Previous Year Question

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?

Both Statement II and Statement III are correct and both of them explain Statement I

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

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

Neither Statement II nor Statement III is correct

Correct Answer: C

Detailed Solution

Porosity indicates the amount of empty space in a rock or sediment, whereas permeability indicates how easily water can flow through interconnected pores, joints or fractures.

Statement I is correct: Under natural conditions, joints and fractures in chalk provide pathways for water, making a chalk rock mass moderately to highly permeable. In contrast, water moves extremely slowly through clay, and its permeability is very low.

Statement II is correct: Chalk is porous and can absorb and store water in its microscopic pores. Interconnected fractures in natural chalk also permit water to flow through it. Therefore, Statement II explains the chalk component of Statement I.

Statement III is incorrect: Clay is porous and may even possess high porosity. However, its pores are extremely small, and water flows through them very slowly. Therefore, low permeability does not mean a complete absence of porosity.

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