Overview
This chapter explains weathering, the slow breaking and rotting of rocks where they lie, by the air, water, heat, cold and living things of the earth's surface. It begins by defining weathering and separating it carefully from erosion and denudation, since the three words are often confused in examinations. It then describes the conditions that decide how fast a rock weathers: the climate of the place, the structure and mineral make-up of the rock, the slope of the land, the cover of plants and the length of time. The heart of the chapter is the three families of weathering. Mechanical or physical weathering breaks rock into pieces without changing what it is made of, through block disintegration, exfoliation, granular disintegration, frost action and the release of pressure. Chemical weathering changes the very minerals of the rock through oxidation, carbonation, hydration, hydrolysis and solution. Biological weathering is the work of roots, burrowing animals, lichens and human beings. The chapter closes with the results of weathering: the loose mantle of regolith, the birth of soil, the shaping of landforms such as tors, domes and karst, and mass movement, the slipping of weathered material down slopes as creep, landslides, rockfalls and mudflows. Understanding weathering is understanding why mountains do not last forever and why the soil under a rice field exists at all.
Learning Objectives
- Define weathering and distinguish it clearly from erosion and denudation.
- Explain the factors that control the rate and type of weathering at a place.
- Describe the processes of mechanical weathering with examples from India.
- Describe the chemical processes of oxidation, carbonation, hydration, hydrolysis and solution with their reactions.
- Explain how plants, animals and human beings bring about biological weathering.
- Relate the type of weathering to the climate of a region.
- Explain how regolith and soil are formed from weathered rock.
- Identify landforms such as tors, exfoliation domes and karst features produced by weathering.
- Classify the forms of mass movement and describe the causes of landslides in the Himalaya and Darjeeling hills.
Topics in this chapter
13 topics · tap a topic title to jump straight to it.
What weathering is
The rocks of the earth's crust were formed deep inside the earth or under the sea, under great heat and pressure. When they are lifted up and exposed at the surface they meet a completely different world: sunshine and frost, rain and wind, oxygen and carbon dioxide, roots and worms. Under these new conditions the rocks slowly break into pieces and decay. This slow breaking-up and decomposition of rocks in the place where they lie (in situ), by the agents of the atmosphere and by living things, is called weathering. The word comes from 'weather', because it is chiefly the elements of weather, temperature, rainfall, humidity and frost, that do the work.
Three ideas are built into this definition. First, weathering is a process of disintegration (breaking into smaller pieces) and decomposition (chemical rotting). Second, it is a static process: the rock is not carried anywhere; it is attacked where it stands. Third, it is slow: a granite boulder in the Chhotanagpur plateau may take thousands of years to crumble into gravel and clay, though a soft shale may weather within a few decades.
Weathering works from the surface downwards. The top of an exposed rock is attacked first, and cracks and joints let the agents enter deeper. The loose, broken and rotten layer that forms over the fresh rock is called the regolith; the boundary below it where fresh rock begins is the weathering front. In the humid tropics, where chemical decay is fast, the regolith may be 30 to 50 metres thick; in cold deserts it may be only a few centimetres.
Why does weathering matter? It is the first step in the whole cycle of denudation. Rivers, glaciers and winds cannot carry solid rock; they carry the fragments that weathering prepares. Weathering also creates soil, without which there would be no plants, no agriculture and no human settlement. It shapes many landforms directly, from the rounded boulders of the Deccan to the caves of Meghalaya. And it slowly destroys human structures too: the darkening and flaking of the sandstone of old temples, the rusting of iron bridges and the crumbling of plaster in a Kolkata monsoon are all weathering at work. The chapter therefore studies a process that is quiet and invisible from day to day but that, over geological time, wears down the highest mountains.
- A freshly cut road through the Darjeeling hills shows fresh grey rock at the base, a brown rotten layer above it and soil on top: fresh rock, regolith and soil, one above the other.
- The Ashokan sandstone pillars, more than 2,200 years old, have flaked and darkened at the surface although they have never moved: weathering without erosion.
- A brick left in the open for years crumbles at the corners; the same brick kept indoors stays sharp-edged.
- Weathering = disintegration (physical breaking) + decomposition (chemical decay) of rocks in situ.
- Regolith = the mantle of loose, weathered rock material lying over fresh bedrock.
Weathering, erosion and denudation
Students often write 'weathering' when they mean 'erosion', and lose marks. The three words describe three different things, and the examination frequently asks for the difference.
Weathering is the breaking and rotting of rock in place, by the atmosphere and by organisms. Nothing is transported. The agents are static: temperature, rain water, frost, oxygen, carbon dioxide, plants and animals.
Erosion is the wearing away of the land surface by moving agents that also carry the material away: running water (rivers), moving ice (glaciers), wind, sea waves and ground water. Erosion therefore always involves transport. A river that cuts a gorge, a glacier that scoops a U-shaped valley, a wind that sand-blasts a mushroom rock: these are erosion. Erosion works fastest on material that weathering has already loosened; the two processes help each other.
Denudation is the widest word. It means the total lowering and laying bare of the land by all the processes together: weathering, mass movement (the downhill slipping of loose material under gravity), erosion and transport. Denudation is the sum; weathering and erosion are parts of it. The Latin root denudare means 'to strip bare'.
| Point | Weathering | Erosion |
| Nature | Static, in situ | Dynamic, involves movement |
| Agents | Temperature, water, frost, gases, organisms | Rivers, glaciers, wind, waves, ground water |
| Transport | None | Always |
| Result | Regolith, soil | Valleys, gorges, cliffs, plains |
| Speed | Very slow | Comparatively fast |
A simple picture helps. Think of a wall of an abandoned house. The plaster cracks in the sun and rain and flakes off: that is weathering. The monsoon wind and rain then carry the flakes away and pile them at the foot of the wall: that is erosion and deposition. The wall growing shorter year by year is denudation. In nature the same sequence lowers the Aravalli hills, once as high as the Himalaya and now worn to stumps a few hundred metres high after hundreds of millions of years.
One more distinction: weathering prepares, erosion removes. Where weathering is fast but erosion slow, as on the flat humid plateaus of the tropics, a thick regolith accumulates. Where erosion is faster than weathering, as on steep young mountain slopes, fresh rock is constantly exposed and the regolith stays thin.
- The rounded boulders on the Deccan lava are produced by weathering; the deep gorge of the Narmada at Bhedaghat is produced by river erosion.
- The Aravalli range, among the oldest fold mountains of the world, has been reduced to low ridges by denudation acting over more than 100 crore years.
- In Kolkata, the peeling of paint and plaster on a building is weathering; the washing away of that debris by rainwater into the drains is erosion.
- Denudation = weathering + mass movement + erosion + transportation.
Factors controlling weathering
Not every rock weathers in the same way or at the same speed. Five conditions decide the type and rate of weathering at a place.
1. Climate. This is the most important factor. Temperature and rainfall decide which processes work. In hot, wet, equatorial and monsoon climates, chemical weathering dominates, because chemical reactions are faster in warmth and need water. In hot deserts with a great daily range of temperature and almost no water, mechanical weathering by heating and cooling dominates. In cold, high-latitude and high-mountain climates, where the temperature crosses 0 °C again and again, frost action dominates. In cold, dry polar regions, weathering of all kinds is very slow.
2. Rock structure and composition. Rocks with many joints and cracks weather faster because water and air enter along them; a massive, unjointed rock weathers only from its outer skin. Mineral composition matters too: rocks rich in iron (basalt) oxidise readily; limestone dissolves in carbonic acid; quartz is almost inert, so quartzite and pure sandstone resist. Coarse-grained rocks such as granite disintegrate grain by grain because their different minerals expand at different rates.
3. Slope of the land. On a steep slope the weathered material is quickly removed by gravity and rain wash, exposing fresh rock to fresh attack, so weathering continues actively though the regolith stays thin. On level ground the debris accumulates and forms a protective blanket, so the rock beneath is attacked more slowly, though the regolith becomes deep.
4. Vegetation. Plant cover has two opposite effects. Roots widen cracks and the decay of leaves produces humic acids that speed chemical weathering. At the same time a canopy of leaves shields the rock from the direct sun and heavy rain drops, and so reduces mechanical weathering.
5. Time. Weathering is slow, so the longer a rock is exposed, the more completely it is weathered. Old, stable land surfaces such as the peninsular plateau carry a deep, mature regolith; young surfaces such as recent lava flows or fresh road cuttings have hardly begun.
To these may be added altitude (higher places are colder and frost is more common), aspect (a slope facing the sun weathers differently from a shaded one) and human activity (quarrying, mining and pollution expose rock and add acids to the rain).
These factors act together. The Meghalaya plateau, with the heaviest rainfall on earth, warm temperatures, limestone and sandstone rocks and a long period of exposure, shows extreme chemical weathering, including the great caves of Cherrapunji. The cold desert of Ladakh, with slight rainfall but severe frost, shows shattered angular rock everywhere.
- Basalt of the Deccan trap, rich in iron, turns rusty and forms the black cotton soil; the quartzite ridges of Delhi have hardly changed in colour.
- Cherrapunji and Mawsynram in Meghalaya, with over 11,000 mm of rain a year, have limestone caves and deeply rotted rock; Jaisalmer, with under 200 mm, has sharp-edged, sun-shattered rock.
- A jointed granite outcrop at Susunia hill in Bankura has weathered into rounded boulders separated by widened joints, while the massive rock between joints stays fresh.
