Many landforms are formed due to the internal movements. Many processes occurring on the earth’s surface also lead to the formation or degradation of landforms continuously. In this lesson, we will be studying the exogenetic (external) processes and the landforms formed by them. External processes occur because of the forces working on the earth’s surface. They are mainly solar energy, gravitational force and kinetic energy associated with the moving objects on the earth’s surface
See the given pictures. Observe the physical appearance of the rocks in each picture. You can see that rocks are broken, fractured and have holes in them. In a picture you can also see that the statue has been deformed. Why are the rocks in such a condition? Think about them and briefly tell the reasons you can think of. Discuss the reasons. Check with the teachers if your reasons are relevant.
Breaking or weakening of rocks is a natural phenomenon. It is called weathering. Weathering can be of three types: mechanical (physical), chemical and biological. In arid climates, mechanical weathering is dominant while in humid climates, chemical weathering is more effective. Biological weathering occurs because of living organisms.
You will notice that just as we can remove each and every outer layer of the onion, similarly, in nature rocks undergo such a process. The exposed part of the rock heats more while the inner part is 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:
Temperature Frost Crystal growth
Release of pressure Water
Water plays an important role in
chemical weathering. Rock is a mixture
of many minerals. Water is a universal
solvent. Many things get dissolved easily
in water. 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. In
areas that receive heavy rainfall, chemical
weathering occurs in the following way.
Carbonation :
The rainwater travels through the atmosphere before reaching the ground. Carbon
dioxide in the air gets mixed in the water
in this process. Dilute carbonic acid gets
formed. Materials like limestone get easily dissolved in such acids. e.g Water +
carbon Dioxide = Carbonic Acid
Oxidation : This process occurs in rocks which have iron present in them. The iron in the rock comes in contact with water and chemical reaction takes place between iron and oxygen. Hence, a reddish coloured layer forms on the rocks. This is called rust. When you kept nails in the wet cloth, you must have noticed a similar thing. Similar process occurs in rocks in areas with high rainfall. Fig. 3.1 (D) There are many more processes involved in chemical weathering besides the ones mentioned above. Daily-life examples include moistening of table salt in rainy season, apple slices turning brown, etc.
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. This process has been happening for years. The weathered particles form a conical heap at the foothills. When weathered particles move down due to gravity alone, the process is called mass movements. Mass movements occur in two ways: it is rapid on steeper slopes while it is slower on gentle slopes.
Rapid mass movements : Rockfalls, landslides, land subsidence occur rapidly. Their effects are very destructive. The probability of these events is more in the regions having humid climates and steeper slopes. A thick layer of weathered material forms on the slope. When it rains in such areas, the rainwater penetrates the weathered materials and their weight increases. The weathered materials move very rapidly and come down the slope e.g., the mudslide at Malin Village of Pune district. Sometimes the weathered materials do not move downward but sink ‘in situ’ (where they are). This is called slumping. See fig 3.10. Such rapid mass movements may also occur because of earthquakes
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. We will study the agents of erosion and the landforms produced by them in the next lesson.
