NDA (II) 2017 General Ability Test · Question 145 of 150

Correct — C, dolomite. Dolomite, also called dolostone, is a sedimentary carbonate rock made up of more than half the mineral dolomite, calcium magnesium carbonate. Most of it formed by the alteration of limestone or lime mud, magnesium gradually replacing calcium in a process called dolomitisation, so it belongs to the same family as limestone and behaves like it, dissolving in weak acid and producing karst landforms. The other three all cooled and crystallised from molten rock, which is the definition of an igneous rock.

• (a)Gabbro — An intrusive igneous rock — the coarse-grained, slowly cooled equivalent of basalt, dark and rich in calcium-bearing feldspar and pyroxene. It is igneous, so it cannot be the answer.
• (b)Granite — The classic intrusive igneous rock, formed when magma crystallises slowly at depth, giving large visible crystals of quartz, feldspar and mica. Also igneous.
• (d)Basalt — An extrusive igneous rock, formed when lava cools quickly at the surface, which is why its grain is fine. The Deccan Traps are basalt, and their weathering gives India's black cotton soil. Igneous again.

Rocks are grouped by how they formed. Igneous rocks crystallise from magma or lava, and are called intrusive or plutonic if they cooled slowly at depth and extrusive or volcanic if they cooled quickly at the surface — granite and gabbro belong to the first, basalt and rhyolite to the second. Sedimentary rocks form at the surface from the weathering, transport and deposition of older material or from chemical and organic precipitation; sandstone, shale, limestone and dolomite are sedimentary. Metamorphic rocks are older rocks changed by heat and pressure — slate from shale, marble from limestone, quartzite from sandstone, gneiss from granite.

The efficient way to answer any odd-one-out on rock types is to look for the carbonate. Anything whose name suggests lime or a carbonate — limestone, dolomite, chalk, marble — is not igneous, and if it has also been altered by heat and pressure it is metamorphic rather than sedimentary. A second useful cue is grain: the igneous names in these questions are usually the familiar building and volcanic rocks, granite, basalt, gabbro, dolerite and rhyolite. Note that dolomite is the name of both a mineral and the rock made largely of it, and that under metamorphism it becomes dolomitic marble.

• Dolomite is a sedimentary carbonate rock, composed of more than half the mineral dolomite, CaMg(CO₃)₂.
• Most dolomite formed by magnesium replacing calcium in limestone or lime mud, a process called dolomitisation.
• Granite and gabbro are intrusive igneous rocks; basalt is extrusive.
• Gabbro is the coarse-grained equivalent of basalt, formed by slower cooling at depth.
• Weathering of Deccan basalt produces the black cotton or regur soil.
• Under heat and pressure limestone becomes marble and shale becomes slate.

• Assuming a hard, heavy rock must be igneous; dolomite and quartzite are not.
• Confusing gabbro with basalt — same composition, different cooling history and grain size.
• Treating dolomite as metamorphic; it is sedimentary, and its metamorphic form is dolomitic marble.

Rock classification comes as an odd-one-out item, as a rock-to-type match, or as a question on the landform an intrusion produces.

• practice — not a real PYQ

  The metamorphic rock formed from limestone is

  • (a)slate
  • (b)marble
  • (c)quartzite
  • (d)gneiss

  Answer(b) marble — slate comes from shale, quartzite from sandstone and gneiss from granite.

• practice — not a real PYQ

  Which one of the following is an extrusive igneous rock?

  • (a)Granite
  • (b)Gabbro
  • (c)Basalt
  • (d)Dolerite intrusion

  Answer(c) Basalt — it cools rapidly at the surface, which is why it is fine-grained.