MPSC 2023 Prelims Paper-I · Question 23 of 100

Correct — option (1), 'only a is correct'. Read the stem before the statements: it asks which of the printed reasons are responsible for the OCCURRENCE of reserves of limestone, gypsum and mineral salts. A reason for the occurrence of a mineral reserve has to be something that put the mineral there, and only statement a is of that kind. Limestone is a sedimentary rock built overwhelmingly of calcium carbonate, laid down on a sea floor from the calcareous remains of marine organisms and from the chemical precipitation of lime out of sea water. Gypsum is calcium sulphate, an evaporite: it crystallises out when a body of saline water evaporates, which is why gypsum beds mark former shallow seas, lagoons and salt flats. Mineral salts are of the same family. All three, in other words, are the residue of sea water, and finding them under a desert is an argument that the desert used to be sea floor. That is exactly what statement a asserts when it says the western part of Rajasthan was submerged under sea water — the paper attaches the Permo-Carboniferous to it, but the force of the statement lies in the submergence, not in the date. Statements b, c and d are all perfectly true descriptions of western Rajasthan today: it does receive low rainfall, it does have a hot dry climate, and its vegetation cover is very thin. None of them is a reason for the reserves. Rainfall figures and temperatures of the present day cannot deposit a rock; the beds were laid down over geological time, long before the present climate existed. The aridity is worth one qualification: it does help concentrate salt at the surface in the playa lakes of the region, since evaporation leaves behind what the water carried. But concentrating salt is not the same as creating it, and the salt has to be present in the rocks and the groundwater before evaporation can gather it — which returns the explanation to the marine origin in statement a. Only statement a survives, so option (1).

• (2)only a and b are correct — This option keeps statement a and adds statement b, low rainfall. Statement b is true — the extreme west of Rajasthan is among the driest tracts in India, because the Aravalli range runs roughly parallel to the Arabian Sea branch of the south-west monsoon instead of standing across it, and so never forces those winds to rise and shed their moisture, while the Bay of Bengal branch is by then far away and spent. But truth is not the same as relevance, and this is the commonest way a 'which of the following reasons' item is failed: the candidate checks each statement for accuracy, finds it accurate, and includes it. The test the stem actually sets is causal. Ask of every statement whether it could have put the limestone in the ground, and low rainfall in the present day plainly could not, because the beds were deposited over geological time under an entirely different set of conditions.
• (3)a, b and c are correct — Here statements b and c are both added, and c — a hot and dry climate — fails for exactly the same reason as b, only more visibly, since it is a restatement of the same aridity in terms of temperature. A hot dry climate produces high evaporation, thin soils, salt crusts on playa floors and sparse vegetation; it does not produce beds of limestone and gypsum, which are marine sediments. It is worth noticing how the option set is built: options (1), (2) and (3) form a ladder of a, then a and b, then a, b and c, each rung adding one more true-but-irrelevant statement. A ladder of this shape is the commission's standard way of finding out whether a candidate stops at the statement that answers the question asked or keeps climbing because every statement reads correctly.
• (4)only d is correct — This option is the only one that discards statement a, and with it the sole statement that offers a cause at all. Statement d, that the area under vegetation is very low, is itself a consequence of the aridity described in statements b and c rather than an independent fact, which places it two steps away from anything geological. There is one honest observation buried in it: thin vegetation and bare rock make mineral beds far easier to find, map and quarry, so sparse cover has a real bearing on how a deposit is exploited. But the stem asks why the reserves are distributed there, not why they are convenient to work, and a candidate who accepts the substitution has answered a different question from the one printed.

Mineral occurrence is controlled by geology, not by present-day weather, and the two are separated by an enormous span of time. Limestone, gypsum and rock salt belong to the sedimentary family, and specifically to the group that forms in and from water. Limestone accumulates on a sea floor, partly from the shells and skeletons of marine organisms and partly by direct chemical precipitation of calcium carbonate. Gypsum and rock salt are evaporites: when a shallow sea, a lagoon or an enclosed basin dries out, the dissolved salts crystallise in a predictable order as the water concentrates, with the less soluble compounds such as calcium sulphate coming out before the more soluble ones such as sodium chloride, so a thick evaporite sequence records repeated flooding and drying of a shallow marine basin. Because all of these require sea water, their presence in a landlocked region is direct evidence of a marine transgression in the geological past, and geologists read such beds, along with the marine fossils in them, as the record of a vanished sea. The Thar is one of the classic Indian examples: an arid surface today, but with marine sedimentary sequences beneath it. The distinction the question turns on is therefore between formative conditions, which are ancient and geological, and present conditions, which describe the landscape a candidate can see but explain nothing about what lies beneath it.

