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Geomorphology: Weathering Processes

Outline

This paper mainly discusses what is weathering and the three processes involved in it. The paper explains the processes involved under each type and the features formed as the result of the processes. In doing so the paper has been broken down onto various sections namely;

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Introduction

Railsback describes weathering as a term that describes all the processes that breaks down the rocks in the environment near the earth surface. This implies that weathering encompasses the disintegration and decomposition of the various kinds of materials which include hard bed rocks, soils and their minerals through direct contact with the planet atmosphere. Weathering therefore occurs without movement or in “Situ” but the various kinds of the hard bedrock facilitates the erosion of the land surface by other several kinds of external agencies that include: running water, waves, wind and glacier. According to Railsback this process entails three main types thus Physical or Mechanical weathering, Chemical weathering and Biological weathering (Railsback, 1993)

Physical Weathering /Mechanical weathering

According to the studies of Richard, Physical or Mechanical weathering can be viewed as the process which involves the breakdown of rocks and soils through direct contact with the atmospheric conditions such as heat, water, ice and pressure. Richard further in his studies states that the breaking process normally involves physically destroying the rocks or the disintegration of rocks. Richard further argues in his studies that the breakdown process of rocks into small particles is therefore caused by factors which include freezing, Exfoliation (onion weathering), thawing, release of pressure, water absorption salt crystal formation, landmass uplift, expansion and contraction from the sun or fire, plant root growth, actions of animals, abrasion, or other means that do not have a direct effect on the rock’s chemistry. Richard further notes that the process normally occurs in areas where there exists a bare rock where there is no vegetation to protect the rock from extreme weather conditions which normally results to the pulling apart of rocks forming cliffs (Richard, 2007).

Richard (2007) identifies frost disintegration as the second process involved in this type of weathering. In his studies Richard states that this process occurs commonly in mountain areas where the temperatures around are assumed be at freezing point and frequently they can fluctuates above and below freezing point. Commonly these areas mainly are alpine and periglacial. Richard in his studies believes that the process is attributed to the expansion of freezing water captured in the cracks which entirely depends on the water-to-ice expansion. Richard argues that the process continues which leads to moist soils to expand or frost heave, after freezing this water then moves from unfrozen areas within thin films which then lead to a growing ice lenses. According to Richard this also applies to pore spaces of rocks which grow larger as they attract liquid water from the surrounding pores to form ice crystals that in the end weakens the rocks making them to beak up with time. Richard argues that this process is normally occurs as a result of the unique water property thus its greatest density at 4 C. According to Richard this implies that ice has a greater volume than water at the temperature below 4 C, because when water freezes, the more ice expands which puts the surrounding environment under intense stress (Richard, 2007).He notes that common example of a rock formed through this process is tors that are commonly seen in Dartmour. Richard also argues that a talus slope or scree slope are other features formed through this process. Richard argues they are formed at the foot of the slope when the water in joints of the rocks freezes to form ice that strains the walls of the joints causing them to deepen and widen. Richard further argues that this occurs when the volume of water expands by 9% hence when it freezes this makes the process to continue and when the ice thaws, water flows further into the rock causing the temperatures to drop below freezing point which again rises making the ice in the joints to enlarge further. In his studies Richard believes that the repeated free-thaw action weakens the rocks causing them to breaks up along the joints into angular pieces which gather at the foot of the slope forming a talus slope (Richard, 2007).

According to Sparks the second process of Mechanical weathering is process of pressure release also referred to as the unloading or the overlying of materials in his studies. Sparks argues that the materials are normally removed by erosion or other processes causing the underlying rocks to expand more hence fracture parallel to the earth surface. Sparks further argues that a moving glacier and Intrusive igneous rocks are examples of features formed through this process. According to Sparks the features are formed when the underlying rock is exposed to tremendous pressure causing the overlying rock material to be removed by erosion. When the process occurs the intrusive rocks are exposed to the high pressure which makes the outer rocks to expand. This expansion then sets up stresses causing fractures parallel to the rock surface to form which then breaks the sheets of rocks making them to be exposed along the fractures (Sparks, 2008).

