Isostasy: Definition, Theories, Key Examples, Differences & UPSC Notes

Isostasy is the equilibrium between the Earth's crust and the dense mantle below it. The crust just sits "on top" of the softer mantle, rising or falling according to its weight, like icebergs floating on the sea's surface. This balance keeps our planet's surface level.

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Isostasy is not just an abstract concept for geologists. It holds significant relevance for those preparing for the UPSC in India. Understanding Isostasy is crucial for the Geography section of the UPSC syllabus, which demands a comprehensive understanding of the Earth's structure and processes.

Isostasy is a critical topic for UPSC IAS exams. It covers a significant portion of geography in the General Studies Prelims syllabus and the General Studies Paper 1 Mains syllabus.

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What Is Isostasy?

Isostasy in geography is the equilibrium in the Earth's crust maintained by the gravitational force and buoyancy. But like any geologic phenomenon, the concept of Isostasy is intricate and intertwined with numerous aspects of the Earth's structure and function. 

• Isostasy is derived from the Greek word 'iso-stasios', which means 'equal standing' (in equipoise). The term isostasy was first proposed by an American geologist, Clarence Dutton, in 1889 to indicate the state of balance between large upstanding areas of the Earth's surface, mountain ranges and plateaus.
• The theory says that the less dense materials of the Earth's surface (sial) must float over the denser magma (sima) of the Earth's interior. Similarly, we see several concentric layers as we go deep into the Earth's interior.
• The densest material forms the core, whereas the Earth's surface is composed of the lightest material. Each layer and the Earth's surface features rest on top of another with an isostatic adjustment. For example, the average density of the Core is 13.5 gm/cm3; the density of the Mantle ranges from 3.3 to 5.7 gm/cm³; the density of the Continental crust is around 2.7 g/cm³.
• Isostasy is extremely useful in explaining the glacial adjustment in Scandinavian countries after the Pleistocene great ice age. The raised beaches of Finland exhibit an uplift of about 250 meters, which has occurred during the last 8000 years due to isostatic adjustment.

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Development of the Isostasy Concept

When you hear 'Isostasy', what springs to your mind? For many, it might be the image of massive tectonic plates balancing on the semi-fluid asthenosphere below. This is the crux of understanding Isostasy. Our Earth is more than just a static sphere floating in space; it's a dynamic, living entity, constantly evolving and reshaping. 

Isostasy may be compared with the floating blocks of wood on water. The wood, the lower it sinks, but never floats. Similarly, continental and oceanic crusts of the Earth also float on semi-fluid asthenosphere. This equilibrium or balance that the earth's lithosphere sits upon is called Isostasy.

• The concept of isostasy came to the minds of geologists, but the idea grew out of the attraction of giant mountainous masses.
• During his expedition of the Andes in 1735-45, Pierre Bouguer found that the towering volcanic peak of Chimborazo was not attracting the plumb line, as it should have done. He thus maintained that the gravitational attraction of the Andes is much smaller than that expected from the mass represented by these mountains.
• Similar discrepancies were noted during the geodetic survey of the Indo-Gangetic plain for determining latitudes under the supervision of Sir George Everest, the then Surveyor General of India, in 1859. The difference in latitude of Kalianpur and Kaliana (370 miles apart) was determined by both the direct triangulation method and the astronomical method.
• Kaliana was only 96 km away from the Himalayas. The difference between the two results was 5.23 seconds, as given below.
• Results obtained through triangulation 5° 23' 42.294".
• Results obtained through the astronomical method = 5° 23'37.058".
• Difference= 5.236"
• This discrepancy between the two methods was attributed to the less attractive of the two.
• Himalayas, due to which the plumb bob used in the astronomical determination of latitude was deflected. Many theories explain the gravitational attraction, deflection, and isostatic balance among the various landforms.

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Key Examples of Isostasy

To learn more about what Isostasy entails, we shall look at a couple of examples in the real world:

• Mountains and Isostasy: Ever wondered why mountains don't sink into the Earth under their weight? Isostasy answers this question. For instance, the Himalayas are massive and heavy, but they don't sink because of the balance maintained by a deep 'root' into the asthenosphere, much like an iceberg floating in water.
• Glaciers and Isostasy: Glaciers also provide compelling examples of Isostasy in action. During the last ice age, the massive weight of the ice pushed down the Earth's crust. When the ice melted, the crust began to rebound slowly - a process that continues even today in some parts of the world.
• Erosion: Erosion, the wearing away of the Earth's surface, also triggers isostatic adjustment. The crust underneath rises to maintain equilibrium as material is eroded from higher elevations.