- Rate of weathering depends on: climate (temperature, rainfall) + rock type (jointing, minerals) + slope + vegetation + time.
Mechanical weathering: block and granular disintegration
Mechanical or physical weathering breaks rock into smaller and smaller pieces without changing its chemical nature: a piece of weathered granite is still granite. It is caused mainly by changes of temperature and by the freezing of water, and it is most active where the weather changes violently and there is little moisture or vegetation: hot deserts, high mountains and cold lands.
Block disintegration. Rocks are poor conductors of heat. In a hot desert the surface of a rock may be heated to 70 °C or more in the afternoon and cool to near 0 °C at night, a daily range of 50 to 60 degrees. Every day the outer layer expands and every night it contracts, while the interior barely changes. Rocks that already have joints (natural cracks) in two or three directions are pulled apart along these joints by the repeated expansion and contraction. In time the rock mass breaks into large rectangular blocks. This is block disintegration, common in the Thar desert and on the jointed sandstones of Rajasthan.
Granular disintegration. A coarse-grained rock such as granite is made of several minerals: white and pink feldspar, glassy quartz and dark mica. Each mineral has its own rate of expansion when heated; feldspar and mica also weather chemically faster than quartz. Under repeated heating and cooling the grains push against one another, the bonds between them loosen and the rock crumbles grain by grain into coarse sand called gruss. The rock surface becomes rough and pitted and the loose grains gather at its foot. Granular disintegration is common on the granite hills of Chhotanagpur and the Deccan.
Thermal shattering and the role of moisture. Laboratory experiments show that heating and cooling alone crack rock only slowly, but if even a little moisture is present, as in dew, the process speeds greatly. Sudden cooling by rare desert rain falling on sun-heated rock also causes rocks to split with a sharp crack. Forest fires and lightning shatter rock in the same way.
All these processes increase the surface area of the rock enormously. A cube one metre on each side has six square metres of surface; broken into cubes of one centimetre it has 600 square metres. Because chemical reactions act on surfaces, mechanical weathering greatly speeds the chemical weathering that follows. That is why the two families rarely work alone.
- In the Thar desert near Jaisalmer, sandstone outcrops lie broken into rectangular blocks along their joints, the result of decades of daily heating and nightly cooling.
- Granite tors of the Ranchi plateau are surrounded by aprons of coarse quartz sand produced by granular disintegration.
- A glass tumbler filled suddenly with boiling water cracks because the inner surface expands faster than the outer: the same principle as thermal shattering of rock.
- Mechanical weathering: rock is broken into smaller pieces; chemical composition unchanged.
- Surface area rule: breaking a 1 m cube into 1 cm cubes raises the surface area from 6 m² to 600 m².
Mechanical weathering: exfoliation and frost action
Exfoliation (from Latin folium, a leaf) is the peeling off of rock in curved sheets, like the layers of an onion, so it is also called onion weathering. It works in two ways. In hot deserts and in the seasonally hot Deccan, the outer skin of a massive rock such as granite or basalt is heated and expands during the day while the interior stays cool; at night the skin contracts. The repeated stress makes the outer shell separate from the mass and fall away as a curved slab. The rock left behind is rounded, because corners and edges, being heated from two or three sides, weather fastest. This produces exfoliation domes and rounded boulders. Moisture entering the thin outer layer and swelling the minerals (hydration) aids the process.
The second, larger kind of exfoliation is pressure release or unloading. Granite is formed many kilometres underground under immense pressure. When the overlying rock is removed by long denudation the granite expands upward, and the expansion produces cracks parallel to the surface called sheeting joints. Great curved sheets then break away, producing bare rounded hills such as the granite domes of southern Karnataka and the Chhotanagpur plateau.
Frost action or freeze-thaw weathering is the most powerful mechanical process in cold climates. Water enters the cracks and pores of a rock during the day. At night, when the temperature falls below 0 °C, the water freezes, and ice occupies about 9 per cent more volume than the water it came from. The freezing water presses on the walls of the crack with a force that can reach 2,000 kilograms per square centimetre. By day the ice melts and the crack, now a little wider, takes in more water. After hundreds of such cycles the rock splits. This is frost wedging. The broken pieces are sharp and angular. Piles of such fragments at the foot of a cliff are called scree or talus; a whole slope covered by them is a blockfield or felsenmeer. In the higher Himalaya, above about 3,500 metres, frost action is the chief agent of weathering, and rockfall from frost-shattered cliffs is a constant danger on mountain roads.
A related process in dry coastal and desert regions is salt weathering: salt solutions enter pores, evaporate and leave crystals whose growth prises the grains apart. Honeycomb-pitted rock surfaces near the sea show its work.
- The bare rounded granite dome of Savandurga near Bengaluru and the sheeted hills near Hazaribagh show exfoliation by pressure release.
- On the Rohtang Pass in Himachal Pradesh, the roadside is littered with angular frost-shattered fragments, and scree slopes lie below every cliff.
- A bottle of water left in a freezer cracks because the ice expands by about one-eleventh of the water's volume: the same expansion splits rock.
- Water expands by about 9 per cent (1/11 of its volume) on freezing.
- Pressure of freezing water in a confined crack: up to about 2,000 kg/cm².
Chemical weathering: oxidation and carbonation
Chemical weathering is the decomposition of rocks by chemical reactions between the minerals of the rock and water, oxygen, carbon dioxide and organic acids. Unlike mechanical weathering, it changes the rock into new substances: hard feldspar becomes soft clay, grey iron minerals become red rust. It works best in warm and humid climates, because reactions run faster at higher temperatures and almost all of them need water. It is therefore the dominant form of weathering in equatorial and monsoon lands, including most of India.
Oxidation. Many rocks contain iron in the ferrous state (Fe2+). Oxygen dissolved in rain water combines with this iron to form ferric oxide (Fe2O3), the reddish-brown substance we know as rust. This is oxidation, the same process that rusts an iron gate in the monsoon. The rock turns red, yellow or brown, its structure is weakened, and it crumbles. Manganese and sulphur minerals are oxidised in the same way. Oxidation explains the red and yellow soils of the peninsular plateau, the red laterite of the Rarh region of West Bengal, and the rusty stains on the Deccan basalt. The reaction may be written 4FeO + O2 → 2Fe2O3.
Carbonation. Rain water dissolves carbon dioxide from the air and from the soil to form weak carbonic acid: H2O + CO2 → H2CO3. This acid attacks rocks containing calcium carbonate, especially limestone, chalk and marble, and converts the insoluble calcium carbonate into soluble calcium bicarbonate, which is carried away in solution: CaCO3 + H2CO3 → Ca(HCO3)2. Carbonation is the origin of karst topography: the widening of joints into grikes, the sinking of the surface into swallow holes and dolines, and the hollowing of underground caves with stalactites and stalagmites, as at the Mawsmai and Krem Liat Prah caves of Meghalaya and the Borra caves of Andhra Pradesh. Where the air is polluted by sulphur dioxide from vehicles and factories, the rain becomes stronger acid and marble monuments weather faster, as the yellowing of the Taj Mahal shows.
Both processes leave the rock softer, more porous and more easily broken by mechanical processes, so chemical and mechanical weathering reinforce each other.
- The red soil of Bankura, Purulia and Birbhum owes its colour to oxidised iron in the laterite.
- Cherrapunji's limestone caves have been hollowed by carbonation in rain water over thousands of years.
- An iron nail left in a wet place turns red-brown with rust in a few weeks; iron in rock does the same over centuries.
- Oxidation: 4FeO + O₂ → 2Fe₂O₃ (ferrous oxide to ferric oxide, rust).
- Formation of carbonic acid: H₂O + CO₂ → H₂CO₃.
- Carbonation: CaCO₃ + H₂CO₃ → Ca(HCO₃)₂ (insoluble limestone to soluble calcium bicarbonate).
Chemical weathering: hydration, hydrolysis and solution
Hydration. Some minerals absorb water molecules into their structure without being chemically broken up. The mineral swells, its volume increases, and the pressure of the swelling cracks the rock around it. Anhydrite (CaSO4) takes in water to become gypsum (CaSO4·2H2O) with an increase of volume of about 60 per cent; haematite (Fe2O3) becomes limonite (2Fe2O3·3H2O); the clay minerals swell in rain and shrink in drought. Hydration thus combines chemical change with mechanical stress and is a chief cause of the flaking of outer rock shells in exfoliation.
Hydrolysis. This is the most important chemical reaction in the weathering of the commonest rocks. Water itself, slightly ionised into H+ and OH-, reacts with the silicate minerals, especially feldspar, which makes up more than half of granite. The hydrogen ions replace the potassium, sodium or calcium in the feldspar, and the mineral is converted into a soft clay mineral called kaolinite, while the potassium is dissolved and washed away and some silica is released. In simplified form: 2KAlSi3O8 + 2H2CO3 + H2O → Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2. The feldspar crystals, once hard and glassy, become chalky and crumble, and the whole granite falls apart into a mixture of clay, sand-sized quartz and mica flakes. The white china clay quarried near Rajmahal, in Birbhum and in Purulia is kaolin formed by the hydrolysis of feldspar in granite and gneiss. In the very hot and wet tropics hydrolysis goes so far that even the silica is removed, leaving a residue of iron and aluminium oxides: this is laterite, from the Latin later, a brick, so named because it hardens like brick on exposure and is cut into building blocks in Kerala and western West Bengal.
