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Civil Engineering Engineering Geology and Seismology Weathering




It is a general term and applied to the combine action of all processes causing rocks disintegration physically and decomposition chemically.


It is the process of weakening rock and producing a rock waste or soil cover. Weathering occurs when rocks and minerals comes in contact with the atmosphere, organic life and suicidal water. Essential feature of weathering is that it affects rocks in-situ (in place) and no transportation of material is involved.

Factors responsible for the weakening of rock are:

  1. A part of rainfall always snips into the soil on the covered rocks and looses the rock particle by dissolving it.
  2. Frost action also takes place in the for-spaces of rocks and causes disintegration and splitting of rocks into fragments.
  3. Chemical reactions are also taking place in rocks internally and causing rock-loosening.
  4. Slow geological process of construction and destruction is going on the surface of earth for millions of years because earth is old enough and has been completely leveled by weathering and erosion. Factors affecting rock weathering.

Rock weathering is greatly affected by the following parameters.


Rock structure and composition:

Rock structure includes joints, beding planes, folds, fractures, and faults. Rocks containing these structures will be easily subjected to weathering.

1. Rock composition:

There are certain elements which are included in rock composition. Some rocks will weather quickly and some slowly e.g. acidic rocks weather more quickly than basic ones.

2. Climate:

It includes the meteorological elements effect on rocks such as moisture, temperature, and wind and air pressure. Climate determines whether physical or chemical weathering will be more active and speedy.

3. Topography and vegetation

Topography directly effects weathering by exposing rocks to the temperature or sun and wind. The elevated areas will be affected more and low level areas will be affected less.

4. Vegetation

Surface covered by the vegetation are protected from weathering but bare surfaces are weathered to great extent.

  • Thin root plants protects weathering
  • Thick root plants accelerates weathering

Types of weathering:

There are two types of weathering:

  1. Physical weathering
  2. Chemical weathering

1. Physical weathering:

In the mechanical disintegration and breaking of the rocks to form particles of smaller size. In physical weathering, the composition of the weathered products remains unchanged. Following are the different agents of physical weathering:

  1. Frost action
  2. Heating and cooling
  3. Organisms

a. Frost action:

Freezing of water in the cracks of rocks tends to disintegrate them because volume of water increases 1-11 times of its actual volume. It exerts a great pressure on the wall having cracks. By this process, angular fragments of rocks are broken off from the main body of the rock, causing physical weathering.

b. heating and cooling:

Heating and cooling of rock masses occur due to daily changes in temperature. Heating of rocks causes expansion and cooling cause contraction of rocks. This repeated heating and cooling helps to develop cracks in rocks and the rock will be disintegrated.

c. Organisms:

Plants and insects like earthworms, ants, termites and snakes play an important role in physical weathering because they convert rock into soil to get their food. Plants also grow in joints and cracks of the rocks and push them further apart. Similarly, man also breaks rocks by making roads, constructing dams and reservoirs by drawing tunnels and mines etc.

2. Chemical weathering:

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