Weathering and its controlling factors

What is Weathering?

The process of disintegration and decomposition of rocks in situ is generally called weathering.

According to C.D.Ollier (1969) “weathering is the breakdown and alteration of minerals near the earth’s surface to products that are more in equilibrium with newly imposed physio-chemical condition.”

CONTROLLING FACTORS OF WEATHERING

Controlling factors of Weathering is listed below.

Composition and structure of rocks

• Since weathering involves disintegration and decomposition of rocks and hence mineral composition, joint patterns, layering system, faulting, folding etc. largely affect the nature and intensity of weathering.

• For eg. Carbonate rocks having more soluble minerals are easily affected by chemical weathering.

• Well jointed rocks are more subjected to mechanical disintegration.

Nature of ground slope

• Ground slope controls mechanical disintegration of rocks and mass movement of weathered products down the slope.

• The rocks in the regions of steep hill slope are easily disintegrated due to mechanical weathering and the weathering materials are instantaneously moved down the hill slope in the form of rock fall, debris fall and slide.

• The regions of gentle and moderate ground slope are less affected by mechanical disintegration.

Climatic variation

The Climatic Geomorphologists are of the view that each climatic type produces definite conditions for a particular type of weathering. For example,

• Chemical weathering is more dominant in humid tropical areas because of more available water and high temperature.

• Mechanical disintegration of rocks is more dominant in the tropical and semi arid region.

• The rocks in dry temperate climates are more susceptible to mechanical weathering than chemical weathering because alternate expansion and contraction of cracks, fractures and joints of rocks due to alternate freeze and thaw of water accumulated in these cracks and fracture weaken the rocks.

• In monsoon climate rocks are subjected to mechanical disintegration during hot and dry summer months whereas chemical and biochemical weathering is more dominant during wet monsoon months.

Floral effect

The nature of weathering is largely determined by the presence or absence of vegetation in a region.

Vegetation is partly a factor of weathering and partly a protector of rocks. As the roots binds the rocks and protect them from weathering but at the same time the penetration of roots weakens the rocks by breaking them into several blocks.

• The micro organisms associated with the roots of plants and trees encourage decomposition and disintegration of rocks through physic-biochemical weathering.

TYPES OF WEATHERING

There are two types of Weathering.

Physical weathering

Block disintegration:Temperature change results into alternate expansion and contraction of which weakens the rock and leads to mechanical disintegration, which known as block disintegration.

• Crystalline rocks like granites are more affected by temperature change.

• Disintegration of rocks into large size blocks due to freeze and thaw of water is of common occurrence in the temperate and cold climate.

Granular disintegration:If the rocks are coarse grained and are of different colours, they absorbs insolation differently so the different parts of rocks are affected by differential expansion and contraction which cause stresses within the rocks as a result the rock disintegrated into smaller particles.

Exfoliation : It is more common over crystalline rocks. Because the outer shell of rocks become loose due to alternate expansion and contraction due to high temperature during day time and low temperature during night. Differential heating of outer and lower shells of a rock mass causes flaking

Chemical weathering

Solution: It refers dissolution of soluble particles and minerals from the rocks with the help of water in motion.

CaO+H₂O =Ca(OH)₂

Oxidation: When water is mixed with oxygen its reaction with minerals of the rocks forms hydroxide. Such as Ferrous oxide produces Ferric oxide.

Fe+H₂O+O₂ = Fe₂O₃.3H₂O

Carbonation: carbonation is the reaction of carbonate or bicarbonate ions with minerals. Wherein atmospheric carbon dioxide after mixing with water form carbonic acid, which react with carbonate rocks and forms calcium bicarbonate.

CaCO₃+H₂O+CO₂ = Ca(HCO₃)₂

Hydration: It is related to the addition of water to the minerals. The rocks after having absorbed water undergo positive change of their volume. Due to hydration Feldsper minerals changes into Kaolinite clays.

Hydrolysis:Hydrolysis is a chemical reaction between mineral and water that is between hydrogen(H)ions and hydroxyl(OH)ions, and the ions of mineral.

Mg₂SiO₄+4H⁺+4OH^- =2Mg⁺⁺+4OH^- +H₄SiO₂

This article is shared by Priyanka Duta. Priyanka is a Guest Lecturer of Geography at New Alipore College, Kolkata.