WEATHERING | DEFINITION, FACTORS, TYPES, PROCESSES, FEATURES AND IMPORTANCE
FACTORS THAT INFLUENCE THE RATE AND NATURE (TYPE AND CHARACTER) OF WEATHERING.
THE NATURE OF THE PARENT ROCK.
Mineralogical composition of the rock: the mineral composition of the rock is very important in determining the rate and character of weathering from rock to rock e.g. igneous rocks are more prone to chemical weathering because they are formed under conditions very different from those operating on the earth’s surface and they are therefore not chemically stable on the earth’s surface.
So when rocks come into contact with water, chemical weathering sets in at a very fast rate when exposed onto the earth’s surface. Examples of such rocks which can easily be weather are basic igneous rocks with low silica content.
On the other hand, rocks with high silica content like granitic rocks are more stable and react to chemical weathering at a slow rate.
Rock jointing
rock jointing also influences the rate and nature of weathering. This is because joints are weak lines which are exposed by agents of weathering to cause rock break down. Water can easily penetrate thus enhancing chemical weathering processes of hydration to take place at a faster rate. Physical weathering processes of freeze thaw action and thermo-expansion are very fast in rocks which are poorly jointed.
Similarly, biological weathering will also take place at a fast rate with the penetration of plant roots and animals which will only take place when cracks only exist on the rock. On the other hand, rocks which are well jointed, chemical, physical and biological weathering processes take place at a slow rate.
Rock color
rocks which are dull and dark colored absorb heat and therefore expand more quickly and with continuous expansion and contraction, cracks are then created and eventually lead to the fracturing of the rocks at a very fast rate by thermal expansion and block disintegration.
On the other hand, rocks which are light and shiny are less affected by weathering processes of thermo-expansion because they reflect heat. Therefore, dark colored rocks are weathered much faster than shiny rocks.
Rock hardness
hard rocks like quartzite are more resistant to weathering processes of carbonation and hydration. On the other hand, rocks which are soft like clay are weathered at a faster rate by chemical weathering processes of oxidation.
Rock permeability
rocks which are permeable like sandstone allow water to penetrate though them. Such rocks that allow water to penetrate though them are weathered at a faster rate by chemical weathering through the process of carbonation. On the other hand, rocks which are impermeable are weathered by chemical weathering processes at a slow rate.
Rock solubility
rocks which are soluble in water like rock salts dissolve in water at a fast rate. So rocks which are soluble in water are easily weathered by chemical weathering processes of solution.
Rock texture
rocks with rough texture can easily allow water to collect on such rocks on giving way for chemical weathering processes of hydration to take place at a faster rate. On the other hand, rocks which are smooth are weathered at a slow rate by physical and chemical weathering processes.
Influence of climate
Climate also determines the rate and nature of weathering. Rainfall and temperature are also the major elements that affect the rate and nature of weathering. Areas with equatorial climate are characterized by high rainfall totals of about 1500mm. this rainfall is high throughout the year with double maxima (two rainfall peaks/seasons).
These rainfall amounts facilitate chemical weathering processes since they require water as a medium of chemical reaction.
Chemical weathering is common in areas which receive high rainfall totals like Northern shores of Lake Victoria, slopes of mountain Elgon and Kilimanjaro (humid areas) among others and very slow in areas with little or no rainfall.
The humid areas are also characterized by high temperatures which acts as a catalyst to chemical weathering processes. Therefore, chemical weathering is more dominant in humid areas i.e. areas with high rainfall and temperatures.
In savannah regions (transition zones), the alternating wet and dry seasons within these areas give way to both chemical and physical weathering to take place. Chemical weathering is more active during the wet season and physical weathering is more dominant in the dry season.
On the hand, arid and semi- arid areas that receive low amounts of low rainfall of less than 500mm per annum and a prolonged dry season characterized by very high temperatures of above 30 0 C provide suitable conditions for physical weathering processes of thermal expansion, block disintegration and granular disintegration. This therefore means that physical weathering is more dominant in such areas like in the Ankole-Masaka dry corridor, north Eastern Uganda, Eastern Kenya in the Turkana and Masai land among others.
