Introduction

The term weathering originates from the weather. Weathering is the process of breaking down rocks under the influence of different elements of weather and in various natural processes and chemical reactions. There are three types of weathering. These are chemical weathering, mechanical weathering or physical weathering, and biological weathering discussed in detail below.

Weathering is a significant process in the environment. The soil is formed, destroyed, and regenerated through this process. Soil originates from rocks through natural and chemical changes such as soil erosion, weathering, etc. in the presence of organic matter.

In weathering, broken or decomposed matter remains in its position, and no removal occurs. In this way rock, sand, and mud, accumulate and form a soft covering of broken and decomposed material on the surface of the earth. It is called regolith. If this regolith is not removed by corrosive energy, it will become a habitat for plants and insects and gradually the regolith will turn into the soil (1) & (4).

Types of weathering

According to Ollier, “Weathering is the breakdown and alteration of materials near the earth’s surface to products that are more in equilibrium with severely imposed physical-chemical conditions”.

In the process of weathering various elements of weather such as rain, sunlight, snow, etc. participate in rock crusting. Weathering is divided into three categories. These are-

1. Chemical weathering
2. Mechanical weathering or physical weathering.
3. Biological weathering (2).

1. Chemical weathering

Definition

Chemical weathering is when rocks are broken down by chemical processes under the influence of various elements of the atmosphere (e.g., carbon dioxide, oxygen, water vapor) and plants and animals.

Chemical weathering changes interstitial minerals in rocks into secondary minerals. So the rocks are easily broken into pieces. This type of weathering is prevalent mainly in equatorial warm humid and tropical climates (1).

Types

1. Oxidation

Oxidation is the dissociation that occurs when oxygen combines with minerals in a rock. These processes are particularly effective on those rock minerals that are usually sulfides, carbonates, and iron silicates. As a result, a type of new compound of yellow and brown color is formed on the iron and corrodes the iron-containing rocks.

2. Carbonation

Carbonic acid is formed when carbon dioxide in the air mixes with rainwater. This acid reacts with the minerals in the rock to cause erosion by disintegrating the rock. Especially those rocks which are rich in minerals such as calcium, magnesium, potassium, sodium, iron, etc. melt easily when exposed to carbonic acid. In this way, the chemical addition of carbon dioxide to various minerals is called carbonation.

3. Solution

Minerals are not dissolved by water directly. But some of the minerals in the rock reach a state as a result of chemical reactions that easily dissolve or melt in water, this is called a solution. This type of dissolution is especially common in limestone. Apart from that, minerals like calcite, gypsum, etc. get mixed with water and dissolve and are called a solution.

4. Hydrolysis

In wet analysis, water breaks down into hydrogen and hydroxyl ions, and the resulting hydroxyl ions react with minerals to cause chemical changes. This is called hydrolysis. Orthoclase feldspar hydrolysis into alumina-silicate and potassium hydroxide.

5. Hydration

The chemical weathering that occurs when pure water combines with a mineral in a rock is called hydration. This causes dry calcium sulfate to become gypsum and hematite to limonite (1) & (3).

2. Mechanical weathering

Definition

Mechanical weathering is the breaking of rocks by natural means under the influence of temperature, precipitation, snowfall, and glaciers. This process results in only physical changes in the rock or inter the rock minerals (2).

According to geologist Perry Reich, mechanical weathering is done mainly in five processes.

1. Thermal expansion

The predominance of climate change processes is observed in hot desert regions. The bedrock continues to contract and expand due to temperature variations throughout the year.

For example, due to the variation in the temperature during the day and night and due to the change of season, it was broken into pieces as a result of gradual contraction and expansion. This process occurs in many ways.

1. Block disintegration

Rocks are poor conductors of heat. Therefore, in the warm region, the upper layer of the rock is heated by the sun’s rays. But the bottom layer doesn’t get as hot. This uneven compression and expansion cause severe internal explosions in the rock.

At one point, the rocks between the various cracks were separated into lumps. This is called block disintegration.

2. Granular disintegration

The temperature range is very high in desert regions. Therefore, when the rock cracks through the mineral-intermediate connective material during rapid contraction and expansion, it becomes small grains. This is called granular disintegration.

3. Exfoliation

The word exfoliation means to come up with the bark or shell on the rock. Rocks are poor conductors of heat. That is, as fast as the upper layer of the rock heats or cools in a given period of time, the interior part of the rock does not heat and cool as fast.

