Weathering of rocks and minerals (Physical and Chemical)
Weathering is a natural process of breakdown and transformation of rocks and minerals into unconsolidated residues, called regolith. Regolith is lying on the surface of the earth, with varying depth. In other words, the process of transformation of solid rocks into soils is known as weathering. Weathering processes are of two types:
(1) Physical weathering brought about by the mechanical action of the various weathering agents, is designated as disintegration, and
(2) Chemical weathering is designated as decomposition.
(1) Physical Weathering
It is a mechanical process, causing disruption of consolidated massive rocks into smaller bits without any corresponding chemical change (or formation of new products).
(a) Temperature. The alternate expansion and contraction of rocks due to variation in temperature, produce cracks. The number of cracks slowly increases and the rock gets broken into pieces.
(b) Water. In cold regions, water freezes in rock joints and cracks. On freezing, the water expands in volume. Due to this tremendous pressure the rock splits and is broken up into a loose mass of stones. Falling rain drops and hailstorms forms with their beating force also cause some abrasion of rocks. The moving water has a tremendous transport capacity which by rolling action grinds the rocks into pieces. Water through its erosion forces removes weathered parts of rock, thereby exposing fresh surface to weathering.
(c) Wind. Wind carrying particles in suspension, like those of sand of rock fragments, and blowing constantly over the rock at great speed exerts a grinding action whereby the rock gets disintegrated. Loosely balanced rock boulders sometimes roll down by the action of wind and break into pieces.
(d) Biological agencies. Physical influences of biological agencies such as plant roots splitting the rock apart; movement of animals; burrowing by rodents; and cultivation by man, also help in mechanical weathering to some extent.
(2) Chemical Weathering
Chemical weathering takes place mainly at the surface of rock minerals with the disappearance of certain minerals and the formation of secondary products. This is called chemical transformation. No chemical weathering is possible without the presence of water. The rate of chemical reactions increases with dissolved carbon dioxide and other solvents in water, and with increases in temperature. The principal agents of chemical weathering are described below:
(a) Solution. Some substances (exp., halite, NaCl) present in the rock are directly soluble in water. When the soluble substances are removed by the continuous action of water,-the rock no longer remains solid and falls to pieces very soon.
(b) Hydration. Hydration means chemical combination of molecules with a particular mineral. Soil-forming minerals occurring in rocks do not contain any water. They undergo hydration when exposed to humid conditions. A large number of minerals, like feldspar, mica etc., become hydrated, forming hydrous compounds, for example:
2Fe203 + 3HOH —> 2Fe203. 3H20
Hematite (red) Water Limonite (yellow)
Due to this reaction the minerals increase in volume and become soft and more readily weatherable.
(c)Hydrolysis. It is one of the most important processes in chemical weathering. Hydrolysis depends on the partial dissociation of water into H-ions and OH-ions. Increases in H-ion concentration, resulting in the accelerated hydrolytic action of water. Water thus, acts like a weak acid on silicate minerals, e.g.,
KAIsi308 + HOH —> HAISi308 + KOH
Orthoclase Water (dissociated) Acidsilicate Clay Potassium hydroxide
The products of hydrolysis are either wholly or partially leached by percolating water. They may also recombine with other constituents to form clay. In a way, hydrolysis may be considered as principal agent of clay formation.
(d) Oxidation. Oxidation means addition of oxygen to minerals. Oxidation is more active in the presence of moisture and results in hydrated oxides. Soil-forming minerals containing iron, manganese etc., are more subjected to oxidation, e.g.,
4FeO + 02 —-à 2Fe203
Ferrous Oxide Oxygen Ferric Oxide (Hematite)
A rusty-looking (red) crust is formed on the surface of the rock. The crust thickens and then slowly gets separated from the parent rock. As process continues, the changes produced in the mineral weakens the rock and ultimately the rock itself crumbles to pieces.
(e) Reduction. This means the removal of oxygen. Under condition of excess water (less or no oxygen), reduction takes place, e.g.,
2Fe203 —> 02 —> 4FeO
Ferric Oxide (Hematite) Oxygen Ferrous Oxide
This reaction is not very important from the point of view of soil formation.
(f) Carbonation. Carbon dioxide dissolved in water it forms carbonic acid:
H2O + CO2 = H2CO3
Water Carbon dioxide Carbonic acid
The carbonic acid or carbonated water attacks many rocks and minerals and brings them into solution. Limestone, which is insoluble in water, is dissolved readily by carbonated water and is thus, removed from the parent rock:
CaCO3 + H2CO3 –> CaH2 (CO3)2
Calcite (slightly soluble) Carbonic acid Calcium bicarbonate (readily soluble)