Component-I (A)- Personal Detail

Role Name Affiliation

Principal Investigator Prof. Masood Ahsan Department of Geography,
Siddiqui Jamia Millia Islamia, New
Delhi
Paper Coordinator Dr.Sayed Zaheen Alam Dyal Singh College,
University of Delhi, New
Delhi
Content Writer Dr. Biplab Biswas Department of Geography,
University of Burdwan,
Golapbag, Burdwan
Content Reviewer Dr.Sayed Zaheen Alam Dyal Singh College,
University of Delhi, New
Delhi
Language Editor Dr.Sayed Zaheen Alam Dyal Singh College,
University of Delhi, New
Delhi

Component-I (B) Description of Module

Items Description of Module
Subject Name Geography
Paper Name Geomorphology
Module Name/Title Isostasy
Module Id GEO-8
Pre-requisites Constitution of the Interior of the Earth
Objectives Total Five
Keywords Iso-Stasios, Geodetic Survey, Sial, Sima,
Triangulation Method, Astronomical
Method, Principle of Floatation, Law of
Compensation, Line of Compensation,
Zone Of Compensation
 The word Isostasy is derived from the Greek word ‘iso-stasios’ which means
‘equal standing’ (in equipoise). The term isostasy was first proposed by an American
geologist Clarence Dutton in 1889 to indicate the state of balance which exists between
large upstanding areas of the Earth’s surface, mountain ranges and plateaus. The theory
says that the less dense materials of the Earth’s surface (sial) must float over the denser
magma (sima) of the Earth’s interior. Similarly, as we go deep interior of the Earth’s, we
see that there are several concentric layers. The densest material forms the core whereas
the Earth’s surface is composed of lightest material. Each layers and the Earth’s surface
features are resting on over another with an isostatic adjustment. For example, the
average density of the Core is 13.5 gm/cm3; density of Mantle ranges from 3.3 to 5.7
gm/cm3; density of the Continental crust is around 2.7 gm/cm3. The concept of Isostasy
is extremely useful to explain ‘glacial adjustment’ taking place in Scandinavian
countries after thsede Pliestocene great ice age. The raised beaches of Finland exhibits
that an uplift of about 250 metres has taken place during the last 8000 years due to
Isostactic adjustment.

In the present module we are going to cover about following aspects of Isostasy:

1. Development of the Concept

2. The Concept of Sir George Airy
3. The Concept of Archdeacon Pratt
4. The Concept of Hayford and Bowie
5. The Concept of Jolly

DEVELOPMENT OF THE CONCEPT

The concept of isostasy came in the mind of geologists but the concept grew out
of attraction of giant mountainous masses (Fig-1).
Pierre Bouguer during his expedition of the
Andes in 1735-45 found that the towering
volcanic peak of Chimborazo was not attracting
the plumb line, as it should have done. He thus
maintained that the gravitational attraction of the
Andes ‘is much smaller than that to be expected
from the mass represented by these mountains’.
Similar discrepancies were noted during the
geodetic survey of the Indo-Gangetic plain for
the determination of latitudes under the
supervision of Sir, George Everest, the then
Surveyor General of India in 1859. The
difference of latitude of Kalianpur and Kaliana
(370 miles apart) was determined by both direct
triangulation method and astronomical method.
 Kaliana was only 96 km away from the
Fig-1: Deflection of plum bob due to Himalayas. The difference between two results
gravity amounted to 5.23 seconds as given bellow-

Result obtained through triangulation= 5° 23’ 42.294”.

Result obtained through astronomical method = 5° 23’37.058”.
Difference= 5.236”

This discrepancy between two methods was attributed to the less attraction of the
Himalayas due to which the plumb bob used in the astronomical determination of latitude
was deflected. There are many theories to explain the gravitational attraction and
deflection and isostatic balance among the various landforms.

