Question.
Compare the concepts of Sir George Airy and A. Pratt regarding Isostasy. (UPPSC 2019)
Answer.
The concept of Isostasy tells us how the major landforms on our earth's crust such as mountains, plateaus, hills, etc. are stable in the upper mantle (asthenosphere) on the rotating earth.
- The stability of the crust on the asthenosphere is determined by the difference between the buoyant force and the gravitational force. The buoyant force is the upward force acting from the asthenosphere to the crust which makes the crust float.
- The force of gravity is applied from the crust to the asthenosphere, which tries to sink.
There are two major views on Isostasy - Sir George Airy and Pratt The following is a comparison of the concepts of Isostasy by Sir George Airy and A Pratt:
- Airy believed that the density of the crust is the same everywhere - plain, mountain, and oceanic crust; While Pratt believed that the density of the crust is not uniform, the highest density is found in oceanic crust and the lowest in mountainous regions.
- According to Airy, where there are high landforms such as mountains or plateaus, the depth of the lithosphere will also be greater and the lower part of the mountains (like the root) penetrates deeper below the asthenosphere for establishing the equilibrium. Whereas Pratt believed that mountains, plateaus, plains, etc. have the same root ( means portion inside the asthenosphere) and Pratt believes that mountains are made of low-density material, therefore not much depth is required for balance.
- Airy's concept of Isostasy works on the basis of the law of floatation, whereas Pratt's concept of Isostasy is based on the law of compensation.
- Airy's concept does not seem practical in the context of the balance of the Himalayan region, because according to Airy its depth in the asthenosphere should be approximately 80 to 100 km below for the equilibrium of the Himalayas, which is not possible because at such depth the material should be in liquid forms; Pratt's concept of land balance seems to be more practical because according to them the root of the crust is the same in all places.
The image below also illustrates the difference between Airy and Pratt's concept of Isostasy.
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