CDS (I) 2021 General Knowledge · Question 54 of 120

Correct — C, Granite. Granite forms when magma cools slowly deep inside the crust, which gives its crystals of quartz, feldspar and mica time to grow large enough to be seen with the naked eye — the coarse-grained texture that marks a plutonic or intrusive igneous rock. Basalt is its extrusive counterpart, the same broad family of melt cooled fast at the surface and therefore fine-grained, as in the Deccan Traps. The other three options are one from each of the remaining possibilities: marble is metamorphic, halite and shale are sedimentary, and between them they cover the ways a rock can form without ever having been molten.

• (a)Marble — Metamorphic. Marble is limestone recrystallised by heat and pressure, so it is a made-over rock rather than a solidified melt.
• (b)Halite — Sedimentary, and of the chemical rather than the clastic kind. Halite is rock salt, precipitated when a body of saline water evaporates, and it is classed as an evaporite.
• (d)Shale — Sedimentary and clastic. Shale is compacted clay and silt, laid down in still water and splitting readily along its bedding planes. Heat and pressure turn it into slate, which is metamorphic.

Rocks fall into three families defined by how they form. Igneous rocks solidify from molten material: intrusive or plutonic if the melt cools slowly at depth, giving coarse crystals as in granite and gabbro; extrusive or volcanic if it cools quickly at the surface, giving fine crystals as in basalt. Sedimentary rocks form from accumulated material — mechanically from fragments, as in sandstone and shale; chemically from precipitation, as in halite and gypsum; organically from remains, as in coal and some limestones. Metamorphic rocks are pre-existing rocks recrystallised by heat and pressure. The rock cycle connects all three, since any of them can be melted, weathered or metamorphosed into another.

The efficient preparation for this recurring question type is a set of transformation pairs rather than a list of rock names, because papers ask them in both directions. Limestone becomes marble; shale becomes slate and then schist; sandstone becomes quartzite; granite becomes gneiss; coal becomes graphite and then diamond under extreme conditions. Once those pairs are held, a rock name in an option carries its family with it. Granite is worth two extra facts because it turns up so often: it weathers into the rounded boulder stacks called tors, and it is the parent rock of the red soils that develop on the crystalline Deccan. Note also that the same word can straddle two families in loose usage — gneissoid granite is described as both igneous and metamorphic — but plain granite is unambiguously igneous.

• Granite is an intrusive or plutonic igneous rock, formed by the slow cooling of magma at depth, and is composed chiefly of quartz, feldspar and mica.
• Basalt is the extrusive counterpart of granite, cooled quickly at the surface and therefore fine-grained; the Deccan Traps are basalt.
• Marble is metamorphosed limestone, and slate is metamorphosed shale.
• Halite is rock salt, an evaporite formed by the precipitation of salts as saline water dries up.
• Shale is a clastic sedimentary rock of compacted clay and silt that splits along its bedding planes.

• Classing marble as igneous because it is hard, crystalline and used in monuments; hardness says nothing about origin.
• Assuming any salt or mineral name must be igneous; halite is an evaporite and therefore sedimentary.
• Confusing shale with slate; shale is the sedimentary parent, slate the metamorphic product.

Asked in both directions — which one is igneous, and which one is not igneous — so it pays to know the family of every rock name that appears, not just the answer.

• practice — not a real PYQ

  Marble is formed by the metamorphism of which rock?

  • (a)Shale
  • (b)Limestone
  • (c)Sandstone
  • (d)Granite

  Answer(b) Limestone — heat and pressure recrystallise limestone into marble, just as they turn shale into slate and sandstone into quartzite.

• practice — not a real PYQ

  Basalt differs from granite chiefly in that basalt

  • (a)is a sedimentary rock
  • (b)cooled rapidly at the surface and is therefore fine-grained
  • (c)contains no silica
  • (d)was formed by metamorphism of shale

  Answer(b) cooled rapidly at the surface and is therefore fine-grained — basalt is extrusive igneous, granite intrusive.