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Weathering What is weathering ? Weathering is the physical, chemical and biological decay of materials at the earth’ earth’s surface into products which are in equilibrium with new imposed physicophysico-chemical conditions. Professor Paul Shaw Weathering takes place in situ in the presence of water. It is not the same as EROSION Weathering What is being weathered ? Rocks – minerals – elements • • • • What is being weathered ? What is being produced ? What are the processes involved ? What are the environmental conditions under which it takes place ? • Elements properties : valency, valency, ionic radius • Mineral properties : chemistry, structure • Rock properties : mineral suite, structure, origins Element properties Properties of rocks • Ionic radius – size of atom • Igneous rocks – Form under great heat and pressure, either intrusively or extrusively. extrusively. Their formational environment does not contain water. – Are crystalline, with complex mineral content. • Valency – surplus electron charge – Anions = negative charge – Cations = positive charge • Sedimentary rocks – Formed from sediments near the earth’ earth’s surface, and in the presence of water. – Occur as layers (strata) and may contain fossils – Have a relatively simple chemical and physical composition – Can form physically, organically or chemically The valency of common elements: Si +4, Al +3, Fe +2/3 Mg +2, Ca +2, K +1, Na +1…… .O –2 +1…….O Common anions = S, SO4, Cl, Cl, NO3, CO3 • Metamorphic rocks – Maintain characteristics of the parent material 1 Properties of minerals • • • • • Chemical composition Mineral structure Crystal size Crystal shape Crystal perfection Silicate structures Processes of chemical Weathering Water disassociates To H20, H2, O, OH Processes of physical weathering • Internal stress – exfoliation of ‘permissive’ permissive’ rock • Internal disintegration through heat and hydration at crystal level • Solar insolation • Fire • Crystal growth – Ice – Salt • Biophysical force Processes of biological weathering Essentially biophysical or biochemical. Includes: • • • • Bioturbation by soil organisms Compaction by trampling Boring Root pressure Rock weathering resistance – by property 2 Grain size Classification The end products of weathering are particles in a size range Rock weathering resistance – by representation ---change ---change in properties Environmental factors • All weathering takes place in the presence of water. • Van’ Van’t Hoft’ Hoft’s Law states that rates of chemical activity double for every rise in temperature of 10 degrees centigrade. • Therefore the primary control on weathering is climate on a large (global/regional) scale. The Strakhov diagram of global zonal weathering The Peltier diagram of weathering type/intensity 3 Environmental factors • However at the local scale factors such as slope and drainage may take precedence Albite (NaAlSi3O8) Anaerobic conditions = Montmorillonite 3NaAlSi3O8 +Mg + 4H2O = 2Na0.5Al1.5Mg0.5Si4O10(OH)2 + 2Na + H4SiO4 Aerobic conditions = Kaolinite • This is particularly important in the humid tropics, where it leads to the formation of soil catenas 2NaAlSi3O8 + 9H2O +2H = Al3Si2O5 (OH)4 + 2Na + 4H4SiO4 Aerobic conditions with rapid drainage = Gibbsite NaAlSi3O8 +7H2O + H = Al(OH)3 + Na + 4H4SiO4 • Different conditions may lead to different end products from a single mineral Environmental factors The Soil Catena A primary control at all scales is the rate of removal of weathered material. If the material is not removed, the weathering process will eventually cease. At a landscape scale rates of removal control the landform suite. Summary • Weathering is a complex universal process, controlled by climate and rock type. Energy sources and states of geomorphological processes • Chemical, physical and biological weathering takes place simultaneously, though some processes may dominate in a given environment. • The end products are sediments of varying sizes. 4 Fluids • Fluids are liquids (e.g. water) and gases (air is a mixture of gases) • Ice is a bit of an odd case • Fluids move because of… – gravity (e.g. streamflow) – pressure differences (airflow) Processes of erosion, transport and deposition Viscosity Fluid flows are... • Viscosity is the capacity of a fluid to resist changes of shape • Laminar • Turbulent Boundary Layers Flow Separation • A boundary layer is the depth of fluid affected by the contact of a fluid stream with a stationary boundary • Logarithmic velocity profile – “Law of the Wall” • also sometimes known as vortex shedding 5 Particle Entrainment • Shear stress is what gets grains going Why aren’t the smallest grains the easiest to entrain? • They rest in the zero velocity layer • They are hidden by larger grains • They stick together (especially clays) Modes of transport • • • • • • Suspension Saltation Traction (Reptation) (Flotation) (Solution) The Hjulstrom Diagram – grain size, velocity and fluid density Dissolved Load • No threshold velocity • Same concentration throughout the fluid depth • Not generally visible • Concentration declines as discharge increases Sediment transport • Four modes of sediment transport • Shear stress entrains particles • There is a threshold shear stress at which motion starts • Fine sand is the most susceptible to entrainment • Sediment transport is roughly proportional to the cube of flow velocity 6 Sorting and grading • • Sorting and grading can be defined by statistical parameters, using phi units instead of measurements You will explore this further in a practical exercise. 7