NDA (I) 2018 General Ability Test · Question 100 of 150

Correct — B, hot and humid climate. Chemical weathering is a set of reactions — solution, carbonation, hydration, hydrolysis and oxidation — and reactions need two things, a reactant and warmth. Water is the reactant, since almost every one of those processes is either water attacking the mineral or water carrying the attacking agent; and heat is the accelerator, because reaction rates rise sharply with temperature. A hot, humid climate supplies both at once, in abundance and all year round, which is why the equatorial and monsoon tropics show weathering profiles running tens of metres deep and why laterite, the residue of intense leaching, is a tropical soil. Take either ingredient away and the rate falls off steeply.

• (a)cold and dry climate — The worst of both worlds for chemical action. There is little liquid water, since what there is stays frozen, and the low temperatures slow every reaction. Cold deserts and polar regions are dominated instead by mechanical breakup through freezing and thawing.
• (c)hot and dry climate — The heat is there but the water is not, and without moisture the reactions have nothing to work with. Hot deserts weather mechanically, through the daily swing between fierce heat and cold nights and through salt crystallising in cracks.
• (d)cold and humid climate — Moisture is available but the temperature holds the reactions back, and for much of the year the water is locked up as ice. Such regions are the classic setting for frost shattering, a mechanical process, rather than for deep chemical decay.

Weathering is the breakdown of rock in place, as distinct from erosion, which carries the debris away. Mechanical or physical weathering breaks rock into smaller pieces without altering its composition — frost wedging, exfoliation, thermal expansion, salt crystallisation, the prising action of roots. Chemical weathering changes the minerals themselves: carbonation dissolves limestone as rainwater carrying dissolved carbon dioxide turns it to soluble bicarbonate, oxidation rusts the iron in rock, hydrolysis turns feldspar into clay, and hydration makes minerals swell as they take up water. The two families reinforce each other, since mechanical breakup exposes more surface for chemical attack.

The reliable way to answer any weathering question is to ask what each process needs and then read the climate off the options. Chemical weathering needs water and warmth, so it peaks in the hot wet tropics. Frost action needs water and a temperature that crosses freezing repeatedly, so it peaks in cold humid uplands and at high latitudes. Thermal and salt weathering need large temperature swings and dryness, so they peak in hot deserts. NDA has set questions from each of those three cells, including a 2022 item that keyed mechanical weathering as the dominant process in hot tropical deserts — the exact complement of this one. A useful corollary to carry is that the depth of soil and of the weathered layer tends to follow the same pattern, deepest in the humid tropics and thinnest in cold or arid regions.

• Chemical weathering includes solution, carbonation, hydration, hydrolysis and oxidation.
• It requires moisture and is accelerated by heat, so it is most intense in hot, humid climates.
• Deep weathering profiles and laterite soils are characteristic of the humid tropics.
• Cold regions and hot deserts are dominated by mechanical weathering — frost action in one, thermal and salt action in the other.
• Weathering breaks rock down in place; erosion is the removal of the resulting material.

• Assuming heat alone drives chemical weathering; without moisture the reactions cannot proceed.
• Confusing exfoliation, which is mechanical, with chemical processes because both are associated with hot regions.
• Treating weathering and erosion as the same thing; weathering acts in place, erosion removes.

NDA asks which climate favours a named type of weathering, which process is not chemical, or which agent produced a named landform.

• 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 and prising the rock apart is mechanical, not chemical.

• practice — not a real PYQ

  Deep laterite soils in India are chiefly the product of

  • (a)wind deposition in arid regions
  • (b)intense chemical weathering and leaching in a hot, wet climate
  • (c)glacial grinding of bedrock
  • (d)the deposition of river silt on floodplains

  Answer(b) intense chemical weathering and leaching in a hot, wet climate — which strips the soluble bases and leaves iron and aluminium oxides behind.