WEATHERING
- Weathering is an action of elements of weather and climate over earth materials.
- 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.
- Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
- As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
- Weathering processes are conditioned by many complex geological, climatic, topographic and vegetative factors.
- The climate is of particular importance.
- There are three major groups of weathering processes
- Chemical
- Physical
- Biological weathering processes
Chemical weathering process
- A group of weathering processes include in this
- Carbonation
- Hydration
- Oxidation
- Reduction
- 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.
- Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions
- 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
Solution
- When something is dissolved in water or acids, the water or acid with dissolved contents is called a solution.
- This process involves the removal of solids in solution and depends upon the solubility of a mineral in water or weak acids.
- This process involves the removal of solids in solution and depends upon the solubility of a mineral in water or weak acids.
- On coming in contact with water many solids disintegrate and mix up as a suspension in water.
- Soluble rock-forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.
- So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
- 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)
- Are carried away in water as a solution. Carbon dioxide produced by decaying organic matter along with soil water greatly aids in this reaction.
Carbonation
- Carbonation is the reaction of carbonate and bicarbonate with minerals and is a common process helping the breaking down of feldspars and carbonate minerals.
- Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
- Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave formation.
Hydration
- 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
- Calcium sulphate takes in water and turns to gypsum, which is more unstable than calcium sulphate
- This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
- Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing.
- The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration.
Oxidation and Reduction
- Oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
- Oxidation occurs where there is ready access to the atmosphere and oxygenated waters.
- The minerals most commonly involved in this process are iron, manganese, sulphur etc.
- In the process of oxidation, rock breakdown occurs due to the disturbance caused by the addition of oxygen.
- The red colour of iron upon oxidation turns to brown or yellow.
- When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place.
- Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- The red colour of iron upon reduction turns to greenish or bluish grey.
- These weathering processes are interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.