CDS (II) 2022 General Knowledge · Question 45 of 120

Correct — D, Geyserites. Sedimentary rocks are sorted by how the sediment came to be there, and the three routes are mechanical, organic and chemical. Mechanically formed rocks are made of fragments of older rock that were broken off, carried by water, wind or ice, dropped and then compacted — the sediment is physically transported debris. Geyserite is not that. It is the pale crust of silica that forms around hot springs and geysers: the water underground is hot enough to dissolve silica out of the surrounding rock, and when it reaches the surface and cools the silica comes out of solution and is laid down as sinter, layer on layer, around the vent. Nothing was broken, carried or piled up. The rock is precipitated from solution, and the first three options are precisely the contrast — three obvious pieces of transported debris standing against one deposit left behind by evaporating and cooling water.

• (a)Sandstone — The textbook mechanical rock. It is sand — quartz grains chiefly — deposited by water or wind and cemented together, and the size of its grains is the definition of the class it belongs to.
• (b)Conglomerate — Mechanical, and even more obviously so. It is made of rounded pebbles and gravel set in a finer matrix; the rounding itself is the evidence of transport by running water. Its angular counterpart is breccia, where the fragments were not carried far enough to be smoothed.
• (c)Loess — Mechanical, and the one people hesitate over because it is so fine. Loess is silt carried by the wind and dropped in thick, unstratified blankets — the great deposits of northern China are the classic case. Fine grain size does not make a deposit chemical; wind is a transporting agent just as water is.

Sedimentary rocks cover about three-quarters of the Earth's land surface but make up only a small fraction of the crust by volume, because they form a thin skin over igneous and metamorphic basement. They are classified by origin into three groups. Mechanically formed, or clastic, rocks are compacted fragments of older rock — conglomerate, breccia, sandstone, shale, loess. Organically formed rocks are built from the remains of living things — chalk and many limestones from shells and skeletons, and coal from plant matter. Chemically formed rocks are precipitated from solution — rock salt, gypsum, potash, chert, and the siliceous sinter that geyserite is.

The question tests one boundary and gives away three of its four terms. Sandstone, conglomerate and loess are the three most quoted mechanical examples in any Indian textbook, so a candidate who can classify even two of them arrives at geyserite by elimination without knowing what geyserite is. That is the practical route, and it is worth taking. There is one honest complication behind the answer that a careful student should know. Some Indian textbooks, following an older convention, place geyserite in the organically formed group rather than the chemically formed one, whereas the geology of the deposit — silica dissolved in hot groundwater and precipitated as sinter when the water cools — is chemical, with microbial mats sometimes trapping the silica but not producing it. Either classification would answer this particular question the same way, because both are 'not mechanical', so the key is safe; but the divergence is real, and a question that asks directly whether geyserite is organic or chemical is on shaky ground. NDA set a version of exactly that question in 2025, offering the pair 'Geyserite — Organically formed', which follows the older textbook convention rather than the geology of siliceous sinter.

• Sedimentary rocks are classified by origin as mechanically formed, organically formed or chemically formed.
• Mechanically formed examples: conglomerate, breccia, sandstone, shale, loess — all compacted fragments of older rock.
• Organically formed examples: chalk and many limestones, from shells and skeletons, and coal, from plant matter.
• Chemically formed examples: rock salt, gypsum, potash, chert, and geyserite or siliceous sinter.
• Geyserite forms where silica-rich hot spring water cools at the surface and deposits silica around the vent.
• Conglomerate has rounded fragments, showing transport by water; breccia has angular ones, showing little transport.

• Assuming fine-grained means chemically formed; loess is silt carried by wind and is mechanical.
• Forgetting that limestone appears in more than one class, since it can be organic or chemical depending on how it formed.
• Treating the organic and chemical labels for geyserite as interchangeable in a question that asks for one of them specifically.

As an odd-one-out on the mode of formation, or as a rock-to-process match list.

• practice — not a real PYQ

  Which one of the following sedimentary rocks is formed by the accumulation of the remains of living organisms?

  • (a)Sandstone
  • (b)Chalk
  • (c)Rock salt
  • (d)Breccia

  Answer(b) Chalk — a limestone built from the calcareous skeletons of microscopic marine organisms.

• practice — not a real PYQ

  Conglomerate differs from breccia chiefly in that its fragments are

  • (a)angular
  • (b)rounded
  • (c)chemically precipitated
  • (d)of volcanic origin

  Answer(b) rounded — the rounding shows that the fragments travelled far enough in water to be worn smooth, whereas breccia's angular pieces did not.