Weathering and Denudation | ICSE Class 9 Geography Notes

This note covers the meaning, types and effects of weathering; physical, chemical and biological weathering; denudation and its agents; river work, stages and associated landforms; and wind action, deflation hollows and sand dunes.

What are weathering and denudation?

Weathering is the mechanical breaking up and chemical decomposition of rocks through the action of weather and climate. Mechanical disintegration means breaking into fragments; chemical decomposition means changes in the minerals that make up the rock. Minerals are the naturally occurring substances composing rocks.

Weathering is an in-situ, or on-site, process. Very little or no movement of material takes place. A rock can therefore become cracked, fragmented or chemically altered while remaining where it was formed or exposed.

Definition: Denudation means the stripping or uncovering of the land surface through weathering, mass movements, erosion and transportation.

Mass movements transfer rock debris down slopes under the direct influence of gravity, the force pulling material towards the Earth. Erosion involves the removal and transportation of rock material by moving agents such as rivers or wind. Transportation is the carrying of that material.

How do these terms differ?

Deposition builds up material in lower areas, whereas weathering, mass movement and erosion help wear down the land. These linked processes change relief, meaning differences in the height and shape of the land surface.

Keep the scale of the terms clear: weathering is part of denudation, not a separate name for the whole process. Breaking a rock at its original site and carrying its fragments elsewhere are related actions, but they describe different parts of landscape change.

What controls weathering, and what are its effects?

Climate, the long-term pattern of weather conditions, is particularly important in weathering. Temperature and precipitation, meaning water reaching the ground as rain, snow or other forms, influence the processes operating on rocks. The depth of weathered material also differs between climates.

Rock structure matters alongside climate. Cracks, joints, hardness and mineral composition influence resistance to weathering. A joint is a fracture in rock. Different rocks offer different resistance, and a rock resistant to one process may be less resistant to another.

Do the types work independently?

The major groups are physical, chemical and biological weathering. Physical weathering breaks rock through applied forces; chemical weathering changes or dissolves minerals; biological weathering involves living organisms. Very rarely does one group operate completely by itself, although quite often one process is dominant.

Physical breakage exposes surfaces to chemical attack. Roots can break rock mechanically, while decaying organic material can supply acids. The categories describe the action involved; they do not require a landscape to experience just one kind of weathering.

Why does weathering matter?

• Weathering produces smaller fragments and helps prepare regolith, the loose covering of weathered material above solid rock.
• It supplies material for soil formation. Soil develops through further physical, chemical and biological changes; weathered fragments alone are not a complete mature soil.
• It aids erosion and mass movements by weakening rock and making material available for removal.
• It can enrich valuable mineral deposits when other materials are removed, increasing the concentration of what remains.

Enrichment means an increase in the concentration of valuable material. Weathering can help concentrate deposits containing iron, manganese, aluminium and copper. This does not mean every weathered rock becomes a useful mineral deposit.

Note: Weathering helps erosion, but it is not a necessary precondition for erosion. It also aids mass movements without being essential for every mass movement.

How do block disintegration, granular disintegration and exfoliation occur?

Physical weathering, also called mechanical weathering, breaks rock into smaller pieces without changing its mineral composition. Most physical weathering processes result from thermal expansion, meaning expansion on heating, and pressure release, meaning reduction of the load pressing on rock.

Repeated expansion and contraction can weaken rock. Heating makes materials expand; cooling makes them contract. These processes are small and slow, but repeated changes can cause great damage through continued fatigue, the weakening produced by repeated expansion and contraction.

What is block disintegration?

Block disintegration is the breaking of rock into blocks along cracks or joints. Repeated heating and cooling can enlarge existing weaknesses until blocks become detached. The result is relatively large pieces rather than the separation of individual mineral grains.

In a block-disintegration explanation, link the applied forces to the joints and then to the separated blocks. The essential observation is how the rock breaks. The fragments retain the mineral composition of the rock from which they came.

What is granular disintegration?

Granular disintegration is the breaking of rock into individual mineral grains. Different minerals respond differently to heating and cooling. Their unequal expansion and contraction weaken the bonds between grains, allowing grains to loosen and separate.

