Chapter 5 — Weathering

Weathering is the breaking down and decay of rocks at or near the surface of the Earth, in the place where they lie, by the action of the elements of the weather, the heat of the Sun, frost, rain and the gases of the air, and of living things. The name says it exactly: rocks are worn by the weather. A rock that seems hard and permanent is in truth under constant attack. Its surface is heated and cooled every day, wetted and dried, frozen and thawed, breathed on by air containing oxygen and carbon dioxide, and pierced by roots. Under this attack the rock loosens, cracks, crumbles and rots, until a mass of solid stone becomes a layer of broken fragments and soil.

Three features define weathering and separate it from the other exogenic processes. First, it is an in situ process, meaning that it happens on the spot; the products remain where they were formed, at least until some other agent removes them. Second, it involves no transporting agent; running water, wind, ice and waves are not needed for weathering, which is why it occurs even on a flat plateau where nothing flows. Third, it is a static or passive process, working slowly and quietly, so that the change over a year may be invisible though the change over a thousand years is great.

Weathering is the first stage of denudation, the general wearing down of the land. It prepares the rock; the agents of erosion then carry the prepared material away. A hard, unweathered rock face resists a river or the wind, but once weathering has loosened its surface into gravel, sand and clay, those agents can remove the debris easily. Without weathering, erosion would be slow indeed; with it, whole mountain ranges are eventually reduced to plains.

Weathering is also the beginning of soil. The loose mantle of broken and decayed rock that covers most of the land, called regolith, is the raw material into which plants root and in which organic matter accumulates to form the soil on which all agriculture depends. Everything grown in the fields of West Bengal is grown in the product of weathering. The chapter examines how rocks weather, why they weather faster in some places than others, and what the results are.

Students often confuse weathering with erosion because both wear away rock, and the examination regularly asks for the distinction. The two are stages of one sequence but are different in nature.

Weathering is the breaking down of rock in place by the weather and by organisms. It needs no moving agent; the rock is simply attacked where it stands, and the fragments stay there. It is a static process. It works on the whole exposed surface of the land at once, on plateau tops as much as in valleys, and its rate depends on climate and rock type rather than on slope or the presence of water flow.

Erosion is the removal and transport of rock material by a moving agent, running water, wind, glaciers, waves or ground water. The agent picks up the loose material, which is usually the product of weathering, carries it away and at the same time uses it as a tool to scrape and grind fresh rock. It is a dynamic process. It is concentrated along the paths of the agents, in river channels, on windward slopes, under glaciers and along coasts, and its rate depends on the energy of the agent, which in turn depends on slope, volume and speed.

The relation between them is one of preparation and removal. Weathering loosens the rock; erosion removes what weathering has loosened. A river cuts its valley much faster through weathered rock than through fresh rock, and a wind can move sand only after weathering has produced it. Conversely, erosion assists weathering by stripping away the weathered cover and exposing fresh rock to attack. The two together, with the eventual deposition of the material somewhere else, make up denudation.

An illustration makes it plain. On a hillside in the Darjeeling hills the rocks crack in the frost and rot in the rain: that is weathering. During the monsoon the loosened material slides and washes into the Teesta, which carries it down to the plains: that is erosion. The one prepared what the other removed.

Rocks do not weather at the same rate everywhere or in the same way. Six factors govern the type and speed of weathering.

Climate is the most important, because weathering is the work of the weather. Temperature and rainfall decide which processes act. Where it is hot and dry, as in the Thar desert, large daily changes of temperature cause physical weathering by expansion and contraction, and chemical weathering is weak for want of water. Where it is hot and wet, as in the Sundarbans or Kerala, chemical weathering is rapid and deep, because chemical reactions speed up with heat and need water; the rock rots to great depths. Where it is cold, as in the high Himalaya, frost action is the main process and chemical weathering is slow. Weathering is most intense in the humid tropics and least in the cold dry polar deserts.

Rock type controls resistance. Soft sedimentary rocks such as shale and poorly cemented sandstone weather quickly; hard igneous rocks such as granite and basalt weather slowly, though granite, with its several minerals of different expansion, crumbles by granular disintegration while basalt rots chemically to black soil. Limestone, though hard, is dissolved by rain water. Rocks made of one mineral, such as quartzite, resist best.

