WEATHERING | Webbpedia

Weathering is the disintegration and decay of rocks which is caused by forces of weather such as frost, rain and temperature changes. It can also be defined as the process by which rocks are broken down into smaller fragments by weather forces/atmospheric forces.

It is a process of rocks breaking and decaying which are caused by climate factors (frost action, rain water and temperature) the nature of rock, relief and living organisms.

TYPES OF WEATHERING

There are three types of weathering: physical, chemical and biological weathering.

PHYSICAL/MECHANICAL WEATHERING

This is the process by which rocks are broken down by the forces of weather without any change in chemical composition of the rocks.

It takes place in three major ways:

Alternate heating and cooling: this process is more prominent in regions such as desert which experience daily extremes of temperature. During the day, the rocks are heated intensely by the scorching sun.

The outer parts which are more intensely heated also expand more and tend to pull away from the inner part. At night when temperature stops rapidly and contrast more rapidly than the inner. When rocks are subjected to this kind of daily stress for a long time, three things may happen.

The outer parts begin to peel off like onion a process known as Exfoliation which leads to the formation of rounded features known as Exfoliation domes.
Well-joints or bedding-planes. The rocks are then broken into large regular blocks, a process known as block disintegration.
Rocks made up of different minerals e.g. granite made up of quartz, mica and feldspar are broken up into small pieces in form of large grains, a process known as granular disintegration.
Alternate wetting and drying: this takes place in tropical regions where heavy rains saturate the rocks and then the hot sun quickly dries them again. Repeated wetting and drying may also affect coastal rocks which are subjected to tides and wares. When the rocks are wet, the absorb water especially the outer layer or part which then expands. When the rocks dry, they contract. If this process is repeated over a very long time, stresses build uo which makes the surface layers to peel off.

iii. Alternative freezing and thawing: in temperature regime, what is at times called frost action is more effective in breaking down rocks. When temperature drops sufficiently at night or during winter, the water in cracks will freeze. As it freezes, it expand by about 10% of its normal volume, thereby exerting more pressure on the walls of cracks and trying to force the rocks apart. When the temperature increases sufficiently, the water thaws. Repeated freezing and thawing is capable of widening and deepen cracks and crevices and leading to the breaking of rocks.

CHEMICAL WEATHERING

This is the decomposition or decay of rocks involving chemical processes or reactions which leads to the changes in chemical composition and coherence of the affected rocks.

The following are the major chemical weathering processes: (i) Solution (ii) Oxidation (iii) Hydrolysis (iv) Hydrate (v) Carbonation

Carbonation: rain water passing through the atmosphere absorbs carbondioxide which produces carbonic acid. This weak acid is capable of dissolving rocks composed of calcium carbonate such as limestone. The limestone dissolved is removed in solutions by running water or percolating water passing through joints or dripping through underground caves.

Hydration: certain rocks are capable of absorbing water into their structure. This takes them to swell, they are therefore weakened and can easily breakdown. The rocks may fracture as internal stresses are set up within them.

Oxidation: this is the reaction that occurs when additional oxygen is taken up by a mineral compound. Oxidation occurs when rocks are exposed to oxygen in air or water. The simplest and most easily recognisable process of oxidation is when iron in a ferrous state is changed by addition oxygen into a ferric state. The rock or soil which might have been blue or grey in colour is dissolved into reddish brown. A process better known as rusting (this occur when oxygen combines with minerals).

Solution: this is a simple chemical process some minerals e.g. rock salt are soluble in water and simply dissolve when it is applied. The rate of solution or dissolution increases with the degree of acidity of water. It is the process by which rain water attacks and dissolves rock salts, calcium carbonate of limestone. Thereby, widening cracks.

Hydrolysis: this process involved by hydrogen combining with certain metals irons that is the water and the mineral then give rise to the formation of different compound.

BIOLOGICAL WEATHERING

This is the weathering process involving the activities of living organisms (plants, animals or even man himself).

Plants and animals also help in the weather process of rocks breaking. Roots of trees sometimes grow in cracks and the roots penetrate through. As they grow, they are as wedge and cause piece of rocks to breakdown from main mass.

Burrowing by animals like earthworm also helps to loosen the soil. The activities of man in read construction, mining and farming also contribute to biological weathering.

Factors that influence weathering of rocks

The five major factors influencing weathering rocks are: