External Processes and Weathering
Walls of an old fort, statues or stone steps, left for years in sun and rain, get loose grains and worn corners. Nature does the same to hills, only more slowly.
Processes caused by forces on the earth’s surface are exogenetic (external) processes. The breaking or weakening of rocks is called weathering.
Key idea 1 : External forces and landforms — External processes occur mainly due to solar energy, gravity and the kinetic energy of moving objects on the surface. Continents, mountains and plateaus made by internal movements are primary and secondary landforms. Weathering, erosion, transport and deposition turn them into tertiary landforms like valleys, dunes, deltas and U-shaped valleys.
Key idea 2 : Three types of weathering — Weathering is of three types: mechanical (physical), chemical and biological. In mechanical weathering the rock only breaks, in chemical its nature changes and in biological living things are responsible.
Key idea 3 : Effect of climate — In dry climates mechanical weathering is more effective because day and night temperatures differ greatly. In humid climates there is more water, so chemical weathering is stronger.
- Easy : Rock only breaks without chemical change; type? — Mechanical.
- Medium : Which weathering is more in a desert? — Mechanical.
- Exam-type : Valleys and deltas are which landforms? — Tertiary.
In the figure: Three types of weathering.
Mechanical = breaking; chemical = changing; biological = living.
Weathering and erosion are not the same; in weathering the rock breaks in place, erosion is done by agents.
Memory clue: Mechanical-Chemical-Biological = break, change, live. Memory clue: "Break, change, live".
Spot the difference
| Climate | Stronger weathering | Reason |
|---|---|---|
| Dry | Mechanical | Big temperature range |
| Humid | Chemical | More water |
Practice questions
- 1What are exogenetic processes?
- 2Which are the main forces behind external processes?
- 3What is weathering?
- 4State the types of weathering.
- 5Why is mechanical weathering more in dry climates?
- 6Give four examples of tertiary landforms.
- 7State the difference between weathering and erosion.
- 8Find one example of weathering in your area.
Show answers (try them yourself first)
- 1Processes caused by forces on the surface.
- 2Solar energy, gravity, kinetic energy.
- 3Breaking or weakening of rocks.
- 4Mechanical, chemical, biological.
- 5Because day-night temperature difference is large.
- 6Valleys, dunes, deltas, U-shaped valleys.
- 7Weathering in place; erosion by agents.
- 8E.g. cracks on an old wall.
Mechanical Weathering
If an onion is cut in half its layers peel off one by one. If cold water is poured in a hot glass it cracks. Rocks too get such shocks of temperature.
Breaking of rock without any chemical change is mechanical weathering. Temperature, frost, crystal growth, release of pressure and water are the main causes.
Key idea 1 : Temperature and frost — In heat the minerals of a rock expand and in cold they contract. Each mineral expands differently, so tension grows, cracks form and the outer layers peel like onion skins; this is exfoliation. In cold regions water in cracks freezes below 0 °C, its volume increases and the rock bursts.
Key idea 2 : Crystal growth and release of pressure — On rocky coasts salty water drops enter cracks, the sun evaporates the water and salt crystals remain. Crystals take more space, tension builds and the rock gets honeycomb-like holes. Also when upper layers are removed the pressure on lower layers drops and they crack.
Key idea 3 : Water and block disintegration — Rocks like sandstone are grains joined by clay. When water enters, grains loosen and separate; this is granular weathering. When temperature change and water act together cracks widen and big blocks separate; this is block disintegration.
- Easy : In a desert with 40° by day and 10° at night what happens to rock? — It cracks and breaks.
- Medium : How does the volume of frozen water change? — It increases.
- Exam-type : Layers peeling like onion skin? — Exfoliation.
In the figure: How temperature breaks a rock.
Hot-cold → tension → cracks → break.
Frozen water does not contract; it expands, so the rock bursts.
Memory clue: Hot-cold = cracks; frozen = swells. Memory clue: "Ice swells, rock bursts".
Spot the difference
| Cause | Action | Result |
|---|---|---|
| Temperature | Expansion-contraction | Exfoliation |
| Frost | Water volume rises | Rock bursts |
| Crystal growth | Salt crystals | Honeycomb holes |
Practice questions
- 1What is mechanical weathering?
- 2State the causes of mechanical weathering.
- 3How does exfoliation occur?
- 4Why does a rock burst due to freezing?
- 5How does crystal growth make holes in rocks?
- 6State the difference between granular weathering and block disintegration.
- 7Why do rocks break due to cold in the high Himalayas?
- 8In cold regions water pipes burst in winter; give the reason.
Show answers (try them yourself first)
- 1Breaking of rock without chemical change.
- 2Temperature, frost, crystal growth, release of pressure, water.
- 3Tension from heat-cold makes outer layers peel like an onion.
- 4On freezing water’s volume increases and puts tension.
- 5Salt crystals take more space.
- 6Granular = grains separate; block = big blocks separate.
- 7Water in cracks freezes and expands.
- 8Frozen water expands and strains the pipe.
Chemical and Biological Weathering
An iron nail kept in a wet cloth gets reddish rust in two days. In the rainy season salt turns damp and a cut apple turns brown. Similar chemical changes occur in rocks too.
