Block 3 Resources

Weathering is the process by which rocks are broken down into smaller fragments at or near the earth’s surface. It is an external geological process that operates under the influence of atmospheric agents. Weathering does not involve transportation of materials but only their disintegration and decomposition. It plays a vital role in soil formation and landscape development.

Objectives

After studying this unit, you should be able to:

Define weathering and its types

Explain physical, chemical, and biological weathering

Understand factors affecting weathering

Describe the importance of weathering

ROCK  WEATHERING

Weathering is defined as a process of decay, disintegration and decomposition of rocks under the influence of certain physical and chemical agencies.

Weathering is a natural process of in-situ mechanical disintegration and/or chemical decomposition of the rocks of the crust of the Earth by certain physical and chemical agencies of the atmosphere.

The most important aspect of this process is that the weathered product remains lying over and above or near to the parent rock unless it is removed from there by some other agency of the nature.

a)     Disintegration:  It may be defined as the process of breaking up of rocks  into small pieces by the mechanical agencies of physical agents.

b)    Decomposition: It may be defined as the process of breaking up of mineral constituents to form new components by the chemical actions of the physical agents.

c)     Denudation: It is a general term used when the surface of the earth is worn away by the chemical as well as mechanical actions of physical agents and the lower layers are exposed.

The process of weathering depends upon the following three factors:

•          Nature of rocks
•          Length of time
•          Climate

Three types of weathering are commonly distinguished on the basis of type of agency involved in the process and nature of the end product. They are:

1) Physical or mechanical weathering

2) Chemical weathering

3) Biological weathering

I. PHYSICAL OR MECHANICAL WEATHERING

Physical or mechanical weathering is the process by which rocks break into smaller fragments without any change in their chemical composition. During mechanical weathering, the fragments produced still contain the same characteristics as the original rock. The breaking of rocks into smaller pieces exposes fresh surfaces to air and water. These fresh surfaces allow chemical reactions to occur more easily.

Therefore, mechanical weathering often supports and accelerates chemical weathering.  In nature, physical and chemical weathering usually occurs together.

There are several important factors responsible for physical weathering.

The main factors include frost action, temperature changes, the mechanical effects of plants and animals, and gravity.

1. Frost Action (Wedge Work of Ice)

Frost action occurs when water enters cracks, joints, and bedding planes in rocks. When the temperature falls below freezing point, the water freezes and expands.The expansion of ice exerts strong pressure on the surrounding rock. This pressure gradually widens the cracks and weakens the rock. Repeated freezing and thawing eventually break the rock into fragments. This process is very common in cold and mountainous regions. During the daytime in mountain areas, rocks may become warm or hot due to sunlight. At night, the temperature may fall below the freezing point. Under such conditions, freeze–thaw action becomes very effective. The broken rock fragments produced by frost action move downward and accumulate at lower slopes.

2. Effects of Changing Temperatures

Temperature changes also play an important role in mechanical weathering. This process is common in desert and semi-arid regions. During the daytime, rocks are heated to very high temperatures by the sun. At night, the temperature drops sharply. Rocks are poor conductors of heat, so only the outer layers are affected. The outer layers expand when heated and contract when cooled. Repeated expansion and contraction cause cracks in the rock surface. Gradually the outer layers separate from the main rock body. These thin layers peel off in curved shells. This process is called Exfoliation. Exfoliation is common in massive rocks such as granite and basalt.

Spheroidal Weathering  

Exfoliation often results in rounded rock masses. This process is called Spheroidal Weathering. The rounded rock masses formed are known as residual boulders. Weathering usually begins at the edges and corners of rocks. Sharp edges and corners wear away first. As weathering continues, the rocks develop rounded shapes. Because of this process, some hills gradually become dome-shaped. On convex slopes, the separated shells may appear overlapping like roof tiles. Good examples of such landforms are Inselbergs, which are isolated rocky hills. These are commonly found in Mozambique and other parts of Africa. Another famous example is Sugar Loaf Hill in Rio de Janeiro, Brazil.

