Outline the main weathering processes and discuss the factors that control their operation PDF

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weathering processes and discuss the factors that control their operation...

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Panuma Rai

Word count: 1643

‘Outline the main weathering processes and discuss the factors that control their operation.’ Introduction Weathering is one of the essential subjects when talking about the earth’s processes. Weathering is the disintegration of rocks and minerals at the earth’s surface; it can be caused by heavy rainfall or extreme fluctuation of temperatures. There are three main types of weathering; mechanical or physical, biological and chemical weathering. Mechanical weathering is gradual breakdown of rocks into smaller fragments. There are six main types of processes for mechanical weathering; unloading, frost action, heating and cooling, wetting and drying and salt crystal growth. (John Huggett, R., 2011) Chemical weathering is when chemical reactions occur with different types of rocks in certain climatic conditions. There are six main types processes of chemical weathering; Solution, hydration, oxidation, oxidation, reduction, carbonation and hydrolysis. (John Huggett, R., 2011) Biological weathering is mechanical, chemical or even combination of both processes that breakdown rocks. An example of this can be rocks being broken by plant roots or moss. (John Huggett, R., 2011) There are a lot of factors which can control weathering operation such as their rate and extent of weathering. These factors can be due to the climate, rock type, rock structure, topography, erosion and time (Hong Kong Geological, 2009). Discussion Mechanical or Physical weathering Mechanical weathering is the breakdown of rocks and minerals by mechanical methods that is caused by many processes. It’s the gradual breakdown of rocks into small pieces due to strain and rupture.

Panuma Rai

Word count: 1643

When there is a tectonic uplift and erosion of overlaying rocks the pressure confined is released and the pressure underneath the rock will exert, forcing the rock to expand. This process is referred as unloading (Pidwirny, M. (2006). Exfoliation is a process of the earth’s surface rocks peeling into fragments due to unloading. This process can create landforms called exfoliation domes such as Half dome in Yosemite national Park (John Huggett, R., 2011). Frost action occurs when water seeps through the pores within rocks and soil, expanding 9 percent when freezing (John Huggett, R., 2011). This causes stress as it expands in the pores causing the rock to break. Frost shattering is when small pieces or big boulders of rock are broken off to make into even smaller fragments. This type of process is common in cold environments where freeze thaw cycles are most dominant. Hydrofracture occurs when unfrozen water is trapped underneath the frozen water moisture at the surface of the pores sealing of the end of the soil/rock (Whalley. W, Warke, P, 2005). The expanding ice will increase the hydrostatic pressure and transfer the force into the hollow space of the unfrozen water making it able to shatter rocks. Thermal fatigue occurs due to rocks having low conductivities. By continuous heating and cooling can lead to disintegration of the rock as there is different level of expansion (Whalley. W, Warke, P, 2005).. This produces a fatigue effect at the surface of the rock leading thermal weathering. Thermal shock is when there is fast increase in temperature on the rock’s surface normally associated with bush fires (Whalley. W, Warke, P, 2005). As the surface of the rock heats it causes stress with the cooler layer underneath. This causes the rocks to flake, form different shapes and sizes.

Panuma Rai

Word count: 1643

Wetting and drying is when clay minerals expand significantly when wetting and leading it to breakdown the rock mass (John Huggett, R., 2011). When it starts drying out, the expanded clay evaporates and caused cracks when it contracts and disintegrates the rocks (Whalley. W, Warke, P, 2005).. This slowly leads to the fatigue effect wet dry weathering. Salt crystal growth occurs in coastal regions and crystals can grow into the pore of the rocks due to evaporation (John Huggett, R., 2011). . This causes stress in the rocks as it widens it leading to disintegration (Whalley. W, Warke, P, 2005). When the salt crystal are heated in the pore it will exert pressure and produces thermal stress which leads to salt weathering.

Chemical weathering Chemical weathering entails of alteration of chemical reaction with the weathered material. There are many different processes which can lead to chemical weathering. Most common ones are Solution, hydration, oxidation, reduction, carbonation, hydrolysis and chelation. When minerals are dissolved into water it makes a good solvent (weak acid) to erode rocks. Solution is the process where a separation of molecules in each ions, anions ( - ion) and cations (+ ion) occurs and then is surrounded by water (John Huggett, R., 2011). This process is normally referred as a mechanical process however chemical is involved in it so therefore declared as chemical process. When exothermic reaction that involves water in minerals occurs the process is called hydration(Whalley. W, Warke, P, 2005). Water molecules are being absorbed by the minerals on the surfaces and edges, but the chemicals in the original matter does not change.

