THEORY OF ISOSTASY BY ARCHDEACON PRATT (1859)
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 explained with an example fig-1
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.
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 ).
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).
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