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The Dynamic Earth Code: 17 TOPIC : WEATHERING PROCESSES By Subject: Earth Science Paper: The Dynamic Earth Code: 17 TOPIC : WEATHERING PROCESSES By Prof. A. Balasubramanian Objectives After attending this lesson, the user would be able to know the mechanisms of weathering that are responsible for the dynamic changes of landforms and relief features on the surface of the earth. The kinds of weathering, their impacts on rocks and minerals and their role as geological agents are also highlighted. 1.0 Introduction: 1.1 Geomorphic processes 2.0 Weathering 2.1 Factors influencing weathering 2.2 Impacts of weathering 3.0 Types of weathering 3.1 Physical weathering 3.2 Chemical weathering 3.3 Topography and climate 3.4 Rock Type 3.5 Rock Structure 3.6 Erosion 3.7 Time 4.0 Physical weathering processes 4.1 Abrasion 4.2 Mechanisms of Physical weathering 4.3 Freezing and thawing 4.4 Frost weathering 4.5 Root Wedging 4.6 Heat spalling 4.7 Exfoliation 4.8 Spheroidal weathering 5.0 Chemical weathering processes 5.1 Effectiveness of chemical weathering 5.2 Rate of chemical weathering 5.3 Impacts of chemical weathering 5.4 Processes of chemical weathering 5.5 Solution 5.6 Hydration 5.7 Hydrolysis 5.8 Oxidation 5.9 Carbonation and Dissolution 6.0 Biological weathering processes 6.1 Man and Animals 6.2 Higher Plants and Roots 6.3 Role of Micro- organisms 7.0 Rates of weathering 7.1 Organisms (Biota) 7.2 Time Page 1 of 11 7.3 Mineral Composition 7.4 Slope and weathering 7.5 Exposure 7.6 Particle Size 7.7 Effect of climate 8.0 Weathering Products 8.1 Behavior of Geologic materials 8.2 The temperature and rainfall 8.3 Unloading 9.0 Conclusion Page 2 of 11 Paper : The Dynamic Earth TOPIC : WEATHERING PROCESSES Objectives After attending this lesson, the user would be able to know the mechanisms of weathering that are responsible for the dynamic changes of landforms and relief features on the surface of the earth. The kinds of weathering, their impacts on rocks and minerals and their role as geological agents are also highlighted. Introduction: The Earth’s lithosphere is composed of varieties of rocks and their related relief features. Relief features are distinct landforms depicting the nature parent rocks and the geological process that have created them. Landforms, their origin, their dynamism and distribution are studied in earth’s physical sciences under the branch of geomorphology. The relief of any land is not permanent on the earth’s surface. Every block of rock is subjected to aerial actions and modifications. Over a period of geological times, these surface blocks and rocks break down into smaller and finer pieces and move away from their sources. Since the rocks present in the uppermost layers of the earth’s curst are in close interaction with the processes of atmosphere, hydrosphere and biosphere, they are dynamic features. Geomorphic processes The Sun’s radiant energy, role of water of the hydrological cycle, role of oxygen and carbon-di-oxide of the atmosphere and the action of organic acids over the rock masses, play a significant role in the material transformation over the surface of the earth. All the rocks exposed at or near the surface of the earth are subjected to various physical and chemical processes. These rocks are mostly unstable and are attacked by aerial agencies both physically and chemically. The processes acting on the surface are called as geomorphic processes. The notable processes are weathering, mass-wasting, erosion, transportation and deposition. In this lesson, the mechanisms of weathering and their impacts on the rocks and relief features are highlighted. Weathering Weathering is an important geological mechanism which can destabilize the earth’s surface materials and remove them by erosive processes. Weathering is the physical disintegration and chemical decomposition of a rock mass on the land. It is a unique phenomena happening on the earth’ surface. Weathering is a collective term used to denote the mechanical, chemical and biological(organic) processes that take place on the earth’s surface. Weathering of rock-forming minerals can create new products from pre-existing rocks. In many regions, soils are the ultimate products of weathering. Weathering of rocks releases chemical compounds that become available for biological processes. It is necessary to study the factors that are influencing the weathering processes. Factors influencing weathering In simple terms, weathering is defined as the natural breakdown of rocks into minor fragments, soils and sediments. There are many factors which influence the weathering of minerals and rocks. They are: 1. Topography 2. Climatic conditions – temperature and humidity 3. Physical characteristics of rocks 4. Chemical and structural characteristics of rocks- their mineralogy and structural features. 