Isostasy & 6 Theories

The literal meaning of the Isostasy is the “stage of balance“. It is the mechanical stability between the earth’s crust( upstanding parts such as plains, plateaus, mountains, etc, and low-lying parts such as the ocean floor ) and mantle on the rotating earth. It maintains an equilibrium state between buoyancy force and gravitational force. Buoyancy force pulls the crust up Ward and gravitational force pushes the crust downward.

Theory of Airy

• Continents are made of lighter material Sial and there are floating on denser material made of Sima.
• Continents have the same density hence to keep balance, the height of mountains is a proposition to depth root underneath.
• He used the boat and iceberg concept of floating, a large part of the boat and iceberg remains in the water to keep its balance.
• In the same concept of boats, the Himalayan is floating on Sima and a large part of Himalayan roots deep down in Sima (denser rock) to keep it afloat.
• The individual peak & valleys of mountains need not be separately balanced, as the crustal rocks have enough strength to maintain minor relief.

Criticism

• The density of mountains, plains, and plateaus need not be the same.
• The very long root of Himalayan is not possible due to intense heat & pressure in Sima.

Theory of Pratt

• The density of mountains, plains, and plateaus is different.
• The density of mountains should have lower than the density of plateaus
• There is an inverse relationship between height and density.
• His idea is the uniform depth with varying thickness

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 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.

Theory of Jolly

• Joly presented his views on isostasy in the year 1925. He disapproved the view of Hayford and Bowie about the existence of a 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, 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 noted that Joly did not mention this, we only infer the idea of floatation from Joly’s concept) and his concept is closer to 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’.

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.

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).