Weakening of rock by chemical reaction with water and gases of the air is chemical weathering. Breaking of rock by plants and animals is biological weathering.
Key idea 1 : Carbonation and solution — Water is a universal solvent. Rain water absorbs carbon dioxide from the air and becomes dilute carbonic acid (water + CO₂ = H₂CO₃). Rocks like limestone dissolve easily in it. Some minerals dissolve in water and make the rock brittle.
Key idea 2 : Oxidation — In rocks containing iron, water and oxygen react and a reddish layer, rust, forms. In heavy rainfall regions this goes on constantly and rocks become weak.
Key idea 3 : Biological weathering — Roots of trees growing on fort towers put tension on rock as they grow and make it crack. Ants, rats, rabbits and worms dig burrows; algae, moss and lichen grow on rocks and weaken them.
- Easy : Rusting of a nail is like which weathering? — Oxidation (chemical).
- Medium : Limestone dissolves in what? — Dilute carbonic acid.
- Exam-type : How does a tree growing in a wall break it? — By root pressure.
In the figure: Kinds of chemical and biological weathering.
At Wadgaon Darya in Ahmednagar district limestone columns (stalactites) are seen in a cave, formed by re-deposition of limestone dissolved in water. (As per the textbook)
Water + CO₂ = acid; iron + oxygen = rust.
In chemical weathering the rock does not just break; its nature and chemistry change.
Memory clue: Iron = rust; limestone = dissolves in acid. Memory clue: "Rust is iron’s friendship with oxygen".
Spot the difference
| Type | Action | Result |
|---|---|---|
| Carbonation | Water + CO₂ → acid | Limestone dissolves |
| Oxidation | Iron + oxygen | Rust |
| Biological | Roots, burrows, lichen | Rock breaks |
Practice questions
- 1What is chemical weathering?
- 2Which are the main types of chemical weathering?
- 3How is carbonic acid formed?
- 4What is seen on a rock in oxidation?
- 5How does biological weathering occur?
- 6Why is chemical weathering more in humid regions?
- 7State the difference between mechanical and chemical weathering.
- 8Give one example of biological weathering near your home.
Show answers (try them yourself first)
- 1Rock weakening through chemical reaction with water and gases.
- 2Carbonation, solution, oxidation.
- 3Rain water absorbing CO₂ from air.
- 4A reddish layer (rust).
- 5By roots, burrowing animals and moss.
- 6Because there is more water.
- 7Mechanical = only breaking; chemical = chemical change.
- 8E.g. a peepal sapling on a wall.
Mass Movements
In the rainy season we see stones and soil fallen on ghat roads. The steeper the slope, the greater the chance of soil coming down.
The sliding of weathered rock material down a slope only due to gravity is mass movement. It is rapid on steep slopes and slow on gentle slopes.
Key idea 1 : Rapid movements — Rockfall, landslide and land subsidence are rapid types and are very destructive. With humid climate and steep slopes a thick layer of weathered material gathers; with rain it becomes heavy with water and comes down fast. Sometimes it sinks in place; this is slumping. Earthquakes too can cause it.
Key idea 2 : Slow movements — In dry climates and on gentle slopes movement is slow. Soil creep is a common example. In periglacial regions thin layers of soil slowly slide down slopes; this is solifluction.
Key idea 3 : Erosion and the next lesson — Like weathering and mass movement, erosion is an external process. Wind, running water, glaciers, sea water and groundwater are the agents. Their work and the landforms they make come in the next lesson.
- Easy : Steep slope and heavy rain; movement? — Rapid.
- Medium : Gentle slope and dry climate? — Slow.
- Exam-type : Soil sinking in place? — Slumping.
In the figure: Rapid and slow movement.
On 30 July 2014 heavy rain brought down part of a hill on Malin village in Ambegaon taluka, Pune district. About 150 people died. (Mentioned in the textbook; check the number with the book)
Steep + rain = rapid; gentle + dry = slow.
Landslides are not due to rain alone; earthquakes can cause them too. The main force is gravity.
Memory clue: Steep slope + water = danger. Memory clue: "Steep slope, wet soil, run".
Spot the difference
| Type | Speed | Example |
|---|---|---|
| Rapid | Very fast, destructive | Rockfall, landslide |
| Slow | Slow and continuous | Soil creep |
Practice questions
- 1What is mass movement?
- 2Where is mass movement rapid?
- 3State the types of rapid mass movement.
- 4State two types of slow mass movement.
- 5Why do rockfalls increase in the rainy season?
- 6Which are the agents of erosion?
- 7State the difference between weathering and mass movement.
- 8Suggest two ways to protect hill villages from landslides.
Show answers (try them yourself first)
- 1Weathered material moving down a slope due to gravity.
- 2On steep slopes in humid climate.
- 3Rockfall, landslide, subsidence, slumping.
- 4Soil creep and solifluction.
- 5Rain makes the soil on slopes heavy and loose.
- 6Wind, running water, glacier, sea, groundwater.
- 7Weathering = rock breaking; mass movement = pieces sliding down.
- 8Plant trees on slopes and avoid construction.