Solution. Some minerals simply dissolve in water: rock salt (halite) and gypsum most easily, limestone slowly in acidic water. The dissolved matter is carried away by rivers and ground water; the world's rivers deliver to the sea nearly 400 crore tonnes of dissolved rock each year. Where soluble rocks lie at the surface, solution hollows them out, producing the same karst features as carbonation. The salt lakes of Rajasthan, such as Sambhar, are fed partly by salts dissolved from the surrounding rocks.
Organic acids from decaying leaves, roots and lichens (chelation) add to all these processes. Together, oxidation, carbonation, hydration, hydrolysis and solution turn hard crystalline rock into the loose, soft, clayey material from which soil is made.
- China clay (kaolin) near Mohammad Bazar in Birbhum was formed by the hydrolysis of feldspar in the ancient granite-gneiss of the Chhotanagpur margin.
- Laterite blocks cut from the ground in the Rarh plain of Bankura are used as building stone; the same rock hardens on exposure to air after being cut soft.
- A lump of jaggery or rock salt placed in a saucer of water disappears into solution; in nature rock salt beds dissolve the same way.
- Hydration: CaSO₄ + 2H₂O → CaSO₄·2H₂O (anhydrite to gypsum, about 60 per cent increase in volume).
- Hydrolysis of feldspar (simplified): 2KAlSi₃O₈ + 2H₂CO₃ + H₂O → Al₂Si₂O₅(OH)₄ (kaolinite) + 2K⁺ + 2HCO₃⁻ + 4SiO₂.
- Solution: NaCl (rock salt) + water → Na⁺ and Cl⁻ ions carried away.
Biological weathering
Living things, from bacteria and lichens to trees, animals and human beings, also break and decay rock. Their work is called biological or organic weathering. It is partly mechanical (forcing rock apart) and partly chemical (dissolving it with acids), which is why some geographers treat it as a part of the other two families rather than a separate one.
Plants. The seed of a peepal or banyan lodges in a crack in a rock or a wall, germinates, and sends down roots. As the roots thicken over the years they press outward on the walls of the crack with great force and widen it until the rock splits. Anyone who has seen a peepal tree splitting the wall of an old building in Kolkata, or the roots of trees breaking the stones of the Angkor temples, has seen mechanical biological weathering. Fine roots also enter the smallest pores. Chemically, roots and decaying plant matter release carbon dioxide and humic acids into the soil water, greatly increasing its power of carbonation and hydrolysis; the carbon dioxide content of soil air may be ten to a hundred times that of the open air. Lichens, the grey-green crusts on bare rock in hills, are the first colonisers of fresh rock; they secrete acids that etch the surface and take iron and other elements out of the minerals. Mosses that follow them hold moisture against the rock and keep chemical weathering going.
Animals. Burrowing animals, earthworms, ants, termites, rats, rabbits, loosen the regolith, mix it, and let air and water enter deeper. Termite mounds in Purulia and Bankura bring fine material from below and expose it to weathering. Earthworms pass soil through their bodies and change it chemically; Darwin estimated that on a hectare of English pasture they bring ten tonnes of soil to the surface each year. Shellfish and boring molluscs drill into coastal rock; bird droppings (guano) supply acids.
Micro-organisms. Bacteria and fungi in the soil break down organic matter, produce acids and take part in the oxidation of iron and sulphur minerals. Their work is invisible but continuous.
Human beings. Human activity is now a major cause of weathering. Quarrying, mining, road cutting and blasting expose fresh rock to the air over vast areas. Ploughing, deforestation and overgrazing remove the protective cover and accelerate weathering of the ground beneath. The burning of coal and petroleum adds sulphur dioxide and nitrogen oxides to the air; these form sulphuric and nitric acids in rain, and this acid rain corrodes limestone, marble and sandstone buildings and statues far faster than natural carbonation would. Human beings are thus, in effect, an agent of chemical weathering on a continental scale.
- Peepal trees growing on old buildings in north Kolkata crack the brickwork with their roots and their leaf litter keeps the walls damp and rotting.
- Lichen patches on the rocks of Susunia hill in Bankura leave pale, etched patches beneath them when scraped off.
- Acid rain from the refineries and vehicles of Mathura and Agra has yellowed and pitted the marble of the Taj Mahal, forcing the Supreme Court to order factories to shift.
- Biological weathering = mechanical action of roots and burrowers + chemical action of organic acids and carbon dioxide from living things.
Weathering and climate: the world pattern
Because climate is the master control, each climatic region of the world has its own characteristic style of weathering. The examination often asks a student to name the type of weathering common in a given region and to give the reason.
Equatorial and humid tropical regions (Amazon and Congo basins, Malaysia, the Western Ghats, Meghalaya). Temperatures stay above 25 °C all year and rainfall exceeds 2,000 mm. Chemical weathering, especially hydrolysis and oxidation, is intense and rapid; water penetrates deep along joints and the regolith may be tens of metres thick, with a rusty, clay-rich laterite at the surface. Mechanical weathering is slight because the daily temperature range is small and the forest cover shields the ground.
Hot desert regions (Sahara, Arabia, Thar). Rainfall is under 250 mm; skies are clear and the daily temperature range is 30 to 50 °C. Mechanical weathering by heating and cooling dominates, producing block disintegration, granular disintegration and exfoliation. Chemical weathering is slow for want of water, though dew and rare rain allow some hydration and salt weathering, and the dark 'desert varnish' of iron and manganese oxides coats exposed rock.
Monsoon and savanna regions (most of India, East Africa). A hot wet season alternates with a dry season. Chemical weathering is strong in the rains, mechanical weathering in the hot dry months; the alternation of wetting and drying is itself very effective. Laterite and deep red soils form on the older surfaces of peninsular India.
Temperate regions (western Europe, eastern USA, northern China). Moderate temperature and rainfall spread over the year give moderate chemical weathering with some frost action in winter. Weathering is of medium intensity and soils are deep and fertile.
Cold and high-mountain regions (Siberia, Canada, the Alps, the Himalaya above the tree line). Temperatures cross the freezing point repeatedly in spring and autumn, so frost action dominates, producing angular scree, blockfields and frost-shattered peaks. Chemical weathering is slow because the cold slows reactions and much water is locked as ice.
Polar regions (Antarctica, Greenland). Weathering of all kinds is minimal: too cold for chemical reactions, too dry for much frost action, and the rock is mostly buried under ice.
Within one country the pattern repeats with altitude and rainfall. In West Bengal itself, the plateau margin of Purulia has strong chemical weathering with laterite, the Darjeeling hills above 3,000 metres have frost shattering, and the wet, warm delta has chemical decay of every exposed brick and stone.
- In the Western Ghats of Kerala, road cuttings show red weathered rock more than 20 metres deep with the fresh rock nowhere visible: deep tropical chemical weathering.
- On Sandakphu (3,636 m) in Darjeeling district, the ground in winter is covered with angular frost-shattered stones.
- In the Thar desert the rounded, exfoliated granite boulders of the Jodhpur region lie among sharp-edged blocks split along joints.
- Rule of thumb: hot + wet → chemical weathering; hot + dry → mechanical (thermal) weathering; cold + moist → frost weathering; cold + dry → very little weathering.
Regolith, soil formation and the results of weathering
The most important product of weathering is soil. Weathering first produces the regolith, the loose mantle of broken and rotted rock resting on fresh bedrock. The regolith is not yet soil; it is merely mineral debris. It becomes soil when organic matter, water, air and living organisms are added to it and it is arranged into layers or horizons. Soil scientists call the weathered mineral material the parent material, and the passage from parent material to soil is called pedogenesis, or soil formation.
Five factors, the same ones that control weathering, control soil formation: parent material, climate, organisms, relief and time. A mature soil shows a soil profile of distinct horizons. The A horizon or topsoil is dark with humus, and from it rain water washes down (leaches) fine clay and soluble minerals. The B horizon or subsoil receives these washed-down materials and is often reddish with iron oxides and richer in clay. The C horizon is the partly weathered parent rock, the regolith proper. Below lies the R horizon, the unweathered bedrock. In the humid tropics the profile may be many metres deep; on a young mountain slope only a few centimetres.
The type of weathering decides the kind of soil. Where hydrolysis and leaching are extreme, laterite soils rich in iron and aluminium but poor in plant food form, as in the Rarh region of West Bengal and the coastal Western Ghats. Where basalt is oxidised and hydrated, the swelling, cracking black cotton soil (regur) forms, as in Maharashtra. Weathering of granite and gneiss gives the sandy red soils of Chhotanagpur. Weathering products carried by rivers and laid down in the plains give the alluvial soils of the Ganga delta, which are young, deep and fertile.
Weathering shapes landforms as well. The rounded tors of the Chhotanagpur and Deccan plateaus are piles of corestones left when the rotted, jointed granite around them was washed away. Exfoliation domes are bare granite hills with curved sheeting. In limestone, carbonation carves karst: grikes, sinkholes, dolines, caverns, stalactites and stalagmites. In dry lands, the different resistance of rock layers produces mesas, buttes and honeycombed cliffs.
Weathering also has an economic face. It concentrates useful minerals: the bauxite of Jharkhand, Odisha and Chhattisgarh is a residue of tropical weathering, and so are many iron, manganese and nickel deposits and the china clay of Birbhum. Building stones, on the other hand, are chosen for their resistance to weathering, and engineers must test the depth of weathered rock before founding a dam or a bridge. The Farakka barrage and every Himalayan road cutting were designed with the regolith in mind.
