निम्नलिखित कथनों पर विचार कीजिए :
- कथन I: शैलों पर जल के प्रभाव के संदर्भ में, चॉक एक अत्यंत पारगम्य शैल के रूप में ज्ञात है, जबकि मृत्तिका एक नितांत अपारगम्य या न्यूनतम पारगम्य शैल के रूप में ज्ञात है।
- कथन II: चॉक सरंध्र होता है, अतः जल अवशोषित कर सकता है।
- कथन III: मृत्तिका बिल्कुल ही सरंध्र नहीं है।
उपर्युक्त कथनों के संदर्भ में, निम्नलिखित में से कौन-सा एक सही है?
Read this question in English
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?
(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
उत्तर और व्याख्या · explanation in English
Answer: (c) कथन II और कथन III में से केवल एक सही है तथा वह कथन I की व्याख्या करता है
Porosity is how much empty space a rock holds; permeability is how easily water passes through it. Chalk is a soft limestone, and limestone is porous and gives up its pore water freely, which explains Statement I. Clay is actually highly porous (USGS puts it at about 50%), but its pores are so tiny that water barely moves, so Statement III ('not at all porous') is wrong.
- ✓ Statement I Chalk is a variety of limestone (Britannica), and USGS shows limestone yielding most of its pore water freely, while groundwater moves very slowly through relatively impermeable materials such as clay and shale.
- ✓ Statement II Chalk, as a limestone, is porous: USGS lists limestone at 20% porosity with 18% specific yield, so it takes up and releases water readily. That is why chalk is permeable, so II explains I.
- ✗ Statement III Clay is not free of pores: USGS lists its porosity at 50%, but only 2% of that drains freely because the pores are tiny. Britannica notes that permeability falls as the average pore size becomes smaller.
Remember · Porosity = pore space; permeability = ease of water flow. Chalk (limestone): porous and permeable. Clay: highly porous but nearly impermeable because its pores are minute.
📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 5 (practise this chapter)
Sources
- U.S. Geological Survey, Water Science School: Aquifers and Groundwater (copy of the USGS page) ↗ “Groundwater moves very slowly through relatively impermeable materials such as clay and shale.”
- Encyclopaedia Britannica: Permeability (geology) ↗ · reference work “The porosity of a soil horizon increases as its texture becomes finer, whereas the permeability decreases as the average pore size becomes smaller.”
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 5 “If the rocks are permeable and soluble and water is available only then karst topography develops.”
- U.S. Geological Survey, Water-Supply Paper 2220: Basic Ground-Water Hydrology (Heath, 1983): porosity, specific yield and specific retention ↗ “Clay ----------------------- 50 2 48 Sand ---------------------- 25 22 3 Gravel --------------------- 20 19 1 Limestone ------------------ 20 18 2”
- Encyclopaedia Britannica: Chalk ↗ · reference work “chalk , soft, fine-grained, easily pulverized, white-to-grayish variety of limestone .”
Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·