Question.
Explain the concept of isostasy with an example. (15 marks) (UPPSC 2021)
What is Isostasy? Describe the views of Airy and Pratt in this context. (UPPSC 2016)
What is Isostasy? Discuss the views of Airy and Pratt in this context. ( UPPSC 2012)
Explain the concept of isostasy. ( UPPSC 2007)
What is the meaning of 'Isostatic state' on the surface of the earth? Explain the views of Airy and Pratt in this context. ( UPPSC 2005)
What is isostasy? Discuss the views of Pratt and Airy. ( UPPSC 2003)
Answer.
As we know that the Earth has four major layers – the crust, upper mantle or asthenosphere, lower mantle, and core. We also know that apart from the crust, the thickness of the other layer is almost the same everywhere. The thickness of the crust is greater in the mountainous region( around 100 km) and is less in the oceanic crust ( around 5 km). We also know that the earth's crust (sial) is floating on the asthenosphere which is in semi-liquid form and made up of denser material than the crust. There are two forces at work that keep the crust floating on the semi-liquid asthenosphere – the buoyant force and the gravitational force.
- The buoyant force is the upward force that keeps objects floating on liquids.
- Gravitational force is the downward force that causes things to sink on liquids.
The big question is how the crust which is very high somewhere (in the Himalayan region) and somewhere very deep (the Mariana Trench) balances itself. To explain this the concept of Isostasy emerged.
The Isostasy term was first used by the American geologist Dutton(1889). The literal meaning of Isostasy is "balance of land". It explains the concept of mechanical stability between the Earth's crust (upper parts like plains, plateaus, mountains, etc., and lower ones like sea level) and the mantle on the rotating Earth.
The concept of isostasy explains the tendency of the Earth's crust to achieve equilibrium which creates a balance between buoyancy and gravitational forces.
There are two major theories that explain the concept of Isostasy.
- Airy's views on Isostasy.
- Pratt's views on Isostasy.
Airy's thoughts on Isostasy
Airy used icebergs and pieces of wood to explain the balance of the crust on the mantle. As large-sized wood is put in water, it sinks more part in water than small-sized wood. Similarly, if that part of the continental crust is high (like the Himalayas) its roots are also deep, to make equilibrium.
The following image explains the concept of isostasy as propounded by Airy -
Pratt's views on Isostasy:
A major drawback of Airy's hypothesis was that the root of the Himalayas would be too long( around 80 to 100 km) for land balance, which is not possible, as the material would become semi-liquid below such depths. To overcome such shortcomings, Pratts gave his views on land balance.
According to Pratt, the density of mountains, plateaus, plains, and oceanic crust is not uniform. Mountains are made up of lighter materials than plateaus, plains, and oceanic crust. Therefore, the roots (line of compensation) of the crust are the same everywhere, but the density of the crust is not the same everywhere.
The following diagram explains the concept of Isostasy by Pratt.
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