The correct option is (b).
Explanation
Black soils, locally known as Regur or Black Cotton soils, are distinct for their clayey texture and moisture-retentive capacity. Their formation is directly linked to the geological history of the Deccan Plateau and the specific parent material available in that region.
Detailed Analysis
- Fissure Volcanic Rock (Correct): The black soils of India are primarily derivatives of the Deccan Trap lava flows. During the Cretaceous period, extensive fissure volcanic eruptions occurred, covering large parts of Peninsular India with basaltic lava. Over millions of years, the weathering and erosion of these igneous basaltic rocks resulted in the formation of black soil. This soil is rich in iron, lime, calcium, potash, aluminum, and magnesium.
- Granite and Schist (Incorrect): The weathering of crystalline igneous rocks like granite and metamorphic rocks like schist typically results in the formation of Red Soil. The red color is due to the diffusion of iron in crystalline and metamorphic rocks.
- Brown Forest Soil (Incorrect): This is a classification of soil found in hill slopes and forest areas, not a parent rock material responsible for creating black soil.
- Shale and Limestone (Incorrect): While limestone may be present in the subsoil of black soil regions (as kankars), the primary parent material defining the soil's genesis is the volcanic basalt, not sedimentary shale or limestone formations.
Key Takeaway
Black Cotton Soil (Regur) is formed in situ due to the weathering of basaltic lava rocks (fissure volcanic rocks) found in the Deccan Trap region.