Weathering and Mass Movement

• Weathering is the in-situ breakdown (disintegration) and alteration (decomposition) of rocks and minerals at or near the Earth’s surface, with no large-scale transport of the products.
• Mass movement (mass wasting) is the downslope movement of that loosened material as a mass under gravity, helped but not carried by water, ice or air.
• Together they open the sequence of denudation among the exogenic forces: weathering prepares material, mass movement delivers it to rivers, glaciers, wind and waves, and both shape slopes directly.

Weathering

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Meaning and Concept

• Cliff David Ollier (1969) defined weathering as the breakdown and alteration of minerals near the surface into products more in equilibrium with new physico-chemical conditions.
  • Minerals formed under high pressure and temperature are unstable at the surface; weathering is their adjustment to air, water and life.
• Parry Reiche (1950) stressed the contact of the lithosphere with the atmosphere, hydrosphere and biosphere; Walter David Keller (1957) objected that rocks are never in permanent equilibrium, so “which were in equilibrium” should be dropped.
• Boris Polynov (1937) put it most briefly: the change of rocks from the massive to the clastic state.
• Arthur Holmes excluded rainwash and wind, since these are erosional: weathering holds only while no large-scale transport occurs.
• The working definition therefore has three parts:
  • Mechanical and chemical change by water, temperature, gases and organisms.
  • In situ operation, making weathering a static process.
  • No transport except gravity-driven mass movement of the rock waste.
• Regolith is the blanket of weathered material over fresh bedrock; the weathering front is its lower boundary.
  • Its depth depends on climate, the water table and duration of weathering, from a few centimetres in deserts to tens of metres in the humid tropics.
• Key terms are collected in the weathering terminology list.

Controlling Factors of Weathering

• The type and rate of weathering vary with five interacting controls; no single one explains a landscape.

Rock Composition and Structure

• Mineral composition sets chemical susceptibility.
  • Carbonate rocks dissolve readily; quartz-rich rocks resist; ferromagnesian minerals (olivine, pyroxene) weather faster than quartz, following the reverse of their crystallisation order.
• Joints, bedding planes, faults and folds give water and roots entry.
  • Well-jointed rocks disintegrate into blocks; vertically dipping strata loosen faster than compact horizontal beds.
• Texture and colour matter: coarse-grained, multi-coloured rocks such as granite heat unevenly.
• Indian example: the jointed Deccan basalts weather into rounded corestones, while the massive Dharwar and Peninsular gneisses stand out as bare domes.

Climate

• Climate is the master control because it fixes water and heat, the two drivers of every reaction.
  • Hot humid tropics: abundant water and high temperature make chemical weathering and leaching dominant and weathering profiles deep.
  • Hot deserts and semi-arid lands: large daily temperature range and scarce water favour mechanical disintegration and salt crystallisation.
  • Cold temperate and periglacial lands: repeated freeze–thaw dominates.
  • Permanent ice cover: both processes nearly cease.
• Rock behaviour changes with climate: limestone is weak in humid regions but forms bold scarps in deserts.
• Monsoon lands alternate: mechanical processes in the dry hot summer, chemical and biochemical ones in the wet season.
• Louis C. Peltier (1950) plotted mean annual temperature against mean annual rainfall to divide the world into weathering regions.
  • Strong chemical weathering in hot wet climates; moderate chemical in warm moist ones.
  • Frost (mechanical) weathering strongest where it is cold with moderate moisture; chemical weathering with frost action where cool and wet.
  • Very slight weathering in hot, very dry and in very cold, dry climates.
  • The same logic underlies his morphogenetic regions in climatic geomorphology.

Slope and Topography

• Steep slopes keep rock surfaces fresh because debris is removed at once by falls, slides and creep, exposing new rock.
• Gentle slopes and plateaus retain a thick regolith that shields bedrock mechanically but holds water for deep chemical weathering.
• Aspect and altitude alter insolation, frost frequency and moisture.
• Indian example: thin, stony regolith on Himalayan scarp faces against deep weathered mantles on the Meghalaya and Chotanagpur plateaus.