- A soil profile in a Rarh laterite field shows a thin grey topsoil, a thick mottled red-and-yellow B horizon of iron-rich laterite, and pale kaolinitic weathered rock beneath.
- The bauxite of the Bagru hills near Lohardaga in Jharkhand is a laterite crust formed by the tropical weathering of aluminium-rich rocks, and is mined for aluminium.
- Tors at Ayodhya hills in Purulia are stacks of rounded granite corestones that survived when the weathered joint material was washed away.
- Soil = weathered parent material (regolith) + organic matter (humus) + water + air + living organisms.
- Soil profile: A horizon (topsoil, humus, leached) over B horizon (subsoil, clay and iron accumulate) over C horizon (regolith) over R (bedrock).
Mass movement: meaning and slow forms
Weathered material does not stay still on a slope. Under the pull of gravity, aided by water, it moves downhill without the help of any transporting agent such as a river or glacier. This downslope movement of rock, regolith and soil under gravity is called mass movement or mass wasting. It is the link between weathering, which loosens the material, and erosion, which carries it away: mass movement delivers the debris of hill slopes to the rivers below. The chapter treats it here because it is the immediate consequence of weathering.
Whether a slope stays put or moves depends on the balance between the forces pulling the material down (the weight of the material multiplied by the steepness of the slope) and the forces holding it (friction and the cohesion of the particles, strengthened by plant roots). Anything that increases the driving force or reduces the resistance triggers movement: heavy rain that adds weight and lubricates the particles, the cutting of the foot of the slope by a river or a road, earthquake shaking, removal of forest, or the piling of buildings and debris on top of a slope.
Mass movements are classified by their speed and by the amount of water involved. The slow, almost invisible forms are described here; the fast ones in the next topic.
Soil creep is the slowest, a movement of a few millimetres to a few centimetres a year, so slow that it can only be seen by its effects: telegraph poles and fence posts tilt downhill, stone walls bulge and break, tree trunks curve at the base as they try to grow upright while the ground slides beneath them, and the soil piles against the upslope side of walls. Creep is caused by the repeated expansion and contraction of the soil as it wets and dries, freezes and thaws, or is heated and cooled; each time the particles rise perpendicular to the slope and fall vertically, so they inch downhill. Creep affects almost every grassy or wooded slope in the Darjeeling hills.
Solifluction (literally 'soil flow') is a faster form of creep in cold regions, where the thawed, waterlogged upper layer of soil flows slowly over the still-frozen ground beneath in summer, forming tongues and lobes on the hillside. It is common in Ladakh, the higher Himalaya and the tundra.
Earthflow and mudflow are intermediate in speed: saturated clay-rich regolith slides and flows downhill as a thick paste, a mudflow being the more watery and faster of the two. In the Himalaya, mudflows following cloudbursts choke roads and bury villages; the 2013 Kedarnath disaster involved enormous debris and mudflows.
- On the tea-garden slopes above Kurseong, fence posts lean downhill and old tree trunks show a curve at the base: the signs of soil creep.
- In summer at Ladakh, lobes of thawed soil flow slowly over frozen ground on the hillsides: solifluction.
- After the cloudburst of June 2013 at Kedarnath, mudflows of glacial debris and boulders swept through the town within minutes.
- Mass movement occurs when the downslope pull (weight × slope) exceeds the resisting forces (friction + cohesion + root binding).
- Order of speed (slowest to fastest): soil creep < solifluction < earthflow < mudflow < landslide < rockfall.
Mass movement: landslides and rockfalls
The rapid forms of mass movement are the dangerous ones, and the Darjeeling hills of West Bengal suffer from them every monsoon.
A landslide is the sudden sliding of a mass of rock, regolith or soil down a slope along a definite surface of failure. When the mass slides along a flat plane, such as a bedding plane or joint sloping in the direction of the hillside, it is a rock slide or debris slide. When it slides along a curved, spoon-shaped surface and the sliding block rotates backwards as it moves, leaving a steep crescent-shaped scar at the top, it is a slump; slumps are common where clay or shale underlies the slope. A landslide may travel a few metres or many kilometres, and the material may be dry or wet.
A rockfall is the free fall or bouncing of individual rock fragments or blocks from a steep cliff, usually loosened by frost action or by the undercutting of the cliff. The fallen fragments pile up at the foot as scree. Rockfalls are frequent along the Rohtang and Nathu La roads and on the Hindustan-Tibet road.
An avalanche is a rapid mass movement of snow and ice, sometimes carrying rock; it is a hazard of the Kashmir and Sikkim Himalaya.
Causes of landslides. The natural causes are steep slopes; weak rocks such as shale, phyllite and highly jointed gneiss; beds and joints dipping in the same direction as the slope; heavy and prolonged rainfall that saturates the regolith and adds to its weight while reducing friction; undercutting of the slope by rivers such as the Teesta; and earthquakes, which shake the slope loose. The human causes are cutting of roads and house sites into the slope, which removes support from the material above; deforestation, which takes away the roots that bind the soil and lets rain enter freely; unplanned construction that loads the slope; leaking water pipes and blocked drains that saturate it; and quarrying and mining. The Darjeeling hills combine nearly every one of these: young, weak, jointed rocks, slopes of 30 to 60 degrees, more than 3,000 mm of monsoon rain, rapid road building and loss of forest.
Major events. The landslides of October 1968, after 1,000 mm of rain fell in three days, killed nearly a thousand people in Darjeeling district and cut the hill roads and railway in scores of places. The September 1980 slides and the landslides of 2015 in Mirik, Kalimpong and Sukhia Pokhri, which killed more than forty people, repeated the pattern. Elsewhere in India, the Malpa landslide of 1998 in Uttarakhand and the Kedarnath disaster of 2013 are remembered.
Prevention. Landslides can be reduced by afforestation of slopes, terracing, retaining walls and wire-crate (gabion) walls at the foot of slopes, proper surface drainage and lined drains to keep water out of the regolith, restriction of building on unstable slopes, careful cutting of roads with gentle benched slopes, and early warning systems based on rainfall thresholds. The Geological Survey of India maps landslide-prone zones in the Darjeeling and Kalimpong hills.
- The 4 October 1968 Darjeeling landslides, triggered by about 1,000 mm of rain in three days, destroyed long stretches of the Hill Cart Road and killed nearly a thousand people.
- In July 2015 heavy rain caused landslides at Mirik, Kalimpong and Sukhia Pokhri in Darjeeling district that buried houses and killed more than forty people.
- On the Teesta valley road to Sikkim, gabion walls of wire cages filled with stone hold back slopes where the river has undercut them.
- Landslide risk rises with: slope angle + rock weakness + bedding dipping downslope + rainfall + undercutting + deforestation + construction load.
- Landslide prevention = afforestation + drainage + retaining and gabion walls + benched road cuttings + zoning + early warning.
Weathering in daily life and the examination pattern
Weathering is not only a matter of distant mountains. It touches buildings, roads, monuments, farms and health, and the examination likes questions that connect the process to ordinary experience.
Buildings and monuments. Every building material weathers. Bricks and plaster in the humid Bengal climate absorb water, grow algae and moss, and crumble; iron rusts; the sandstone of the Bishnupur terracotta temples has softened and flaked over three centuries; the marble of the Victoria Memorial has been cleaned repeatedly of dark crusts formed by sulphur pollution. Engineers choose granite and quartzite for durability and coat steel against oxidation. Understanding weathering is thus the basis of conservation of heritage.
Agriculture. The soil that feeds Bengal's rice fields is the end product of weathering, either in place (the red and laterite soils of the west) or after transport by rivers (the alluvium of the delta). Weathering releases the potassium, calcium, phosphorus and other nutrients locked in minerals into forms that plants can take. Where weathering has gone too far, as in laterite, the nutrients have been leached away and the soil is poor; where it is young, as in fresh alluvium, the soil is rich.
Minerals. Bauxite, china clay, iron ore of the laterite type and many placer deposits are products of weathering, so it underlies whole industries.
Hazards. Weathering weakens slopes and prepares the material for landslides, rockfalls and mudflows, the hazards of every hill road from Darjeeling to Sikkim.
What the board asks. Questions from this chapter fall into predictable patterns. One-mark and multiple-choice questions ask for the name of a process from a description: 'the peeling of rock in layers is called', 'the weathering of limestone by carbonic acid is called', 'the soil formed by intense leaching in hot wet regions is'. Two-mark questions ask for definitions: weathering, regolith, exfoliation, frost wedging, mass movement, soil creep. Three-mark questions ask for differences, most often weathering versus erosion, or mechanical versus chemical weathering, or for the factors controlling weathering. Five-mark questions ask for a full account of the types of mechanical weathering with diagrams, or the chemical processes with reactions, or the causes and prevention of landslides in the Darjeeling hills. Diagrams score marks: practise the exfoliation dome, the frost-wedged crack, the jointed block, the soil profile and the slump.
A good answer to any of these begins with a clear definition, gives the process step by step, names the climate or region where it is typical, gives an Indian or West Bengal example, and, where asked, ends with a labelled diagram. Learn the chemical equations of oxidation, carbonation and hydrolysis exactly, and remember the numbers: water expands about 9 per cent on freezing, a hot desert's daily temperature range may exceed 50 °C, and the Darjeeling landslides of 1968 followed about 1,000 mm of rain in three days.
