WEATHERING – ADVANCED PHYSICAL GEOGRAPHY
Weathering may be defined as the process whereby the rock decomposes and disintegrate insitu, at or near the earth’s surface. It is mechanical fracturing or chemical decay of rocks insitu (on its original position without involvement of transportation of weathered materials).
Weathering is the first stage of denudation followed by mass wasting and erosion.
TYPES OF WEATHERING
There are three (3) types of weathering. These are: –
i. Physical or mechanical weathering.
ii. Chemical weathering.
iii. Biological weathering.
I: PHYSICAL (MECHENICAL) WEATHERING
Physical weathering refers to the disintegration of rock by physical process. In physical weathering the rocks are broken into smaller pieces retaining their characteristics of original materials without any change in the chemical composition.
It is very common in desert areas, mountaineous areas, and cold areas. In physical weathering the temperature and temperature changes is the major agents and plays a big role for occurrence.
TYPES (WAYS) OF PHYSICAL WEATHERING
Physical weathering is mainly influenced by temperature and temperature change with the following types; Thermal expansion (granular disintegration, block disintegration, exfoliation), frost action, salt crystallization, pressure release (unloading process), and alternate wetting and drying. These are: – 1. Thermal expansion:
It is also known as insolation. It is very common in arid and semi-arid areas due to high daily temperature range.
This occur through the following ways;
1. Exfoliation:
This process dominant in arid and semi-arid regions where diurnal (day) temperature range is very high due to presence of open sky or absence of vegetation and cloud cover.
During the day time outer surface layer of rock expands due to the great heat while in night time the layer contract due to coldness. The alternate expansion and contraction process caused the rocks to develop cracks and finally the outer layer peels off from main body and leaving behind a smooth dome called exfoliation dome surrounding by weathering materials called screes. There are many exfoliation domes in Egypt and Kalahari, Sahara and Sinai deserts.
2. Block disintegration:
This process occurs where by some rocks such as granite rock initially develop both vertical and horizontal joints during the time when they cool down. These joints are widening up by alternate expansion and contraction causing the rock to disintegrate in lager size blocks. The broken blocks are called tors. They are very common in Mwanza and Iringa – Tanzania.
3. Granular disintegration:
this is the minute disintegration of rocks composed by different type of minerals which respond differently by temperature change, hence leading the internal fracture and disintegration into small particles.
4. Frost action (congelifraction):
This is the effective process in cold climatic regions and mountainous regions where temperature fluctuates just below and above 00C and thus water within joints and cracks freezes. The repeated melting and freezing of water cause the joints and cracks to become wider and deeper and the pressure of freezing water cause the rock to crumble.
5. Pressure (load) release / Unloading:
This process occurs where by some rocks like igneous and metamorphic rocks release pressure when they exposed on the earth surface. When the pressure released, their outer surface layer expands and ultimately detach from the main body in sheeting or spalling. It is common in Mwanza and Iringa in Tanzania.
6. Salt crystallization:
This process is dominant in arid and semi-arid areas, as well as coastal areas of tropical dry climate where salts crystals accumulates in the rock cavities or in the pore spaces between rocks particles. This process occurs when high temperature cause evaporation or land surface leading formation of salt crystals which grows in size with the rock cavities.
The crystals exert pressure against rocks walls the cavities leading the rocks to crumbles.
7. Spheroidal weathering (alternate wetting and drying):
This is the physio-chemical weathering process. It occurs when some rocks such as Anhydrite and Hematite rocks absorbs water and swell leading their outer surface layer to peel off from the main body like onion. It is common in wet climates and when rocks absorb water.
II: CHEMICAL WEATHERING
Chemical weathering refers to the decaying or decomposition of rocks through chemical process. in chemical weathering the rocks minerals react with water or other weathering agents such as moisture etc. thus turning the chemical composition of rocks into different types. The major agents of chemical weathering are water, humidity, and moisture. In chemical weathering temperature is an important factor as it speeds up the rate of chemical reactions. Chemical weathering is dominant process in the tropical wet climate and where water is a abundantly available.
TYPES / WAYS OF CHEMICAL WEATHERING
In chemical weathering the water player a major role for occurrence. The water plays their role in weathering through following ways;
1. Hydrolysis:
It refers to the addition of hydrogen to a compound. It involves the reaction between the hydrogen ions (H+) in water with the minerals in a rock such as decomposition of feldspar minerals in igneous rocks example Granite and Diorite rocks. The end product are silica and clay minerals.
2KALSi3O8 + 2H2O + CO2 AL2Si2O5(OH)4 + K2CO3 + 4SiO2
(Feldspar) (water) (carbondioxide) (clay minerals) (potassium carbonate) (silica)
2. Hydration:
It occurs to the addition of water to a rock. This occurs when certain rocks such as calcium sulphate (Anhydrite rock) absorb water, the minerals then change to Gypsum rock which is weak rock.
CaSO4 + 2H2O CaSO4 * 2H2O
(Anhydrite) (water) (Gypsum)
Another example of hydration occurs when Hematite rocks (Iron oxide) combine with water to give limonite rock.