Rock structure, the pattern of joints, cracks and bedding planes, decides how far water and air can enter. A closely jointed rock offers many surfaces for attack and breaks up fast; a massive rock with few joints resists. Weathering follows the joints, which is why jointed granite breaks into blocks and why limestone with many cracks is honeycombed with caves.

Slope matters because on steep slopes the weathered material is quickly removed by gravity and rain, exposing fresh rock to continued attack, while on gentle slopes and flat land the debris accumulates as a thick blanket that shields the rock below from further weathering, though chemical weathering continues within the wet mantle.

Vegetation works both ways. Plant roots pry rocks apart and the acids of decaying leaves attack minerals, speeding chemical and biological weathering; but a plant cover also shields rock from direct sun and frost and reduces the temperature changes that drive physical weathering.

Time is the sixth factor. Weathering is slow, and the depth of the weathered layer depends on how long the rock has been exposed. The ancient rocks of the Deccan and Chota Nagpur, exposed for hundreds of millions of years, are weathered many metres deep, while a fresh lava flow or a newly cut road face has only a skin of decay. The interaction of these six factors gives every region its own weathering pattern.

Physical or mechanical weathering is the breaking of rock into smaller pieces without any change in its chemical composition. The pieces are the same material as the parent rock, only smaller; a boulder of granite is reduced to blocks, gravel and grains of granite. It is a matter of force rather than chemistry, and the force most often comes from changes in temperature, called thermal or insolation weathering, because the heat comes from the Sun's insolation.

Rock is a poor conductor of heat. In a hot desert the surface of a rock is heated to 60 or 70 degrees Celsius by day and expands, while the interior a few centimetres below stays cool; at night the surface cools rapidly, sometimes to near freezing, and contracts, while the interior is still warm. Day after day the outer layer expands and contracts against an inner mass that does not, and the strain eventually cracks the surface. Three forms of this process are distinguished.

Exfoliation, from the Latin for stripping off leaves, occurs in massive rocks of uniform composition such as granite and sandstone. The outer shells crack loose from the mass and peel away in curved sheets, one beneath another, like the skins of an onion, so that a dome-shaped hill or a rounded boulder results. Exfoliation domes are found in the granite country of the Deccan and Chota Nagpur, around Hyderabad and Bengaluru, and on a giant scale in the Sierra Nevada of California. The removal of overlying rock, which reduces the pressure on rock once deeply buried, also helps the sheets to separate, a process called pressure release or unloading.

Granular disintegration occurs in coarse-grained rocks made of several minerals, such as granite with its quartz, feldspar and mica. Each mineral has its own rate of expansion and its own colour, dark minerals absorbing more heat than light ones, so the grains push against one another unequally with each change of temperature until they loosen and fall away one by one, leaving a rounded surface and a heap of coarse sand. Much of the sand of the desert is produced in this way.

Block disintegration occurs in rocks already divided by joints and bedding planes, such as sandstone and basalt. Expansion and contraction widen the joints until the rock breaks along them into rectangular or polygonal blocks, which pile up as a rubble of angular fragments. The columnar basalts of the Deccan and the jointed sandstones of the Vindhya are broken up thus.

Insolation weathering is strongest where the daily range of temperature is largest and the sky is clearest, in hot deserts and on bare mountain tops, and weakest in cloudy, humid regions where the rock is never strongly heated. Desert travellers have reported the sharp cracks of rocks splitting in the cold of the night, a sound that is the process itself.

Besides heat, three other physical forces break rock.

Frost action or frost wedging is the most powerful physical weathering process in cold climates. Water seeps into cracks and pores in the rock by day. At night, when the temperature falls below 0 degrees Celsius, the water freezes, and because ice occupies about 9 per cent more volume than the water it came from, it presses outward on the walls of the crack with a force that can exceed 2,000 kilograms per square centimetre, more than any rock can resist. The crack widens; the ice thaws by day and water penetrates deeper; the next night it freezes again. After many cycles the rock is split apart. Frost action needs both water and repeated crossing of the freezing point, so it is strongest not in the coldest places, where the ice never thaws, but where the temperature swings across 0 degrees every day, as in high mountains in spring and autumn and in the cold temperate lands in winter. The ridges of the Himalaya above the tree line are littered with angular blocks broken by frost, and the sharp peaks, arêtes and pinnacles of high mountains are frost-shattered forms. The loose angular fragments that gather at the foot of a frost-shattered cliff form a sloping heap called scree or talus, well seen below the cliffs of Ladakh and Lahaul. Freezing of water within the soil, called frost heaving, lifts stones to the surface and breaks roads and walls in cold countries.