Vegetation, Biota and Time

• Vegetation plays a dual role.
  • Roots bind the regolith and canopies shade rock from sharp heating.
  • Roots also wedge joints open and release organic acids and CO₂ that speed chemical attack.
• Time allows slow processes to accumulate.
  • Old, stable surfaces such as the Peninsular plateau carry deeper profiles than young, rapidly eroding Himalayan slopes.

Types of Weathering

• Weathering is grouped as physical, chemical and biological, but the processes are intimately linked.
  • Richard John Chorley and co-authors (1985) noted that no chemical weathering occurs without physical stress, thermal cracking probably needs water, and wherever plants grow chemical weathering becomes partly biochemical.
  • The grouping is by the dominant agent only.

Physical (Mechanical) Weathering

• Physical weathering is the disintegration of rock without change in mineral composition, producing blocks, boulders, grit and sand.
• It increases surface area, readying rock for chemical attack.

Unloading: Sheeting and Exfoliation

• Unloading (pressure release) occurs when erosion removes the overburden from rocks formed at depth, such as granite batholiths.
  • The rock expands upward and cracks parallel to the ground surface; this is sheeting.
  • Sheets peel off as curved shells, producing exfoliation domes.
• Richard H. Jahns (1943) listed seven possible causes of sheeting: cooling strains, tectonic stress, insolation, hydration, fire/frost/plants, unloading and their combinations; unloading is now the accepted main cause.
• Related forms of stress release:
  • Rockbursts in deep quarries and mines, when freshly cut faces expand violently.
  • Spalling into platy fragments, and cambering of brittle sandstone caps over weak clays.
• Indian example: Kanke dome near Ranchi and the granite domes and inselbergs of the Karnataka plateau such as Savandurga.

Thermal (Insolation) Weathering

• Block disintegration: the daily heating and cooling of bare rock sets up stresses that open joints parallel and normal to the surface, and large blocks break away.
• Granular disintegration: in coarse-grained, multi-mineral rocks, dark minerals absorb more heat than light ones, so grains expand unequally and crumble into grus (coarse sand).
• Exfoliation (onion weathering) by heat: rock is a poor conductor, so only the outer few centimetres expand; thin shells flake off and wind removes them.
• Rain-shock shattering: sudden showers on sun-heated rock cause surface spalling.
• Evaluation:
  • Laboratory tests by Eliot Blackwelder and David Tressel Griggs (1936) heated and cooled dry rock through cycles equal to centuries of desert days without cracking it, but cracks formed quickly when water was used for cooling; for decades insolation weathering was dismissed and moisture credited instead.
  • Field studies since 2005 in the deserts of the south-western United States and the Gobi found crack orientations aligned with the Sun’s directional daily heating, and monitoring of exfoliation sheets in Yosemite links rockfalls to thermal cycling.
  • The current view: insolation causes fatigue and slow crack growth in natural, unevenly heated rock, usually working together with moisture.
• Indian example: angular debris fields and grus in the Thar Desert and the Aravalli foothills of Rajasthan.

Frost Weathering (Congelifraction)

• Water expands by about 9 per cent on freezing, wedging open joints and pores.
• Frost wedging splits rock along joints into angular blocks; frost shattering breaks weak zones into sharp fragments.
• Effect depends on freeze–thaw frequency, not on extreme cold; porous sedimentary rocks suffer more than massive granite.
• Kirk Bryan (1946) proposed the term congelifraction for frost shattering in periglacial areas, where it also produces frost-riven polygons.
• The debris collects below cliffs as scree (talus).
• Indian example: scree slopes and talus cones of Ladakh, Lahaul–Spiti and the Zanskar Range.

Salt Crystallisation, Wetting–Drying and Other Processes

• Salt weathering: saline water enters pores, evaporates and leaves crystals that grow and expand on heating or hydration, prising grains apart.
  • Most active in hot deserts and on coasts with salt spray; it forms honeycomb (tafoni) cavities.
  • Indian example: saline flats of the Rann of Kutch and western Rajasthan.
• Slaking (wetting and drying): alternate swelling and shrinking of clay-rich rocks such as shales and mudstones disaggregates them.
• Minor processes: boulder cleaving by heating, dirt cracking of boulders, and fire that flakes rock surfaces during forest or brush fires.

