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Weathering & Erosion

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The process of disintegration of rocks in situ (static) is generally called weathering. Weathering is the breakdown and alternation of minerals . Near the earth's surface to products that are more in equilibrium with newly imposed physico-chemical conditions. Thus, weathering may be defined as the mechanical frac-turing or chemical decomposition of rocks by natural agents at the surface of the earth. It is obvious that weathering involves two types of changes in the rocks, for example: physical and mechanical changes, wherein rocks are disintegrated through temperature changes, frost-action, biological activities (biotic factor) and wind actions. Chemical changes, wherein rocks are decomposed through static water, oxygen, carbon dioxide and biological activities.

Provided that:

  1. The breakdown of rocks occurs in situ;

  2. There is no large-scale transport of weathered materials except mass-movement or mass-transaction of weathered materials down the slope under the force of gravity. The products of rock weathering tends to accumulate in a short surface layer called regolith; and the regolith grades downward into solid or altered rocks known as bed rocks. Therefore, regolith is the layer of loose broken rocky material mantling the surface of the undecomposed bedrock. The regolith produces the source of sediments consisting of detached mineral particles deposited and transported a fluid medium which may be water, air or glacier ice.

Agents of Weathering

  1. Transportation: The wind, running water, moving ice and sea waves also carry away particles, thus removing one part and settling it in other part is called the process of transportation.

  2. Deposition: The material carried out by winds, running water, and moving ice are deposited in some other place is called deposition.

  3. Erosion: It is same as weathering, but, it is very much different at the same time. Weathering is a static process, there is no displacement and movement of rocks. It does not constitute depositional features. Whereas erosion is basically more a process of denudation than weathering. Denudation is a term used to denote the action of laying bare by the process of washing away of the surface materials, such that all surface inequalities would be reduced to uniformity. It is ba-sically the process of aggradation. Erosion is basically a dynamic process, there is always displacement of rocks,

Denudation

thus, denudation = weathering + transportation. The depositional feature is also included in the erosion. Therefore, we see erosion in a broader concept, because it includes both transportation and deposition.

(1) Block Disintegration Due to Temperature Change: The repetition* of expansion and contraction of outer rock layers due to diurnal rangeof temperature in the hot desert areas causes tension and stresses which introduce parallel joints in the Rocks.

(2)Granular Disintegration Due to Change in Temperature: The coarse grained rocks are more affected by shattering process in those hot deserts which are characterised by high range of daily temperature.

(3) Shattering Due to Rain Shower and Heat: The outer shells of the rocks are shattered hot climatic regions mainly in hot desert areas.

(4) Block disintegration due to frost: Frost action weakens the rocks in two ways:

  1. Due to freeze and thaw of water between the particles of the rocks.

  2. Due to freeze and thaw of water in the crevices and spaces.

(5)Exfoliation due to temperature and wind:

Exfoliation weathering, also known as onion weathering, refers to peeling off concentrjc shells of rocks due to combined actions of heat and wind in hot

arid and semi-arid regions and monsoon lands. The outer shells of rocks become loose due to alternate expansion and contraction due to high temperature during daytime and comparatively low temperature during night respectively, and these loosened shells are removed (peeled off by strong winds).

(6) Disintegration and exfoliation due to unloading: Sheeting refers to the development of cracks and fractures

Parallel to the surface caused bv removal of super-incurnbent load resulting into reduction of conflicting pres-sure.

Chemical weathering

Decomposition and disintegration of rocks due to chemical reaction is called chemical weathering wherein the minerals of the rocks weather away. Water vapour and water are the media which activate several types of chemical reactions within the rocks. Pure water, distilled water, is chemically inert but when it mixes with the atmospheric gases, mainly with C02, it becomes potent solvent. Following are the important chemical reactions:

  1. Solution: It refers to the dissolution of soluble particles and minerals from the rock with the help of water in motion but a thin film of water around a solid particle also leads to chemical dissolution. Common salts are most soluble whereas carbonate rocks are of moderate solubility. Limestones are more susceptible to solution process which depends on temperature, C02 content of water and PH of the solution.

  2. Oxidation: The chemical process of oxidation simply means a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen, its reaction with the minerals of the rocks forms hydroxide.

  3. Carbonation: It is the reaction of carbonate or bicarbonate ions with minerals. The process of carbonation is also known as 'solution' wherein atmospheric C02 after mixing with water forms Carbonic acid (H2C03), i.e.,

  4. C02 + H20 >H2C03,

  5. which after reacting with carbonate rocks, say limestones (CaC03) forms Calcium carbonate [Ca(HC03)2] which is easily dissolved in water.

  6. Hydration: The process of hydration is related to the addition of water to the minerals. The rocks after having absorbed water undergo the process of positive change of their volume. The process of hydration changes feldspar minerals into Kaolinite clays, this process is known as 'Kaolinization'.

  7. Hydrolysis: It is a chemical reaction between minerals and water, that is, between hydrogen ions or hydroxyl (OH) ions, and the ions of the mineral in order to form mineral compounds. Silicate minerals are most affected by hydrolysis.

  8. Chelation: Chelation is a complex organic process by which metallic cations are incorporated into hydrocarbon molecules. Chelation is a form of chemical weathering by plants.

Biotic Weathering

1. Faunal weathering: The burrowing of animals, worms and other organisms help in gradual breakdown of rocks or fragments thereof.

2.Floral weathering: Floral weathering does not take place independently, rather it helps the physical and chemical process of weathering. Larger plants affect and control weathering in a number of ways:

  1. Cracks are widened by root penetration and consequent root pressure.

  2. Dense vegetation cover generates distinct micro climate at the ground surface.

3.Anthropogenic weathering: The economic and technological man lashed with modern technologies was becoming the most powerful weathering and erosion agent. Biochemical Weathering:

  1. It refers to decomposition and disintegration of rocks due to organic materials of both flora and fauna.

  2. A complex set of different biochemical processes such as Cation exchange in roots, chelation, solution by root exudates and production of different kinds of organic acids such as humic acids, bacterial acids, microfaunal acids, etc. produced by organic materials.

  3. Humic Acid active chelation and helps in the decomposition of silicate minerals. Fulvic Acids derived from peat, help in the decomposition of rock materials.

Mass Movement-Mass wasting Phenomena

Direction of Movement of rock is divided into three categories:

  1. Vertical: rockfall, collapse (of roofs of underground caves or cavities or lava tubes) earthfall, debris fall, topple (rotational fall of rock slab's, or of earthen material) and settlement (collapse of ground surface due to withdrawal of water, crude oil, etc.);

  2. Lateral: Blockslide (movement of materials along a horizontal fracture or interface between two rock strata), spread (lateral displacement of a series of rock blocks), cambering (draping of sedimentary units), sackung (lateral spreading away from anticlinal crests CREEPING: very slow and imperceptible downslope movement of materials i.e colluvium); and

  3. Diagonal: soil creep (movement of moistened soils downslope), rockcreep, talus creep, rockslide, debris slide, slumping (movement of fine materials along a curved plane), flow (dominant role of water, downslope transport of water-soaked fine debris), debris flow, mud flow, solifluction and avalanche.