ICSE Class 6 Geography Chapter 9 Weathering

Chapter Nine: Weathering

Syllabus

Weathering and Denudation

Meaning, types and effects of weathering. Types: Physical Weathering - block and granular disintegration, exfoliation. Chemical Weathering - oxidation, carbonation, hydration and solution; Biological Weathering - caused by humans, plants and animals.

The internal forces operating in the interior of the earth are described as 'Constructive Forces' because they lead to the formation of landforms on the surface of the earth. Some forces also operate on the surface of the earth. They are known as exogenic forces (exo in Greek means outside). These external forces act on the surface of the earth through several processes such as weathering, mass wasting, erosion, transportation, deposition and formation of soils. These forces derive their energy from the sun. In all these cases, climate acts as the driving agent.

Weathering

Weathering is the process of breaking down of rocks but not its removal. It is described as disintegration or decomposition of a rock in size by natural agents at or near the surface of the earth. It is so called because the disintegration or decomposition of rocks take place only by being exposed. It is the change in weather conditions like temperature, moisture, precipitation that disintegrate or break down the rocks. Weathering cannot take place on the moon's surface where such conditions do not exist. The footprints left on the moon by astronauts will last for millions of years. Thus, weathering is the basic process and an essential phase of denudation as described above.

Chief Characteristics of Weathering

1. Weathering involves disintegration or decay of solid rocks.

2. It depends on climatic elements and on the characteristics of rocks. For example, the rock's chemical composition, hardness, texture and its permeability (allowing water to penetrate) determines the weathering pattern.

3. Weathering affects the surface of the earth.

4. Weathering causes formation of soil.

5. Weathering involves not one but several processes to cause disintegration of rocks.

6. Weathering changes hard massive rocks into finer materials.

7. It prepares rock materials for transportation by agents of gradation.

Types of Weathering

Weathering, as already stated, is a static process. The disintegration of rocks may occur through physical forces, chemical forces or by roots of plants, animals, insects, etc. Weathering is of three types - physical, chemical, and biological.

Physical Weathering

Physical weathering is also known as Mechanical Weathering. It involves rock disintegration without any change in the chemical constituents of the rock. Main factors responsible for physical weathering are temperature changes, moisture, frost action and wind.

Weathering due to changes in temperature is more rapid in deserts. It is because temperature changes are the sharpest in a desert.

Block Disintegration

The sudden rise and fall of temperature causes expansion and contraction of rocks. The repeated action causes their breakdown. At night the temperature suddenly falls and during the day it rises suddenly. This leads to tension and splitting of rocks known as block disintegration.

Granular Disintegration

Since rocks are made up of different minerals, weathering may also reduce rocks to pieces and fragments. This type of disintegration is called granular disintegration. The granular disintegration may also take place in cold places and at high altitudes. The cracks and joints of rocks are filled with water during the day and at night the water freezes and expands in the cracks.

Water collects in cracks.

Water freezes and expands in the cracks.

Exfoliation

Sudden temperature changes may also cause into which water penetrates. Most rocks are not good conductors of heat. The sudden contraction and expansion creates stress in the outer layer known as exfoliation.

Chemical Weathering

In chemical weathering, some or all of the minerals in the rocks suffer decay or alteration by such agents as water, oxygen, carbon or various organic acids. It leads to decomposition of rocks and minerals as chemical reaction transform and minerals into different chemical combinations.

The chemical weathering of rocks is accomplished in a number of ways and each one is given a name to distinguish the different processes involved. Four main processes are - oxidation, carbonation, hydration and solution.

Oxidation

It means the reaction of minerals in rocks to atmospheric oxygen. The rainwater also contains atmospheric oxygen. On coming in contact with iron-compounds in rocks, the iron starts rusting or crumbling. The colour of the rock also changes to red, yellow or brown.

Carbonation

Many mineral constituents of rocks such as carbon dioxide while coming in contact with water produce a slight acid effect on rocks. This action dissolves most calcareous rocks such as limestone, gypsum, marble and limestone. However, this action should not be mistaken for solution. In the case of carbonation, rainwater converts calcium carbonate into calcium bicarbonate which is taken away by the ground water.

Hydration

In this process, expansion of minerals occurs on coming into contact with rainwater. These minerals become heavy and start disintegrating. This action, though can be termed as physical or mechanical, its effect is that of a chemical process. Rocks like feldspar are converted into kaolin.

Solution

It is the process in which minerals in the rocks dissolve in water. All rocks are subjected to solution to some extent. The rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure. For example, sedimentary rocks have pore-spaces between the grains in which air and water can lodge and thus attack the rocks. Some rock minerals such as salts are more soluble in water than others. For example, rainwater causes chemical disintegration of rock-salt and gypsum.

Biological Weathering

It is also known as Organic Weathering. Its main agents are animals, insects, plants and man. In all the cases, however, both physical disintegration and/or chemical decomposition are involved. This is because all biological matter is made up of oxygen and water, the two substances that set off reaction of minerals in rocks.

Weathering In Different Climatic Regions

From the processes of weathering discussed above, it is clear that climate is the most important factor. For example physical weathering is more rapid in desert climates and chemical weathering occurs in moist and cold climates.

In Equatorial climates, humidity and temperature are constantly high. Weathering especially chemical weathering is more active in these regions.

In Tropical climates, there is a marked dry season and wet season. Differences in heating leads to consistently high rate of evaporation. During wet season, precipitation of oxides of iron and aluminium takes place upon rocks. This promotes formation of laterite soils.

In Dry climates or deserts, mechanical weathering is most dominant due to high diurnal range of temperature.

In Mid-latitudes or Temperate Climates, the most powerful agent of weathering is the frost action. In limestone areas, 'Solution' of chemical weathering operates on a large scale.

In Polar climates, physical weathering due to frost action is more prevalent. Chemical and biological weathering in these areas is negligible. However, recent studies have shown that the melting water of polar regions contains more carbonic acid content because carbon dioxide is more soluble at low temperatures than at high temperatures. This suggests that chemical weathering is more active in polar regions.

Agents of Biological Weathering

Humans: Man is the most active agent of both physical and chemical weathering of rocks. For example: mining, excavations, construction of roads, buildings, etc. cause physical disintegration of rocks. Agriculture, dumping of chemicals and wastes underground promote both physical and chemical weathering.

Plants: Roots of large trees reach deep into rocks and cause physical disintegration due to pressure. Most of vegetation, however, prevents disintegration of rocks because it binds the surface layer and does not allow exposure of rocks beneath to the elements of weathering.

Animals and Insects: Animals like rabbits, worms, moles and insects bring large quantities of fine material to the surface. Burrowing animals help to loosen the surface materials around the rocks facilitating their physical disintegration. Upon death, the decaying animals also provide many chemicals and acids for rock disintegration.

Teacher's Note

Weathering is happening all around us - from rust forming on metal fences to cracks appearing in concrete sidewalks - and it helps explain why old buildings and monuments gradually lose their original appearance over time.

This is a preview of the first 3 pages. To get the complete book, click below.