The distinction from block disintegration concerns the units released. One produces blocks separated along larger weaknesses; the other loosens the grains making up the rock. Both involve physical breakage rather than the creation of different minerals.

What is exfoliation?

Exfoliation is the flaking off of more or less curved sheets or shells from rock. It can leave smooth, rounded surfaces. It is a result of weathering, rather than a separate force that acts on rock.

Exfoliation can occur through expansion and contraction caused by temperature changes. Pressure release can also produce exfoliation: removal of overlying material allows underlying rock to expand, encouraging sheets to separate near the exposed surface.

Photograph: Exfoliation and granular disintegration (NCERT Class 11 Figure 5.3). This is a photograph of exposed, broken rock with irregular surfaces and loose fragments. It is not a labelled sequence showing the stages of heating or pressure release.

Remember the visible outcomes together: blocks for block disintegration, grains for granular disintegration and curved sheets for exfoliation. A description of a rounded rock alone does not establish which force caused the exfoliation.

How do oxidation, carbonation, hydration and solution weather rocks?

Chemical weathering involves reactions that decompose or dissolve rock minerals. Water, acids, and the gases oxygen and carbon dioxide take part. Heat, water and air help speed chemical reactions, while decomposing plants and animals add carbon dioxide to the soil environment.

How does oxidation work?

Oxidation in weathering is the reaction of oxygen with susceptible minerals. Iron-bearing minerals can form iron oxides, commonly producing rusty red or brown colours. These chemical changes can weaken the rock containing the altered minerals.

The identifying action is combination with oxygen. A colour change can support recognition of oxidation, but the explanation should connect that observation to the chemical alteration of minerals rather than to simple physical cracking.

How does carbonation work?

Carbonation involves carbon dioxide dissolved in water forming weak carbonic acid. This acidic water reacts with calcium carbonate, the main chemical constituent of limestone, and helps carry it away in soluble form. Soluble means capable of dissolving in water.

1. Water takes up carbon dioxide from the air or the soil environment.
2. The dissolved carbon dioxide forms weak carbonic acid in the water.
3. The acidic water reacts with calcium carbonate in limestone.
4. Soluble products can be removed in water, gradually enlarging openings in the rock.

Limestone is a rock rich in calcium carbonate. Carbonated water, meaning water containing dissolved carbon dioxide, can dissolve its calcium carbonate. Cracks and joints provide routes along which water can enter and act.

How do hydration and solution differ?

Hydration is the chemical combination of water with a mineral. The resulting change can increase mineral volume and help weaken or break the surrounding rock. It means more than water merely wetting a rock surface.

Solution is the dissolving of soluble mineral matter in water. Rock salt is a simple example: its soluble material enters the water. The essential distinction is incorporation of water in hydration and dissolution of mineral matter in solution.

These terms describe different chemical actions, even though water is involved in several of them. Identify what the water does before naming the process: reacting with carbon dioxide, entering a mineral chemically, or dissolving soluble material.

How do plants, animals and humans cause biological weathering?

Biological weathering includes physical changes caused by the growth or movement of organisms and changes to minerals in the weathering environment. Organisms can break material mechanically, expose new surfaces or help produce chemicals that attack minerals.

How do plants affect rock?

Plant roots exert pressure on earth materials and mechanically break them apart. A root growing within a crack can press against its sides. This shows why biological weathering can include physical breakage without requiring the immediate chemical decomposition of the rock.

Decaying plant and animal matter also helps produce acids, including carbonic acid and humic acids, acids associated with decaying organic material. These acids enhance the decay and solubility of some substances. Thus biological activity can assist chemical weathering as well.

How do burrowing animals help?

Earthworms, termites and rodents burrow through earth materials. Their movements expose fresh surfaces and assist the entry of moisture and air. Burrowing therefore changes the contact between rock or soil particles and the substances that can weather them.

How do human activities contribute?

People disturb vegetation, plough land and cultivate soils. These actions mix earth materials and create new contacts between minerals, water and air. The contribution is not limited to breaking a rock directly: exposing or mixing material can also assist subsequent weathering.