- The Rasmancha and Jor Bangla terracotta temples at Bishnupur show flaked and softened laterite and brick surfaces after three centuries of monsoon weathering.
- The Victoria Memorial's Makrana marble has been cleaned of black sulphur crusts and is protected by a ban on polluting industries nearby.
- A typical board question: 'Distinguish between weathering and erosion' (3 marks); 'Describe with diagrams the processes of mechanical weathering' (5 marks).
Key Concepts
- Weathering
- The slow disintegration and decomposition of rocks in the place where they lie, by the agents of the atmosphere and by living things, without transport.
- Erosion
- The wearing away of the land and removal of material by moving agents such as rivers, glaciers, wind, waves and ground water.
- Denudation
- The total lowering of the land surface by weathering, mass movement, erosion and transportation acting together.
- Regolith
- The mantle of loose, broken and chemically rotted rock material lying over fresh bedrock.
- Mechanical weathering
- The breaking of rock into smaller pieces by physical forces such as temperature change and frost, without change in its chemical composition.
- Block disintegration
- The splitting of jointed rock into large rectangular blocks by repeated expansion and contraction with daily heating and cooling.
- Granular disintegration
- The crumbling of a coarse-grained rock such as granite grain by grain because its different minerals expand and weather at different rates.
- Exfoliation
- The peeling away of curved outer shells of rock, like the layers of an onion, through heating and cooling or release of pressure.
- Frost wedging
- The splitting of rock by water that freezes in its cracks and expands by about nine per cent, widening the crack with each freeze-thaw cycle.
- Scree or talus
- The pile of angular, frost-shattered rock fragments that collects at the foot of a cliff or steep slope.
- Chemical weathering
- The decomposition of rock through chemical reactions with water, oxygen, carbon dioxide and organic acids, producing new minerals.
- Oxidation
- The reaction of oxygen with iron and other minerals in rock, forming rust-like oxides that colour the rock red or brown and weaken it.
- Carbonation
- The dissolving of limestone and other carbonate rocks by carbonic acid formed when rain water absorbs carbon dioxide.
- Hydrolysis
- The reaction of water with silicate minerals such as feldspar, converting them into soft clay minerals like kaolinite.
- Hydration
- The absorption of water into the structure of a mineral, causing it to swell and crack the rock around it.
- Laterite
- A red, iron- and aluminium-rich residual material formed by intense tropical weathering and leaching, which hardens on exposure to air.
- Biological weathering
- The breaking and decay of rock by plants, animals, micro-organisms and human activity, through both physical force and organic acids.
- Karst
- A landscape of sinkholes, caves and underground drainage formed by the solution of limestone through carbonation.
- Mass movement
- The downslope movement of rock, regolith and soil under gravity without a transporting agent, ranging from soil creep to landslides.
- Landslide
- The sudden sliding of a mass of rock or soil down a slope along a surface of failure, often triggered by heavy rain, undercutting or earthquakes.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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What is weathering? Distinguish between weathering and erosion. / अपक्षय क्या है? अपक्षय और अपरदन में अंतर बताइए।
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Weathering is the slow disintegration and decomposition of rocks in the place where they lie, brought about by the agents of the atmosphere such as temperature, rain water, frost, oxygen and carbon dioxide, and by living organisms. It does not involve any transport of material. Erosion, on the other hand, is the wearing away of the land surface by moving agents such as rivers, glaciers, wind, sea waves and ground water, which carry the loosened material away with them. Thus weathering is a static process and erosion a dynamic one; weathering produces regolith and soil while erosion produces valleys, gorges and cliffs; weathering is very slow while erosion is comparatively rapid; and weathering prepares the material that erosion then removes. Together with mass movement they make up denudation. / अपक्षय वह धीमी प्रक्रिया है जिसमें चट्टानें अपने स्थान पर ही तापमान, वर्षा-जल, तुषार, ऑक्सीजन और कार्बन डाइऑक्साइड जैसे वायुमंडलीय कारकों तथा जीवों द्वारा टूटती और सड़ती हैं; इसमें पदार्थ का कोई परिवहन नहीं होता। अपरदन में नदी, हिमनद, पवन, समुद्री लहरें और भूमिगत जल जैसे गतिशील कारक भूमि की सतह को घिसते हैं और ढीले पदार्थ को अपने साथ बहा ले जाते हैं। अतः अपक्षय स्थैतिक प्रक्रिया है और अपरदन गतिशील; अपक्षय से रेगोलिथ और मिट्टी बनती है जबकि अपरदन से घाटियाँ, गॉर्ज और कगार बनते हैं; अपक्षय अत्यंत धीमा है जबकि अपरदन अपेक्षाकृत तेज़; और अपक्षय उस पदार्थ को तैयार करता है जिसे अपरदन हटाता है। ये दोनों, संचलन के साथ मिलकर, अनाच्छादन कहलाते हैं।
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Explain the factors that control the rate of weathering. / अपक्षय की दर को नियंत्रित करने वाले कारकों की व्याख्या कीजिए।
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The rate and type of weathering depend on five main factors. Climate is the most important: hot and wet climates favour chemical weathering because reactions need warmth and water, hot dry deserts favour mechanical weathering by heating and cooling, and cold climates favour frost action. Rock structure and composition matter: jointed and porous rocks admit water and air and weather faster, iron-rich rocks oxidise, limestone dissolves, while quartz-rich rocks resist. Slope decides whether the weathered material is removed to expose fresh rock (steep slopes) or accumulates as a protective blanket (flat land). Vegetation speeds chemical weathering through roots and humic acids but shields the rock from sun and rain. Time is the last factor: the longer a rock is exposed, the deeper it is weathered, so old plateau surfaces carry a thick regolith and fresh cuttings a thin one. / अपक्षय की दर और प्रकार पाँच मुख्य कारकों पर निर्भर करते हैं। जलवायु सबसे महत्वपूर्ण है: गर्म और आर्द्र जलवायु रासायनिक अपक्षय के अनुकूल है क्योंकि अभिक्रियाओं को गर्मी और जल चाहिए, गर्म शुष्क मरुस्थल तापन-शीतलन द्वारा यांत्रिक अपक्षय के अनुकूल हैं, और ठंडी जलवायु तुषार क्रिया के अनुकूल है। चट्टान की संरचना और संघटन का महत्व है: संधियुक्त और सरंध्र चट्टानें जल और वायु को भीतर आने देती हैं और शीघ्र अपक्षयित होती हैं, लौह-युक्त चट्टानें ऑक्सीकृत होती हैं, चूना पत्थर घुल जाता है, जबकि क्वार्ट्ज़-युक्त चट्टानें प्रतिरोध करती हैं। ढाल तय करता है कि अपक्षयित पदार्थ हटकर ताज़ी चट्टान उजागर होगी (तीव्र ढाल) या सुरक्षात्मक परत के रूप में जमा होगा (समतल भूमि)। वनस्पति जड़ों और ह्यूमिक अम्लों से रासायनिक अपक्षय को तेज़ करती है परंतु चट्टान को धूप और वर्षा से बचाती भी है। अंतिम कारक समय है: चट्टान जितने अधिक समय तक उजागर रहेगी, उतनी ही गहराई तक अपक्षयित होगी, इसलिए पुराने पठारी धरातलों पर मोटा रेगोलिथ और नई कटानों पर पतला रेगोलिथ मिलता है।
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Describe with a diagram the process of exfoliation. In which regions is it common? / रेखाचित्र सहित अपपत्रण (अपशल्कन) की प्रक्रिया का वर्णन कीजिए। यह किन क्षेत्रों में सामान्य है?
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Exfoliation is the peeling off of the outer layers of a rock in curved sheets, like the skins of an onion, and is therefore also called onion weathering. In hot regions with a great daily range of temperature, the outer skin of a massive rock such as granite expands strongly in the daytime heat while the interior, rock being a poor conductor, stays cool; at night the skin contracts. The repeated expansion and contraction sets up stresses between the skin and the core, so the outer shell cracks parallel to the surface and falls away in curved slabs. Corners and edges, heated from more than one side, weather fastest, so the rock becomes rounded. Moisture that enters the outer layer and swells the minerals by hydration helps the peeling. On a larger scale, the removal of overlying rock releases pressure on deep-seated granite, which expands and develops sheeting joints from which great curved sheets break away, forming bare exfoliation domes. The diagram shows a rock mass with concentric curved shells, the outermost partly detached. Exfoliation is common in hot deserts such as the Thar and the Sahara, in the seasonally hot Deccan and Chhotanagpur plateaus, and on the granite domes of southern Karnataka. / अपपत्रण चट्टान की बाहरी परतों का प्याज़ के छिलकों की तरह वक्र पत्रकों में उतरना है, इसलिए इसे प्याज़-छिलका अपक्षय भी कहते हैं। अधिक दैनिक तापांतर वाले गर्म क्षेत्रों में ग्रेनाइट जैसी ठोस चट्टान की बाहरी परत दिन की गर्मी में बहुत फैलती है जबकि भीतरी भाग, चट्टान के ऊष्मा का कुचालक होने के कारण, ठंडा रहता है; रात में बाहरी परत सिकुड़ती है। बार-बार के इस फैलाव और सिकुड़न से बाहरी परत और भीतरी भाग के बीच तनाव उत्पन्न होता है, जिससे बाहरी खोल सतह के समानांतर चटकता है और वक्र पट्टिकाओं के रूप में गिर जाता है। कोने और किनारे, कई ओर से गर्म होने के कारण, सबसे तेज़ी से अपक्षयित होते हैं, अतः चट्टान गोलाकार हो जाती है। बाहरी परत में प्रवेश कर खनिजों को जलयोजन से फुलाने वाली नमी इस प्रक्रिया में सहायक है। बड़े पैमाने पर, ऊपर की चट्टानों के हटने से गहराई में बने ग्रेनाइट पर दबाव कम होता है, वह फैलता है और उसमें पत्रक-संधियाँ बनती हैं जिनसे बड़ी वक्र चादरें टूटकर अलग होती हैं और नंगे अपपत्रण गुंबद बनते हैं। रेखाचित्र में संकेंद्री वक्र खोलों वाली चट्टान दिखाई जाती है जिसका बाहरी खोल आंशिक रूप से अलग हो चुका है। अपपत्रण थार और सहारा जैसे गर्म मरुस्थलों, मौसमी रूप से गर्म दक्कन और छोटानागपुर पठारों तथा दक्षिणी कर्नाटक के ग्रेनाइट गुंबदों पर सामान्य है।
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How does frost action weather rocks? Why is it dominant in the higher Himalaya? / तुषार क्रिया चट्टानों का अपक्षय कैसे करती है? यह उच्च हिमालय में प्रमुख क्यों है?
