Consider the following statements with reference to salt weathering : 1. Salts in rocks expand due to thermal action, hydration and crystallization. 2. Salts like calcium, sodium, magnesium and barium have a tendency to expand. 3. Salt weathering is a typical example of chemical weathering. Which of the statements given above is/are correct?

Correct — B, 1 and 2 only. Statements 1 and 2 are the textbook account. Salts held in the pores of a rock expand for three reasons — heating, taking up water as hydration, and growing as crystals when a solution evaporates — and the salts named, those of calcium, sodium, magnesium and barium, together with potassium, are the ones with the strongest tendency to do so. Statement 3 is the false one. Salt weathering is classed with the physical or mechanical processes, because the salt does the damage by forcing the pore walls apart; the mineral composition of the rock is not changed. Chemical weathering is a different list altogether — solution, carbonation, hydrolysis and oxidation, in which the rock minerals themselves react and become new compounds.

• (a)1 only — Drops a correct statement. Calcium, sodium, magnesium, potassium and barium salts are precisely the ones named as prone to expansion in the standard account of salt weathering.
• (c)2 and 3 only — Keeps the misclassification and discards the mechanism. Thermal action, hydration and crystallisation are the three ways salts expand, and together they define the process.
• (d)1, 2 and 3 — Would make salt weathering a chemical process. Hydration and crystallisation involve water, which makes the label tempting, but nothing in the rock's own mineralogy is altered — the work is mechanical.

Weathering divides into physical, chemical and biological. Physical processes break rock into smaller pieces without changing its composition: frost wedging, unloading and exfoliation, thermal expansion and contraction, and salt weathering. Chemical processes change the minerals themselves: solution, carbonation, hydration of minerals, hydrolysis and oxidation. Salt weathering sits on the border in a candidate's mind because it involves water and dissolved compounds, but the rock is prised apart, not transformed.

The test that settles any borderline case is simple — ask whether the rock's minerals are chemically different afterwards. Salt crystals growing in a pore push the walls apart and leave the sandstone still sandstone, so the process is mechanical. Salt weathering is at its most effective in hot deserts and along coasts, where evaporation concentrates salts near the surface, and it is the process behind honeycomb hollows in coastal cliffs and the flaking of stone monuments. Note also that the printed statement 2 omits potassium from the usual list; leaving a member out does not make a statement false.

• Salt weathering is a physical or mechanical process: salts expand by thermal action, hydration and crystallisation.
• Calcium, sodium, magnesium, potassium and barium salts are the ones usually named as prone to expansion.
• Chemical weathering covers solution, carbonation, hydration of minerals, hydrolysis and oxidation.
• The test for physical weathering is that the rock's mineral composition is unchanged after it.
• Salt weathering is most effective in hot deserts and coastal zones, where evaporation concentrates salts.

• Classing salt weathering as chemical because it involves dissolved salts and water.
• Rejecting statement 2 because potassium is missing from the list.
• Confusing hydration of a salt with hydrolysis of a silicate mineral.

A three-statement item where two sentences are quoted description and the third misfiles the process.

• practice — not a real PYQ

  Which one of the following is not a process of chemical weathering?

  • (a)Carbonation
  • (b)Oxidation
  • (c)Frost wedging
  • (d)Hydrolysis

  Answer(c) Frost wedging — water freezing in cracks breaks rock apart mechanically.

• practice — not a real PYQ

  Salt weathering is most effective in which environments?

  • (a)Equatorial rainforests
  • (b)Hot deserts and coastal zones
  • (c)Glaciated valleys
  • (d)Deep ocean floors

  Answer(b) Hot deserts and coastal zones — high evaporation concentrates salts in surface pores.