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Weathering Chapter 7.1 Earth-Environmental WZPP Introduction Read page 153 on Cleopatra’s needle and see figure 7-1 WZPP ees 7.1 2 Weathering of Cleopatra’s needle. The photograph taken to the left in Egypt before 1880, on the right was taken in New York in 1918 (courtesy of the Metropolitan Museum of Art) WZPP ees 7.1 3 Introduction Weathering Is the process by which rocks near the earth’s surface break down and change Erosion The removal and transport of weathered material These processes have been going on for a long time WZPP ees 7.1 4 Mechanical Weathering Materials change size or shape Breakdown of material into smaller pieces Backcountry lake Tetons NP J B Photos Holly Lake Tetons NP J B Photos WZPP ees 7.1 5 Mechanical Weathering Temperature Water freezes and expands by 9% Exerts pressure on the rock Frost wedging Causes potholes WZPP ees 7.1 6 Mechanical Weathering Pressure When rock is deep underground and is exposed at the surface, the pressure of the rock is reduced Rocks break apart like layers in an onion (exfoliation) Example: Half Dome in Yosemite NP 7.1a ws WZPP ees 7.1 7 Mechanical Weathering Pressure Roots wedge themselves into cracks and exert pressure as they grow WZPP ees 7.1 8 Chemical Weathering Rocks and minerals undergo changes due to chemical reactions Chemical reactions form new minerals and release dissolved substances WZPP ees 7.1 9 Chemical Weathering Composition of the rock determines the effects of chemical weathering WZPP ees 7.1 10 Chemical Weathering Temperature determines the rate of chemical weathering Increase temperature means increased chemical weathering WZPP ees 7.1 11 Chemical Weathering Agents of erosion: Water Oxygen Carbon dioxide Acids WZPP ees 7.1 12 Chemical Weathering Water Dissolves many kinds of minerals and rock Serves as a medium through which other reactions occurs WZPP ees 7.1 13 Chemical Weathering Oxygen Combines with other substances Iron (& other minerals) readily combine with atmospheric O2 WZPP ees 7.1 14 Chemical Weathering Carbon Dioxide + H2O weak carbonic acid Reacts with minerals (like calcite) in limestone, marble to dissolve rock CO2 7.1b ws WZPP ees 7.1 15 Chemical Weathering Acid precipitation Caused mainly by oxidation of sulfur dioxide and nitrogen oxide in atmosphere due to human activities Is precipitation below 5.5 pH Is harmful to living organisms WZPP ees 7.1 16 Rate of Weathering Climate has a major influence Variables include Precipitation Temperature Evaporation Interaction between temperature and precipitation is the greatest effect WZPP ees 7.1 17 Rate of Weathering Chemical weathering occurs readily in areas that have warm temperatures, abundant rainfall and lush vegetation WZPP ees 7.1 18 Rate of Weathering Physical weathering occurs readily in cool, dry climates Is highest where water undergoes repeated freezing and thawing Remember, cool temperatures inhibit (slows) chemical reactions WZPP ees 7.1 19 Rate of Weathering Harder rock is going to be more resistant to weathering than soft rock This depends type of rock and their composition (what they are made of) Sedimentary rock is generally going to be more easily weathered than igneous or metamorphic WZPP ees 7.1 20 Rate of Weathering Surface area Increased surface area equals increased weathering Both physical and chemical WZPP ees 7.1 21 Rate of Weathering Topography The steeper the slope, the greater tendency for a material to move downhill Indian Paint brush canyon Tetons NP J B Photos WZPP ees 7.1 22 Rate of Weathering Organisms Decaying organic matter and plant roots release carbon dioxide for chemical weathering 7.1c ws WZPP ees 7.1 23