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Weathering and Soils Chapter 7 Weathering Vocabulary • Exfoliation: outer layers of rock are stripped away like an onion • Hydrolysis: reaction of water with substance • Frost wedging: repeated thawing and freezing of water in the cracks of rocks • Oxidation: reaction of oxygen with a substance Earth’s surface processes First a definition: • Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface Types of weathering • Chemical: breakdown of material using changes in chemical composition • Physical: breakdown of material without chemical change. Physical or Mechanical Weathering • Physical weathering –Physical weathering is the disintegration of earth material without undergoing a chemical change. Frost wedging – freezing and thawing of water in cracks disintegrates rocks Exfoliation– exfoliation of igneous rocks at Earth’s surface due to erosion of “overburden” and reduction in pressure Biological activity – disintegration resulting from plants and animals - root wedging, animal burrows Frost Wedging Evidence of Frost Wedging in Wheeler Park, Nevada Source: Tom Bean/DRK Photo Rockfall caused by frost wedging Salt Wedging Mechanical Exfoliation in Yosemite National Park Source: Phil Degginger/Earth Scenes Mechanical Exfoliation of granite Tree roots growing in rock fractures, plus animal burrows, expose deep rocks to water Source: Runk/Schoenberger/Grant Heilman Physical weathering increases the rate of chemical weathering by increasing surface area. Joint-controlled weathering in igneous rocks Joints in sandstone - Canyonlands Natl Park, Utah Chemical Weathering • Breaks down rock and minerals • Important agent in chemical weathering is water • Water dissolves and transports ions and molecules • The dissolved ions later bond & form the cements in sedimentary rocks Chemical Weathering • Major processes of chemical weathering • Dissolution – Soluble ions contained in underground water • Oxidation – Chemical reaction where compound loses electrons – Important in breaking down mafic minerals – Rust colored weathering of Basalt • Hydrolysis H+ or OH- ions – Reaction of any substance with water – Water’s ions replace different ions in mineral – Feldspars, most abundant crust minerals, become fine clay particles. Easily transported. Dissolution Weathered and Unweathered Limestone Boulders Source: Ramesh Venkatakrishnan Angular Boulder Decomposes and Rounds Source: Paul McKelvey/Tony Stone Images Hydrolysis Cleopatra’s Needle, (Egypt) Granite in a Dry Climate Source: New York Public Library, Locan History and Genealogy Division Hydrolysis Cleopatra’s Needle, (Central Park, NYC) Granite in a Wet Climate Source: Runk/Schoenberger/Grant Heilman Oxidation of Basalt Rust (Iron Oxide) forms Weathering • Rates of weathering • Mechanical weathering aids chemical weathering by increasing surface area • Others factors affecting weathering • Rock characteristics e.g. minerals – Marble and limestone easily dissolve in weak acidic solutions -Dissolution Soils Our food grows in soils. Also very important in recognizing past climates Soil • Soil - combination of mineral and organic matter, water, and air • It is that portion of the regolith (weathered rock and mineral) that supports the growth of plants Components in soil that support 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 (Thicker, with greater differences between layers) 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 Soil Profile • The soil profile • Soil forming processes operate from the surface downward • Vertical differences are called horizons – zones or layers of soil Soil Horizons and Profiles Soil Horizons • Layers in Soil • Not Deposited, but Zones of Chemical Action Soil Profile • Suite of Horizons at a Given Locality An idealized soil profile with horizons Soil • The soil profile • O horizon – organic matter • A horizon – organic and mineral matter – High biological activity (animals live here) – Together the O and A horizons make up topsoil • B horizon – zone of accumulation • C horizon – partly altered parent material Mnemonic: Only Active Educators Become Champions Soil Profiles A mature soil profile with well developed A, B, and C horizons takes hundreds to thousands of years to develop. Horizon “A” Horizon “B” Horizon “C” Image: NRCS Soils-3-6 Remember the different horizons O A Organic Animal Activity Soluble Minerals E B C Exited Soluble Minerals Back Crushed Rock Animal Activities in “A” horizon Source: Runk/Schoenberger/Grant Heilman • Soil types • The characteristics of each soil type primarily depend on the prevailing climatic conditions • Three very generic soil types • Pedalfer Moist All Year • Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon – Best developed under forest landscapes • Pedocal - High Evaporation – White calcium carbonate (caliche) in B horizon – Associated with dry grasslands and dry brush vegetation • Laterite e.g. Monsoonal Climate – – – – Alternating Hot dry and Cool Wet tropical climates Intense chemical weathering Red Iron oxide – horizons not distinct Very thick soils but thin useful topsoil-Trees are Buttressed pedalfer Evergreen forests pedocal Shortgrass laterite tropics Pedocal with Caliche in the B horizon O A E Organic Activity Leached B Accumulation C Crushed Rock Caliche Source: Teaching Collection/University of Washington, Department of Geological Science Laterite in Sarawak, Borneo Source: Fletcher & Baylis/Photo Researchers, Inc. Earth’s surface processes • Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity Soil Erosion • Soil erosion • Recycling of Earth materials • Natural rates of soil erosion depend on –Soil characteristics –Climate –Slope –Type of vegetation Erosion Headed for the Sea Source: Ramesh Venkatakrishnan 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 • Terraces Limits of Soil Development Balance Between: • Downward Lowering of Ground Surface • Downward Migration of Soil Horizons If erosion rapid or soil evolution slow, soils may never mature beyond a certain point. Extremely ancient soils may have lost everything movable Soil Developed on a Lava Flow End of Chapter 7 Source: Stanley Chernicoff/Patrick Spencer