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Geography · Weathering & Mass Wasting

UPSC Prelims 2025, Question 28

The concept being tested: Rock porosity and permeability in water infiltration.

Consider the following statements: Statement I: In the context of 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?

Options

  1. ABoth Statement II and Statement III are correct and both of them explain Statement I
  2. BBoth Statement II and Statement III are correct but only one of them explains Statement I
  3. COnly one of the Statements II and III is correct and that explains Statement ICorrect answer
  4. DNeither Statement II nor Statement III is correct

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

Porosity and permeability are distinct properties. Porosity is the percentage of pore space a rock holds, while permeability is the capacity to transmit water through those pores, which depends on pore size and interconnection. Chalk has many fine, interconnected pores and fissures, so it is both porous and permeable, absorbing and passing water freely, which makes Statement II correct and explains Statement I. Clay, however, is highly porous, its plate-like particles pack loosely, but the pore spaces are extremely small and the molecular attraction between clay particles and water is strong enough to resist gravity's pull, so water gets held rather than transmitted. Clay is therefore porous but impermeable, not non-porous as Statement III claims. Since only Statement II is factually correct and it accounts for chalk's permeability, the option stating that only one of the two statements is correct and explains Statement I is right.

What UPSC was testing

UPSC is testing the difference between porosity, how much pore space a rock has, and permeability, whether that pore space is interconnected enough to transmit water, a distinction where clay is the classic porous-but-impermeable exception that trips up students who assume the two properties always move together.

How often this comes up

This concept has appeared 6 times in UPSC Prelims General Studies Paper I between 2015 and 2025 — in 2021, 2022, 2024 and 2025.

The option-by-option autopsy for this question — why each of the other three is wrong, and which neighbouring concept each one was built from — is in the app.

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Other past-year questions touching Weathering & Mass Wasting.

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