THEORY OF ISOSTASY BY 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.
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.
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.
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