Institute of Lifelong Learning , University of Delhi

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DISCIPLINE COURSE -1

SEMESTER -1Paper: 101: GEOMORPHOLOGY

Unit 1: Geomorphology and Internal Structure ofthe Earth1.2: Earth: Internal Structure

Lesson Developer:Dr. Prabuddh Kr. Mishra

Bhim Rao Ambedkar College University of Delhi, Delhi





Table of Contents

1.2: Earths: Interior Structure 1.2.1: Objective

1.2.2: Background

1.2.3: Evidences about Earth’s Interior

1.3: Scientific profile of Internal Structure of the Earth

(A) Chemical Sub-division

1.3.1: Crust

1.3.2: Mantle

1.3.3: Core

(b) Mechanical Sub-division

1.3.4: Lithosphere

1.3.5: Aesthenosphere

1.3.6: Mesosphere

1.3.7: Barysphere

Summary

Exercises

Glossary

References





1.2 Earth: Interior Structure

1.2.1 Objectives

This unit entitled Earth: Interior Structure tries to understand the structure, conditions and

material properties within the Earth’s interior. An understanding of the earth’s interior is

essential to follow the nature of changes going on over the earth surface which are related

to the deep laid internal forces operating from within the earth.

1.2.2 Background

During the 1800’s there was keen interest on the interior of the earth. Several scientific

experiments happened during this period. Till this period the details of the interior structure

of the earth, viz. the information regarding the core mantle and crust (their compositions,

thickness, etc) was very limited and had not been discovered in a holistic manner. During

this period it was made clear through scientific evidences of volcanic eruptions that the

earth’s interior is hot enough to melt the rocks. However, the temperature and the

existence of radioactivity of the interior were unknown. Jules Verne’s book, A Journey to

the Center of the Earth (1864, 272 pages; originally published in France as Voyage au Centre

de la Terre), epitomized the interest of the earth and its interior during this period and

wrote an adventurous and exciting science fiction story that is still popular today. The

understanding of the earth’s interior is based mainly on indirect sources, because so far it

has not been possible to have access to the inner levels of the earth’s structure.





Fig. 1 Interior Structure of the Earth

(Source: http://web.ics.purdue.edu/~braile/edumod/earthint/earthint.htm)

To watch video on the Earth’s interior click on the following site: 1. https://www.youtube.com/watch?v=0mWQs1_L3fA

2. https://www.youtube.com/watch?v=N9ncfAsmiSg

1.2.3 Evidences about Earth’s Interior

a) High level of Temperature and Pressure Downwards

The recurrent volcanic eruptions throwing out extremely hot, molten material from the

earth’s interior and the existence of hot springs, geysers etc. point to an interior which is

http://web.ics.purdue.edu/~braile/edumod/earthint/earthint.htm


https://www.youtube.com/watch?v=0mWQs1_L3fA


https://www.youtube.com/watch?v=N9ncfAsmiSg









very hot. Although the average rise in temperature from the surface downwards is 320 C per

meter, this rise is not uniform throughout. In the upper 100km the increase is estimated at

300 km and 10 C per km after that. As per this calculation, the temperature is 2000

0 C at the

earth’s core. The high temperature is attributed to internal forces, automatic disintegration

of the radioactive substances, chemical reactions and other sources.

Although ideally the innermost part of the earth should be in liquid or a gaseous state due

to the high temperature, yet because the pressure also increases with depth, the core is a

rigid mass. The layer enveloping the core is in semi-solid or plastic state.

B) Behaviour of the Earthquake Waves

The earthquake waves are measured with the help of a seismograph and are of three types-

the ‘P’ wave 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. Thus, the velocity of P waves decreases towards the interior pointing

to less solid layer ( a characteristics of longitudinal waves), but increases for a while when

passing through the inner core only to decrease as it comes out of the core. This point to a

solid core surrounded by a partially molten layer. Similarly, the ‘S’ wave cannot pass through

a liquid medium and are only transmitted through a rigid or solid medium. The ‘S’ waves

cannot deflected while travelling inwards and come out at the earth’s surface. This, again

points to a molten, semi molten layer below the crust and mantle. The ‘L’ waves do not pass

and do not go deeper inside the earth.

3) Evidences from the Meteorites

The meteorites are solid bodies freely travelling in space which accidently come under the

sphere of influence of the earth’s gravity and as a result fall on earth (or collide with it).

