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Ch 14-Weathering and Erosion
Objectives Identify agents of mechanical weathering Compare mechanical and weathering processes Describe chemical reactions that decompose rock Explain how rock composition affects the rate of weathering Discuss how surface area affects the rate at which rock weathers Describe the effects of climate and topography on the rate of weathering Summarize how soil forms Explain how the composition of parent rock affects soil composition Describe the characteristics layers of mature residual soils Predict type of soil that will form in artic and tropical climates Define erosion, list agents of erosion Identify farming methods that conserve soil Discuss 2 ways gravity contributes to erosion Describe the major land forms shaped by weathering and erosion
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Weathering Processes What is weathering?
The natural process in which wind, rain, and temperature changes, disintegrates and decomposes rock 2 types of weathering-mechanical and chemical Mechanical weathering-rocks break down into smaller pieces by physical means Do not cause change to composition to rock Ice, plants, animals, gravity, running water, wind Physical changes may affect mech weathering Joints-long, curved cracks that develop in rock Exfoliation-joints are parallel to surface of rock, breaks into curved sheets
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Mechanical Weathering
Ice Wedging Occurs in cold climate, high elevations Water seeps into cracks Abrasion Collision of rocks Gravity, running water, wind Organic Activity Weathering caused by animal and plants Can weather rocks dramatically over a long period of time
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Chemical Weathering What is chemical weathering?
Process by which rocks break down as a result of chemical reactions Takes place when chemical reactions act on minerals in rock Occur between rock, water, CO₂, oxygen, acids May lead to formation of new minerals Changes both chemical composition and physical appearance of rock Oxidation, hydrolysis, carbonation, organic acids, acid precipitation
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Types of Chemical Reactions
Oxidation Reaction that removes 1 or more electrons from substance. Metallic element combines w/ oxygen Occurs in rock w/ iron bearing minerals Hydrolysis Reaction between water and another substance to form 2 or more new substances Carbonation Conversion of a compound into a carbonate Organic Acids Produced naturally by certain living organisms Lichens and mosses produce weak acids Acid Precipitation Precipitation that contains high concentration of acids Rain, sleet, snow Weathers rock faster than ordinary
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Sec 2-Rates of Weathering
The rate at which rock weathers-rock composition, climate, topography Differential Weathering-Softer, less weather resistant rocks wear away at faster rate than harder, more weather resistant rocks do Rock Composition Weathering of rocks depends on the material that holds the rocks together Limestone and sedimentary rocks Shales and sandstones
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Surface Area Important factors in determining rate of weathering-rock’s surface area, amount of time rock is exposed Surface Area Part of rock that is exposed to air, water and other agents of erosion Factures and joints increase surface area
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Effects of Climate Alternating periods of hot and cold weather allow fastest rates of weathering Freezing and thawing=mechanical breakdown Rising temps=chemical weathering Humid climates=fairly rapid chemical Hot, dry climates=slow weathering
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Topography and Elevation
What is topography? Elevation and slope of land surface Ice wedging more common at high elevation Gravity pulls rock fragments downhill Washed out by heavy rains Human Activities Mining and construction expose rock surfaces to agents of weathering
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Sec 4-Erosion Define erosion Soil erosion Gullying and sheet erosion
Materials of Earth’s surface are loosened, dissolved or worn away and transported from one place to another by natural agent Soil erosion Greatest environmental problem Farming and ranch practices increase erosion Gullying and sheet erosion Farming technique increases erosion Strips away layers of topsoil, exposes sub soil What is the result of erosion? Reduces fertility of soil, removes A horizon-contains fertile humus
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Farming Methods Contour plowing Strip cropping Terracing Crop rotation
Soil is plowed in curved bands that follow contour, or shape of land Prevents water from flowing directly down slopes Strip cropping Crops planted in alternating bands Corn and alfalfa planted next to each other Combined w/ contour plowing, reduces erosion by 75% Terracing Construction of step like ridges, follow contours of sloped field Prevent down slope of movement of water Crop rotation Plant 1 crop one yr and different one next yr How does this stop erosion?
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Gravity and Erosion Mass movement-movement of large mass of sediment or section of land down a slope Rockfalls and Landslides Rocks fall from steep cliff Masses of loose rock combined w/ soil suddenly fall down a slope Triggered by heavy rain, spring thaws, volcanic eruptions, earthquakes Mudflows and Slumps Rapid movement of large amount of mud Dry, mountainous regions during sudden, heavy rainfall Large block of soil and rock becomes unstable and moves downhill in one piece
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Soilfluction Slow, downslope of soil saturated w/ water in areas surrounding glaciers at high elevations Creep Slow downhill movements of weathered rock material Moves most soil of all mass movements
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Erosion and Landforms What is a landform?
Physical feature of Earth’s surface Major landforms shaped by erosion Mts, plains, plateaus Hills, valleys, dunes Processes landforms go through Bends, breaks, lifts Earth’s crust, creates elevated or uplifted landforms Weathering and erosion, wears down land
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Erosion of Plains and Plateaus
Erosion of Mts. Tectonic forces uplift the mountain, faster than erosion Tend to be rugged, sharp peaks, deep narrow valleys Forces stop=erosion and weathering Erosion of Plains and Plateaus Plain-relatively flat landform near sea level Plateau-broad, flat landform w/ high elevation Erosion depends on climate and composition and structure of rock
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Glacial Erosion What kind of landforms do glaciers create?
Large lakes, solitary boulders on flat plains, jagged ridges Landforms created by glaciers Mt ranges Cirque-bowl shaped depression Arete-sharp jagged edge that forms between cirques Horn-sharp, pyramid peak, forms when several aretes join U-shaped valleys
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Effects of Wind Erosion
Deserts and coastlines-most affected by wind Few plants, low moisture. How would this affect erosion? Desert pavement Deflation-wind removes top layer of fine, very dry soil or rock particles and leaves behind large rock particles Deflation Hollows Wind strips off topsoil, shallow depression forms Expand width of several km and to depth of 5-20 m
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Wave Erosion How are beaches formed? Sea Cliffs
Waves slowly erode base of rock Waves cut under underlying rock, rock collapses to form seep sea cliff Headlands-high and steep formation of rock that extends out from shore into water Areas w/ less resistant rock form bays Sea Caves, Arches, Stacks Waves cut deep fractured and weak rock along base of cliff to form large hole Sea arch forms when waves cut completely through headland Sea stacks are offshore columns of rock that once were connected to sea cliff or headland How are beaches formed? Created by water eroding the land and depositing sediment
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