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Weathering and Soil Learning Goals: Weathering & Soils 1. Define: weathering, mass wasting, erosion, differential weathering. 2. Explain how and why rocks disintegrate to form sediment by mechanical processes 3. Compare and contrast the major processes and effects of chemical and mechanical weathering, and describe how they interact. 4. Explain how climate and rock type affect weathering rates. 5. List the factors influencing soil formation characteristics 6. Describe and illustrate major characteristics of common soil horizons. 7. Describe the characteristics of the three primary types of soil, and explain important factors in the formation of each type. 8. Explain the importance of soil for human societies, and describe how human activities affect soils. Earth’s surface processes Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface Mass wasting – transfer of rock and soil downslope by gravity Earth’s surface processes Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity Weathering Two types of weathering • Mechanical weathering – breaking of rocks into smaller pieces • Four types of mechanical weathering – Frost wedging – freezing and thawing of water in cracks disintegrates rocks Frost Wedging Movie QuickTime™ and a Cinepak decompressor are needed to see this picture. Rockfall along I-70 Weathering • Mechanical Weathering continued – Unloading – exfoliation of igneous rocks at Earth’s surface due to reduction in pressure – Like peeling layers off an onion – Thermal expansion – alternate expansion and contraction due to heating and cooling – Biological activity – disintegration resulting from plants and animals Exfoliation of granite Thermal Expansion and Salt Weathering Weathering Chemical Weathering • Breaks down rock and minerals • Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions) Chemical Weathering of Boulder (core stone) Weathering Major processes of chemical weathering • Dissolution – Aided by acid in water (occurs in limestone) – Soluble ions contained in underground water • Oxidation – Chemical reaction where compound loses electrons – Important in breaking down mafic minerals – Almost like the rocks are rusting • Hydrolysis – Reaction of any substance with water – Hydrogen ion replaces other positive ions Why do rocks such as granite appear so fresh (as they originally formed) in recently excavated quarries A) Radioactive elements deep in the Earth cause them to melt and reform B) Weathering occurs mostly at the surface of the Earth, where water and atmospheric gasses exist (the weathered rocks have been removed in the quarry to access the fresh rock) C) Workers in the quarry purposely scrub the rocks so they appear fresh D) Electrical charges from lightning strikes cause the weathering products to vaporize and disappear Why do rocks such as granite appear so fresh (as they originally formed) in recently excavated quarries A) Radioactive elements deep in the Earth cause them to melt and reform B) Weathering occurs mostly at the surface of the Earth, where water and atmospheric gasses exist (the weathered rocks have been removed in the quarry to access the fresh rock) C) Workers in the quarry purposely scrub the rocks so they appear fresh D) Electrical charges from lightning strikes cause the weathering products to vaporize and disappear Weathering Rates of weathering • Mechanical weathering aids chemical weathering by increasing surface area Others factors affecting weathering • Rock characteristics – Marble and limestone easily dissolve in weak acidic solutions QuickTime™ and a Cinepak decompressor are needed to see this picture. Increase in surface area by mechanical weathering Calculate the amount the surface area increases as grain size is cut in half A) Does not change B) Is half as large C) Is twice as large D) Is four times as large E) None of the above Increase in surface area by mechanical weathering Calculate the amount the surface area increases as grain size is cut in half A) Does not change B) Is half as large C) Is twice as large D) Is four times as large E) None of the above Limestone weathers faster than granite Granite Grave Marker 1888 Limestone Grave Marker 1885 Clicker Question Limestone in humid regions often forms valleys, while in arid regions it more commonly forms steep hills and ridges. Why is this so? 1. 2. 3. 4. Other kinds of rock weather very quickly under desert conditions Frost shattering occurs more frequently under humid conditions Limestone dissolves much more readily in humid conditions Desert plants shield limestone from the erosive power of rain drops Clicker Question Limestone in humid regions often forms valleys, while in arid regions it more commonly forms steep hills and ridges. Why is this so? 1. 2. 3. 4. Other kinds of rock weather very quickly under desert conditions Frost shattering occurs more frequently under humid conditions Limestone dissolves much more readily in humid conditions Desert plants shield limestone from the erosive power of rain drops Weathering Others factors affecting weathering • Rock characteristics continued – Silicate minerals weather more rapidly in the same order as their order of crystallization (Bowens rxn) • Climate – Temperature and moisture most crucial factors – Chemical weathering most effective in warm, moist climates Differential weathering controlled by jointing patterns Joint-controlled weathering in igneous rocks Joints in sandstone - Canyonlands and Arches Natl Parks, Utah Why do we care about soils? A) B) C) D) Food Production Air Quality Water Quality Weathering products Soil (otherwise known as dirt) Soil - combination of mineral and organic mater, water, and air • Portion of regolith (weathered rock and mineral) that supports growth of plants Components in soil that supports plant growth Soil Factors controlling soil formation • Parent material – parent material is the underlying bedrock composition affects soil types Soil Factors controlling soil formation • Time – Soils get better developed with more time • Climate – Biggest control on soil formation – Key factors are temperature and precipitation Soil Factors controlling soil formation • Plants and animals – Organisms influence soil properties – Also furnish organic matter to the soil (especially plants) • Slope – Steep slopes have poorly developed soils (due to faster erosion and downslope transport – Flatter terrain accumulates soil faster Variations in soil development due to topography Note location of agriculture In the Rocky Mountain region of the United States, northfacing slopes (downhill direction is toward the north) are typically more moist and heavily forested than south-facing slopes. Why is this? 1. North-facing slopes receive more sunlight in the summer; snow melts faster and more soil moisture is available for the trees. 