For mountainous areas where the temperatures are low like in areas of mountain Rwenzori, Mountain Kenya and Kilimanjaro, physical weathering processes of frost action / freeze and thaw is dominant. In this case, water freezes in the cracks and since ice is bigger in volume, cracks are widened and deepened leading to eventual breakdown of rocks therefore physical weathering is very fast in areas with low temperatures.
Effect of plants and animals (living organisms)
Living organisms like plants act as agents of weathering by the action of their roots in the process of sapping water underground where they dig dip and create cracks and keep on expanding the joints within the rocks in that way. In this process they therefore facilitate physical, chemical and biological weathering. However, thick vegetation may act as a protective layer against physical weathering and it may slow down the removal of the weathered layers.
Animals such as rats, termites etc. loosen the compactiveness of the rocks thus causing physical fracturing of the rocks. Water easily penetrates through the rocks giving room for chemical weathering processes of hydration. Therefore, where animals exist, physical, biological and chemical weathering take place at a faster rate than where animals are absent.
When living organisms (plants and animals) die, they decompose into humus and organic matter. With the addition of water, the humus turns into humic acid which decomposes the rocks plus organic acids.
Therefore, in such areas, chemical weathering processes of chelation take place at a faster rate.
Man himself does his activities of rock blasting with explosives, mining, road construction among others facilitate physical break down of rocks (physical weathering/ fracturing). He also aids chemical weathering processes through his activities of application of irrigation, fertilizers, spraying with pesticides, insecticides, herbicides etc. since they involve the element of water. Therefore, there is chemical weathering in such areas.
Relief
Relief also determines the rate and type of weathering. The rate at which weathering takes place is related to the speed at which weathered rocks are removed. On steep slopes, the weathered rocks are quickly removed and the parent rock is exposed to further physical weathering. In other words, steep slopes encourage faster rates of physical weathering because of high rates of erosion that exposes the underlying rocks to further physical weathering.
However, gentle slopes and low lying areas promote the occurrence of chemical weathering due to the high retention capacity of water. In other words, this results into the occurrence of chemical weathering processes of hydration, oxidation, chelation among others at a faster rate because water accumulates on gentle slopes and low lying areas than on the steep slopes.
Time
It takes time for rocks to be broken down by physical and chemical weathering processes. The longer the time the rock has been exposed to the weathering, the more easily such a rock will be weathered and the shorter the time, the more resistant the rock will be to weathering agents.
Natural catastrophes
Natural catastrophes like earth quakes, lightning and thunder, volcanic eruptions etc. result into mechanical or physical breakdown of rocks. These catastrophes can further expose the underlying rocks to both physical chemical weathering.
TYPES OF WEATHERING
physical or mechanical weathering
This is the breaking down of rocks into successively smaller fragments or particles.
It doesn’t involve mineralogical change or change in the chemical bond of the rocks. Its common in arid and semi-arid areas of Ankole-Masaka dry corridor, Karamoja area, northern Kenya and on the lee ward side of mountains like mountain Kilimanjaro, Kenya, Rwenzori and mountain tops which involve temperature changes and have a wide diurnal range and also on the mountains with very cold temperatures where rocks are broken down by frost action.
6 processes of physical/mechanical weathering.
.png "Frost action (freezethaw effect)")
This takes place almost in the same way as block disintegration except that in this process, rocks break down or disintegrate into smaller particles called granules. This occurs when the rocks have different types of minerals of different heating and expansion co-efficient. So the various minerals heat and expand at different rates causing the rock to break into angular blocks.
Aridity shrinkage
This type of weathering occurs in areas experiencing extremes of weather i.e. wet and dry seasons. It occurs when drought succeeds a rainy season such that the rock loses the water that was previously taken up during the rainy season. As a result, the rock crumbles into smaller elongated fragments.
