The black cotton soil, also known as Regur soil, predominantly forms from the weathering of fissure volcanic rocks, particularly basalt. This soil is rich in minerals like iron, lime, calcium, and magnesium, which are essential for cotton cultivation, making it particularly suitable for the crop. The volcanic activity in regions like the Deccan Plateau has contributed to the formation of these soils, which are characterized by their moisture-retaining capacity and fertility, aligning with the geological and agricultural aspects relevant to the UPSC syllabus.