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Earth & Space ScienceChapter 7
Weathering, Erosion, and Soil
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Mechanical/Physical Weathering
Changes size and shape of rocks, not composition
• Temperature– Water freeze/thaw cycle (water expands when it
freezes): frost wedging• Pressure– Plant roots enlarge cracks in rocks– Exfoliation of layers
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Examples:
Frost wedging, exfoliation, and root wedging
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Chemical WeatheringChanges the chemical identity of the rock• Water– Dissolves minerals– Chemical reaction: hydrolysis (decomposes silicates)
• Oxygen– Chemical reaction: oxidation (especially iron)
• Carbon dioxide– CO2 + H2O Carbonic acid (dissolves calcite)
• Acid precipitation– Nitric and sulfuric acid from air pollution
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Examples:
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Effect of temperature and rainfall
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What affects the rate of weathering?
• Climate– Warm/rainy = more chemical weathering– Cool/dry = more physical weathering
• Rock type/composition– Sedimentary = most weathering
• Exposed surface area– Greater surface area = greater weathering
• Topography: slope of the land– Steeper = greater weathering
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Exposed surface area affects weathering
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Erosion and Deposition
• Erosion transports weathered material until it is deposited in a new location.
• Wind, running water, glaciers, glaciers, ocean currents, and waves
• The importance of gravity:– Loose rock falls downhill– Water flows downhill– Glaciers flow downhill
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Water Erosion
• Greatest when water is moving rapidly• Rivers/streams: billions of tons of sediment
carried downstream, deposited at deltas• Waves: erosion and deposition of sand forms
barrier islands
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Glacial Erosion
• Move as dense rivers of ice
• Grind up, transport and deposit large amounts of rock and debris
• Much of the northern U.S. landscape has been shaped by glaciers
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Wind Erosion
• Especially important in arid (dry) locations and coastal regions without much vegetation
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Soil Formation
• Soil consists of weathered rock and organic material
• Soil texture is determined by the amount and type of different sized particles
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Soil Horizons
• O: organic material and leaf litter
• A: weathered rock and organic material
• B: zone of accumulation/ clay and minerals
• C: broken down bedrock
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Factors in Soil Formation
• Climate—rock weathers more rapidly in humid, temperate climates
• Topography—slope affects deposition of soil • Parent material—type of rock that forms the
soil• Biological organisms—help break down soil
and add to humus• Time—soil is renewable, but can take long
periods of time