1.3 isostasy .pdf

Institute of Lifelong Learning , University of Delhi

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DISCIPLINE COURSE -1

SEMESTER -1

Paper: 101: GEOMORPHOLOGY

Unit 1: Geomorphology and Internal Structure of

the Earth

1.3: Isostasy

Lesson Developer:

Dr. Prabuddh Kr. Mishra

(Bhim Rao Ambedkar College)

University of Delhi, Delhi



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Table of Contents

1.3: Isostasy

1.3.1 Introduction

1.3.2 Discovery of the concept

1.3.3 The concept of Sit George Airy

1.3.4 The concept of Archdeacon Pratt

1.3.5 Conclusion

Summery

Glossary

References



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1.3.1 Introduction

Isostasy simply means a mechanical stability between the upstanding parts and the low-

lying basins on a rotating earth. Isostasy has been derived from a German word isostasios

(meaning in equipoise in a state of balance). American geologist Dutton proposed the principle

of isostasy in the paper On Some of the Greater Problems of Physical Geology (1892). He was

the first person to propose and express his view to indicate the state of balance which he

thought must have exist between large upstanding areas of the earths surface, mountain

ranges, plateaus and contiguous lowlands, etc. According to Dutton, the upstanding parts of

the earths surface (mountains, plateaus, plains and ocean basins) must be compensated by

lighter rock materials from beneath so that the crustal reliefs should remain in mechanical

stability. According to J.A. Steers (1961) this doctrine states that whatever equilibrium exists

on the earths surface, equal mass must underlie equal surface area.Isostatic equilibrium is an

ideal state where the crust and mantle would settle into in absence of disturbing forces. The

waxing and waning of ice sheets, erosion, sedimentation, and extrusive volcanism are examples

of processes that perturb isostasy.

1.3.2 Discovery of the Concept

Though the concept of isostacy came much later in the mid 19th century, however, its

concept grew out of gradual thinking in terms of gravitational attraction of giant mountainous

masses. Pierre Bouguer during his expedition of the Andes in 1735 found that the towering

volcanic peak of Chimborazo (one of the ranges of Andes) was not attracting the plumb line as

it should have done. He thus maintained that the gravitational attraction of Andes is much

smaller than that to be expected from the mass represented from these mountains. Similar

discrepancies were noted during the geodetic survey of the Indo-gangetic plain for the



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determination of latitudes under the supervision of Sir George Everest, the then surveyor

general of India in 1859. The debate on the discrepancies of the gravitational deflections of the

plumb line and numerous discrepancies resulted into the postulation of the concept of isostacy.

There are two important hypothesis proposed to explain the concept of Isostasy by Sir George

Airy and Archdeacon Pratt. There are other concepts of Isostasy given by Hayford and Bowie

and their concepts were almost similar to Pratt.

Click on the link to watch the Video on Concept of Isostasy

http://www.youtube.com/watch?v=G9rN7qhUQZg

1.3.3 The concept of Sir George Airy (hypothesis proposed in 1855)

According to Airy, the inner parts of the mountains are not hollow; rather the excess

weight is compensated by the lighter materials below. According to him the crust of relatively

lighter material is floating in the substratum of dense material. Therefore, the continents which

are made of lighter sial are floating over the sub-stratum which is built of the denser sima.Thus,

Himalaya is also floating in the denser glassy magma. He suggested that the lighter sial of the

Himalaya is floating over the denser material of sima lying underneath. Not only this, but the

Himalaya is floating in the denser magma with their maximum portion sunk in magma in the

same way as a boat floats in water with its maximum parts sunk in the water.

This concept in fact involves the principle of floatation. For example, an iceberg floats

in water in such a way that for every one part to be above water-level nine parts has to be

below the water. If it is assumed the average density of the crust and substratum to be 2.67

and 3.00 respectively, then, for every one part of the crust to remain above the substratum,

nine parts of the crust must be in the substratum. In other words, law of floatation demands

that the ratio of freeboard to drought is 1 to 9. If the law of floatation is applied as stated



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above, in the case of the concept of Airy, then we have to assume that for the 8,848 meters

height of the Himalaya, there must be a root, 9 times more in length than the height of the

Himalaya, the substratum. Thus, for 8,848 meters part of the Himalaya above, there must be

downward projection of lighter material beneath the mountain reaching the depth of roughly

80,000 meters.

Thus, according to Airy, Himalaya was exerting their real attraction force because their

existed a long root of lighter materials in the substratum which compensate the material above.

Based on the above observation Airy postulated that if the land column above the substratum

is larger, its greater part would be submerged in the substratum and if the land cover is lower,

its smaller part would be submerged in the substratum. A per Airy, the density of different

column does not change with depth, i.e., uniform density with varying depth (Fig.1).

He showed that the landmasses could be conceived as wooden flocks of different height

floating in water. These blocks are said to be in the state of hydrostatic balance or equilibrium.

