Weathering is the process of disintegration and decomposition of rocks and minerals at or near the earth’s surface. It is the initial stage of denudation and the static part of the process of erosion. Its products are equilibrium in position within the new physio-chemical condition. Weathering products have been accumulated in insitu and forming a soft layer called regolith.
Controlling Factors of Weathering
There are different factors that contribute to weathering. Some of them are described below:
Structure, texture and mineral composition of rocks
- Structures: Joints, Cracks, Fissures, Beddings, Foliations.
- Textures: Fine grained rocks are more Susceptible to weathering than coarse grained
- Mineral Composition: Rocks with hight silica content have less weathering while rocks with low Silica content have more weathering. Also dark coloured minerals are weathered fast as compared to light coloured minerals. It depends upon Bowen’s reaction series: First crystallized minerals are weathered fast in comparison to last crystalized minerals.
Climate is the total sum of temperature, moisture(humidity, precipitation), atmospheric pressure, wind, etc. Here’s how climate affects weathering.
- Tropical and arid climate: Prominent physical weathering
- Temperate and humid climate: Prominent chemical weathering
- Temperature: Diurnal and seasonal temperature changes results expansion of rocks than mostly weatherd rocks physically. High temperature within humid condition affects rate of chemical weathering
- Rainfall: High precipitation saturate rocks increase pore pressure. H+, OH–, HCO3– ion with water react unstable minerals causes chemical decomposition
Topography is the gradient of the area. The most effective weathering happens in average topography due to higher percolation and infiltration of water. In high relief, there is less effective weathering but a high rate of erosion and transportation.
The most effective weathering is in north-facing land in comparison to the south and east-facing land.
Plant roots on joints and fissures cause the disintegration of rocks. Also, organic acid from plant decay causes decomposition.
The longer the time period, the degree of weathering is maximum and affects the greater rock volume.
Types of Weathering
There are three types of weathering: physical, chemical, and biological.
Physical weathering, also known as mechanical weathering takes place when rock is split into smaller pieces of the same material without changing its composition. Example: breaking of rock cliff into boulders and pebbles. Here are some of the common types of physical weathering:
- Insolation: Due to thermal expansion, it results exfoliation and granular disintegration.
- Exfoliation: It is the separation of successive thin, onion like shell from bar surfaces of massive rock. It is common in moderate rainfall. It is resulted due to temperature variation.
- Granular Weathering: The rock masses are broken down grain by grain in coarse-grained rocks i.e in granite, pegmatite etc. The process occurs in region of great temperature extreme.
- Dilation: When over broaden load is removed, the underlying rocks are exposed in new physio-chemical condition than start physical or chemical weathering. All intrusive rock are formed under great pressure. When they exposed to the air through erosion process, they will break-up for pressure release.
- Crystal Growth: The soluble constituents (HaCl, CaCO3, MgSO4 etc) on fractures and joints result growth of crystals and exert pressure on either side than broken down the rocks.
- Colloidal Plucking: Soil colloids have power loosen rocks.
- Frost Action: The freezing and thawing cause alternate expansion and contraction of rocks eventually breaking them apart. This is mainly dominant in the mountain and polar regions. Also, it is more likely to occur in winter.
Chemical weathering also known as decomposition takes place when the rock’s minerals are changed into different substances. Water and water vapor are important agents of chemical weathering. For example: formation of clay minerals from feldspar. Here are different types of chemical weathering:
- Hydrolysis: The chemical reaction of water with other substance is called hydrolysis. Chemical reaction occurs between ions of minerals and H+ or OH- ions of water. H+ ion and OH- ions are measured by PH values. Common materials undergond hydrolysis are feldspar, hornblende, augite, etc.
- Oxidation: The chemical reaction of oxygen with other substances is called oxidation. Iron-bearing minerals are the ones most easily attacked by oxygen. Examples: magnetite, pyrite, dark-colored ferromagnesian silicates. Oxidation of these minerals results in kinds of rust, or iron oxides. Most elements at earth’s surface exist in an oxidized state.
- Reduction: It is the opposite process of oxidation. It occurs below water table. Reduced form of elements aer more mobile than oxidized because they’re more soluble.
- Carbonation: Carbon dioxide dissolves easily in water. It forms a weak acid called carbonic acid. This is the same compound that is in carbonated drinks. It attacks many common minerals such as feldspar, hornblende, augite and biotite mica. The original mineral is changed into a clay mineral. It has the greatest effect on calcite than any other mineral. It dissolves it completely, with no clay left over.
- Solution: Mineral is dissolved by water and removed in solution.
Biological weathering is the disintegration and decomposition of rocks by the action of plants and animals. Biological weathering occurs due to the following causes:
- Plants root on cracks results wedge.
- Activities of burrowing animals.
- Production of reducing gasses (H2S) results chemical weathering.
- Organic acids i.e. humic acids in rain water increase its solvent power.
- Some bacteria are active in reducing condition and making Sulphide.
- Other bacteria can convert N2 to NH4 which affect the PH value.
Significance of Weathering of Rocks
- It acts an aid to mass wasting and erosion.
- It helps in general lowering of land surface.
- It contributes to the creation and modification of land forms.
- As a major process involved in the formation of regolith or soil.
- Facilitates soil nutrients within earth crust. Most of the plant nutrients are derived from weathering processes.
- Suitable site for plant growth and forest development.
- It reflects the paleo-environment.
- Gives idea to estimate underlying rocks.