INTRODUCTION: The rock surface of the continents of the Earth, on which we are living, is undergoing constant and continuous destruction, a process called denudation. Denudation is the process by which the land areas are continually being reduced and their shape modified by weathering and erosion. Rocks exposed at Earth’s surface are constantly being altered by water, air, changing temperature and other environmental factors. The term weathering refers to the group of destructive processes that change the physical and chemical character of rock at or near Earth’s surface. The tightly bound crystals of any rock can be loosened and altered to new minerals by weathering. It is important to distinguish between weathering and erosion and between erosion and transportation. Weathering breaks down rocks that are either stationary or moving. Erosion is the picking up or physical removal of rock particles by an agent such as streams or glaciers. Weathering helps break down a solid rock into loose particles that are easily eroded. Most eroded rock particles are at least partially weathered, but rock can be eroded before it has weathered at all. A stream can erode weathered or un-weathered rock fragments. After a rock fragment is picked up (eroded), it is transported. Transportation is the movement of eroded particles by agents such as rivers, waves, glaciers, or wind. Weathering processes continue during transportation. A boulder being transported by a stream can be physically worn down and chemically altered as it is carried along by the water. Types of weathering: These are three types of weathering namely:- i) Mechanical weathering or disintegration. ii) Chemical weathering or decomposition iii) Biological weathering. Mechanical weathering or disintegration This is the breakdown of rocks into small particles by the action of temperature, by impact from rain drops and by the abrasion from mineral particles carried in the wind. Products of mechanical weathering The products of mechanical weathering include everything from huge boulders found beneath the cliffs to the smallest silt. Processes most commonly involved in mechanical weathering. 1. Mechanical unloading This is the vertical expansion due to the reduction of vertical load by erosion. This will open existing fractures and may permit the creation of new fractures. 2. Mechanical loading This is impact on rock and abrasion, by sand and silt size. Wind borne particles that occur in deserts, impact on soil and weak rocks, by raindrops during intense rainfall storms. 3. Thermal loading. This is expansion by freezing water in pores and fractures in cold regions or by the heating of rocks in hot regions. 4. Welting and drying. Expansion and contraction associated with the repeated absorption and loss of water molecules from mineral surfaces and structures. 5. Crystallization. This is expansion of pores and fissures by crystallization within them, of minerals that were originally in solution. Note: Expansion is only severe when crystallization occurs within a confined place. 6. Pneumatic loading. The repeated loading by waving of air trapped at the head of fractures exposed in the wave zones of a sea cliff. Chemical weathering This is the breakdown of minerals into new compounds by the action of chemical agents; such as acid in the air, in rain and in river water; although they act slowly, produce noticeable effects especially in soluble rocks. The rate of chemical weathering depends on temperature, the surface area and the amount of water. Chemical weathering causes the old minerals to disintegrate and to form new minerals. Minerals which are originally formed at lower temperatures in the original igneous rocks during the process of cooling of magma prove more resistant to chemical weathering compared to those which were formed at high temperatures during the cooling of Magma. Processes of chemical weathering Processes are most commonly involved in chemical weathering are listed below and their rate of operation depends upon the presence of water and is greater in wet climates than in dry climates. Some commonly occurring processes in chemical weathering are: 1. Solution. This is dissociation of minerals into ions greatly aided by the presence of carbon dioxide (CO2) in the soil profile, which forms carbonic acid (H2Co3) with percolating rain water. 2. Oxidation This is the combination of oxygen (O2) with a mineral to form oxides and hydroxides or any other reaction in which the oxidation number of the oxidized elements is increased. 3. Reduction. The release of oxygen (O2) from a mineral to its surrounding environment; ions leave the mineral structure as the oxidation number of the reduced element is decreased. 4. Hydration. This is the absorption of water molecules into the mineral structure. Note: This normally results in expansion; some clay expands as much as 60% and by admitting water hastens the process of solution, oxidation reduction and hydrolysis. 5. Hydrolysis Hydrolysis is the reaction with water. Hydrogen ions in percolating water replace mineral cations; no oxidation reduction occurs. In other words, hydrolysis is a chemical reaction in which a compound reacts with water causing decomposition and the production of two or more other compounds. 6. Leaching This is the migration of ions produced by the above processes. Leach. (To drain away from soil when dissolved in rain water, lose a mineral or chemical dissolved in rain water. Note: The mobility of irons depends upon their ionic potential. Calcium (Ca), Magnesium (Mg), Sodium (Na), and Potassium (K) are easily leached by moving sodium water, Iron (Fe) is more resistant, Silicon (Si) is difficult to leach and Aluminum (Al) is almost immobile. 7. Cation exchange. This is the absorption onto the surface of negatively clay of positively charged cations in solution especially Calcium (Ca), Hydrogen (H), Potassium (K), and Magnesium (Mg). Biological Weathering This describes those mechanical and chemical changes of the ground that are directly associated with their activities of animals and plants. When present, microbial activities can change the chemistry of the ground close to ground level.

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