Weathering & Erosion
Weathering & Erosion
Weathering is the in-situ breakdown and decomposition of rocks at or near the Earth's surface by physical, chemical, and biological processes. Erosion is the removal and transport of weathered material by agents like water, wind, ice, and gravity. Together, they are the primary exogenous (external) processes that shape landforms, create soils, and sculpt landscapes.
Key Dates
James Hutton published "Theory of the Earth" — established that geological processes (weathering, erosion) operate uniformly over vast time (uniformitarianism)
Charles Lyell's "Principles of Geology" popularized uniformitarianism: "the present is the key to the past"
W.M. Davis proposed the Geographical Cycle of Erosion — youth, maturity, old age stages of landscape evolution
Walther Penck proposed that uplift and erosion happen simultaneously — contradicting Davis's sequential model
L.C. King proposed the Pediplanation Cycle for arid/semi-arid regions — landscapes reduced through parallel slope retreat
John T. Hack proposed dynamic equilibrium — landscapes maintain a steady state where erosion rate equals uplift rate
Running water is the most powerful agent of erosion globally; wind dominates in deserts; glaciers in polar/high-altitude areas
Denudation = weathering + mass wasting + erosion; the overall wearing away of the land surface by exogenous processes
Chambal ravines (MP/Rajasthan/UP) — India's worst gully erosion covering 6,000+ sq km of badlands
India loses 5,334 million tonnes of soil annually; 29% lost permanently to the sea; 10% deposits in reservoirs
Siachen (76.4 km) is India's longest glacier; Gangotri Glacier (source of Ganga) has retreated ~1.5 km over the last century
India's 7,516.6 km coastline: about 40% experiencing erosion; Chennai loses 2-3 m/year; Sundarbans losing land to rising seas
Karst topography from carbonation weathering of limestone: Meghalaya caves (Mawsmai, Krem Liat Prah — India's longest cave at 31+ km)
Physical (Mechanical) Weathering
Physical weathering breaks rocks into smaller fragments without altering their chemical composition. It is most effective in arid and semi-arid climates with large temperature variations and limited vegetation. Main types: (1) Thermal Expansion and Contraction (Insolation Weathering) — repeated heating (expansion) and cooling (contraction) of rock surfaces due to diurnal temperature changes creates stress, causing the outer layers to peel off (exfoliation or onion-skin weathering); most effective in deserts where day-night temperature range can exceed 40°C; granite domes and tors in Rajasthan and Deccan Plateau show this; (2) Frost Weathering (Freeze-Thaw) — water seeps into rock cracks, freezes and expands by about 9%, widening the cracks; repeated cycles shatter rocks; dominant in cold climates and high altitudes; responsible for scree slopes in the Himalayas; particularly effective in the Ladakh region; (3) Pressure Release (Unloading) — when overlying rock is removed by erosion, the underlying rock expands upward, creating sheet joints parallel to the surface; leads to exfoliation; common in granite areas; (4) Salt Weathering (Salt Crystallization) — in arid regions, saline water enters rock pores and evaporates, leaving salt crystals that grow and exert pressure; very effective in the Thar Desert and coastal areas; (5) Biological Physical Weathering — tree roots grow into rock cracks and widen them (root wedging); burrowing animals (rodents, earthworms) physically break up soil and rock. In India, physical weathering is dominant in the Thar Desert (Rajasthan), the high-altitude regions of Ladakh and the Himalayas, and the Deccan Plateau.