For example, when a sample of clay dries out, it shrinks and its surface becomes filled with cracks hence facilitating its break up and its subsequent removal especially on a slope.
Chemical weathering
The following are 5 processes of chemical weathering
10 FACTORS AFFECTING CHEMICAL WEATHERING
Nature of the parent rock
Mineral composition of the parent rock: some rocks like those having calcium carbonate react with carbonic acids which are due to the combining of rainwater with carbon dioxide in the atmosphere to produce calcium bicarbonate by a process known as carbonation e.g. at Nyakasura. The calcium-bi-carbonate can easily be dissolved in water.
Some rocks have minerals like feldspar which when mixed with water decompose to produce other mineral compounds like potassium hydroxide and aminocilic acids through the process of hydrolysis. However, in the absence of water, feldspar is a very hard element to weathering.
Some rocks have mineral compounds which react with oxygen in the presence of water to form new compounds or oxides through the process of oxidation e.g. ferrous rocks (rocks rich in iron compounds) are turned into brown or red ferric compounds or laterite soils.
Some rocks have minerals that can easily dissolve in water and the solution is carried away leading to the decomposition of the rocks through a process of solution e.g. limestone rocks, rock salts etc.
Jointing of the rock: the presence of joints or cracks increase the surface area for chemical reactions to take place and also allow water to penetrate to the deeper layers of the rocks to chemically weather the rock.
Permeability of the rock: when a rock is permeable, it allows water to penetrate and weather the deeper rock layers through the processes like carbonation, hydration and hydrolysis etc.
Climate:
The nature of the climate experienced in area determines the type of weathering as indicated below;
Rainfall or precipitation provides the water needed for chemical weathering to take place. Many areas in East Africa receive heavy rainfall amounts almost year throughout (equatorial climate). Other areas like the savannah regions receive moderate rainfall and hot temperatures and such humid conditions are conducive for chemical weathering to take place for most of the year.
Areas having hot temperatures for most of the year have physical weathering as the most dominant weathering process. However, most of the humid areas in East Africa have hot temperatures of over 20oC which increase the rate of chemical reactions thus promoting chemical weathering.
Relief:
chemical weathering is more dominant on gentle slopes and low-lying areas as water accumulates and percolates to chemically weather the rock than on steep slopes. However, erosion on the steep slopes exposes the rocks to chemical weathering.
Drainage:
leaching occurs on flat lands because of poor drainage i.e. rock minerals are dissolved and taken away in solution to deeper layers of the soil profile.
This leaves behind residual soils which are rich in iron, magnesium, and calcium compounds. The iron compounds are oxidized in the process of oxidization to form laterite soils.
Poorly drained areas like flat plains have a high dominance of chemical weathering inform of hydrolysis, hydration, reduction and solution which help to decompose the rock. This is because of the stagnant water in valleys and other low lying areas.
Living organisms:
man’s influence; man may influence chemical weathering through a number of ways e.g.
Emission of industrial gases in the atmosphere which increases acidity in rainy water which accelerates the rate of chemical weathering processes of carbonation.
Dumping of industrial or domestic or agricultural influence on land or water which directly react or increase the activity in the environment thus increasing the rate of chemical weathering by carbonation etc.
Man carries out activities that directly involve the breakdown of rocks e.g. mining, quarrying, road and other activities like agriculture which expose the underlying rocks to chemical weathering processes. Also, irrigation avails water that increases chemical weathering processes like hydration, hydrolysis and solution.
Vegetation: the dead decaying organic matter produce humic acids that assist in rock decomposition. These humic acids react with minerals in the rocks and eventually decompose.
Plant roots release mineral substances into the rock while extracting other mineral substances from the rock in a process known as chelation. This weakens the rock and it eventually breaks up.
Other living organisms like animals secrete acids that chemically decompose the rocks e.g. uric acids. Barrowing animals make holes through the soil e.g. moles, termites etc. and through these holes water penetrates to the deeper layers of rocks which aids chemical weathering through processes like solution, hydration, hydrolysis, carbonation etc.
