CDS Geography · Earth's Structure, Landforms and Geological Time
Weathering, Mass Movement and Soils
How rock breaks down where it stands, how loose material slides downhill, and the soil orders that form from what is left.
Why this matters
Thirteen questions, six of them on soils. Histosols, entisols and vertisols have each been named in three different questions, so the soil orders table pays off more than any other here.
Concept 1 of 4: Physical weathering
Physical weathering breaks a rock into pieces without changing what it is made of. Something pushes the rock apart from inside: ice in a crack, salt crystals in a pore, or the rock's own swelling and shrinking as it heats and cools.
Definition
The main kinds:
- Frost weathering (freeze-thaw): water in cracks freezes, expands by about 9% and wedges the rock apart. It needs pores and joints, so compact, well consolidated rocks are least affected.
- Thermal stress: hot days and cold nights make the outer layer expand and shrink. In hot deserts this shatters rocks into blocks.
- Exfoliation: curved sheets peel off the surface like onion skins. Crystalline rocks such as granite are prone to it.
- Salt weathering: salts in pores grow, swell or take up water. Salt crystallisation is the most effective of these, and it attacks porous sedimentary rocks (sandstone, limestone) more than dense igneous rocks.
| Process | What pushes the rock apart | Where it is strongest |
|---|---|---|
| Frost weathering | Water freezing in cracks | Cold regions with many freeze-thaw days |
| Thermal stress and shattering | Daily heating and cooling | Hot deserts |
| Exfoliation | Outer layers expanding and peeling | Crystalline rocks such as granite |
| Salt crystallisation | Salt crystals growing in pores | Porous sedimentary rocks, dry climatesQ CDS 2025 (I): both statements, most effective process and stronger on sedimentary rock, were correct. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q70Moderate
Example 1 · Earth's Structure, Landforms and Geological Time · Weathering, Mass Movement and Soils
Consider the following statements about Mechanical Weathering :
1. More compact and highly consolidated rocks are least affected by freeze-thaw weathering.
2. Shattering of rocks is more common in hot deserts.
3. Crystalline rocks are often prone to exfoliation.
Which of the statements given above is/are correct?
Salt crystal growth is physical, not chemical
Growing salt crystals prise rock apart by pressure without changing its minerals, so salt crystal growth is physical weathering.
Concept 2 of 4: Chemical weathering
Chemical weathering changes the minerals themselves. Water, oxygen and weak acids react with the rock and turn its minerals into new, softer ones. It works fastest where it is warm and wet.
Definition
The processes NCERT names:
- Solution: a mineral dissolves in water, as rock salt does.
- Carbonation: carbonic acid (carbon dioxide in rainwater) attacks limestone and dissolves it. This is how caves form.
- Hydration: a mineral takes up water and expands. Anhydrite becomes gypsum. The swelling sets up stress in the rock and helps break it.
- Oxidation: oxygen combines with iron in the rock, as in rusting. The rock turns red or brown and crumbles.
- Hydrolysis: a mineral reacts chemically with water and breaks down into new substances.
- Acid action of all kinds is chemical. Salt crystal growth is not.
| Process | What happens | Example |
|---|---|---|
| Solution | Mineral dissolves in water | Rock salt |
| Carbonation | Carbonic acid dissolves calcium carbonate | Limestone caves |
| Hydration | Mineral takes up water and swells | Anhydrite to gypsum |
| Oxidation | Oxygen joins iron, like rusting | Red, crumbly iron-rich rock |
| Salt crystal growth | Physical pressure, no new mineral | Not chemical at allQ CDS 2024 (II): it was the odd one out among acid action, hydrolysis and oxidation. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q91Moderate
Example 2 · Earth's Structure, Landforms and Geological Time · Weathering, Mass Movement and Soils
Which one of the following processes leads to expansion of certain minerals as they take up water, causing additional stress in the rock ?
Hydration is not hydrolysis
In hydration a mineral takes water into itself and swells, as anhydrite does when it becomes gypsum. Hydrolysis is a different reaction in which the mineral breaks down. The swelling and stress belong to hydration.
Concept 3 of 4: Mass movement and soil erosion
Once rock is weathered, gravity pulls the loose material downhill. Some of it slides in seconds, some creeps so slowly you only see it in tilted posts. Running water then carries soil away, and on soft ground it cuts deep gullies.
Definition
From slow to fast:
- Creep: very slow movement of soil and loose rock down a gentle slope. Seen in tilted poles, bent tree trunks and leaning walls.
- Solifluction: slow flow of water-soaked soil, common where the ground is frozen below.
- Earthflow and mudflow: faster flows of wet soil and mud.
- Landslides: rapid falls or slides of rock and debris: slump, debris slide, rockslide, rockfall.
- Gully erosion: running water cuts deep channels in soft soil. The Chambal valley (Madhya Pradesh, Rajasthan, Uttar Pradesh) is India's best known example, with its ravines or badlands.
| Movement | Speed | Material |
|---|---|---|
| Creep | Very slow | Soil and loose rock |
| Solifluction | Slow | Water-soaked soil over frozen ground |
| Mudflow | Fast | Wet mud |
| Rockfall | Very fast | Rock falling free from a cliff |
| Gully erosion | Grows over years | Soft alluvial soilQ CDS 2019 (I): the Chambal valley was the one under intensive gully erosion. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q77Easy
Example 3 · Earth's Structure, Landforms and Geological Time · Weathering, Mass Movement and Soils
Which kind of mass wasting processes are commonly associated with creep ?