The third process is Exfoliation which can be described as the process in which rocks are broken down along the joints which are parallel to the earth surface into sheets (John, 1961).John argues that the process happens in plutonic rocks which when exposed under great pressure cools at the depth and finally de-pressuring once the pressure is removed this causes sheets of rock to peel off sub parallel to the earth’s surface, or whatever is the least pressurized surface. John further notes that this occurs mostly in large mass of rocks where the mass is brought on the surface as a result of tectonic movements which thrusts the rocks upwards. John states that the process continues even after the rock layer comes on the surface of the earth which then exerts pressure on the earth surface resulting in the formation of cracks or sheet joints (John, 1961).

The fourth process is Hydraulic action according to John’s studies and in his studies he argues that it is a process where water commonly powerful waves rushes in the rock face rapidly, trapping a layer of air at the bottom, this compresses the rock and finally weakens it. This process leads to fragments at the rock which widens the crack caused by explosive release of highly pressurized air cracks (John, 1961).

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. Another process of Physical or Mechanical weathering is Salt-crystal (haloclasty) where it occurs on the surface pattern of pedestral rocks. It is also referred to as honey comb weathering and it heavily caused by salt crystallization(John,1961), In his studies John believes that it is a process which is usually caused by disintegration of rocks when saline or salinity solutions seeps into the cracks of rocks and joints in the rocks eventually evaporating leaving salt crystals behind. The crystals are heated up making them to expend this exerts more pressure on the confined rocks. These rocks decompose forming solutions. According to John it is commonly done in arid climates where strong heating occurs, causing strong evaporation hence salt crystallization and along coastal areas. This process is common in honeycombed stones in sea walls which is a type of tafoni a class of cavernous rock (John, 1961).

The sixth process involved here is Thermal expansion and Contraction also commonly referred to as onion-skin weathering and it is mostly enhanced by presence f moisture (Sparks, 2008. According to his studies this process is common in deserts areas where large diurnal temperature range occurs. Normally the temperatures rise high during the day and lowers during the day. This causes the expansion and contraction of the rocks up which exerts stress on the outer layers making them to peel off into very thin sheets. (Sparks, 2008).

According to Sparks abrasion is viewed as the last type of mechanical weathering which is usually very common in the deserts. It normally entails the physical grinding of the rocks fragments also known as sandblasting process. Sparks further clarifies that during the process all the surface rocks are weathered due the action of an abrasive agent this removes the upper layer of the rocks by means of friction which happens when the wind carries all the sand particles that finally chafes against the rock making it to wear down (Sparks, 2008).

Chemical weathering

The second type of weathering process is chemical weathering. Colmann and Detheir describe this as a process through which the internal structure of a mineral is altered by through the additional or removal of elements (Colmann & Dethier, 1986). According to their studies the process occurs on the available surface due to the reaction of temperatures and presence of chemically active fluids. Colmann and Dethier believes that during the process small particles found on the earth surface weather faster than larger particles by means of chemical which results in new or secondary minerals to develop from original. The process entails the combination of water and other various chemicals to create an acid which directly breaks down the material in the end. Colmann and Dethier further analyzes that the main processes involved include: dissolution, carbonation, oxidation or hydrolysis of rocks and minerals (Colmann & Dethier, 1986)

To begin with the first process here is known as Dissolution (Colmann &Dethier, 1986). In their studies they argue that it is common in the areas that have a great deal of limestone. The water which is dissolved is acidic water which comes from pollution or naturally in limestone. The two dissolves producing a weak carbonic acid. According to Colmann and Detheir the acidic rain comes from ever present gases in the atmosphere which reacts with rain water producing stronger acids that can cause the power of water point to lower to 4.5 or even 3.0.Colmann and Dethier believes that the Sulfur dioxide which comes from volcanic eruption or from the fossils fuels can become sulfuric acid when mixed with rain water causes weathering in the rocks on which they fall resulting to formation of features such as sinkholes and karsts (Colmann & Dethier, 1986).

The second important process is Carbonation (Colmann & Dethier, 1986). Colmann and Detheir argue that the process occurs when carbon dioxide in the atmosphere dissolves in calcium carbonate. They further argue that gas mixes with water to form a weak carbonic acid which reacts with calcium calcium carbonate (the limestone) forming calcium bicarbonate. This is speed up with a decrease in temperatures forming a large feature example as a glacier and limestone pavements that are effective along the joints (Colmann & Dethier 1986).

