Weathering Weathering the disintegration or breakdown of rock material Mechanical Weathering no change in chemical composition just disintegration into smaller pieces Chemical Weathering breakdown as a result of chemical reactions CaCO3 CO2 H2O Ca2 2HCO3 Mechanical Weathering Physical breakup pressure release water freeze thaw cycles crystallization of salt in cracks thermal expansion and contraction All this increases the total surface area exposed to weathering processes Mechanical Weathering Exfoliation Rock breaks apart in layers that are parallel to the earth s surface as rock is uncovered it expands due to the lower confining pressure resulting in exfoliation Weathering Mechanical Weathering Sheet Joints Exfoliation Stone Mountain GA Half Dome Yosemite CA Exfoliated Domes Yosemite Stone Mountain Georgia showing the product of exfoliation due to unloading Stone Mountain GA Frost Wedging rock breakdown caused by expansion of ice in cracks and joints Shattered rocks are common in cold and alpine environments where repeated freezethaw cycles gradually pry rocks apart Weathering Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks Role of Physical Weathering 1 Reduces rock material to smaller fragments that are easier to transport 2 Increases the exposed surface area of rock making it more vulnerable to further physical and chemical weathering Surface Area and Weathering Rates of weathering Joints in a rock are a pathway for water they can enhance mechanical weathering Chemical Weathering Definition transformation decomposition of one mineral into another Mineral breakdown carbonate dissolves primary minerals secondary minerals mostly clays Net loss of elements retained in the soil Chemical Weathering Water is the main operator Dissolution Many ionic and organic compounds dissolve in water Silica K Na Mg Ca Cl CO3 SO4 Acid Reactions Water carbon dioxide carbonic acid Water sulfur sulfuric acid H effective at breaking down minerals Dissolution H2O CO2 CaCO3 Ca 2 2HCO3water carbon dioxide calcite dissolve into calcium ion and bicarbonate ion Biological activity in soils generates substantial CO2 Bicarbonate is the dominant ion in surface runoff Chemical Weathering Oxidation Oxygen dissolved in water promotes oxidation of sulfides ferrous oxides native metals Organic Activity Plant material makes H ions available Chemical Weathering Hydration attachment of water molecules to crystalline structure of a rock causing expansion and weakness Hydrolysis combination of hydrogen and oxygen in water with rock to form new substances Chemical Weathering Solution process by which rock is dissolved in water Is strongly influenced by pH and temperature When water becomes saturated chemicals may precipitate out forming evaporite deposits Calcium carbonate calcite limestone sodium chloride salt and calcium sulfate gypsum are particularly vulnerable to solution weathering Resistance to Weathering First to Crystallize Fast Weathering Bowen s Reaction Series Goldrich Stability Series Last to Crystallize Slow Weathering Olivine pyroxene to clay H2CO3 acid Feldspars to clay H2CO3 acid Quartz to quartz anything Calcite to anything nothing This photo of Lime Sink was taken on 20 July 1932 over a week after the drawdown which occurred over the night of 9 10 July Karst landforms develop in areas underlain with limestone Biological Weathering Can be both chemical and mechanical in nature roots split rocks apart roots produce acids that dissolve rocks tree throw burrowing animals Weathering Rates of weathering Climate Temperature and moisture characteristics Chemical weathering Most effective in areas of warm moist climates decaying vegetation creates acids that enhance weathering Least effective in polar regions water is locked up as ice and arid regions little water Mechanical weathering Enhanced where there are frequent freeze thaw cycles Mechanical and Chemical Weathering Fracturing disintegration caused by mechanical weathering exposes more surface area Greater surface area means more places for chemical action to occur Alaska Seattle Amazon Altiplano
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