Isostasy | Geomorphology | Principle Of Geography

 

Geomorphology

Principle Of Geography

Geography Complete Study Material

(Paper - I)

Isostasy

                Different relief features of varying magnitudes e.g. m ountains, plateaus, plains, lakes, seas and oceans, faults and rift valleys etc. standing on the earth’s surface are probably balanced by certain difinite principle, otherwise these would have not been m aintained in their present form. W henever this balance is disturbed, there start violent earth m ovements and tectonic events. Thus, ‘isostasy sim ­ ply means a m achanical stability between the upstanding parts and lowlying basins on a rotating earth’.

                The word isostasy, derived from a German word ‘isostasios’ (meaning thereby ‘in equipoise’), was first proposed by American geologist Dutton in 1859 to express his view to indicate ‘the state of balance which he thought must exist between large upstanding areas of the earth's surface, mountain ranges and plateaus, and contiguous lowlands, etc.’ (S.W . W ooldridge and R.S. Morgan, 1959). A ccording to Dutton the upstanding parts of the earth (m ountains, plateaus, plains and ocean basins) must be com pensated by lighter rock material from beneath so that the crustal reliefs should remain in m echanical stability. According J.A. Steers (1961), ‘this doctrine states that wherever equilibrium exists on the earth's surface, equal mass must underlie equal surface areas.’ 

DISCOVERY OF THE CONCEPT 

              Though the concept o f isostasy came in the mind o f geologists all of sudden but its concept grew out of gradual thinking in term s of gravitational attraction of giant m ountainous m asses. Pierre Bouguer during his expedition of the Andes in 1735 found that the towering volcanic peak of Chimborazo was not attracting the plum b line as it should have done. He thus m aintained that the gravitational attraction of the Andes ‘is much sm aller than that to be expected from the mass represented by these mountains’. Sim ilar discrepencies were noted during the geodetic survey of the Indo-G angetic plain for the determination of latitudes under the supervision of Sir George Everest, the then Surveyor General of India, in 1859. The difference oflatitude of Kalianpur and Kaliana (603 km due northw ard) was determined by both direct triangulation method and astronomical method. Kaliana was only 96 km away from the Himalayas. The difference between two results amounted to 5.23 seconds as given below— 

• 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 attraction of the Himalayas due to which the plum b-bob used in the astronom ical determination of latitude was deflected. This interpretation, thus, brought the fact before the scientists that the enormous mass of the Himalaya was responsible, through its attractional force, for the difference in the results of two methods. Later on the m atter was referred to Archdeacon Pratt for further investigation and clarification. He attempted to estim ate the am ount of attraction of the Himalayas on the basic assumption that all the mountains had the average density of 2.75. Thus, Pratt based on m inim um estim ate of the mass of the Himalayas calculated the gravitational effects on the plumbob at two places (Kaliana and Kalianpur) and to his dismay he discovered that the difference was surprisingly more than actually worked out during the survey.

• Gravitational deflection at Kaliana = 27.853" 
• Gravitational deflection at Kalianpur = 11.968” 
• difference = 15.885"

                Thus, the difference of 15.885" was in fact more than 3 times the observed deflection of 5.236" during the survey. Pratt's calculation of the difference of the gravitational deflections brought another fact before the scientists that the Himalaya was not exerting the attraction according to its enormous mass. This interpretation gave birth to another problem-What reason is behind low attractional force of the Himalayas ? The following explanations were offered for this question. 

(1) 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 thus their gravitational force would also be low. This was the reason for the difference in the results o f two locations as referred to above. This explanation cannot be accepted because such a high mountain, if com posed of bubbles, cannot stand on the earth's surface. 

(2) If the mountains are not hollow, the visible mountain mass must be compensated by defficiency of mass from below. In other words, the density of the rocks o f the mountains ‘must be relatively low down to considerable depth.’ Thus, the total weight would be low and consequently the attractional force would also be low. 

(3) The rocks of the Himalayas are of low density in themselves and thus their attraction is also low.

(4) It was suggested ‘that there is such a level below the surface of the earth below which there is no change in the density of the rocks’, density varies only above this level. Thus, all columns have equal mass along this level. It was therefore suggested on this basis that ‘bigger the colum n, lesser the density, and smaller the column, greater the density.

              Thus, the debate on the discrepancies o f the gravitational deflections o f the plumb-line and numerous explanations for these discrepancies resulted into the postulation of the concept of isostasy by different scientists, the views of a few of them are presented below.

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 th at must be balance at local level, it maybe 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 exam ple, a newly form ed 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 becom es 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 superincum bent 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 o f relatively heavier materials of substratum (from beneath the seafloor) towards the lighter materials o f the rising column of the mountain at or below the level of compensation (fig. 7.10). Thus, the process o f 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, W ooldridge 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’ (S.W. W ooldridge and R.S. Morgan, 1959, p. 26).

             Some times 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, some times 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 Pleistocene glaciation but the landmasses began to rise suddenly because of release of pressure of superincumbent thick load of ice sheets due to deglciation and consequent melting of ice sheets about 25,000 years ago and thus the isostatic balance was disturbed. A ccording 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.