As a result, there is a temperature difference between the outer and inner parts of the rock. Then the top layer opens up like a pea shell. Such weathering is more common in granite rocks.

4. Boulder cracking

Sometimes large boulders or stone blocks are embedded in rocks made up of smaller particles. Under the influence of heat, the upper part of such rock expands, but the part enclosed by the ground does not expand.

The uneven pressure then causes the rocks to detach from the ground. This process is called boulder cracking (1).

2. Weathering caused by pressure decreases

If a rock layer is initially under pressure in the upper layer rock and then the upper layer rock is removed by erosion, the rock layer breaks due to the decrease of pressure. Sheeting and spalling are mainly seen due to pressure decrease.

1. Sheeting

A rock is divided into thin sheets or layers by cracks parallel to the surface. This splitting tendency is called sheeting. According to Gilbert, it is mainly due to the removal of the erosion of the upper layer of rock and is mainly due to the expansion of the upper layer of the granite rock, resulting in sheet formation.

2. Spalling

If the upper rock layer is weak, pressure decrease can cause sheeting with explosions. However, in some cases, even if the upper rock layer is removed, sheeting may not occur. In these cases, the rock layer separates into small pieces in a somewhat disorderly manner called spalling (3).

3. Crystal-induced weathering

1. Snow or frost shattering

In high mountain areas or cold climates, rainwater or snowmelt water accumulates in various cracks in rock layers and turns into ice in extreme cold. When water freezes and turns into ice, it increases in volume by 9%. As a result, the rock layer is under high pressure and it breaks into small pieces. This form of weathering in the snow process is called snow or frost shattering.

2. Growth of salt crystals

In arid and sub-arid regions and in coastal regions, dissolved salts in groundwater rise by cellular processes and evaporate to form salt crystals. These salt crystals accumulate in the cracks in the bedrock, causing tremendous pressure and eventually breaking the rock into pieces (1).

Weathering caused by bioactive processes

Weathering that takes place through biologically active processes is generally called organic weathering. Various animals, plants, microorganisms, and humans help in weathering through mechanical processes. For example- earthworms, termites, mice, rabbits, etc. burrow in the soil. Plant spikes penetrate the rock and cause cracks in the rock.

Colloid production process

Soil minerals and inorganic substances break down into very fine particles to form a sticky substance called a colloid. When the soil dries up, some parts of the bedrock are eroded due to colloid formation. The resulting rock deformation is called the colloid production process.

Slacking

The alteration of rocks or minerals by successive wetting and drying is called slaking. Sandstones, siltstones, and mudstones are first soaked for a day and then dried for a long time will show that these rocks are altered, but other fine-grained rocks are more fragmented. This gradual wetting and drying increase the tendency of the rock to fracture (1) & (3).

Two types of weathering

1. Mechanical weathering

Mechanical weathering is the external alteration of surface rocks by different elements of weathering without any chemical change.

2. Chemical weathering

Chemical weathering occurs when surface rocks are weathered or dissolved by chemical processes in warm and humid climates (1).

Types of physical weathering

When a rock is broken or fragmented mainly by a mechanical process, it is called mechanical weathering. This mechanical weathering is called physical weathering.

There are mainly three types of physical weathering. These are

1. Physical weathering due to the release of pressure

• Sheeting
• Spalling

2. Physical weathering due to crystallization

• Weathering from crystallization of snow
• Weathering from crystallization of salt

3. Physical weathering due to insolation

• Boulder cracking
• Exfoliation
• Granular disintegration
• Block disintegration
• Dirt cracking (1).

Q&A

1. What are the two types of weathering?

Mechanical and chemical weathering are two types of weathering.

2. What are two types of weathering?

There are two types of weathering. These are-

• Mechanical or physical weathering
• Chemical weathering

3. What types of rocks are formed by weathering?

Sedimentary rocks are formed by weathering.

4. Which of these types of weathering requires the presence of water?

The solution required the presence of water.

5. What are the types of chemical weathering?

There are 5 types of chemical weathering. These are-

• Oxidation
• Carbonation
• Solution
• Hydrolysis

Reference

1. Savindra Singh. Geomorphology. Pravalika Publications, Allahabad. Chapter 14: Weathering and Mass movement. Page No: 247- 266.

Written By: Manisha Bharati