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Frost action or freeze-thaw weathering works through the expansion of water when it freezes. Rain water or melt water enters the cracks, joints and pores of a rock during the day. When the temperature falls below 0 °C at night the water turns to ice, and ice occupies about nine per cent more volume than the water from which it formed. The growing ice presses on the walls of the crack with a force that may reach 2,000 kilograms per square centimetre and widens it a little. By day the ice melts and the enlarged crack admits more water, which freezes again the next night. After hundreds of such cycles the rock is split into sharp, angular fragments, a process called frost wedging; the fragments fall and collect at the foot of cliffs as scree or talus, and whole slopes may be covered by blockfields. It is dominant in the higher Himalaya above about 3,500 metres because there the temperature crosses the freezing point again and again, especially in spring and autumn, there is abundant moisture from rain and snow, the rocks are highly jointed by mountain building, and the cold slows chemical weathering so that frost is the chief agent left. / तुषार क्रिया या हिम-गलन अपक्षय जल के जमने पर होने वाले प्रसार से कार्य करती है। दिन में वर्षा या हिम के पिघले जल चट्टान की दरारों, संधियों और छिद्रों में प्रवेश करता है। रात में जब तापमान 0 °C से नीचे गिरता है तो जल बर्फ बन जाता है, और बर्फ का आयतन उस जल से लगभग नौ प्रतिशत अधिक होता है जिससे वह बनी है। बढ़ती बर्फ दरार की दीवारों पर 2,000 किलोग्राम प्रति वर्ग सेंटीमीटर तक का दबाव डालती है और उसे थोड़ा चौड़ा कर देती है। दिन में बर्फ पिघलती है और चौड़ी दरार में और जल भरता है, जो अगली रात फिर जमता है। ऐसे सैकड़ों चक्रों के बाद चट्टान नुकीले, कोणीय टुकड़ों में टूट जाती है, जिसे तुषार-कीलन कहते हैं; टुकड़े गिरकर कगारों के नीचे स्क्री या टेलस के रूप में जमा होते हैं और पूरे ढाल शिलाखंड-क्षेत्र से ढक सकते हैं। यह लगभग 3,500 मीटर से ऊपर उच्च हिमालय में प्रमुख है क्योंकि वहाँ तापमान बार-बार हिमांक को पार करता है, विशेषकर वसंत और शरद में, वर्षा और हिम से पर्याप्त नमी मिलती है, पर्वत-निर्माण के कारण चट्टानें अत्यधिक संधियुक्त हैं, और ठंड रासायनिक अपक्षय को धीमा कर देती है जिससे तुषार ही मुख्य कारक बचता है।
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Explain oxidation and carbonation with chemical equations and Indian examples. / रासायनिक समीकरणों और भारतीय उदाहरणों सहित ऑक्सीकरण और कार्बोनेटीकरण की व्याख्या कीजिए।
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Oxidation is the combination of oxygen dissolved in water with the iron and other minerals of a rock. Ferrous iron in the rock is converted into ferric oxide, the rust-coloured substance that stains the rock red, yellow or brown and weakens its structure so that it crumbles: 4FeO + O₂ → 2Fe₂O₃. The red laterite soils of Bankura, Purulia and Birbhum and the rusty colour of the Deccan basalt are its result. Carbonation begins when rain water absorbs carbon dioxide from the air and soil to form weak carbonic acid, H₂O + CO₂ → H₂CO₃. This acid attacks limestone, chalk and marble, converting insoluble calcium carbonate into soluble calcium bicarbonate that is carried away in solution: CaCO₃ + H₂CO₃ → Ca(HCO₃)₂. Carbonation widens joints into grikes, opens sinkholes and hollows out caves with stalactites and stalagmites, producing karst landscapes such as the Mawsmai caves of Cherrapunji in Meghalaya and the Borra caves of Andhra Pradesh; polluted, more acidic rain has also yellowed the marble of the Taj Mahal. / ऑक्सीकरण जल में घुली ऑक्सीजन का चट्टान के लोहे और अन्य खनिजों से संयोग है। चट्टान का फेरस लोहा फेरिक ऑक्साइड में बदल जाता है, जो जंग के रंग का पदार्थ है और चट्टान को लाल, पीला या भूरा रंग देकर उसकी संरचना को कमज़ोर कर देता है जिससे वह भुरभुरी हो जाती है: 4FeO + O₂ → 2Fe₂O₃। बाँकुड़ा, पुरुलिया और बीरभूम की लाल लैटेराइट मिट्टियाँ और दक्कन बेसाल्ट का जंग जैसा रंग इसी का परिणाम हैं। कार्बोनेटीकरण तब शुरू होता है जब वर्षा-जल वायु और मिट्टी से कार्बन डाइऑक्साइड सोखकर दुर्बल कार्बोनिक अम्ल बनाता है, H₂O + CO₂ → H₂CO₃। यह अम्ल चूना पत्थर, खड़िया और संगमरमर पर आक्रमण करता है और अघुलनशील कैल्शियम कार्बोनेट को घुलनशील कैल्शियम बाइकार्बोनेट में बदल देता है जो घोल में बह जाता है: CaCO₃ + H₂CO₃ → Ca(HCO₃)₂। कार्बोनेटीकरण संधियों को चौड़ा कर ग्राइक बनाता है, विलयन-रंध्र खोलता है और स्टैलेक्टाइट-स्टैलेग्माइट वाली गुफाएँ खोखली करता है, जिससे मेघालय के चेरापूंजी की मॉसमाई गुफाओं और आंध्र प्रदेश की बोर्रा गुफाओं जैसे कार्स्ट भूदृश्य बनते हैं; प्रदूषित, अधिक अम्लीय वर्षा ने ताजमहल के संगमरमर को भी पीला कर दिया है।
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What is hydrolysis? How does it convert granite into clay and laterite? / जलअपघटन क्या है? यह ग्रेनाइट को मृत्तिका और लैटेराइट में कैसे बदलता है?
