MPSC 2024 Prelims Paper-I · Question 20 of 100

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.

• (1)Particles of many sizes are organized in a single rocks layer. — 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.
• (3)Fossils are mainly found in rocks. — 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.
• (4)Rocks are soft, so they wear out quickly. — 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.

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.

MPSC's physical-geography questions on rocks are almost always property-based and very often negative, since a negative stem allows three true properties of one rock class to be printed alongside one property borrowed from another. The borrowed property is nearly always hardness, crystallinity or the absence of layering, all of which belong to igneous rock. Two habits make the format safe: mark each printed sentence true or false before reading the question again, and watch for a pair of options that contradict one another, since in a well-set question one of the pair is the answer. The topic also has direct Maharashtra relevance, because the state is dominated by Deccan Trap basalt — an igneous rock — while its Gondwana sedimentary formations in the east carry the coal seams of the Wardha valley, and that contrast is itself examinable. The rock classification also joins the soil and mineral chapters, since Maharashtra's black regur soil is the weathering product of the Deccan basalt and since coal and petroleum occur in sedimentary formations, so a single three-class table with its properties and examples serves the rock, soil and mineral questions alike.

• Sedimentary rocks form by the deposition, compaction and cementation of weathered material and organic remains, and are therefore stratified.
• A single sedimentary bed may contain particles of many sizes cemented together, of which conglomerate is the clearest example.
• Fossils are found chiefly in sedimentary rocks, because heat and pressure destroy organic remains in igneous and metamorphic rocks.
• Sedimentary rocks are comparatively soft and porous, so they erode relatively quickly and can hold groundwater, coal, petroleum and natural gas.
• Hardness, massiveness, a crystalline texture and the absence of strata and fossils are characteristics of igneous rocks such as granite and basalt; the Deccan Trap basalt underlies most of Maharashtra.

• Missing the bolded 'not' and selecting the sentence that best describes sedimentary rock instead of the one that does not
• Attributing hardness and massiveness to sedimentary rock, when those are properties of igneous rock
• Being unsettled by a printing slip in an option, such as 'a single rocks layer', and treating an awkward sentence as a false one

Rock questions in MPSC papers turn on properties rather than on definitions and are set as negative items far more often than as positive ones, because the negative form lets the commission mix classes. Expect the same construction applied to igneous rock, with a fossil or a stratification property inserted, and to metamorphic rock, with a parent-rock pairing inserted. Companion questions ask for the metamorphic equivalent of a named rock, for the class to which a named rock belongs, or for the rock type in which coal and petroleum occur, so the three-class table with four or five properties and three examples in each row answers the whole family.

No directly related past PYQ was found.

• practice — not a real PYQ

  Fossils are found chiefly in sedimentary rocks rather than in igneous rocks for which of the following reasons ?

  • (a)Sedimentary rocks are older than igneous rocks
  • (b)Organic remains are buried and preserved with the sediment, whereas molten rock destroys them
  • (c)Sedimentary rocks are always found near the sea
  • (d)Igneous rocks are too soft to retain impressions

  Answer(b) Organic remains are buried and preserved with the sediment, whereas molten rock destroys them — plants and animals settle along with the sediment and are preserved as it is compacted into rock, while the heat of magma or lava, and the heat and pressure of metamorphism, obliterate such remains. Igneous rocks are in fact hard rather than soft.

• practice — not a real PYQ

  The metamorphic rock formed from limestone is which of the following ?

  • (a)Quartzite
  • (b)Slate
  • (c)Marble
  • (d)Gneiss

  Answer(c) Marble — limestone becomes marble under heat and pressure. The other standard pairs are sandstone to quartzite, shale to slate and granite to gneiss, and all four are worth learning together because MPSC asks the transformation in both directions.