5. Vegetation- their abundance and type, including the micro and macro vegetation. Weathering involves no moving agent of transport. Impacts of weathering The impacts of weathering are very phenomenal. The nature and magnitude of weathering differs from place to place. The first impact is the physical modification of pre-existing rocks. Several modifications are made on the pre-existing rocks by mechanical forces, chemical reactions and biological interactions, during the process of weathering. These changes are expected in the pre-existing geologic materials that are exposed at or near the surface of the Earth. The second impact is destabilization of masses. Weathering destabilizes the surface materials and encourage their removal by erosion. Page 3 of 11 Erosion is done by geological agents like running water, wind, glaciers and water waves. The third impact of weathering is the formation of soils and sediments. The soils are the products of weathering. Weathering creates both soils and other loose rock fragments. The fourth impact of weathering is the chemical modification. Weathering releases several chemical compounds from the parent rocks and leave them to support the biological processes. Types of weathering Weathering is a general term for several processes. In general, weathering is of three types as : a) Physical weathering b) Chemical weathering and c) Biological weathering. The chemical and physical weathering often go hand in hand. Physical weathering can occur due to temperature, pressure, frost, etc. Living organisms may contribute to mechanical weathering, as well as chemical weathering. Lichens and mosses that are growing on bare rock surfaces may create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances the physical as well as chemical breakdown of the surface layer of the rocks. These are all very slow and silent processes. Physical weathering Physical weathering is also called as mechanical weathering. Mechanical weathering is related to the physical breakup of rocks into small pieces and fragments. In physical weathering, there is no change in the chemistry of the parent rock. The physical characteristics of rocks also influence the process of physical weathering. The characteristics are differential composition , particle size , the hardness and degree of cementation. Some rock bodies are very resistant due to their strong physical properties. They may not be attacked by physical weathering processes. The structure of rocks are fully favourable to weathering activities. Presence of joints in rock bodies are typical examples. Chemical weathering Chemical and structural characteristics of minerals present in rocks are also expected to play a significant role in weathering. Chemical weathering changes the composition of rocks, often transforming them when water interacts with their minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. For minerals of given particle size, chemical and crystalline characteristics determine the ease of decomposition. (e.g.) gypsum – sparingly soluble in water, is dissolved and removed in solution form under high rainfall. Ferro magnesium minerals are more susceptible to chemical weathering than feldspar and quartz. Tightness of packing of ions in crystals is yet another property for encouraging mineral weathering processes. Less tightly packed minerals like olivine and biotitic are easily weathered as compared to tightly packed zircon and muscovite (resistant). Topography and climate Topography is an important factor in relating rocks with the atmospheric pressure, temperature and water vapor. The climatic condition tends to control the kind and rate of weathering. Under conditions of low rainfall, there is a dominance of physical weathering which reduces the size and increases the surface area with little change in volume. The increase in moisture content encourages both chemical as well as mechanical changes. This also creates new minerals and soluble products. The rates of weathering are generally fastest in humid tropical regions as there is sufficient moisture and warmth to encourage chemical decomposition. The easily weatherable minerals disappear on account of intense chemical weathering and more resistant products (hydrous oxides of Fe and Al) tend to accumulate . Climate controls the dominant type of vegetation which in turn controls the biochemical reactions in soils and mineral weathering. Rock Type The rock type determines the resistance of the rock to the weathering processes that operate in that particular environment. Each rock type is composed of a particular set of minerals, which are joined
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