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Hydrolysis is the chemical reaction between water, slightly ionised into hydrogen and hydroxyl ions, and the silicate minerals of a rock, in which the hydrogen ions replace the potassium, sodium or calcium in the mineral and new, softer minerals are formed. Its chief victim is feldspar, which makes up more than half of granite. Feldspar is converted into the soft white clay mineral kaolinite, while potassium and some silica are dissolved and washed away: 2KAlSi₃O₈ + 2H₂CO₃ + H₂O → Al₂Si₂O₅(OH)₄ + 2K⁺ + 2HCO₃⁻ + 4SiO₂. As the feldspar crystals turn chalky the granite loses its coherence and falls apart into a mixture of clay, loose quartz sand and mica flakes; the white china clay of Birbhum and Purulia is such kaolin. In very hot and wet climates hydrolysis and leaching go further still: even the silica is removed and only the insoluble oxides of iron and aluminium remain, forming laterite, the red material of the Rarh region that hardens like brick on exposure to air and is the source of bauxite in Jharkhand and Odisha. / जलअपघटन जल, जो थोड़ा-सा हाइड्रोजन और हाइड्रॉक्सिल आयनों में विघटित होता है, और चट्टान के सिलिकेट खनिजों के बीच की रासायनिक अभिक्रिया है, जिसमें हाइड्रोजन आयन खनिज के पोटैशियम, सोडियम या कैल्शियम का स्थान ले लेते हैं और नए, नरम खनिज बनते हैं। इसका मुख्य शिकार फेल्सपार है, जो ग्रेनाइट का आधे से अधिक भाग बनाता है। फेल्सपार नरम सफेद मृत्तिका खनिज केओलिनाइट में बदल जाता है, जबकि पोटैशियम और कुछ सिलिका घुलकर बह जाते हैं: 2KAlSi₃O₈ + 2H₂CO₃ + H₂O → Al₂Si₂O₅(OH)₄ + 2K⁺ + 2HCO₃⁻ + 4SiO₂। फेल्सपार के क्रिस्टल खड़िया जैसे होते ही ग्रेनाइट अपनी बंधन-शक्ति खो देता है और मृत्तिका, ढीली क्वार्ट्ज़ बालू और अभ्रक की पपड़ियों के मिश्रण में बिखर जाता है; बीरभूम और पुरुलिया की सफेद चीनी मिट्टी ऐसा ही केओलिन है। अत्यंत गर्म और आर्द्र जलवायु में जलअपघटन और निक्षालन और आगे बढ़ते हैं: सिलिका भी हट जाता है और केवल लोहे और एल्युमिनियम के अघुलनशील ऑक्साइड बचते हैं, जिससे लैटेराइट बनता है, राढ़ क्षेत्र का वह लाल पदार्थ जो हवा में आने पर ईंट की तरह कठोर हो जाता है और झारखंड तथा ओडिशा में बॉक्साइट का स्रोत है।
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Describe how plants, animals and human beings bring about weathering. / पौधे, जीव-जंतु और मनुष्य अपक्षय कैसे करते हैं, वर्णन कीजिए।
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Living things weather rock both mechanically and chemically. Plant roots grow into cracks, thicken over the years and force the rock apart, as a peepal tree splits an old wall; fine roots enter the smallest pores, and decaying leaves and roots release carbon dioxide and humic acids that make soil water far more powerful in carbonation and hydrolysis. Lichens, the first colonisers of bare rock, secrete acids that etch the surface, and mosses hold moisture against it. Burrowing animals such as earthworms, ants, termites, rats and rabbits loosen and mix the regolith and let air and water enter deeper; termite mounds bring fresh material to the surface, and boring molluscs drill coastal rock. Bacteria and fungi produce acids and help oxidise iron and sulphur minerals. Human beings are today a major agent: quarrying, mining, blasting and road cutting expose fresh rock; deforestation, ploughing and overgrazing strip the protective cover; and the sulphur dioxide and nitrogen oxides from burning coal and petroleum produce acid rain, which corrodes limestone and marble buildings such as the Taj Mahal far faster than natural rain. / जीव चट्टानों का यांत्रिक और रासायनिक दोनों प्रकार से अपक्षय करते हैं। पौधों की जड़ें दरारों में बढ़ती हैं, वर्षों में मोटी होकर चट्टान को अलग कर देती हैं, जैसे पीपल का पेड़ पुरानी दीवार को फाड़ देता है; महीन जड़ें सूक्ष्मतम छिद्रों में घुसती हैं, और सड़ती पत्तियाँ और जड़ें कार्बन डाइऑक्साइड और ह्यूमिक अम्ल छोड़ती हैं जो मिट्टी के जल को कार्बोनेटीकरण और जलअपघटन में कहीं अधिक शक्तिशाली बना देते हैं। नंगी चट्टान पर सबसे पहले बसने वाले लाइकेन अम्ल स्रावित कर सतह को खुरचते हैं, और काई उस पर नमी बनाए रखती है। केंचुए, चींटियाँ, दीमक, चूहे और खरगोश जैसे बिल बनाने वाले जीव रेगोलिथ को ढीला और मिश्रित करते हैं तथा वायु और जल को और गहराई तक जाने देते हैं; दीमक की बाँबियाँ ताज़ा पदार्थ सतह पर लाती हैं, और छेद करने वाले मोलस्क तटीय चट्टानों में छेद करते हैं। जीवाणु और कवक अम्ल बनाते हैं और लोहे तथा गंधक के खनिजों के ऑक्सीकरण में सहायक होते हैं। मनुष्य आज एक प्रमुख कारक है: खदान, खनन, विस्फोट और सड़क कटाई ताज़ी चट्टान को उजागर करते हैं; वनोन्मूलन, जुताई और अतिचारण सुरक्षात्मक आवरण छीन लेते हैं; और कोयले तथा पेट्रोलियम के जलने से निकले सल्फर डाइऑक्साइड और नाइट्रोजन ऑक्साइड अम्लीय वर्षा उत्पन्न करते हैं, जो ताजमहल जैसे चूना पत्थर और संगमरमर के भवनों को प्राकृतिक वर्षा से कहीं तेज़ी से गलाती है।
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Why is chemical weathering dominant in the equatorial regions and mechanical weathering in hot deserts? / भूमध्यरेखीय प्रदेशों में रासायनिक अपक्षय और गर्म मरुस्थलों में यांत्रिक अपक्षय प्रमुख क्यों है?
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In the equatorial regions the temperature stays above 25 °C throughout the year and rainfall exceeds 2,000 mm, so the two conditions for chemical reactions, warmth and abundant water, are present all the time; oxidation, hydrolysis and carbonation proceed rapidly, water penetrates deep along the joints, and the regolith may be tens of metres thick with laterite at the surface. The daily range of temperature is small and the dense forest shields the rocks from the direct sun, so the heating and cooling that drive mechanical weathering are weak, and there is no frost. In hot deserts the opposite holds: rainfall is below 250 mm, so there is hardly any water for chemical reactions, but clear skies give a daily temperature range of 30 to 50 °C, with rock surfaces reaching 70 °C by day and cooling sharply at night. The repeated expansion and contraction breaks jointed rocks into blocks, crumbles granite grain by grain and peels massive rock in exfoliation shells, while the absence of vegetation leaves the rock fully exposed. / भूमध्यरेखीय प्रदेशों में तापमान पूरे वर्ष 25 °C से ऊपर रहता है और वर्षा 2,000 मिमी से अधिक होती है, अतः रासायनिक अभिक्रियाओं की दोनों शर्तें, गर्मी और प्रचुर जल, सदैव उपस्थित रहती हैं; ऑक्सीकरण, जलअपघटन और कार्बोनेटीकरण तेज़ी से चलते हैं, जल संधियों के साथ गहराई तक प्रवेश करता है, और रेगोलिथ दसियों मीटर मोटा हो सकता है जिसकी सतह पर लैटेराइट होता है। दैनिक तापांतर कम है और घना वन चट्टानों को सीधी धूप से बचाता है, अतः यांत्रिक अपक्षय को चलाने वाला तापन-शीतलन दुर्बल है, और तुषार नहीं पड़ता। गर्म मरुस्थलों में इसका उल्टा है: वर्षा 250 मिमी से कम है, अतः रासायनिक अभिक्रियाओं के लिए जल लगभग नहीं है, परंतु साफ आकाश 30 से 50 °C का दैनिक तापांतर देता है, चट्टान की सतह दिन में 70 °C तक तपती है और रात में तेज़ी से ठंडी होती है। बार-बार का यह फैलाव और सिकुड़न संधियुक्त चट्टानों को खंडों में तोड़ता है, ग्रेनाइट को कण-कण भुरभुरा करता है और ठोस चट्टान को अपपत्रण खोलों में छीलता है, जबकि वनस्पति के अभाव में चट्टान पूरी तरह उजागर रहती है।
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What is a soil profile? Describe its horizons. / मृदा परिच्छेदिका क्या है? इसके संस्तरों का वर्णन कीजिए।
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A soil profile is the vertical section of a soil from the surface down to the unweathered rock, showing the distinct layers or horizons that soil-forming processes have produced. The A horizon or topsoil at the surface is dark because it contains humus from decayed plants, holds most of the roots and organisms, and is the zone from which rain water washes down, or leaches, fine clay and soluble minerals. The B horizon or subsoil below receives this leached material and is therefore richer in clay and often coloured red or yellow by accumulated iron oxides; it is more compact and has fewer roots. The C horizon is the partly weathered parent material, the regolith proper, consisting of broken rock fragments with little organic matter. Beneath it lies the R horizon, the solid unweathered bedrock from which the whole profile has been made. In the humid tropics the profile may be many metres deep, while on a young mountain slope it may be only a few centimetres. / मृदा परिच्छेदिका मिट्टी का सतह से लेकर अनपक्षयित चट्टान तक का ऊर्ध्वाधर काट है, जो मृदा-निर्माण प्रक्रियाओं द्वारा बनी पृथक परतों या संस्तरों को दिखाता है। सतह पर A संस्तर या ऊपरी मिट्टी गहरे रंग की होती है क्योंकि इसमें सड़े पौधों से बना ह्यूमस होता है, इसमें अधिकांश जड़ें और जीव रहते हैं, और यही वह क्षेत्र है जहाँ से वर्षा-जल महीन मृत्तिका और घुलनशील खनिजों को नीचे बहा ले जाता है, जिसे निक्षालन कहते हैं। नीचे का B संस्तर या अवमृदा इस निक्षालित पदार्थ को प्राप्त करता है और इसलिए मृत्तिका में समृद्ध होता है और प्रायः जमा हुए लौह ऑक्साइडों से लाल या पीला रंग लिए होता है; यह अधिक सघन है और इसमें जड़ें कम हैं। C संस्तर आंशिक रूप से अपक्षयित मूल पदार्थ है, वास्तविक रेगोलिथ, जिसमें टूटी चट्टान के टुकड़े हैं और जैविक पदार्थ बहुत कम है। इसके नीचे R संस्तर है, ठोस अनपक्षयित आधार चट्टान, जिससे पूरी परिच्छेदिका बनी है। आर्द्र उष्णकटिबंध में परिच्छेदिका कई मीटर गहरी हो सकती है, जबकि किसी युवा पर्वतीय ढाल पर केवल कुछ सेंटीमीटर।
