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Isostasy


Q. Explain the concept of Isostasy as postulated by Airy and Pratt

Isostasy refers to mechanical stability that exist between all the high lying features like mountains and low lying features like ocean basin on fast rotating earth.

Sir George Airy’s theory
Airy adopted Edward Suess Scheme of earth’s interior structure and assumed that it is made up of three layered structure- Sial,Sima and Nife. Sial is uppermost layer containing crust with density of 2.67 and Sima with density of 3.0.Since, Sima is denser, semi molten and just below sial, sial is floating on sima.
REASON FOR ISOSTASY
  • Sial can be imagined to be divided into vertical sections containing mountains,plateaus,plain,ocean basin,etc separately..
  • Section with higher feature like mountain has deeper root while section with lower feature has shallow root.Though,same density of all the sections .
  • This arrangement is mechanically stable due to principle of flotation.
  • Airy demonstrated this through an experiment
CRITICISM
  • Iceberg Example: Principle of flotation results in 1:9 ratio of fraction above surface to submerged.In this manner, himalayas would have 81 Km of root.This can’t be the case as at such depth sialic material will melt.
  • Density: Different crustal features like mountains, plateau, plains can’t have similar density.Pratt after survey pointed out that mountains have lesser density than plains.

Archdeacon Pratt Theory
He also used Edward Suess scheme of earth’s interior.His theory differ from Airy as he suggested various crustal features have different densities. This was based on his survey of geological structures of mountains and plains.
According to him density is inversely related to the height of the relief feature. Thus,  mountains would be less denser than plateaus, and plateaus would be less denser than plains.
According to him, no flotation occurs but sima layer exert molecular resistance to sialic layer.There is a line of compensation, where all the crustal features exert same weight. This he demonstrated through an experiment where he used mercury in container and placed in it various metals(different density) with same weight.

CRITICISM
Bowie suggested Pratt’s theory also follow principle of flotation like airy.Also, there is phenomenon of root formation in Pratt’s theory like Airy’s.

Heiskanen Theory
His theory has some element of both Airy’s and Pratt’s theory as well as some of his contribution in such manner that it eliminates most of the limitations of previous theories.
According to him:

  1. Larger the sialic section, deeper is its root.
  2. Larger the section, lesser is the density.
  3. Density increases with depth.
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