📖 PART 1: Weathering
Weathering is the process of disintegration (breaking up) and decomposition of rocks in
situ (in place) by natural agents like temperature, frost, water, and living organisms —
without any transport of the broken material. No movement occurs in weathering.
Types of Weathering
1. Physical (Mechanical) Weathering
- Breaking up of rocks by physical forces without changing chemical composition.
- Block Disintegration (Thermal Expansion): In desert regions, day temperatures are very
high (rocks expand) and night temperatures very low (rocks contract). This repeated expansion and
contraction creates stress cracks → rock breaks into blocks. Common in hot deserts.
- Granular Disintegration: Mineral grains in rocks expand/contract at different rates →
outer grains separate → gritty, sandy surface forms. Especially in rocks with different coloured
minerals (absorb heat differently).
- Exfoliation (Onion Weathering): Alternate heating and cooling causes outer sheets of
rock to peel off like layers of an onion → produces rounded rocky hills called inselbergs or
bornhardts.
- Frost Action / Freeze-Thaw: Water seeps into rock cracks → freezes (expands ~9% in
volume) → wedges cracks apart → rock breaks. Common in polar, sub-polar and high mountain areas.
2. Chemical Weathering
- Decomposition of rocks through chemical reactions — changes the mineral composition of rocks. Speeded up
by moisture and warm temperatures.
- Oxidation: Oxygen in air/water reacts with iron minerals → iron oxide (rust) → brown,
crumbly rock surface. E.g., Laterite soil formation.
- Carbonation: CO₂ in rainwater forms weak carbonic acid (H₂CO₃) → attacks limestone and
chalk → dissolves them. Forms karst topography — caves, sink holes, stalactites, stalagmites.
- Hydration: Minerals in rocks absorb water molecules → expand and weaken → rock
structure breaks down. E.g., feldspar + water → clay minerals.
- Solution: Soluble minerals (rock salt, gypsum) are dissolved and carried away by water.
3. Biological Weathering
- Caused by the activity of living organisms (plants and animals).
- Roots: Tree roots penetrate rock cracks and wedge them open as they grow (root
wedging).
- Burrowing animals: Earthworms, rabbits, moles, etc. mix soil and expose fresh rock
surfaces.
- Lichens: Grow on bare rock surfaces; produce acids that chemically attack minerals.
- Humans: Mining, road building, quarrying exposes fresh rock to weathering.
🌊 PART 2: Denudation
Denudation is the combined process of weathering, erosion, transportation, and
deposition — the wearing down of the land surface over time. Unlike weathering, denudation involves
removal and transport of material.
Agents of Denudation
- Running Water (Rivers): The most powerful agent of denudation; erodes, transports, and
deposits sediment.
- Wind: Important in arid/desert regions; carries sand and dust → deflation, abrasion,
attrition.
- Glaciers: Ice masses that carve valleys by plucking and abrasion.
- Sea waves: Erosion of coastlines.
🏔️ PART 3: Work of a River — Stages and Landforms
| Stage |
Location |
Activity |
Landforms Produced |
| Upper Course (Youth) |
Mountains; steep gradient; fast flow |
Mainly EROSION (vertical cutting = downcutting) |
V-shaped valley, Gorge, Canyon, Waterfall (e.g., Niagara, Angel Falls), Rapid, Pot holes |
| Middle Course (Maturity) |
Foothills; moderate gradient; moderate speed |
EROSION + DEPOSITION (lateral erosion widens valley) |
Meander (S-bends in river path), Ox-bow Lake (cut-off meander loop), Flood plain (flat fertile
land alongside river) |
| Lower Course (Old Age) |
Plains; gentle gradient; slow, wide river |
Mainly DEPOSITION (river loses energy) |
Delta (triangular deposition at river mouth — e.g., Ganga-Brahmaputra Delta = world's largest
delta / Sundarbans), Estuary, Natural levees |
🌬️ PART 4: Work of Wind — Desert Landforms
Erosional Features
- Deflation Hollows: Wind removes fine particles (sand, dust) from dry areas → lowers the
surface → creates shallow depressions called deflation hollows. E.g., Qattara Depression (Egypt).
- Ventifacts: Pebbles shaped into multi-faced stones by wind abrasion in deserts.
- Mushroom / Inselberg Rock: Isolated rock mass eroded by wind-blown sand at base more
than top → mushroom shape.
Depositional Features
- Sand Dunes: Mounds of wind-deposited sand. Types: Barchan (crescent-shaped; most
common), Seif (linear/longitudinal dune), Star dunes (complex shape). Found in Sahara, Arabian, Thar
deserts.
- Loess: Very fine wind-blown dust deposited far from deserts → forms very fertile soil.
E.g., Loess Plateau (China), Rhine Valley (Germany), Great Plains of USA.
📌 Chapter Summary
- Weathering = breakdown of rock IN PLACE (no transport). Types: Physical (block disintegration,
granular, exfoliation, freeze-thaw); Chemical (oxidation, carbonation, hydration, solution);
Biological (roots, animals, lichens).
- Denudation = weathering + erosion + transport + deposition. Agents: rivers (most powerful), wind,
glaciers, sea waves.
- River stages: Upper (V-valley, waterfall, gorge); Middle (meander, ox-bow lake, flood plain); Lower
(delta, estuary). Ganga-Brahmaputra delta = world's largest delta.
- Wind erosion: deflation hollows, ventifacts. Wind deposition: sand dunes (barchan, seif, star),
loess (fertile fine dust deposits).