Introduction

• Refers to the process of disintegration (by physical or mechanical process) and decomposition (by chemical process) of rocks and minerals which are brought about by physical agent and chemical processes, leading to the formation of Regolith (unconsolidated residues of the weathering rock on the earth’s surface or above the solid rocks).

Products of Weathering:

The process of weathering can result in the following three outcomes on rocks and minerals:

(1). the complete loss of particular atoms or compounds from the weathered surface.

(2). the addition of specific atoms or compounds to the weathered surface.

(3). A breakdown of one mass into two or more masses, with no chemical change in    the mineral or rock.

Physical weathering

Mechanical disintegration of exposed surface of rocks; accomplished by following process:

• Heat & cold: Expansion& contraction of non-conductor rocks-loosens the rocks to crumble.
• Freezing & thawing: Freezing creates massive force which enlarges crevices; Alternate freezing & thawing breaks off fragments.
• Grinding & rubbing: Grinding & rubbing of moving rocks against each other by water, wind, glaciers results in rock disintegration.
• It is slow process; causes to change only in size but hasten chemical weathering by exposing more reactive surface.

Chemical weathering

• Involves the alteration of the chemical and mineralogical composition of the weathered material.
• The most common chemical weathering processes are:

a. Hydration:

b. Hydrolysis

c. Solution

NaCl+ H₂O  <——>   Na⁺+ Cl^–  (dissolved ions with water)

d. Oxidation

4 FeO (Ferrous oxide)  +  O2 ——->2 Fe2O3 ( Ferric oxide)

4 Fe3O4(Magnetite)     +   O2——–>6 Fe2O3 (Hematite)

e. Reduction

2Fe2O3 (Hematite)  – O2 —– > 4FeO( Ferrous oxide) -reduced form

f. Carbonation

2 H2O + CO2 —–> H2CO3

CaCO3+ H2CO3——- > Ca(HCO3)2

(Calcite) 

Biological weathering

• Biological agents (veg., animal & human act.) accelerate P & C weathering.
• Plant roots widen cracks & crevices in the rocks-make fragments.
• Farming, mining, constructions etc. hasten weathering process.
• Burrowing animals (Earthworm) exposes subsoil to weathering (heat, water, wind)
• Soil microbes continuously weather rocks by their biochemical process & enzymatic action.