CDS (II) 2024 General Knowledge · Question 82 of 120

Correct — C, Salt Crystal Growth. Chemical weathering decomposes a rock by altering the minerals in it, and NCERT's list of those processes is short and fixed: solution, carbonation, hydration, oxidation and reduction. Physical or mechanical weathering breaks a rock apart without changing its chemistry, and NCERT groups its causes as gravitational forces such as overburden pressure and shearing stress, expansion forces due to temperature changes, crystal growth or animal activity, and water pressures from wetting and drying cycles. Crystal growth sits in that second list. When saline water seeps into pores and cracks and then evaporates, the salt crystallises, and the growing crystals push outward on the pore walls hard enough to prise grains apart. Nothing about the rock's mineralogy changes; the rock simply comes to pieces. The other three options are all chemical. Acid action is the dissolving of rock by acids in rain and soil water, hydrolysis is a reaction with water that converts feldspars to clay, and oxidation is the reaction with oxygen that reddens iron-bearing minerals and leaves them weaker.

• (a)Acid action — Chemical. Rainwater carries dissolved carbon dioxide and soil water carries humic and carbonic acids, which dissolve limestone and attack other minerals.
• (b)Hydrolysis — Chemical. Water reacts with silicate minerals, breaking feldspar down into clay minerals — the reaction behind deep tropical weathering profiles.
• (d)Oxidation — Chemical. Oxygen dissolved in water reacts with iron in the rock to form oxides, which is why weathered rock surfaces turn red or brown and crumble more easily.

Weathering is the breakdown of rock in place, without transport. It comes in three groups. Chemical weathering decomposes minerals through solution, carbonation, hydration, oxidation and reduction, and needs water, air and heat to work. Physical weathering shatters rock through applied forces — the release of overburden pressure as rock above is stripped away, expansion and contraction under temperature change, growth of salt crystals in pores, and wetting-and-drying pressures. Biological weathering covers what plants and animals do, from earthworms and termites exposing new surfaces to roots wedging rock apart and decaying matter producing acids. NCERT notes that a dominance of one process can usually be seen, though rarely does one operate entirely alone.

The test is whether the mineral is changed or only the block. Salt crystallisation is the classic borderline case, since it involves a dissolved salt and therefore looks chemical, but the salt does not react with the rock — it simply crystallises in a pore and levers the grains apart, so the process belongs with frost wedging and thermal expansion. This is also the process behind the honeycombed rock faces called tafoni that are common on coasts and in deserts. Exfoliation, the flaking of curved shells from a rock surface, is another effect NCERT attributes to unloading, thermal expansion and salt weathering, and it is worth noting that exfoliation is described there as a result rather than a process in its own right.

• NCERT lists the chemical weathering processes as solution, carbonation, hydration, oxidation and reduction.
• Among the applied forces of physical weathering NCERT names expansion forces due to temperature changes, crystal growth or animal activity.
• Salt crystallising in pores exerts outward pressure on the pore walls and prises grains apart without altering the rock's minerals.
• Chemical weathering needs water and air together with heat, and decaying plants and animals raise the carbon dioxide content of soil water.
• Exfoliation is described as a result of unloading, thermal contraction and expansion and salt weathering rather than as a separate process.

• Classing salt weathering as chemical because a dissolved salt is involved.
• Treating exfoliation as a process; it is the outcome of several.
• Assuming chemical weathering stops in cold or dry climates; it slows but does not vanish.

As an odd-one-out among weathering processes, or as a match between a process and the chemical or mechanical group it belongs to.

• practice — not a real PYQ

  Which weathering process turns feldspar into clay minerals?

  • (a)Hydrolysis
  • (b)Frost wedging
  • (c)Exfoliation
  • (d)Salt crystallisation

  Answer(a) Hydrolysis — a chemical reaction with water that breaks silicate minerals down and produces the clays of deep weathering profiles.

• practice — not a real PYQ

  The flaking off of curved shells of rock from a bare rock surface, leaving it smooth and rounded, is known as which of the following?

  • (a)Exfoliation
  • (b)Carbonation
  • (c)Solifluction
  • (d)Deflation

  Answer(a) Exfoliation — a result of unloading, of thermal expansion and contraction, and of salt weathering rather than a process of its own.