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Isostasy

ICSE · Class 11 · Geography

Quick revision notes for Isostasy — ICSE Class 11 Geography. Key concepts, formulas, and definitions for last-minute revision.

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Key Topics to Revise

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1. Introduction and Definition of Isostasy

  • Isostasy comes from the German word 'Isostasios' meaning 'in equipoise' or 'state of balance'
  • The term was first used by American geologist Dutton in 1889
  • Isostasy explains the gravitational equilibrium of Earth's landforms (mountains, plateaus, plains, oceans)
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2. Historical Background: The Basis of Isostasy Theory

  • Pierre Bouguer (1735) first observed that mountains don't pull gravity as strongly as expected—seeds of isostasy were sown here
  • During the Andes Expedition, Bouguer found that the deflection of the plumb line near Chimborazo was much less than calculated
  • Sir George Everest's Indo-Gangetic Plain Survey (1854) provided crucial evidence supporting the theory
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3. Causes of Less Than Expected Gravitational Pull

  • Airy and Pratt identified four main causes for the anomaly in gravitational pull:
  • First cause (REJECTED): Lower density of mountains—assumed density was 2.67, but if lower, mountains would be 'hollow bubbles' (geologically impossible)
  • Second cause (ACCEPTED): No heavy material beneath mountains; light material extends to great depth; mountains sink into heavy liquid below like floating objects
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4. Airy's Concept of Isostasy

  • Sir George Airy (1801-1892) was the Astronomer Royal who formulated his theory in 1855
  • Airy followed the Law of Floatation—continents are composed of lighter sial floating on denser sima
  • Key principle: Mountains are stable because lighter rocks extend as 'roots' deep into the denser substratum below

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Full Notes

Key Concepts

Isostasy refers to the stateSial refers to the lighter rockyCrustal roots are deep extensionsThe level of compensation isSir George Airy proposed in 1855

Frequently Asked Questions

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Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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