Geography Question in UPSC Prelims 2025 On Chalk vs Clay Permeability
Q. Consider the following statements:
Statement I: In the context of the effect of water on rocks, chalk is known as a very permeable rock, whereas clay is known as quite an impermeable or least permeable rock. Statement II: Chalk is porous and hence can absorb water. Statement III: Clay is not at all porous.
Which one of the following is correct in respect to the above statements?
✅ Correct Answer: C. Only one of the Statements II and III is correct, and that explains Statement I
Explanation:
Statement I: ✅ Correct. Chalk is a soft, sedimentary rock composed primarily of calcium carbonate and has a structure with large, well-connected pores. These pores allow water to pass through easily, making chalk a highly permeable rock. In contrast, clay is made up of very fine particles that are tightly packed together, with extremely small pores. As a result, water moves through clay very slowly, making it one of the least permeable types of soil or rock.
Statement II: ✅ Correct. Chalk is not only permeable but also highly porous, meaning it has many tiny holes or voids within its structure. These pores can hold water, allowing chalk to absorb and retain moisture. This is one of the reasons why chalk feels damp in humid conditions and can crumble easily when wet. Its porous nature is directly responsible for its ability to absorb water, which also contributes to its use in agriculture and construction, where water interaction is important.
Statement III:Incorrect. Clay is highly porous, as it contains many tiny pores between its fine particles. However, these pores are so small that water moves through them very slowly, making clay poorly permeable. This is why clay retains water and drains slowly, despite having high porosity.
Hence, only Statement II is correct and explains Statement I.