Chemical Weathering
Chemical weathering involves the decomposition of rocks through chemical reactions that alter their mineral composition. It is most effective in warm, humid climates where water and organic acids are abundant. Main processes: (1) Oxidation — iron-bearing minerals react with oxygen and water to form iron oxides (rust); converts ferrous iron (Fe2+) to ferric iron (Fe3+); responsible for the reddish-brown colour of laterite soils and red soils in India; e.g., biotite → limonite; (2) Hydrolysis — the most important chemical weathering process; water reacts with silicate minerals to form clay minerals; e.g., feldspar + water → clay (kaolinite) + dissolved silica; responsible for the deep weathering profiles in tropical India; (3) Carbonation — carbon dioxide dissolves in rainwater to form weak carbonic acid (H2CO3), which dissolves limestone (CaCO3); responsible for karst landforms — caves, sinkholes, stalactites, stalagmites; in India, karst topography is found in the Vindhyan limestone region (MP), Cherrapunji-Mawsynram area (Meghalaya), and parts of Andhra Pradesh and Karnataka; (4) Hydration — minerals absorb water and expand; anhydrite → gypsum; contributes to rock disintegration; (5) Solution — some minerals dissolve directly in water; rock salt (NaCl) and gypsum are highly soluble. Chemical weathering produces regolith (weathered rock material) and residual soils. In tropical India, intense chemical weathering has produced deep laterite profiles (up to 30 m in Kerala and Karnataka) and thick clay-rich soils.
Biological Weathering
Biological weathering involves the breakdown of rocks through the activities of living organisms. It can be both physical and chemical in nature. Key agents: (1) Plant Roots — roots penetrate rock cracks and exert tremendous pressure as they grow, widening joints and fractures (physical); root exudates (organic acids) chemically attack minerals; (2) Lichens — pioneer colonizers of bare rock surfaces; they secrete organic acids that dissolve minerals (chemical) and physically penetrate the rock surface; crucial in initiating soil formation; (3) Mosses — retain moisture on rock surfaces, promoting chemical weathering; their root-like structures (rhizoids) penetrate micro-cracks; (4) Burrowing Animals — earthworms, termites, ants, rodents, and rabbits physically break up rock and soil, increasing the surface area exposed to chemical weathering; Charles Darwin estimated that earthworms in England brought about 10 tonnes of soil per acre per year to the surface; in Indian tropical soils, termite mounds are significant agents of biological soil mixing; (5) Microorganisms — bacteria and fungi secrete organic acids and chelating agents that dissolve minerals; they play a crucial role in humus formation and nutrient cycling; (6) Human Activities — mining, quarrying, deforestation, and construction are forms of accelerated biological weathering. Biological weathering is particularly significant in India's tropical forests (Western Ghats, Northeast India) where high biodiversity and warm, moist conditions create intense biological activity. The dense forest floor accelerates rock breakdown and soil formation, while deforestation removes this protective cover, exposing rock to accelerated physical and chemical weathering.
Erosion by Running Water
Running water (rivers and streams) is the most powerful and widespread agent of erosion on Earth, responsible for shaping most of India's landforms. Erosional processes of running water include: (1) Hydraulic Action — the force of moving water dislodges and removes loose material from the channel bed and banks; particularly effective during floods; (2) Abrasion (Corrasion) — sediment carried by the river acts like sandpaper, grinding against the bed and banks; responsible for V-shaped valleys, potholes, and river terraces; (3) Attrition — rock fragments carried by the river collide with each other and are progressively broken down and rounded; (4) Solution (Corrosion) — river water chemically dissolves soluble minerals from the bed and banks. The stages of river erosion (following Davis's model): Youth stage — steep gradient, vigorous vertical erosion, V-shaped valleys, waterfalls, rapids, gorges, interlocking spurs; Mature stage — lateral erosion becomes dominant, valley widens, meanders develop, floodplains form, oxbow lakes appear; Old stage — very gentle gradient, wide floodplain, river deposits more than it erodes, deltas form at the mouth. In India, dramatic examples include: the Ganga cutting through deep gorges in the Himalayas, the Chambal ravines in MP/Rajasthan (severe gully erosion affecting over 6,000 sq km), the Sundarbans Delta (world's largest), Shivasamudram Falls on the Kaveri, Jog Falls on the Sharavathi (highest plunge waterfall in India at 253 m).