Chemical Weathering

• Chemical weathering is the decomposition of minerals by chemical reaction, forming new minerals and soluble products.
• Water is the medium: pure water is almost inert, but with dissolved CO₂, O₂ and organic acids it becomes a strong solvent.
• Reaction rates rise with temperature and moisture, so it is fastest in the humid tropics.

Solution and Carbonation

• Solution is the direct dissolving of soluble minerals such as rock salt and gypsum; it is the first step in chemical attack.
• Carbonation is the reaction of carbonic acid (rainwater plus atmospheric and soil CO₂) with carbonate rocks.
  • CO₂ + H₂O → H₂CO₃; then CaCO₃ + H₂CO₃ → Ca(HCO₃)₂, a soluble calcium bicarbonate carried away in solution.
  • CO₂ dissolves better in cold water, while reaction speed rises with warmth; soil CO₂ from roots and microbes greatly raises acidity.
• Robert Minard Garrels (1960) listed the variables controlling limestone solution: CO₂ partial pressure, pH, temperature and the concentrations of the ions involved.
• The result is karst: sinkholes, dolines, uvalas, poljes and caves, detailed in karst landforms.
• Indian example: the caves of the Meghalaya plateau (Krem Liat Prah), Borra Caves in the Eastern Ghats, and the Himalayan limestones around Dehradun.

Hydration and Hydrolysis

• Hydration is the addition of water molecules to a mineral’s structure.
  • It increases volume, as in anhydrite changing to gypsum, causing physical stress and granular disintegration.
  • It also prepares mineral surfaces for oxidation and carbonation.
• Hydrolysis is the reaction between mineral ions and the H⁺ and OH⁻ ions of water; it is the most important process in silicate rocks.
  • Potassium feldspar + carbonic acid + water → kaolinite (clay) + silicic acid + potassium and bicarbonate ions in solution.
  • This kaolinisation turns granite into clay-rich regolith, leaving quartz grains as sand.
• Indian example: china clay (kaolin) deposits of Kerala and the Rajmahal area of Jharkhand, formed by hydrolysis of feldspar-rich rocks.

Oxidation and Reduction

• Oxidation is the combination of minerals, chiefly iron, with oxygen dissolved in water.
  • Ferrous iron in pyrite, siderite and silicates changes to ferric oxides and hydroxides (hematite, limonite), giving red and yellow colours and weakening the rock.
• Reduction takes place in waterlogged, oxygen-poor conditions, giving grey and greenish gley colours.
• Indian example: oxidation of iron-rich Vindhyan sandstones helps block disintegration along the Kaimur Range and Rewa scarps; the red soils of Tamil Nadu and Chotanagpur owe their colour to oxidation.

Chelation and Base Exchange

• Chelation is the binding of metal cations within ring-shaped organic molecules, which pulls ions out of mineral lattices; it is essentially chemical weathering by plants and humus.
• Base (cation) exchange swaps cations between clay surfaces and soil water, altering mineral properties and nutrient supply.
• Products of chemical weathering fall into three groups:
  • Solutes (Ca, Na, K, Mg ions) carried to seas and re-precipitated as limestones and evaporites.
  • Clays from feldspars and ferromagnesian minerals, the raw material of shales.
  • Resistant residues such as quartz, which become sandstones.

Biological and Biochemical Weathering

• Boris Polynov held that completely sterile weathering is impossible: organisms take part in all weathering, everywhere.
• Faunal weathering: burrowing animals, termites and earthworms mix the regolith and expose fresh material.
  • Charles Darwin (1881) measured how earthworms move soil to the surface; termite mounds do the same in the tropics.
• Floral weathering: root wedging widens cracks, and dense vegetation creates a moist, CO₂-rich soil atmosphere.
• Biochemical weathering: humic and fulvic acids, bacterial acids (oxalic, acetic, lactic) and acids from lichens and fungi attack silicates and carbonates.
  • Micro-organisms help form desert varnish by concentrating iron and manganese oxides on rock surfaces.
• Anthropogenic weathering: quarrying, blasting, mining and deforestation break rock in days that nature takes millennia to weather; human effects are covered in anthropogenic geomorphology.
• Indian example: lichen growth on the rock-cut monuments of the Deccan, and termite mounds across the peninsular plateau.