3. Mechanical Work of Plants and Animals

Plants and animals also contribute to mechanical weathering. Roots of plants and trees grow into cracks and crevices of rocks. As the roots grow thicker, they exert pressure on the rock walls. This pressure widens the cracks and breaks the rocks apart. Burrowing animals also play an important role in weathering. Animals such as earthworms and rodents dig into the soil and rock. Their activity loosens the rock material. They bring partly weathered rock fragments to the surface. These fragments are then exposed to air, water, and further weathering. It is estimated that earthworms may bring more than 10 tons of soil particles per acre to the surface every year.

Role of Microorganisms

Plants such as lichens and fungi contribute to weathering. They extract certain minerals and elements from rocks. Water containing bacteria attacks rock minerals actively. Dead organisms in soil decompose due to the action of soil bacteria and fungi. This process produces a brown, jelly-like organic material called humus. Humus is an important organic component of soil. It can dissolve small quantities of minerals such as limonite.

4. Action of Gravity

Gravity also plays an important role in weathering. It helps in the removal of weathered rock materials. When weathered debris moves downward, fresh rock surfaces are exposed for further weathering. Gravity causes the slow downward movement of loose rock fragments. In steep mountain regions, rock fragments fall down from cliffs. These fragments accumulate at the base of the slopes. Such accumulations of broken rock fragments are called Talus. The slope of a talus deposit is usually about 25° to 30°.

Chemical weathering is the process by which rocks are decomposed through chemical reactions. These reactions change the internal structure and composition of minerals. During chemical weathering, the original rock transforms into new substances. These new substances remain stable under the existing environmental conditions. Water is the most important agent of chemical weathering. Water acts as a solvent and carrier of dissolved gases such as oxygen and carbon dioxide. It may also contain acids and organic substances from the soil. The rate of chemical weathering depends on several factors.

These factors include:

• Composition of the rock
• Temperature
• Concentration of chemical solutions
• Presence of bacteria
• Pressure conditions

Most minerals decompose during chemical weathering. However, some minerals such as quartz and muscovite are resistant to weathering. Minerals like calcite dissolve completely in water. Many silicate minerals break down into clay minerals. At the same time, soluble substances are carried away in solution.

2. CHEMICAL WEATHERING

The main processes of chemical weathering include:

1. Solution

2. Oxidation

3. Hydration

4. Carbonation

1. Solution 

Solution occurs when minerals dissolve directly in water. Rainwater contains dissolved carbon dioxide from the atmosphere. This forms carbonic acid.  Carbonic acid reacts with rocks and dissolves certain minerals. Limestone is particularly susceptible to solution weathering. In this reaction, calcium carbonate is converted into soluble calcium bicarbonate. Example:

CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂

This soluble compound can be carried away by water. In limestone regions, solution weathering forms grooves and furrows on rock surfaces. Such features are commonly found in the Pennines of England near Ingleborough. When limestone contains impurities such as clay and quartz, these materials remain behind. They accumulate and form residual soil deposits.

Examples include:

Terra Rossa soil in karst regions

Clay with flints in chalk landscapes

2. Oxidation

Oxidation occurs when oxygen reacts with minerals in rocks. Water containing dissolved oxygen attacks iron-bearing minerals. Iron changes from ferrous iron to ferric iron. This produces minerals such as hematite, limonite, and goethite. Oxidation causes colour changes in rocks. Rocks may change from green or black to red, yellow, or brown. Because of oxidation, many soils in warm humid regions appear red or yellow.

3. Hydration

Hydration occurs when water molecules combine chemically with minerals. This process forms new hydrous minerals. For example, feldspar minerals in granite are converted into clay minerals such as kaolin. The reaction also produces silica and other dissolved substances.

Other hydrous minerals formed by hydration include: Serpentine, Talc, Chlorite, Zeolite.