Erosion by Wind, Glaciers, and Waves
Wind Erosion — Wind is a significant agent of erosion in arid and semi-arid regions. Processes include: deflation (lifting and removal of loose particles), abrasion (sandblasting effect of wind-blown particles — most effective at ground level), and attrition (collision of particles mid-air). In India, wind erosion is dominant in the Thar Desert (Rajasthan, parts of Gujarat and Haryana), affecting about 5.3 lakh hectares annually; it creates desert landforms like mushroom rocks (pedestal rocks), ventifacts, yardangs, and deflation hollows. Glacial Erosion — Glaciers erode through plucking (freezing onto rock and pulling fragments away) and abrasion (rock-studded glacier base acts as giant sandpaper). Glacial landforms include cirques (armchair-shaped hollows), aretes (knife-edged ridges), horns (pyramidal peaks), U-shaped valleys, hanging valleys, fjords, and moraines. In India, glaciers are found in the Greater Himalayas: Siachen Glacier (76.4 km — largest in India, in Ladakh), Gangotri Glacier (source of the Ganga), Zemu Glacier (Sikkim), and Kolahoi Glacier (J&K). These Himalayan glaciers are retreating due to climate change — the Gangotri Glacier has receded by about 1.5 km over the last century. Wave Erosion — Sea waves erode coastlines through hydraulic action, abrasion, and attrition, creating cliffs, wave-cut platforms, sea caves, arches, stacks, and stumps. India's 7,516.6 km coastline shows active wave erosion, particularly along the Konkan coast, Kerala coast, and parts of Tamil Nadu. The Chennai coast loses about 2-3 m per year to wave erosion.
Mass Wasting and Gravitational Processes
Mass wasting (mass movement) refers to the downslope movement of rock, soil, and debris under the direct influence of gravity, without a transporting agent like water or wind. It is a critical process in mountainous terrain like the Himalayas. Types of mass wasting: (1) Landslides — rapid downslope movement of a mass of rock or earth; includes rockfalls (free-falling rock from cliffs), rockslides (sliding along a plane), and debris slides; common in the Himalayas during the monsoon season; major landslide-prone areas include Uttarakhand, Himachal Pradesh, Sikkim, Meghalaya, and the Western Ghats; the 2013 Kedarnath disaster involved massive debris flows triggered by cloudbursts; (2) Mudflows and Debris Flows — rapid flow of water-saturated earth material; common in steep, unvegetated terrain after heavy rain; frequent in Himalayan valleys; (3) Creep — extremely slow (mm to cm per year), imperceptible downslope movement of soil and rock; indicated by tilted trees, fence posts, and retaining walls; affects slopes throughout India; (4) Slump — rotational movement of a mass of rock or soil along a curved surface; leaves a crescent-shaped scarp; common along road cuts in hilly terrain; (5) Solifluction — slow flow of water-saturated soil over frozen ground; relevant in high-altitude areas of Ladakh and the Greater Himalayas. Factors affecting mass wasting: slope angle, rock type, water content (rain saturation is the most common trigger), vegetation cover (roots bind soil), earthquakes, and human activity (deforestation, mining, road construction). India's National Landslide Susceptibility Mapping (NLSM) by the Geological Survey of India identifies about 12.6% of India's land area as landslide-prone.
