Top 6 Theories of Isostasy | Isostasy | Theories | Geology

The following points highlight the top six theories of isostasy. The theories are: 1. Theory of Sir George Airy 2. Theory of Archdeacon Pratt 3. Theory of Hayford and Bowie 4. Theory of Joly 5. Theory of Heiskenen 6. Theory of Holmes.

1. Theory 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 in ‘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 to floation.

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 floation 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 8848m 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 8848 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 floation 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 floation to the concept of Airy, there would be downward projection of the Himalaya upto 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.’

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 upto varying depths depending on their lengths. The same pattern may be demonstrated by taking wooden pieces of varying lengths. If put into the basin of water these would sink in the water according to their lengths (fig. 6.1).

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 increases with increasing depth at the rate of 1°c per 32m.

“Quite recently, however, the fundamental con­cept of Airy, the continental masses floating as lighter (sial) blocks in a heavier (sima) substratum, has been rejuvenated, largely through the influence of Heiskanen’s work, so that is now probably true to say that most geologists favour Airy’s explanation’.

2. Theory 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.

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 plateaux, 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 explained with an example (fig. 6.2).

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. 6.3).

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. Fig. 6.4 explains the fundamen­tal difference between the concepts of Airy and Pratt on isostasy.

3. Theory 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 Archdeacon 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. 6.5.

There are four imaginary columns (interior plain, plateau, coastal plain and offshore region) in fig. 6.5 which reach 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’.

Fig. 6.6 explains the above concept. It is appar­ent from fig. 6.6 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.

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.

4. Theory of Joly:

Joly, while presenting his views on isostasy in 1925, contradicted the concept of Hayford and Bowie. 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, appears that Joly assumed the level of compensa­tion as not a linear phenomenon but a zonal phenom­enon. 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 remembered 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 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. 6.7).

5. Theory of Heiskenen:

Heiskenen presented a new concept of isostasy in 1933 in which he combined the concepts of both Airy (uniform density with varying thickness) and Pratt (varying density in different columns). Accord­ing to him density of rocks varies within the column (section of the earth) and between the columns. For example, rocks of a column at sea level have higher density (say 2.76 gram cm^-3) than at higher elevation of the same column (say 2.70 gram cm^-3) which means as we go downward the rocks of a section of the earth’s crust become denser i.e. density increases downward. Similarly, density of rocks of different sections (col­umns) of the earth’s crust also varies. Thus, it appears that density of rocks varies both vertically and horizon­tally.

6. Theory of Holmes:

The views of Arthur Holmes on isostasy, to a greater extent, are compatible with the views of Airy. Following Airy Holmes has also assumed that upstanding crustal parts are made of lighter materials and in order to balance them major portions of these higher columns are submerged in greater depth of lighter materials (of very low density).

According to Holmes the higher columns are standing because of the fact that there is lighter material below them for greater depth whereas there is lighter material below the smaller columns upto lesser depth, (fig. 6.8).

A. Holmes and D.L. Holmes (1978) have tried to explain and illustrate the concept of isostasy through a diagram (fig. 6.9) which ‘shows characteristic exam­ples of crustal columns, each of which has the same area and extends downward to the same depth below sea level, the same depth at which the weight of each column exerts approximately the same pressure on the underlying material, irrespective of its surface eleva­tion’.

They have taken the depth of 50 km for isostatic compensa­tion in those areas which have not been disturbed by geological events for fairly longer duration. A. Holmes and D.L. Holmes have attempted to explain and illus­trate the concept of equal weight along the ‘level of equal pressure’ through the examples of 4 columns of equal cross-section through characteristic parts of the continents and ocean floor (fig. 6.9).

These four col­umns are:

(i) Plateau, 4 km high;

(ii) Plateau, 1 km high;

(iii) Plain at sea level and

(iv) Ocean, 5 km deep.

Each column has a thickness of 50km. The figures to the right of each column denote density (average). M indicates Mohorovicic Discontinuity. The weight of each column along the level of equal pressure is almost the same, ranging between 150.0 to 151.2.