In an explanation, connect each organism or activity to its effect. Roots apply pressure; burrowing exposes and loosens material; decay supplies acids; cultivation mixes material. These links explain the mechanism more fully than a list of plants, animals and humans alone.

How do denudation agents erode, transport and deposit material?

A geomorphic agent is a moving natural medium that removes, transports and deposits earth material. Geomorphic means related to the shaping of the Earth's surface. Running water, groundwater, glaciers, wind, waves and currents act as such agents.

Groundwater is water beneath the ground. A glacier is a moving mass of ice. Waves and currents are movements of water. Their detailed landforms differ, but each can participate in changing the surface by removing or redistributing material.

How are a process and an agent different?

Abrasion is the wearing of a surface by rock fragments carried against it. The moving agent supplies motion, while the transported fragments help scrape or grind exposed material. This assists erosion by rivers and wind.

1. Rock fragments become available through weathering or other processes.
2. A moving agent acquires material and removes it from its earlier position.
3. The agent transports the material; carried fragments can also abrade surfaces.
4. When the agent loses speed and energy, material begins to settle as deposition.

This sequence connects the processes without making weathering essential before all erosion. Likewise, gravity-driven mass movement belongs to denudation but differs from erosion by a transporting medium. In mass movement, the debris itself moves down the slope under gravity.

The same agent can contribute to erosion in one setting and deposition in another. Therefore, naming a river or wind is only the beginning of an explanation. State whether material is being removed, carried or laid down at the place being discussed.

How does river work change through the upper, middle and lower course?

A river's course is its path from its source, where it begins, to its mouth, where it enters another water body. The upper, middle and lower course describe successive parts of that path. The gradient is the steepness of its channel.

Steep gradients favour strong downward erosion. As channel slopes become gentler, downward cutting becomes less dominant and sideways erosion increases. Lateral erosion means erosion of the channel sides or banks; vertical erosion means downward cutting into the bed.

Which features are associated with each part?

A V-shaped valley has sides converging towards a narrow valley floor. A waterfall is a fall of river water over a steep or abrupt drop. A meander is a loop-like bend in a river channel.

A floodplain is a plain built by river deposits in and beside its channel. A delta is an accumulation of river deposits at the mouth where the material is retained rather than dispersed away. These features are explained further below.

How should youthful, mature and old stages be understood?

Youthful landscapes show dominant downward cutting and narrow valleys. Mature landscapes have better-developed river networks and wider floodplains. In old landscapes, rivers meander across extensive floodplains and most of the land is at or slightly above sea level.

These stages describe landscape development; upper, middle and lower course describe locations along a river. They are useful associations, not interchangeable definitions. Meanders may exist in youthful landscapes, and deposition can occur on steep slopes, though on a smaller scale than along gentler channels.

Note: Avoid assigning each process exclusively to one course. Erosion and deposition can occur within the same river system, and local rock structure and slope affect the resulting features.

How do V-shaped valleys and waterfalls develop?

River valleys develop as flowing water concentrates and cuts into the land. Small, narrow channels called rills can grow into larger channels called gullies. Continued deepening, widening and lengthening help produce valleys.

How does a V-shaped valley form?

On steep gradients, erosion is normally concentrated on the channel bed. Downward cutting deepens the valley, while weathering and the movement of material down its sides contribute to the sloping valley walls. The cross-section becomes V-shaped around a relatively narrow floor.

The characteristic V shape is seen across the valley, not by tracing the river's course from above. Valley shape also depends on the type and structure of the rock. Downward erosion alone does not make every valley identical.

Draw a cross-section with two slopes descending towards a narrow floor. Label the valley sides and the river at the bottom. Add a downward arrow at the river bed to indicate vertical erosion.

How can unequal rock resistance produce a waterfall?

A common waterfall setting has resistant rock overlying less resistant rock. Differential erosion, meaning erosion at unequal rates, allows the weaker rock to wear away faster. A steep drop is maintained where the more resistant layer forms a ledge.

1. River water flows across a resistant layer above a weaker layer.
2. The weaker rock wears away more rapidly, creating or enlarging the drop.
3. Falling water and rotating rock fragments deepen a hollow below the fall.
4. Continued undercutting can leave the upper ledge unsupported, so it collapses and the waterfall retreats upstream.