The commission builds a whole family of questions on the difference between a correlation and a cause, and this is one of the cleanest examples in the paper. Four statements are printed; all four are true; only one answers the question. A candidate working through them as a truth-checking exercise will mark every statement correct and then choose the option that includes the most of them, which is exactly what options (2) and (3) are placed there to collect. The method that defeats this design is to write the stem's demand at the top of the rough sheet in three or four words — here 'why are the minerals THERE' — and then to test each statement against that demand rather than against the world. A second habit worth building for the geography section is to keep the two timescales apart. Questions about India's mineral belts, its rock systems and its physiography are answered from geological history; questions about its rainfall, its crops and its settlement patterns are answered from present conditions. A statement drawn from the wrong timescale is almost always a distractor, however impeccably true it may be.

• Limestone is a sedimentary rock composed mainly of calcium carbonate, formed on sea floors from the calcareous remains of marine organisms and from the chemical precipitation of lime; its presence far inland is evidence that the area once lay under the sea.
• Gypsum is calcium sulphate and rock salt is sodium chloride; both are evaporites, deposited when a body of saline water evaporates and its dissolved salts crystallise out in order of increasing solubility.
• Statement a of this question places western Rajasthan under the sea in the permocarboniferous period, as the paper spells it; whatever the exact age assigned, the argument the statement carries is that these minerals are of marine sedimentary origin.
• Western Rajasthan is arid largely because the Aravalli range lies roughly parallel to the Arabian Sea branch of the south-west monsoon and so does not force it to ascend, while the Bay of Bengal branch has largely exhausted itself before reaching the region.
• Sambhar Lake in Rajasthan is India's largest inland saline lake and a long-established source of common salt; evaporation in an arid climate concentrates the salt, but the salt itself is derived from the rocks and groundwater of the basin.

• Marking a statement as an answer because it is true, when the stem asks for a reason and truth without causal bearing is exactly what the distractor statements are built from
• Explaining a mineral deposit from present-day climate, when mineral occurrence is set by geological history and the two timescales have nothing to do with one another
• Climbing a nested option ladder of a, then a and b, then a, b and c, on the assumption that the fuller option must be safer
• Confusing a consequence of aridity, such as thin vegetation, with a cause of anything, when it is itself an effect two steps removed from the question asked

MPSC's physical and economic geography questions on minerals are asked in three recurring shapes. The first is straight recall of which state or region leads in a named mineral. The second is a pairing of mineral with rock system or with geological period. The third, which this question uses, prints a set of statements and asks which of them explain an observed distribution, and it is the one that separates candidates, because it cannot be answered by memory alone. The statements offered are usually a mixture of one geological cause with two or three accurate descriptions of the region's present climate, vegetation or economy, and the discipline required is to ask of each statement whether it could have produced the thing being explained. Expect the option set to be nested rather than independent, so that the difference between two options is a single statement, and settle that statement before reading any further.

No directly related past PYQ was found.

• practice — not a real PYQ

  Gypsum, which is chemically calcium sulphate, is formed mainly by which of the following processes ?

  • (a)Evaporation of a body of saline water
  • (b)Cooling and solidification of molten magma
  • (c)Metamorphism of limestone under heat and pressure
  • (d)Chemical weathering of granite in humid conditions

  Answer(a) Evaporation of a body of saline water — gypsum is an evaporite, crystallising out as a shallow sea, lagoon or enclosed basin dries and its dissolved salts concentrate. Cooling magma yields igneous rocks such as granite and basalt; the metamorphism of limestone yields marble, not gypsum; and the chemical weathering of granite yields clay minerals rather than a sulphate deposit.

• practice — not a real PYQ

  The extreme western part of Rajasthan receives very little rain from the Arabian Sea branch of the south-west monsoon mainly because of which of the following ?

  • (a)The Aravalli range lies roughly parallel to the direction of these winds
  • (b)The region lies in the rain shadow of the Western Ghats
  • (c)The Arabian Sea branch does not cross the Gujarat coast at all
  • (d)The region lies too far north to be reached by any monsoon current

  Answer(a) The Aravalli range lies roughly parallel to the direction of these winds — a range set across a moist current forces it to rise and shed rain, but one aligned along the current merely lets it pass, so the Arabian Sea branch crosses western Rajasthan without being lifted. The rain shadow of the Western Ghats accounts for the dryness of the interior Deccan rather than of the Thar, and the region is reached by the monsoon current; it is simply not made to rise.