Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation

1. Why the surface is uneven: two opposing groups of forces

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• The crust is always moving, both up and down and sideways. Two groups of forces act on it.
• Endogenic and exogenic forces: endogenic forces come from inside the earth and build land up. Exogenic forces act at the surface, are driven mainly by the sun's energy, and wear land down.
• Gradation means wearing down relief through erosion. It has two parts: degradation (high areas are worn down) and aggradation (basins and hollows are filled up).
• Endogenic forces keep raising land, so exogenic forces never make the surface flat. The unevenness lasts as long as both act. Class 11 calls the earth "a playfield for two opposing groups of geomorphic processes".
• Geomorphic processes are the forces that cause physical stress and chemical action on earth materials and change the shape of the surface. Diastrophism and volcanism are endogenic. Weathering, mass wasting, erosion and deposition are exogenic.
• A geomorphic agent is a moving medium that picks up, carries and drops material: running water, groundwater, glaciers, wind, waves and currents. The process is the force; the agent is the medium. Class 9 calls these "agents of gradation".
• Gravity switches on every downslope movement. Without gravity and slopes (gradients), there would be no erosion, transport or deposition. All movement goes from high to low, whether level or pressure.

2. Endogenic processes: internal energy, diastrophism, volcanism and isostasy

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• Sources of the earth's internal heat: radioactivity, rotational and tidal friction, and primordial heat (heat left over from when the earth formed). This heat flows outwards and drives diastrophism and volcanism.
• Heat flow, crustal thickness and crustal strength differ from place to place. So endogenic action is uneven, and the original surface it shapes is uneven too.
• Diastrophism covers all processes that move, raise or build up the crust:
• Orogeny: mountain building by severe folding of long, narrow belts (Himalayas, Alps, Andes).
• Epeirogeny: broad uplift or gentle warping of large areas with simple deformation. It is a continent-building process.
• Earthquakes: local, fairly minor movements.

• Plate tectonics: sideways movement of crustal plates.

• All four can fault and fracture rocks. The pressure, volume and temperature changes they cause lead to metamorphism.

• Volcanism is the movement of magma onto or towards the surface, forming intrusive and extrusive features. Volcanism is the process; a volcano is the vent and the landform.
• Isostasy (beyond NCERT): the crust floats on the denser mantle like wood on water. Airy (1855) explained height differences by different crustal thickness (mountains have deep "roots"). Pratt (1855) explained them by different crustal density. Scandinavia and Hudson Bay are still rising now that their ice sheets have melted.

3. Folding and faulting: fold mountains, block mountains and rift valleys

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• Folding: sideways squeezing bends rock layers into upfolds (anticlines) and downfolds (synclines). Severe folding along narrow belts builds fold mountains such as the Himalayas. Anticlines can also trap petroleum.
• Faulting: pulling apart (tension) or squeezing breaks rocks along cracks called faults.
• Block mountain (horst): a block of crust rises between two faults, or stays standing while the land around it sinks. It has a flat top and steep sides.
• Rift valley (graben): a strip of crust sinks between two parallel faults.

Feature	World example	Indian example
Block mountain	Vosges, Black Forest	Satpura range (cited by NCERT)
Rift valley	Rhine (between Vosges and Black Forest), East African Rift, Dead Sea	Narmada (between Vindhyas and Satpuras), Tapi, Damodar

• The Narmada and Tapi flow west through rifts. As a result they form estuaries rather than large deltas and have few meanders. Gondwana coal lies in Damodar-type grabens.
• Map practice: mark the Narmada and Tapi rifts, the Satpura range and the Damodar valley.

4. Exogenic processes: stress, denudation and what controls it

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• Exogenic processes get their energy from the atmosphere. That energy comes ultimately from the sun, helped by slopes that tectonic forces have created.
• Stress is force per unit area. Pushing or pulling deforms rock. Shear stress (a force acting along a surface) breaks rocks and makes them slip. Temperature change, crystal growth and melting add molecular stress. Chemical action loosens grains and dissolves cements. Stress is the basic cause of weathering, mass movement and erosion.
• Denudation ("to strip off") is the umbrella term for all exogenic processes: weathering, mass wasting, erosion and transportation. Each has its own driving force (Figure 5.1). Weathering and mass movement are driven by gravity, temperature and chemical action. Erosion and transport are driven by the kinetic energy (energy of motion) of water, ice, wind and waves.
• Climatic controls: temperature and rainfall matter most. Others are latitude, season, land-sea spread, vegetation, altitude, aspect (which way a slope faces), wind, rain intensity, the rain-evaporation balance, daily temperature range and how often water freezes and thaws.
• Rock controls (when climate is the same): folds, faults, dip of beds, joints, mineral hardness, chemical make-up and permeability. A rock may resist one process but give way to another. This uneven wear creates differences in topography.

