· The term “Isostasy” is derived from “Isostasios”, a word of Greek language meaning the state of being in balance.
· You already know and must have seen that the mountain have many peaks and relatively great heights.
· Similarly plateau and plain have flat surfaces. They have moderate and lower height, respectively.
· On the contrary oceanic beds and trenches have greater depths. There is a great difference in height among these features.
· You also know that the earth is rotating while keeping perfect balance among its various features.
· Thus, our earth is considered to be in isostatic equilibrium.
· Example:- Suppose you are holding one stick each in your both hands vertically with varying heights, say 5’ and 15’ and you are moving in a particular direction.
· Do you have any difficulty in maintaining a balance in congruence with your body as well as two sticks together? Definitely, smaller stick will be easy to make a balance than the longer one. It is just because of the centre of gravity.
· The centre of gravity with smaller stick will be nearer to your holding hand in comparasion to the longer stick. In the same way smaller surface features like plains are more stable than the tall mountains.
· Airy, a geologist, considered the density of different columns (plains, plateaus, mountains, etc.) to be the same. Hence, he proposed the idea of ‘uniform density with varying thickness’.
· We know that the upper crust of the earth is made up of lighter material. In this layer, silica and aluminium are found in abundance, hence it is known as ‘Sial’.
· It is less denser than the lower one. Airy assumed that the Sialic crust is floating over the Sima (silica and magnesium, lower denser layer).
· Crustal layer is uniform in terms of density with varying length of columns. Therefore, those columns are projecting down into the asthenosphere depending upon the proportions of the column.
· It is due to this reason that the root has developed or the sima has been displaced from below.
· To prove this concept, Airy took an example of wooden blocks of various sizes and immersed them into water .
· All blocks are of same density. They get immersed differently in proportion to their sizes.
· In the same way higher features with great height seen on the surface of the earth have deeper roots whereas short in length has shorter roots beneath.
· It is the concept of root which is sustaining the higher elevation. He is of the openion that the landmasses are floating like a boat in the substratum (magmatic asthenosphere).
· According to this concept, the root beneath the Mt. Everest would be 8848X8 = 70784 metre below the sea level.
· On this bases Airy has been criticized that the root is not possible to be at such a great depth. Because the root material will melt due to higher temperature found at that depth.
· Pratt considered land blocks of various heights to be different in terms of their density.
· The taller landmass has lesser density and smaller height features to be denser. In other words, there is an inverse relationship between height and density.
· If there is a higher column, density will be lesser and if there is a shorter column, density will be higher.
· Assuming this to be true, he accepted that all blocks of different height get compensated at a certain depth into the substratum.
· In this way a line is being demarcated above which there is equal pressure with varying heights.
· Thus, he denounced the root concept of Airy and accepted the ‘concept of a level of compensation’.
· For proving his concept he took a number of metal bars of varying density with same weight and put them into mercury .
· In this way they form a line by all those bars, which he regarded to be the level of compensation.
Differences between the views of Airy and Pratt :
Views of Airy
Views of Pratt
Uniform density of crustal material.
Varying density of crustal material.
Varying depth upto which root penetrates. crustal material reaches.
Uniform depth upto which crustal material reaches.
Deeper root below the mountain and smaller beneath plain.
No root formation, but a level of Compensation.
C. Global Isostatic Adjustment :
· It is quite apparent that there is no complete isostatic balance over the globe. The earth is unstable.
· Endogenetic forces often disturb the crustal balance.
· The regular earthquakes and volcanic eruptions along a particular belt do not signify any balance but a sort of adjustment is needed continuously.
· Endogenetic forces and their tectonic effects are the causes of imbalance on the surface but nature always tries to make an isostatic adjustment with itself.
· Exogenetic forces are trying to eliminate the differences on the surface of the earth and in this process they are peeling off, transporting down to far flung places, and depositing them.
· In this process, isostatic balance is maintained by the underneath flowage of material by subsidence at the place of deposition and upliftment at the peeling of place in their proportion to the denudation ..