Weathering is defined as the gradual breaking down or disintegration of rocks by either physical (mechanical) or chemical process.
FACTORS AFFECTING WEATHERING
The following factors affect the weathering of rocks:
- Climate: Climatic elements like temperature and rainfall are the major factors affecting weathering. While physical (mechanical) weathering is aided by temperature and water to break down rocks, chemical weathering is aided by water and other gases in the atmosphere. Generally, chemical weathering is common in hot and wet climate (i.e. humid regions with high temperature) while mechanical weathering is common in arid and cool temperate climates.
- Types of rocks: Rocks are made up of different minerals. While some are resistant to weathering, others are not simply because of their differences in structure, texture, composition and size of the rocks.
- Relief: Very steep slopes like high mountains encourage weathering, especially frost action while gentle and even slopes favour chemical weathering.
- Living organisms: Plants and animals have great influence on weathering. Roots of plants promote physical weathering as the roots penetrate into rocks and break them, and chemical weathering occurs due to vegetative cover which conserves water to produce weak acids in the soil. Animals bore holes into rocks and break them in addition to allowing other agents of chemical weathering to take place.
TYPES OR PROCESSES OF WEATHERING
There are two major types of 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:
- Temperature changes: This is common in arid regions like, hot deserts where rocks are exposed to 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 rock later results in the peeling off of the rock surface. The process involved in the peeling off of the rock surface is called The pieces of rocks that peel off are called screes or tallus which are deposited at the base of the rock. The remaining parent rock is smoothened into a dome-shaped structure called exfoliation.
- 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.
- Frost action: This occurs in polar and temperate regions of the world where rocks on high mountains with cracks or joints collect water or snow in them. When the temperature drops during the night or winter, the water in the cracks freezes and melts during the day or summer. Such repeated freezing and melting widen and deepen the cracks which eventually breakdown the rock. The pieces of rocks which peel off from the rock surface accumulate at the base of the mountain to form screes.
- 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 roots 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 climates or humid regions with high temperature. The processes or agents involved in chemical weathering include:
- Solution: Rain water attacks and dissolves rock salts to form solution e.g. rain water dissolves calcium carbonate of limestone, thereby, widening the cracks and joints of the rocks.
- Oxidation: Oxidation is the reaction of oxygen in air or water with minerals in the rock. Oxidation takes place actively in rocks which contain iron. When oxygen combines with iron, it forms iron oxides.
- Carbonation: Atmospheric carbon dioxide mixes with rain water to form a weak carbonic acid. This dissolves rocks, resulting in their breakdown.
- Hydration: This occurs when rock minerals absorb water, resulting in the chemical alteration of the minerals, e.g. the conversion of iron (ii) rocks to (iii) hydrated rocks i.e. Haematite rock is changed to liminite and absorption of water by calcium sulphate to give gypsum.
- Hydrolysis: This is the reaction of water with rock minerals to produce a rock entirely different from the original one e.g. olivine rock is changed to serpentine. 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.