Question 82 from UPSC CSE 2025 - Geography

Question

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 of the above statements?

IASCODEX · Accepted Answer

Correct Option: C. Only one of the Statements II and III is correct and that explains Statement I.

Strategy: Focus on the physical characteristics of chalk and clay. Chalk is porous, so it is permeable, and this property explains its behavior in Statement I. For clay, despite being impermeable, saying it is 'not at all porous' is an extreme and factually inaccurate claim, since clays have very low porosity but are not absolutely nonporous. Therefore, only Statement II is correct, and it directly explains Statement I. Carefully eliminate options where both II and III are correct or where Statement III alone is considered correct.

Key Info: Chalk is a sedimentary rock predominantly made of calcium carbonate, characterized by high porosity and well-connected pore spaces. These properties make chalk highly permeable, facilitating the movement and storage of groundwater, which is why chalk formations often serve as aquifers. The porosity of chalk results from its composition of microscopic shells (coccoliths). In contrast, clay is a fine-grained natural material composed of hydrous aluminum phyllosilicates. The particles in clay are extremely small and closely packed, leading to very low porosity and minimal interconnected voids. This microscopic structure impedes water movement, making clay effectively impermeable although not entirely nonporous. Clay layers often act as aquitards, restricting groundwater flow between permeable layers. Porosity is the percentage of a rock's volume that is open space, and permeability is the ability of a material to transmit fluids. While porous rocks are generally permeable, the arrangement and connectivity of pores is critical. In the context of hydrogeology, these properties determine how different rock layers interact with water and control water movement underground.

IASCODEX · Answer

Option A: Both Statement II and Statement III are correct and both of them explain Statement I.

IASCODEX · Answer

Option B: Both Statement II and Statement III are correct but only one of them explains Statement I.

IASCODEX · Answer

Option D: Neither Statement II nor Statement III is correct.