Weathering and Erosion of Rocks Presentation

Weathering and erosion of rocks The evolution of landscapes Stefano FURLANI Department of Mathematics and Geosciences Program of the lecture • Definition of weathering, erosion and denudation • Mechanical weathering • Chemical weathering • Biological weathering • Erosion • Coastal vs inland erosion rates • Bedrock lowerings of shore platforms • Genesis and evolution of coastal landforms • Erosion and preservation of human structures Definitions Weathering and erosion Weathering and erosion • Weathering is the decay of rocks, or the chemical alteration and physical breakdown of rock material in response to environmental conditions; it is a consequence of the exposure of rocks to the weather • Chemical weathering is a process that breaks down rock by altering its structure through any chemical change • Mechanical (or physical) weathering is a physical process that breaks down rock without causing any chemical change • Biological weathering involves both chemical and mechanical processes acted by organisms • Erosion is the breakdown and removal of rock by the same force. It implies movement. • Denudation is the long-term sum of all weathering and erosion processes. It results in the lowering of rock surfaces. What is the difference? • Weathering involves in situ processes • Erosion involves Relations between geomophological processes (from Fairbridge, 1965) movement of altered rocks https://socratic.org/questions/what-is-weathering-and-how-is-it-different-from-erosion …breaking and movement… From: https://sites.google.com/a/gx.camden.k12.ga.us/vurnakes6-1/science-units/weathering-and-erosion Sketch of weathering and erosion processes (from Atkinson, 2004) Some examples of weathering processes Some examples of erosion processes Related terms • Abrasion is the process of scouring, scratching, wearing down, marring, or rubbing away of rocks. • Corrosion in physical geography include chemical and biological processes • Corrasion indicates the process of mechanical erosion of the earth's surface caused when materials are transported across it by running water, waves, glaciers, wind or gravitational movement downslope, resulting in abrasion Studies of weathering and erosion (Trudgill, 1983) • The potential for weathering and erosion inherent in the environment; • The susceptibility of the material involved to the environmental potential; • The rates of operation of processes at the site of reaction; • The rates and amounts of losses of material from the site of reaction; • The residual material left behind after reaction Factors affecting weathering processes From: http://greenfieldgeography.wikispaces.com/IGCSE+and+GCSE+Weathering Weathering factors https://www.chegg.com/homework-help/definitions/weathering-factors-2 Concepts • Materials and processes involved are different in weathering and erosion, and the principles are the same; • Weathering (or erosion) potential: It indicates the ability of a system to change in response to changing weather • Weathering (or erosion) susceptibility is a quantification for the change of an extensive property of the rock under variation of an intensive property. It is affected by the mineralogy and structure of the rock Susceptibility • Very high susceptibility: halite, gypsum • High susceptibility: halite, pyrite, calcite, dolomite • Medium susceptibility: Ca-plagioclase, pyroxene, amphibole, biotite, Na-plagiocalse, K-feldspar, muscovite • Low susceptibility: quartz, clay, bauxite, iron oxides Study model of erosion and weathering Potential Loss materials RATES Material Residuals Modified by Trudgill (1983) Mechanical weathering Principles and types of mechanical (or physical) weathering Principles • Mechanical weathering involves the breakdown of large masses of solid rock into smaller fragments. • It provides material for sediments and fresh rock surfaces for chemical and biological weathering • It occurs under stress along lines of weakness, or crystal cleavage planes, joints, fractures • Summerfield (1991) highlighted that «mechanical weathering encompasses a range or mechanisms the relative effectiveness of which are not accurately known but clearly vary significantly as a function of environmental conditions Processes • Wetting and drying • Insolation weathering • Salt weathering • Freeze-thaw weathering (Ice wedging) • Pressure release • Sheeting or exfoliation Ustica (I) Wetting and drying (slacking) • Wetting and Drying is effective at breaking up rocks that contain clay. • If clays get wet they swells up. When they dry they shrinks again. • Rocks are repeatedly swelling and shrinking causing them to crack and fall apart. Salt weathering (hasloclasty) • Salt crystallization, also called haloclasty, causes disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. The