Soil Erosion in India — Extent and Control
Soil erosion is the removal of the top layer of soil by water, wind, or other agents faster than it can be replenished by natural soil-forming processes. India faces severe soil erosion: about 130 million hectares (nearly 40% of India's geographic area) are affected by various forms of land degradation. India loses approximately 5,334 million tonnes of soil annually, of which about 29% is permanently lost to the sea, 10% is deposited in reservoirs (reducing storage capacity), and the rest is redistributed. Types of soil erosion in India: (1) Sheet Erosion — thin layers of topsoil removed uniformly by surface runoff; most widespread form; affects agricultural lands across the Indo-Gangetic Plains; (2) Rill Erosion — water concentrates into small channels (rills) that cut into the soil; progresses to gully erosion if unchecked; (3) Gully Erosion — deep channels carved by concentrated runoff; the Chambal Ravines (MP, Rajasthan, UP) are the most extreme example in India, covering over 6,000 sq km of barren ravine land; similar ravines exist along the Yamuna and other rivers; (4) Wind Erosion — dominant in the Thar Desert and adjoining areas; about 5.3 lakh hectares affected annually in Rajasthan; (5) Coastal Erosion — affects India's 7,516.6 km coastline; about 40% of India's coastline is experiencing erosion. Conservation measures: contour ploughing, terrace farming, bunding, strip cropping, afforestation, shelterbelts (wind erosion), check dams, gabion structures, riparian buffer zones, and integrated watershed management. Key programs: Integrated Watershed Management Programme (IWMP), National Mission for Green India, MGNREGA (includes land development works), and PM Krishi Sinchayee Yojana (watershed development component).
Cycle of Erosion — Davis vs Penck
The concept of the cycle of erosion attempts to explain how landscapes evolve over time under the influence of erosional processes. W.M. Davis's Geographical Cycle (1899) — Davis proposed that landform evolution follows a predictable sequence: (1) Youth — initial uplift; rivers cut deep V-shaped valleys; steep gradients; waterfalls and rapids; narrow valley floors; poor drainage integration; (2) Maturity — maximum relief; well-integrated drainage; valleys widen; meanders develop; floodplains appear; topography is diverse; (3) Old Age — landscape is worn down to a gently undulating plain (peneplain) close to base level; rivers are sluggish with wide meanders, oxbow lakes, and broad floodplains; residual hills called monadnocks stand above the peneplain. Davis assumed: initial rapid uplift followed by prolonged erosion with no further tectonic activity; humid temperate climate; this makes his model idealistic. Walther Penck's Model (1924) — Penck rejected Davis's sequential approach and argued that uplift and erosion happen simultaneously, not sequentially. He proposed: if uplift rate > erosion rate → convex slopes (waxing development); if uplift rate = erosion rate → straight slopes (uniform development); if uplift rate < erosion rate → concave slopes (waning development). Penck's model is more realistic for tectonically active regions like the Himalayas, where uplift and erosion occur simultaneously. L.C. King (1953) proposed the Pediplanation Cycle for arid/semi-arid regions, where landscapes are reduced through parallel retreat of slopes rather than downwearing, producing pediments and inselbergs. In India, elements of all three models can be observed: the Himalayas show Penckian simultaneous uplift and erosion; the Peninsular Plateau approximates a Davisian peneplain with residual hills; and the Thar Desert shows King's pediment landscapes.
Depositional Landforms — Fluvial, Aeolian, Glacial, and Marine
Erosion, transport, and deposition are three interconnected phases of landscape sculpting. Deposition occurs when the transporting agent loses energy: (1) Fluvial Depositional Landforms — alluvial fans (fan-shaped deposits at mountain fronts — Dehradun sits on a massive alluvial fan where the Ganga system exits the Himalayas); floodplains (flat areas flanking rivers built by repeated flood deposition — the Indo-Gangetic Plain is the world's largest floodplain system); natural levees (raised banks alongside rivers built by flood deposits — prominent along the Mississippi and Ganga); deltas (fan-shaped deposits at river mouths — Sundarbans/Ganga-Brahmaputra delta is the world's largest at 100,000+ sq km; other major Indian deltas: Mahanadi, Godavari, Krishna, Kaveri); oxbow lakes (crescent-shaped lakes formed by meander cutoff — found in Ganga floodplain in Bihar and Assam); point bars (deposits on the inner bank of meanders); braided channels (multiple channels separated by bars — Brahmaputra in Assam is one of the most braided rivers globally). (2) Aeolian (Wind) Depositional Landforms — sand dunes (barchans: crescent-shaped dunes with horns pointing downwind; seif/longitudinal dunes: parallel to wind direction; star dunes; the Thar Desert has all types); loess deposits (wind-blown silt — not prominent in India but covers large areas of China and Central Asia). (3) Glacial Depositional Landforms — moraines (till deposited by glaciers: terminal/end moraine, lateral moraine, medial moraine, ground moraine — the Himalayan valleys show these features); drumlins (streamlined hills of glacial till); eskers (sinuous ridges of sand/gravel deposited by subglacial streams); outwash plains (sorted sediment deposited by meltwater beyond the glacier front). (4) Marine/Coastal Depositional Landforms — beaches (sand and gravel deposited by wave action — Marina Beach in Chennai is India's longest natural urban beach); spits (elongated deposits of sand extending from headlands — Dhanushkodi near Rameshwaram); bars (offshore ridges — Chilika Lake in Odisha is a lagoon formed behind a sandbar); tombolo (sand bar connecting island to mainland). In India, the vast alluvial deposits of the Northern Plains (up to 2,000 m deep) form one of the most fertile and productive agricultural regions on Earth, entirely created by Himalayan river deposition.
