NDA Geography · Earth's Structure, Landforms and Geological Time

Weathering and Denudation

Weathering breaks rock down in place — physically by stress, chemically by reaction with water and air — and together with erosion and transport it wears the land down, a process called denudation.

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Why this matters

10 PYQs and a GUARANTEED-MARKS pocket — 0% HARD over ten years. Almost every question is one of two kinds: sort a process as mechanical vs chemical, or spot the odd one out. Lock the two lists (mechanical processes; chemical processes) and the climate rule (chemical loves hot+humid, mechanical loves hot+dry), and this subtopic is free.

Concept 1 of 3: Weathering, erosion and denudation

Three words are easy to mix up. WEATHERING is the breakdown of rock where it sits — no movement. EROSION is the wearing away AND removal of that broken material by an agent (river, wind, ice). DENUDATION is the umbrella term: the whole wearing-down of the land by weathering + mass wasting + erosion + transport. These are EXOGENIC (external, surface) processes, the opposite of the endogenic forces that build the land up.

Definition

Weathering — in-place breakdown of rock (no transport).
Mass wasting — downslope movement of rock/soil under gravity.
Erosion — wearing away + removal by a moving agent (water, wind, ice).
Denudation — the overall lowering of the land: weathering + mass wasting + erosion + transportation taken together. It is an exogenic (external) process.

Worked example

Weathering, mass wasting, erosion and transportation together indicate which overall process?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2022 · Q139Easy

Example 1 · Earth's Structure, Landforms and Geological Time · Weathering and Denudation

Weathering, mass wasting, erosion and transportation are indicators of which one of the following processes?

Drill 1 more on weathering, erosion and denudation

Concept 2 of 3: Mechanical (physical) weathering

Mechanical weathering shatters rock WITHOUT changing its chemistry — pure physical stress. Water freezing in cracks wedges them open; salt crystals growing in pores push grains apart; daily heating and cooling makes outer layers peel off (exfoliation). It dominates where chemistry is sluggish — hot DRY deserts and cold climates.

Definition

Physical processes (no chemical change):

Frost wedging — water freezes in cracks and expands.
Salt-crystal growth — salts crystallise in pores and prise rock apart (a PHYSICAL process).
Thermal expansion / exfoliation — repeated heating/cooling flakes off outer shells.
Driving forces: gravity, expansion force, and water-pressure force.
Mechanical weathering is most prevalent in hot dry deserts (and cold regions), where chemical reactions are slow. Granite hills weather into rounded tors.

Worked example

Which type of weathering dominates in a hot tropical desert, and why?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2022 · Q134Moderate

Example 2 · Earth's Structure, Landforms and Geological Time · Weathering and Denudation

Which one of the following is likely to be the most prevalent form of weathering in hot-tropical desert areas?

Salt-crystal growth is PHYSICAL

Because it involves salt, students label salt-crystal growth 'chemical'. It is physical (mechanical) — the salt crystals exert a mechanical force; they don't react with the rock minerals.

Drill 4 more on mechanical (physical) weathering

Concept 3 of 3: Chemical weathering

Chemical weathering ROTS the rock — water, oxygen and acids react with the minerals and change them into new substances. Solution dissolves minerals away; carbonation eats limestone; hydration makes minerals swell; oxidation rusts iron and reddens the rock. It thrives where it is hot AND wet, because warmth and water speed reactions. Watch the odd-one-out traps: thawing and exfoliation are PHYSICAL, not chemical.

Definition

Chemical processes (mineral change):

Solution — soluble minerals dissolve, leaching them away.
Carbonation — carbonic acid attacks carbonates/feldspar.
Hydration — minerals absorb water, swell and change (increases rock volume).
Oxidation — oxygen reacts with iron, changing the rock's colour (reddening).
Chemical weathering is strongest in a hot + humid climate.
NOT chemical: thawing, exfoliation, frost action (these are physical).

Process	What it does
Solution	Dissolves and leaches minerals
Carbonation	Acid attacks limestone / feldspar
Hydration	Minerals absorb water, swell, increase volume NDA 2021 — absorbing water and expanding is hydration.
Oxidation	Oxygen reddens iron-bearing rock

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q111Moderate

Example 3 · Earth's Structure, Landforms and Geological Time · Weathering and Denudation

The process whereby certain minerals absorb water, expand and change is called as

Thawing and exfoliation are NOT chemical

Odd-one-out questions slip thawing or exfoliation into a chemical-weathering list. Both are PHYSICAL. The genuinely chemical processes are solution, carbonation, hydration and oxidation.

Drill 4 more on chemical weathering

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Chemical weathering4 rows

Process	What it does
Solution	Dissolves and leaches minerals
Carbonation	Acid attacks limestone / feldspar
Hydration	Minerals absorb water, swell, increase volume NDA 2021 — absorbing water and expanding is hydration.
Oxidation	Oxygen reddens iron-bearing rock

Watch out for (2)

Salt-crystal growth is PHYSICAL
→ Mechanical (physical) weathering
Thawing and exfoliation are NOT chemical
→ Chemical weathering

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