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Weathering and Erosion. Weathering and Erosion. Weathering: The disintegration or decomposition of rocks on the Earth's surface. Two types: Mechanical and Chemical. Mechanical Weathering. Mechanical Weathering:
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Weathering and Erosion Weathering: The disintegration or decomposition of rocks on the Earth's surface. Two types: Mechanical and Chemical
Mechanical Weathering Mechanical Weathering: Breaks rocks by physical forces into smaller and smaller pieces, each retaining the characteristics of the original piece. Four important processes: Frost wedging, Unloading, Thermal expansion, and Organic activity.
Increasing Surface Area Increased surface area More surfaces available for weathering
Talus Slope Water expands 9% when frozen
Rock Slide Pic I-40 North Carolina
Sheeting Unloading Sheeting or exfoliation Reduction of overlying pressure causes fractures to develop parallel to surface topography
Half Dome Half Dome, Yosemite “Exfoliation Dome”
Table Rock Table Rock, South Carolina
Thermal Expansion Heating and cooling of rocks in very hot desert regions cause stress on the outer surface of the rock. Mineral boundaries and stressed due to heating and cooling. Eventually, the outer shell will crack and fall off. Death Valley, California
Organic Activity • Plant roots grow into rock fractures in search of water and mineral nutrients. • As roots grow, fractures widen. • Burrowing animals move fresh material to the surface, allowing it to weather quicker • than it would undergound. • - Decaying organisms produce acids, which contribute to chemical weathering.
Chemical Weathering • - alters internal structure of minerals by removing/adding elements. • - original material changes into something stable in the surface environment. • - Water is the most important agent in chemical weathering. • - can oxidize (like rust on a fender) and make a material weak & friable. • feldspar weathers into clay. • combines w/CO2 to form carbonic acid: H2CO3 H2O+CO2=H2CO3 >CO2= >Acidity Acid Rain
Haze over northeastern US Power plants & automobiles Phytoplankton bloom; probably pollution
Haze over India India Ganges River
Smog over China Beijing
Arctic warming 11oF warming in winter months during last 30 years
Retreating Iceland Glaciers Iceland 2km retreat since 1973 All 40 of Iceland’s glaciers are retreating
Mt. Kilimanjaro 82% decrease since 1978 Ice cap will be gone in 15 yrs.
Thinning Greenland ice cap Thinning up to 3 feet (!) per year
Silicate Mineral Susceptibility Rocks rich in quartz are very resistant Silica-poor rocks weather easily & quickly quartz --> quartz feldspars --> soft clay minerals amphibole --> clay, hematite olivine --> hematite
Green Sand Beach, Hawaii Weathered olivine minerals, not quartz sand
Bora Bora Bora Bora, Tahiti Basalt; silica-poor Knife-edged ridge
Monument Valley, Utah Quartz-rich, highly-resistant sandstone/quartzite layers
Controls on soil formation: • 1) Parent material • source of the weathered material • - quartz-rich rocks will weather slower than an olivine-rich rock. • 2) Time • - soils need time to evolve. The longer a soil has had time to form, the thicker it'll be. Soils need time to evolve before they are washed away. • 3) Climate • - most important control on the formation of soils. • - variations in temp and precip determine which process will dominate: mechanical or chemical weathering. And the rate of weathering. • - Hot & Wet = thick layer of chemically weathered soil in same time as: Cold & Dry = thin mantle of mechanically weathered debris. • 4) Plants and Animals • - supply organic matter to soils • - bog soil is almost entirely organic, while desert soil lacks organics. • - plants supply acids which increase the weathering/soil forming process. • - microorganisms like fungi, bacteria also help • - end product of organic decay is called humus. • 5) Slope • - steep slopes encourage washing away of soil, so they tend to be very thin. • - flats produce thick, dark, humus-rich soils due to the retaining of water and organic debris.
Humus Decaying organic matter In tropical regions, bacteria consume humus in the soil, so the soil itself is nutrient poor. Soil is slow to regenerate.
Deforestation Central South America 9/19/2001 Tan = deforestation Fires Smoke
Bolivia 1986 2002