5. Weathering: definition, controls and the weathering mantle

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• Weathering is the mechanical breaking (disintegration) and chemical rotting (decomposition) of rocks by the elements of weather and climate.
• It happens in situ (in place). Little or no material moves. The small movement that sometimes happens is not transportation. Class 9 puts it simply: weathering breaks rock, while erosion moves it.
• Weathering depends on geology, climate, topography and vegetation. Climate matters most, because it changes both the type of process and the depth of weathering.
• The weathering mantle is the layer of loose, weathered material lying over solid bedrock. Figure 5.2 (after Strakhov, 1967) shows it is deepest in humid tropics and thin in deserts and polar or tundra zones.
• There are three groups of weathering: chemical, physical (mechanical) and biological. They rarely act alone, but one usually dominates.
• Physical and chemical weathering are not independent. Breaking rock exposes fresh surfaces to chemical attack, and chemical rotting weakens rock so it breaks more easily.
• Activity: mark the latitudes of each climate zone on Figure 5.2 and compare mantle depths.

6. Chemical weathering

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• Chemical weathering breaks down rocks through reactions with oxygen, water, carbon dioxide and acids. It works by solution, carbonation, hydration, oxidation and reduction. Water, air and heat speed it up. Rotting plants and animals add extra carbon dioxide underground.
• Solution: minerals dissolve in water or weak acids and are carried away. Examples are rock salt, gypsum and (with acid) limestone.
• Carbonation: rainwater absorbs CO₂ and forms weak carbonic acid. This acid turns limestone and dolomite into soluble bicarbonates. Unusually, it works faster in cold water, because cold water holds more CO₂. It creates karst topography: caves and sinkholes.
• Hydration: water is chemically added to minerals, so they swell and weaken. Anhydrite becomes gypsum, and feldspar becomes clay. Repeated wetting and drying makes rock crumble. Clay is the material most affected (Class 11 exercise).
• Oxidation: minerals combine with oxygen, especially iron, giving red, brown or yellow oxides. This explains the red of laterite and red soils. Reduction happens below the water table where oxygen is missing, giving grey-green colours.
• Chemical weathering dominates in hot, humid climates. It is slow in deserts and cold lands, except for carbonation.

7. Physical and biological weathering

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• Physical weathering breaks rock without changing it chemically. The forces behind it are:
• gravity (the weight of overlying rock, load, shear stress);
• expansion from temperature change, crystal growth or animals;
• water pressure in wetting and drying cycles.

Most of it comes from thermal expansion and pressure release. Each change is small, but repeated fatigue does great damage.

• Unloading: erosion removes overlying rock, so buried rock expands upwards and cracks into curved sheets parallel to the surface.
• Thermal expansion and contraction: daytime heating and night-time cooling stress the outer layer more than the inside, causing flaking and granular disintegration. It matters most in hot deserts with large daily temperature ranges.
• Frost weathering: water in cracks expands by about 9% when it freezes. Repeated freezing and thawing splits rock into angular pieces. It is common in high mountains and cold regions.
• Salt weathering: as water evaporates, salts in rock pores crystallise and expand, pushing grains apart. It is common in deserts and on coasts.
• Biological weathering is weathering by living things:
• earthworms, termites and rodents burrow and expose fresh surfaces;
• ploughing and clearing by humans mix the soil;
• rotting matter produces humic and carbonic acids;
• plant roots wedge rocks apart with great pressure.

8. Special effects and significance of weathering: exfoliation, tors and ore enrichment

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• Exfoliation is the peeling off of curved sheets or shells of rock, leaving smooth, rounded surfaces (Figure 5.3). It is a result, not a process. It is caused by unloading, thermal expansion and contraction, and salt weathering.
• Exfoliation dome: a large, smooth granite dome formed by unloading. The domal granite hills of the Karnataka plateau (laccoliths or batholiths, Class 11, Interior of the Earth) are examples.
• Tor: a rounded pile of boulders on a hilltop. It forms by weathering along joints in granite, mainly through thermal expansion. Hampi's boulder landscape is an Indian example.
• Significance of weathering:
• It produces regolith (loose rock cover) and soils, and prepares the ground for mass movement and erosion.
• Without weathering, erosion cannot be significant. However, weathering is not a precondition for erosion.