Weathering and Erosion in the Indian Himalayas
The Himalayas are geologically young (about 50 million years old), tectonically active, and undergoing simultaneous uplift and erosion — making them a living laboratory for geomorphic processes: (1) Uplift Rates — the Himalayas are rising at 5-10 mm per year (offset by erosion of about 2-5 mm per year, so net elevation gain is slow); this ongoing collision between the Indian and Eurasian plates generates earthquakes, drives metamorphism, and creates one of the highest sediment yields in the world (the Ganga-Brahmaputra system carries about 2.5 billion tonnes of sediment annually to the Bay of Bengal). (2) Frost Weathering — dominant above 4,000 m; repeated freeze-thaw cycles create scree slopes and rock glaciers; particularly intense in Ladakh, Spiti, and upper Sikkim; the Himalayas have an estimated 15,000 glaciers covering 40,000+ sq km. (3) Landslides — the most devastating geomorphic hazard in the Himalayas; triggered by: heavy monsoon rainfall, earthquakes, road construction (cutting into steep slopes destabilizes them), and deforestation; major events: Kedarnath disaster 2013 (cloudburst + glacial lake drainage killed 5,000+), Chamoli rockslide 2021 (rock-ice avalanche destroyed Dhauliganga hydropower project, killed 200+), Joshimath subsidence 2023 (slow landslide causing building damage — town sits on ancient landslide deposits). (4) Glacial Erosion — Himalayan glaciers have carved spectacular U-shaped valleys, cirques, aretes, and moraines; the Gangotri Glacier (30.2 km) has retreated about 1.5 km since 1817; most Himalayan glaciers are retreating due to global warming (IPCC AR6 projects 1/3 to 2/3 of Himalayan glacial ice could be lost by 2100); this threatens water security for 1.6 billion people downstream. (5) Himalayan Rivers and Deep Gorges — antecedent rivers (Indus, Sutlej, Brahmaputra) have carved gorges through the Himalayas as the mountains rose around them; the Indus Gorge near Nanga Parbat is 4,500 m deep (one of the deepest in the world); the Kali Gandaki Gorge between Dhaulagiri and Annapurna (Nepal) is the world's deepest. (6) Geomorphic Zonation — from south to north: Siwalik Hills (young, easily erodible sedimentary rocks — severe erosion), Lesser Himalayas (moderate erosion of metamorphic rocks), Greater Himalayas (slow glacial and frost erosion of crystalline rocks), Trans-Himalayas (arid, wind erosion dominant in Ladakh and Spiti).