Their outer layer is burnt during their fall due to extreme friction and the inner core is

exposed. The heavy material composition of their core confirms the similar composition of

the inner core of the earth, as both evolved from the same star system in the remote past.





From the above scientific evidence a fairly convincing picture of the earth’s interior can be

drawn (Fig. 2a)

Fig 2 Earth’s interior structure. The silicic (contains silicon and oxygen) crystalline rocks make up the

earth’s crust. The mantle is rich in Iron- and magnesium-rich silicate rocks. The core is primarily

made up of iron and nickel. The outer core is in molten state and the inner core is in solid state. (

http://web.ics.purdue.edu/~braile/edumod/journey/journey.htm)

http://web.ics.purdue.edu/~braile/edumod/journey/journey.htm







Fig. 2a Earth's shallow (0 to 600 km depth) structure showing chemical layer and mechanicalclassification (http://web.ics.purdue.edu/~braile/edumod/earthint/earthint.htm)

Did You Know

Some Numerical Facts about Earth’s Interior

Crust= 0- 100 KM (0.5% of Earth’s volume) - SIAL

Mantle= 100-2900 KM (16% of Earth’s volume) - SIMA

Core= 2900-6400 KM (83% of Earth’s volume) -NIFE

Earth’s Density= 5.517 g/cm3









1.2.3 A Sectional Profile of the Earth’s Structure

The structure of the earth’s structure is layered and broadly three layers can be identified-

Crust, Mantle and Core (Fig. 3).

(A) Chemical Sub-division

1.3.1: Crust

The outermost layer or cover of the earth is crust with an approximate thickness of 100 km.

In fact it is the earth’s surface on which we are standing on. It is composed of igneous,

sedimentary and metamorphic rocks. It forms 0.5% of the earth’s crust volume.

Geologists have discovered, with the help of indirect evidences like seismology, volcanic

eruption, etc., that the interior of the earth has very high temperature. However, the

temperature of the earth’s core is influenced by the atmosphere above it and thus, the crust

would be same temperature as the air above. It might be, as hot as 50°C in the desert and

might be below freezing near Greenland and Antarctica.

The outer covering of the crust is of sedimentary materials and below that lie crystalline,

igneous and metamorphic rocks which are acidic in nature. The lower layer of the crust

consists of basaltic and ultra-basic rocks. The continents are composed of lighter silicate=

silica + aluminium (also called SIAL)- while the oceans have the heavier silicates- silica +

magnesium (also called SIMA) which form the part of the mantle.

The rigid zone of the earth’s crust is also known as Lithosphere. The layer below it is the

zone of Asthenosphere, which is a part of mantle and helps in the movement of the

lithospheric plates.

1.3.2: Mantle

Earth's mantle lies between 100- 2900km below the earth’s surface and forms 16% of the

earth’s volume. The outer layer of the mantle is partly somatic and behaves like plastic mass

while the inner layer is composed of wholly somatic ultra-basic rocks. Temperature of the

earth’s mantle varies and increases with increase in depth. Lowest temperature is found

immediately below the crust and highest immediately above the heat-producing core. This





phenomenon of steady increase of temperature with depth in the interior of the earth is

known as geothermal gradient . The geothermal gradient changes the behaviour of the rock

at different zones and the different rock behaviours are captured to divide the mantle in

two different zones. The boundary between the crust and the mantle is a surface

discontinuity which was discovered by A. Mohorovicic and is, therefore named after him.

This is often referred to simply as the Moho, or as M-discontunity . Thus, the continent of

the lighter material is floating in a sea of heavier and denser material.

1.3.3: Core

Earth's Core lies between 2900 km and 6400 km below the earth surface and accounts for

83% of the earth’s volume. The central core has the heaviest mineral materials of high

density. It is composed of nickel and iron (ferrous) and is therefore called as ‘nife’, while a

zone of mixed heavy metals and silicates separates the core from outer layers. Geologists

have assumed this composition based on the calculation of the density and upon the fact

that many meteorites (which are thought to be portions of the interior of a planetary body)

are iron-nickel alloys.

