UPSC Prelims 2025 Q28: Chalk vs Clay Permeability — Explained
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
Answer: (c) Only one of the Statements II and III is correct and that explains Statement I
Statement-wise explanation
- STATEMENT-I: correct — Permeability is how easily water passes through a rock. Standard geography texts class chalk as a very permeable rock: water soaks into it and moves through it. That is why chalk country has few surface streams and dry valleys. Clay holds water but hardly lets it pass, so it is treated as impermeable or the least permeable.
- STATEMENT-II: correct — Chalk is a soft, fine-grained limestone made mostly of calcium carbonate. It is full of pore spaces and fissures, so it absorbs water and lets it soak downward. Its porosity, together with its joints and fissures, is the reason it is called permeable. So Statement II is correct and explains Statement I.
- STATEMENT-III: incorrect — Clay is highly porous. Its total pore space is often 40% or more by volume, which is higher than most sandstones. But its pores are extremely small and poorly connected, and water clings to the clay particles. So water cannot flow through it. Clay's low permeability comes from pore size and connectivity, not from having no pores, so 'not at all porous' is wrong and cannot explain Statement I.
How to eliminate
Look at the absolute words. Statement III says clay is 'not at all porous'. That is an extreme claim, and anyone who has seen wet clay swell and hold water knows clay takes in water, so it has pore space. Once Statement III is struck out, the two options that need both II and III to be correct go. The option saying neither is correct also goes, because chalk being porous is basic knowledge. Only one option is left. The fact to remember: porosity is how much empty space a rock has; permeability is whether water can flow through that space. Clay is high in porosity but low in permeability.
Know more
Porosity vs permeability
- Porosity is the share of a rock's volume that is empty space (pores). It tells you how much water the rock can store.
- Permeability is how easily water can move through the rock. It depends on how large the pores are and how well they connect.
A rock can be porous and still be impermeable. Clay is the classic case: it has many pores, but they are tiny, and water sticks to the flat clay particles. So clay holds water but does not pass it on. This is why clay layers act as aquicludes. They trap water above them, which forms springs and perched water tables.
Some rocks have low porosity but pass water easily through cracks. Granite and basalt are examples: they are pervious because of their joints and fractures. Limestone and chalk also let water through their joints. Limestone, in addition, dissolves along those joints and forms karst landscapes.
| Rock | Porosity | Permeability | Behaviour with water |
|---|---|---|---|
| Chalk | High | High (pores + fissures) | Absorbs water; few surface streams; good aquifer |
| Sandstone | Moderate–high | High | Classic aquifer |
| Clay | Very high | Very low | Holds water, blocks flow; waterlogging, aquiclude |
| Granite | Very low | Low (only via joints) | Water moves only along cracks |
| Limestone | Variable | High via joints | Dissolves, forms karst |
Why it matters: Permeability controls groundwater recharge, aquifer location, how prone a soil is to waterlogging, and landforms. For example, chalk downs have dry valleys, while clay vales are marshy.
NCERT Concepts to Revise
- Sedimentary rock: Chalk and clay are both sedimentary rocks. How their particles settle and are cemented sets their pore space and how water moves through them.