Weathering Types at a Glance

Geomorphic Significance of Weathering

• Produces rock waste: weathering generates the regolith that every other exogenic process works on.
• Prepares for erosion and mass movement: loosened material is easily removed by rivers, wind, ice and waves, and a deep weathered mantle is the usual failure zone of landslides.
  • Deforestation-accelerated weathering in the Garhwal and Kumaun Himalaya feeds heavy sediment loads to the Ganga, Yamuna, Ghaghara and Kosi, raising their beds and flood frequency in the plains.
• Lowers the land surface over time, together with mass movement and erosion.
• Creates landforms by differential weathering: weaker rock goes first, leaving resistant forms standing.
  • Tors: joint-bounded granite blocks left after the weathered mantle is stripped, as in the boulder hills of Hampi.
  • Exfoliation domes and inselbergs.
  • Karst from solution.
  • Duricrusts (laterite, calcrete, silcrete): hardened crusts that protect plateau tops and form mesas and scarps.
  • Talus cones and fans, stone lattices, honeycomb weathering and pedestal rocks.
• Supplies economic resources: residual deposits of bauxite, laterite iron ore and kaolin, and nutrients to soils.
• Evaluation: weathering and erosion act together, so the share of each in a landform is hard to separate; climate-driven weathering depth is now seen as a key control on landscape evolution in the tropics.

Weathering and Soil Formation

• Soil begins as regolith: weathering supplies the mineral skeleton (sand, silt, clay) and releases plant nutrients.
• Mechanical weathering sets texture by breaking rock into particles; chemical weathering forms clays and iron oxides that give soil structure, colour and exchange capacity.
• Biological processes add humus and turn inert regolith into living soil; horizons (O, A, B, C) then develop through leaching, eluviation and illuviation.
• Hans Jenny (1941) expressed soil as a function of climate, organisms, relief, parent material and time, the same factors that control weathering.
• Indian illustrations:
  • Laterite: intense hydrolysis and leaching in hot, wet climates remove silica and bases, leaving iron and aluminium oxides; Francis Buchanan (1807) named it from Angadipuram, Kerala, now a national geological monument.
  • Black (regur) soils from weathering of Deccan basalt, rich in montmorillonite clay.
  • Red soils from oxidised weathering of granites and gneisses.
  • Thin, stony mountain soils in the Himalaya, where slopes remove regolith before soils mature.

Mass Movement

Meaning and Concept

• Richard John Chorley and co-authors (1985) defined mass movement as the detachment and downslope transport of soil and rock under gravity, accelerated by water, ice and air, at all scales and rates.
• Arthur L. Bloom (1978) used mass wasting for gravitational movement without a transporting medium, while noting that water steepens slopes by undercutting and raises seepage pressure.
• Mass movement is the broader term as it covers both detachment and transport; the two are used interchangeably in practice.
• Key features:
  • Gravity is the driving force; water, ice or air act only as lubricants.
  • Material moves en masse, not grain by grain.
  • Deposits collect at slope bases as scree, talus cones and colluvium.
• Where rocks are shattered and relief is high, mass movement may lower mountains more than rivers or glaciers do, which links it closely with tectonics.
• Landslides have a preparatory phase with precursors such as cracks, tilting and seepage that usually go unnoticed.
• Terms are listed in the mass movement and landslides terminology post.

Factors of Mass Movement

• Slope stability is expressed by the factor of safety (Fs): shear strength of material ÷ shear stress acting on it.
  • Fs > 1: stable; Fs < 1: failure and movement.
  • Movement follows when shear stress rises, shear strength falls, or both.
• David J. Varnes (1978) grouped the factors accordingly.

• Preparatory factors: steep slopes, weak or sheared lithology, adverse dip of joints and bedding, deep weathering.
• Triggers: intense or prolonged rain (the commonest in India), earthquakes, snowmelt and toe erosion.
• Human factors: road cutting, hydropower tunnels, deforestation, slope loading by buildings, blocked drainage and changed farming practices.

Classification of Mass Movement

• Classifications use speed, type of motion, material and water content; each has limits because real movements are often complex.

