Chapter 6: Weathering and Soils

Weathering is categorized into mechanical and chemical pathways that operate simultaneously to break down rock and alter mineral composition. Mechanical weathering physically fragments rock without changing its mineralogical identity through frost wedging, where water expansion in fractures exerts tremendous pressure, salt crystal growth that forces mineral grains apart, sheeting or exfoliation that creates dome-shaped landforms as overlying weight is removed, and biological activity from plant roots and animal burrowing. By exponentially increasing rock surface area, mechanical processes dramatically accelerate chemical weathering. Chemical weathering operates primarily through aqueous solutions and includes dissolution of soluble minerals like calcite in acidic water, oxidation of iron-bearing minerals that produces rust-colored compounds and acid mine drainage, and hydrolysis of silicate minerals that breaks down feldspars into clay minerals while releasing ions to groundwater. The relative resistance of different minerals to weathering and the influence of climate, particularly warm and moist conditions, create differential weathering patterns that sculpt distinctive landforms. Soil represents the most significant product of weathering and functions as a critical boundary layer where geosphere, hydrosphere, atmosphere, and biosphere interact. Soil development depends on five controlling factors: parent material composition, climate regime, biological communities, elapsed time allowing horizon differentiation, and topographic position influencing water movement and erosion. Mature soil profiles display distinct horizons from organic-rich surface layers through enriched subsurface accumulation zones to partially weathered parent material. Soil classification systems recognize orders adapted to different climatic and environmental conditions, with properties like texture and structure determining hydrological and agricultural behavior. The chapter concludes by addressing anthropogenic impacts on soils, particularly the accelerated erosion and degradation resulting from deforestation, inappropriate cultivation practices, and overuse that can render soils unproductive within decades despite formation requiring millennia.