Karst Topography — Processes and Indian Examples
Karst topography develops on soluble rocks (primarily limestone and dolomite) through chemical weathering by carbonation — the process where slightly acidic rainwater (H2O + CO2 = H2CO3, carbonic acid) dissolves calcium carbonate (CaCO3). Key karst landforms: (1) Surface Features — sinkholes/dolines (circular depressions where limestone dissolves or collapses — primary karst feature), poljes (large flat-floored basins surrounded by steep walls), uvulas (compound sinkholes), karren/lapies (grooved and furrowed rock surfaces), tower karst (isolated steep-sided limestone hills — best developed in Guilin, China; found in a limited form in Meghalaya). (2) Underground Features — caves (formed by groundwater dissolving limestone along joints and bedding planes; Mawsmai Cave and Krem Liat Prah in Meghalaya's Jaintia Hills are India's most significant caves; Krem Liat Prah at 31+ km is the longest cave in the Indian subcontinent); stalactites (calcium carbonate deposits hanging from cave ceilings — formed as CO2 escapes from dripping water, precipitating CaCO3); stalagmites (grow upward from cave floors from dripping water); pillars/columns (when stalactites and stalagmites meet). (3) Karst in India — Meghalaya: the "Abode of Caves" — contains India's longest, deepest, and most numerous caves; the Jaintia Hills, East Khasi Hills, and South Garo Hills have extensive karst systems; the limestone is Eocene age; cave exploration by Meghalaya Adventurers' Association has documented over 1,700 caves. Vindhyan Region (MP): limestone areas around Panna and Rewa show karst features including Pandav Caves. Chhattisgarh: Kutumsar Cave (1,327 m long, near Jagdalpur) in the Kanger Valley National Park — India's best-studied cave; has unique cave-adapted (troglobitic) fauna including blind fish. Andhra Pradesh/Telangana: Belum Caves (3,229 m — the second longest cave in India) and Borra Caves (in Araku Valley, Vizag). Karnataka: limestone areas near Dharwad, Belgaum; coral limestone along the coast. Karst presents challenges for construction (sudden collapses, unstable foundations) but caves also serve as important water reservoirs and biodiversity refugia.
Coastal Erosion and Coastal Geomorphology
India's 7,516.6 km coastline is shaped by wave, tidal, and current processes: (1) Wave Erosion Processes — hydraulic action (wave impact, especially in confined spaces like joints and caves — compressed air in cracks exerts immense pressure), abrasion (waves hurl sediment against cliffs — the "sandblasting" effect), corrosion (chemical dissolution of coastal rocks, especially limestone), and attrition (rock fragments are rounded and reduced in size as they collide). (2) Erosional Landforms — sea cliffs (steep rock faces cut by wave action — common along the Konkan coast, Kerala coast, and parts of Tamil Nadu), wave-cut platforms (flat rocky surfaces at the base of retreating cliffs), sea caves (formed by selective erosion of weaker rock — Ajanta-like caves at Elephanta Island, Mumbai; Panaiyur cave, TN), arches (when caves on opposite sides of a headland meet — limited examples in India), sea stacks (isolated rock pillars left after arch collapse). (3) India's Coastal Erosion — about 40% of India's coastline is experiencing erosion: Kerala coast (losing about 0.6 m/year; Chellanam village near Kochi severely affected; sea walls being built), Chennai coast (losing 2-3 m/year; sand mining from rivers reduces sediment supply to coast); West Bengal (Sundarbans islands like Ghoramara and Mousuni are disappearing due to rising seas and erosion); Odisha (Pentha coast severely eroded). (4) Coastal Regulation Zone (CRZ) — CRZ Notification 2019 classifies coastal areas into CRZ-I (ecologically sensitive — mangroves, coral reefs, sand dunes; no construction), CRZ-II (developed urban areas; limited construction), CRZ-III (rural and undeveloped; minimal construction), CRZ-IV (aquatic area up to 12 nautical miles). National Centre for Coastal Research (NCCR, Chennai) monitors shoreline changes using satellite data. (5) Coastal Depositional Landforms — beaches (Marina Beach, Chennai; Juhu, Mumbai; Puri, Odisha; Goa beaches), spits (Dhanushkodi), barrier islands (along Odisha-WB coast), lagoons (Chilika — India's largest coastal lagoon; Vembanad, Kerala; Pulicat, AP-TN), and mangrove deltas (Sundarbans). (6) Tsunamis — the 2004 Indian Ocean Tsunami (caused by a 9.1 magnitude earthquake off Sumatra) devastated Tamil Nadu, Andaman Islands, and parts of Kerala/AP coasts; Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad established in 2007 provides tsunami alerts for the Indian Ocean.