2. South-facing slopes receive more moisture and sunlight; rock weathering is slower. 3. North-facing slopes receive about the same amount of precipitation as south-facing slopes; less moisture evaporates from north-facing slopes. 4. South-facing slopes receive less moisture, yet rock weathering is faster In the Rocky Mountain region of the United States, northfacing slopes (downhill direction is toward the north) are typically more moist and heavily forested than south-facing slopes. Why? 1. North-facing slopes receive more sunlight in the summer; snow melts faster and more soil moisture is available for the trees. 2. South-facing slopes receive more moisture and sunlight; rock weathering is slower. 3. North-facing slopes receive about the same amount of precipitation as south-facing slopes; less moisture evaporates from north-facing slopes. 4. South-facing slopes receive less moisture, yet rock weathering is faster Given the earlier slides on where form best in form regions of slow Given the earlier slidessoils on where soils best in regions of erosion, slow or deposition wheresediments, should a erosion,oforfine-grained depositionsediments, of fine-grained where wheat farmer plant his crops in Colorado near Boulder? should a wheat farmer plant his crops in Colorado near Boulder? 1. In topographically high regions, such as along the Peak to Peak Highway between Nederland and Estes Park 2. In river valleys, such as along the Boulder Creek path on the road between Boulder and Nederland 3. In low rolling hills near Lafayette and Louisville where Cretaceous strata are exposed 4. Along floodplains, such as along Coal Creek between A. In high regions, such as along the Peak to Peak Hwy between Louisville/Lafayette and Broomfield Nederland and Estes Park B. In river valleys, such as along Boulder Creek on the road between Boulder and Nederland C. In low rolling hills near Lafayette and Louisville where shaley strata are exposed D. Along modern floodplains, such as along Coal Creek between Louisville/Lafayette and Broomfield Given the earlier slides on where form best in form regions of slow Given the earlier slidessoils on where soils best in regions of erosion, slow or deposition wheresediments, should a erosion,oforfine-grained depositionsediments, of fine-grained where wheat farmer plant his crops in Colorado near Boulder? should a wheat farmer plant his crops in Colorado near Boulder? 1. In topographically high regions, such as along the Peak to Peak Highway between Nederland and Estes Park 2. In river valleys, such as along the Boulder Creek path on the road between Boulder and Nederland 3. In low rolling hills near Lafayette and Louisville where Cretaceous strata are exposed 4. Along floodplains, such as along Coal Creek between A. In high regions, such as along the Peak to Peak Hwy between Louisville/Lafayette and Broomfield Nederland and Estes Park B. In river valleys, such as along the Boulder Creek path on the road between Boulder and Nederland C. In low rolling hills near Lafayette and Louisville where shaley strata are exposed D. Along modern floodplains, such as along Coal Creek between Louisville/Lafayette and Broomfield Soil Profile The soil profile • Soil forming processes operate from the surface downward • Vertical differences are called horizons – zones or layers of soil Soil The soil profile • O horizon – organic matter • A horizon – organic and mineral matter – High biological activity (critters live here) – Together the O and A horizons make up topsoil • E horizon – little organic matter – Zone of leaching • B horizon – zone of accumulation • C horizon – partly altered parent material An idealized soil profile A soil profile showing different horizons Soil types • The characteristics of each soil type primarily depend on the prevailing climatic conditions Three very generic soil types • Pedalfer – Accumulation of iron oxides and Al-rich clays in the B horizon – Best developed under forest landscapes • Pedocal – High accumulations of calcium carbonate – Associated with dry grasslands and brush vegetation • Laterite – Hot and wet tropical climates – Intense chemical weathering pedocal pedalfer laterite Deserts Dry and Arid Forests Temperate Tropics Hot and Wet What sort of soil do you think forms in the mountains and plains along the Front Range? A. pedocal B. pedalfer D. Depends on where you are looking… C. laterite What sort of soil do you think forms in the mountains and plains along the Front Range? A. pedocal B. pedalfer D. Depends on where you are looking… C. laterite Why does deforestation in tropical rainforests result in such barren-looking and unproductive landscapes? A) High rainfall erodes soil away B) Cycling of organic material in tropics is rapid C) Levels of organic material in tropics is low D) All of the above Soil Soil erosion • Recycling of Earth materials • Natural rates of soil erosion depend on – Soil characteristics – Climate – Slope – Type of vegetation Soil Soil erosion • In many regions the rate of soil erosion is significantly greater than the rate of soil formation • Farmers now level fields with lasers to slow loss of topsoil 06.04 In terrain with steep hill slopes, which crop and cultivation technique will minimize soil erosion? 1. Corn; rows trending straight down the slope, frequent cultivation. 2. Apples; land between the trees is planted in grass and not cultivated. 3. Winter wheat; after the harvest, the field is plowed and left idle until next fall. 4. Beans; rows are spaced wider than on a level field. 06.04 In terrain with steep hill slopes, which crop and cultivation technique will minimize soil erosion? 1. Corn; rows trending straight down the slope, frequent cultivation. 2. Apples; land between the trees is planted in grass and not cultivated. 3. Winter wheat; after the harvest, the field is plowed and left idle until next fall. 4. Beans; rows are spaced wider than on a level field. End of Chapter 5