THE CONCEPT OF SIR GEORGE AIRY

According to Airy the inner part of the mountains cannot be hollow; rather the
excess weight of the mountains is compensated (balanced) by lighter materials below.
According to him the crust of relatively lighter material is floating in the substratum of
denser material. In other words, ‘sial’ is floating over ‘sima’.
Thus, the Himalayas are floating in denser glassy magma. According to Airy ‘the
great mass of the Himalayas was not only a surface phenomenon – the lighter rocks of
which they are composed do not merely rest on the level surface of denser material
beneath, but, as a boat in water, sink into the denser material.
In other words, the Himalayas are floating in the denser magma with their
maximum portion sunk in the magma in the same way as a boat floats in water with its
maximum part 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 of the iceberg remain below water level. If we assume the
average density of the crust and the substratum to be 2.67 and 3.0 respectively, 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, the law of floatation demands that ‘the ratio of freeboard to
draught is 1 to 9. It may be pointed out that Airy did not mention the example of the
floatation of iceberg. He simply maintained that the crustal parts (landmasses) were
floating, like a boat, in the magma of the substratum.
It we apply the law of floatation, as stated 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, in the substratum.
Thus, for 8,848 m part of the Himalaya above, there must be downward projection of
lighter material beneath the mountain reaching a depth of 79,632m (roughly 80,000 m).
Joly applied the principle of floatation for the crust of the Earth taking the
freeboard to draught ratio as 1 to 8. According to him ‘for every emergent part of the
crust above the upper level of the substratum there are eight parts submerged’. If we
apply Joly’s view of floatation to the concept of Airy, there would be downward
projection of the Himalaya up to a depth of 70,784m (8848m x 8) in the substratum.
Thus, according to Airy the Himalayas were exerting their real attractional force
because there existed a long root of lighter material in the substratum which compensated
the material above. Based on 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 column is lower, its smaller part would be submerged in the substratum.’
According to Airy, the density of different columns of the land (e.g. mountains, plateaux,
plains etc.) remains the same. In other words, density does not change with depth, that is,
‘uniform density with varying thickness’ .(fig. -2).
This means that the continents are
made of rocks having uniform
density but their thickness or
length varies from place to place.
In order to prove this concept Airy
took several pieces of iron of
varying lengths and put them in a
basin full of mercury. These pieces
of iron sunk up to varying depths
depending on their lengths. The
same pattern may be demonstrated
by taking wooden pieces of
varying lengths. If we put them
into the basin of water these would
sink in the water according to their Fig- 2: Model of Isostasy of Airy
lengths (Fig-2).
Though the concept of Sir George Airy commands great respect among the
scientific community but it also suffers from certain defects and errors. If we accept the
Airy’s views of isostasy, then every upstanding part must have a root below in
accordance with its height.

Thus, the Himalayas would have a

root equivalent to 79,632m (if we
accept the freeboard to draught ratio
as 1 to 9) or 70,784m (if the
freeboard to draught ratio is taken as
1 to 8). It would be wrong to assume
that the Himalaya would have a
downward projection of root of
lighter material beneath the mountain
reaching such a great depth of
79,634m or 70,784m because such a
long root, even if accepted, would
melt due to very high temperature
prevailing there (as temperature Fig-3: Comparison between the views of Airy
increases with increasing depth at the and Pratt on Isostasy
rate of 1°c per 32m).

So, there are some alternative models (Fig-3) to explain the isostatic adjustment
of the landforms of the Earth’s surface and related gravitational deflection.

THE CONCEPT OF ARCHDEACON PRATT

While studying the difference of gravitational deflection of 5.236 seconds during
the geodetic survey of Kaliana and Kalianpur, Archdeacon Pratt calculated the
gravitational force of the Himalaya after taking the average density of the Himalaya as
2.75 and came to know that the difference should have been 15.885 seconds. Paratt said
that “In order to measure angle to a star a surveyor must determine the horizontal plane
if horizontal plane between two sites were askew so must be the vertical direction”.
He, then, studied the rocks (and their densities) of the Himalaya and neighbouring
plains and found that the density of each higher part is less than a lower part. In other
words, the density of mountains is less than the density of plateau, that of plateau is less
than the density of plain and the density of plain is less than the density of oceanic floor
and so on. This means that there is inverse relationship between the height of the
reliefs and density.
According to Pratt there is a level of compensation above which there is variation
in the density of different columns of land but there is no change in density below this
level. Density does not change within one column but it changes from one column to
other columns above the level of compensation.

Thus, the central theme of the concept of

Pratt on isostasy may be expressed as
‘uniform depth with varying density’.
According to Pratt equal surface area must
underlie equal mass along the line of
compensation. This statement may be
Fig- 4: Line of Compensation explained with an example (Fig-4).

There are two columns, A and B, along the line of compensation. Both the
columns, A and B, have equal surface area but there is difference in their height. Both the
columns must have equal mass along the line of compensation, so the density of column
A should be less than the density of column B so that the weight of both the columns
become equal along the line of compensation.
Thus, Pratt’s concept of inverse relationship between the height of different
columns and their respective densities may be expressed in the following manner- ‘bigger
the column lesser the density and smaller the column, greater the density.’ According to
Pratt density varies only in the lithosphere and not in the pyrosphere and barysphere.

