Weathering and Mass Movements
This article deals with ‘Weathering and Mass Movements.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here
- Weathering is the action of elements of weather over earth materials to reduce them to fragmental state.
- Very little or no motion take place in them & process is in-situ.
- There are three major groups of weathering processes :
- Physical or mechanical
- Biological weathering processes.
1. Chemical Weathering
- When something is dissolved in water , it is called solution.
- Hydration is the chemical addition of water. Minerals take up water and expand
- Calcium sulphate takes in water and turns to gypsum, which is more unstable
c. Oxidation & Reduction
- Oxidation means a combination of a mineral with oxygen to form oxides or hydroxides. Eg : Oxidation of iron to form rust
- When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place. Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- Carbonation leads to dissolution of Carbon Dioxide into water to form Carbonic Acids which will dissolve calcium and magnesium compounds
2. Physical Weathering
Physical Weathering is the disintegration of rock mainly induced by elements of weather and natural forces.
Physical weathering can be further divided into following categories :-
- Gravitational forces such as overburden pressure, load and shearing stress.
- Expansion forces due to temperature changes => Rocks expand during day and contract during night in arid and semi-arid regions=> the rocks crack and eventually splits up.
- Exfoliation : Rocks generally heat or cool more on the surface layers. The alternate changes in temperature could cause their outer layers to peel off from the main mass of the rock in concentric layers just as the skin of an onion.
- Frost wedging : when water freezes, it expands. As water expands between the rock wedges expand, it puts great pressure on rocks resulting in weathering.
- Water pressures controlled by wetting and drying
3. Biological Weathering
Weathering due to growth or movement of organisms.
- Burrowing &wedging by organisms like earthworms, termites, rodents etc. => exposing new surfaces to chemical attack
- Human beings by disturbing vegetation, ploughing and cultivating soils=> this creates new contacts
- Plant roots exert great pressure breaking rocks apart.
Importance of Weathering
- Weathering is the initial stage in the formation of soil. It breaks down the initial rock mass into smaller fragments thus preparing the rock material for the formation of soil.
- Trees are able to ‘mine’ essential nutrients such as calcium through their association with symbiotic mycorrhizae through small pores in the mineral soil, which is possible only due to weathering.
- Erosion, with the aid of weathering, helps in mass wasting and reduction of relief. This leads to modifications in various landforms.
- It leads to the formation of various natural resources such as clay used in making bricks.
- Placer deposits are formed due to weathering . These placer deposits are source of rare earth metals, thorium etc
- It weakens the rocks, thus facilitating the mining and quarrying activities
Hence, we can say that although weathering is a disintegrating process yet it plays an integral role in sustaining life on earth.
- These movements transfer the mass of rock debris down the slopes under the direct influence of gravity ( happens only under influence of gravity & no other geomorphic agent is involved)
- Weathering is not a prerequisite for mass movement although it aids mass movements. Mass movements are very active over weathered slopes .
Mass Movement can be grouped under two classes
1 . Slow Movement
- Creep : Occur on moderately steep, soil covered slopes. Movement of materials is extremely slow and imperceptible except through extended observation.
- Solifluction : Slow downslope movement of soil mass saturated with water. Quite common in moist temperate areas
2. Rapid Movement
Mostly prevalent in humid climatic regions with gentle to steep slopes.
- Earthflow : Movement of water-saturated earth materials down hillsides. Arcuate scarps at heads & accumulation bulge at the toe are observed in this.
- Mudflow : Mudflow is a liquid mass of soil, rock debris and water that moves quickly down a well defined channel. Mudflow originating on a volcanic slope is called a lahar.
- Debris avalanche : characteristic of humid regions with steep slopes. These are rapidly churning mass of rock debris, soil, water, and air that moves down steep slopes. The trapped air may increase the speed of an avalanche by acting as a cushion between the debris and the underlying surface. They are much faster and deadlier than Earthflow & Mudflow
- Rock falls : Rock falls occur when pieces of rock break from a cliff. It may result due to Frost wedging . Accumulation of rock debris at the base of a steep slope is called talus.
- Landslides : Landslides occur when a large piece of rock breaks off and slides down hill. It can be initiated by heavy rainfall or earthquake.
- Slump : Great mass of bed rock moves downward by rotational slip from a high cliff
Question – Why more Landslides & Debris Avalanches occur in Himalayas compared to Western Ghats?
- There are many reasons for
- One, the Himalayas are tectonically active.
- They are mostly made up of unconsolidated and semi-consolidated deposits.
- The slopes are very steep.
Question : Compared to the Himalayas, the Nilgiris bordering Tamilnadu, Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks; but, still, debris avalanches and landslides occur although not as frequently as in the Himalayas, in these hills. Why?
- Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiris.
- Mechanical weathering due to temperature changes and ranges is pronounced.
- They receive heavy amounts of rainfall over short periods. So, there is almost direct rock fall quite frequently in these places along with landslides and debris avalanches.