Similarly, land masses of similar density of rocks, penetrate deep into the substratum in

proportion to their height. To prove the concept he took several pieces of copper of varying

length and put them in a basin full of mercury. These pieces of copper sunk upto varying depths

depending on their lengths. The longest block will float with greatest amount rising above the

level of mercury surface and the shortest block has upper surface lowest. With all block now

floating at rest, it is obvious that block rising highest also extends to grater depths.



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Fig.1 Airys Concept of Isostasy (Image Source: Wikipedia)

This concept of Airy also suffers from certain defects and errors. If we accept the Airys concept

of isostasy then, every upstanding part must have a root below with its height. That means

Himalayas have the root equivalent to 79,632 meters, which is not possible. It is wrong to

assume that Himalaya have a downward projection of lighter material beneath the mountain,

which will have such a great depth. Moreover, such a long root would melt due to very high

temperature prevailing at such great depth. Though there is an error on Airys concept the

geologists favour Airys explanation the most than others.

1.3.4 The concept of Archdeacon Pratt (hypothesis proposed in 1859)

According to Pratt the gravitational attraction of the Himalayas was less than the mass

represented by these mountains because they are made up of much lighter materials, i.e. rocks

of much lower density. According to Pratt, there is a level of compensation above which there is

a variation in the densities of different columns of land but there is no change in density below

that level. Thus, the central theme of the concept of Pratt on Isostasy may be expressed as

Uniform depth with varying densities. According to Pratt equal surface area must underlie

equal mass along the line of compensation. This statement may be explained with the help of

an example.

There are two coloumns A and B, along the line of compensation. Both the coloumns, A

& B, have equal surface area but there is difference in height (B is lower in height than A). Both

the coloumns must have equal mass along the line of compensation. So the density of coloumn

B is more than the density of Coloumn A so that weight of both the coloumns becomes equal

along the line of compensation. So Pratts concept of inverse relationship between the height of



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different coloumns and their respective densities may be expressed in the following manner

bigger the coloumn, lesser the density and smaller the coloumn, greater the density. Hence

materials are less dense under mountains than under low plains and are most dense under

ocean floors. According to Pratt density varies only in the Lithosphere and barysphere. Thus as

per Pratt, different relief features are standing only because of the fact that their respective

mass is equal along the line of compensation of varying density (Fig.2).

Fig.2 Pratts concept of Isostasy (Image Source: Wikipedia, eps.mq.edu.au )

1.3.5 Conclusion

Bowie has opined that Pratt does not believe in the law of floatation as stated by Airy,

but if we look closely we will find, the glimpse of the law indirectly. Similarly, though Pratt does

not believe in the concept of root formation but close perpetual of his concept of isostasy,

does indicate glimpse of such idea. While making comparative analysisd between Airys and

Pratts concept, Bowie observed that fundamental difference between their concept is that

Line of Compensation



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former postulated a uniform density with varying thickness and the latter one postulated a

uniform depth with varying density.

Fig. 3 Pratts Theory (left) and Airy's Theory (right). (source: large.stanford.edu )

Summary

The concept of Isostasy has been proposed by Clarence Edward Dutton.

Its historical evolution has been as old as in 1735, when Pierre Bouguer launched an

expedition on the Andes Mountain.

Later the discrepancies of the gravitational deflections of the plumb line (during the

survey of Andes and Himalayan Mountains) and numerous discrepancies resulted into

the postulation of the concept of isostasy.

Two major concepts of Isostasy have been given by Sir George Airy and Archdeacon

Pratt. The major difference between the two concepts were that former one postulated

a uniform density with varying thickness and the latter one postulated a uniform

depth with varying density.

Glossary



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Isostasy

The state of balance/ equilibrium exists between large upstanding areas of the earths surface,

mountain ranges, plateaus and contiguous lowlands. A balance between lithosphere and

aesthenosphere.

Line of compensation

The concept has been given by Pratt and according to him there is a level of compensation

above which there is a variation in the densities of different columns of land but there is no

change in density below that level. According to him equal surface area must underlie equal

mass along the line of compensation.

References

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

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

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

First Indian Reprint. Delhi: Pearson Education (Singapore) Pvt. 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.

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.



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

Pan,S.A. - December 8, 2007The Theory of Isostasy, ( Submitted as coursework for Physics 210, Stanford University, Fall 2007). Accessed on 26th August 2013 http://large.stanford.edu/courses/2007/ph210/pan2/

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

http://eps.mq.edu.au/courses/GEOS260/Lithosphere1.html

http://large.stanford.edu/courses/2007/ph210/pan2/ http://commons.wikimedia.org/wiki/File:Isostasy.Airy%26Pratt.Scheme.png

http://www.britannica.com/EBchecked/topic/174692/Clarence-Edward-Dutton

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