Salt weathering or salt crystal growth happens in dry climates and along coasts. Water carrying dissolved salts enters the pores of a rock; as it evaporates, the salts crystallise, and the growing crystals press on the pore walls exactly as ice does, loosening grains and flaking off the surface. Rocks along the sea shore, sprayed with salt water, and the rocks of the desert, where ground water evaporates at the surface, are honeycombed and pitted by it. Salt weathering is also a cause of decay of stone buildings and monuments in coastal cities, including old buildings of Kolkata and the Sun Temple at Konark.

Pressure release or unloading is not the work of weather at all but of the removal of weight. Rocks such as granite that formed deep in the crust under enormous pressure are brought to the surface as erosion strips the cover away. Freed from the load, the rock expands slightly and cracks parallel to the surface in sheets, which then break away as exfoliation. The great domes of massive granite owe much of their sheeting to this process.

Finally, the wetting and drying of clay-rich rocks such as shale makes them swell and shrink, and the repeated change crumbles them into small flakes, a process sometimes called slaking. All these physical processes have the same result: rock is reduced to smaller pieces of unchanged material, with a far larger surface area on which chemical weathering can then work.

Chemical weathering is the decay of rock by chemical reactions between its minerals and the water, oxygen and carbon dioxide of the atmosphere and soil. Unlike physical weathering it changes the composition of the rock: the original minerals are converted into new minerals, usually softer, and into substances that dissolve and are carried away in solution. Water is essential to every chemical weathering process, and heat speeds every reaction, so chemical weathering is strongest in hot, wet climates and weak in deserts and polar lands. It is aided by physical weathering, which by breaking rock into small pieces vastly increases the surface exposed to attack. Five processes are recognised, of which two are described here.

Oxidation is the combination of a mineral with oxygen, the same reaction as the rusting of iron. Many rocks contain iron in compounds such as pyrite and the dark silicate minerals. When oxygen dissolved in rain water reaches them, the iron is converted to iron oxides, which are the red, brown and yellow colours of rust. The oxides are larger in volume, softer and more crumbly than the original minerals, so the rock is weakened, and its surface takes on a reddish or brown stain that is the surest sign of oxidation. The red soils of Purulia, Bankura and West Midnapore, the red and yellow soils of the Chota Nagpur plateau and the red sandstone of the Vindhya owe their colour to this process; so does the laterite of the same districts, which is a mass of iron and aluminium oxides. Oxidation is strongest in warm climates with alternate wetting and drying.

Carbonation is the reaction of minerals with carbonic acid, a weak acid formed when carbon dioxide from the air and from decaying vegetation in the soil dissolves in rain water. Carbonic acid attacks calcium carbonate, the mineral of limestone, chalk and marble, and converts it into calcium bicarbonate, which unlike the carbonate is soluble in water and is carried away in solution. In this way limestone is eaten away along its joints to form the sinkholes, caves, underground rivers and jagged surfaces of limestone country, known as karst, of which the caves of the Meghalaya plateau at Cherrapunji and Mawsynram, the Borra caves of Andhra Pradesh and the limestone gorges near Dehradun are Indian examples. Carbonation also attacks the feldspar of granite, helping to convert it to clay. It is fastest in cool, wet conditions, because carbon dioxide dissolves better in cold water, though the abundant vegetation of the tropics supplies extra carbon dioxide to the soil water. Marble buildings such as the Taj Mahal and the old marble monuments of Kolkata are slowly corroded by carbonation from rain, a process made worse by the acid rain of polluted cities.

The remaining three chemical processes all involve water directly.

