Which of the following is/are the applied forces in mechanical weathering process? 1. Gravitational force; 2. Expansion force; 3. Force due to water pressure.
- (a)1 only
- (b)1 and 2 only
- (c)2 and 3 only
- (d)1, 2 and 3
Correct — D, 1, 2 and 3 (all three). In physical (mechanical) weathering, rock is broken by applied physical stresses without any change in its chemical make-up. Standard physical geography lists three kinds of applied force: gravitational forces (the load or overburden pressure), expansion forces (from temperature change and the growth of ice or salt crystals), and water pressures (from wetting-and-drying cycles). All three act in mechanical weathering, so the answer is 1, 2 and 3.
- (a)1 only — Gravitational (load) force is one agent, but not the only one. Expansion forces and water pressures also drive mechanical weathering, so restricting it to gravity alone leaves out two genuine forces.
- (b)1 and 2 only — This adds expansion force but omits water pressure. Wetting-and-drying cycles set up pressures inside rock that help shatter it, so force due to water pressure must be included.
- (c)2 and 3 only — This drops gravitational (overburden) force, which is a listed applied force in mechanical weathering. Removal of overlying load lets rock expand and crack, so gravity belongs on the list.
Weathering is the in-place breakdown of rock. Mechanical (physical) weathering disintegrates rock into smaller pieces without altering its chemistry, unlike chemical weathering which changes mineral composition. It is driven by physical stresses applied to the rock.
The three options each name a genuine physical force, so the task is to recognise that mechanical weathering has several drivers, not one. Gravitational load (unloading/overburden), expansion from heat and crystal growth, and water-pressure changes from wetting and drying all apply, giving all three, option (d).
- Mechanical weathering breaks rock physically without changing its chemistry.
- Gravitational (overburden/unloading) force is one applied force.
- Expansion forces come from temperature change and ice or salt crystal growth.
- Water-pressure changes from wetting-and-drying cycles also apply force.
- Thinking mechanical weathering has a single cause.
- Confusing mechanical weathering with chemical weathering (which changes composition).
- Ignoring water-pressure and unloading forces as physical agents.
NDA/UPSC list agents or forces and ask which belong to mechanical/physical weathering — the physical stresses (load, expansion, water pressure) all qualify, chemical changes do not.
Consider the following statements: Statement-I: Rainfall is one of the reasons for weathering of rocks. Statement-II: Rain water contains carbon dioxide in solution. Statement-III: Rain water contains atmospheric oxygen. Which one of the following is correct in respect of the above statements?
- (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
- (b) Both Statement-II and Statement-III are correct, but only one of them explains Statement-I
- (c) Only one of the Statements II and III is correct and that explains Statement-I
- (d) Neither Statement-II nor Statement-III is correct
Answer(a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
The wider weathering theme. It probes how rainfall weathers rock (dissolved CO2 and O2), a companion to this NDA question on the physical forces behind mechanical weathering.
Which one among the following processes is NOT part of chemical weathering?
- (a) Solution
- (b) Hydration
- (c) Reduction
- (d) Thawing
Answer(d) Thawing
The exact physical-versus-chemical weathering distinction. There thawing is singled out as mechanical, not chemical; here the question asks which physical forces drive that same mechanical weathering.
- practice — not a real PYQ
Which one of the following is a process of physical (mechanical) weathering?
- (a)Oxidation
- (b)Carbonation
- (c)Frost action
- (d)Hydrolysis
Answer(c) Frost action — freeze-thaw physically shatters rock; oxidation, carbonation and hydrolysis are chemical weathering.
- practice — not a real PYQ
Mechanical weathering differs from chemical weathering in that it
- (a)changes the mineral composition of rock
- (b)breaks rock without changing its chemistry
- (c)only occurs underwater
- (d)requires living organisms
Answer(b) breaks rock without changing its chemistry — mechanical weathering is purely physical disintegration.