Exogenetic Forces — Weathering & Mass Movement
Learn the Concept
AllExogenetic (external) forces draw their energy from the Sun and gravity and work to wear down, transport and level the relief built by endogenetic forces — the process of gradation (degradation + aggradation). The attack begins with weathering (the in-situ break-up and decay of rock), continues with mass movement (the downslope transfer of material under gravity), and is completed by erosion, transportation and deposition by rivers, glaciers, wind, waves and groundwater. Fix the distinction: weathering involves no transport (rock decays where it lies), mass movement is transport purely under gravity, and erosion is transport by a moving agent. Weathering prepares the raw material (regolith) on which erosion then operates.
- Weathering: in-situ disintegration (physical) & decomposition (chemical) of rock — no transport.
- Mass movement: downslope movement of material under gravity (creep → flows → slides → falls).
- Erosion: wearing-away + transport by a moving agent (river, ice, wind, waves).
- Product: weathering yields regolith; the balance of processes shapes slopes & soils.
- Weathering: the breakdown of rock in place, by mechanical, chemical and biological action.
- Regolith: the layer of loose, weathered material (broken rock + soil) over solid bedrock.
- Mass movement (mass wasting): the downslope movement of rock/soil under gravity, without a transporting medium.
- Angle of repose: the steepest slope at which loose material stays stable.
- Base level of erosion: the lowest level to which an agent can erode (ultimately sea level).
1. Physical (mechanical) weathering
- Rock is broken into fragments without change in chemical composition. Chief processes: frost action (frost shattering / freeze–thaw) — water in joints expands ~9% on freezing and prises rock apart (produces scree/talus in cold & high mountains); thermal expansion (insolation weathering) — repeated day–night heating/cooling in deserts fatigues rock (block disintegration, granular disintegration); exfoliation (unloading/sheeting) — release of confining pressure peels off curved shells, producing exfoliation domes (e.g. granite domes); and salt crystallisation (haloclasty) — salt crystals growing in pores wedge rock apart (deserts, coasts).
2. Chemical weathering
- Rock is decomposed by chemical reaction, usually involving water; most effective in warm, humid climates. Processes: oxidation (iron minerals + oxygen → rusting, reddening); carbonation (CO₂ + water → carbonic acid dissolving limestone → karst); hydration (minerals absorb water & swell, e.g. anhydrite → gypsum); hydrolysis (water reacts with silicates, e.g. feldspar → clay — the key to deep tropical weathering); and solution (direct dissolving of soluble salts). Chemical weathering produces deep regolith, clays, laterite.
3. Biological weathering
- Both physical and chemical, performed by organisms: root wedging (tree roots prising joints), burrowing animals (earthworms, termites mixing & exposing material), and microbial/organic acids and chelation (lichens, bacteria, humic acids accelerating chemical decay).
4. Factors controlling weathering; weathering profile & regolith
- Controls: climate (temperature & moisture — chemical weathering peaks in the hot-wet tropics, frost action in cold lands), rock type & mineralogy, jointing/structure (more joints = faster), relief, vegetation and time.
- Repeated weathering produces a vertical weathering profile: fresh bedrock at base → partially weathered saprolite → mobile regolith → soil at the top. Regolith is the whole mantle of loose weathered debris; where it is transported it feeds erosion and soil formation.
5. Mass movement (mass wasting)
- Material moves downslope under gravity once the driving force (shear stress) exceeds the resisting force (shear strength). Types are graded by speed and water content:
- Slow (creep): soil creep — imperceptible downslope creep of soil (tilted poles, curved tree trunks); solifluction — slow flow of water-saturated regolith over frozen ground in periglacial areas.
- Flows: earthflow (moderate, wet); mudflow (very wet, fast, channelled); debris flow (coarse + water, fast, destructive).
- Slides: landslide (rapid downslope slip of a rock/soil mass along a plane); slump (rotational slide along a curved surface, leaving a back-tilted step); rockslide.
- Falls: rockfall (free-fall of fragments from a cliff → talus/scree); avalanche (rapid fall of snow/ice ± debris).