Hydration is the absorption of water into the crystal structure of a mineral, forming a new hydrated mineral that is larger in volume and softer. The mineral anhydrite takes up water and becomes gypsum, expanding by more than half; iron oxide, haematite, absorbs water to become limonite, the yellow-brown rust; and the feldspar of granite absorbs water as the first step of its decay. Because the hydrated minerals swell, they set up stresses inside the rock that crack its surface, so hydration is partly a physical as well as a chemical process. The swelling and shrinking of clay minerals with wetting and drying is a related effect, which cracks the black soil of the Deccan in the dry season and makes it sticky in the rains. Hydration is common wherever rocks are alternately wet and dry.

Hydrolysis is the chemical reaction of a mineral with water itself, in which the hydrogen ions of water replace the metal ions of the mineral and produce an entirely new mineral. It is the most important chemical weathering process for the silicate minerals that make up most rocks. Its classic case is the decay of feldspar, the commonest mineral of granite, which reacts with water and carbonic acid to form kaolin, a white clay, together with soluble potassium or sodium carbonate and silica that are washed away. Since feldspar is about 60 per cent of granite, a granite that has undergone hydrolysis becomes a soft, crumbly mass of quartz grains held in clay, which the rain easily washes into sand and mud. The white china clay of Birbhum and Bankura, quarried for pottery, is kaolin produced by the hydrolysis of feldspar in the local granite and gneiss. In the humid tropics hydrolysis goes deeper still, removing even the silica and leaving a residue of iron and aluminium oxides, which is laterite, the hard red rock of the Rarh region of West Bengal and of the coastal plateaus of Kerala and Goa, and bauxite, the ore of aluminium.

Solution is the simplest process: some minerals dissolve directly in water. Rock salt and gypsum dissolve in pure water; limestone, dolomite and chalk dissolve in water containing carbonic acid, which is why solution and carbonation are often described together. The dissolved material is carried away invisibly by rivers and ground water, so that in limestone regions whole hills disappear leaving hollows, sinkholes and caves, and the world's rivers deliver about four billion tonnes of dissolved rock to the oceans every year, which is the source of the sea's salt. Solution is faster in wet climates and in water made acid by vegetation or pollution.

In nature the five chemical processes act together, and their combined result is a rock that has lost its cohesion, changed colour, softened and partly dissolved, ready to be washed away or to become soil. Chemical weathering works below the surface as well as on it, following the joints into the rock so that the fresh rock is left as rounded cores within the rotted mass, the corestones that appear as boulders when the soft material is removed.

Living things are the third agent of weathering. Biological or organic weathering is the breakdown of rock by the physical and chemical action of plants, animals and human beings. It is partly mechanical and partly chemical, and it is most active where life is most abundant, in the humid tropics and in any soil-covered land.

Plants act mechanically through their roots. A seed lodged in a crack sends down roots that thicken as the plant grows, and their expanding pressure prises the crack open exactly as ice does; the roots of a large tree can split a boulder or lift a pavement, and the ruins of temples in Bengal and the jungle-covered monuments of Cambodia have been torn apart by the roots of banyan and pipal. Plants act chemically through the organic acids released by their roots and by the decay of their fallen leaves, humic acid among them, which dissolve minerals far more effectively than rain water alone; the carbon dioxide breathed out by roots and by the bacteria of the soil makes the soil water rich in carbonic acid. Lichens and mosses, the first plants to colonise a bare rock, cling to its surface, secrete acids that etch it, and hold moisture against it, beginning the formation of a soil in which larger plants can follow; the grey and orange patches on old temple stones are lichens at work. Algae and fungi contribute in the same way.

Animals weather rock chiefly by burrowing. Earthworms, ants, termites, rats, rabbits and other burrowers dig through the regolith, bring fresh material to the surface where the weather can reach it, and open passages for air and water to penetrate. Darwin estimated that earthworms in an English field pass ten tonnes of soil per acre through their bodies every year; in India termites build mounds several metres high from soil brought up from below. Grazing animals trample and loosen the surface, and along the shore boring molluscs and sea urchins drill into rock. The decay of animal remains adds acids to the soil water.