Sharpe’s Classification (1938)

• Charles Farquharson Stewart Sharpe (1938) gave the first systematic scheme, based on type of movement, material, water or ice content and velocity.

• Sharpe also divided mudflows into arid, alpine and volcanic types.

Varnes’ Classification (1978)

• Varnes classified slope movements by type of movement and type of material, and is the most widely used scheme.
  • Movement types: falls, topples, slides (rotational and translational), lateral spreads, flows, and complex combinations.
  • Materials: rock, debris (20–80 per cent fragments coarser than 2 mm) and earth (80 per cent or more finer than 2 mm).
  • He added topples and spreads and a velocity scale from extremely slow to extremely rapid.

• Oldrich Hungr, Serge Leroueil and Luciano Picarelli (2014) updated the scheme to 32 landslide types, redefining materials in standard geotechnical terms.
• A simpler scheme by direction separates vertical (falls, subsidence), lateral (block slides, spreads, cambering, sackung) and diagonal (creep, slides, flows) movements.

Types of Mass Movement

Slow Movements: Creep and Solifluction

• Creep is the very slow, imperceptible downslope movement of regolith, a few millimetres to centimetres a year.
  • Soil creep is driven by wetting–drying, freeze–thaw and heating–cooling, which heave particles normal to the slope and let them settle vertically.
  • Talus creep rearranges scree; rock creep moves joint blocks slowly, and deep-seated creep (sackung) deforms whole mountainsides.
  • Signs: tilted trees, poles and walls, curved tree trunks and terracettes.
• Solifluction is the slow flow of water-saturated regolith; the term was coined by Johan Gunnar Andersson (1906).
  • In periglacial areas, a thawed active layer over permafrost flows as gelifluction, forming lobes and terraces.
• Indian example: leaning trees and walls on Himalayan hill towns, and solifluction lobes in Ladakh.
• Rock creep (deep-seated creep) moves whole slopes of jointed rock, often tens to hundreds of metres deep.
  • Michael J. Selby (1982) set it apart from soil creep by its great depth and isolation from daily and seasonal climatic changes, and from a landslide by the lack of a single, clearly defined failure plane.
  • It works through bending, buckling and fracturing of inclined beds at millimetres to centimetres a year, leaving uphill-facing scarps, trenches and double ridges, and may later turn into a rapid slide.

Flows

• Flows move like a viscous fluid without a single failure plane; water content rises from debris flow through earthflow to mudflow.
• Earthflow: slow to moderate flow of saturated, clay-rich material, often at the toe of a slump.
• Mudflow: fast flow of fine sediment with abundant water, common in semi-arid lands after rare downpours, where Eliot Blackwelder (1928) described them, and in young mountains.
• Debris flow: coarse, bouldery slurry that follows existing channels and can travel several kilometres.
• Debris avalanche: an extremely rapid, often dry or partly saturated flow on steep slopes.
• Lahar: a volcanic mudflow; the 1919 Kelud eruption in Java released crater-lake water that killed more than 5,000 people.
• Indian example: the channelised debris flows of Wayanad (2024) and Kedarnath (2013).

Slides

• A slide moves along a discrete shear surface.
• Slump (rotational slide): a block rotates backward on a curved, spoon-shaped surface; common in thick, cohesive clays and alluvium undercut at the toe.
  • Indian example: bank slumping along the Ganga, Kosi and Brahmaputra during monsoon floods.
• Translational slide: movement along a planar surface such as bedding, foliation or a joint dipping downslope; includes debris slides and rock (block) slides.
  • Examples: the Frank Slide, Alberta (1903) and the Gros Ventre Slide, Wyoming (1925); in India, the Nainital landslide of 18 September 1880, which killed 151 people.
• Favourable sites: steep hillslopes and valley sides, fault scarps, deeply incised rejuvenated valleys, sea cliffs, undercut banks of alluvial rivers, and hills degraded by deforestation, road cutting and settlement.

Falls and Topples

• Falls: free fall, bounce and roll of rock or debris from cliffs and steep cuts; the fastest movement.
  • Promoted by frost wedging, stress release, root wedging and undercutting of a weak underlying layer.
• Topples: forward rotation of blocks about a low pivot, usually in steeply jointed rock.
• Indian example: rockfalls along Himalayan highways such as the Char Dham roads during the monsoon.