According to Holmes and Holmes the total weight of each column along the level of equal pressure can be obtained by summing up the product of the density and corresponding thickness down to the depth of 50 km as given below:

(i) For the plateau (4 km high from sea level (fig. 6.9 A) – 54 x 2.8 (average density) = 151.2

(the whole section is continental crust)

(ii) For the plateau (1 km high) (fig. 6.9 B) – 36 x 2.8 (continental crust) + 15.33 (mantle sima, prob­ably basaltic rock) = 150.3

(iii) For the plain near the sea level (fig. 6.9 C).

30 x 2.8 (continental crust) x 20 x 3.3 (mantle sima) = 150.0

(iv) For the ocean (5 km deep, fig. 6.9 D)-

5 x 1.03 (sea water) + 1 x 2.4 (sediments) + 5 x 2.9 (crustal sima, probably basaltic rock) + 39 x 3.3 (mantle sima) = 150.75

Global Isostatic Adjustment:

It may be pointed out that there is no complete isostatic adjustment over the globe because the earth is so unresting and thus geological forces (endogenetic forces) coming from within the earth very often disturb such isostatic adjustment. Moreover, recently a few scientists have even questioned the concept of isostasy.

Even there is disagreement among the scientists about local or regional nature of isostasy. It appears from the result of various expeditions, experiments and obser­vations that if the isostatic adjustment does not occur at local level, it does exist at extensive regional level. It is necessary that there must be balance at local level, it may be and it may not be.

The endogenetic forces and resultant tectonic events cause disturbances in the ideal condition of isostasy but nature always tends towards the isostatic adjustment.

For example, a newly formed mountain due to tectonic activities is subjected to severe denudation. Consequently, there is continuous lowering of the height of the mountain. On the other hand, eroded sediments are deposited in the oceanic areas, with the result there is continuous increase of weight of sediments on the sea floor.

Due to this mechanism the mountainous area gradually becomes lighter and the oceanic floor be­comes heavier, and thus the state of balance or isostasy between these two areas gets disturbed but the balance has to be maintained. It may be stated that the superincumbent pressure and weight over the mountain decreases because of continuous removal of material through denudational processes.

This mechanism leads to gradual rise in the mountain. On the other hand, continuous sedimentation on the sea floor causes gradual subsidence of the sea floor. Thus, in order to maintain isostatic balance between these two features there must be slow flowage of relatively heavier materials of sub­stratum (from beneath the sea floor) towards the lighter materials of the rising column of the mountain at or below the level of compensation (fig. 6.10).

Thus, the process of redistribution of materials ultimately restores the disturbed isostatic condition to complete isostatic balance. Commenting on the validity of the above mechanism of the isostatic adjustment, Wooldridge and Morgan (1959) have remarked, ‘That some such mechanism operates is indeed very likely; geologists have irrefutable evidence that sediments can depress the floor of a loaded sea to a limited extent, and some species of sub-crustal flow has been invoked on many other grounds.

But clearly we are not justified in regarding the crust as composed of columns, moving up and down independently; such a conception flouts the facts of observation, and even it did not, it would on the geological side, create many more problems than it solved.’

Sometimes the endogenetic forces act so sud­denly and violently that the state of isostatic balance is thrown out of gear all of sudden and hence the isostatic adjustment through the process of flowage of materials from the substratum is not maintained. Similarly, sometimes climatic changes occur at such an extensive global scale that there is accumulation of thick ice sheets on the land surface and thus increased burden causes isostatic disturbance.

For example, extensive parts of North America and Eurasia were subsided under the enormous weight of accumulation of thick ice sheets during Pleistoncene glaciation but the land- masses began to rise suddenly because of release of pressure of superincumbent thick load of ice sheets due to deglaciation and consequent melting of ice sheets about 25,000 years ago and thus the isostatic balance was disturbed.

According to an estimate major parts of Scandinavia and Finland have risen by 900 feet. The land masses are still rising at the rate of one foot per 28 years under the process of isostatic recovery. The isostatic adjustment in these areas could not be achieved till now.