Mass Movement and Landslides: Terminology for UPSC Geography Optional

Mass movement is the downslope transfer of rock and soil under gravity, from creep of a few millimetres a year to avalanches faster than a car. It links weathered hillsides to rivers and valley floors, and it is India’s most frequent geomorphic disaster, from the Himalaya to the Western Ghats. This post covers the mechanics, the classification, each movement type and the vocabulary of landslide hazard.

Each entry gives an exam-ready definition first, then mechanism, features, examples and a sketch line. UPSC has framed these terms inside questions on Himalayan hazards (2018) and on weathering and mass wasting; dated Indian cases from Malin (2014) to Wayanad (2024) are placed under the movement type they illustrate.

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Mechanics and Classification

Mass wasting and mass movement

Mass movement is the downslope movement of soil, regolith and rock as a body under the direct pull of gravity, without a transporting medium such as flowing water, ice or wind, although water within the mass usually helps. Mass wasting is the older, near-synonymous term that stresses the wearing away of slopes; rates range from millimetres a year to over 100 km an hour.

• Mechanism: gravity resolves into a component down the slope that drives movement and a component into the slope that holds material in place. Movement starts when driving stress exceeds the strength of the material (see factor of safety below).
• Role of water: water adds weight, raises pore pressure that cuts friction, lubricates clays and, at high contents, turns a slide into a flow.
• Types: by speed and water content, from slow creep, through slides and falls, to rapid flows and avalanches; the standard scheme is the Varnes classification.
• Geomorphic significance: mass movement delivers weathered debris (weathering products) to rivers, shapes hillslope profiles, widens valleys and, in high mountains, may lower relief faster than rivers or glaciers.
• Hazard link: Melanie J. Froude and David N. Petley counted 4,862 fatal non-seismic landslides worldwide between 2004 and 2016, killing 55,997 people, with the Himalayan arc among the densest clusters. About 0.42 million km², some 12.6 per cent of India’s land area, is rated landslide-prone.
• Don’t confuse with: solifluction, the frost-related flow of thawed soil over frozen ground, and lahar, a volcanic debris flow; both are owned elsewhere.

Shear stress, shear strength and factor of safety

Shear stress is the component of a slope material’s weight acting parallel to a potential slip surface, and shear strength is the material’s resistance to sliding on that surface. The factor of safety is strength divided by stress: above 1 the slope stands, at 1 it is on the point of failure, and below 1 it moves.

• Equations (plain text): shear stress on an infinite slope = W sin β, where W is weight and β slope angle. Strength follows the Mohr–Coulomb relation s = c′ + (σ − u) tan φ′, where c′ is cohesion, σ normal stress, u pore-water pressure and φ′ the angle of internal friction.
• Key idea — effective stress: Karl Terzaghi showed that pore-water pressure (u) is subtracted from normal stress, so a rising water table weakens a slope without any added load. This is why most landslides follow heavy or prolonged rain.
• What raises stress: steepening, undercutting at the toe, added load (rain, buildings, fill), earthquake shaking.
• What lowers strength: weathering, saturation, loss of root cohesion after deforestation, swelling clays, adverse joints.
• Indian example: modelling of the Mundakkai slope in Wayanad found the factor of safety falling from about 1.35 in normal conditions to about 0.85 under full saturation, matching its failure on 30 July 2024.
• Use: engineers design permanent cut slopes with factors of safety of about 1.3–1.5.
• Sketch: a block on an inclined plane, weight resolved into components parallel and normal to the slope, with pore pressure arrows pushing up from the slip surface.

Angle of repose

The angle of repose is the steepest angle, measured from the horizontal, at which a heap of loose, cohesionless material such as sand, gravel or rock fragments can stand without its surface grains sliding. For most dry, angular debris it lies between about 30 and 40 degrees, and slopes of loose debris adjust to it.