Lateral Spreads and Subsidence

• Lateral spreads: extension of a mass on gentle slopes over a weak or liquefied layer, usually earthquake-triggered.
  • Indian example: liquefaction and lateral spreading in the Rann of Kutch and at Kandla and Navlakhi ports during the 2001 Bhuj earthquake.
• Subsidence: vertical sinking by collapse over caves, mines or lava tubes, or settlement after withdrawal of groundwater, oil or gas.
  • Indian example: Joshimath (2022–23) and the coal-mining areas of Jharia.

Geomorphic Significance of Mass Movement

• Main agent of slope retreat and evolution, shaping slope form in step with weathering, as seen in slope elements and slope development.
• Links slopes to rivers, supplying most of the sediment load in mountains.
• Creates landforms: scars, scarps, hummocky debris, terracettes, talus cones, landslide terraces, and widening of valley meanders by bank slumping.
• Dams rivers, forming landslide lakes that may later burst and flood the valley below.
• Lowers mountain relief in tectonically active ranges such as the Himalaya.

Landslides in India

Distribution

• About 0.42 million km², nearly 12.6 per cent of India’s land area, is landslide-prone, covering 22 states and 2 Union Territories.
• The Geological Survey of India has completed National Landslide Susceptibility Mapping at 1:50,000 over about 4.3 lakh km², with roughly 15 per cent classed as highly susceptible.
• The Landslide Atlas of India (2023) by the National Remote Sensing Centre mapped about 80,000 landslides between 1998 and 2022, with Rudraprayag and Tehri Garhwal the most exposed districts.

Recent Landslide Events

• Joshimath sits on thick, unconsolidated old landslide material, as a committee chaired by Mahesh Chandra Mishra warned in 1976.

Causes of Landslides in the Himalaya

• Natural causes:
  • Young, fragile rocks crushed along the Main Central and Main Boundary Thrusts.
  • Steep slopes and high relief from rapid uplift and river incision.
  • High seismicity: most prone areas lie in seismic zones IV and V (Uttarkashi 1991, Chamoli 1999, Sikkim 2011).
  • Intense monsoon rain and cloudbursts, snowmelt and glacier retreat leaving loose moraine.
  • Toe erosion by rapidly incising rivers.
• Human causes:
  • Road widening and blasting that steepen and destabilise slopes.
  • Hydropower tunnels, dams and reservoirs that alter groundwater and load slopes.
  • Deforestation and unplanned construction on steep slopes with poor drainage.
  • Rising tourism and pilgrimage pressure in hill towns.
• Emerging factor: more frequent extreme rainfall under a warming climate is raising landslide frequency in both the Himalaya and Western Ghats.

Mitigation and Management

• Structural measures:
  • Drainage: surface ditches and subsurface drains, as water is the main trigger.
  • Slope reshaping: removing load at the head, benching and flattening.
  • Support: toe buttresses, retaining walls, piles, rock bolts, anchors and shotcrete.
  • Rockfall protection: catch ditches, wire mesh, barriers and sheds.
  • Bio-engineering: grasses such as vetiver, shrubs and afforestation to bind soil.
• Non-structural measures:
  • Hazard zonation and land-use regulation, keeping settlement off the most susceptible slopes.
  • Early warning based on rainfall thresholds and slope monitoring.
  • Building codes, community awareness and drills.
• Institutional framework:
  • National Landslide Risk Management Strategy (NDMA, September 2019): hazard mapping, monitoring and early warning, awareness, capacity building, mountain zone regulations, and stabilisation with a proposed special purpose vehicle.
  • National Landslide Risk Mitigation Project (November 2024): about ₹1,000 crore for 15 states from the National Disaster Mitigation Fund.
  • GSI’s Landslide Early Warning System: operational from the 2025 monsoon in Darjeeling, Kalimpong and the Nilgiris, with experimental bulletins for 18 more districts; maps are shared through the Bhusanket portal and the Bhooskhalan app.
• Evaluation: mapping has advanced fast, but enforcement of building and land-use rules, carrying-capacity limits for hill towns and local warning dissemination remain the weak links.