World Geography Part 9 – Weathering


  1. Weathering is an action of elements of weather and climate over earth materials.
  2. There are a number of processes within weathering which act either individually or together to affect the earth materials in order to reduce them to fragmental state.
  3. Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
  4. As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
  5. Weathering processes are conditioned by many complex geological, climatic, topographic and vegetative factors.
  6. The climate is of particular importance.
  7. There are three major groups of weathering processes
    1. Chemical
    2. Physical
    3. Biological weathering processes

Chemical weathering process

  1. A group of weathering processes include in this
    1. Carbonation
    2. Hydration
    3. Oxidation
    4. Reduction
  2. They act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids.
  3. Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions
  4. Over and above the carbon dioxide present in the air, decomposition of plants and animals increases the quantity of carbon dioxide underground.

Steps involved in chemical weathering


  1. When something is dissolved in water or acids, the water or acid with dissolved contents is called a solution.
  2. This process involves the removal of solids in solution and depends upon the solubility of a mineral in water or weak acids.
  3. This process involves the removal of solids in solution and depends upon the solubility of a mineral in water or weak acids.
  4. On coming in contact with water many solids disintegrate and mix up as a suspension in water.
  5. Soluble rock-forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.
  6. So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
  7. Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones are soluble in water containing carbonic acid (formed with the addition of carbon dioxide in water)
  8. Are carried away in water as a solution. Carbon dioxide produced by decaying organic matter along with soil water greatly aids in this reaction.


  1. Carbonation is the reaction of carbonate and bicarbonate with minerals and is a common process helping the breaking down of feldspars and carbonate minerals.
  2. Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
  3. Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave formation.


  1. Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock
  2. Calcium sulphate takes in water and turns to gypsum, which is more unstable than calcium sulphate
  3. This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
  4. Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing.
  5. The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration.

Oxidation and Reduction

  1. Oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
  2. Oxidation occurs where there is ready access to the atmosphere and oxygenated waters.
  3. The minerals most commonly involved in this process are iron, manganese, sulphur etc.
  4. In the process of oxidation, rock breakdown occurs due to the disturbance caused by the addition of oxygen.
  5. The red colour of iron upon oxidation turns to brown or yellow.
  6. When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place.
  7. Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
  8. The red colour of iron upon reduction turns to greenish or bluish grey.
  9. These weathering processes are interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.

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