Model Answer
0 min readIntroduction
Isostasy, derived from the Greek words ‘isos’ (equal) and ‘stasis’ (standstill), refers to the state of gravitational equilibrium between the Earth’s lithosphere and asthenosphere. It explains how different densities of crustal materials cause them to ‘float’ at different levels within the mantle. This concept is crucial for understanding phenomena like mountain building, post-glacial rebound, and the varying elevations of continents and ocean basins. Proposed independently by George B. Airy and Joseph Barrell in the early 20th century, isostatic models attempt to explain the vertical movements of the Earth’s crust. While both sought to explain the same phenomenon, their approaches differed significantly in their assumptions about the nature of the crust and mantle.
Airy’s Isostasy
Sir George Biddell Airy, in 1855, proposed that the Earth’s crust is composed of materials of uniform density, but varying thickness. He likened the crust to a series of blocks of wood floating in water. According to Airy’s hypothesis:
- The crustal blocks have different thicknesses to compensate for variations in density.
- Thicker crustal blocks, like mountains, extend deeper into the mantle, displacing a greater volume of mantle material.
- The weight of the crustal block is equal to the weight of the displaced mantle material.
- The root of the mountain extends deep into the mantle, providing buoyancy.
Airy’s model assumes that the density of the crust is constant, and variations in elevation are solely due to differences in crustal thickness. This model is often referred to as the ‘root’ hypothesis, as it predicts a deep root beneath mountain ranges. The model works best in areas with relatively uniform crustal composition, like continental shields.
Pratt’s Isostasy
Joseph Barrell, in 1920, proposed an alternative model known as Pratt’s Isostasy. Unlike Airy, Pratt suggested that the crust is composed of materials of varying density, but uniform thickness. His hypothesis states:
- The crustal blocks have uniform thickness.
- Variations in elevation are due to lateral variations in density within the crust.
- Denser crustal materials underlie lower elevations, while less dense materials underlie higher elevations.
- The crust ‘floats’ because of the density contrast between the crust and the mantle.
Pratt’s model doesn’t require a deep root beneath mountains. Instead, it suggests that mountains are supported by less dense crustal material. This model is more applicable to regions with complex geological structures and varying crustal compositions, such as areas with significant metamorphic or igneous activity.
Comparison of Airy and Pratt Isostasy
The key differences between Airy and Pratt isostasy can be summarized in the following table:
| Feature | Airy’s Isostasy | Pratt’s Isostasy |
|---|---|---|
| Crustal Density | Constant | Variable |
| Crustal Thickness | Variable | Constant |
| Root Formation | Deep root beneath mountains | No deep root |
| Supporting Mechanism | Displacement of mantle material | Density contrast |
| Applicability | Continental shields | Complex geological regions |
In reality, neither model perfectly explains isostatic compensation globally. A more accurate representation often involves a combination of both Airy and Pratt mechanisms, known as the Vening Meinesz-Moritz isostatic model, which considers both variations in crustal thickness and density. This model acknowledges that isostasy is a dynamic process influenced by various geological factors.
Examples of Isostatic Adjustment
- Post-glacial rebound in Scandinavia and Canada: The removal of the massive weight of ice sheets during the last glacial period caused the land to slowly rise, demonstrating isostatic adjustment.
- Himalayan Orogeny: The formation of the Himalayas, due to the collision of the Indian and Eurasian plates, resulted in a thickened crust and a deep isostatic root.
- East African Rift Valley: The thinning of the crust in the rift valley leads to lower elevations and isostatic uplift in surrounding areas.
Conclusion
In conclusion, both Airy’s and Pratt’s isostasy provide valuable insights into the vertical movements of the Earth’s crust. While Airy’s model emphasizes variations in crustal thickness, Pratt’s model highlights density differences. Modern understanding recognizes that isostatic compensation is a complex process involving both mechanisms, influenced by regional geological settings. Continued research, utilizing advanced geophysical techniques, is refining our understanding of isostasy and its role in shaping the Earth’s surface.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.