3. Carbonation:
It is the addition of carbondioxide to a rock. In rocks, it is added through a reaction with weak carbonic acid. It is occur when carbondioxide reacts with rain water in the atmosphere to produce a weak carbonic acid. The weak acid reacts with rocks composed of calcium carbonate such as limestone, chalk, and dolomite rocks. Calcium carbonate changes to calcium bicarbonate which is soluble. This process is very important in the formation of Karst sceneries such as stalagmites, stalactites caves and natural pillars.
CaCO3 + H2O + CO2 Ca(HCO3)2.
(calcium carbonate) (water) (carbondioxide) (calcium bicarbonate)
4. Solution:
It is the dissolving of a rock. It is happening to rocks like salt rock and calcium carbonate rocks after carbonation process.
5. Oxidation:
It refers to the addition of oxygen to a rock. Oxidation occurs when rocks are exposed to oxygen in the air or in water. It is involving reaction of oxygen and iron minerals. It occurs when oxygen is added to ferromagnesium rock minerals such as Iron, Magnesium, etc. Example the reaction between oxygen with ferrous oxide minerals gives to Hematite rocks ie reddish brown colour is rusting which is weak.
4Fe + 3O2 2Fe2 O3
(Iron) (oxygen) Iron oxide (Hematite)
6. Reduction:
This is the reversal of oxidation. It takes place in the water-logged area where amount of oxygen in the rock or soil is reduced ie oxygen ions are removed and hydrogen ions are added to the rocks or soils. Example olive rocks. When both reduction and oxidation occur sequentially the process is called REDOX.
III: BIOLOGICAL WEATHERING / ORGANIC / BIOTIC WEATHERING
Biological weathering is the type of weathering where by living organisms disintegrate and decompose rocks. OR
Is the decomposition or disintegration of rocks into smaller particles due to the influence of living organisms such as plants, animals and man.
The main agent of biological weathering is living organism both plants and animals, hence play a great role for occurrence this type of weathering.
TYPES OF BIOLOGICAL WEATHERING
Biological weathering can be subdivided into two types. These are: –
1. Bio-physical weathering:
Is the disintegration of rocks into smaller particles due to the influence of living organisms.
2. Bio-chemical weathering:
Is the decomposition or decaying of rocks into smaller particles due to the influence of living organisms.
WAYS (PROCESSES) OF BIOLOGICAL WEATHERING
The biological weathering can take place through the following ways. These are;
1. Biotic weathering by plants:
It occurs through action of plants for example when the plant roots growing in rock joints and cracks.
2. Growing tree roots
When a tree grows in a crack as it grows in a cracks or joints, its roots will exert pressure on the crack as it grows bigger. This will widen the cracks hence gradually shattering. This is the bio-physical weathering.
3. Decaying of vegetation
When the vegetation decaying produces organic acids, which cause further breaking and decaying of minerals. Hence leading bio-chemical weathering.
4. Release of chemical through plant roots
Also plant roots release chemical substance like acid which then breaks down the rocks. Hence leading bio-chemical weathering.
5. Biotic weathering by animals:
This occur through the following activities;
6. Burrowing animals
Animals like rabbits, moles, etc. burrow the top soil as they create their roosting or dwelling places. This paves a way for air and water to enter underground hence a possibility for biological chemical weathering to occur on the surrounding rocks.
7. Role of rodents
Movement of animals such as mice, squirrel and porcupine which use their strong incisors to disintegrate rocks gradually hence promote bio-physical weathering for softer rocks like shale rocks.
8. Role of humic acid
Generally, when living organisms die and decompose in the soil they release organic acid such as lactic acid. This acid attack rocks and cause the rocks to decompose chemically. This is bio-chemical weathering.
9. Role of man
The human activities such as road construction, mining, quarrying, transportation, and bombing can cause rocks to disintegrate physically. Also, agricultural activities such as cultivating of the land can break rocks physical hence biophysical weathering. Moreover, uses of chemical fertilizers and pesticides in agriculture activities decaying of rock chemically.
FACTORS INFLUENCING THE RATE AND TYPE OF WEATHERING
There are several factors affecting the rate and types of weathering. These are: –
1. Organisms:
Plants and animals can bring about biotic weathering or both physical and chemical weathering through their activities on the land surface. For instance, when plants and animals die they produce humic acids which attacks the rock chemically, hence chemically weathering. Also, action of human activities such as road construction, overgrazing, cultivation, etc. disintegrate rocks physically, hence physically weathering.
On the other hand, the quantity of these organisms present on the land surface is very important in the rate of rock weathering.
2. Climate:
Through the elements of rainfall, humidity, and temperature, climate is said to be the most important factor in rock weathering both types and rates of it. For instance, temperature and frost, pressure, etc. are responsible for physical weathering while humidity and rainfall are responsible for chemical weathering.
