Sedimentary Processes 1) Weathering & erosion 2) Transport & 3) deposition 4) Lithification
Weathering: decomposition and disintegration of rockProduct of weathering is regolith or soilRegolith or soil that is transported is called sedimentMovement of sediment is called erosion
Chemical Weathering- Decomposition of rock as the result of chemical attack. Chemical composition changes. Mechanical Weathering - Disintegration of rock without change in chemical composition Weathering Processes
Mechanical Weathering • Decompression causes jointing • Frost wedging • Alternate heating and cooling
Chemical Weathering Processes • Hydrolysis - reaction with water (new minerals form) • Oxidation - reaction with oxygen (rock rusts) • Dissolution - rock is completely dissolved Most chemical weathering processes are promoted by carbonic acid: H2O +CO2 = H2CO3 (carbonic acid)
Oxidation Oxidation can affect any iron bearing mineral, for example, ferromagnesian silicates which react to form hematite and limonite
Oxidation of pyrite and other sulfide minerals forms sulfuric acid which acidifies surface water and rain Pyrite + oxygen + water = sulfuric acid + goethite (iron sulfide) (iron oxide)
Hydrolysis Feldspar + carbonic acid +H2O = kaolinite (clay) + dissolved K (potassium) ion + dissolved bicarbonate ion + dissolved silica Clay is a soft, platy mineral, so the rock disintegrates
Products of weathering Clay minerals further decompose to aluminum hydroxides and dissolved silica.
Controls on rates of weathering • Relative resistance of primary minerals to chemical weathering • Climate – especially: • availability of water (for chemical weathering), • temperature (increases reaction rates), and • vegetation (which increases the availability of carbonic acid)
Soil classification1) Laterite or oxisol Forms in a hot, humid climate. Soil is deep red, hard and infertile. Plants recycle nutrients in a thin A and O horizon.
Deforestation removes the fertile organic layer. The underlying soil is infertile, dries to brick-like hardness when it dries out, and is difficult to cultivate. Aluminum (from bauxite) and iron (from limonite) can be mined from these soils.
Soil classification 2) Alfisols and mollisols: Form in warm or cool, temperate climates. Soil is grey and clay-rich, fertile.Alfisols are forest soils, mollisols form in grasslands
Soil map of world Greens – Alfisols & mollisols Red – Oxisols Tan - Aridosols
Minerals formed by weathering:(pages 191-192) Clay minerals: kaolinite, smectite (Na, Ca), illite (K) (pages 255-256) Aluminum oxides and hydroxides: gibbsite, diaspore, boehmite (consituents of bauxite) (pages 371-374) Iron oxides and hydroxides: hematite, goethite, lepidocrosite, limonite Mn oxides and hydroxides (bog ores or wad): pyrolusite, romanechite, manganite, others