WEATHERING AND SOIL FORMATION
SOIL FORMATION
WEATHERING PROCESSES
Weathering is the physical or chemical breakdown of rock. It is this process by which rock is converted into soil. Weathering is generally thought of as a variety of physical or chemical processes that are dependent on theenvironmental conditions present.
(i) Physical Processes
Physical weathering occurs when rocks are broken in to smaller pieces without changing the chemical composition of the rock. Think of a physical change (e.g., ripping a piece of paper) where the sample will change in size but all its other characteristics will remain the same. Physical weathering is the disintegration of rock. Physical weathering processes break rock masses into smaller and smaller pieces without altering the chemical composition of the pieces. Therefore, the disintegrated fragments of rock exhibit the same physical properties as their sources. There are a few types of physical weathering such as:
· Unloading.
· Frost action.
· Organism growth.
· Temperature changes.
· Crystal growth.
· Abrasion.
· Unloading. When rock layers are buried under the surface, they are under compressive stress from the weight of overlying materials. When these materials are removed, the resulting stress reduction may allow the rock unit to expand, forming tensional cracks (jointing) and causing extensive fracturing.
· Frost Action. Most water systems in rocks are open to the atmosphere, but freezing at the surface can enclose the system. When the enclosed water freezes, it expands nearly one-tenth of its volume, creating pressures up to 4,000 pounds per square inch (psi). The expanding ice fractures the rock.
· Organism Growth. Trees and plants readily grow in the joints of rock masses near the surface. The wedging action caused by their root growth hastens the disintegration process.
· Temperature Changes. Daily or seasonal temperature changes can cause differential expansion and contraction of rocks near the earth’s surface. This results in a tensional failure called exfoliation. As the rock’s surface heats up, it expands; as it cools, it contracts. The jointing patterns of igneous rock are
often the result of temperature changes.
· Crystal Growth. The growth of minerals precipitating from groundwater can apply pressure similar to that of expanding ice. Soluble minerals, such as halite (salt), readily crystallize out of solution.
· Abrasion. Sediments suspended in wind or fast-moving water can act as abrasives to physically weather rock masses. Rock particles carried by glacial ice can also be very abrasive.
(ii) Chemical Processes
Chemical weathering is the decomposition of rock through chemical processes. Chemical reactions take place between the minerals of the rock and the air, water, or dissolved or suspended chemicals in the atmosphere. Processes that cause chemical weathering are
· Oxidation.
· Hydration.
· Hydrolysis.
· Carbonation.
· Solution.
· Oxidation. Oxidation is the chemical union of a compound with oxygen. An example is rusting, which is the chemical reaction of oxygen, water, and the iron mineral pyrite (FeS2) to form ferrous sulfate (FeSO4). Oxidation is responsible for much of the red and yellow coloring of soils and surface rock bodies. This type of reaction is important in the decomposition of rocks, primarily those with metallic minerals.
· Hydration. Hydration is the chemical union of a compound with water. For example, the mineral anhydrite (CaSO4) incorporates water into itsstructure to form the new mineral gypsum (CaSO4·2H2O).
· Hydrolysis. This decomposition reaction is related to hydration in that it involves water. It is a result of the partial dissociation of water during chemical reactions that occur in a moist environment. It is one of the types of weathering in a sequence of chemical reactions that turns feldspars into clays. An example of hydrolysis is the altering of sodium carbonate (Na2CO3) to sodium hydroxide (NaOH) and carbonic acid (H2CO3).
· Carbonation. This is the chemical process in which carbon dioxide from the air unites with various minerals to form carbonates. A copper penny eventually turns green from the union of copper with carbon dioxide in the air to form copper carbonate. Carbonate rocks, in turn, are susceptible to further weathering processes, namely solution.
Solution. Carbon dioxide dissolved in water forms a weak acid called carbonic acid (H2CO3). Carbonic acid acts as a solvent to dissolve carbonates, such as limestone, and carry them away. This creates void spaces, or caves, in the subsurface. Areas that have undergone extensive solutions are known as karst regions.
(iii) Biological Weathering
Biological weathering involves the disintegration of rock and mineral due to the chemical and/or physical agents of an organism. The types of organisms that can cause weathering range from bacteria to plants to animals.
Biological weathering involves processes that can be either chemical or physical in character. Some of the more important processes are:
1. Simple breaking of particles, by the consumption of soils particles by animals. Particles can also fracture because of animal burrowing or by the pressure put forth by growing roots.
2. Movement and mixing of materials. Many large soil organisms cause the movement of soil particles. This movement can introduce the materials to different weathering processes found at distinct locations in the soil profile.
3. Simple chemical processes like solution can be enhanced by the carbon dioxide produced by respiration. Carbon dioxide mixing with water forms carbonic acid.
4. The complex chemical effects that occur as a result of chelation. Chelation is a biological process where organisms produce organic substances, known as chelates, that have the ability to decompose minerals and rocks by the removal of metallic cations.
5. Organisms can influence the moisture regime in soils and therefore enhance weathering. Shade from aerial leaves and stems, the presence of roots masses, and humus all act to increase the availability of water in the soil profile. Water is a necessary component in several physical and chemical weathering processes.
6. Organisms can influence the pH of the soil solution. Respiration from plant roots releases carbon dioxide. If the carbon dioxide mixes with water carbonic acid is formed which lowers soil pH. Cation exchange reactions by which plants absorb nutrients from the soil can also cause pH changes. The absorption processes often involves the exchange of basic cations for hydrogen ions. Generally, the higher the concentration of hydrogen ions the more acidic a soil becomes.