Desertification in India — Causes, Extent, and Remediation
Desertification is land degradation in arid, semi-arid, and dry sub-humid areas resulting from climate variations and human activities. India is particularly vulnerable: (1) Extent — according to ISRO's Desertification and Land Degradation Atlas (2021), about 97.85 million hectares (29.7% of India's total geographic area) is undergoing degradation; Rajasthan, Gujarat, MP, Maharashtra, and Jharkhand are most affected. (2) Types of Desertification in India — (a) Wind erosion and sand encroachment in the Thar Desert belt (Rajasthan, Gujarat, Haryana); the Thar Desert is expanding eastward and southward; sand dune migration threatens agricultural land and villages; (b) Water erosion creating ravines and badlands (Chambal, Yamuna, Mahi, Sabarmati river basins); (c) Vegetation degradation from overgrazing and fuel-wood collection (throughout semi-arid India); (d) Salinization from irrigation (Punjab, Haryana, western UP, Gujarat, Rajasthan); (e) Mining degradation (Jharkhand, Odisha, Chhattisgarh, Goa). (3) Human Causes — overgrazing (India has the world's largest livestock population — 535 million; carrying capacity of pastures exceeded in most states), deforestation, unsustainable agriculture (monoculture, chemical-intensive farming depleting soil organic matter), excessive groundwater extraction (lowering water tables in Rajasthan, Punjab, Haryana), and industrial pollution. (4) Climate Causes — increasing drought frequency and intensity, changing monsoon patterns, rising temperatures. (5) Remediation — shelterbelts and windbreaks (Jodhpur-Barmer belt of Prosopis trees reduces wind erosion); sand dune stabilization (planting grasses and shrubs, micro-windbreaks); afforestation (National Mission for Green India targets 10 million hectares); community-managed grazing rotation; watershed management (PMKSY-WDC component); MGNREGA land development works. (6) India and UNCCD — India is a signatory to the UN Convention to Combat Desertification; hosted COP 14 of UNCCD in 2019 at Greater Noida; committed to achieving Land Degradation Neutrality (LDN) by 2030 and restoring 26 million hectares of degraded land under the Bonn Challenge. The India Meteorological Department monitors droughts using the Standardised Precipitation Index (SPI) and remote-sensing-based vegetation indices.
