Soil Formation

Soils are formed by either physical disintegreation or chemical decomposition od rocks.

Physical Distintegration. Physical disintegration or mechanical decomposition weatheringof rocks occurs due to the following physical processes :

1. Temperature changes. Different minerals of a rock have different cofficint of thermal expansions. Unequal expansion and contractions of these minerals occure due to temperature changes. when the stresses induced due to such changes are repeated many times, the particles get detached from the rocks and the soils are formed.
   
2. wedging action of ice. water in the pores and minute cracks of rocks gets frozen in very cold climates. As the volume of the ice formed is more than that of water, expansions occurs. Rocks get broken into pieces when large stresses develop in the cracks due to wedging action of the ice formed.
   
3. spreading of roots of plants. As the roots of plants and shrubs grow in the cracks and fissures of the rocks, forces act on the rock. The segments of the rock are forced apart and disintegration of rocks occurs .
   
4. Abrasion. As water, wind and glaciers move over the surface of rock, abrasion and scouring takes place. it results in the

In all the processes of physical disintegration, there is no change in the chemical composition. The soil formed has the properties of the parent rock. Coarse-grained soils, such as gravel and sand, are formed by the process of physical disintegration.

Chemical Decomposition.when chemiacal decomposition of weathering of rocks takes place, original rock minerals are transformed into new minerals by chemical reaction. the soil formed do not have the properties of the parent rock. the follwing chemical processes generally occure in nature.

1. Hydration. in hydration. water combines with the rock minerals and results in the formation of a new chemical compound . The chemical reaction causes a change in volume and decomposition of rock into small particles.
   
2. Carbonation. it is a type of chemical decomposition in which carbon dioxide in the atmosphere combines with water to form carbonic acid. The carbonic acid reacts chemically with rocks and causes their decomposition.
   
3. Oxidation. Oxidation occurs when oxygen ions combine with minerals in rocks. Oxidation results in decomposition of rocks. Oxidation of rocks is some what similar to rusting steel.
   
4. Solution. some of the rock minerals form a solution with water when they get disolved in water. Chemical reaction takes place in the solution and the soils are formed.
   
5. Hydrolysis. It is a chemical process in which water gets dissociated int H+ and OH- ions. The hydrogen cations H+ replace the metallic ions such as calcium, sodium and potassium in rock minerals and soils are formed with a new chemical composition.

Chemical decomposition of rocks results in the formation of clay minerals. these clay minerals impart plastic properties to soils. Clavey soils are formed by chemical decomposition 

Reference: INTRODUCTORY SOIL ENGINEERING