Theories of Isostasy

Two major theories explaining the mechanism of Isostasy are Airy's Isostasy and Pratt's Isostasy. Theory of Isostasy includes the following:

Isostasy: Concept of Sir George Airy

The early model of isostasy was by George Biddell Airy, a 19th-century British Astronomer; we commonly refer to the Airy model. The model presumes that the lithosphere of the Earth, which is the outermost shell, is composed of blocks of constant density. Although they have the same density, these blocks have different thicknesses.

Imagine an iceberg on the sea. A lot of the iceberg, the source/root, lies under the surface, and the tip sticks out. The bigger the tip of the iceberg, the more the root descends below the surface. Equally, in Airy Isostasy, geographically high areas of the earth contain a thicker layer of crust (or a "root") which extends down into the mantle where it is denser. This additional root works to balance the mountain's extra mass above the surface.

When erosion wears down a mountain over time, reducing its mass, the crust underneath rises in response, maintaining isostatic balance. Airy Isostasy describes a 'floating' lithosphere, where the thicker parts extend deeper into the mantle, just as larger icebergs sink further into the sea.

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Isostasy: Concept of Archdeacon Pratt

British geologist Archdeacon John Henry Pratt presented a different approach to explaining isostatic equilibrium with his model known as Pratt Isostasy. Pratt's model does not focus on the thickness of the lithospheric blocks but instead hinges on their density.

Imagine a wooden block and a sponge of the same size floating in a tub of water. The sponge will float higher because it is less dense, despite being the same size as the wooden block. Similarly, in Pratt Isostasy, areas of the Earth's crust that are less dense (such as those composed of less dense rock types or those underlain by significant sedimentary deposits) 'float' higher on the denser mantle than areas of thicker material.

Thus, Pratt's model suggests that the various elevations on Earth's surface, from plains to plateaus, result from these density variations within the crusf.

Acrust and Pratt Isostasy do not exclude each other. Variations in both thickness and density are allowable variations in the crust of the Earth, able to affect the balancing of the planet in the real world, given a different geographical and geological environment. The two models together give us a fuller picture of what the concept of Isostasy is all about, and, by extension, we are now in a better position to realize how dynamic our planet can be.

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Difference Between George Airy and Archdeacon Pratt's Concept of Isostasy

Although both models developed in depth by George Airy and Archdeacon Pratt give us clues on how to regard the theory of isostasy, their frames of reference give us different views on how to consider the theory of isostasy. The models offer different tinctures to view the complicated relations of the lithosphere and asthenosphere of Earth.

Divergent Assumptions

The model of isostasy introduced by Airy is guided by the fact that a variable crust thickness exists on Earth. It represents the lithosphere of the Earth as slabs of uniform density that vary in thickness. Conversely, the model developed by Pratt is grounded on the theory of different densities. It even visualizes the earth's surface as being on the surface in blocks of the same thickness, yet with different densities. Whereas the model advanced by Airy is compared with icebergs, the model presented by Pratt may be compared with other objects floating on the surface of a water mass. The degree of their floating depends on their density.

The Balance Factor

In Airy's concept, equilibrium is achieved by the varying thickness of the lithospheric 'roots' extending into the denser mantle. The greater the mass above the surface (like a high mountain range), the thicker the 'root' below. Conversely, in Pratt's model, the balance is struck by the varying density of the crustal blocks. Areas with lower density (such as those of lighter rock types) 'float' higher on the mantle than the denser ones.

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Isostatic Adjustment

The isostatic adjustments vary in the two models, too. According to the Airy model, as the weight on top of the crust varies due to erosion or deposition, the crust adjusts itself by increasing and decreasing thickness correspondingly - becoming less thick with erosion and thicker with deposition. However, in the Pratt model, such adjustments come into effect in alterations in the density of the crustal blocks.

Real-world Applications

Although the two models have different views about it, both usually exist simultaneously in practice. In the actual world, the topography has a higher chance of being a combination of variations in the density and thickness of the Earth's crust. So, combining these ideas of Airy plus Pratt, one can gain a more holistic picture of this complicated geological balance.

Although the assumptions driven by these two individuals differ, the ultimate idea behind them is to describe a similar phenomenon: the state of gravitational equilibrium or Isostasy, which enables the earth's crust to adapt and float under loads imposed in all directions. Through the innuendoes of these two models, we widen our horizons in terms of forces that have constituted the dynamic topography of our planet.

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Conclusion

Isostasy explains how the Earth's crust stays balanced on the mantle by varying thickness (Airy's theory) or density (Pratt's theory). This natural balance helps maintain the Earth's surface stability and explains why mountains and plains have different heights but remain supported over time.

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