Panuma Rai

Word count: 1643

Oxidation is when an electron is lost from an atoms or ions making it negatively charged. Oxygen is then dissolved in the water oxidising the rocks. Whereas reduction is the opposite of oxidation (Bernard J. Smith. 2012). This is when an atom or an ion gains an electron making it positively charged and encourages changes called ‘gleying’. Carbonation is when minerals are mixed with carbon dioxide to make ‘carbonic acid’ (Bernard J. Smith. 2012). This then separates into hydrogen ion and bicarbonate ion. In limestone a mineral called calcite is commonly found which is very sensitive to carbonation. Hydrolysis is considered as one of the main processes of chemical weathering. It alters the elements of the minerals in rocks or fully disintegrates it. Water is split into two section; hydrogen cation (+) and hydrogen anions (-) and reacts with silicate minerals (Bernard J. Smith. 2012). Chelation is process where metallic cation is removed by decomposing minerals and rocks, this is by mixing with organic acids (like fulvic and humic acid) to produce organic mattermetal complexes (John Huggett, R., 2011). Decomposition products from plants and its roots are part of chelating; it promotes chemical weathering and transmission of metals in soils or rocks.

Biological weathering Biological weathering entails disintegration of rocks and minerals by either chemical or mechanical processes or even the combination of both. There are different types of organisms which can cause biological weathering. For example it can be bacteria, plants or even animals.

Panuma Rai

Word count: 1643

Plant roots can exert stress and pressure rocks to fracture as they grow. Even though this process is mechanical however it is still considered as biological weathering due to biological forces fracturing the rocks (biomechanical effect). Plucking is a process where fungi and algae together breaks down rock minerals to break down rock or minerals as they cause gaps and holes. When animals burrow, rocks can be exposed nearer to the surface which can enhance weathering to more intense chemical and mechanical process (John Huggett, R., 2011).

Factors that control weathering Before considering which factor is the most important we need to know which weathering processes occur in which climate. Physical or mechanical weathering is dominated in the cold and dry conditions. Chemical weathering normally dominates the warm and humid conditions whereas Biological weathering tends to dominate the active warm and humid conditions. There are several factors which can control weathering operation such as the climate, rock composition, topography, erosion and time (Hong Kong Geological, 2009). Rock composition includes the type and the structure of the rock. It determines the weathering process in a certain environment as rock type varies depending on the composition of the rock. It also determines the penetration of water into the rock due to the structure of the rock. Topography is one of the factors that can control weathering as it determines the speed and energy rate which water can absorb through the rock. This factor normally controls weathering process where there are high gradient slopes and dynamic weathering systems are taken place.

Panuma Rai

Word count: 1643

Factors like erosion can determine how fast the materials can be disintegrates and how much the rock is exposed to weathering. Chemical weathering is mainly controlled by this factor as it occurs with different types of rocks in certain conditions. Time also play a huge factor in weathering as the duration of how long the weathering last will play a huge part on the degree of the weathering. Climate is one of the main factors which that controls weather processes as it determine what type of weathering process should be taken place. It largely determines the quantity of water obtained and the temperature of the process occur. For example warmer and humid conditions are needed for chemical reactions occur for chemical weathering. While for cold and dry conditions are needed for frost action to occur for mechanical weathering. Conclusion Weathering processes are controlled by different factors. All have different levels of extent to how much is being controlled. Climate is the main factor on determining chemical, mechanical and biological processes. This is because climate controls the temperature and water availability. It is fare to say that without climate, weathering processes are not likely to be as effective and therefore not be able to process. Increase in temperature every 10˚C speeds up chemical reactions (John Huggett, R., 2011)..

Panuma Rai

Word count: 1643

References Bernard J. Smith. (2012). Weathering. In: Holden, J An Introduction to Physical Geography and the Environment. Essex, England: Pearson Education Limited. 185-214. Gutiérrez, M, 2005. Chapter 11 Weathering processes and resulting forms. Climate geomorphology, 8, 241–258 Hong Kong Geological Survery, Civil Engineering and Development Department. (2009). Weathering & Erosion - Introduction to Geomorphological Processes. Available: http://hkss.cedd.gov.hk/hkss/eng/education/GS/eng/hkg/chapter4.htm Last accessed 10th November 2017. John Huggett, R., 2011. Fundamentals of Geomorphology. 3rd ed. New York: Routledge Taylor & Francis Group. Pidwirny, M. (2006). "Weathering". Fundamentals of Physical Geography, 2nd Edition. Available : http://www.physicalgeography.net/fundamentals/10r.html. Last accessed 10th Novemeber 2017. Whalley. W, Warke, P, 2005. Encyclopedia of Grology. 1st ed. Belfast, UK: Elsevier....