Time:
it takes time for the rock to undergo chemical weathering. The longer the time, the more the rock is chemically weathered and the shorter the time, the lesser the rock is chemically weathered.
Biological weathering
This refers to the breakdown of rocks with the help of living organisms. Plants and animals help in rock weathering by both chemical and mechanical means i.e. bio-physical and bio-chemical weathering.
Plants like algae, mosses, lichens and other hydrophytes retain water on rock surfaces which result into chemical decomposition.
The roots of plants in the process of sucking water from the rocks release some acids (humic acids) that may react with rocks and disintegrate them.
Animals and plants can also disintegrate the rock or break the rocks. The roots of plants which are growing in the rocks may enlarge the cracks or joints which are already existing in the rocks thus making them to breakdown physically. The animals also physically break down the rocks as they move on the rock surfaces due to the pressure exerted on them.
Barrowing animals like rabbits, rats, termites and other animals drill holes into the rocks and therefore directly disintegrate the rocks.
Man through his activities of cultivation, mining, quarrying, rock blasting, road construction, etc. directly breaks down rocks and then disintegrates them physically.
Living organisms whether living or dead play a positive role. Urine of animals once exposed on rocks, reacts with rocks making them to breakdown.
The chemicals man uses in agriculture like herbicides, insecticides, pesticides and fertilizers also weaken the rocks and lead to their break up.
Decomposing organic matter release organic acids which are absorbed by the rocks hence making them to decay and decompose thus weathering.
However, a layer of decaying organic matter may prevent disintegrating since it mulches the soil and underlying rocks hence preventing them from exposure to agents of weathering.
NOTE: All the three types of weathering are interdependent because physical weathering may open up some areas through disintegration and chemical weathering act deeper in the rock.
While physical weathering is at its maximum in arid and semi-arid areas because of high temperatures onto which the rocks are exposed to during day time and low temperatures at night which leads to a lot of stress and strain, chemical weathering is at its maximum in humid areas because of the presence of water.
However, it’s important to remember that in general, all types of weathering operate hand in hand and are usually complimentary although in a given area, one maybe more important than the other.
DIFFERENCES BETWEEN PHYSICAL WEATHERING AND CHEMICAL WEATHERING
Physical weathering is the disintegration or breakdown of rocks into smaller particles or fragments instu while chemical weathering is the decomposition or decay or rotting of rocks instu at or near the earth’s surface.
Physical weathering occurs due to temperature fluctuations (temperature changes) like alternate heating and cooling while chemical weathering occurs as a result of heavy or adequate rainfall and high humidity that provide water to act as a medium of chemical reactions and the hot temperatures to accelerate the rate of chemical reactions.
In physical weathering, there is no change in the chemical composition of the rock but instead the rock is broken down into smaller particles while in chemical weathering there is a change in the chemical composition of the rock i.e. new compounds are formed.
Physical weathering occurs inform of block disintegration, thermal expansion, granular disintegration, frost action etc. while chemical weathering occurs inform of carbonation, solution, hydrolysis, hydration etc.
Importance of weathering to human life.
Weathering is the initial stage in the formation of soil.
Weathering produces other natural resources such as clay which is used for making bricks.
Another significance of weathering is that weathering weaken rocks making them easier for people to exploit for example by mining and quarrying
Also weathering is significant since it prepares the land on which the agents of erosion and transportation act.
Last but not least, some weathered rocks like the granitic tors are very fascinating. They, therefore, act as a tourist attraction.
8 LANDFORMS RESULTING FROM WEATHERING IN EAST AFRICA
Although weathering is all about wear and tear of the earth’s surface, in the process it leads to the formation of landforms, especially within the karst regions.
Karst regions are regions made up of limestone rocks.
These landforms include;
Glikes and clints:
These are formed during the process of carbonation where by rainfall dissolves CO2 in the atmosphere to form weak carbonic acids.
These carbonic acids act on limestone rocks to form calcium carbonates or converted to a more readily dissolvable form of calcium bi carbonates.