The occurrence of different types of weathering differ from one climate to another like as follow;
i. In the equatorial areas; Chemical weathering predominates due to heavy rainfall, high temperature and high humidity.
ii. In the tropical savannah areas; Both chemical and physical weathering dominates due to seasonal changes of rains and temperature in the year.
iii. In the desert and semi-deserts areas; Physical weathering predominates due to extreme temperature changes and little or no rainfall and vegetation.
iv. In the mountaineous and temperate climate areas; Physical weathering dominates due to coldness. In these areas chemical weathering take place in summer where there is melting of ice occurs.
The rate of chemical and biological weathering is high in the equatorial area due to heavy rains, high humidity, and high temperature while its rate is low in the Arid climate and elsewhere. In fact, warm temperature accelerates the water for chemical weathering while cold temperature slows it.
The rate of physical and biological weathering in the arid and mountaineous climate are high due to extreme high temperature and very low temperature respectively.
3. Relief:
Generally, the role of relief on weathering is mainly indirect like as follow;
i. On wind ward side moist condition exist because of orographic rains. Therefore, chemical and biological weathering are dominant. However, in the lee ward side the physical weathering dominant due to little rains or no.
ii. Moreover, in the area with height below 2500m precipitation occurs in the form of water droplets (rainfall) but above 2500m it occurs as snow. Therefore, at lower altitude chemical weathering predominates while above the snow line the physical weathering is dominates.
iii. The rate of weathering by relief is revealed by the fact that, on the gentle slope weathering materials accumulates and therefore burying the weathered surface. In this case the rate of weathering slowed down. On the other hand, on steep slopes weathered materials are quickly removed by erosion and mass wasting. The fresh surface of rock is then exposed to the weathering agents thus increasing the rate of rock weathering.
4. The nature of rock:
The influence of parent rocks on the nature and rate of weathering is marked by the following;
5. Composition or types of minerals:
Some rocks are composed by minerals which make them stable to weathering while other are not stable to it. For example, granite rocks composed by quartz are stable and resistance to chemical weathering than the basalt rock which are unstable to chemical weathering. Some rocks with iron minerals and calcium carbonate etc are easily affected by chemical weathering. Example ferromagnesian and limestone rocks respectively.
6. Rock structure (planes of weakness):
Some rocks have many joints and cracks allowing easy penetration of weathering agents such as water, temperature, and moisture.
Therefore, easily to be attacked by chemical weathering faster than rocks without such planes of weakness. Frost action is faster in well jointed rocks and therefore the rate of frost action as types of physical weathering is faster due to rock jointing.
7. Rock colour:
Rock with dark colour example olivine and basalt rocks absorb heat faster than the light coloured rocks such as chalk and limestone rocks. Therefore, easily to be affected by physical weathering. Moreover, differentiated heating of rocks due to differentiation in minerals characteristics can lead to different rate of expansion and contraction, hence disintegration. d. Rock origin:
Very often for igneous and metamorphic rocks are susceptible to chemical weathering because very formed under great heat and pressure very different to that on the earth surface.
8. Time:
This factor depends upon time where agents of weathering such as water, temperature, etc. concentrate on the particular place. Therefore, the rate of weathering agents operates for a long period of time.
SIGNIFICANCE (EFFECTS) OF WEATHERING
The weathering process has the following social economic benefits to the life on earth:
a) It leads to soil formation. Weathering breaks rocks into small particles which after being mixed with organic matter lead to the formation and development of soil.
b) It provides materials for pottery and bricks making. The weathering process produces new substances such as clay, which is used in pottery and brick making.
c) It offers opportunity for tourism. The weathering process results in some attractive features like granite tors. These are tourist attractions that bring local income and foreign currency to a country. A good example is Bismarck rock in Mwanza Region, in Tanzania on the shore of Lake Victoria.
d) It provides building materials. Weathering is a source of building materials. For example, scree and rock blocks can be used in the construction of houses, bridges and dams.
e) It can contribute to the modification of landforms. Weathering is significant because it weakens the surface rocks and in so doing it facilitates the agents of erosion and transportation hence modifying landforms. Most of the depositional features either by water or wind benefit from weathering processes. For example, deltas and estuarine features are formed as a result of deposition of weathered materials.
REVISION QUESTIONS
1. Explain six factors determining resistance of rock to denudations
2. Explain the processes involved in denudation
3. Geologist and Geomorphologists agree on the modifications of the earth’s surface through internal and external forces are still going on. Please! May you show the influence of erosion and weathering on the development of modification on the earth’s surface by giving relevant examples about the results (6 points)
4. Assess the importance of erosional and depositional features to human being.
5. The surface of the earth’s crust is under constant sculpturing due to multiple geomorphic agents. Examine endogenic and exogenic geomorphic processes responsible for development of landscapes.
6. Describe the factors that control the rate and type of weathering (Necta 1991 & 1992)
7. Explain the role of water in weathering process. (Necta 1982)
8. How does temperature a paramount important in weathering?
9. Explain the significance of temperature in mechanical weathering (Necta 1995)
10. Explain the influence of climate on the weathering (Necta 2003 and Necta 1980)
11. What are the main factors that determine the resistance of rocks to weathering and erosion? (Necta 1977)
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