A plunge pool is the large, deep hollow at the foot of a waterfall. The impact of falling water and rotation of boulders help excavate it. The pool is an erosional feature, even though loose fragments may lie within it.

Waterfalls may exist where local hard rock bodies are exposed. Their presence depends on conditions along the channel, so a waterfall description should explain the actual drop and rock arrangement rather than assuming every upper course contains one.

How do meanders grow through erosion and deposition?

Meanders are loop-like channel patterns commonly found on floodplains and delta plains with very gentle gradients. A meander describes the shape of a river's channel. It does not mean that the whole valley has a winding cross-section.

What happens at the two banks?

With a very low gradient, flowing water works laterally on its banks. Slight bank irregularities can develop into curves. Deposition on the inside of a curve and erosion on its outside deepen that curvature and help the bend grow.

The convex bank is the inner bank projecting into the bend. The concave bank is the outer, inward-curving bank around it. Normally, deposition occurs along the convex bank and undercutting along the concave bank.

The undercut outer bank presents a steep face, while the inner bank has a long, gentle profile. Sediment accumulated along the inside is called a point bar, or meander bar. Sediment means material such as sand, silt or clay transported and deposited by an agent.

What can happen as loops grow?

As a meander develops into a deep loop, erosion may cut it off from the main channel. The abandoned, water-filled loop is an oxbow lake. This is a possible development of a meander, not a necessary outcome for every bend.

The slip-off bank is the gently sloping inner bank.

The drawing shows a winding channel, point bars and detached oxbow lakes. Labels identify the slip-off bank as convex and the cut-off bank as concave. Arrows indicate the direction of flow.

See Fig. 6.7 in your NCERT textbook

Sketch the winding river channel in plan view, with the nearby detached loops. Label the Burhi Gandak river, meanders, oxbow lakes and cut-offs. Use a simple key to distinguish the main channel from the abandoned loops.

Reference: sketch map to draw; NCERT Class 11 Figure 6.6

The contrasting banks demonstrate how erosion and deposition can operate close together. A meander is therefore best explained through both actions rather than erosion alone.

How does deposition build a delta?

A delta develops where a river deposits material at its mouth and the material accumulates. River load means the material carried by the river. When this load reaches the sea, it may settle and spread into a low accumulation.

The condition matters: a delta grows if the deposited material is not carried far out to sea or distributed along the coast. The arrival of sediment alone does not guarantee delta formation. Enough material must remain at the mouth to build up the deposit.

What is the sequence of formation?

1. The river transports its load towards the mouth.
2. Material begins to settle as the river loses carrying energy.
3. The coarsest material settles first, while finer silt and clay are carried farther into the sea.
4. Retained deposits accumulate, and the delta builds out into the sea as its channels lengthen.

Distributaries are channels that branch away from the main river across its deposits. They differ from tributaries, which join a larger river. As a delta grows, its distributaries continue to increase in length.

Delta deposits are well sorted and show clear stratification, meaning arrangement in layers. Sorting means separation of sediment by grain size. These properties reflect the settling of different sizes of material as the water loses transporting energy.

Satellite image: Part of the Krishna river delta, Andhra Pradesh (NCERT Class 11 Figure 6.4). The satellite image shows part of the delta in Andhra Pradesh. Dark river channels branch across the land towards the coast. This is a satellite view, not a labelled formation diagram.

Sketch the coastline and the river channels branching across the delta towards the sea. Label the Krishna river, distributaries, delta and sea. Use a simple key for the channels and coastline.

Reference: sketch map to draw; NCERT Class 11 Figure 6.4

Explain a delta through deposition, retention and outward growth. Its connection with the lower course follows from the river-mouth location and the accumulation of transported material; it does not require every river mouth to have the same outline.

How does wind create deflation hollows?

Wind can lift, remove and transport loose material, particularly where dry surfaces provide fine particles. Its work includes deflation, abrasion and the impact of wind-blown grains. These actions help produce erosional features in desert landscapes.

Deflation is the lifting and removal of dust and smaller particles from the land surface. Wind can carry away weathered material lying above rock or loose particles from bare soil. Removal exposes or lowers the surface from which the particles came.