4. Carbonation

Carbonation occurs when carbon dioxide reacts with minerals in rocks.  Carbon dioxide dissolves in water and forms carbonic acid. This weak acid reacts with minerals such as calcium, magnesium, sodium, and potassium oxides.  These reactions produce carbonates or bicarbonates. Carbonated water can dissolve minerals more easily than pure water. Therefore, it is a very active agent of chemical weathering. Weathered rock surfaces often show rusty brown stains due to iron compounds.

Importance of Chemical Weathering

Chemical weathering contributes to the breakdown of rocks in several ways. It weakens the bond between mineral grains. It produces soluble compounds that are washed away by rainwater. This makes rocks porous and weak. Chemical weathering may also produce new minerals with larger volume. These expansion processes can cause the outer layers of rocks to peel away. This effect may contribute to exfoliation of rocks.

Influence of Climate on Weathering

Climate is one of the most important factors controlling the type and intensity of weathering. Temperature, rainfall, moisture, and vegetation determine whether mechanical or chemical weathering dominates. Different climatic regions show different weathering characteristics.

1. Equatorial Climate

Equatorial regions have high temperature and heavy rainfall throughout the year. Chemical weathering is very intense because heat and moisture accelerate chemical reactions. Dense vegetation also contributes organic acids that help in rock decomposition. Silicate minerals break down rapidly and silica is removed in solution. The typical product of weathering in this region is laterite, rich in iron and aluminum oxides.

2. Desert Climate

Desert regions are hot and dry with very little rainfall and vegetation. Mechanical weathering is dominant due to large daily temperature variations causing expansion and contraction of rocks. Salt crystallization also breaks rocks apart. Evaporation of groundwater may cement surface materials forming hardpan. A common feature of desert rocks is desert varnish, a dark coating of iron and manganese oxides.

3. Temperate Climate

Temperate regions experience moderate climate with seasonal variations. In winter, frost action causes mechanical weathering as water freezes and expands in rock cracks. In summer, chemical weathering becomes active due to moisture and moderate temperature. Thus, both mechanical and chemical weathering occur in these regions.

4. Arctic Climate

Arctic regions are extremely cold and snow-covered for most of the year. Meltwater repeatedly freezes and thaws in rock cracks, producing frost wedging. Mechanical weathering is dominant and produces sharp peaks and ridges. Large quantities of rock debris accumulate due to continuous disintegration.

3. BIOLOGICAL WEATHERING

Products of Weathering – Soils

Weathering of rocks produces soil, which is an important natural resource. When rock is weathered, it forms loose material called regolith above the bedrock. If the bedrock is exposed at the surface, it is called an outcrop.

Soil is a mixture of mineral particles, organic matter, water, and air. Soil formation depends on several factors such as parent rock material, climate, living organisms, land slope, and time.

Types of Soils

1. Residual Soil

Residual soils are formed directly from the weathering of underlying bedrock and remain in the same place. They gradually change downward into subsoil and then into bedrock.

2. Transported Soil

Transported soils are formed from weathered materials that have been moved from their original place by agents such as wind, water, glaciers, or gravity.

3. Major Soil Types

1. Lateritic Soil: Formed in hot and humid climates; rich in iron and aluminum oxides but poor in nutrients.

2. Black Soil (Chernozem / Black Cotton Soil): Derived from basalt; rich in minerals and suitable for cotton cultivation.

3. Podsol Soil: Developed in cold and moist climates under coniferous forests; generally acidic and less fertile.

4. Loamy Soil: Contains balanced amounts of sand and clay; very fertile and suitable for agriculture.

Mass Wasting

Mass wasting refers to the down slope movement of weathered rock materials under the influence of gravity. It is an important process in landscape development and occurs after weathering has weakened the rocks.

Gravity is the main force responsible for mass wasting, but other factors such as water also play an important role. When soil becomes saturated with water, its weight increases and the cohesion between particles decreases. This makes it easier for the material to slide or flow downhill.

Mass wasting includes various movements such as rock falls, landslides, and soil creep, which transport weathered materials from higher slopes to lower areas.