Which of the following statements with reference to Salt Weathering is/are correct? 1. Salt crystallisation is the most effective of all the salt weathering processes. 2. Salt crystal growth is more effective over sedimentary rocks as compared to igneous rocks. Select the answer using the code given below:

Correct — C, Both 1 and 2. Salt attacks rock in three ways — it expands when heated, it expands when it takes up water, and it expands as it crystallises out of solution in a pore. Of those three, crystal growth does the most damage, because a salt crystal growing in a confined pore exerts pressure directly on the grains around it and pushes them apart; that is statement 1. Statement 2 follows from where salt can get in. Crystallisation needs a solution to soak into the rock first, so the more porous and permeable the rock, the more salt it can hold and the more crystals can grow inside it. Sedimentary rocks — chalk, limestone, sandstone, shale — are porous, cemented aggregates of grains; igneous rocks such as granite and basalt are dense interlocking crystal mosaics with almost no pore space. The standard textbook ranking makes exactly this point: with salt crystal growth chalk breaks down most readily, then limestone, sandstone and shale, with gneiss and granite last.

• (a)1 only — Rejects the rock-type ranking, which is well established: porous sedimentary rocks take up the salt solution that crystal growth needs, while dense igneous rocks resist it and sit at the bottom of the damage order.
• (b)2 only — Rejects statement 1, but crystallisation is indeed the most effective of the salt-weathering processes; thermal expansion and hydration of salts act alongside it and do less work.
• (d)Neither 1 nor 2 — Both are correct. This option would only fit if salt weathering acted mainly by chemical decomposition, which it does not — it is a physical process that levers grains apart.

Salt weathering is a physical, not a chemical, process: the salt does not react with the rock, it simply grows inside it and forces the grains apart. Salt-laden water seeps into pores and joints, evaporates, and leaves crystals that go on drawing more solution in and enlarging. Individual grains are split off, giving granular disintegration, and in extreme cases the honeycomb hollows and hollowed-out cavities seen on desert and coastal rock faces. High desert surface temperatures, roughly 30 to 50 degrees Celsius, and alternating wetting and drying favour it.

Two habits of thought can mislead here. The first is to file anything involving salts and water under chemical weathering; salt weathering is squarely physical, and older UPSC-family papers have tested exactly that. The second is to expect hard rock to fail first. Hardness is not what matters — porosity is. Chalk is soft and extremely porous and disintegrates fastest; granite is hard and effectively non-porous and lasts longest. The same logic explains why salt damage is so visible on sandstone monuments and on limestone buildings near the sea while a granite plinth beside them stays clean.

• Salt weathering acts through three routes — thermal expansion of the salt, hydration, and crystallisation — and crystallisation is the most effective of them.
• Salt crystals growing in near-surface pores split individual grains off the rock, producing granular disintegration.
• Salt crystal growth is favoured in areas of alternating wetting and drying, and in deserts sodium chloride and gypsum crystals can heave up the overlying layer until polygonal cracks form across the surface.
• The order in which rocks break down under salt crystal growth runs chalk, limestone, sandstone, shale, gneiss and then granite — the porous sedimentary rocks first, the crystalline rocks last.
• Surface temperatures of about 30 to 50 degrees Celsius, common in deserts, favour salt expansion.

• Classing salt weathering as chemical because a solution is involved — it is a physical process.
• Assuming the hardest rock resists best; the property that decides salt damage is porosity, so soft chalk fails first and hard granite last.
• Confusing salt crystal growth with hydration, which is a separate and weaker route by which salts expand.

Either as a two-statement item on which salt-weathering process dominates and where, or as a 'which of these is not chemical weathering' item in which salt crystal growth is the odd one out.

• practice — not a real PYQ

  Under salt crystal growth, which one among the following rocks is likely to break down most readily?

  • (a)Granite
  • (b)Gneiss
  • (c)Chalk
  • (d)Basalt

  Answer(c) Chalk — the standard order of breakdown under salt crystal growth runs chalk, limestone, sandstone, shale, gneiss and granite, because porosity, not hardness, decides how much salt solution the rock can take in.

• practice — not a real PYQ

  Salt weathering of rocks is best classified under which one of the following?

  • (a)Chemical weathering
  • (b)Physical weathering
  • (c)Biological weathering
  • (d)Mass movement

  Answer(b) Physical weathering — the salt does not react with the rock; growing crystals exert pressure that levers the grains apart.