Human beings have become the most powerful biological agent of all. Quarrying, mining, road cutting, ploughing, deforestation and construction expose fresh rock to the weather on an enormous scale, and the deforested hills of the Darjeeling district and Chota Nagpur weather and erode far faster than they did under forest. Industrial pollution produces acid rain, which speeds carbonation and solution, and the exhaust of vehicles and factories corrodes stone buildings; the yellowing of the marble of the Taj Mahal and the decay of old stone in Kolkata's colonial buildings are human-accelerated weathering.

Biological weathering rarely acts alone; it works with and speeds the physical and chemical processes, and its chief importance is in the making of soil, where the mixing of organic matter with weathered rock by roots, worms and microbes turns barren regolith into fertile earth.

Because climate is the master control, each climatic region of the world has its own characteristic weathering, and the examination often asks the student to match region and process.

In hot deserts such as the Thar, the Sahara and Arabia, rainfall is under 250 millimetres and the sky is clear, so the daily range of temperature is enormous, often 30 to 40 degrees. Physical weathering by insolation dominates: exfoliation rounds the granite hills, granular disintegration feeds the dunes with sand, and block disintegration shatters the jointed sandstones into angular rubble. Salt weathering pits the rock where ground water evaporates. Chemical weathering is slight for want of water, though the thin dark desert varnish on rock surfaces is a product of slow oxidation. The weathered mantle is thin and the landscape is one of bare rock, rounded domes and sharp cliffs.

In the humid tropics, the equatorial and monsoon lands such as the Western Ghats, Kerala, the Sundarbans, the Congo and the Amazon, temperature is high all year and rainfall exceeds 2,000 millimetres, so chemical weathering is intense and continuous. Hydrolysis, oxidation and solution rot the rock to depths of 30 metres or more, forming deep red and yellow soils and laterite. The dense vegetation adds organic acids and carbon dioxide and speeds the decay. Physical weathering is weak because the temperature hardly changes and frost never occurs, though root action is strong. The landscape is one of deeply weathered, rounded hills with few rock outcrops except corestones.

In cold regions, the high Himalaya above the snow line, the Tibetan plateau, the Arctic and Antarctic, the water is frozen for most of the year and chemical reactions are slow. Frost action dominates wherever the temperature crosses freezing: the rocks are shattered into angular fragments, scree slopes gather below every cliff, and peaks are sharpened into horns and ridges into arêtes. In the coldest polar deserts, where the ice never thaws and there is no liquid water, weathering of every kind is very slow, and rock surfaces may survive unchanged for a million years.

In temperate humid regions, such as Europe, eastern North America and the Himalayan foothills, both physical and chemical weathering are moderate: frost acts in winter, carbonation and hydrolysis in the wet seasons, and vegetation is abundant; the weathered mantle is of medium depth and the soils are fertile brown and grey earths.

The pattern can be summed up thus: physical weathering is greatest where the temperature range is greatest and water is scarce, chemical weathering where heat and water are both abundant, and frost weathering where water is present and the temperature oscillates about the freezing point. West Bengal, a hot humid monsoon land with a dry winter, is a region of dominant chemical weathering, seen in the laterite of the Rarh and the deep alluvial soils, with some physical weathering on the bare rocks of Purulia in the hot dry months.

Weathering produces a loose mantle of rock waste on every slope, and gravity does not leave it there. Mass wasting or mass movement is the downslope movement of weathered material under the direct pull of gravity, without the help of a transporting agent such as a river or a glacier, though water often assists by adding weight and reducing friction. It is the bridge between weathering and erosion: what weathering loosens, mass wasting delivers to the valley floor, where rivers carry it away. It ranges from movement so slow that it is measured in millimetres a year to catastrophes that bury villages in seconds.

Soil creep is the slowest form. The particles of the regolith on a slope move downhill a little with every expansion and contraction, every wetting and drying, every freeze and thaw, and every disturbance by animals and roots. It cannot be seen happening, but its results can: fence posts and telegraph poles tilt downslope, tree trunks bend at the base, walls bulge, and the soil at the foot of a slope thickens. Creep is universal on soil-covered slopes.

Solifluction is a faster flow of water-saturated soil, typical of cold regions where the surface thaws in summer over frozen ground below and the sodden layer oozes downhill in lobes and terraces, and also of tropical slopes after heavy rain.

