Weathering and Soil
ICSE Class 7 · Geography · Chapter 8
What this chapter covers
- What weathering means
- Factors affecting weathering
- Mechanical weathering
- Chemical weathering
- Biological weathering
- Soil formation as a result of weathering
- The soil profile
- Why soil must be conserved
- Methods of soil conservation
Key definitions
Weathering is the breaking down and decay of rocks at or near the Earth's surface by the action of the weather, water and living things, while the broken pieces stay in the same place.
Erosion is the wearing away of the land and the removal of loose rock material by moving agents such as running water, wind, glaciers and sea waves.
Mechanical weathering is the breaking of rocks into smaller pieces by physical forces, without any change in the minerals that make up the rock.
Exfoliation is the peeling off of thin, curved sheets of rock from the surface of a rock mass because of repeated heating and cooling.
Frost action is the breaking of rocks by the repeated freezing and melting of water in their cracks.
Chemical weathering is the decay of rocks by chemical changes in their minerals, caused mainly by water, oxygen and carbon dioxide.
Solution is the process in which minerals in a rock are dissolved by water and carried away in it.
Oxidation is the process in which minerals in a rock combine with oxygen, usually with water present, forming new, weaker substances.
Hydration is the process in which minerals in a rock absorb water and swell, which weakens the rock.
Biological weathering is the breaking down of rocks by the actions of living things such as plants, animals, micro-organisms and human beings.
Soil is the thin layer of loose material on the Earth's surface, made of weathered rock particles, humus, water and air, in which plants can grow.
Humus is the dark, decayed remains of plants and animals in the soil, which makes the soil fertile and helps it hold water.
Key points
- Weathering is the breaking down and decay of rocks at or near the surface, with the pieces left in place; erosion carries the pieces away.
- The rate of weathering depends on climate, the type and structure of the rock, the slope, living things and time.
- Mechanical weathering breaks rocks without changing their minerals; it works through heating and cooling (exfoliation), frost action, pressure release and salt crystals.
- Chemical weathering changes or dissolves minerals through solution, oxidation and hydration; it is strongest in hot, wet regions.
- Biological weathering is caused by plant roots, burrowing animals, micro-organisms and human activities.
- Soil forms when weathered rock is mixed with humus, water and air through the work of living things over a very long time.
- The parent rock decides much of a soil's colour, texture and minerals.
- A soil profile has four horizons: A (topsoil), B (subsoil), C (weathered parent rock) and R (bedrock).
- Soil must be conserved because it forms slowly, feeds all life on land and can be lost quickly by erosion.
- Methods of soil conservation include afforestation, terrace farming, contour ploughing, strip cropping, shelter belts, controlled grazing, check dams, crop rotation and mulching.