6. Triggers and slope instability
- Triggering factors: heavy rainfall / saturation (raises pore-water pressure, reduces strength), steep slopes / undercutting (rivers, waves, roads), earthquakes, deforestation, weak/weathered rock & adverse dip, and human activity (mining, construction, loading). Instability occurs when shear stress > shear strength — often when a slope exceeds its angle of repose or a weak layer is lubricated. Landslides are a major hazard in the Himalaya & Western Ghats.
7. Erosion, transportation and deposition
- Erosion = the wearing-away and removal of material by a moving agent (river, glacier, wind, waves, groundwater); its sub-processes include hydraulic action, abrasion (corrasion), attrition and solution (corrosion). Transportation moves the load (as traction, saltation, suspension and solution). Deposition lays it down when energy falls. The uneven removal of weaker rock is differential erosion; all erosion works toward the base level of erosion (ultimately sea level), below which an agent cannot cut.
| Process | Nature | Transport? |
|---|---|---|
| Weathering | Break-up/decay in situ | No |
| Mass movement | Downslope under gravity | Yes (gravity only) |
| Erosion | Wear + removal by an agent | Yes (agent) |
| Deposition | Laying down of load | End of transport |
- Weathering = no transport; mass movement = gravity only; erosion = moving agent.
- Physical: frost action, thermal expansion, exfoliation, salt crystallisation.
- Chemical: oxidation, carbonation, hydration, hydrolysis, solution (peaks in hot-wet tropics).
- Mass movement by speed/water: creep → solifluction → flows → slides (slump/landslide) → falls.
- Trigger when shear stress > shear strength; profile = soil→regolith→saprolite→bedrock.
- Factors controlling landforms: climate controls which weathering dominates.
- Soil genesis (Biogeography): weathering is the first step of soil formation.
- Landslides (hazards & India): mass movement in the Himalaya & Western Ghats.
- Karst & geomorphic agents: carbonation and fluvial/glacial/aeolian erosion.
Prepare for the Exam — Prelims
PrelimsTested on the weathering types & reactions, climate control, the weathering profile/regolith, and the mass-movement classification & triggers.
- Physical: frost action, thermal expansion, exfoliation, salt crystallisation.
- Chemical: oxidation, carbonation (karst), hydration, hydrolysis, solution.
- Chemical weathering peaks in hot-wet tropics; frost action in cold lands.
- Mass movement: creep, solifluction, flows, slump, landslide, rockfall.
- Regolith = loose weathered mantle; profile = soil→regolith→saprolite→bedrock.
- Weathering = in situ (NO transport).
- Hydrolysis (feldspar→clay) drives deep tropical weathering.
- Carbonation dissolves limestone → karst.
- Slump = rotational (curved); landslide = translational (planar).
Prepare for the Exam — Mains
MainsOptional Paper I / GS Paper I. The examiner rewards linking weathering type to climate and mass movement to slope stability/hazards.
- Discuss the processes of weathering and their climatic control. (15)
- Classify mass movements and examine their causes. (15)
- Distinguish between weathering, mass movement and erosion. (10)
- Examine the causes of landslides in the Himalaya. (10)
- Weathering: physical/chemical/biological.
- Controls: climate, rock, jointing, time.
- Mass movement: types + triggers.
- Erosion/transport/deposition.
- Cite hydrolysis & laterite (tropics).
- Shear stress vs shear strength.
- Himalaya/Western Ghats landslides.
How to Write the Answer
Mains“Classify mass movements and examine the factors that trigger them.” (15)
Classify by speed/water content, then analyse the stress–strength balance and triggers.
- Calling weathering a transport process.
- Confusing slump and landslide.
- Ignoring pore-water pressure.
Prelims Questions
Prelims30 interactive MCQs — previous-year questions are included and tagged with their year. Select an answer for instant feedback and an explanation; your score is tracked and you can review wrong answers at the end.
Mains Questions
Mains15 mains questions — previous-year questions are included and tagged. Write a full answer, then click Evaluate My Answer for paragraph-level feedback. Reveal the model structure after you attempt.
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