Factors Controlling Weathering — Climate, Rock, Relief, and Time
Weathering is not a uniform process — its type, rate, and intensity vary with multiple controlling factors: (1) Climate — THE most important factor; temperature determines whether physical or chemical weathering dominates: hot-wet climates (tropical India) → intense chemical weathering producing thick residual soils (laterite in Kerala can be 30+ m deep); hot-dry climates (Thar Desert) → physical weathering by thermal expansion, salt weathering; cold climates (Ladakh, Himalayas above 4,000 m) → frost weathering dominant; temperate climates → moderate chemical weathering. Precipitation is equally critical: water is essential for chemical weathering (hydrolysis, carbonation, oxidation all require water); the Western Ghats windward side (3,000+ mm rainfall) has deep chemical weathering profiles while the rain-shadow side (500-600 mm) has shallower profiles. (2) Rock Type — mineral composition, porosity, permeability, and structure control susceptibility: quartz is highly resistant to chemical weathering (remains as sand); feldspar is easily hydrolysed (breaks down to clay); olivine oxidises rapidly; limestone dissolves through carbonation; granite (with quartz, feldspar, mica) weathers to form grus (loose granular material) in arid areas but deep clay-rich saprolite in humid areas. Rock structure matters: well-jointed and fractured rocks weather faster (water penetrates along joints); massive, joint-free rocks are more resistant. (3) Relief — slope angle affects water retention: steep slopes shed water quickly (less chemical weathering) but are prone to mass wasting; gentle slopes retain water longer (deeper chemical weathering); altitude creates temperature changes (lapse rate: 6.5 degrees Celsius per 1,000 m) affecting weathering type. (4) Vegetation — dense vegetation promotes chemical weathering (root exudates, increased CO2 from decomposition, moisture retention) but protects against physical erosion; bare rock surfaces are more exposed to physical weathering. (5) Time — given enough time, all rocks weather; the ancient Peninsular Plateau of India (3+ billion years) has deep weathering profiles (100+ m in some areas) while young Himalayan rocks have shallow, immature profiles. The concept of "differential weathering" — where rocks of varying resistance weather at different rates within the same area — creates spectacular landforms: the granite tors of Jawai (Rajasthan), the basalt pillars of Lonar Crater (Maharashtra), and the sandstone formations of Pachmarhi (MP).
Weathering Products and Their Economic Significance
Weathering transforms primary minerals into secondary minerals and residual deposits with significant economic value: (1) Bauxite (Aluminium Ore) — formed by intense tropical chemical weathering (laterisation) of aluminium-rich rocks; silica is leached out leaving aluminium hydroxides (gibbsite, boehmite); found on flat-topped plateaus in high-rainfall areas: Odisha (largest deposits — Koraput, Kalahandi), Gujarat (Kutch), Maharashtra (Kolhapur), Jharkhand, Chhattisgarh; India has the 5th largest bauxite reserves globally. (2) Laterite — the residual product of tropical weathering; iron and aluminium oxides remain after silica and bases are leached; hardened laterite (also called "murram") is used as building material across South India; laterite was historically quarried in blocks for temple construction in Kerala and Karnataka. (3) Kaolin (China Clay) — high-quality clay mineral (kaolinite) produced by hydrolysis of feldspar in granite; used in ceramics, paper coating, and pharmaceuticals; deposits in Rajasthan (Bhilwara), Kerala, and Jharkhand. (4) Regolith and Soil — the most fundamental product of weathering; all soils originate from weathering of bedrock; the relationship between parent rock and soil type determines agricultural productivity across India (basalt → black soil; granite → red soil; alluvium → alluvial soil). (5) Weathering-Enriched Mineral Deposits — supergene enrichment occurs when weathering concentrates ore minerals near the surface: iron ore deposits in the Dharwar rocks are enriched by leaching of silica from banded iron formations (BIF), leaving high-grade hematite ore (65%+ Fe content) — this process created the rich iron ore deposits of Singhbhum (Jharkhand), Keonjhar (Odisha), and Bellary (Karnataka). (6) Manganese — supergene concentration of manganese by weathering produces high-grade manganese oxide deposits in Odisha, Maharashtra, and MP. (7) Nickel Laterite — weathering of ultrabasic rocks concentrates nickel in laterite profiles; significant deposits identified in Odisha (Sukinda — also has chromite). Understanding weathering products is essential for both physical geography (soil formation, landform evolution) and economic geography (mineral deposits, construction materials).
Relevant Exams
Weathering and erosion is a core geomorphology topic tested across all competitive exams. UPSC asks about types of weathering, erosional and depositional landforms, soil erosion in India (Chambal ravines, coastal erosion), Davis vs Penck models, and desertification. Karst topography (Meghalaya caves) and Himalayan geomorphic processes (glacial retreat, landslides) are frequently tested. SSC and banking exams focus on factual recall — agents of erosion, types of weathering, examples of Indian landforms. Questions on the Chambal ravines, Himalayan glaciers, Joshimath subsidence, coastal erosion, and soil conservation measures are frequently asked.