• Forests depend on the depth of the weathering mantle, and biodiversity depends on forests. So weathering supports biodiversity.

• Enrichment: weathering removes some material and leaves valuable ores of iron, manganese, aluminium and copper concentrated enough to mine.

• Leaching is the removal of soluble minerals by water seeping downwards. In wet tropics it removes bases and silica and leaves iron and aluminium oxides. This forms laterite and bauxite, for example at Panchpatmali in Odisha.

9. Mass movements and landslides

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• Mass movement is the movement of rock debris downslope under the direct pull of gravity. No agent carries the debris, though the debris may carry water or ice with it. It is therefore not erosion. Weathering helps it but is not essential.
• Favourable conditions: weak or loose material, thin beds, faults, steeply dipping beds, steep slopes or cliffs, heavy rain and little vegetation.
• Activating causes of mass movement (triggers):
• loss of support from below;
• steeper or higher slopes;
• overloading by fill, or by rain causing saturation and lubrication;
• removal of load from the slope;
• earthquakes, explosions or machinery;
• heavy seepage;
• rapid drawdown of reservoirs;

• clearing of vegetation.

• There are three forms: heave (soil pushed up by frost growth and other causes), flow and slide.

Speed	Types
Slow	Soil creep (shows as tilted poles and fences); solifluction (saturated soil sliding over permafrost)
Rapid flow	Earth flow; mudflow (volcanic mudflow = lahar, e.g. Armero, Colombia, 1985); debris avalanche (wet debris rushing down steep, narrow channels)
Landslides (relatively dry, perceptible)	Slump (backward rotation); debris slide; debris fall; rockslide (along bedding, joint or fault planes, deep); rock fall (free fall from surface layers only)

• India:
• Himalayas: tectonically active, soft sedimentary rock and steep slopes, so landslides are very frequent.
• Western Ghats and Nilgiris: stable, hard rock, but with near-vertical cliffs, strong mechanical weathering and heavy short bursts of rain.
• Recent cases: Kedarnath (2013), Malin (2014), Joshimath (2023), Wayanad (30 July 2024). The ISRO-NRSC Landslide Atlas (2023) ranks Rudraprayag and Tehri Garhwal among the most exposed districts.

10. Erosion and deposition

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• Erosion is the picking up and carrying away of rock debris by agents: running water, groundwater, glaciers, wind and waves. Weathering helps erosion but is not a precondition for it. Erosion is the main cause of continuous change on the surface.
• Abrasion is the scraping and grinding of surfaces by the debris an agent carries. It greatly increases erosive power.
• Erosion and transport are driven by kinetic energy (energy of motion).
• Wind, running water and glaciers are controlled by climate. They represent the three states of matter: gas, liquid and solid.
• Waves are controlled by location at the land-sea edge. Groundwater is controlled by rock type: karst forms only where rock is permeable and soluble and water is available.
• Deposition happens when agents lose speed on gentler slopes and drop their load. Coarse material settles first, fine material later, and hollows fill up. Deposition is a result of erosion, not really the "work" of any agent.
• Human impact (Class 9): erosion strips fertile topsoil, destroys homes and roads along rivers and coasts, and harms tourism and fishing.
• Traditional conservation: contour trenches, bunding, terracing, check dams and Nagaland's Zabo system.

11. Soil and the process of soil formation

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• Soil is a dynamic medium of minerals, organic matter, water and air. Chemical, physical and biological activity goes on in it all the time. It is both a result of decay and a medium for growth.
• Soil changes with the seasons. Biological activity slows or stops when the soil is too cold or too dry, and organic matter rises when leaves fall.
• Pedology is the science of soils, and a soil scientist is a pedologist. V.V. Dokuchaev (Russia, 1880s) is regarded as the founder of soil science.

• Soil formation (pedogenesis) begins with weathering. The weathering mantle is the basic input. The steps are: 1. Bacteria, mosses and lichens colonise weathered or transported material. 2. Small organisms shelter in it, and their dead remains build humus. 3. Grasses and ferns grow, then bushes and trees from seeds brought by birds and wind. 4. Roots and burrowing animals make the mass porous and sponge-like, so it holds water and lets air through. 5. A mature soil forms: a mixture of mineral and organic matter.

• Weathering is necessary but not enough on its own. Living things are essential.

• A mature soil has horizons (layers): O, A, E, B, C, R. According to the FAO, forming 1 cm of soil can take up to about 1,000 years.

