Weathering
ICSE Class 9 · Geography · Chapter 9
What this chapter covers
- Meaning of weathering
- Physical weathering
- Chemical weathering
- Biological weathering
- Effects of weathering
Key points
- Weathering is the breaking down and decay of rocks in their original place by temperature changes, water, air and living things; erosion removes the weathered material.
- The rate of weathering depends on climate, type and structure of rock, slope, vegetation and time.
- Physical weathering breaks rock without changing its chemical composition; its causes are temperature changes and frost action.
- Block disintegration breaks jointed rock into rectangular blocks; granular disintegration crumbles coarse-grained rock into grains; exfoliation peels off curved shells and forms rounded domes.
- Physical weathering is strongest in hot deserts with a large daily range of temperature and on cold mountains where water freezes and thaws.
- Chemical weathering changes the minerals of rock by oxidation, carbonation, hydration and solution, and is strongest in hot, wet regions.
- Oxidation forms red and brown iron oxides; carbonation dissolves limestone into caves and sinkholes; hydration makes minerals swell; solution dissolves soluble minerals such as rock salt.
- Biological weathering is caused by plants (roots and acids), animals (burrowing, mixing, trampling) and human beings (mining, construction, deforestation, acid rain).
- Weathering forms soil, prepares rock for erosion and denudation, creates landforms such as scree, tors, domes and caves, and produces clay, sand, laterite and bauxite.
- Weathering also damages buildings and monuments, causes landslides and weakens foundations.
Questions answered
Define weathering.
Weathering is the breaking down and decay of rocks in their original place, at or near the earth's surface, by changes of temperature and by the action of water, air, plants, animals and human beings.
State two differences between weathering and erosion.
In weathering the rock is broken or decayed but stays in place, while in erosion the rock material is worn away and carried off. Weathering is caused by temperature changes, water, air and living things, while erosion is done by moving agents such as rivers, wind, glaciers and waves.
Name the three types of weathering and state the main agent of each.
Physical weathering, caused mainly by temperature changes and frost; chemical weathering, caused by water, oxygen, carbon dioxide and acids; and biological weathering, caused by plants, animals and human beings.
What is block disintegration? Where is it most common?
Block disintegration is the breaking up of a rock along its joints and bedding planes into large rectangular blocks, caused by repeated expansion and contraction or by frost in its cracks. It is most common in hot deserts with a large daily range of temperature and on cold mountains with strong frost action.
Why does granite undergo granular disintegration but quartzite does not?
Granite is coarse-grained and made of different minerals (quartz, feldspar and mica) of different colours, which absorb heat and expand by different amounts, so the grains loosen and separate. Quartzite is made of a single mineral whose grains expand together, so no stress builds up between them.
Explain the formation of exfoliation domes.
In large masses of uniform rock such as granite, the outer layer is heated and cooled far more than the inside, and the release of pressure from overlying rock also opens cracks parallel to the surface. Thin curved shells peel away one after another. Corners and edges are attacked from more than one side and peel fastest, so the mass becomes a smooth, rounded dome.
How does frost action break rocks?
Water enters cracks and pores and freezes at night. Water expands by about 9 per cent on freezing, so the ice presses on the walls of the crack and widens it. When the ice melts, water seeps deeper; repeated freezing and thawing finally splits the rock.
Give the chemical equations for carbonation and name one landform it produces.
CO2 + H2O → H2CO3, and CaCO3 + H2CO3 → Ca(HCO3)2. The soluble calcium bicarbonate is washed away, forming limestone caves such as the Borra Caves, and sinkholes.
Past board exam questions
ICSE 2019 sample paper · 2 marks
Explain the following terms: (i) Exfoliation (ii) Oxidation
Show answer
(i) Exfoliation: a type of physical (mechanical) weathering in which the outer layers of rocks peel off in curved sheets like the layers of an onion, because of repeated heating by day and cooling by night (common in hot deserts); it produces rounded exfoliation domes.
(ii) Oxidation: a type of chemical weathering in which the oxygen in air or water combines with minerals in rocks, especially iron, forming oxides (rust); the rock becomes weak, reddish-brown and crumbles.