The Chambal, not the Kosi or Damodar, is the gully river
The Kosi and Damodar are known for floods. The Chambal valley is known for its deep ravines cut by gully erosion.
Concept 4 of 4: Soil orders and humus
Soil scientists sort soils into orders by how far they have developed and what they are rich in. A brand new soil has no layers at all; a very old tropical soil has been weathered almost to the bone. Learn each order by its one stand-out feature.
Definition
Key orders (the USDA system, also used by ICAR for Indian soils):
- Entisols: the youngest; no horizons (layers).
- Inceptisols: a step older; weakly developed horizons.
- Histosols: very rich in organic matter; the peaty soils of bogs and marshes.
- Oxisols: very old and highly weathered; humid tropics.
- Vertisols: rich in clay, highly basic; swell when wet and crack when dry (India's black soil).
- Mollisols: dark, fertile grassland soils. Aridisols: desert soils. Gelisols: soils with permafrost. Spodosols: acid forest soils of cool regions.
- Humus is the dark, amorphous remains of decayed plants and animals. Being colloidal, it holds nutrients and acts as their store.
- Soil impoverishment means the soil has become very deficient in plant nutrients.
| Order | Stand-out feature |
|---|---|
| Entisols | Youngest, no horizons |
| Inceptisols | Weakly developed horizonsQ |
| Histosols | Very high organic matter, peat and bogsQ CDS 2020 (I): the soil with very high organic matter was the histosol. |
| Oxisols | Very old, highly weathered, tropical |
| Vertisols | Clay-rich, basic, swell and crack |
| Mollisols | Dark, fertile grassland soilsQ CDS 2021 (II): inceptisols, entisols, vertisols and mollisols were named as orders of soil. |
| Gelisols | Permafrost within the soil |
| Humus | Dark, amorphous, colloidal store of nutrientsQ |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (II) 2019 — General Knowledge · Q103Hard
Example 4 · Earth's Structure, Landforms and Geological Time · Weathering, Mass Movement and Soils
Match List I with List II and select the correct answer using the code given below the lists :
Code :
| List I (Soil type) | List II (Major characteristic) |
|---|---|
| A. Oxisols | 1. Very rich in organic matter |
| B. Vertisols | 2. Soil lacking horizons |
| C. Histosols | 3. Very old and highly weathered |
| D. Entisols | 4. Rich in clay content and highly basic |
Entisols and inceptisols are easy to swap
Entisols have no horizons at all. Inceptisols have weakly developed horizons. The 'in' of inceptisols is the beginning of layers.
Impoverished soil has lost nutrients
Soil impoverishment is the loss of plant nutrients, not erosion of the soil itself and not enrichment.
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 (4)
Physical weathering4 rows
| Process | What pushes the rock apart | Where it is strongest |
|---|---|---|
| Frost weathering | Water freezing in cracks | Cold regions with many freeze-thaw days |
| Thermal stress and shattering | Daily heating and cooling | Hot deserts |
| Exfoliation | Outer layers expanding and peeling | Crystalline rocks such as granite |
| Salt crystallisation | Salt crystals growing in pores | Porous sedimentary rocks, dry climatesQ CDS 2025 (I): both statements, most effective process and stronger on sedimentary rock, were correct. |
Chemical weathering5 rows
| Process | What happens | Example |
|---|---|---|
| Solution | Mineral dissolves in water | Rock salt |
| Carbonation | Carbonic acid dissolves calcium carbonate | Limestone caves |
| Hydration | Mineral takes up water and swells | Anhydrite to gypsum |
| Oxidation | Oxygen joins iron, like rusting | Red, crumbly iron-rich rock |
| Salt crystal growth | Physical pressure, no new mineral | Not chemical at allQ CDS 2024 (II): it was the odd one out among acid action, hydrolysis and oxidation. |
Mass movement and soil erosion5 rows
| Movement | Speed | Material |
|---|---|---|
| Creep | Very slow | Soil and loose rock |
| Solifluction | Slow | Water-soaked soil over frozen ground |
| Mudflow | Fast | Wet mud |
| Rockfall | Very fast | Rock falling free from a cliff |
| Gully erosion | Grows over years | Soft alluvial soilQ CDS 2019 (I): the Chambal valley was the one under intensive gully erosion. |
Soil orders and humus8 rows
| Order | Stand-out feature |
|---|---|
| Entisols | Youngest, no horizons |
| Inceptisols | Weakly developed horizonsQ |
| Histosols | Very high organic matter, peat and bogsQ CDS 2020 (I): the soil with very high organic matter was the histosol. |
| Oxisols | Very old, highly weathered, tropical |
| Vertisols | Clay-rich, basic, swell and crack |
| Mollisols | Dark, fertile grassland soilsQ CDS 2021 (II): inceptisols, entisols, vertisols and mollisols were named as orders of soil. |
| Gelisols | Permafrost within the soil |
| Humus | Dark, amorphous, colloidal store of nutrientsQ |
Watch out for (5)
- Salt crystal growth is physical, not chemical→ Physical weathering
- Hydration is not hydrolysis→ Chemical weathering
- The Chambal, not the Kosi or Damodar, is the gully river→ Mass movement and soil erosion
- Entisols and inceptisols are easy to swap→ Soil orders and humus
- Impoverished soil has lost nutrients→ Soil orders and humus
Test yourself on Earth's Structure, Landforms and Geological Time
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.