FACTORS OF ROCK WEATHERING
Weathering of rocks is as a result of several factors, and these factors are responsible for determining the rate and the nature of the weathering processes. The factors are mainly environmental because they are within the environment where the affected rocks are found. These factors include:
The climatic factors
The aspects of the climate that have to do with rock weathering are temperature and precipitation. The temperature affects weathering in the sense that high temperatures increase the rate of chemical reactions, which quicken the process of chemical weathering. Similarly, if high temperature is accompanied by alternate low temperature within the same vicinity, the processes of physical weathering are accelerated. Temperature therefore causes and accelerates both physical and chemical weathering.
Where there is high precipitation, the rate of both physical and chemical weathering processes are accelerated. This is because there is availability of water within the rocks to cause the decay of rock minerals thereby causing breakdown of the rock chemically. The freezing of the water increases the sizes of rock crevices and this causes the rock to breakdown physically. Precipitation, just as temperature causes and accelerate both physical and chemical weathering All these lead to rock breakdown.
The biotic factors
The biotic factors relate to the effects of plants and animals in causing and accelerating the processes of both chemical and physical weathering. The role of the plant cover is that of providing organic acids which hasten the process of rock decomposition. The plant roots also widen cracks in the rocks as they grow and this tears the rocks apart. Animals such as micro-organisms facilitate humification and libration of acid radicals which cause rock decomposition. Also, the same micro — organisms serve as translocation agents. Activities of other animal species such as rodents, rabbits and earthworms burrow holes in sedimentary rocks and this weaken such rocks and facilitate the rate of their weathering. According to Emielu, S.A. (2004), oxygen passes through the burrows or holes and initiates the oxidation process of weathering in such rocks. Water also sips down into the rocks through these burrows and this causes the decomposition of the rock.
The various activities of man such as construction work, cultivation, quarrying, farming activities and movement of animals initiate physical breakdown of both rock types.
The Geologic factors
The various rock types have varying characteristics. Igneous rocks contain minerals and some of these minerals are susceptible to alteration. When the minerals are altered, this facilitates and promotes early disintegration of the rocks. Another characteristic of rocks is the rock texture. Any rock with coarse texture easily disintegrates. Whereas, rocks which are of fine-grained texture take longer time before they are affected by either physical or chemical weathering processes. The fissures, that is, the joints, faults (cracks), which either develops along boundaries or in the body of the rock, promote the weathering process of such rocks. This happens with time when these fissures, joints and cracks gradually become open and weaken the’ rock which eventually breaks along these openings.
The Geomorphic factors
The geomorphic factors of rock weathering relate to the land surface stability of an area, in areas where the landform or the topography is stable, especially for a long time, the rate of weathering process is very low. The effect of this low weathering rate is what leads to deep, weathering profiles. A gentle slope or topography is the type of landform where low rate of weathering takes place and where the deep weathering profile is enhanced.
Topographic factors
The topographic factors relate to the slope of the land. The position of the slope of the topography determines the rate of weathering along such slope. Similarly, the angle of the slope is another factor that determines the process of weathering of such a slope. Also, the slope form and the slope aspect determine the nature and rate of weathering along such slope.
Generally, the topographic factors are factors that promote physical weathering more than chemical weathering.
Weathering of igneous rocks
The weathering of all the different types of rocks is carried out by the general factors of rock weathering. These factors have been discussed under the heading, “factors of rock weathering”. The rock types could be igneous under which we have granite and basalt, the sedimentary or metamorphic. The controlling weathering factors are the same. When discussing the weathering factors these are the same. Whenever discussion is centered on the weathering of any of these rocks, reference should always be made to the particular rock-type.
Products of weathering
The weathering of rocks always results in three groups of end products. The end products of each group differ from the others.
- There are the coarse debris which are mainly sand and quartz. There are also big boulders of different sizes and shapes which scatter at the foot of an igneous rock that is being weathered. The boulders scatter at the foot because they roll down from a high elevation where the main rock attains some height. These boulders form the most conspicuous products of weathering.
- There are products which are made up of silt particles and clay minerals. This group of products is easily removed by agents of erosion such as wind and running water because the products are lighter.
- The third group of products is known as soluble substances. The products also referred to as residual material or sciprolite. This consists of substances which are removed either in solution or in colaidal suspension.
The nature of the products of weathering are usually a reflection of the type of weathering. The products of physical weathering are mainly the coarse products. Products of chemical weathering on the hand are fine grained products.