UPSC Previous Year Question | Prelims GS | 2025

Consider the following statements: Statement I: In the context of the effect of water on rocks, chalk is known to be a very permeable rock, whereas mudstone (clay) is known to be a quite impermeable or least permeable rock. Statement II: Chalk is porous, hence can absorb water. Statement III: Mudstone (clay) is not at all porous. Which one of the following is correct in respect of the above statements?

  1. Both Statement II and Statement III are correct and both of them explain Statement I
  2. Both Statement II and Statement III are correct but only one of them explains Statement I
  3. Only one of the Statements II and III is correct and that explains Statement I
  4. Neither Statement II nor Statement III is correct

Answer

Correct Answer

Only one of the Statements II and III is correct and that explains Statement I

Explanation

Chalk is a porous rock and readily absorbs water, which explains why it is highly permeable, making Statement II correct and explanatory of Statement I. However, clay is actually porous too (it has a high porosity due to fine particles) but has very low permeability because its pores are poorly connected, so Statement III ('not at all porous') is incorrect. Hence only one of II/III is correct and explains I.

Topics: geography, geology, rocks