Thus, Pratt’s concept of isostasy

 was related to the ‘law of compensation’
and NOT to ‘the law of floatation.’
According to Pratt different relief features
are standing only because of the fact that
their respective mass is equal along the
line of compensation because of their
varying densities. This concept may be
explained with the help of an example
(Fig- 5 ).
Fig-5: Pratt’s Model of Isostasy

Bowie has opined that though Pratt does not believe in the law of floatation, as
stated by Sir George Airy but if we look, minutely, into the concept of Pratt we certainly
find the glimpse of law of floatation indirectly. Similarly, though Pratt does not believe
directly in the concept of ‘root formation’ but very close perusal of his concept on
isostasy, does indicate the glimpse of such idea (root formation) indirectly.
While making a comparative analysis of the views of Airy and Pratt on isostasy
Bowie has observed that ‘the fundamental difference between Airy’s and Pratt’s views
is that the former postulated a uniform density with varying thickness, and the latter a
uniform depth with varying density (Steers,1937). Fig-3 explains the fundamental
difference between the concepts of Airy and Pratt on isostasy.

THE CONCEPT OF HAYFORD AND BOWIE

Hayford and Bowie have propounded their concepts of isostasy almost similar to
the concept of Pratt. According to them there is a plane where there is complete
compensation of the crustal parts. Densities vary with elevations of columns of crustal
parts above this plane of compensation.
The density of the mountains is less than the ocean floor. In other words, the crust
is composed of lighter material under the mountains than under the floor of the oceans.
There is such a zone below the plane of compensation where density is uniform in lateral
direction.

Thus, according to Hayford and Bowie there is inverse relationship between the
height of columns of the crust and their respective densities (as assumed by Pratt) above
the line of compensation. The plane of compensation (level of compensation) is
supposedly located at the depth of about 100 km. The columns having the rocks of lesser
density stand higher than the columns having the rocks of higher density. This statement
may be understood with the help of fig. 6.
There are four imaginary columns (interior plain, plateau, coastal plain and
offshore region) which reach at the level of compensation. Their height varies but they
are balanced by their varying densities. ‘The assumption is that the varying volume of
matter in the several columns is compensated by their density, in such a fashion that they
exert equal downward pressure at the level of compensation and thus balance one
another’

Below given figure explains the above concept. It is apparent from fig. 6. that different
columns of equal cross-section cut from various metals and ores having varying
densities are seen floating in a basin of mercury but all of them reach the same line
(level of compensation) and thus exert equal weight along the line of compensation.

Fig-6: The Concept of Isostasy given by Hayford and Bowie

Bowie made a comparative study of the views of Airy and Pratt on isostasy and
concluded that there was a great deal of similarity in their views. In fact, ‘both the views
appeared to him similar but not the same.’
Bowie could observe a glimpse of the concept of root formation and law of floatation of
Airy, though indirectly, in the views of Pratt. The concept of Hayford and Bowie, that the
crustal parts (various reliefs) are in the form of vertical columns, is not tenable because
the crustal features are found in the form of horizontal layers.

THE CONCEPT OF JOLLY

Joly, presented his views on isostasy in the year 1925. He disapproved the view of
Hayford and Bowie about the existence of level of compensation at the depth of about
100 km on the ground that the temperature at this depth would be so high that it would
cause complete liquefaction and thus level of compensation would not be possible.
He further refuted the concept of Hayford and Bowie that ‘density varies above
the level of compensation but remains uniform below the level of compensation’ on the
ground that such condition would not be possible in practice because such condition
would be easily disturbed by the geological events and thus the level of compensation
would be disturbed.
According to Joly there exists a layer of 10-mile (16 km) thickness below a shell of
uniform density. The density varies in this zone of 10-mile thickness. It, thus, Joly
assumed the level of compensation as not a linear phenomenon but a zonal phenom-
enon. In other words, he did not believe in a ‘line (level) of compensation’ rather he
believed in a ‘zone of compensation’ (of 10-mile thickness).

Thus, we also find a glimpse of the

law of floatation (it may be noted that Joly
did not mention this, we only infer the idea
of floatation from Joly’s concept) and his
concept is closer to the Airy’s concept
rather than the concept of Hayford and
Fig-7: Jolly’s view Bowie.

‘This is in close agreement with floatation idea; the areas of low density in the 10-inile
layer correspond with downward projections of the light continental crust, while those of
high density represent the intervening areas filled with material of the heavier under-
stratum’ (Fig-7).