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What is mass movement? Distinguish between soil creep and landslide. / संचलन (वृहत क्षरण) क्या है? मृदा विसर्पण और भूस्खलन में अंतर बताइए।
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Mass movement or mass wasting is the downslope movement of weathered rock, regolith and soil under the direct pull of gravity, aided by water but without the help of a transporting agent such as a river, glacier or wind. It occurs when the downslope force, the weight of the material on a slope, exceeds the resisting forces of friction, cohesion and root binding. Soil creep is the slowest form: the soil moves only a few millimetres or centimetres a year, driven by repeated wetting and drying, heating and cooling or freezing and thawing that lift the particles and let them settle a little downhill each time; it is invisible in action and known only from tilted poles and fence posts, curved tree trunks and bulging walls. A landslide is a sudden, rapid sliding of a large mass of rock or soil along a definite surface of failure, either a flat bedding plane or joint (rock slide) or a curved surface with backward rotation of the block (slump); it is triggered by heavy rain, undercutting of the slope, earthquakes or human cutting, moves in minutes, and can destroy roads and villages, as in the Darjeeling hills in 1968 and 2015. / संचलन या वृहत क्षरण अपक्षयित चट्टान, रेगोलिथ और मिट्टी का गुरुत्वाकर्षण के सीधे खिंचाव से, जल की सहायता से परंतु नदी, हिमनद या पवन जैसे किसी परिवहन कारक के बिना, ढाल के नीचे की ओर संचलन है। यह तब होता है जब ढाल पर पदार्थ के भार से उत्पन्न नीचे की ओर का बल घर्षण, संसंजन और जड़ों के बंधन के प्रतिरोधी बलों से अधिक हो जाता है। मृदा विसर्पण सबसे धीमा रूप है: मिट्टी वर्ष में केवल कुछ मिलीमीटर या सेंटीमीटर खिसकती है, जिसे बार-बार का भीगना-सूखना, गर्म-ठंडा होना या जमना-पिघलना चलाता है जो कणों को उठाकर हर बार थोड़ा नीचे बैठा देता है; यह क्रिया में अदृश्य है और केवल झुके खंभों और बाड़ों, मुड़े वृक्ष-तनों और उभरी दीवारों से जानी जाती है। भूस्खलन चट्टान या मिट्टी के बड़े पिंड का किसी निश्चित विफलता-सतह के साथ अचानक, तीव्र फिसलन है, चाहे वह समतल संस्तर-तल या संधि हो (शैल-स्खलन) या पिंड के पीछे की ओर घूर्णन वाली वक्र सतह (स्लंप); यह भारी वर्षा, ढाल की कटान, भूकंप या मानव द्वारा कटाई से प्रेरित होता है, मिनटों में घटित होता है, और सड़कों तथा गाँवों को नष्ट कर सकता है, जैसा 1968 और 2015 में दार्जिलिंग की पहाड़ियों में हुआ।
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Discuss the causes of landslides in the Darjeeling hills and suggest measures to prevent them. / दार्जिलिंग की पहाड़ियों में भूस्खलन के कारणों की विवेचना कीजिए और उनकी रोकथाम के उपाय सुझाइए।
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Landslides in the Darjeeling hills have both natural and human causes. The natural causes are the steep slopes of 30 to 60 degrees; the young, weak and highly jointed rocks such as phyllite, schist and gneiss of the Himalayan foothills; bedding and joints that often dip in the direction of the slope; the very heavy monsoon rainfall of more than 3,000 mm a year, which saturates the regolith, adds to its weight and reduces the friction between particles, especially in cloudbursts such as the 1,000 mm in three days of October 1968; the undercutting of slopes by rivers like the Teesta and Rangit; and earthquakes such as that of September 2011. The human causes are the cutting of roads and house sites into the slopes, which removes support from the material above; large-scale deforestation for tea gardens, timber and settlement, which destroys the root binding of the soil; unplanned multi-storeyed construction that loads unstable slopes; leaking pipes and blocked drains that saturate the ground; and quarrying. Prevention requires afforestation of bare slopes with deep-rooted trees; construction of retaining walls and wire-crate gabion walls at the toe of slopes; terracing and benching of road cuttings; lined surface drains and sub-surface drains to keep water out of the regolith; strict zoning that forbids building on mapped unstable slopes; and rainfall-based early warning and evacuation, using the landslide hazard maps prepared by the Geological Survey of India. / दार्जिलिंग की पहाड़ियों में भूस्खलन के प्राकृतिक और मानवीय दोनों कारण हैं। प्राकृतिक कारण हैं 30 से 60 डिग्री के तीव्र ढाल; हिमालय की तलहटी की युवा, कमज़ोर और अत्यधिक संधियुक्त चट्टानें जैसे फिलाइट, शिस्ट और नाइस; संस्तर और संधियाँ जो प्रायः ढाल की दिशा में झुकी होती हैं; वर्ष में 3,000 मिमी से अधिक की अत्यंत भारी मानसूनी वर्षा, जो रेगोलिथ को संतृप्त करती है, उसका भार बढ़ाती है और कणों के बीच घर्षण घटाती है, विशेषकर अक्टूबर 1968 के तीन दिन में 1,000 मिमी जैसे बादल फटने में; तीस्ता और रंगीत जैसी नदियों द्वारा ढालों की कटान; और सितंबर 2011 जैसे भूकंप। मानवीय कारण हैं ढालों में सड़कों और मकानों की कटाई, जो ऊपर के पदार्थ का सहारा छीन लेती है; चाय बागानों, इमारती लकड़ी और बस्तियों के लिए बड़े पैमाने पर वनोन्मूलन, जो मिट्टी के जड़-बंधन को नष्ट करता है; अस्थिर ढालों पर बोझ डालने वाला अनियोजित बहुमंज़िला निर्माण; ज़मीन को संतृप्त करने वाले रिसते पाइप और अवरुद्ध नाले; और खदानें। रोकथाम के लिए नंगे ढालों पर गहरी जड़ों वाले वृक्षों का वनरोपण; ढालों के तल पर प्रतिधारक दीवारें और तार-जालीदार गैबियन दीवारें; सड़क कटानों की सीढ़ीनुमा बनावट; रेगोलिथ में जल न जाने देने के लिए पक्के सतही और भूमिगत नाले; मानचित्रित अस्थिर ढालों पर निर्माण पर रोक लगाने वाला कड़ा ज़ोनिंग; और भारतीय भूवैज्ञानिक सर्वेक्षण द्वारा तैयार भूस्खलन-संकट मानचित्रों के आधार पर वर्षा-आधारित पूर्व चेतावनी और निकासी आवश्यक हैं।
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Name the type of weathering responsible for each of the following: (a) the tilting of telegraph poles on a hillside, (b) the formation of caves in limestone, (c) the reddening of rocks rich in iron, (d) the splitting of rock by ice in cracks. / निम्नलिखित में से प्रत्येक के लिए उत्तरदायी अपक्षय के प्रकार का नाम लिखिए: (क) पहाड़ी ढाल पर तार के खंभों का झुकना, (ख) चूना पत्थर में गुफाओं का बनना, (ग) लौह-युक्त चट्टानों का लाल होना, (घ) दरारों में बर्फ से चट्टान का टूटना।
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(a) The tilting of telegraph poles on a hillside is the sign of soil creep, the slowest form of mass movement, in which the weathered soil inches downhill under gravity through repeated wetting and drying or heating and cooling, carrying the base of the poles with it. (b) Caves in limestone are formed by carbonation, a chemical weathering process in which rain water charged with carbon dioxide forms carbonic acid that converts insoluble calcium carbonate into soluble calcium bicarbonate and hollows out the rock along joints. (c) The reddening of iron-rich rocks is oxidation, a chemical process in which oxygen dissolved in water combines with the ferrous iron of the rock to form ferric oxide or rust. (d) The splitting of rock by ice in cracks is frost wedging or freeze-thaw action, a mechanical weathering process in which water in the cracks expands by about nine per cent on freezing and forces the rock apart over many cycles. / (क) पहाड़ी ढाल पर तार के खंभों का झुकना मृदा विसर्पण का लक्षण है, जो संचलन का सबसे धीमा रूप है, जिसमें अपक्षयित मिट्टी बार-बार भीगने-सूखने या गर्म-ठंडे होने से गुरुत्व के अधीन धीरे-धीरे ढाल के नीचे खिसकती है और खंभों के आधार को अपने साथ ले जाती है। (ख) चूना पत्थर में गुफाएँ कार्बोनेटीकरण से बनती हैं, एक रासायनिक अपक्षय प्रक्रिया जिसमें कार्बन डाइऑक्साइड-युक्त वर्षा-जल कार्बोनिक अम्ल बनाता है जो अघुलनशील कैल्शियम कार्बोनेट को घुलनशील कैल्शियम बाइकार्बोनेट में बदल देता है और संधियों के साथ चट्टान को खोखला कर देता है। (ग) लौह-युक्त चट्टानों का लाल होना ऑक्सीकरण है, एक रासायनिक प्रक्रिया जिसमें जल में घुली ऑक्सीजन चट्टान के फेरस लोहे से मिलकर फेरिक ऑक्साइड या जंग बनाती है। (घ) दरारों में बर्फ से चट्टान का टूटना तुषार-कीलन या हिम-गलन क्रिया है, एक यांत्रिक अपक्षय प्रक्रिया जिसमें दरारों का जल जमने पर लगभग नौ प्रतिशत फैलता है और अनेक चक्रों में चट्टान को अलग कर देता है।
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