The term “weathering” refers to the mechanical disintegration and chemical deterioration of rocks caused by various weather and climate factors. Weathering is an in-situ or on-site process because there is little to no material motion. Rock, climatic, topographic, and vegetative factors all influence weathering. Climate-to-climate variations affect both the depth of the weathering and the weathering processes. There are three types of weathering:
(i) chemical weathering
(ii) physical or mechanical weathering
(iii) biological weathering
Chemical weathering
Through chemical reactions involving oxygen, surface and/or soil water, and other acids, various weathering processes, including solution, carbonation, hydration, oxidation, and reduction, act on the rocks to break them down, dissolve them, or reduce them to a fine clastic state.
All chemical reactions must occur in the presence of water, air, and heat. The amount of carbon dioxide underground rises as plants and animals decompose, adding to the amount of carbon dioxide already in the air.
Solution
Many minerals and carbon dioxide are dissolved in water to make it a weak acid. This decomposed rocks.
Solution is the most potent weathering process in limestone regions where rainwater attacks and dissolves the calcium carbonate.
The dissolved calcium carbonate is carried away by the water.
Joints and cracks in the rock are quickly widened and whole systems of caves and passages are formed.
Oxidation
When rocks with iron content comes in contact with air, it is changed to iron oxide that crumbles easily and far more easily eroded than the original iron.
It is thus removed, loosening the overall structure of the rocks and weakening them.
Hydrolysis
It involves the reaction between mineral ions and the ions of water.
It results in the decomposition of the rock surface by forming new compounds and by increasing the pH of the solution involved through the release of the hydroxide ions.
Hydrolysis is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charged crystal surfaces.
Reduction
It is simply the reverse of oxidation. It is caused by the addition of one or more electrons producing a more stable compound.
The newly formed compounds are usually eroded away easily.
Hydration
It involves the rigid attachment of ions of water and mineral ions to a reacted compound.
In many situations, the H and OH ions become a structural part of the crystal lattice of the mineral.
Hydration also allows for the acceleration of other decomposition reactions by expanding the crystal lattice offering more surface area for reaction.
Carbonation
It is the reaction of carbonate and bicarbonate ions with minerals. The formation of carbonates usually takes place as a result of other chemical processes.
The most important factor affecting all of the above-mentioned chemical weathering processes is climate.
Climatic conditions control the rate of weathering that takes place by regulating the catalysts of moisture and temperature.
Physical Weathering
Physical or mechanical weathering processes depend on some applied forces.
The applied forces are:
(i) gravitational forces such as overburden pressure, load and shearing stress
(ii) expansion forces due to temperature changes, crystal growth or animal activity
(iii) water pressures controlled by wetting and drying cycles.
Physical weathering processes are caused by thermal expansion and pressure release.
It causes damage to the rocks by repetition of contraction and expansion.
Abrasion: It occurs when some force causes two rock surfaces to come together causing mechanical wearing or grinding of their surfaces. Collision between rock surfaces normally occurs through the erosional transport of material by wind, water or ice.
Crystallisation: It causes the necessary stresses needed for the mechanical rupturing of rocks and minerals.
Crystal Growth: It causes stress as a result of a compounds or an element’s change of physical state with change in temperature.
Transformation: It is from liquid to solid crystalline form produces a volumetric change. It causes the necessary mechanical action for rupture. There are primarily two types of crystal growth i.e., ice and salt. Upon freezing, the volumetric change of water from liquid to solid is 9%. This relatively large volumetric change upon freezing has potentially a great rupturing effect.
Insolation: The physical breakdown of rock by their expansion and contraction due to diurnal temperature changes is known as insolation weathering. The surface of the rock expands more than its interior and this stress will eventually cause the rock to rupture. The disintegration of rocks is caused due to alternate wetting and drying of rocks. It leads to expansion and contraction of rocks. It occurs by the mechanism of ordered water which is the accumulation of successive layers of water molecules in between the mineral grains of a rock. The increasing thickness of the water pulls the rock grains apart with great tensional stress.
Slaking in combination with dissolved sodium sulphate can disintegrate samples of rock in only twenty cycles of wetting and drying. Pressure release of rock can cause mechanical weathering due to unloading. The majority of igneous rocks were created deep under the Earth’s surface at much higher pressures and temperatures.
Biological weathering
Biological weathering is the removal of minerals and ions and physical changes due to growth or movement of organisms.
Burrowing and wedging by organisms like earthworms, termites, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials.
Decaying plant and animal matter help in the production of humus, carbonic and other acids which enhance decay and solubility of some elements.
Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