• Controls: particle shape (angular fragments interlock and stand steeper than rounded ones), size and sorting, and moisture. A little moisture adds apparent cohesion, while saturation lowers the stable angle sharply.
• Key features: talus slopes, the slip faces of sand dunes, spoil heaps and cinder cones all stand near this angle.
• Significance: slopes of loose material steeper than the angle of repose are unstable and shed debris by grain flow and small slides until they return to it.
• Examples: scree slopes below the frost-shattered crags of Ladakh and Lahaul; the slip faces of barchans in the Thar.
• Don’t confuse with: the threshold slope, the limiting angle of a regolith-mantled hillslope, which depends on cohesion and pore pressure as well as friction.

Varnes classification of mass movements

The Varnes classification is the standard scheme for naming landslides, devised by David J. Varnes of the United States Geological Survey in 1978 and revised by David M. Cruden and Varnes in 1996. It combines the type of movement with the type of material, so every landslide gets a two-part name such as “rock fall” or “debris flow”.

• Movement types: fall, topple, slide (rotational or translational), lateral spread and flow, plus complex movements that combine two or more, such as a slide that turns into a flow downslope.
• Material types: rock (bedrock) and engineering soil, divided into debris (coarse, with a substantial gravel-and-boulder fraction) and earth (mostly sand and finer).
• The grid: five movement types times three materials give names such as rock topple, debris slide and earth flow.
• Velocity scale (Cruden and Varnes, 1996): seven classes from extremely slow (under 16 mm a year) to extremely rapid (over 5 m a second), which link speed to the likely damage and the chance of escape.
• Activity terms: active, suspended, reactivated, dormant and relict describe the state of a landslide over time.
• Update: Oldřich Hungr, Serge Leroueil and Luciano Picarelli (2014) refined the scheme into 32 types using geotechnical material terms (clay, silt, sand, gravel, boulders, peat, ice).
• Earlier approaches: classifications by direction of movement (vertical, lateral, diagonal) and by water content are still met in older accounts, but the Varnes grid is the one to use in an answer.

• Sketch: five small profile sketches in a row — fall, topple, rotational slide, translational slide, flow — each with its failure surface or path marked.

Creep, Slides, Falls and Topples

Creep (soil creep, rock creep and terracettes)

Creep is the extremely slow, usually imperceptible downslope movement of soil, regolith or rock, commonly a few millimetres a year at the surface and decreasing with depth. Soil creep moves the surface mantle; rock creep is slow, deep-seated deformation of rock masses; terracettes are small, step-like ridges running along the contour of steep grassy slopes.

• Mechanism of soil creep: repeated expansion and contraction — by wetting and drying, heating and cooling, or freezing and thawing — lifts particles at right angles to the slope and lets them settle back vertically, ratcheting them downhill. Burrowing, root growth and trampling add to it.
• Evidence: tilted posts and poles; bulging retaining walls; soil banked upslope of walls; bedrock layers bent downslope near the surface.
• Terracettes: steps a few tens of centimetres high on slopes of roughly 25–40 degrees; creep, small shallow slips and animal tracks all contribute, and their origin is debated.
• Rock creep: slow, deep sagging of steep mountain slopes, sometimes ahead of catastrophic failure.
• Examples: tilted poles and cracked retaining walls on hill roads of the Lesser Himalaya; terracettes on the chalk downs of southern England.
• Don’t confuse with: solifluction and gelifluction, which depend on seasonal freezing and thawing over frozen ground; creep works in all climates.

Slump (rotational slide)

A slump is a rotational slide in which a coherent block of soil, clay or weak rock moves downward and outward along a curved, concave-upward (spoon-shaped) failure surface, rotating backward as it goes. It is typical of thick, fairly homogeneous, cohesive materials such as clay, shale, deeply weathered regolith and alluvium.