How does a hollow develop?

1. Loose weathered material or bare soil is exposed at the surface.
2. Persistent wind lifts and removes particles from that surface.
3. Repeated removal reduces the amount of loose material in the affected area.
4. A shallow depression may develop; this is a deflation hollow.

The word may matters: persistent removal can produce a hollow, but the process should not be described as producing an identical basin everywhere. A deflation hollow is an erosional feature because material is taken away rather than accumulated.

How does abrasion differ from deflation?

Deflation removes particles. Abrasion uses wind-borne sand and silt as tools that wear rock surfaces. The impact of grains against rock also contributes to wear. These mechanisms can work together but name different actions.

Desert erosion is not entirely the work of wind. Although rainfall is scarce, rain falls torrentially over a short period. Water flowing across the surface as sheet wash removes weathered material, and general mass erosion is accomplished mainly through sheet floods or sheet wash.

This distinction prevents a common mistake: a dry landscape can contain features made by running water as well as wind. Wind is particularly effective at moving fine material; its presence does not explain every desert landform.

How do sand dunes form, and how do they differ from deflation hollows?

Sand dunes are accumulations of sand deposited by wind. Dry, hot deserts provide favourable conditions, and obstacles that initiate accumulation are important. A source of sand and suitable wind conditions are needed to supply and redistribute the grains.

Wind transports grains by rolling, saltation, meaning repeated hopping or bouncing, and suspension, meaning carriage within the air. Different grain sizes move differently as wind speed changes. This helps make wind a good sorting agent.

Why is a dune a depositional feature?

When wind slows or begins to die down, grains begin to settle according to their size and the wind speed needed to keep them moving. Accumulation produces a dune. Deflation hollows, by comparison, develop through the removal of loose material.

What shapes can dunes take?

A barchan is a crescent-shaped dune whose wings point downwind, meaning in the direction towards which the wind blows. Barchans form with a constant, moderate wind direction over an almost uniform surface.

Transverse dunes lie across the wind direction. Longitudinal dunes appear as long ridges. Dune form reflects sand supply and wind conditions; where sand is plentiful, regularly shaped dunes quite often join and lose their individual characteristics.

A seif resembles a barchan with one wing, while a parabolic dune is a reversed barchan associated with partially vegetated sandy surfaces.

The drawing labels barchan, seif, parabolic, transverse and longitudinal forms. Arrows show wind direction. It contrasts curved dunes with transverse bands and elongated longitudinal forms.

See Fig. 6.14 in your NCERT textbook

Most desert dunes shift, while a few become stabilised, especially near human habitations.

Dunes are not confined to deserts. Wind also lifts sand from beaches and deposits it behind them. The basic distinction remains the same in either setting: dunes record accumulation, whereas deflation hollows record removal.

Glossary

• Weathering — Mechanical disintegration and chemical decomposition of rocks through the action of weather and climate.
• Denudation — Stripping of the land surface through weathering, mass movements, erosion and transportation.
• In situ — At the original site, with very little or no movement of the material.
• Block disintegration — Physical breaking of rock into blocks along existing cracks or joints.
• Granular disintegration — Physical breakdown of rock as its individual mineral grains loosen and separate.
• Exfoliation — Flaking away of more or less curved rock sheets, leaving smoother rounded surfaces.
• Oxidation — Chemical reaction of oxygen with susceptible rock minerals, changing their composition.
• Carbonation — Chemical weathering involving carbonic acid formed when carbon dioxide dissolves in water.
• Hydration — Chemical combination of water with a mineral, which can change its volume.
• Solution — Dissolving of soluble mineral matter in water, allowing its removal in dissolved form.
• Plunge pool — A deep hollow excavated by falling water and rotating rock fragments below a waterfall.
• Meander — A loop-like river channel bend commonly developed across floodplains and delta plains.
• Delta — Accumulation of river deposits at its mouth where deposited material is retained.
• Deflation — Lifting and removal of dust and smaller loose particles by the wind.
• Sand dune — An accumulation of wind-deposited sand whose form reflects sand supply and wind conditions.

