Category: Weathering and Slope Development

• Denudation refers to the wearing away of the land by various natural agencies: the wind, the rain, frost, running water, moving ice and the sea.
• The term weathering refers to the loosening, decaying and breaking up of the rocks. Weathering produces rock wastes in “situ” i.e no movement is involved.
• Weathering is the process by which rocks and minerals are broken down or altered by physical, chemical, and biological processes, while denudation is the process of erosion and transportation of rock and soil by water, wind, or ice.

• Weathering occurs at the Earth’s surface, while denudation involves the removal of weathered material from the surface.
• Weathering is a gradual process that occurs over long periods of time, while denudation can occur rapidly, such as in a flash flood.
• Weathering can happen in both cold and warm temperatures, while denudation is mainly caused by running water, wind, and ice.
• Weathering can be caused by both chemical and mechanical processes, while denudation is caused by the physical movement of rock and soil particles by natural agents.

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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 that are growing in the rocks may enlarge the cracks or joints which are already existing in the rocks thus making them to break down physically. The animals also physically break down the rocks as they move on the rock surfaces due to the pressure exerted on them.

Burrowing 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.

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Physical weathering is the disintegration or breakdown of rocks into smaller particles or fragments insitu 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.

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Chemical weathering refers to the decomposition or rotting of rocks using water involving a change in the chemical composition of the rock to form new compounds or minerals.

Chemical weathering is most common in the humid tropical areas that receive heavy rainfall although it can also take place in deserts since they also receive occasional showers.

Chemical weathering involves a chemical reaction between the minerals of the rock as well as the atmospheric gases like carbon-dioxide, oxygen and acids from rotten plants and animals.

In chemical weathering, water acts as a medium of chemical reaction and the hot temperatures accelerate the rate of chemical reactions. Chemical weathering involves a number of processes or types and these include;

Carbonation

This takes place when rainwater mixes with carbonates in the atmosphere to form weak carbonic acids.

These carbonic acids react with minerals in the rocks like limestone rocks which contain calcium to form calcium carbonate which can easily be dissolved in water and hence carried away.

This leads to creation of holes (solution hollows) on the surface and caves beneath. When the calcium carbonate is removed in solution by groundwater, karst landforms are formed like stalactites and stalagmites, underground caves etc. at Nyakasura in fort portal, Bamburi and kilifi in Kenya.

Ca(s) + H₂CO_3(aq)                                                                 CaCO_3(s) + H_2(g)

Calcium carbonate + H₂O + CO₂                    calcium hydrogen carbonate.

Limestone + weak carbonic acids                  calcium hydrogen carbonate.

Hydration

This is a process of chemical weathering by which rocks absorb water and expand in size.

This expansion reduces the cohesiveness of the rock particles hence internal stress is created within the rock and therefore crumbling and fracturing of the rock and changing their chemical compounds.

Some rocks like hematite absorb water changing to limonite.

Others are calcium sulphate, after absorbing water changes to gypsum and feldspar mineral in granite absorbs water and disintegrates to form mud.

Hydrolysis.

This is the reaction between water and mineral elements i.e. the hydrogen ions of water and ions of the minerals (in the rocks). This water and mineral ions react chemically which gives rise to the formation of new compounds e.g. feldspar is broken down to produce potassium hydroxide and aluminosilic acid.

The aluminosilic acid is further decomposed into clay minerals while potassium hydroxide reacts with water CO₂ to produce potassium carbonate.

The potassium carbonate is removed in solution leaving silica and residual clay minerals as the end product for example in the broad valleys of Buganda like at Kajjansi.

2KAlSO₃O₈ + 2H₂O + CO₂                               Al₂Sl₂O₅(OH)₄ + K₂CO₃ + 4SIO₂

Oxidation

This is the reaction that occurs when additional oxygen is taken up by mineral compounds within the rocks.

The oxidation of minerals nearly always occurs in association with water in which atmospheric oxygen has been dissolved.

It is most active in the zone above the water table and particularly in sedimentary rocks such as clay which contain ion compounds.

In this zone, water oxidizes the ferrous compounds into red or brown ferric compounds.

In the zone below the water table, the ferrous compounds in clay are oxidized to give a grey or blue color.

This form of chemical weathering is common in rocks which contain mineral elements like ion, calcium and magnesium which can easily be oxidized to form oxides. The laterite soils / marrum soils on many hill tops in Buganda where we get murram for road construction were formed through this process.

Mineral oxides can be formed as below.

Fe + O₂                             FeO (iron oxide)

2Ca(s) + O₂                              2CaO(s)

2Mg(s) + O₂                                2MgO(s)

Chelation.

In this process of chemical weathering, organic acids and humic acids from decayed materials/ leaf litter and other remains of plants react with certain minerals.

The exchange of organic acids with mineral elements or rock minerals causes the decomposition and change in the chemical composition in the rock particles hence chemical weathering (fracturing and decomposition).

Solution.

Under this process of chemical weathering, some soluble rock particles are dissolved and weathered in solution form e.g. water may mix with rock salts and form a solution. This means that rock salts have been weathered in that way.

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Chemical weathering is the process by which chemical reactions break down rocks. This type of weathering is caused by the interaction of water, oxygen, and carbon dioxide with the minerals in rocks.

THE FOLLOWING ARE FACTORS THAT AFFECT THE RATE AND PROCESS OF 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 20^oC 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.

1. Emission of industrial gases in the atmosphere which increases acidity in rainy water which accelerates the rate of chemical weathering processes of carbonation.
2. 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.
3. 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.

In conclusion, there are many factors that affect the rate of chemical weathering. The type of rock, the climate, the presence of water, oxygen, and carbon dioxide, the presence of living organisms, the acidity of the water, the amount of sunlight, and the amount of salt in the water all play a role in how quickly rocks are weathered. By understanding these factors, we can better understand how chemical weathering shapes the Earth’s surface.

Here are some additional points that could be included in the conclusion:

• Chemical weathering is a slow process, but it can have a significant impact on the landscape over time.
• It is one of the main processes that contribute to the formation of soils and the evolution of landscapes.
• Chemical weathering can be accelerated by human activities, such as pollution and deforestation.
• It is important to understand the factors that affect chemical weathering in order to mitigate its negative impacts and protect the environment.

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