• Mechanism: undercutting of the toe by a river, waves or excavation, combined with saturation, lets the mass rotate about an axis parallel to the slope; the displaced block often breaks into several back-tilted steps.
• Key features: a crescent-shaped main scarp at the head; back-tilted blocks with ponded depressions behind them; transverse cracks; a bulging toe that often breaks into an earthflow.
• Examples: Holbeck Hall, Scarborough, England, where a cliff-top hotel collapsed in a rotational slide in June 1993; monsoon bank slumps along the alluvial cliffs of the Ganga in Uttar Pradesh and Bihar.
• Significance: slumps recur on the same slopes because the back-tilted blocks trap water; coastal and river-bank towns lose land this way.
• Don’t confuse with: a translational slide, which moves on a flat plane without rotation.
• Sketch: a profile with a curved slip surface, a back-tilted block with a pond behind it, the main scarp and a lobate toe.

Translational slide (rock slide, block glide and debris slide)

A translational slide is a landslide in which a mass moves downslope along a roughly planar surface — a bedding plane, joint, fault, foliation or the contact between regolith and bedrock — with little rotation. A rock slide moves bedrock, a block glide moves a single coherent block, and a debris slide moves a shallow sheet of weathered mantle over bedrock.

• Mechanism: failure is most likely where planes of weakness dip out of the slope at an angle gentler than the slope face but steeper than their friction angle, so that they “daylight” in the face; pore pressure on the plane reduces friction.
• Key features: long, shallow slip surfaces; slabs intact at first, then breaking up; debris slides only a few metres thick.
• Examples: Vajont, Italy, 9 October 1963, when about 270 million m³ of rock slid along clay-bearing beds into a reservoir and the displaced water killed about 2,000 people; Irshalwadi, Raigad district, Maharashtra, 19 July 2023, where a rain-soaked debris slide buried much of a tribal hamlet below Irshalgad, with 27 bodies recovered and dozens never found.
• Significance: debris slides are the commonest monsoon failure in the Western Ghats and Lesser Himalaya.
• Sketch: a dip slope with bedding daylighting at a river-cut toe, a slab sliding along one bedding plane.

Lateral spread (spreading, cambering and sackung)

A lateral spread is the sideways extension of a cohesive mass of rock or soil over a weaker underlying layer that deforms, flows or liquefies, so that the upper mass breaks into blocks that subside, tilt and drift apart on very gentle slopes. In rock the process produces cambering and sackung; in soils it often follows earthquake liquefaction.

• Mechanism: the lower layer (liquefied sand, sensitive clay, or soft shale beneath hard caprock) squeezes out and flows; the brittle upper layer fractures into strips and grabens.
• Cambering and sackung: in cambering, hard beds drape downward over valley sides as underlying clay squeezes out; in sackung, ridge-tops spread and sag, leaving double crests and uphill-facing scarps.
• Examples: Turnagain Heights, Anchorage, Alaska, in the 1964 earthquake; cambered Jurassic limestone over clay in the Cotswolds, England; liquefaction spreads in the Rann of Kachchh during the Bhuj earthquake of 26 January 2001 (see liquefaction).
• Significance: spreads damage pipelines, bridges and embankments on near-flat ground that looks safe.
• Sketch: a hard layer broken into tilted blocks over a flowing soft layer, arrows pointing outward.

Rockfall and debris fall

A rockfall is the free fall, bouncing and rolling of rock fragments or blocks detached from a cliff or very steep slope, the fastest of all mass movements. A debris fall is the similar fall of mixed, weathered debris, soil and vegetation from steep river banks, road cuts or cliffs. Little or no water is involved.

• Mechanism: blocks are loosened along joints and bedding by frost wedging, thermal stress, root wedging, pressure release and undercutting of weaker rock below; rain, earthquakes and blasting provide the final trigger.
• Key features: fresh scars on the cliff, impact craters and scattered blocks below, and a growing talus at the foot.
• Examples: repeated rockfalls from El Capitan, Yosemite, in September 2017; the Malpa disaster of 18 August 1998 in the Kali valley, Pithoragarh district, Uttarakhand, where rock and debris falling from the valley wall buried a camp and killed more than 200 people, including Kailash–Mansarovar pilgrims.
• Significance: the main hazard on Himalayan highways, managed with rock bolts, wire meshes, catch fences and rockfall galleries.
• Sketch: a cliff with a detaching block, its bouncing trajectory, and a talus cone below.

