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ECU GEOL 1500 - Weathering - Landforms - Soils - Notes - Fall 2014

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WEATHERING: THE BREAKDOWN OF ROCKSOVERVIEW- Mechanical Weathering is the physical breakdown or disintegration of rocks and minerals into smaller and smaller pieces with no chemical change.- Chemical Weathering is the decomposition of rocks and minerals into new minerals and substances due to the presence of water (H2O), oxygen (O2), and/or hydrogen ions (H+).- Both Mechanical Weathering and Chemical Weathering occur under surface/near-surface conditions.- Note the role of Weathering in the Rock Cycle in that weathering attacks any earth materials that are exposed by uplift and erosion- The source of energy for weathering is Solar Energy/Radiation ROLE OF WATER IN WEATHERING1. Chemical Weathering - Because water is polar molecule, it is a very good solvent.2. Mechanical Weathering - Because the water molecule expands by about 9% in volume upon freezing, it can also promote mechanical weathering. Liquid H2O with no Open Spaces Frozen H2O with Open Spaces MECHANICAL WEATHERING PROCESSES- Mechanical Weathering is the physical breakdown or disintegration of rocks and minerals into smaller and smaller pieces with no chemical change.Frost Action or Frost Wedging - Need moisture, permeability in materials, and temperatures that fluctuate across the freezing point. Increase in volume when water freezes to ice physically breaksrocks into smaller pieces. Often produces Talus at the base of cliffs. Talus = coarse, angular rock material that accumulates at the base of a slope or cliff. Results from a combination of Frost Wedging and Rock Falls.2Mechanical Weathering by Frost Wedging or Frost ActionTalus Slope that Results from Frost Action and Rock Falls in Fractured Quartzite near Taos, NMStress Release/Pressure Release - Opens large fractures in rocks and allows other processes to operate. Rocks originally form under high confining pressure, which reduces the volume of therock. With erosion of the overburden, rocks physically expand, and fractures open within rocks. These fractures are usually large or small joints parallel to the outcrop surface. Stress or Pressure Release or Unloading can continue to considerable depths and cause Exfoliation Domes and Sheeting especially in Granites. 3.Exfoliation and Sheeting Caused by Stress Release4Exfoliation Domes in Granite at Ko Samui, THAILANDSalt Weathering→Crystal Growth - Mechanical disintegration of rocks due to growth of salt crystals in pore spaces in rocks. Most often occurs in semi-arid to arid environment where evaporation is a major process or in coastal environment where there is salt spray. Produces Tafoni or Cavernous Weathering Features (See Landforms later in these notes) Salt Weathering in a Sandstone at Abiquiu, New MexicoThermal Expansion and Contraction - Alternate heating and cooling of rock surfaces under direct solar heating physically disintegrates rock material.5Mechanical Breakdown of Rocks due Thermal Expansion and ContractionOrganic Activity or Bio-Mechanical Activity- Tree roots and burrowing/boring animals disintegrate materials.6Bio-Mechanical Weathering in THAILAND Bio-Mechanical Weathering by Marine Boring Organisms in a Limestone at Phra Nang Headland, THAILANDRELATIONSHIP BETWEEN MECHANICAL AND CHEMICAL WEATHERINGMechanical weathering breaks rocks and minerals down into smaller and smaller pieces,which provides more surface area for chemical weathering to take place on. So, the 2 types of weathering actually work together to break down rocks.“Surface Area to Volume Ratio”Chemical weathering is dependent on available surface for reaction temperature and presence of chemically active fluids. Smaller particle sizes weather by chemical means more rapidly than large particles due to an increase of surface area. Look at the diagram below and you will see that as the particles get smaller, the total surface area available for chemical weathering increases.7The smaller we subdivide particles, the more surface area is exposed. Things that happen at surfaces, like heat transfer or chemical reactions, happen faster. Some common applications of thisfact: - Crushed ice cools faster than an ice cube - Granulated sugar dissolves faster than a large lump - A carburetor or fuel injector sprays a fine mist of gasoline into an engine cylinder CHEMICAL WEATHERING - Chemical Weathering is the decomposition of rocks and minerals into new minerals and substances due to the presence of water (H2O), oxygen (O2), and/or hydrogen ions (H+).- Chemical Weathering is like a Differentiation Process in that it breaks rocks and minerals into both soluble and insoluble substances. AVAILABILITY OF SUBSTANCES NECESSARY FOR CHEMICAL WEATHERINGThe primary substances necessary for chemical weathering are Water (H2O), Oxygen (O2),and Hydrogen ions (H+), The Hydrogen Ions (H+) are supplied primarily by dissociation of carbonic acid or water.In the chemical weathering equations (separate file), Note the GAS that is removed from the atmosphere for each reaction, the MATERIALS/SUBSTANCES necessary to make the reaction take place, and the NEW PRODUCTS that are a result of the chemical weathering reaction/process.8THE 3 TYPES OF CHEMICAL REACTIONS OR PROCESSES ARE:Dissolution = Chemical reaction whereby ions pass from a mineral into a solution {See Equation Sheet}.Hydrolysis = Chemical reaction with water in which an O-H bond is broken {See Equation}.Oxidation = Chemical reaction in which oxygen as an oxidizing agent causes a loss of electrons by an atom or ion and thus involves an increase in oxidation number {See Equation Sheet}.FACTORS INFLUENCING CHEMICALWEATHERING1. Climate (Warm and wet conditions enhance and cold and dry conditions retard Chemical Weathering)Effect of Climate on Weathering92. Rock Structure (Fractures and Voids)Fractures in Rocks Increase Surface Area and thus Enhance Chemical Weathering3. Organic Activity (Plants and animals supply carbon dioxide and oxygen and organic acids) 4. Time (Longer a rock is exposed on the surface, the more chemical breakdown it experiences) 5. Mineral and Rock composition (Bowen’s Reaction Series – High Temperature Minerals are less Resistant to and Low Temperature Minerals are more Resistant to Chemical Weathering/Breakdown respectively)“Bowen’s Reaction Series and Mineral Stability - Susceptibility to Chemical Weathering”The stability of minerals can be predicted using the Bowen's reaction


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