Denudational Processes/Weathering II

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Welcome to class! 

In today’s class, we will be talking more about denudational processes/weathering. Enjoy the class!

Denudational Processes/Weathering II

Types/processes of weathering

There are two major types or processes of weathering. These are physical (mechanical) weathering and chemical weathering.

Physical or mechanical weathering: This process promotes the physical disintegration of rocks. Agents of physical weathering include:

1. Temperature Changes: This is common in arid regions like the hot desert where rocks are exposed to the blazing sun during the day. The outer layers of the heated rocks expand much faster than the cooler interior of the rocks. At night, the temperature drops and the outer layers of rocks cool more rapidly than the interior. This alternate expansion and contraction of the rock later result in the peeling off of the rock surface. The process involved in the peeling off of the rock surface is called exfoliation. The pieces of rock that peel off are called screes or tallus which are deposited at the base of the rock. The remaining parent material is smoothened into a dome-shaped structure called exfoliation dome.
2. Alternate Wetting and Drying: This occurs in tropical areas of the world where rocks absorb water, resulting in swelling. When the rocks dry out, the outer surface of the rocks shrinks. The alternate wetting and drying weaken the rocks and they begin to crack.
3. Frost action: This occurs in polar and temperate regions of the world where rocks on high mountains with cracks or joint collect water or snow in them. When the temperature drops during the night or in winter, the water in cracks freezes and then melts during the day or summer. Such repeated freezing and melting widen and deepen the cracks will eventually breakdown the rock. The pieces which peel off from rock surface accumulate at the base of the mountain to form screes.
4. The action of Plants and Animals: This is often referred to as biological weathering. As plants grow, their roots penetrate the rocks below usually along the joints and other areas of weakness, thereby breaking the rocks apart. Some animals like rodents, earthworm, etc. bore holes into rocks and cause them to break into pieces in addition to allowing other agents of chemical weathering to take place. The activities of man in the course of mining, road construction and farming also contribute to the breaking down of rocks mechanically.

Chemical weathering: Chemical weathering is the gradual disintegration of rocks due to their exposure to air and water. It is common in hot-wet climate or humid regions with high temperature. The processes or agents involved in chemical weathering include:

1. Solution: Rainwater attacks dissolve rock salts to form solution e.g. rainwater dissolves calcium carbonate of limestone, thereby, widening the cracks and joints of the rocks.
2. Oxidation: Oxidation is the reaction of oxygen in air or water with minerals in the rocks. Oxidation takes place actively in rocks which contain iron. When oxygen combines with iron, it forms iron oxides.
3. Carbonation: Atmospheric carbon dioxide mixes with rainwater to form a weak carbonic acid. This acid dissolves rocks, resulting in their breakdown.
4. Hydration: This occurs when rock mineral absorbs water, resulting in the chemical alteration of the minerals, e.g. the conversion of iron in rocks to hydrated rocks.
5. Hydrolysis: This is the reaction of water with rock minerals to produce a rock entirely different from the original one.

The purpose of weathering is to form the soil. The remains of the rock i.e. weathered materials which eventually form the soil are called regolith.

Evaluation

1. Explain mechanical weathering.
2. What is talus?

General evaluation

1. Define weathering.
2. Mention the processes of weathering.
3. Explain physical weathering.
4. What is exfoliation?
5. Explain biological weathering.

Weekend assignment

1. An extensive area of level or gentle undulating land which is usually a few meters above the sea level is known as (a) Plain (b) Mountain (c) Valley (d) Plateau
2. Which of these plain is associated with glacier (a) Deltaic plain (b) Flood plain (c) Outwash plains (d) Lacustrine plains
3. Plains formed by agents of denudation are known as (a) Dissected plain (b) Residual plain (c) Erosional plains (d) Structural plain
4. Which of the following is not a depositional plain (a) Outwash plain (b) Pedi plain (c) Deltaic plain (d) Flood plain
5. Great plains of USA is a good example of (a) Volcanic plains (b) Structural plains (c) Erosional Plains (d) Depositional plains

Theory

1. Explain briefly the difference between chemical weathering and physical weathering.
2. State three effects of weathering.

 

In our next class, we will be talking about Mass Movement.  We hope you enjoyed the class.

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