8.1 Physical and Chemical Weathering Mechanisms

Rocks and minerals break down through physical and chemical weathering processes. Physical weathering splits rocks without changing their chemistry, while chemical weathering alters their composition. These processes work together to shape Earth's surface and create soils.

Chemical weathering involves reactions like hydrolysis, oxidation, and carbonation. These reactions produce clay minerals, metal oxides, and dissolved ions crucial for soil fertility and ecosystem health. Understanding these mechanisms is key to grasping Earth's geochemical cycles.

Physical Weathering Mechanisms

Physical vs chemical weathering

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  5.4 Weathering and the Formation of Soil – Physical Geology View original

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  Rock Weathering CO2 Cycle (with annotations) View original

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  5.4 Weathering and the Formation of Soil – Physical Geology View original

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• Physical weathering mechanically breaks down rocks and minerals without altering chemical composition resulting in smaller fragments and increased surface area for chemical weathering
• Chemical weathering alters rock and mineral composition through reactions with water, air, or biological agents producing new minerals or dissolved substances transforming original material

Mechanisms of physical weathering

• Frost wedging occurs when water enters rock cracks, freezes, and expands exerting pressure causing fragmentation through repeated freeze-thaw cycles
• Thermal expansion stresses rocks as different minerals expand and contract at varying rates during daily temperature fluctuations leading to fracturing over time
• Salt crystallization exerts pressure on rock structures when salt-laden water enters pores, evaporates, and leaves growing crystals (common in coastal and arid environments)

Chemical Weathering Mechanisms

Chemical weathering reactions

• Hydrolysis replaces mineral cations with H+ and OH- ions from water (feldspar to clay minerals) $KAlSi_3O_8 + H_2O \rightarrow HAlSi_3O_8 + K^+ + OH^-$
• Oxidation occurs between minerals and oxygen, common in iron-bearing minerals (pyrite oxidation) $4FeS_2 + 15O_2 + 14H_2O \rightarrow 4Fe(OH)_3 + 8H_2SO_4$
• Carbonation dissolves minerals using carbonic acid formed by dissolved CO2 in water (limestone dissolution) $CaCO_3 + H_2CO_3 \rightarrow Ca^{2+} + 2HCO_3^-$

Products of chemical weathering

• Clay minerals result from silicate weathering, have high cation exchange capacity, retain nutrients and water in soil
• Metal oxides and hydroxides form from oxidation reactions, contribute to soil color and structure (iron oxides in red soils)
• Dissolved ions released during mineral dissolution provide essential nutrients for plant growth (Ca, Mg, K, phosphate)
• Carbonates and bicarbonates buffer soil pH and influence water hardness in aquatic systems
• Organic acids produced by plant roots and soil microorganisms enhance mineral weathering rates and nutrient availability