Weathering

Weathering is the group of processes whereby rocks and mineral, on exposure to weather, change in character, disintegrate, decompose, and synthesize new compounds. It generally involves two stages or phases.

The first stage involves the accumulation of parent material for the soil, while the second involves the formation of soil from the parent material. Parent materials are formed when rocks are broken down into smaller but bulky fragments. This is generally described as rock weathering.

Weathering is grouped into two types or categories: physical weathering and chemical weathering – each consisting of a number of distinct processes. Physical weathering, which results mainly in visible changes to the structure of rocks and minerals, can be observed through physical analyses of rocks, while chemical weathering, which mostly affects the chemical structure of rocks and minerals, is better observed through chemical analyses of rocks and soils. The changes brought about by weathering are accompanied by a continued decrease in particle size, and the release of soluble constituents, most of which are subject to loss in drainage waters.

Weathering of soil materials precedes soil development in hard rock and accompanies soil development in soft rock. It is a continuous process during soil development and it takes place both within and below the solum.

Weathering below the solum is called geochemical weathering and that which takes place within the solum is called pedochemical weathering.

Weathering, which expresses the physical and chemical disintegration of rocks and minerals, is therefore a combination of destruction and synthesis.

Weathering is accomplished in two ways:

a)           Physical and Mechanical changes:

Otherwise known as physical weathering, it is enforced by factors which exert mechanical forces on rocks e.g. temperature variations, frost actions, penetrating plant roots, burrowing animals etc. Over time, the actions of these forces would disintegrate the rocks, breaking them down into finer sized particles. The grains that dominate the coarser fraction of soil texture (sands and silt) are formed through this method. Where physical weathering is dominant, the resultant grains will retain much of the chemical composition of the rocks from which they were formed.

b)           Chemical changes:

Also known as chemical weathering, this process involves the decomposition, dissolution and loosening of minerals by water (H₂O), Oxygen (O₂), Carbon dioxide (CO₂) and living organisms – their secretions and products of decay. It mostly affects the chemical composition and structure of rocks and minerals and it can completely transform the chemistry of a mineral, creating a new mineral altogether – a secondary mineral. Chemical weathering is therefore responsible for the formation of the finer fraction of soil texture (mostly the clays).

Weathering is important for a number of reasons being:

i)             It is the major process by which mineral soils (soils containing less than 15% organic matter) are formed.

ii)            It facilitates the derivation and release of plant nutrients into the soil, thereby making them available for plant use.

iii)          It helps Soil Scientists to access the natural fertility status of soils through the assessment of the presence or absence of weatherable soil minerals. This also indicates the stage of soil development.

Further studies in this theme:

Physical Weathering

Chemical weathering