12. Soil-forming factors: parent material, topography, climate, biological activity and time

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• There are five soil-forming factors, and they act together. Active factors are climate and biological activity. Passive factors are parent material, topography and time.
• Parent material: weathered rock in place gives residual soils. Material moved by rivers, wind or ice gives transported soils, such as the alluvium of the Indo-Gangetic plains (residual and transported soils). Young soils show a strong link to the parent rock.
• Topography: soils are thin on steep slopes and thick on flat uplands. Gentle slopes are the most favourable. Flat areas may build dark, clay-rich layers.
• Climate as a soil-forming factor works through moisture and temperature:
• Excess water washes material down (eluviation) and deposits it in lower layers (illuviation).
• In wet equatorial areas, calcium, sodium, magnesium, potassium and most of the silica are removed. Removing silica is called desilication (laterite).
• In dry areas, capillary rise and evaporation leave salt crusts called hardpans.

• Where rainfall is intermediate, kanker (calcium carbonate nodules) forms.

• Biological activity:

• Dead plants give humus (fine, dark organic matter). Humus builds up in cold climates where bacteria work slowly, and stays low in humid tropics where it oxidises fast.
• Peat is half-rotted matter in sub-arctic and tundra areas, and the first stage of coal.

• Rhizobium bacteria in legume root nodules fix nitrogen.

• Time decides how mature a soil is. Young alluvium or glacial till shows weak horizons. No fixed time can be stated for soil to mature. Hans Jenny (1941) formalised these factors.

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Exam angles

Prelims — high-yield facts and traps

• Endogenic = land-building (diastrophism, volcanism). Exogenic = land-wearing (weathering, mass wasting, erosion, deposition). "Diastrophism is a gradational process" is FALSE. Erosion and deposition are gradational.
• Internal energy comes from radioactivity, rotational and tidal friction and primordial heat. The sun is the sole ultimate driver of exogenic processes.
• Orogeny = severe folding of narrow belts (mountain building). Epeirogeny = broad uplift (continent building).
• Weathering is in situ. Erosion needs an agent. "Mass movement is a type of erosion" is FALSE.
• Carbonation is faster in cold water. Hydration affects clay most. Anhydrite turns into gypsum. Oxidation gives laterite its red colour. Water expands about 9% on freezing.
• "Exfoliation is a process" is FALSE: it is a result. Exfoliation domes come from unloading; tors come from thermal expansion.
• A debris avalanche is a rapid flow mass movement. Slump has backward rotation. Rock fall affects surface layers only; a rockslide goes deep. Creep and solifluction are slow.
• Narmada and Tapi flow in rift valleys, so they have estuaries, not deltas. Satpura is cited as a block mountain. Airy explained isostasy by thickness, Pratt by density.
• Wind, water and ice are controlled by climate and represent gas, liquid and solid. Waves and groundwater are not controlled by climate.
• Active soil-forming factors: climate and biology. Passive: parent material, topography and time. Desilication happens in wet equatorial areas. Kanker forms under intermediate rainfall. Hardpans form in dry climates. Peat forms in tundra.

Mains — GS-I themes

1. "Our earth is a playfield for two opposing groups of geomorphic processes." Discuss.
2. Exogenic processes get their ultimate energy from the sun. Explain with reference to denudation and its driving forces.
3. Are physical and chemical weathering independent? Explain with examples from deserts, tundra and humid tropics.
4. Why are landslides frequent in both the tectonically active Himalayas and the stable Western Ghats? Suggest measures.
5. Distinguish the process of soil formation from soil-forming factors, and assess the role of climate and biological activity.
6. "Weathering is responsible for biodiversity and for valuable ore deposits." Explain, with reference to laterite and bauxite.
7. Explain how rift valleys shape the drainage and mouths of the Narmada and Tapi.

Current-affairs hooks

• Monsoon landslides in the Western Ghats and Himalayas: Wayanad (30 July 2024), Himachal and Uttarakhand events (verify latest).
• Land subsidence in Himalayan towns (Joshimath 2023) and debates on hill roads and hydropower (verify current).
• GSI National Landslide Susceptibility Mapping, landslide early-warning systems and the ISRO-NRSC Landslide Atlas (2023) (verify current status).
• Soil health and erosion data: Soil Health Card, land degradation atlases (verify current).
• Glacial lake outburst floods (GLOFs) and permafrost thaw in the Himalaya: South Lhonak, Sikkim (2023) (verify current).
• Coastal erosion along India's coasts: NCCR shoreline change assessments (verify current).