In areas where limestone rocks alternate with rocks with different minerals, the weak CaCO3 are removed in solution to form ditch-like depressions called grikes while the hard resistant rocks remaining behind form round topped ridges called clints e.g. at Kajjansi near Kampala.
Stalactites.
These are features found mostly in karst regions (areas with limestone rocks).
They are protrusions found on the roof of a chemically weathered limestone cave.
Stalactites are therefore formed when rainwater mixes with carbon dioxide in the atmosphere to form weak carbonic acids.
These carbonic acids dissolve the limestone rock on the roof of the cave and the solution starts dripping on the floor.
When water evaporates, dripping stops and finger like projections are left on the cave and are called stalactites.
Best examples are found at Nyakasura in Fort-Portal in kabarole district in Uganda and Tanga in Tanzania.
Stalagmites
These are formed together with stalactites in the karst regions.
A stalagmite is a landform that arises from the floor of the cave due to the accumulation of materials deposited on the floor from ceiling drippings.
These are therefore formed on the floor of the cave by dripping water from the roof of the cave.
When water evaporates from the dissolved calcium carbonate, it leaves behind a dry and compact mass of limestone protruding upwards and this forms a stalagmite and are also found at Nyakasura.
Pillars.
A pillar is an upright shaft or structure of stone or any other mineral relatively slender in proportion to its height and can be of any shape in section.
Pillars are formed hen stalagmites and stalactites meet in a limestone cave. They appear as vertical stands of calcium carbonates.
Underground caves.
These are natural hollows formed under ground by rivers. They are formed when a river flowing into a karst region disappears underground hence removing the limestone rocks in solution.
They are formed due to the solution. Solution is the removal of rock in solution by acidic rain.
Some rocks are chemically weathered by being dissolved in water for example limestone and after weathering off such rocks, it forms impressive features like widened hollows for example the caves at Nyakasura.
Sinkholes.
A sinkhole is a depression or hole in the ground caused by some form collapse of the surface area.
Sinkholes occur due to erosion or underground water. They start developing longtime due before it actually appears.
Most of the sinkholes occur due to karst processes of chemical dissolution of carbonate rocks.
The formation of sinkholes involves the process of erosion or gradual removal of slightly soluble bedrock such as limestone by percolating water, the collapse of cave roof, or a lowering of water table.
Sinkholes often from through the process of suffusion for example groundwater may dissolve the carbonate cement holding the sandstone particles together and then carry away the lax particles gradually forming a void or sinkhole.
Exfoliation domes
An exfoliation dome is a large dome-shaped form, developed in massive homogenous coarse-grained rocks especially granite by exfoliation.
Exfoliation is a term used to describe the peeling away of sheets of rock millimeters to meters in thickness from a rock’s surface due to a range of physical and chemical processes during exhumation and weathering.
Therefore, if a form of mechanical weathering in which curved plates of rocks are stripped from the rock below.
Exfoliation domes therefore a result of physical weathering by exfoliation. The rapid heating and cooling especially in the areas of high temperatures cause expansion and contraction of rocks.
The outer most layer of the rock is eventually detached from the original and it peels off.
This leaves behind a round topped rock called an exfoliation dome. Best examples are seen at Mubende along Mubende- kyenjojo road.
Arenas
these are lager circular depressions on the earth’s surface. They are formed in areas of alternating bands of hard and soft rocks.
So the soft rocks are weathered away leaving behind the hard and resistant rocks.
The soft rocks are removed leaving behind circular depressions which may or may not be filled with water to form arenas.
Examples can be seen on the slopes of mountain Rwenzori in the areas of Ntoroko.
Tors
these are remnants of weathered rocks rooted in the bed rocks.
These appear onto the earth’s surface as basaltic remnants.
They are common in jointed rocks which are weathered deep and when denudation forces remove these debris, tors remain as resistant rocks bedded in as un-weathered.
Best examples can be seen in Kenya at Kitmikaye near Seme, Bismark rock in Lake Victoria near Mwanza.
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