Topple

A topple is the forward rotation of a rock column, slab or block about a pivot point at or near its base, out of a steep slope face. It occurs where joints, foliation or bedding dip steeply into the slope, isolating tall, thin columns whose centre of gravity moves outside their base.

• Types: block toppling of columns cut by cross-joints; flexural toppling of continuous slabs that bend forward like the pages of a book; debris and earth topples in soft banks.
• Mechanism: water pressure in cracks behind a column, undercutting at its base, weathering and earthquakes push the column past its tipping point; many topples end as falls or slides.
• Key features: open tension cracks parallel to the cliff top; columns leaning outward; a bench of rotated blocks at the foot.
• Examples: toppling of steeply foliated phyllites and schists in road cuts of the Lesser Himalaya; toppling of columnar-jointed basalt on the cliffs of the Deccan Traps.
• Don’t confuse with: a fall, which begins with detachment rather than rotation.
• Sketch: tall columns bounded by steep joints dipping into the slope, the outer one pivoting about its base.

Flows and Avalanches

Debris flow

A debris flow is a rapid, often surging flow of water-saturated, poorly sorted debris — a mixture of boulders, gravel, sand and mud — that moves like wet concrete down steep channels and gullies. It is denser than a flood, can carry house-sized boulders, travels several kilometres, and deposits a lobate, bouldery fan at its end.

• Formation: a small debris slide on a saturated slope, or channel-bed collapse during intense rain, mobilises into a flow; the flow grows by scouring its bed (bulking) as it descends.
• Key features: a bouldery front or snout, lateral levees along the path, surges in pulses, and poorly sorted, often inversely graded deposits; speeds of several metres to more than 10 m per second.
• Examples: Vargas state, Venezuela, December 1999; Mundakkai–Chooralmala, Wayanad, Kerala, 30 July 2024 — after about 200 mm of rain and then about 370 mm in successive 24-hour periods, a debris slide near the headwaters at about 1,500 m turned into a debris flow that ran more than 6 km down the Punnappuzha valley through Mundakkai and Chooralmala. It was one of India’s deadliest landslide disasters, with published estimates of more than 400 dead or missing. Pettimudi, Munnar (Idukki), 6 August 2020, killed about 70 plantation workers. At Kedarnath on 16–17 June 2013, extreme rain and the Chorabari lake outburst sent debris flows through the town (the lake breach is covered under GLOF).
• Significance: most deadly because flows travel far beyond the source slope into settled valley floors.
• Sketch: source scar, channel with levees, and depositional lobe at the valley mouth.

Mudflow

A mudflow is a rapid flow of fine-grained sediment, dominantly silt and clay, with a very high water content, which gives it the fluidity of a slurry. It moves faster and more fluidly than an earthflow, follows valleys and gullies, and spreads into thin sheets and lobes when it reaches gentle ground.

• Formation: heavy rain on bare, fine-grained or deeply weathered slopes, especially after long dry spells; loose ash or loess is ideal material. Eliot Blackwelder (1928) described mudflows as a major geomorphic agent in semi-arid mountains.
• Key features: fluid behaviour, mud-coated valley sides, lobate deposits with few large boulders.
• Types: mudflows of semi-arid mountains, alpine mudflows fed by snowmelt, and volcanic mudflows, the last treated as lahar.
• Examples: the Sarno mudflows in pyroclastic soils of Campania, Italy, in May 1998; the Kavalappara mudslide in Malappuram district, Kerala, on 8 August 2019, which buried a hamlet and killed 59 people.
• Don’t confuse with: a debris flow, which has a coarser load of gravel and boulders; the series debris flow–earthflow–mudflow is one of rising fineness and water content.

Earthflow

An earthflow is the slow to moderately rapid flow of fine-grained, clay-rich soil or weathered rock that is partly saturated, moving as a tongue or lobe down a hillside over days to years. It commonly issues from the toe of a slump and is plastic rather than fluid.