Earthflows and mudflows occur when soil or fine debris is so saturated with water that it flows like a thick liquid. Mudflows on the slopes of volcanoes, called lahars, and on the deforested hills of the Himalaya after cloudbursts move with great speed and destructive power.

Landslides are the rapid sliding of a mass of rock or soil down a slope along a definite surface of failure. They follow heavy rain, which saturates the ground and lubricates the slip plane, earthquakes, which shake the mass loose, and the undercutting of slopes by rivers, waves or road building. The Darjeeling Himalaya suffer landslides every monsoon; those of 1968 and 2015 killed hundreds and cut the road and rail links to the hills. A rockfall is the free fall of blocks from a cliff, building the scree slope below; an avalanche is a fall of snow and ice with rock, common on the steep faces of the high Himalaya.

Mass wasting is favoured by steep slopes, heavy rainfall, weak or weathered rock, layers dipping towards the valley, earthquakes, and the removal of vegetation, whose roots bind the soil. Its control is a matter of keeping forest on slopes, draining slopes so that water does not saturate them, building retaining walls and avoiding cutting into unstable hillsides. It is the subject of the next chapter's discussion of landslide hazards, and it explains why the products of weathering do not simply accumulate where they are made but are steadily fed to the rivers.

The first product of weathering is regolith, from Greek words meaning blanket of stone, the loose layer of broken and decayed rock fragments that lies between the fresh bedrock and the surface. It may be a few centimetres thick on a steep mountain slope, where mass wasting removes it as fast as it forms, or tens of metres thick on the old, gently sloping surfaces of the tropics. It is made of the fragments produced by physical weathering, from boulders to sand, and the clays and oxides produced by chemical weathering, together with whatever dissolved matter has not yet been washed out. Regolith is not soil; it is the parent material from which soil is made.

Soil is the upper part of the regolith that has been further changed by living things, by water and by time into a medium in which plants can grow. Its formation, called pedogenesis, needs five things: a parent material, the weathered rock, which decides the soil's texture and minerals, sandy from sandstone, clayey from shale, red from iron-rich basalt; a climate, whose temperature and rainfall govern the rate of weathering and the washing of materials through the soil; organisms, the plants whose roots and leaf litter supply humus, the animals that mix it, and the bacteria and fungi that decompose it; a relief, since flat and gently sloping land keeps its soil while steep slopes lose it; and time, because a mature soil takes thousands of years to develop, a centimetre of topsoil forming in roughly two hundred to a thousand years.

A mature soil shows layers, called horizons, in a vertical section, the soil profile. The A horizon or topsoil is dark with humus and is the zone from which rain washes fine particles and dissolved matter downward; the B horizon or subsoil is where those materials accumulate, often as clay or iron-stained layers; the C horizon is the partly weathered parent material; and below it lies the fresh bedrock, sometimes called the R horizon. The depth and clarity of the horizons record the age of the soil.

The soils of West Bengal illustrate the link to weathering and parent material. The alluvial soils of the Ganga plain and delta are formed on river silt itself the weathered product of the Himalaya; the laterite soils of the Rarh are the residue of deep chemical weathering of the old rocks of the plateau edge; the red soils of Purulia are the weathered granite and gneiss coloured by oxidised iron; the terai soils at the foot of the Darjeeling hills are coarse, gravelly weathered debris washed from the mountains; and the mountain soils of Darjeeling are thin, stony and rich in humus from the forest. Every field in the state is a page in the story of weathering.

Weathering is slow and unspectacular, yet its consequences are among the most far reaching in geography, and the chapter closes by weighing them.

Soil formation. The greatest gift of weathering is soil. All the food of humanity is grown in the thin layer of weathered rock and humus that covers the land. Without weathering there would be bare rock and no agriculture; the fertility of the alluvium of Bengal, the black soil of the Deccan and the red soils of the plateau all rest on it.

Landform development. Weathering is the first stage of the wearing down of mountains and the shaping of the land. It prepares the material for erosion, without which rivers, wind and ice would work far more slowly; it produces its own landforms, the exfoliation domes and tors of granite country, the blockfields and screes of cold mountains, the sinkholes and caves of limestone, and the corestone boulders of the tropics; and it rounds off the sharp edges of every cliff and peak. The gently rolling surface of an old plateau like Chota Nagpur is the work of hundreds of millions of years of weathering.

