Soil is formed from parent materials which is formed by weathering of rocks. Different weathering processes are discussed here.
Weathering is a process in which different physical and chemical changes are produced in rocks at or near the earth’s surface by atmospheric agents. There are two basic weathering processes.
- Mechanical/Physical weathering (disintegration): It causes decrease in size of rocks and minerals without appreciably affecting their chemical composition.
- Chemical weathering (decomposition): It is a process in which chemical changes take place in rocks and minerals. In this process, soluble materials are released, new minerals are synthesized and some resistant products remain as such.
Mechanical/Physical Weathering (Disintegration)
Rocks heat up during the day and cool down at night, causing alternate expansion and contraction of their constituent minerals. As some minerals expand more than others, temperature changes set up differential stresses that eventually cause the rock to crack apart. Further the outer surface of a rock is often warmer or colder than the inner resulting in the peeling away of outer layers. This process is called exfoliation.
Abrasion by Water, Wind, and Ice
When flowing water is loaded with sediments, it has tremendous cutting power. These sediments colloid with each other and breakdown into smaller particles. The rounding of riverbed rocks and beach sand grains is an evidence of the abrasion that accompanies water movement.
Wind-blown dust and sand can break down rocks by abrasion in certain arid regions. In glacial areas, huge moving ice masses carrying soil and rock fragments, grind down rocks in their path and carry away large volumes of material.
Action of Microorganisms, Plants and Animals
Lower plants like mosses, lichens grow on exposed rocks, catch dust particles, accumulate organic matter and encourage further plant growth. Pressure by roots of higher plants assist disintegration by opening up spaces for the penetration of water which may freeze and expand later.
Burrowing animals such as earthworms, ants and rodents and also hoofed animals through their action contribute slowly to the disintegration of rocks. Humans accelerate the slow process of physical weathering by ploughing and cultivating.
Chemical Weathering (Decomposition)
Hydration: It is the addition of water molecule in chemical combination with a mineral to form hydrated compounds. Hydrated minerals are more prone to decomposition due to their increased volume which makes it softer and more stressed.
Hydrolysis: In hydrolysis reactions, water molecules split into their hydrogen and hydroxyl components and the hydrogen often replaces a cation from the mineral structure.
Dissolution: Water dissolves many minerals by hydrating the cations and anions until they become dissociated and surrounded by water molecules. These dissolved minerals are converted into solution form which permits greater chemical changes than could occur in an unionized (generally solid) state.
Acid reactions: Weathering is accelerated by the presence of acids, which increase the activity of hydrogen ions in water. Soils contain stronger acids, such as nitric acid (HNO3), sulfuric and (H2SO4), and many organic acids like carbonic acid. Hydrogen ions are also associated with soil clays. Each of these sources of acidity is available for reaction with soil minerals resulting in enhanced weathering.
Oxidation-reduction: Minerals that contain iron, manganese, or sulfur are especially susceptible to oxidation-reduction reactions. The variation in oxidation-reduction ability cause weathering.
The various chemical weathering processes occur simultaneously and are interdependent.
Syed Shabbar Hussain Shah
Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan
M +92 304 6020238 E [email protected]