• Mechanism: rain or snowmelt raises pore pressure in clay-rich material until it deforms internally; movement is concentrated along shear zones at the base and sides.
• Key features: a bowl-shaped source, a hummocky, tongue-shaped body with lateral ridges, and a bulging toe; speeds from millimetres a day to metres an hour.
• Examples: the Slumgullion earthflow in the San Juan Mountains, Colorado, which has moved for centuries at up to several metres a year; slow earthflows on clay-rich shale and weathered-mantle slopes of the Siwaliks during the monsoon.
• Significance: earthflows rarely kill but steadily wreck roads, pipelines and houses.
• Don’t confuse with: solifluction, which depends on frozen ground; and with creep, which has no discrete body or boundaries.
• Sketch: a slump scar at the head feeding a tongue-shaped, hummocky flow with a lobate toe.

Avalanche (snow, debris and rock–ice avalanche)

An avalanche is a very rapid, flowing mass of snow, ice, rock or debris that descends a steep mountain slope at tens of metres per second, often preceded by an air blast. Snow avalanches are the commonest; debris avalanches involve unchannelled flows of rock and soil; and rock–ice avalanches combine collapsing rock with glacier ice.

• Snow avalanches: loose-snow (point-release) and slab avalanches, dry powder and wet avalanches; they release on slopes of roughly 30–45 degrees after heavy snowfall, wind loading or thaw.
• Debris and rock avalanches: very large rock slides (over about a million m³) fragment and flow, travelling far beyond expected run-out; the flank collapse of Mount St Helens on 18 May 1980 was the largest historic debris avalanche.
• Rock–ice avalanches: the Huascarán avalanche of 31 May 1970 in Peru, set off by an earthquake, buried the town of Yungay.
• Chamoli, 7 February 2021: about 27 million m³ of rock and glacier ice, roughly 80 per cent rock, collapsed from a ridge below Ronti peak in the Nanda Devi region. It fell about 3.2 km vertically, turned into a debris flood down the Ronti Gad, Rishiganga and Dhauliganga, destroyed the Rishiganga hydropower project, badly damaged the Tapovan–Vishnugad project and left about 200 dead or missing. It was not a glacial lake outburst (GLOF).
• Indian snow avalanches: an avalanche on the Siachen Glacier on 3 February 2016 killed ten soldiers; NDMA’s guidelines of 2009 cover landslides and snow avalanches together.
• Sketch: a steep slope with release zone, track and run-out zone, powder cloud ahead of the dense flow.

Deposits, Dams and Subsidence

Talus (scree) and talus cone

Talus, or scree, is the accumulation of angular rock fragments at the foot of a cliff or free face, supplied mainly by rockfall. A talus cone is the fan-shaped heap that builds below a gully or chute concentrating the falling debris, and adjacent cones merge into a talus apron or sheet along the cliff base.

• Formation: blocks loosened by frost, thermal stress and pressure release fall and roll; the slope builds up at close to the angle of repose, about 30–40 degrees.
• Key features: fall sorting — large blocks roll farthest and collect at the base, finer fragments stay near the apex; a straight or slightly concave profile; slow talus creep and occasional debris flows rework the surface.
• Examples: the Wastwater Screes, English Lake District; talus cones lining the valley walls of Lahaul, Spiti and Ladakh along the Manali–Leh road.
• Significance: talus buries the cliff foot; as a slope element it becomes the debris or constant slope of hillslope theory. Ice-cemented talus can evolve into a rock glacier.
• Don’t confuse with: an alluvial fan, built by running water at a lower angle.
• Sketch: cliff, chutes and coalescing talus cones with large blocks at the base.

Landslide dam and landslide-lake outburst

A landslide dam is a natural barrier formed when a landslide, rock avalanche or debris flow blocks a river valley and impounds a lake. A landslide-lake outburst is the sudden flood released when that loose, unconsolidated dam is overtopped and breached, usually by rapid erosion of the crest, sending a flood wave far downstream.