Mineral deposits. Chemical weathering concentrates valuable materials. Laterite and bauxite, the ore of aluminium mined in Odisha and Jharkhand, are the residues of deep tropical weathering; the china clay of Birbhum is weathered feldspar; the iron ores of some regions have been enriched by the removal of other minerals in solution; and placer gold and gems are freed from their rock by weathering before being sorted by rivers.

Building materials. The sand, gravel, clay and laterite blocks used in building are weathering products, and the soft weathered rock of the plateau edge is easily quarried.

Water supply. The porous weathered mantle absorbs rain and stores it as ground water, feeding wells and springs and keeping rivers flowing in the dry season.

Against these benefits weathering has harmful effects. It destroys buildings and monuments: the salt and moisture that attack the stone of Konark and the acid rain that corrodes the marble of the Taj Mahal are weathering, and old bridges, roads and dams need constant repair against it. It weakens slopes, and the weathered mantle on hillsides is the material of the landslides that plague Darjeeling and the Himalaya. It loosens the soil so that, once vegetation is removed, erosion can strip it away, the cause of the gullies of the Chambal and the bare hills of the Rarh. Deep chemical weathering in the tropics can also leach the soil of its nutrients, leaving the infertile laterites that support only scrub.

For the geographer, then, weathering is the quiet foundation of the visible landscape and of human livelihood, and for the engineer, the farmer and the conservator it is a force to be worked with and guarded against.

Questions on weathering ask for definitions, the difference between weathering and erosion, the description of named processes with examples, the relation of weathering to climate, and the effects of weathering. The following summary is arranged for revision.

Definitions. Weathering: the disintegration and decomposition of rock in situ by the elements of the weather and by organisms, without transport. Physical weathering: breaking into smaller pieces without chemical change. Chemical weathering: decay by chemical reactions changing the minerals. Biological weathering: breakdown by plants, animals and humans. Exfoliation: peeling of curved sheets from a massive rock. Granular disintegration: loosening of individual grains. Block disintegration: breaking along joints into blocks. Frost action: splitting by the freezing of water in cracks. Oxidation: combination with oxygen. Carbonation: reaction with carbonic acid. Hydration: absorption of water into minerals. Hydrolysis: reaction with water forming new minerals. Solution: direct dissolving. Mass wasting: downslope movement of weathered material by gravity. Regolith: the mantle of weathered rock. Scree: angular fragments at the foot of a cliff.

Classification. Physical: insolation (exfoliation, granular, block), frost, salt, pressure release, wetting and drying. Chemical: oxidation, carbonation, hydration, hydrolysis, solution. Biological: plants, animals, humans.

Process and rock. Exfoliation in granite and sandstone; granular disintegration in granite; block disintegration in basalt and sandstone; frost in all jointed rocks in cold climates; oxidation in iron-rich rocks; carbonation and solution in limestone, chalk and marble; hydrolysis in feldspar-bearing rocks such as granite.

Process and climate. Hot desert: insolation and salt. Humid tropics: hydrolysis, oxidation, solution, deep laterite. Cold mountains: frost. Temperate: mixed. West Bengal: chemical, with laterite in the Rarh.

West Bengal examples to quote. Laterite of Bankura, Purulia, Birbhum and West Midnapore; red soil of Purulia from oxidised iron; china clay of Birbhum from hydrolysed feldspar; frost-shattered scree in the Darjeeling Himalaya; landslides of Darjeeling; roots splitting the ruins of Bishnupur and Gaur.

Reasons often asked. Why chemical weathering is dominant in the tropics: heat and water speed reactions. Why physical weathering is dominant in deserts: large temperature range and no water for chemistry. Why frost action is strongest where the temperature crosses zero repeatedly: because water must freeze and thaw many times. Why weathering is called a static process: because nothing moves the material. Why limestone regions have caves: carbonation along joints. Why granite forms rounded domes: exfoliation and corestone weathering.

Answer pattern. For a process, give the definition, the mechanism in two or three sentences, the rock and climate in which it is common, and one named example, and draw a small diagram where possible. For a difference, use a table with at least four points. For the effects, give both benefits and harm.