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?

Question

Accepted Answer

Only one of the Statements II and III is correct and that explains Statement I Quick Verdict: A rock's ability to hold water (porosity) is different from its ability to let water flow through it (permeability). Clay is actually highly porous; it is simply impermeable because its pores are disconnected. Since only Statement II is correct (and explains the chalk aspect of Statement I), Option (c) is the right choice. Direct Breakdown of the Rock Types: Although both chalk and clay are sedimentary rocks, they differ fundamentally in formation, composition, porosity, and permeability. Chalk (Statements I & II): Chalk is a soft, highly porous form of calcium carbonate, typically formed from the remains of marine organisms and mined from ancient sea beds. Crucially, the many pores of this organically formed sedimentary rock are highly interconnected. Because the water can easily pass from one pore to the next, chalk is highly permeable to water. Hence, Statement II is correct. (Note: This natural chalk is different from manufactured school chalk, which is often mixed with gypsum or other materials). Clay (Statements I & III): In contrast, clay is made of very fine-grained (primarily silicate) mineral particles formed by the mechanical weathering of other rocks. This rock is creamier, denser, and more layered than chalk. Although clay contains a massive number of pores, these pores are microscopic, tightly packed, and very poorly interconnected. Water gets trapped inside rather than flowing through, making clay mostly impermeable to water. Therefore, Statement III is completely incorrect for stating clay is "not at all porous."