Denudation
ICSE Class 9 · Geography · Chapter 10
What this chapter covers
- Meaning and agents of denudation
- Work of river
- Stages of a river course
- Landforms associated with a river course
- Work of wind
Key definitions
Hydraulic action is the wearing away of the banks and bed by the sheer force of moving water, which pushes into cracks, compresses the air inside and loosens pieces of rock.
Abrasion is the wearing away of the bed and banks by the sand, pebbles and boulders that the river hurls and rolls against them, much as sandpaper wears away wood.
Attrition is the wearing down of the load itself as the pebbles and boulders knock against one another and against the bed.
Solution is the dissolving of soluble rock minerals, especially limestone, in the slightly acidic river water.
Deflation is the lifting and blowing away of loose, dry, fine particles of sand and dust from the ground by the wind.
Key points
- Denudation is the wearing down and lowering of the land by weathering, mass movement and erosion, with the removal of the loosened material; it is the work of exogenic forces.
- The agents of denudation are running water, wind, glaciers, sea waves and underground water; each erodes, transports and deposits.
- The work of an agent depends mainly on its speed and volume: fast, full agents erode, slow ones deposit.
- A river erodes by hydraulic action, abrasion, attrition and solution, and transports its load by traction, saltation, suspension and solution.
- Vertical erosion deepens a valley; lateral erosion widens it.
- The upper course (youthful stage) is steep and fast and does vertical erosion; the middle course (mature stage) does lateral erosion and transportation; the lower course (old stage) is almost level and deposits.
- A V-shaped valley forms in the upper course where the river cuts down faster than the sides are worn back; a very deep and narrow one is a gorge.
- A waterfall forms where a river crosses hard rock over soft rock or the edge of a plateau; undercutting and collapse make it retreat upstream and leave a gorge.
- A meander is a loop-like bend with a steep river cliff on the outer bank and a gentle slip-off slope on the inner bank; a cut-off meander becomes an ox-bow lake.
- A delta is triangular land built of fine sediment at a river's mouth in a shallow, calm sea; the Ganga-Brahmaputra delta is the world's largest, while the Narmada and the Tapi form estuaries.
- Wind is the chief agent in deserts, where rain and plants are scarce and loose sand is plentiful; it erodes by deflation, abrasion and attrition.
- A deflation hollow is a depression scooped out by wind and stops deepening at coarse pebbles or moist ground; an oasis may form where water reaches the surface.
- A sand dune has a gentle windward slope and a steep leeward slope and moves downwind; barchans are crescent-shaped with horns pointing downwind, and seifs are long ridges parallel to the wind.
Questions answered
Define denudation. Name the three processes it includes.
Denudation is the general wearing down and lowering of the earth's surface by weathering, mass movement and erosion, together with the transportation of the loosened material to other places. It includes weathering, mass movement and erosion.
Distinguish between weathering and erosion.
Weathering breaks down and decays rock in its original place, without moving it. Erosion wears away rock material and removes it from one place by a moving agent such as running water, wind, a glacier or sea waves.
Name five agents of denudation. Which is the most important on land, and which is dominant in deserts?
Running water, wind, glaciers, sea waves and underground water. Running water is the most important over most of the land; wind is dominant in hot deserts.
Explain any three ways in which a river erodes its bed and banks.
By hydraulic action, the force of the moving water loosens rock from the bed and banks. By abrasion, the sand, pebbles and boulders carried by the river scrape and grind the bed and banks. By solution, the river water dissolves soluble rocks such as limestone. (Attrition, the wearing down of the load itself, may also be given.)
Name the four ways in which a river transports its load.
Traction (rolling of boulders), saltation (hopping of pebbles and coarse sand), suspension (fine silt and clay held in the water) and solution (dissolved minerals).
State two differences between the upper course and the lower course of a river.
In the upper course the slope is steep and the river flows fast, while in the lower course the slope is almost level and the river flows slowly. The main work of the upper course is vertical erosion, while the main work of the lower course is deposition.
Explain how a V-shaped valley is formed. Why does it form in the upper course?
The fast river cuts down into its bed by abrasion and hydraulic action, while weathering and mass movement wear back the valley sides and send the loose material into the river. Because downcutting is faster than the wearing back of the sides, the valley is narrow at the bottom and wide at the top. It forms in the upper course because the steep slope gives the river the speed needed for strong vertical erosion.
Describe the formation of a waterfall where a band of hard rock lies over soft rock. Why does the waterfall retreat upstream?
The river wears away the soft rock faster than the hard rock, so a step forms and the water falls over the edge of the hard rock. The falling water digs a plunge pool and undercuts the soft rock, leaving the hard rock overhanging. The overhang collapses from time to time, so the edge of the fall moves upstream and a gorge is left below it.
Past board exam questions
ICSE 2019 sample paper · 3 marks
Give a geographical reason for each of the following: (i) Volcanic activity and earthquakes occur in the same belt. (ii) All rivers do not form a delta. (iii) Chemical weathering is more common in the tropical regions.
Show answer
(i) Both occur along the weak zones of the Earth's crust - the margins of the plates where plates collide or move apart (e.g. the Circum-Pacific belt or Ring of Fire); movement of plates there causes earthquakes and lets magma escape, causing volcanic activity.
(ii) A delta forms only where a river brings a large load of sediment, the sea at the mouth is shallow and calm with weak tides and currents; where the tides or currents are strong they carry the sediments away, or the river carries little load, so no delta forms (an estuary forms instead).
(iii) Tropical regions have high temperature and heavy rainfall/high humidity; heat and moisture speed up chemical reactions such as oxidation, …
ICSE 2019 sample paper · 3 marks
Explain the meaning of the following terms: (i) Deflation hollow (ii) River meander (iii) Block disintegration.
Show answer
(i) Deflation hollow: a depression formed in deserts when wind removes (blows away) loose sand and dust particles from an area; e.g. the Qattara Depression in Egypt.
(ii) River meander: a large loop or bend formed by a river in its middle/lower course on gentle slopes, by erosion on the outer (concave) bank and deposition on the inner (convex) bank.
(iii) Block disintegration: physical weathering in which rocks break into rectangular blocks along their joints, owing to repeated expansion and contraction caused by great daily changes of temperature (common in hot deserts).