UPSC Prelims 2025Question 28Geography

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

(a)Both Statement II and Statement III are correct and both of them explain Statement I
(b)Both Statement II and Statement III are correct but only one of them explains Statement I
(c)Only one of the Statements II and III is correct and that explains Statement I
(d)Neither Statement II nor Statement III is correct

Correct answer: b

Decision rule

Porosity is pore volume, permeability is flow: a fine-pored rock can be highly porous yet impermeable, so 'not at all porous' overreaches.

Explanation

Correct Answer:

(b) Both Statement II and Statement III are correct but only one of them explains Statement I

Explanation of the Correct Answer:

Statement I:

Chalk is known as a very permeable rock whereas clay is known as quite an impermeable or least permeable rock.

– Correct.

Permeability refers to the ability of a material to allow fluids to pass through it.
Chalk is a sedimentary rock that is both porous and permeable, meaning it can absorb and transmit water.
Clay, although it may be porous, has very small pores that retain water through surface tension, making it effectively impermeable to water flow.

Statement II:

Chalk is porous and hence can absorb water.

– Correct.

Porosity refers to the volume of pore spaces in the rock.
Chalk has high porosity and permeability, allowing it to absorb and transmit water, explaining why it is considered permeable.

Statement III:

Clay is not at all porous.

– Partially incorrect.

Clay does have porosity, but the pores are extremely small, making the rock practically impermeable to water flow.
Therefore, while technically porous, it is functionally impermeable. Hence, this statement is not entirely accurate, even if the intent matches the functional truth.

Thus, only Statement II correctly explains Statement I. Hence, option (b) is correct.

Why Other Options Are Incorrect:

(a): Incorrect – Statement III is not entirely correct; hence it cannot be used to explain Statement I.
(c): Incorrect – Both II and III are correct in intent, but only one of them (II) explains Statement I.
(d): Incorrect – Statement II is definitely correct.

Relevant Supplementary Information

Table: Comparison of Chalk and Clay

Property	Chalk	Clay
Type of Rock	Sedimentary (Carbonate)	Sedimentary (Fine-grained)
Porosity	High	High (but with very fine pores)
Permeability	High	Very low (functionally impermeable)
Water Absorption	Absorbs and transmits water easily	Absorbs water but does not transmit it easily
Practical Use	Aquifers, filters	Used to block water (linings, dams)

Related Concepts

Term	Explanation
Porosity	The percentage of a rock’s volume that is pore space
Permeability	The ability of a rock to allow fluids to pass through it
Aquitard	A layer (e.g., clay) that slows the flow of groundwater
Aquifer	A permeable rock (e.g., chalk, sandstone) that stores and transmits groundwater

Current Relevance

Understanding rock permeability is essential in:
- Groundwater management
- Civil engineering and construction
- Environmental impact assessment
- Agriculture (drainage and water retention)

Table 1: Porosity and Permeability – Key Definitions

Concept	Definition
Porosity	The percentage of void (pore) space in a rock or sediment
Permeability	The ability of a material to transmit fluids (like water) through its pores
Impermeable	A material with very low or negligible permeability (blocks fluid movement)
Aquifer	A rock layer that is both porous and permeable; stores and transmits groundwater
Aquitard	A rock or sediment layer with low permeability; restricts water movement

Table 2: Comparison of Chalk and Clay

Property	Chalk	Clay
Type of Rock	Sedimentary (Limestone variant)	Sedimentary (Fine-grained)
Porosity	High	High (but with very tiny pores)
Permeability	High	Very Low (functionally impermeable)
Water Movement	Water can both enter and flow through	Water may enter slowly but cannot flow easily
Common Use	Aquifers, filters	Clay liners in dams, landfills (to stop flow)
Soil Water Retention	Low (drains quickly)	High (retains moisture)

Table 3: Common Rock Types by Permeability

Rock Type	Porosity	Permeability	Typical Role
Chalk	High	High	Aquifer
Sandstone	Moderate	Moderate to High	Aquifer
Limestone	Variable	Variable	May be aquifer if fractured
Clay	High	Very Low	Aquitard or seal layer
Granite (unfractured)	Very Low	Very Low	Not a useful aquifer

Table 4: Functional Classification of Rocks in Hydrogeology

Term	Criteria	Examples
Aquifer	Porous and permeable rock storing/transmitting water	Chalk, Sandstone
Aquitard	Porous but not permeable; retards water flow	Clay, Shale
Aquiclude	Neither porous nor permeable (rare)	Dense igneous rocks
Aquifuge	Impervious and non-porous (e.g., solid granite)	Basalt (non-vesicular)

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