Chapter 6 Notes - Chapter 6 Weathering and Soil External...

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Chapter 6: Weathering and Soil I) External processes: occur at or near earth’s surface and are powered by Sun’s energy A) Responsible for transforming solid rock into sediment B) Include: 1) Weathering: physical breakdown and chemical alteration of a rock at or near earth’s surface 2) Mass wasting: transfer of rock and soil downslope under influence of gravity 3) Erosion: physical removal of material by mobile agents (i.e. water, wind, ice) C) All interact with each other II) Interior processes: derive energy from earth’s interior (i.e. volcanic activity, mountain building) III) Weathering A) Slow and subtle process B) Mechanical weathering: physical forces break rock into smaller pieces w/o changing rocks  mineral composition, increases surface area available for chemical weathering 1) Frost wedging: rock is broken into angular fragments by many freeze-thaw cycles a) Water works its way into cracks in rock, when it freezes it expands and enlarges  openings b) Most pronounced in mountainous regions where daily freeze-thaw cycle exists c) Talus slopes: large piles of rock that has been wedged loose that forms at base of steep  rock outcrops 2) Unloading: great reduction in pressure when overlying rock is eroded away a) Due to sheeting: onion like layers that occur when large masses of igneous rock (granite  especially) are exposed to erosion and concentric slabs break lose b) Outer layers expand more than rock below and separate from rock body c) Continued weathering causes slabs to separate and spall off forming exfoliation domes d) Some produced by contraction during crystallization of magma or tectonic forces— fractures have definite pattern--joints 3) Thermal expansion: daily cycle of temp. can weaken rocks a) Especially in hot deserts b) Heating a rock causes expansion, cooling causes contraction.  Repeated  swelling/shrinking of minerals with different rates exert stress on rock’s outer shell c) Rocks must be weakened by chemical weathering before they can be broken down by  thermal activity 4) Biological activity a) Plant roots grow into fractures and wedge rock apart b) Burrowing animals break down rock by moving fresh material to surface where  physical/chemical processes attack it c) Decaying organisms produce acids that contribute to chemical weathering C) Chemical weathering: a chemical transformation of rock into one or more new compounds,  original rock decomposes to substance that is stable in surface environment.  Water is the most  important agent 1) Dissolution: when minerals dissolve in water
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a) Surrounding molecules are polar which disrupts attractive forces within a crystal and 
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