According to Colmann and Detheir Mineral Hydration is the third process which involves the absorption of minerals in the rock where the minerals absorb water and expand creating stress finally causing disintegration of rocks (Colmann & Dethier 1986).They further describes it as the process through which a form chemical weathering that involves the rigid attachment of hydrogen and hydroxide ions to the atoms of the molecules of the mineral in question. Colmann and Dethier further note that this makes the rocks to find a chance to take the water, which increases the volume creating physical stress within the rock. Colmann and Deithier classifies iron oxides as an example which are converted to iron hydroxides and this leads to the hydration of anhydrite which then forms gypsum and other features such as sandstones (Colmann & Dethier, 1986).

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Hydrolysis is also a major process involved in this type of chemical weathering (Colmann & Dethier, 1986). According to Colmann and Detheir the process involves the chemical reaction between the minerals in the rocks and the hydrogen in the rain water that affects the silicate minerals ionizing the silicate minerals to form alkaline solutions. This reaction results completely in a complete dissolution which is obtained from the original minerals. This reaction normally assumes availability of enough water to speed up the reaction.. An example of a featured formed by this process is the lay minerals where the feldspars in granite rocks accumulates they then crumbles and in the process the rock is weakened causing it to break down forming clay minerals (John,1961).

The last process involved in this type of weathering is Oxidation (John, 1961). This is also a chemical reaction where it entails the oxidation of a variety of metals. Commonly ferrous irons combine with oxygen forming iron oxide hematite and other oxides such as goethite and limonite (John, 1961). John argues that the reaction forms rust which gives the rocks a reddish-brown coloration on the surface which slowly crumbles and hence weakens the rock during which more metallic ores and minerals are oxidized and hydrated to producing colored deposits such as chalcopyrite’s or oxidizing to copper hydroxide and iron oxides (John,1961).

Biological weathering

According to Pidwimy Biological weathering is the finally type of weathering (Pidwimy, 2006). This can be described as the process through a number of animals and plants are involved in the disintegration of rocks and minerals due to their chemical or physical agents contained in their bodies. Pidwimy describes bacteria and plants as the main organisms causing biological weathering which can be physical or chemical.. The processes involved here are very simple (Pidwimy, 2006). Pidwimy emphasizes that the first process involved here is the simple breaking of particles, by the consumption soil particles by the animals that is then followed by the fracture of the particles by animal burrowing or by the pressure put forth by growing roots (Pidwimy, 2006).

Pidwimy identifies movement and mixing of materials as the second process of Biological weathering caused by many large soil organisms that moves through the soil particles (Pidwimy, 2006). Pidwimy argues that the movement introduces the material to different weathering process that is found at distinct locations in the soil profile (Pidwimy, 2006). This results in a simple chemical solution which is enhanced by the carbon-dioxide produced by respiration that mixes with water to form carbonic acid. According to Pidwimy the complex chemical effects come as a result of chelation which is described as the process where organisms produce organic substances, known as chelates that have the ability to decompose minerals and rocks by the removal of metallic cations (Pidwimy, 2006).

In biological weathering the third process involved is where the Organisms influence the moisture regime in the soils which enhances weathering (Pidwimy, 2006). Pidwimy describes that the increase in availability of water is caused by leaves obtained from trees, stems, presence of roots masses and humus. Pidwimy states that the Organisms have a great influence on the power of hydrogen from the soil solution (Pidwimy, 2006).In his studies Pidwimy states that during the respiration process plants roots, release carbon dioxide that mixes with water forming carbonic acid that lowers the soil power of hydrogen since the cation exchange reactions are absorbed by the plant nutrients from the soil which eventually causes the power of hydrogen to change. The process which involves exchange of cations for hydrogen ions extremely affects the surrounding rocks and soils leading to podsolisation (Pidwimy, 2006).

References

Richard J. Huggett, (2007), Fundamentals of Geomorphology, 2nd Edition ISBN 0415390834, 9780415390835.

Sparks.B.W, (1961), Geomorphology, 3rd edition.

John Wiley & Sons, (1961), Analysis of Landforms: An Introduction to Geomorphology.

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Pidwimy, M. (2006), “Weathering”, Fundamentals of Physical Geography, 2nd Edition.

Railsback, L, B, (1993), A geochemical view of weathering and the origin of sedimentary rocks and natural waters: Journal of Geological Education.

Colmann, S. Dethier, P, (1986), Rates of Chemical Weathering of Rods and Minerals: New York: Academic Press.

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