UPSC Prelims 2021 Q64: Black Cotton Soil Parent Rock — Explained
The black cotton soil of India has been formed due to the weathering of
- brown forest soil
- fissure volcanic rock
- granite and schist
- shale and limestone
Answer: (b) fissure volcanic rock
Statement-wise explanation
- a: incorrect — Brown forest soil is a soil, not a parent rock. Soils form when rock weathers, so one soil type cannot be the 'parent' that weathers into black soil. In India, forest soils form on hill and mountain slopes such as the Himalaya under forest cover. They are far from the Deccan lava plateau where black soil dominates.
- b: correct — Black cotton soil (regur) developed on the basalt of the Deccan Traps. That basalt formed from very fluid lava that poured out of long cracks, or fissures, near the end of the Cretaceous, about 66 million years ago. As the basalt weathered, it gave a clayey soil, rich in montmorillonite, iron, lime and magnesia. That is why the soil is concentrated on the Maharashtra, Malwa, Saurashtra and Madhya Pradesh plateaus.
- c: incorrect — Granite and schist are crystalline igneous and metamorphic rocks. Their weathering produces red and yellow soils, not black soil. Red soil develops on crystalline rocks in the low-rainfall eastern and southern parts of the Deccan Plateau. It is red because iron is spread through it.
- d: incorrect — Shale and limestone are sedimentary rocks. They are not the parent material of India's black cotton soil. The black soil belt matches the extent of the Deccan Trap basalt, an igneous volcanic rock, not sedimentary basins.
How to eliminate
Start with option structure. Brown forest soil is itself a soil. The question asks what was weathered to form a soil, which means a rock, so this option drops out. That leaves three rock types. Basic NCERT knowledge removes granite and schist, because these crystalline rocks are the textbook parent of red and yellow soils. Next, use the map. The black soil belt (Maharashtra, Malwa, Saurashtra, western Madhya Pradesh) sits on the same area as the Deccan Traps. Its dark colour and lime-rich clay point to a dark, iron- and magnesium-rich igneous rock (basalt), not to pale sedimentary shale and limestone. Fact to remember: black (regur) soil = weathered Deccan Trap basalt, which came from fissure (flood) eruptions. Red soil = weathered granite, gneiss and schist.
Know more
Parent rock decides the soil type in Peninsular India. In the Indo-Gangetic plain, rivers carry and deposit the soil (alluvium). On the Peninsula, most soils form in place, on the rock beneath them. So the geology map is a good guide to the soil map.
Deccan Traps. Near the end of the Cretaceous, very fluid basalt lava came out of long cracks (fissure eruptions), not from cone-shaped volcanoes. It spread in near-flat sheets, layer after layer, and built a step-like ('trap', from the Swedish word for stairs) flood basalt province. Today it covers about 5 lakh sq km of the north-western Deccan.
Black soil (regur, Vertisol)
- Very clayey, mostly montmorillonite clay. It swells when wet and shrinks when dry.
- Deep cracks open in summer. Topsoil falls into them, so the soil turns itself over ('self-ploughing').
- Holds moisture well, which suits rain-fed cotton. Hence the name 'black cotton soil'.
- Rich in lime, iron, magnesia and potash. Poor in nitrogen, phosphorus and organic matter.
- Dark colour comes from iron–titanium compounds and the clay–humus mix, not mainly from high humus.
| Soil | Parent material | Core region |
|---|---|---|
| Black (regur) | Deccan Trap basalt | Maharashtra, Malwa, Saurashtra, MP, along Godavari–Krishna |
| Red and yellow | Granite, gneiss, schist (crystalline) | Eastern and southern Deccan, Odisha, Chhattisgarh |
| Laterite | Leaching of many rocks under heavy rain and high heat | Western Ghats, Odisha, NE hills |
| Alluvial | River-deposited sediment | Indo-Gangetic plain, deltas |
| Forest/mountain | Weathered rock under forest cover | Himalaya, Western/Eastern Ghats slopes |
Some black soil in the Godavari and Krishna valleys and in Tamil Nadu has been carried by rivers or formed on other rocks. The classic regur of the Deccan, though, is basalt-derived.
NCERT Concepts to Revise
- Black soil: Defines regur or black cotton soil, the soil whose origin the question asks about.
- Deccan Traps: The Deccan Traps are the lava province whose rock type the options test you on.
- Flood basalt province: Explains how lava pouring from long cracks builds vast sheets, linking 'fissure' eruptions to a whole plateau.
- Basalt: Knowing basalt is a dark volcanic rock helps you sort the volcanic, crystalline and sedimentary options.
- Deccan Plateau: Locates the plateau where Deccan soils formed on different rocks, red on some and black on others.