Which of the following is not a characteristic of sedimentary rock ?

Question

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Correct — option (2), 'The rocks are huge and hard.' The stem prints 'not' in bold, so the question asks which of the four printed sentences is NOT true of sedimentary rock, and here the four sentences are themselves the options — there is no separate statement list. Sedimentary rocks are made by deposition: weathered fragments, dissolved matter and organic remains are carried by rivers, wind, ice or waves, laid down in basins, and then compacted and cemented into rock. Everything characteristic about them follows from that manner of formation. They are stratified, laid down bed upon bed, and a single bed can hold particles of many sizes cemented together, which is what option (1) describes — a conglomerate is the extreme case, with rounded pebbles set in a finer matrix. They are the rocks in which fossils are found, as option (3) says, because the remains of plants and animals settle with the sediment and are preserved in it, whereas the heat of molten rock and the heat and pressure of metamorphism destroy such remains; this is why the fossil record, and with it the whole of stratigraphy, is read from sedimentary sequences. And they are comparatively soft and porous, as option (4) says, because grains held together by a cement are weaker than an interlocking crystalline mass, so they weather and erode relatively quickly and they can hold water, coal, oil and gas within their pore spaces. What is not characteristic of them is being massive and hard. That description belongs to igneous rocks, which crystallise from cooling magma or lava into a compact, interlocking, unstratified and fossil-free mass — basalt and granite are the standard examples, and the Deccan Trap basalt that underlies most of Maharashtra is the one every candidate in this state has walked on. Option (2) is therefore the sentence that does not belong.

Why the other options are wrong — (a) This is a genuine characteristic of sedimentary rock, so it cannot be the answer to a question asking what is not one. Sediment is deposited as it is delivered, and a single bed may contain grains of widely differing size cemented into one layer — a conglomerate, with pebbles set in a finer matrix, is the clearest illustration. The sentence as printed reads awkwardly, with 'a single rocks layer' where 'a single rock layer' is meant, and a candidate can be thrown by the wording; but a printing slip is not a factual error, and the property described is real.  (c) This too is a real characteristic — indeed it is the property that makes sedimentary rocks the foundation of historical geology. Remains of plants and animals are buried along with the sediment and preserved as the sediment turns to rock, whereas molten rock destroys organic material and metamorphism generally obliterates whatever fossils the parent rock contained. Selecting this option usually means the bolded negation in the stem has been missed and the candidate has picked the sentence that most strongly characterises sedimentary rock rather than the one that does not.  (d) Relative softness is a characteristic of sedimentary rocks, not an exception to them. Because their grains are bound by a cement rather than locked into an interlocking crystalline framework, they are generally weaker and more porous than igneous rocks, they weather and erode comparatively quickly, and their pore space is what allows them to hold groundwater and to serve as reservoirs for coal, petroleum and natural gas. This option and option (2) are direct opposites, and noticing that two of the four printed sentences contradict each other is itself a route to the answer: only one of them can be a characteristic.

Concept: The three great rock classes are defined by how they form, and every property tested in questions of this kind follows from that. Igneous rocks solidify from magma or lava; they are crystalline, massive, hard, without layers and without fossils, and they are called primary rocks because all other rocks are ultimately derived from them — granite is the intrusive example, basalt the extrusive one. Sedimentary rocks form from the deposition and compaction of weathered material and organic remains; they are stratified, often porous and comparatively soft, they may contain particles of mixed size in one bed, and they are the only rocks that regularly preserve fossils — sandstone, shale, limestone and conglomerate are the standard examples. Metamorphic rocks form when existing rocks are altered in place by heat and pressure, becoming harder and often foliated: limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss. The rock cycle links the three, and it is the cycle rather than the lists that makes the properties memorable.

State PCS 2024 Q20 — Geography PYQ with Answer

State PCS 2024 Q20 — Geography PYQ with Answer