Common errors and misconceptions

• Misconception: Weathering and erosion are identical. Correct: Weathering involves very little or no movement; erosion removes and transports material through a moving agent.
• Misconception: Physical weathering creates different minerals. Correct: It breaks rock mechanically; chemical weathering changes or dissolves mineral matter.
• Misconception: Exfoliation must be caused by heating alone. Correct: Exfoliation is a result that can arise through temperature changes or pressure release.
• Misconception: Biological weathering means root action alone. Correct: Burrowing animals, acids from decay and human cultivation also contribute to weathering.
• Misconception: A meander grows through erosion alone. Correct: Normally its outer bank is undercut while deposition builds its inner bank.
• Misconception: Every river mouth must develop a delta. Correct: Sediment must accumulate rather than be carried far offshore or distributed along the coast.
• Misconception: Deflation hollows and dunes are both deposits. Correct: Deflation removes loose material; dunes accumulate sand deposited by wind.
• Misconception: Wind makes every desert feature. Correct: Running water during short, intense rainfall and sheet wash also remove weathered desert material.

Exam-style questions with model answers

Q1. A rock breaks into fragments at its original site with no transport. Later, a stream removes those fragments. Identify the process in each stage. [2 marks]

1. The first stage is weathering: the rock breaks into fragments at its original site without transportation.
2. The second stage is erosion: moving stream water removes and transports the fragments from their earlier position.

Q2. Rock breaks along joints into blocks, another rock sheds separate mineral grains, and a third loses curved sheets. Name and explain the physical-weathering result shown by each observation. [3 marks]

1. Breaking along joints into blocks shows block disintegration. The separated pieces follow larger weaknesses in the rock rather than individual grain boundaries.
2. Shedding separate mineral grains shows granular disintegration. The rock breaks down as bonds between its constituent grains weaken and the grains become detached.
3. Losing curved sheets shows exfoliation. Outer shells flake away from the rock, potentially leaving smoother, rounded surfaces beneath them.

Q3. Name the chemical-weathering process in each observation: oxygen reacts with iron-bearing minerals; water containing dissolved carbon dioxide attacks limestone; water combines chemically with a mineral; soluble rock salt dissolves in water. Explain each identification. [4 marks]

1. The oxygen reaction is oxidation, because oxygen chemically alters susceptible minerals in the rock. Iron-bearing minerals can form iron oxides.
2. The attack on limestone is carbonation, because carbon dioxide dissolved in water forms carbonic acid, which reacts with calcium carbonate.
3. The combination of water with a mineral is hydration, involving incorporation of water rather than mere surface wetting.
4. The dissolving of rock salt is solution, because soluble mineral matter enters the water and can be removed in dissolved form.

Q4. A river flows over resistant rock lying above weaker rock. Falling water and rotating boulders strike the base of the drop, while the weaker layer erodes faster. Explain waterfall development and retreat in five points. [5 marks]

1. The contrasting rock layers undergo differential erosion: the weaker layer wears away faster than the resistant rock above it, helping maintain an abrupt drop.
2. The resistant upper layer forms a ledge over which the river falls. It remains projecting as erosion removes more of the weaker material.
3. Falling water and rotating boulders erode the base of the fall, producing a deep hollow called a plunge pool.
4. Continued erosion of the weaker layer undercuts the resistant ledge. Removal of support can cause the projecting upper rock to collapse.
5. Repeated undercutting and collapse move the position of the waterfall upstream. This backward movement of the fall is called waterfall retreat.

Q5. A river crosses a plain with a very gentle gradient. Its bends show erosion on the outside and deposition on the inside. Identify the channel pattern, name both banks, and explain how the bends grow. [4 marks]

1. The loop-like pattern consists of meanders, commonly associated with rivers crossing floodplains and delta plains where channel gradients are very gentle.
2. The outer bank is the concave bank. The stated erosion undercuts this bank and produces a relatively steep face along the bend.
3. The inner bank is the convex bank. The stated deposition builds sediment there, producing a gentler profile and a point bar.
4. Continued outer-bank erosion and inner-bank deposition deepen the curvature. Together, these actions explain growth of the bends rather than erosion acting alone.