SOIL FORMATION
WEATHERING PROCESSES
Weathering is the physical or chemical breakdown of rock. It is this process by which rock is converted into soil. Weathering is generally thought of as a variety of physical or chemical processes that are dependent on theenvironmental conditions present.
(i) Physical Processes
Physical weathering occurs when rocks are broken in to smaller pieces without changing the chemical composition of the rock. Think of a physical change (e.g., ripping a piece of paper) where the sample will change in size but all its other characteristics will remain the same. Physical weathering is the disintegration of rock. Physical weathering processes break rock masses into smaller and smaller pieces without altering the chemical composition of the pieces. Therefore, the disintegrated fragments of rock exhibit the same physical properties as their sources. There are a few types of physical weathering such as:
· Unloading.
· Frost action.
· Organism growth.
· Temperature changes.
· Crystal growth.
· Abrasion.
· Unloading. When rock layers are buried under the surface, they are under compressive stress from the weight of overlying materials. When these materials are removed, the resulting stress reduction may allow the rock unit to expand, forming tensional cracks (jointing) and causing extensive fracturing.
· Frost Action. Most water systems in rocks are open to the atmosphere, but freezing at the surface can enclose the system. When the enclosed water freezes, it expands nearly one-tenth of its volume, creating pressures up to 4,000 pounds per square inch (psi). The expanding ice fractures the rock.
· Organism Growth. Trees and plants readily grow in the joints of rock masses near the surface. The wedging action caused by their root growth hastens the disintegration process.
· Temperature Changes. Daily or seasonal temperature changes can cause differential expansion and contraction of rocks near the earth’s surface. This results in a tensional failure called exfoliation. As the rock’s surface heats up, it expands; as it cools, it contracts. The jointing patterns of igneous rock are
often the result of temperature changes.
· Crystal Growth. The growth of minerals precipitating from groundwater can apply pressure similar to that of expanding ice. Soluble minerals, such as halite (salt), readily crystallize out of solution.
· Abrasion. Sediments suspended in wind or fast-moving water can act as abrasives to physically weather rock masses. Rock particles carried by glacial ice can also be very abrasive.
(ii) Chemical Processes
Chemical weathering is the decomposition of rock through chemical processes. Chemical reactions take place between the minerals of the rock and the air, water, or dissolved or suspended chemicals in the atmosphere. Processes that cause chemical weathering are
· Oxidation.
· Hydration.
· Hydrolysis.
· Carbonation.
· Solution.
· Oxidation. Oxidation is the chemical union of a compound with oxygen. An example is rusting, which is the chemical reaction of oxygen, water, and the iron mineral pyrite (FeS2) to form ferrous sulfate (FeSO4). Oxidation is responsible for much of the red and yellow coloring of soils and surface rock bodies. This type of reaction is important in the decomposition of rocks, primarily those with metallic minerals.
· Hydration. Hydration is the chemical union of a compound with water. For example, the mineral anhydrite (CaSO4) incorporates water into itsstructure to form the new mineral gypsum (CaSO4·2H2O).
· Hydrolysis. This decomposition reaction is related to hydration in that it involves water. It is a result of the partial dissociation of water during chemical reactions that occur in a moist environment. It is one of the types of weathering in a sequence of chemical reactions that turns feldspars into clays. An example of hydrolysis is the altering of sodium carbonate (Na2CO3) to sodium hydroxide (NaOH) and carbonic acid (H2CO3).
· Carbonation. This is the chemical process in which carbon dioxide from the air unites with various minerals to form carbonates. A copper penny eventually turns green from the union of copper with carbon dioxide in the air to form copper carbonate. Carbonate rocks, in turn, are susceptible to further weathering processes, namely solution.
Solution. Carbon dioxide dissolved in water forms a weak acid called carbonic acid (H2CO3). Carbonic acid acts as a solvent to dissolve carbonates, such as limestone, and carry them away. This creates void spaces, or caves, in the subsurface. Areas that have undergone extensive solutions are known as karst regions.
(iii) Biological Weathering
Biological weathering involves the disintegration of rock and mineral due to the chemical and/or physical agents of an organism. The types of organisms that can cause weathering range from bacteria to plants to animals.
Biological weathering involves processes that can be either chemical or physical in character. Some of the more important processes are:
1. Simple breaking of particles, by the consumption of soils particles by animals. Particles can also fracture because of animal burrowing or by the pressure put forth by growing roots.
2. Movement and mixing of materials. Many large soil organisms cause the movement of soil particles. This movement can introduce the materials to different weathering processes found at distinct locations in the soil profile.
3. Simple chemical processes like solution can be enhanced by the carbon dioxide produced by respiration. Carbon dioxide mixing with water forms carbonic acid.
4. The complex chemical effects that occur as a result of chelation. Chelation is a biological process where organisms produce organic substances, known as chelates, that have the ability to decompose minerals and rocks by the removal of metallic cations.
5. Organisms can influence the moisture regime in soils and therefore enhance weathering. Shade from aerial leaves and stems, the presence of roots masses, and humus all act to increase the availability of water in the soil profile. Water is a necessary component in several physical and chemical weathering processes.
6. Organisms can influence the pH of the soil solution. Respiration from plant roots releases carbon dioxide. If the carbon dioxide mixes with water carbonic acid is formed which lowers soil pH. Cation exchange reactions by which plants absorb nutrients from the soil can also cause pH changes. The absorption processes often involves the exchange of basic cations for hydrogen ions. Generally, the higher the concentration of hydrogen ions the more acidic a soil becomes.
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