Black cotton soil, also known as regur soil, is a type of soil found in the Deccan region of India. It is characterized by its deep black color and high clay content. This soil is highly fertile and is widely used for agricultural purposes in the region.

Black cotton soil is associated with volcanic rocks in the Deccan region. Here is a detailed explanation of this association:

1. Formation of Black Cotton Soil:
- Black cotton soil is formed by the weathering of basaltic rocks that are found in the Deccan Plateau.
- The Deccan Plateau is known for its extensive volcanic activity in the past, which led to the deposition of basaltic lava flows.
- Over time, the basaltic rocks undergo chemical weathering and breakdown, forming the black cotton soil.

2. Basaltic Rocks:
- Basaltic rocks are igneous rocks that are formed from solidified lava flows.
- These rocks are typically dark-colored and have a fine-grained texture.
- Basaltic rocks are rich in minerals such as feldspar, pyroxene, and olivine.

3. Weathering of Basaltic Rocks:
- The weathering of basaltic rocks involves the breakdown of minerals and the release of clay particles.
- The clay particles are responsible for the high clay content in black cotton soil.
- The weathering process also contributes to the deep black color of the soil.

4. Characteristics of Black Cotton Soil:
- Black cotton soil has a high water holding capacity due to its clay content.
- It swells when wet and shrinks when dry, leading to the formation of deep cracks.
- These cracks can be problematic for construction and infrastructure development.
- The high clay content also makes the soil sticky and difficult to work with.

In conclusion, black cotton soil in the Deccan region is associated with volcanic rocks, specifically basaltic rocks. The weathering of these rocks over time leads to the formation of the highly fertile black cotton soil.