Q6. A river carries coarse and fine sediment into the sea. It loses carrying energy at its mouth, and the deposited material is neither carried far offshore nor distributed along the coast. Explain delta formation in four points. [4 marks]

1. As the river loses carrying energy at its mouth, transported sediment starts to settle and accumulate in the river-mouth area.
2. The coarsest material settles first, while finer silt and clay are carried farther into the sea before settling.
3. The stated retention of sediment allows the deposits to build up instead of being dispersed away, producing a delta.
4. As the delta grows outward into the sea, its branching channels, called distributaries, continue to increase in length across the deposit.

Q7. At one bare site, persistent wind removes loose fine material and leaves a shallow depression. Nearby, wind carrying sand slows beside an obstacle and sand accumulates. Explain the processes, identify both features, and distinguish them in five points. [5 marks]

1. At the bare site, the lifting and removal of loose fine material is deflation. Wind takes particles away from their earlier position on the ground.
2. The shallow depression produced by this removal is a deflation hollow. It is an erosional feature because material has been removed from the site.
3. At the nearby obstacle, the wind loses speed. Its reduced transporting ability allows carried sand grains to settle out of the moving air.
4. The accumulation of deposited sand forms a sand dune. The obstacle helps initiate accumulation, while the moving wind supplies the sand.
5. The features record opposite local changes: the hollow reflects removal and lowering of loose material, whereas the dune reflects deposition and accumulation of sand.

Q8. Roots press against rock cracks, burrowing animals expose fresh material to moisture and air, and cultivation mixes soil minerals with water and air. Explain how each observation contributes to biological weathering. [3 marks]

1. Growing roots exert pressure on the sides of cracks, mechanically breaking earth materials apart. Their growth therefore contributes directly to physical breakdown.
2. Burrowing animals expose fresh surfaces and allow moisture and air to penetrate. This creates additional opportunities for chemical attack on minerals.
3. Cultivation mixes earth materials and creates new contacts between minerals, water and air. Human activity therefore assists weathering by changing the conditions of exposure.

Key takeaways

• Weathering breaks or decomposes rock in situ, with very little or no movement of the affected material.
• Denudation includes weathering, mass movements, erosion and transportation, linking rock breakdown with the wearing of the land.
• Block disintegration releases blocks, granular disintegration separates mineral grains, and exfoliation removes more or less curved sheets.
• Oxidation, carbonation, hydration and solution describe different chemical actions, although water is involved in several of them.
• Plants, animals and humans assist weathering through pressure, exposure, mixing and chemical changes associated with organic decay.
• River gradients influence the balance between downward erosion, lateral erosion and deposition; processes are not exclusive to particular courses.
• Meanders normally combine outer-bank erosion with inner-bank deposition, while deltas require river-mouth sediment to accumulate.
• Wind removes particles to create deflation hollows and deposits sand to build dunes; running water also shapes deserts.

Test yourself

What does in situ mean when describing weathering?

It means at the original site, with very little or no movement of material. Weathering can break down rock without a transporting agent carrying it away.

How can you distinguish block from granular disintegration?

Block disintegration separates rock into blocks along larger cracks or joints. Granular disintegration loosens and separates the individual mineral grains composing the rock.

Why does the presence of water not identify a single chemical-weathering process?

Water can combine chemically with minerals in hydration, dissolve soluble matter in solution, or contain carbon dioxide that forms carbonic acid in carbonation.

How can biological activity assist both physical and chemical weathering?

Roots can press rock apart mechanically. Burrowing exposes fresh surfaces, while decaying plant and animal material supplies acids that assist chemical alteration.

Why does a plunge pool form below a waterfall?

The impact of falling water and the rotation of boulders erode the base of the fall, enlarging a deep hollow called a plunge pool.

Which banks normally undergo erosion and deposition in a meander?

Erosion normally undercuts the outer, concave bank. Deposition builds sediment along the inner, convex bank, which has a gentler profile.

Why is sediment supply alone insufficient to guarantee a delta?

Deposited sediment must remain and accumulate at the river mouth. If it is carried far offshore or distributed along the coast, a delta need not build up.

What separates a deflation hollow from a sand dune?

A deflation hollow develops through removal of loose material by wind. A sand dune forms through accumulation of sand deposited when transporting conditions weaken.