Isostasy is one of those geomorphology topics UPSC never really lets go of. It keeps coming back in different forms. Sometimes as a straight definition, sometimes as a comparison between theories, and sometimes linked with plate tectonics and crustal movements.
If you look at past questions, UPSC has asked isostasy in 2007, 2011, and 2021. That pattern alone makes it a topic you cannot afford to ignore.
This blog breaks down isostasy, its core meaning, and the three major interpretations you must prepare for Geography Optional.
Contents
- 1 What Is Isostasy?
- 2 Why Is Isostasy Important for UPSC?
- 3 Pratt’s Theory of Isostasy (Compensation Theory)
- 4 Airy’s Theory of Isostasy (Flotation or Root Theory)
- 5 Plate Tectonics Interpretation of Isostasy
- 6 Key Differences Between Pratt and Airy (For Answer Writing)
- 7 Conclusion
- 8 Check out our GS Geography Course – Click Here
- 9 FAQs
What Is Isostasy?
The term isostasy comes from the Greek word isostasios, meaning equal standing.
It was proposed by Clarence Dutton in 1889 to explain the state of balance between landforms and the Earth’s interior.
In simple terms, isostasy explains how landforms remain mechanically stable despite differences in height, mass, and erosion.
What this really means is:
- The Earth’s crust is not rigid
- It adjusts itself vertically to maintain gravitational balance
- This adjustment helps explain mountains, plateaus, uplift, and subsidence
Isostasy also helps address gravitational anomalies observed across the Earth’s surface.
Why Is Isostasy Important for UPSC?
From an exam perspective, isostasy:
- Explains mountain building and crustal stability
- Links geomorphology with plate tectonics
- Helps answer questions on uplift, subsidence, and rebound
- Is repeatedly asked in Geography Optional (Paper I)
UPSC expects clarity on:
- Pratt’s theory
- Airy’s theory
- Plate tectonics interpretation of isostasy
Pratt’s Theory of Isostasy (Compensation Theory)
Pratt’s theory is also called the law of compensation.
Core Idea
- Equal masses underlie equal areas
- Landforms have equal thickness but varying densities
- Compensation occurs up to a fixed depth called the depth of compensation
In Pratt’s model:
- Higher landforms have lower density
- Lower landforms have higher density
- Density variation exists only up to a certain depth
- Beyond this depth, material has uniform density
This theory explains why areas with similar crustal thickness can have different elevations.
Modern Modification
Later, Hayford and Bowie refined Pratt’s idea by introducing the concept of a plane of compensation at around 100 km depth, which aligns closely with the thickness of lithospheric plates.
Pratt’s model fits best with mid-oceanic ridges, where density variation plays a major role.
Airy’s Theory of Isostasy (Flotation or Root Theory)
Airy’s theory is based on the principle of flotation.
Core Idea
- All landforms have the same density
- Elevation differences arise due to variation in crustal thickness
- Taller landforms have deeper roots, just like icebergs floating in water
In this model:
- The crust floats on a denser substratum
- Greater height above the surface means greater depth below it
- Hence, it is also called the root theory of isostasy
Joly’s Contribution
Joly suggested a 1:8 ratio, meaning:
- For every 1 unit of crust above the surface
- There are 8 units extending below as roots
Airy’s model is best suited for continents and mountain ranges, where thick crustal roots support high relief.
Plate Tectonics Interpretation of Isostasy
Modern plate tectonics integrates both Pratt and Airy principles.
Key Modifications
- Isostatic balance is regional, not local
Landforms do not float independently. Entire lithospheric plates adjust together. - Lithosphere behaves like a flexural membrane
The concept of flexural rigidity explains how plates bend under load.
Role of Flexural Rigidity
- Excess load (glaciers, dams, urbanisation) causes bending
- Mantle material adjusts to redistribute stress
- Removal of load leads to isostatic rebound
Examples
- Scandinavian Shield uplift after glacial retreat
- Alaska uplift due to ice melting
- Associated phenomena include earthquakes, subsidence, and uplift
This interpretation makes isostasy dynamic and closely linked to plate movements and climate change.
Key Differences Between Pratt and Airy (For Answer Writing)
- Pratt: Equal thickness, varying density, law of compensation
- Airy: Equal density, varying thickness, law of flotation
- Pratt: Suitable for mid-oceanic ridges
- Airy: Suitable for continents and mountains
- Pratt: No concept of roots
- Airy: Deep crustal roots support tall landforms
Conclusion
Isostasy is not just a theoretical concept. It is the foundation for understanding:
- Crustal stability
- Mountain building
- Plate tectonics
- Isostatic rebound
For UPSC Geography Optional, mastering Pratt, Airy, and plate tectonics interpretations is non-negotiable. Questions may change form, but the core idea remains the same.
Check out our GS Geography Course – Click Here
FAQs
- #IsostasyUPSC: What is isostasy in geomorphology for UPSC Geography Optional?
- #PrattTheoryUPSC: Explain Pratt’s theory of isostasy with suitable examples.
- #AiryTheoryUPSC: How does Airy’s theory of isostasy explain crustal roots?
- #IsostaticReboundUPSC: What is isostatic rebound and how is it linked to plate tectonics?
