Authentic Classroom Explanation by IAS Monk

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📍 The Question

The black cotton soil of India has been formed due to the weathering of:

(a) Brown forest soil
(b) Fissure volcanic rock
(c) Granite and schist
(d) Shale and limestone

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✅ Correct Answer: (b) Fissure volcanic rock

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🎯 Theme of the Question

Indian Geography | Pedology | Soils of India | Deccan Trap Volcanism

This is a classic source-based soil question testing the parent rock–soil relationship.

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🧠 Classroom Explanation

🌋 Origin of Black Cotton Soil (Regur Soil)

• Black cotton soil is formed from the weathering of basalt, an extrusive igneous rock
• Basalt was deposited through fissure volcanic eruptions during the formation of the Deccan Traps
• Hence, the correct geological source is fissure volcanic rock

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🧪 Why Basalt Matters

Basalt is:

• Rich in iron and magnesium
• Fine-grained and dark coloured
• Produces clay-rich soil with:
  • High moisture retention
  • Swelling when wet and cracking when dry

These properties make it ideal for cotton cultivation, hence the name black cotton soil.

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🌾 Distribution in India

Major regions:

• Maharashtra
• Madhya Pradesh
• Gujarat
• Telangana
• Karnataka

All lie within or adjacent to the Deccan Trap region.

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❌ Why Other Options Are Incorrect

• Brown forest soil ❌
  → A soil type, not a parent rock
• Granite and schist ❌
  → Form red or lateritic soils
• Shale and limestone ❌
  → Typically form alluvial or calcareous soils

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🧩 Prelims Memory Hook

“Regur remembers the Deccan Traps.”

Black soil = Basalt = Fissure volcanism

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🧭 IAS Monk Whisper

Every soil carries the memory of ancient fire.
Black cotton soil is India’s volcanic signature, still feeding fields centuries later.