• Mechanism: with no spillway, core or compaction, an overtopped dam is breached within hours; most that fail do so within a year, many within days.
• Key features: a lake upstream, a hummocky debris barrier, and, after failure, a breach channel and flood deposits downstream.
• Examples: Gohna Tal on the Birahi Ganga, Chamoli, dammed by a huge rock slide in September 1893 and breached in August 1894, when a forecast of the outburst limited loss of life; the Attabad landslide of January 2010, Hunza valley, which impounded a lake that drowned villages and a stretch of the Karakoram Highway; the Usoi dam of 1911, Pamirs, which holds Sarez Lake; the Parechu lake in Tibet, formed in 2004, which flooded the Sutlej valley of Himachal Pradesh in June 2005.
• Don’t confuse with: a glacial lake outburst flood, in which the dam is a moraine or glacier.
• Sketch: a valley blocked by slide debris, a lake upstream and a breach with flood arrows downstream.

Natural land subsidence

Natural land subsidence is the gradual settling or sudden sinking of the ground surface caused by natural processes — compaction of young sediments, solution of soluble rock below, thawing of ground ice, or tectonic and volcanic movement — as distinct from subsidence caused by pumping groundwater or mining. It is the vertical member of the mass-movement family.

• Types: compaction (deltas and floodplains settling under their own weight); collapse over dissolved limestone, gypsum or salt, as covered under sinkholes; thaw settlement of permafrost (thermokarst); tectonic subsidence during earthquakes; and deflation of volcanoes as magma withdraws.
• Key features: slow subsidence is measured in millimetres to centimetres a year; collapse can be instant.
• Examples: natural compaction of the Mississippi and Ganga–Brahmaputra deltas, which adds to relative sea-level rise; the Kachchh earthquake of 1819, when the ground around Sindri sank and was flooded while the Allah Bund ridge rose.
• Significance: delta subsidence magnifies coastal flooding and storm-surge risk in the Sundarbans.
• Don’t confuse with: human-induced subsidence, as at Joshimath in 2023, which is covered under human-induced land subsidence.

Landslide Triggers and Hazard Zonation

Landslide triggers (rainfall thresholds, earthquakes, undercutting)

Landslide triggers are the external events — intense or prolonged rainfall, earthquakes, snowmelt, rapid undercutting or human loading — that push a slope already weakened by preparatory factors past its failure threshold. The distinction between slow preparatory causes and a sudden trigger is central: a trigger only works on a slope that is already close to failure.

• Preparatory factors: steep relief, weak or sheared rock, adverse joints, deep weathering, deforestation, road cutting and loss of toe support. David J. Varnes grouped all causes into those that increase shear stress and those that reduce shear strength.
• Rainfall thresholds: the minimum rainfall that has been followed by landslides, expressed as intensity against duration. Nels Caine’s global threshold (1980) is I = 14.82 D^−0.39, with I in mm per hour and D in hours. At Wayanad in 2024 this threshold was exceeded repeatedly from the afternoon of 29 July, after two weeks of rain had saturated the slopes.
• Antecedent rain: pore pressure peaks after the soil is saturated, so failures cluster late in the monsoon. The Malin landslide of 30 July 2014, Ambegaon taluka, Pune district, which killed 151 people, followed 108 mm of rain on 29 July on slopes levelled for cultivation.
• Cloudbursts: rainfall of 100 mm an hour or more over a small area, as the India Meteorological Department defines it, trigger debris flows in the Himalaya.
• Earthquakes: shaking adds transient stress; the Gorkha earthquake of 25 April 2015 triggered more than 25,000 landslides, and the Kashmir earthquake of 2005 created the Hattian Bala landslide dam.
• Undercutting: river and wave erosion or road cuts remove toe support. The Nainital landslip of 18 September 1880, after heavy rain, killed 151 people.
• Human loading: on the Char Dham highway corridor, a 2025 study by Soumik Saha and Biswajit Bera found 81 per cent of landslides within 100 m of the road.
• Sketch: a graph of rainfall intensity against duration on log axes with a sloping threshold line, landslide events plotted above it.

Landslide hazard zonation

Landslide hazard zonation is the division of land into zones ranked by the relative likelihood of landslides, based on the factors that control slope stability. It separates susceptibility (where slides are likely), hazard (where, how often and how large) and risk (hazard combined with the people and property exposed and their vulnerability).

