Notes
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Topics to be Learn :
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Landforms formation :
- Many primary and secondary landforms are formed due to the internal movements. Examples: continents, mountains, plateaus, plains, etc.
- External processes occur because of the forces working on the earth’s surface.
- The external processes like weathering, erosion, transportation and deposition occur due to solar energy, gravitational force and kinetic energy associated with the moving objects on the earth’s surface.
- Because of these external processes, the primary and secondary landforms give way to the tertiary landforms. Examples : sand dunes, delta, U-shaped valleys, etc.
Weathering:
Breaking or weakening of rocks is a natural phenomenon. It is called weathering.
Mechanical (physical) weathering, chemical weathering and biological weathering are the three types of weathering.
- Mechanical weathering is dominant in arid climates.
- Chemical weathering is more effective in humid climates.
- Biological weathering occurs because of living organisms.
Mechanical weathering :
Breaking or weakening of rocks without any change in their chemical composition is called mechanical weathering
- The exposed part of the rock heats more while the inner part remains comparatively cooler.
- As a result, the outer layers of the rocks fall apart from the main rock. This is called exfoliation of the rock.
Mechanical weathering mainly occurs because of the following reasons : Click here to View Figure-1
Temperature : The minerals in the rocks expand because of heat and contract when temperature decreases. Due to such continuous contracting and expanding, tension develops in the rock particles. As a result, cracks develop in rocks and they break. In areas where the diurnal range of temperature is higher, weathering of this type is common. For example, it is very common in hot desert. Frost : In areas where the temperature drop below 0°C for quite some time, the water accumulated in the cracks and crevices in the rocks freezes. This leads to tension in the rocks and they shatter. Crystal growth : In the alkaline water of sea, the soluble materials in the rocks get dissolved. This leads to formation of small holes in the rocks. Because of the heat, only crystals of alkaline materials remain in the rocks. Gradually, holes are formed in the rocks and rocks look like a honeycomb. Release of pressure : The outer layer of the rocks exert pressure on the lower layers. When this pressure ceases to exist, the lower or inner layers are freed from the pressure. This also leads to weathering. Water: Some areas experience more rainfall than others. In such areas, soaking of rock water also causes granular weathering of some rocks like sandstone, conglomerate, etc. Cumulative effect of temperature and water : Sometimes both temperature and water are responsible for weathering.
Chemical weathering : The process of decomposition of rocks by changes in the chemical composition of it is called chemical weathering.
Water is a universal solvent. The solubility increases because the matter has dissolved in water. And those materials which do not dissolve easily in water get dissolved in such solutions. As its effect, chemical weathering occurs. In areas that receive heavy rainfall, chemical weathering occurs in the following way :
- Carbonation : Carbon dioxide in the air gets mixed in the rainwater leading to formation of dilute carbonic acid. Materials like limestone get easily dissolved in such acids. e.g. Water + carbon Dioxide = Carbonic Acid (H2O + CO2 = H2CO3 )
- Solution : Some minerals in the rock get dissolved in water. Limestone is formed due to chemical precipitation between water and alkalis. Similarly, because of solution, alkalis in the rock dissolve and make them brittle.
- Oxidation : The process of oxidation occurs in rocks which have iron present in them. Due to oxidation, rust is formed on the rocks.
Click here to View Figure-2
Biological weathering : Biological weathering occurs due to human beings, animals and plants.
- Trees grow in cracks and crevices of rocks of old forts or buildings. Roots of trees grow bigger, they create tension in the rocks and start breaking them.
- Rats, mice, rabbits, etc. make burrows in the ground. Ants make large anthills. Because of the activities of these burrowing animals, biological weathering of rocks occurs.
- Algae, moss, lichen and other flora grow in the rocks. They also help in biological weathering.
Click here to View Figure-3
| Know this :
Oxidation : Reaction of any element with oxygen is called oxidation. i Universal solvent : A solvent in which many soluble materials can get dissolved is called universal solvent. Water can dissolve number of materials. So water is called universal solvent. Moss : Moss are the flowerless plants that grow in the areas of humid climates and moist shaded locations. Moss can be seen on the trunks of trees. |
Mass movements (Mass wasting) :
- The weathered rock materials start moving along the slopes due to gravity and accumulate near the foothills or the gentler slopes
- Gradually, the weathered particles form a conical heap at the foothills.
- The process of moving down of weathered particles due to gravity alone, is called mass movement or mass wasting.
Mass movements occur in the following two ways :
Rapid mass movements : Slower mass movements :
Erosion :
- Like weathering and mass movements erosion is also an external process. Erosion occurs through various agents.
- Wind, running water, glaciers, sea Water and groundwater cause erosion.