Skip to main content

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.

Comments

Popular posts from this blog

DAVIES,PENCK & KING

Geographical Cycle of Davies William Morris Davies was an american geographer who gave first general theory on landform development.. Davis' most influential scientific contribution was the "geographical cycle",  which he defined in an article,’ The Rivers and Valleys of Pennsylvania ,’ published at the end of 19th century. According to him, uplifted landmass undergoes sequential development or erosion till base level in various stages.This sequential development referred as cycle of erosion. FUNDAMENTAL PRINCIPLES 1.Cyclic nature of landform evolution. 2 Uniformitarianism:The same physical processes and laws that operate today, operated throughout geologic time, although not necessarily always with the same intensity as now BASIC POSTULATES Upliftment takes place on featureless plain which he modified 10 yrs later to accept it can occur from geosyncline. Upliftment on geological timescale is sudden.In later works, he accepted it to be episodic. ...

Rimland Theory

It was given by John Spykman which was published posthumously. It was a reaction to Mackinder’s Heartland Theory.He also believed in historical struggle between land and sea powers.But, his view were similar to Alfred Mahan’s idea of supremacy of sea power. CRITICISM OF HEARTLAND 1.Climatic hazards and physiographic difficulties 2.Non-ecumen region devoid of most important human resource.Thus,Resources remain unutilized. 4.Accessible from west and south West and merely few hours distance from N America RIMLAND It was similar to Mackinder’s inner crescent which comprised 3 sections 1.European Coast 2.Arabian middle east desert land 3.Asiatic monsoon land ADVANTAGES OF RIMLAND 1.¾ th of population and most of world resources like coal,petroleum,iron ore,etc. 2.Largest agricultural tract. 3.Suitable climate. 4.Variety of human race. According to him,those who control rimland,rules eurasia.And who rules eurasia controls destinies of world. APP...

Heartland Theory

Concept was given by Mackinder in 1919 in his book ,”Democratic Ideals and reality”.It was modification of Pivot concept given by him in 1909 in his book,”Geographical pivot of history”.Concept of heartland was influenced by Bolshevik revolution and emergence of USSR as world power.                        According to him, geography is history and geography is cause.He tried to  explain how geographical advantages results in geopolitical domination.His theory successfully explains emergence of USSR as great power which he thinks is result of geographical advantages. 3 TIER DIVISION OF WORLD 1. Pivot Area (Ural-arctic-siberian highland-central asian mountains; Rich natural resources;natural fortress;Symbolizes Land power) 2. Inner crescent (Europe except UK,West asia,South asia,East and Far east asia;Sea power;accessible and prone to invasion) 3. O...