• Factors mapped: lithology, geological structure, slope angle and aspect, relative relief, land use and land cover, drainage and groundwater, and past landslides from an inventory.
• Methods: heuristic rating schemes, statistical methods (frequency ratio, logistic regression), machine-learning models, and physically based stability models that apply the factor of safety cell by cell.
• Indian standard: BIS IS 14496 (Part 2): 1998 guides macro-zonation maps using a Landslide Hazard Evaluation Factor rating — lithology, structure, slope morphometry and land cover up to 2 points each, relative relief and hydrogeology up to 1 each — summed into zones from very low to very high hazard.
• National programmes: the Geological Survey of India is the nodal agency for landslides; its National Landslide Susceptibility Mapping covers the landslide-prone parts of the country, about 0.43 million km², at 1:50,000. NDMA issued National Guidelines on Management of Landslides and Snow Avalanches in 2009.
• Landslide Atlas of India (2023): prepared by ISRO’s National Remote Sensing Centre, it compiled about 80,000 landslides for 1998–2022 and ranked 147 districts in 17 states and two union territories by exposure, with Rudraprayag and then Tehri Garhwal in Uttarakhand at the top.
• Limits: zonation maps show relative, not absolute, hazard and have seldom been made binding on road alignment or building permission. Joshimath, flagged by the Mishra Committee in 1976, is the lesson (see human-induced subsidence).
• Sketch: a small watershed map shaded into five hazard classes, with the factor layers stacked beside it.

PYQs Built on These Terms

• “Geomorphological changes are largely responsible for environmental hazards in the Himalayan region.” Comment with relevant examples. (2018)
• Explain weathering and mass wasting, and describe their geomorphic significance. (model answer)

Frequently Asked Questions

What is the difference between a landslide and mass wasting?

Mass wasting is the umbrella term for every gravity-driven movement of rock and soil, from slow creep to rapid avalanches. A landslide, strictly, is a relatively rapid movement of a discrete mass — a fall, topple, slide, spread or flow — with a recognisable source and boundary. In common usage “landslide” covers most rapid mass movements, while creep is not usually called one.

What is the difference between a debris flow and a mudflow?

Both are rapid flows of saturated sediment, but they differ in grain size and water content. A debris flow carries coarse material, with a large share of gravel and boulders, and moves in bouldery surges with levees. A mudflow is dominated by silt and clay, holds more water and behaves like a slurry. Debris flows are generally more destructive because of their boulder load.

What caused the Wayanad landslide of 2024?

Extreme rain was the trigger. About 570 mm fell in two days on slopes already saturated by two weeks of monsoon rain, pushing pore pressure up until the factor of safety dropped below 1. A debris slide near the headwaters above Mundakkai turned into a debris flow that ran more than 6 km down the valley, destroying Mundakkai and Chooralmala on 30 July 2024.

Why are landslides so frequent in the Himalaya?

The Himalaya combines every preparatory factor: steep, rising relief, rock crushed along major thrusts, deep weathering in the Lesser Himalaya, and rivers that keep undercutting valley sides. The monsoon, cloudbursts and earthquakes supply triggers, and road building, tunnelling and settlement on old landslide debris add load and remove toe support, so failures recur every year.

What is the factor of safety of a slope?

The factor of safety is the ratio of a slope’s shear strength to the shear stress acting on it. A value above 1 means the slope is stable, exactly 1 means it is on the verge of failure, and below 1 means it will move. Heavy rain lowers it by raising pore-water pressure, which reduces friction along a potential slip surface.

Was the Chamoli disaster of 2021 a glacial lake outburst flood?

No. The Chamoli disaster of 7 February 2021 was a rock–ice avalanche: about 27 million m³ of rock and glacier ice broke from a ridge below Ronti peak, fell some 3.2 km vertically and turned into a debris flood down the Rishiganga and Dhauliganga. No glacial lake burst, which is why it is classed as mass movement, not a GLOF.