The earth’s core has the highest temperature of all the layers and is a major source of heat

for the earth’s interior. The core receives heat from radioactive materials present within the

core, which releases heat when it breaks down into more stable substance. The core has

been divided into two different zones, the outer core (liquid state) and the inner core (solid

state). The outer core is in liquid state as the temperature in this zone is high enough to

melt the iron and nickel alloy. However, the inner core is in solid state inspite of its

temperature higher than the outer core. This is mainly due to the immense pressure of the

overlying rocks exerting on the inner core, which keeps this zone in solid state.

Discontinuities within the Earth

Conrad Discontinuity- between outer and inner crust

Mohorovicic Discontinuity- between crust and mantle

Repetti discontinuity- between outer and inner mantle

Wheichart-Gutenberg Discontinuity -between mantle and core

Lehmann Discontinuity - between outer and inner core





Fig . 3 Earth Interior through 3D view

(b) Mechanical Sub-division

1.3.4: Lithosphere

The rocky, solid portion of the earth’s crust, from the top of Mount Everest to the bottom of

Mariana trench, is called as lithosphere. Lithosphere also includes the solid portion of the

upper layer of the mantle. It is composed of variety of minerals. It is divided into several

large fragments, called plates, and the motion of these surface plates is over the years is

referred to as plate tectonics (Fig. 2a & 4).

Web link

http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/tectonic_plates_rev1.shtml








Fig . 4 Earth Interior- Sectional profile

1.3.5: Aesthenosphere

The zone, below the lithosphere extending between the depth of 100 km and 700 km is

aesthenosphere. It is a low velocity zone. The word aesthenosphere has been taken from a

greek word ‘asthenēs’ which means weak. it is highly viscous and mechanically weak and

deforming zone of the upper mantle of the earth. Compared to the lithosphere above, this

region is more plastic or less viscous, that is, it is softer, more pliable and capable of bending

or deforming without breaking.

The aesthenosphere is involved in plate tectonic movements and isostatic adjustments. The

pressure of the overlying rocks keeps this zone in plastic state, inspite of the relative high

temperature. It is relatively low density zone. The scientists and seismologists gathered

information and evidences of this zone. The major information about the physical properties

of this zone came from the earthquake waves. Theses waves decreases with decreasing

rigidity. The seismic waves pass through this zone in a relatively low velocity than the upper

lithosphere mantle. Thus, this zone is also known as low-velocity zone (LVZ). This low

Web link: http://www.universetoday.com/73597/what-is-lithosphere/

http://www.wikipedia.org/wiki/Plate_tectonics

http://www.wikipedia.org/wiki/Isostasy

http://www.wikipedia.org/wiki/Low-velocity_zone

http://www.universetoday.com/73597/what-is-lithosphere/




http://www.wikipedia.org/wiki/Low-velocity_zone

http://www.wikipedia.org/wiki/Isostasy

http://www.wikipedia.org/wiki/Plate_tectonics





velocity zone starts at a depth of 180 –220 km (forming the upper part of aesthenosphere)

and ends at a depth of about 700 km (forming the base of the aesthenosphere).

The aesthenosphere is usually much closer to the sea floor surfaces and mid-oceanic ridge.

The rigid lithosphere is thought to "float" or move about on the slowly flowing

aesthenosphere, creating the movement of crustal plates (Fig. 2a & 4).

1.3.6: Mesosphere

The term Mesosphere (“mesospheric shell”) derieved from was given and used for the first

time by Reginald Aldworth Daly, a geology professor in Harvard University, for the zone

immediately below the aesthenosphere and extending down to the mantle-core boundary

at 2830 km. This layer should not be confused with the Mesosphere, which is also a name of

one of the atmospheric layer. The study of the seismic waves passing through this zone

indicates prominent boundaries at 400 km and 670 km (Fig. 2a & 4).

1.3.7: Barysphere

The term barysphere is used to denote both the mantle and the core. However, it is

sometimes used to refer only to the core or only to the mantle. The term is now less used.

Summary/Key Points for Revision

There are many good reasons for studying earth interior. Certain things about the

surface simply cannot be explained by surface and near surface processes.

The interior of the earth can be known by the study of its density, seismic properties

and analogy from outer space.

The average Specific gravity of the earth is 5.5.

The best evidences on the physical and chemical properties of the interior of the

earth come from the study of seismic waves- P, S, and L.

On the basis of chemical composition earth can be divided into crust, mantle and

core.

