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Q28 (IAS/2025) Geography › World Physical Geography › Rocks and minerals Answer Verified

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?

Result
Your answer: —  ·  Correct: C
Explanation
Here is the breakdown: Statement III is incorrect because clay is actually highly porous (it holds a lot of water in microscopic spaces), but it is impermeable because those spaces are not well-connected, blocking water flow. Statement II is correct because chalk is porous and permeable, allowing it to absorb and transmit water. Since Statement III is false and Statement II is true (and explains why chalk is permeable as mentioned in Statement I), Option C is the correct choice.
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Q. Consider the following statements : Statement I : In the context of effect of water on rocks, chalk is known as a very permeable rock whe…
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Origin: Mixed / unclear origin Fairness: High fairness Books / CA: 8/10 · 0/10
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This is a textbook concept check directly from GC Leong (Chapters 4 & 8). It tests the precise scientific distinction between 'Porosity' (storage capacity) and 'Permeability' (flow capacity). The trap lies in the common misconception that if a rock stops water (impermeable), it must have no holes (non-porous)—Clay proves this wrong.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
In the context of the effect of water on rocks, is chalk a highly permeable rock (allows easy water flow)?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 8: Limestone and Chalk Landforms > Limestone and Chalk > p. 76
Presence: 4/5
“Limestone and chalk are sedimentary rocks of organic origin derived from the accumulation of corals and shells in the sea. In its pure state, limestone is made up of calcite or calcium carbonate, but where magnesium is also present it is termed dolomite. Chalk is a very pure form of limestone, white and rather soft. Limestone is soluble in rain-water which, with carbon dioxide from the air, forms a weak acid.”
Why this source?
  • Identifies chalk as a very pure form of limestone, linking chalk to the properties of carbonate rocks.
  • Establishes chalk's composition (calcium carbonate) which is relevant to its interaction with water.
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 17: Major Landforms and Cycle of Erosion > Conditions for The Formation of Karst Topography > p. 227
Presence: 5/5
“• Surface or subsurface strata that are made up of porous water-soluble rocks such as limestone.• Thinly bedded and highly jointed and cracked rock strata that make it easy for the water to seep in.• Moderate to abundant rainfall for chemical weathering of limestone.• A perennial source of water and a low water table to erode the weathered rock.”
Why this source?
  • Explicitly lists limestone among porous, water-soluble rocks used in karst formation, implying good capacity for water movement.
  • Notes that thin bedding and heavy jointing/cracking make such rocks easy for water to seep into.
Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 4: Weathering, Mass Movement and Groundwater > Groundwater > p. 42
Presence: 5/5
“Various rocks and soils differ greatly in their porosity and permeability; the amount of groundwater present and the depth at which it lies are governed by these characteristics. Porous rocks are those, like sandstone, which have many pore-spaces between the grains. Water is easily absorbed by such rocks and may be stored in the pore-spaces. Permeable or pervious rocks are those which allow water to pass through them easily (Fig. 4.6). Thus most porous rocks are also permeable. Fig. 4.7 Groundwater table and its relationship to the curvature of the land no water but the rock may have numerous joints or cracks through which the water can pass, rendering it pervious or permeable.”
Why this source?
  • Defines the relation between porosity and permeability and states most porous rocks are also permeable.
  • Explains that permeability can result from pore-spaces or from joints and cracks, providing the mechanism for water flow through rocks like chalk/limestone.
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