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

Correct — B, Salt crystal growth. Chemical weathering changes the mineral itself, leaving a substance different from the one the rock started with. Salt crystallisation does nothing of the kind: salty water seeps into pores and joints, evaporates, and the growing crystals push outward with enough force to prise grains apart. The rock is broken but not altered, so this belongs with frost shattering, exfoliation and thermal expansion under physical weathering — haloclasty is the technical name. The other three change chemistry. Carbonation dissolves limestone as rainwater charged with carbon dioxide turns calcium carbonate into soluble calcium bicarbonate. Hydrolysis breaks feldspar down into clay minerals. Oxidation attacks iron-bearing minerals and leaves the reddish-brown stain of iron oxide.

• (a)Carbonation solution — The reaction that carves karst country. Rainwater plus carbon dioxide makes weak carbonic acid, which converts limestone into a bicarbonate that the water carries away — chemical by definition, and responsible for caves, sinkholes and poljes.
• (c)Hydrolysis — Water reacts with silicate minerals and replaces cations in the crystal lattice, so feldspar in granite becomes kaolinite clay. The product is a wholly new mineral, which is the test of chemical weathering.
• (d)Oxidation — Oxygen dissolved in water combines with iron in minerals such as olivine and pyroxene to form iron oxides. The rusty red of laterite and of weathered basalt is the visible result.

Weathering breaks rock in place, without transport. Physical weathering fragments it — frost wedging, salt crystallisation, thermal expansion and contraction, unloading and exfoliation, and the pressure of roots. Chemical weathering alters it — solution, carbonation, hydration, hydrolysis and oxidation with its reverse, reduction. Biological weathering runs through both, since roots split rock physically while lichens attack it chemically.

Climate decides which process dominates, and that is often the second half of an examination question. Chemical weathering runs fastest where it is hot and wet, which is why the humid tropics carry deep weathered profiles and laterite. Physical weathering dominates where water freezes and thaws or where salt is abundant and evaporation is strong, so deserts and cold high mountains are its home ground. Salt weathering sits at the boundary in a way worth remembering: the salt arrives in solution, a chemical route, but the damage it does is mechanical.

• Chemical weathering alters the mineral; physical weathering only breaks the rock.
• Salt crystal growth, or haloclasty, is physical — crystallising salts exert outward pressure in pores and joints.
• Carbonation converts limestone to soluble calcium bicarbonate and produces karst landforms.
• Hydrolysis turns feldspar into clay minerals such as kaolinite.
• Oxidation of iron-bearing minerals gives the red and brown colours of weathered rock and of laterite.

• Classing salt weathering as chemical because salt arrives dissolved in water.
• Confusing weathering, which happens in place, with erosion, which involves transport.
• Assuming chemical weathering needs high temperature alone; moisture matters at least as much.

An odd-one-out item across the weathering taxonomy. Ask of each process whether the mineral survives it.

• practice — not a real PYQ

  The weathering process that converts feldspar into clay minerals is

  • (a)oxidation
  • (b)hydrolysis
  • (c)carbonation
  • (d)exfoliation

  Answer(b) hydrolysis — water reacts with the silicate lattice and leaves kaolinite.

• practice — not a real PYQ

  Frost shattering is most effective in regions where

  • (a)rainfall is heavy and temperatures are uniformly high
  • (b)temperatures fluctuate about the freezing point
  • (c)the air is dry and salt is abundant
  • (d)rocks are rich in iron

  Answer(b) temperatures fluctuate about the freezing point — repeated freezing and thawing widens joints.