Weathering and Soil Formation

(a) Weathering is the breaking down of rocks by agents like temperature, rain, etc.

(b) Erosion is the wearing away of rocks by running water.

(c) Mechanical weathering is disintegration of rocks by elements of weather such as heat, frost, wind, etc.

(d) Carbonation is the reaction of carbonic acid on minerals.

(e) B-Horizon of soil profile contains gravel, clay and sand

(a) Weathering

(b) Denudation

(c) Mechanical weathering

(d) Exfoliation

(a) Exfoliation — The changes in temperature during summer and winter and in case of arid region, during day and night, cause expansion and contraction in the rock surfaces. This causes the rocks to break-up or peel off. Rectangular rock blocks are thus, gradually rounded by splitting away of sharp corners. This process is called exfoliation.

(b) Hydration — Hydration is the chemical reaction of water with minerals present in rocks. It changes the proportion and composition of rocks and minerals. It weakens the rocks and ultimately the rock breaks-up.

(c) Weathering — Weathering is described as the breaking down or dissolving of rocks and minerals on the surface of the earth. Water, ice, acids, salts, plants, animals and changes in temperature are all agents of weathering. It is a static process and leads to the formation of soil.

(d) Rocks — Rocks are aggregates of minerals and have no definite chemical composition. Rocks makeup most of the solid materials of the lithosphere. On the basis of their formation rocks are classified into three types:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks.

(e) Minerals — A mineral is a naturally occurring, homogenous solid that has definite chemical composition with its own chemical and physical properties, e.g., silicates and quartz.

(f) Denudation — Denudation is a dynamic process that includes the disintegration and decomposition of rocks, transportation of the eroded material and deposition of it at some other place. Therefore, process of denudation is two-fold process that is both destructive and constructive.

(g) Mechanical weathering — Mechanical weathering is responsible for the disintegration of rocks by the elements of weather such as heat, frost and wind. Mechanical weathering does not change the composition of rocks. It depends on the following factors:

1. Extremes of temperature
2. Nature of rocks
3. Structure of rocks
4. Frost
5. Wind
6. Slope of lands

(a) Difference between chemical and mechanical weathering:

(b) Difference between weathering and denudation:

Mechanical weathering refers to the disintegration of rocks by the elements of weather such as heat, frost and wind. Mechanical weathering does not change the composition of rocks.

Mechanical weathering depends on the following factors:

1. Extremes of temperature
2. Nature of rocks
3. Structure of rocks
4. Frost
5. Wind
6. Slope of land

Denudation is a dynamic process that includes disintegration and decomposition of rocks as well as the wearing away of the rocks. It is a two-fold process, both destructive and constructive. It is destructive as it breaks down the rocks. It is constructive as it transports the eroded material and deposits it at some other place.

Biological weathering refers to the breakdown of rocks by living organisms. This can occur through various means, such as root growth, burrowing activities, acid secretion by certain organisms, and the physical actions of organisms like mosses and lichens.

Biological weathering is caused by following agents:

1. Plants — The roots of plants grow into the cracks and crevices in search of water or nourishment. They exert pressure on rocks and break them. Sometimes, the roots release carbonic acid which changes the chemical composition of minerals and help in weathering.
2. Animals — Burrowing animals cause loosening or weakening of the rocks and expose them to other agents of weathering.
3. Human activities — Construction of roads, mining, farming and deforestation lead to weathering of rocks.

The various factors responsible for weathering are:

1. Extremes of temperature
2. Frost
3. Wind
4. Slope of land
5. Water
6. Oxygen
7. Acids
8. Plants
9. Animals
10. Human activities

The changes in temperature during summer and winter and also during day and night (in case of arid region), cause expansion and contraction in the rock surfaces. This causes the rocks to break-up or peel off. Rectangular rock blocks are thus, gradually rounded by splitting away of sharp corners. This process is called exfoliation.

Climatic factors like rainfall, wind, frost and extremes of temperature affect weathering.

• Wind has sand particles that erode the softer rocks.
• Water or frost when present in crevices of rocks exert pressure on rocks and breaks it.
• Extreme high temperature leads to expansion in rocks and extreme low temperature causes contraction. The repetitive expansion and contraction lead to weathering of rocks.
• Rain, snow and other forms of precipitation contribute to weathering by providing water that can dissolve and transport minerals and rocks. For example, water that contains dissolved carbon dioxide can create a weak acid that can dissolve limestone and erode them leading to the formation of caves and other features.

The thin topmost layer of the earth's crust, made up of fine rock particles and organic matter is termed as soil. It is a layer of unconsolidated materials at the earth's surface.

Some methods to prevent soil erosion are:

1. Afforestation
2. To check overgrazing
3. Constructing dams and barrages
4. Improve techniques of agriculture

Two importance of rocks are:

1. Rocks are used as building material.
2. Rocks supply precious metals like gold and silver.

On the basis of their formation rocks are classified into following three types:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

Two types of igneous rocks are:

1. Intrusive rocks — E.g., granite.
2. Extrusive rocks — E.g., Basalt.

Igneous rocks were formed as the molten magma solidified either inside the earth or on the surface of the earth

Sedimentary rocks are formed from sediments, accumulated over long periods in seas, lakes, rivers and glaciers. They are deposited in distinct layers or strata. Therefore, they are also called stratified rocks.

Sedimentary rocks are divided into three classes according to their formation:

1. Mechanically formed sedimentary rocks
2. Organically formed sedimentary rocks
3. Chemically formed sedimentary rocks

Examples of each type of sedimentary rocks are:

1. Mechanically formed sedimentary rocks — E.g., Sandstone, Shale, Clay.
2. Organically formed sedimentary rocks — E.g., Coal, Limestone, Coral.
3. Chemically formed sedimentary rocks — E.g., Rock Salt, Gypsum, Dolomite .

When igneous or sedimentary rocks experience intense heat and pressure, often due to geological forces like tectonic movements or volcanic activity, they undergo a complete change in form and characteristics features. These transformed rocks are referred to as metamorphic rocks.

Oxidation

Mountainous regions

Erosion

Igneous

Igneous

Sediments in layers

Remains of marine animals

Metamorphosis

Metamorphosis

Soil

Wind speed

Terrace farming

Solution

Denudation

Coal