Different relief features of varying magnitudes e.g. mountains, plateaus plains, lakes, seas, and oceans, faults, and rift valleys, etc. standing on the earth’s surface are probably balanced by certain definite principle, otherwise, this would have not been maintained in their present form. Whenever this balance is disturbed, there start violent earth movement and tectonic events. Thus, isostasy simply means “mechanical stability between the upstanding parts and low-lying basins on a rotating Earth.”
The word isostasy derived from the German word ‘isostasios’ (meaning thereby ‘in equipoise’), was first proposed by American geologist Clarence Edward Dutton.
According to Dutton, the upstanding parts of the earth must be compensated by lighter rock material from beneath so that the crustal reliefs should remain in mechanical stability.
Discovery of the Concept:
The concept of isostasy 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 Chimborazo was not attacking the plumb line as it should have done. He does maintain 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 latitude under the supervision of Sir George Everest, the then Surveyor General of India in 1859.
This discrepancy was attributed to the attraction of the Himalayas due to which the plumb-bob used in the determination of latitude was deflected.
Later on, the matter was referred to Archdeacon Pratt for further investigation and clarification.
Pratt’s calculation brought another fact before the scientist that the Himalaya was not exerting the attraction according to its enormous mass.
This interpretation gives birth to another problem- What reason is behind the low attractional force of the Himalayas? The following explanations were offered for this question.
- The Himalayas are hollow and are composed of bubbles and not the rocks. Due to this fact, the weight and density of the Himalayas would be low and their gravitational force would also below.
- If the mountains are not hollow, the density of the rock of the mountains must be relatively low down to considerable depth. Thus the total weight would be low and consequently, the attraction force would also be low.
- The Rocks of the Himalayas are of low density themselves and does their attraction is also low.
Thus, the debate on the discrepancies of the gravitational deflection of the plumb line and numerous explanations for these discrepancies resulted in the postulation of the concept of isostasy by different scientists.
Theory of Isostasy by Sir George Airy
According to Sir George Airy, the inner part of the mountains cannot be hallow rather the excess weight of the mountains is compensated 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 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 involves the principle of floatation.
Thus, according to Airy, the Himalayas were exerting there real attractional force because there existed a long route of lighter material in the substratum which compensated the material above.
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, it smaller part would be submerged in the substratum.’
According to Airy the density of different columns of the land (e.g. mountains, plateaus, plains, etc.) remains the same. In other words, density does not change with the 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 vary from place to place. To prove this concept array to several pieces of iron of varying lengths and put them in a basin full of Mercury. The species of and sunk up to varying depths depending on their lengths.
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 79,632 m (if we accept the principle of floatation). 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 that of 79,623 m 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.
Theory of Isostasy by Archdeacon Pratt
While studying the gravitational deflections Pratt’s calculation brought another fact before the scientist that the Himalaya was not exerting the attraction according to its enormous mass.
He then studied the rocks (and their densities) of the Himalaya and neighboring 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 Plateaus, that of Plateau is less than the density of plain and the density of plain is less than the density of ocean floor and so on.
This means that there is an 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 the 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 line 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.
For example, 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 a difference in their height. Both the columns must have equal mass along the line of compensation, so the density of column B should be more than the density of column A so that the weight of both the columns become equal along the line of compensation.
Thus, the Pratt’s concept of the 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.’
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 their respective mass is equal along the line of compensation because of their varying densities.
Bowie has opined that through Pratt does not believe in the law of floatation, as stated by Sir George airy but if you look minutely into the concept of Pratt we certainly find the glimpse of the law of floatation indirectly. Similarly, Pratt does not believe directly in the concept of ‘root formation’ but a very close perusal of his concept on isostasy does indicate the glimpse of such an 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 pustulated a uniform density with varying thickness, and the latter a uniform depth with varying density.’
Global Isostatic Adjustment
It may be pointed out that there is no complete isostatic adjustment over the globe because the earth.
The endogenetic forces and resultant tectonic events cause a disturbance in the ideal condition of isostasy but nature always turns towards the isostatic adjustment.
For example, a newly formed mountain due to tectonic activities is subjected to severe denudation.
Consequently, there is a 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 a continuous increase of weight of sediment 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 of 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 decrease because of continuous removal of material through denudational processes. This mechanism leads to a gradual rise in the mountain.
On the other hand, continuous sedimentation on the seafloor causes gradual subsidence of the sea-floor. Thus, to maintain the isostatic balance between these two features there must be slow flowage of relatively heavier materials of substratum (from Beneath The Seafloor) towards the lighter materials of the rising column of the mountain at or below the level of compensation.
Thus, the process of redistribution of materials ultimately restores the disturbed isostatic condition to complete isostatic balance.
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.
Similarity, sometimes climatic changes occur at such an extensive global scale that there is an accumulation of thick ice sheets on the land surface and does increase burden cause isostatic disturbance.
For example, extensive parts of North America and Eurasia were subsided under the enormous weight of accumulation of thick ice sheets during Pleistocene glaciation but the landmasses began to rise suddenly because of the release of pressure of superincumbent load of ice sheets about 25000 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 landmasses 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.