LESSON 20: WEATHERING
Denudation is the wearing away of the earth surface by various elements like weathering, erosion, mass movement.
Weathering is the decomposition, decay and the disintegration of rocks insitu when exposed to the elements of the weather.
There are three types of weathering, namely; physical, chemical and biological weathering.
- Physical weathering: it is the disintegration or breakdown of rocks into smaller particles which does not lead to a change in mineral composition of the rock.
- Chemical weathering: this is the decomposition and the decay of rocks exposed to weather resulting to a chemical change in the mineral composition of the rock. It occurs everywhere on earth. Especially in forest regions.
- Biological weathering: this refers to the breakdown of rocks due to the activities of plants and animals.µ
Factors influencing the rate of weathering
- Nature of the rock: if the rock is resistant, weathering will be slow. E.g. granite and vice versa.
- Relief: weathering will be effective in a gentle slope
- Vegetal cover: plants provide chemical and biological weathering.
- Physical weathering processes:
- Frost action: this is the breakdown of rocks by frozen water. It occurs in artic and cool regions and on top of high Mt of the tropics.
When water enters the rock’s cracks, it freezes and expands. When temperature rises, water thaws leading to contraction. These repeated contraction expansion leads to the physical breakdown of rocks.
- Repeated wetting and drying: this occurs in humid tropical regions where rain wet the rock and then sunshine quickly dries up.
The wetting does the rock to expand while the drying does the rock to contract, when this happens repeatedly, the rock squeeze up causing the screes and the talus.
- Chemical weathering process
- Oxidation: this occurs when oxygen in water reacts with iron and weakens the rock structure to form oxides.
- Hydration: here, minerals take up water and expand by swelling resulting to rot and decomposition;
- Hydrolysis: this occurs when oxygen in water reacts with rock minerals to produce different compounds.