Weathering and Sedimentary Rock Formation
Weathering
Weathering is the process of physical and chemical transformation of parent rocks and primary minerals. This process generates secondary minerals, such as clays, that form soil. Weathering involves chemical reactions where products become reagents for subsequent synthesis. Weathering at the soil surface is called edaphochemical weathering, while weathering in deeper layers (like the C horizon) is called geochemical weathering.
Schistosity
Schistosity is a physical change involving the separation of metamorphic rocks into near-parallel layers. Examples include various schists (phyllites) and gneiss.
Weathering Processes
Weathering is the alteration of rocky material exposed to air, humidity, and organic matter. It involves mechanical disintegration and chemical decomposition, often interacting. Humidity and temperature variations influence both weathering forms, affecting rocks mechanically and promoting chemical reactions.
Physical Weathering
Physical weathering, primarily occurring in desert and periglacial environments, is caused by physical processes. Desert climates experience significant temperature differences between day and night, and the lack of vegetation allows direct sun exposure on rocks. Periglacial environments see temperatures fluctuate above and below the melting point of ice daily or seasonally.
Chemical Weathering
Chemical weathering disintegrates rocks and minerals when they react with nearby substances, mainly those dissolved in water. This creates new minerals with different chemical compositions and greater stability in external conditions. Minerals with weaker bonds and formed in conditions distant from Earth’s surface environment are generally more susceptible to weathering.
Factors of Physical or Mechanical Weathering
Mechanical weathering factors include thermal stress, frost wedging, pressure release, exfoliation, root action, and crystal growth.
Sedimentary Rocks
Source
Weathering products can be transported along the bottom of flows (dragging, rolling, or saltation) or within the fluid (suspension, solution, or flotation).
Abundance
Sedimentary rocks are common in engineering projects. The top 15 km of the Earth’s crust consists of 5% sedimentary rocks and 95% igneous and metamorphic rocks, with the latter dominating deeper environments.
Diagenesis
Diagenesis, the formation of sedimentary rocks from sediments, involves processes that generally reduce porosity and increase material compactness.
Major Lithification Processes
The main diagenetic processes are cementation, consolidation, desiccation, and crystallization. Lithification is the process of rock formation, in this case, from sediment consolidation.
- Cementation: Minerals like calcite and dolomite, dissolved in water, fill sediment interstices. This process requires porous and permeable material and is dominant in sandstone, limestone, and dolomite formation.
- Consolidation and Desiccation: These linked processes involve water expulsion during consolidation. In wind-transported sand deposits, evaporation can lead to lithification. This process doesn’t necessarily require porous and permeable material. Clays at a depth of one kilometer lose 60% of their volume due to confining pressure, forming shale.
Characteristics of Sedimentary Rocks
The main characteristics of sedimentary rocks are stratification, facies, and color.
Stratification
Stratification is the most important characteristic. Each layer represents a completed event. The internal geometry of a layer and the overall geometry of the layers reveal the formation environment. Layers can be horizontal (quiet environments like lakes), wavy (dune environments), sloped (detrital environments), rippled (beach environments), cross-bedded (swamp environments), or geode-like (concentric spherical shells formed due to osmotic salinity differences).
Sedimentary Facies
Sedimentary facies refers to the accumulation of deposits with specific characteristics that grade laterally into other sedimentary accumulations formed simultaneously but with different characteristics.
Organic Sediments
Organic compounds decompose quickly through anaerobic bacterial action (putrefaction) or oxidize in the presence of air through aerobic bacterial action. If materials are covered by oxygen-poor water, they ferment anaerobically, increasing their free carbon content.
Soil Formation
Weathering is the chemical transformation of rocks into soil. Soil formation is synonymous with weathering. In igneous and metamorphic rocks, weathering transforms dense, solid materials into soft, porous particles with different chemical compositions and structures. Weathering has a less intense effect on sedimentary rocks.
When rocks are exposed to surface conditions, erosion, water freezing and thawing, and heating and cooling slowly break them down. However, the most significant changes are caused by chemical reactions with water, oxygen, carbon dioxide, and organic compounds.