latter expand as they are heated up, exerting pressure on the confining rock. • The most effective salts, such as sodium sulfate and calcium chloride, can expand up to three times or even more. • Salt weathering is usually associated with arid climates and along coasts. • An morphological example of salt weathering are the honeycombes, tafoni, which are related to both chemical and salt weathering processes. Examples of salt weathering Sardinia (I) Sardinia (I) Frost weathering (ice wedging) • Freeze-thaw weathering, also called frost weathering or ice wedging is the general name involving ice degradation of rocks. • These processes include frost shattering, frost- wedging and freeze-thaw weathering. • Frost weathering is particularly effective in mountains when the temperature ranges around the freezing point of water. • Frost weathering produces large amounts of rock fragments located at the foot of mountains or along slopes. Frost weathering From: http://alliance.la.asu.edu/ Arizona Geographic Alliance Frost weathering Debeli Rtic (Slovenia) Exfoliation • It occurs in rocks with homogeneous strucure • Repeated heating and cooling by daily temperature changes affect weathering • Repeated expansiona dn contraction create stress in rock and produce radial and concentric cracks • The outer layers eventually peel off to form exfoliation Little Shuteye Pass (USA) Chemical weathering Methods of evaluation of chemical weathering Principles • Chemical weathering involves the alteration of the minerals of rocks by chemical reactions • They can be influenced by biological processes, so often it is very difficult to distinguish them • It is related to the surface area available for chemical and biological processes Variables • Water: in general, the more precipitation, the greater the rate os weathering. As a consequence, dry climates show slow rates • Temperature: the higher rates of chemical weathering occur in warm, wet climates, while very cold climates show slow rates. Dissolution • Many minerals can be dissolved by water because it is a bipolar molecule. • The bonds of ions at the mineral surface are loosen • Salt and gypsum are easily dissolved • Also some silicates, such as pyroxene, can be dissolved in carbonic acid • Carbon dioxide forms carbonic acid in water. It can dissolve calcite following the formula: Lapiez, pinnacles (Ahrax Point, Malta) Solution pan dissected by cliff retreat (Malta) Weathering potential from acids • Acids react with many types of minerals in rocks dissolving them. • Acids in contact with rocks can dissolve itself in different ways. • One common way occurs when CO2 is dissolved in water and produces carbonic acid. • Also some silicates, such as pyroxene, can be dissolved in carbonic acid. Hydration • A mineral reacts with either the H+ or the OH- from water to produce a new mineral • Aluminium silicates do not dissolve in water • Weathered feldspar, such as plagioclase, forms clay NaAlSi3O8 + H20 + H2CO3 + - Na + HCO3 + H4SiO4 + Al2Si2O5(OH)4 Oxidation and reduction • The chemical combination of oxygen with a mineral • It is fundamental in weathering of iron-rich silicates, such as pyroxene, olivine, biotite, amphibole • The final products are hematite or lemonite 2Fe2SiO4 + 4H2O + O2 2Fe2O3 + 2H4SiO4 Fe2O3 + H2O 2FeO(OH) (limonite) Results of chemical weathering • Na, K, Ca and Mg, such as in limestones, are moved into solution • Al and Si are concentrated in clays • Fe is incorporated in oxides Rock weathering • Granite: exfoliation, quartz to clay. It provide the source for quartz sand grains • Basalt: weathers totally to clay and iron oxides to form red or brown soils • Sandstone: quartz is resistant to weathering andcan be found into new sand deposits. The type of cement controls the erosion (e.g. calcite, silica) • Limestone: dissolution to form karst features Differential weathering • Different rocks weather at different rates • This create different landforms at different scales (e.g. canyon slopes, mesa, etc) Results of weathering • Smaller blocks • Regolith • Soil • Ions in solution Shapes of weathered rocks • Fractures and joints can accelerate weathering • Bedding planes – rocks break into slabs or sheets • Metamorphic rocks break along leavage or exfoliation planes • Shattering in dense rocks such as quartzite • Granular disintegration in sandstone or granite • Spheroidal weathering, such as subrounded spheres • Exfoliation, breaking into concentric layers Biological weathering How living organisms destroy rocks Biological processes • Biomechanical weathering (plant roots, animal burrows, etc) • Biochemical weathering (solution, etc) • Some plants like mosses and lichens are capable of growing without soil

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