Lithosphere – II Exogenetic Processes — Class 9 Social Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 9 Social Science, Geography — Chapter 2. Weathering, erosion, and landforms created by rivers, glaciers, wind, and waves.
1. About this lesson
This chapter covers exogenetic (external) processes — the forces acting on the Earth's surface that wear down and reshape the landscape. You will learn about weathering, erosion, gradation, and the landforms created by rivers, glaciers, wind, and sea waves.
2. Key Concepts
Weathering vs Erosion
| Process | Definition |
|---|---|
| Weathering | The disintegration or decomposition of rocks in place (no movement) |
| Erosion | The wearing away and removal of rock material by natural agents (involves movement) |
| Transportation | Movement of eroded material from one place to another |
| Deposition | Dropping/accumulation of transported material |
Gradation
| Term | Definition |
|---|---|
| Gradation | The process of levelling up of land by natural agents (combined degradation + aggradation) |
| Degradation | Wearing down/ lowering of the land surface (erosion) |
| Aggradation | Building up/ raising of the land surface (deposition) |
3. Types of Weathering
Physical (Mechanical) Weathering
- Disintegration of rocks without chemical change.
- Types:
- Frost action / Freeze-thaw: Water in cracks freezes → expands → breaks rocks.
- Exfoliation: Outer layers of rock peel off due to temperature changes.
- Block disintegration: Rocks break along joints/cracks.
- Granular disintegration: Individual mineral grains separate.
Chemical Weathering
- Decomposition of rocks through chemical reactions.
- Types:
- Oxidation: Oxygen + minerals (especially iron) → rust; rocks become weak and crumble.
- Carbonation: Carbon dioxide + water → carbonic acid → dissolves limestone (karst topography).
- Hydration: Water absorbed by minerals → expansion → weakening.
- Solution: Minerals dissolve in water (especially in limestone regions).
- Hydrolysis: Water reacts with minerals to form new compounds.
Biological Weathering
- Caused by living organisms.
- Plant roots grow into cracks → widen them; burrowing animals; human activities (mining, quarrying); lichens and mosses produce acids.
4. Agents of Erosion and their Landforms
A. Running Water (River)
Erosional Landforms:
| Landform | Description |
|---|---|
| V-shaped Valley | Deep, narrow valley formed by river erosion in upper course |
| Waterfall | Sudden fall of river water over a steep/vertical slope (e.g., Jog Falls, Niagara Falls) |
| Gorge / Canyon | Deep, narrow valley with steep sides |
| Meander | A bend/curve in a river's course |
| Oxbow Lake | A crescent-shaped lake formed when a meander is cut off from the main river |
Depositional Landforms:
| Landform | Description |
|---|---|
| Floodplain | Flat land alongside a river formed by deposition during floods; very fertile |
| Levee | Natural embankment along river banks formed by deposition |
| Delta | Triangular-shaped landform at a river's mouth formed by deposition (e.g., Ganga-Brahmaputra delta, Nile delta) |
| Alluvial Fan | Fan-shaped deposit at the base of mountains where a river enters a plain |
B. Glacier
- A glacier is a large, slow-moving mass of ice.
- Erodes by plucking (pulling out rock fragments) and abrasion (scraping like sandpaper).
Erosional Landforms:
| Landform | Description |
|---|---|
| U-shaped Valley | Wide, steep-sided valley formed by glacial erosion (vs V-shaped by rivers) |
| Cirque | Bowl-shaped depression at the head of a glacial valley |
| Arete | Sharp ridge between two cirques |
| Horns | Pyramid-shaped peak formed by several cirques (e.g., Matterhorn) |
| Fjord | A drowned glacial valley; deep, narrow inlet of the sea (e.g., Norway) |
Depositional Landforms:
| Landform | Description |
|---|---|
| Moraine | Accumulation of rock debris deposited by a glacier |
| Drumlin | Smooth, elongated hill of glacial deposit |
| Esker | Long, winding ridge of sand and gravel deposited by meltwater streams |
C. Wind
- Wind erosion is most effective in arid (dry) and semi-arid regions (deserts).
- Processes: Deflation (removal of loose material), Abrasion (sand-blasting), Attrition (particles hitting each other).
Erosional Landforms:
| Landform | Description |
|---|---|
| Mushroom Rock | Rock with a narrow base and wide top, shaped by wind abrasion near the ground |
| Yardang | Ridge of rock shaped by wind abrasion |
| Zeugen | Tabular mass of hard rock resting on softer rock; eroded by wind |
Depositional Landforms:
| Landform | Description |
|---|---|
| Sand Dune | Mound/hill of sand deposited by wind (e.g., Barchan, Seif) |
| Loess | Fine, wind-blown silt deposit (e.g., Loess Plateau, China) |
D. Sea Waves
Erosional Landforms:
| Landform | Description |
|---|---|
| Sea Cliff | Steep rock face along the coast formed by wave erosion |
| Wave-cut Platform | Flat surface at the base of a cliff formed by wave erosion |
| Sea Cave | Hollow cavity in a cliff formed by wave action |
| Sea Arch | Natural arch formed when two sea caves from opposite sides meet |
| Stack | Isolated pillar of rock left standing when an arch collapses |
Depositional Landforms:
| Landform | Description |
|---|---|
| Beach | Accumulation of sand, gravel along the shore |
| Sand Bar | Ridge of sand deposited offshore |
| Spit | Narrow ridge of sand extending from the coast into the sea |
| Lagoon | Shallow water body separated from the sea by a sand bar/spit (e.g., Chilika Lake, Pulicat Lake) |
5. Worked examples
Example 1. What is the difference between weathering and erosion? Weathering is breakdown in place; erosion involves movement and removal.
Example 2. What shape valley does a river form? V-shaped valley. Glaciers form U-shaped valleys.
Example 3. What is an oxbow lake? A crescent-shaped lake formed when a meander is cut off from the main river.
Example 4. Name a depositional landform of wind. Sand dunes (Barchan, Seif) and Loess.
6. Common mistakes
- Mistake: Weathering and erosion are the same thing. Fix: Weathering = breakdown in place. Erosion = breakdown + movement/removal.
- Mistake: Rivers form U-shaped valleys. Fix: Rivers form V-shaped valleys. Glaciers form U-shaped valleys.
- Mistake: Delta is an erosional landform. Fix: Delta is a depositional landform formed at a river's mouth.
- Mistake: Gradation means only erosion. Fix: Gradation = degradation (erosion/wearing down) + aggradation (deposition/building up).
7. Quick revision
- Geography Ch 2 · Exogenetic Processes: Weathering → Erosion → Transportation → Deposition.
- Weathering: Physical (frost, exfoliation), Chemical (oxidation, carbonation, hydration, solution), Biological (plants, animals, humans).
- River landforms: V-shaped valley, waterfall, meander, oxbow lake, floodplain, delta.
- Glacial landforms: U-shaped valley, cirque, moraine, fjord.
- Wind landforms: Mushroom rock, sand dunes (Barchan), Loess.
- Sea wave landforms: Sea cliff, cave, arch, stack, beach, lagoon.
- Gradation = degradation + aggradation.
