Question 17 from UPSC CSE 2021 - Geography

Question

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

IASCODEX · Accepted Answer

Correct Option: B. fissure volcanic rock

Strategy: Focus on the origin of black cotton soil. The crucial keyword is black cotton soil, which is associated with volcanic rocks, specifically basaltic fissure volcanic rocks of the Deccan Plateau. Eliminate options related to other rock types and soils not known to yield this characteristic black soil. This points directly to fissure volcanic rock as the correct answer.

Key Info: ## Black Cotton Soil Overview
Black cotton soil, also known as Regur soil, is most extensive across the Deccan Plateau, covering parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, Karnataka, and Tamil Nadu.

### Geological Origin
This soil forms from the intensive weathering of basalt, a dark-colored, fine-grained igneous rock. The basalt originates from fissure volcanic activity that occurred during the late Cretaceous to early Eocene period, an event known as the Deccan Traps.

### Characteristics and Composition
The soil's black color and high moisture retention are due to the parent basalt rock's rich content of iron, magnesium, and clay-forming minerals.

**Chemical Profile:**
- **Rich in:** Calcium carbonate, magnesium, potash, and lime.
- **Deficient in:** Nitrogen, phosphoric acid, and organic matter.

The soil is ideal for cotton cultivation, hence its name. It is also known for its self-ploughing nature, a result of its high swell-shrink capacity. Other rock types like granite, schist, and shale do not produce this characteristic expansive soil.

IASCODEX · Answer

Option A: brown forest soil

IASCODEX · Answer

Option C: granite and schist

IASCODEX · Answer

Option D: shale and limestone