The process in which massive consolidated rocks are broken down into fine, smaller particles is termed weathering.
It is combination of destruction and synthesis processes. The rocks are first broken into smaller rocks and then into minerals which they are composed (primary minerals) such as feldspars. The rock fragments and minerals are attacked by chemical forces which release soluble components that either leach down or changed into new minerals (secondary minerals) such as clay minerals. Plants, animals and micro-organisms accelerate the breakdown of rocks and minerals by bringing both mechanical and chemical changes.
Types of Weathering
On the basis of the forces involved in weathering, it may be: mechanical weathering, chemical weathering or biological weathering.
Mechanical Weathering — Disintegration of Regolith
The weathering brought about by mechanical or physical forces such as temperature. wind and ice; plants and animals and fire is called mechanical weathering,
During mechanical weathering the rocks are cracked and crushed resulting in formation of materials of the size from boulders to very fine particles. No significant change in the chemical composition of parent rock material takes place during mechanical weathering. Physical forces involved in mechanical weathering and their action is as under:
Temperature brings mechanical weathering in the following ways.
Differential Expansion of Rock Minerals: During day, the rocks are heated and cause warming and subsequent expansion of some minerals. At night, the rocks ere cooled and some minerals contract more than others. This differential expansion and shrinkage produce cracks and remove flakes and rock grains.
Exfoliation: Since the heat is conducted slowly therefore the outer layers of the rock are warmer during day and cooler at night than the inner layers. This differential heating and cooling results in peeling away of surface layers causing the breakdown of rocks. The phenomenon is called exfoliation.
Frost Action: The water present among the pores and crevices of the rocks freezes and expand when the rocks are cooled. The water expands by about 9% on freezing at 0°C and result in development of pressure equivalent to about 150 tons/ft2. This pressure causes widening of cracks and shattering of rocks. This is called frost action.
Water, Wind and Ice
Water, wind and ice are eroding and carrying agents. They carry the parent material to various places where it undergoes decomposition.
Plants and Animals
Plants and animals also bring about mechanical weathering up to some extent. The lichens grow on the surfaces of exposed rocks. They catch dust particles and their parts dead decay. This results in accumulation of a thin layer of fertile soil on the surface of the rock. This encourages the higher plants to establish themselves. Their roots penetrate the crevices in the rocks and the pressure exerted by these roots causes disintegration of rocks. Animals, especially rodents and burrowing animals also establish themselves in the habitat and bring further disintegration of rock.
Fire is an important physical force in forest areas. During fires the outer surface layers of rocks are much more heated as compared to inner layers, therefore expand. This differential expansion results in rupture of rocks into thin sheets that fall to the ground. Repeated fires weaken the rock surface and disintegrate the rock over a period of time.
The alteration of rocks and minerals that are stable at temperatures and pressures at the surface of the earth by chemical processes is called chemical weathering. The chemical weathering is accelerated by the presence of water and its solutions, oxygen, and organic and inorganic acids produced by the breakdown of plant residues. The following chemical processes bring about chemical weathering.
Water is the main solvent in rock-soil system. The dipole nature of water allows the dissolution of ionic solids in the rock such as chlorides and sulphates of Na, K and Mg to produce a stable Ionic solution. The solvent action of water Is increased in the presence of C02 which reacts with water to form carbonic acid. The carbonic acid containing water dissolves certain minerals easily, for example. CaCO3
CaCO3 H2Co3 ——————————- Ca (HCO3)2
The solvent power of water is further increased by the presence of traces of organic and mineral acids. It is estimated that 37 tons of material per sq km is removed in solution annually in the forms of CaC03, CaS04, NaCl, SIO2, alkali carbonates and sulphates, MgCO3 and Fe2O3.
Hydration is the chemical association of water with a mineral without actual decomposition or modification of mineral itself. The water molecules are adsorbed on mineral surfaces and at the broken edges of minerals. The adsorbed water allows entry of hydronium (H3O+) ions to attack the structure. The common examples of hydration are formation of gypsum, goethite and gibbsite.
CaSO4 + 2H2O ——————————- CaSO4 + H2O
Fe2O3 + H2O ——————————- 2FeOOH
Al2O3 + 3H2O ——————————- Al2O3.3H2O
Hydration involves a considerable change in volume which helps in physic weathering. It also prepares the mineral surface for the oxidation and hydrolysis processes.
Hydrolysis is a chemical reaction in which exchange of constituents occurs between water and mineral that helps to decompose rock crystal lattice. For example, during hydrolysis of orthoclase the soluble K+ is removed and unstable aluminosilicate is formed which is further hydrolyzed and changes into either a clay mineral kaolite or into silca and aluminum hydroxide.
Carbonation (Acid Solution Weathering)
The common acids present in the water are carbonic acid (produced by union of carbon dioxide in contact with water) and organic acids produced by the decay of organic Plant residues. These acids help in chemical weathering.
Oxidation is a reaction with oxygen to form oxides. Oxidation followed by hydrolysis is useful in disintegration of rocks. The process of oxidation is more common in rocks that contain iron and manganese. The Iron as ferrous (Fe2+) is oxidized to ferric (Fe3+). These ionic adjustments result in loss of cations from crystal lattice of the mineral making it less stable. These unstable minerals are then attacked by other weathering processes. An example pure oxidation process is oxidation of magnetite to hematite when exposed to air.
4Fe3O4 + O2 ——————————- 6Fe2O3
Oxidation followed by hydrolysis results in acceleration of weathering.
Reduction takes place in waterlogged geochemical environments and low oxygen supply. The reduction results in formation of more soluble and mobile forms of minerals such as iron which either react to form sulphides or lost by leaching. Much reduction is done by bacteria, for example, sulphates in the organic matter are reduced by bacteria to sulphides. Many silicate minerals contain Fe2+/Fe3+ or Mn2+/Mn+ cations that are easily converted into one or the other ionic form. In order to maintain neutrality of the crystal structure, some ions enter or leave the lattice. This makes the mineral unstable so it is attacked by weathering reactions such as hydrolysis, etc.
Physical and chemical weathering by biological agents is considered another form of weathering by certain geologists. It was named biological weathering.
The breakdown of rocks and minerals is accelerated by plants, animals, micro-organisms. Plant roots penetrate into the crevices and cracks of rocks. As the grow these exert pressure, widen the cracks, force the rock blocks apart, and rupture the rocks in certain cases as well. Thus, roots bring about physical weathering,
Lichens growing on rock surfaces also contribute to the physical weathering. These add to the organic contents of the soil upon their death and in this way, help in building the soil substratum. The lichens produce carbonic acid during respiration which helps in chemical weathering of the rocks. Large plants growing and covering the surface of the rock change the microclimate by controlling the temperature and soil erosion. The cyclic movement of plant nutrients changes the nature of weathering in rocks or soils.
Some plants especially grasses accumulate silica in solid form as phyfoliths (SiO2), The quartz (SiO2) is more resistant but the phytolith is soluble and can be converted into other mineral forms.
The organic acids and compounds secreted by plant roots, lichens, mosses, bacteria or other soil microorganisms that grow on the surface of the rocks, act chemically, corrode the rocks and cause cracks. Vegetation litter accumulating on the surface of rocks decomposed by soil organisms to form humus which improves water holding capacity of soil. The increase in moisture contents of the rocks promote weathering.
Soil fauna also contribute to weathering but less than the soil flora and micro-flora. The burrowing animals contribute to physical weathering of the rock or fragments to some extent. However, the main role played by these burrowing animals in weathering is to expose new surfaces of weathered materials to chemical reactions by mixing the soil.