The theory of Isostasy explains the gravitational equilibrium between different relief features on Earth's surface. This fundamental concept helps us understand why mountains, plateaus, and ocean basins maintain a state of balance despite their varying heights and masses. The study of isostasy is crucial for understanding Earth's crustal structure and dynamics.
THEORY OF ISOSTASY
Introduction
There are different physical features such as mountains, plateaus, and plains on the surface of the Earth which vary greatly in size. Despite this variation, these features are balanced or are in equilibrium. The study of equilibrium or the state of balance between the upstanding part and the low-lying basin is known as the state of Isostasy.
The term was first given by "Dutton" to determine the state of gravitational equilibrium between the different relief features on the rotating Earth.
According to Dutton, the higher features such as mountains are made up of rocks of lower density, while the lower features such as ocean basins are made up of rocks of greater density.
Below a certain depth, the pressure along the horizontal plane line is uniform everywhere, and this line is known as the level of uniform pressure or better known as level of compensation.
All the landmass have mass and exert the gravitational force, and because of the gravitational force they influence the Plumb bob that measures the centre of gravity and is used in levelling and surveying of land.
Pierre Bouguer found an anomaly or discrepancy in the deflection of Plumb Bob between the calculated value and actual observation in Andes.
Similar observation was seen in India when the survey was done under George Everest, the gravitational pull of the Himalayas between Kalyana and Kalianpur.
Sir Everest concluded along with Sir Pratt that Himalayas were made up of much lighter Sialic material, and hence there is discrepancy in the gravitational attraction of Himalayas.
The reasons given were:
1. Himalayas are hollow mountains & have bubbles inside, and therefore they exert much less gravitational force. But the majority of scientists rejected this view.
2. Himalayas are made up of rocks which have low density, and therefore their gravitational pull is also low.
AIRY'S CONCEPT OF ISOSTASY
Airy opposed or Airy gave his concept of isostasy in which the stress was on the law of floatation. He suggested that continents are made up of lighter SIAL which is floating on denser SIMA.
According to him, the Himalayas were not just surface features, but the lighter rocks of the mountain extend up to considerable depth into the SIMA. He used Archimedes principle, taking an example of an iceberg in which he pointed that only one part of iceberg floats on the surface but nine parts are below the surface.
→ To show this, he took wooden blocks of different heights and suspended them in a block of water.
→ By showing this, he said that all the blocks are made up of wood and have the same density, but their heights differed. The longest block emerged to the greatest height.
The same experiment was done with the copper blocks, and Airy came to the conclusion that continents are resting or floating on denser SIMA, and the highest mountains have the deepest roots.
Criticism of Airy
1. His concept that the tallest mountains have the deepest roots is controversial because, if we take the case of Mt. Everest which is 8885 meters tall, then its roots will be around 71 km deep. At such depth, the temperature will be so high that all the roots will melt.
2. Rocks cannot have the same density.
THEORY OF PRATT
Pratt presented his concept in 1855 based on the studies in the Himalayan region. According to Pratt, there is a variation in the densities of the Earth's crust. This variation in the density occurs only up to a certain depth, below which the density is uniform. This depth was called by Pratt as the level of compensation.
→ Above the level of compensation, the density changes from one column to another, and thus Pratt gave the concept of uniform depth with varying density.
→ To show this, he took the blocks of lead, iron, zinc, and antimony and suspended them in the vessel containing mercury.
→ Pratt is of the view that different relief features are standing on the Earth's surface because of the different density.
The highest relief features of the mountains have the lowest density, while the lower relief features such as ocean basins have the highest density.
→ According to Pratt: Lighter the rocks, greater is the height; denser the rocks, lower is the height.
Question:-
Explain the concept of Isostasy and bring out the differences between Airy and Pratt.
COMPARISON BETWEEN AIRY & PRATT
1. Though both the theories look different, they are basically based on the concept of isostatic adjustment. Airy believes in the law of floatation, while Pratt believes in the law of compensation.
2. Airy argued that densities of the rocks are similar, but their height varies because higher relief features have the deepest roots. Thus, his theory is based on root formation.
On the other hand, Pratt does not believe in the law of floatation and crustal similarities in the density. He believes that different relief features have different densities. Higher the rocks, lesser the density, and vice versa. But this variation in the densities is only up to a certain depth known as the level of compensation. But on close scrutiny, even the Pratt theory has a slight hint of root formation.
CONCLUSION
Thus, the fundamental difference between Airy and Pratt, according to "Bowie", is: Airy talks about uniform density with varying depth, while Pratt talks about uniform depth with varying density.
HAYFORD & BOWIE
Hayford & Bowie presented the view similar to Pratt, i.e., bigger the column, lesser is the density, but concluded that the depth of the compensation level is 100 km.
Joly rejected the view of Hayford & Bowie. He supported Airy's concept of floatation but said that the roots of the highest mountains will be up to depth which varies, below which the density is uniform. This region of uniform density was called the zone of compensation, which he said is around 10 miles thick.
Arthur Holmes agrees with the view of Airy because the study of earthquakes shows that most of the mountains are made up of lighter sialic rocks.
These mountains have deep roots up to 60 km, but the plains have roots up to 30 km.
Holmes also studied that below the ocean, SIAL is absent or very thin. Thus, from the study of Holmes, we can conclude that higher relief features on the SIAL have roots which are around 50-60 km deep, while lower relief features such as ocean basins or ocean floors will have no or very thin roots. And SIAL is separated from SIMA by Moho discontinuity.
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