On the basis of chemical composition earth can be divided into lithosphere,

aesthenosphere, mesosphere, and Barysphere (Core).

http://www.wikipedia.org/wiki/Tectonic_plate

http://en.wikipedia.org/wiki/Reginald_Aldworth_Daly

http://en.wikipedia.org/wiki/Reginald_Aldworth_Daly

http://www.wikipedia.org/wiki/Tectonic_plate





Glossary

Crust

The Earth’s surface or the outer shell, composed of solid rock and is having thickness in

between 0 to 100 kilometres.

Mantle

The layer within the earth’s interior lying beneath the crust and above the core. This

extends from beneath the core and goes deep down to 2,900 km inside the earth. This

layer is mostly solid state.

Core

The innermost part of the interior of the earth is the core, lying beneath the mantle and

extends deep down to approximately 6,300 km inside the earth.

Oceanic crust

The part of the Earth’s surface that lies below the oceans. Oceanic crust is denser than

the continental crust and hence generally lies below sea level.

Plate Tectonics

The theory that explains the movement of the Earth’s plates.

Lesson review Questions

List two ways that scientists learn about what makes up the Earth’s interior?

How do we know that density increases towards the centre of the earth?

What type of rock makes up the oceanic crust?

What types of rock make up the continental crust?

Of what is the mantle generally thought to be composed?

Name two lines of evidence that suggest that interior is vey hot.

The boundary that separates the crust from the mantle is called as…………………………..





The inner core is most likely composed of……………………………………..

The interior composition and structure of Earth have been deduced in part from………

The composition of the upper mantle is known because…………….

The largest portion of Earth's volume is…………………………. The average thickness of the crust is………………………………..

Think Critically

1. List two reasons that scientists know that the outer core is liquid.

2. Suppose that Earth’s interior contains a large amount of lead. Lead is very dense:

11.34 g/cm3. Would the lead be more likely to be found in the crust, mantle, or

core?

References

Ronald L. Turcotte and Gerald Schubert. Geodynamics, 2nd ed., (2001) Cambridge

University Press

GEMS Institute of higher education, Nepal.Environment Management Club, Kushal.

An Introduction to the Solar System; McBride and Gilmour; Cambridge University

Press 2004

Ollier, C.and Pain, C. (2000), The Origin of Mountains ,Routledge.

S. Singh (1998), Geomorphology , Prayag Pustak Bhawan, Allahabad.

Dayal, P. (1976), A Text Book of Geomorphology, Shukla Book Dept, Patna.





A.N. Strahler & A.R. Strahler (1978), Modern Physical Geography , John Wiley.

J.A. Steers, (1961). The Unstable Earth, Lyell Book Dept, Ludhiana.

Kale V. and Gupta, A. (2001) Elements of Geomorphology . Oxford University Press,

Calcutta.

Bharatdwaj,K. (2006). Physical Geography: Introduction to Earth.Discovery Publishing

House,Delhi.

Bloom, Arthur L., (2003) Geomorphology: A Systematic Analysis of Late Cenozoic

Landforms. First Indian Reprint. Delhi: Pearson Education (Singapore) Pte. Ltd.

Chorley, Richard J., Schumm, Stanley A. and Sugden, David E., (1984)Geomorphology .

London: Methuen & Co. Ltd.

Engeln, O. D. von, Geomorphology . (1960) New York: The Macmillan Company.

Suggested Readings

Hugget, R. J. (2011) Fundamentals of Geomorphology, Routledge, Oxon.

Strahler, A. and Strahler, A. (2002) Physical Geography: Science and Systems of the

Human Environment, John Wiley and Sons, New York.

Holmes, A and Holmes, D.L. (1978) Holmes Principles of Physical Geology, BAS

Printers, Hampshire.

Web Links

1. http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Asthenosphere.html

2. http://www3.geosc.psu.edu/~jte2/geosc20/lect07.html

3. http://en.wikipedia.org/wiki/Mesosphere_(mantle)

http://www.google.co.in/search?tbs=bks:1&tbo=p&q=+inauthor:%22Richard+J.+Chorley%22

http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Asthenosphere.html


http://www3.geosc.psu.edu/~jte2/geosc20/lect07.html


http://en.wikipedia.org/wiki/Mesosphere_(mantle)





http://www.google.co.in/search?tbs=bks:1&tbo=p&q=+inauthor:%22Richard+J.+Chorley%22

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1.2 Earth Interior Structhbjure

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