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1 Weathering the physical breakdown and chemical alteration of rock at or near Earth s 2 Mass wasting the transfer of rock and soil downslope under the influence of gravity 3 Erosion the physical removal of material by mobile agents such as water wind ice or Earth s External Processes surface gravity Weathering 1 Two types of weathering a Mechanical weathering breaking rocks into smaller pieces b Four types of mechanical weathering i Frost wedging alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks ii Sheeting exfoliation of igneous and metamorphic rocks at the Earth s surface as a result of a reduction in confining pressure iii Salt crystal growth in rocks 1 Sea spray or salty groundwater penetrates crevices and pore spaces 2 As the water evaporates salt crystals form and enlarge the crevices iv Biological activity disintegration resulting from plants and animals c Chemical weathering i Breaks down rock components and the internal structures of minerals ii Most important agent involved in chemical weathering is water responsible for transport of ions and molecules involved in chemical processes 2 Major processes of chemical weathering a Dissolving b Oxidizing i Aided by small amounts of acid in the water ii Soluble ions are retained in the underground water supply i Any chemical reaction in which a compound or radical loses electrons ii Important in decomposing ferromagnesian minerals c How granite weathers i Chemically attacked by carbonic acid ii Hydrogen ion attacks and replaces other positive ions iii Most abundant product is clay minerals 3 Alterations caused by chemical weathering a Decomposition of unstable minerals b Generation or retention of materials that are stable c Physical changes such as the rounding of corners or edges spheroidal weathering 4 Differential weathering a Masses of rock do not weather uniformly because of regional and local factors b Results in many unusual and spectacular rock formations and landforms Soil 1 Soil is a combination of mineral and organic matter water and air a That portion of the regolith rock and mineral fragments produced by weathering that supports the growth of plants 2 Factors controlling soil formation a Parent material i Residual soil parent material is the underlying bedrock ii Transported soil forms in place on parent material that has been carried from elsewhere and deposited b Time i ii Amount of time for soil formation varies for different soils depending on Important in all geologic processes geologic and climatic conditions c Climate i Most influential control of soil formation ii Key factors are temperature and precipitation d Plants and animals i Organisms influence the soil s physical and chemical properties ii Also furnish organic matter to the soil i Steep slopes often have poorly developed soils ii Optimum terrain is a flat to undulating upland surface e Slope 3 The soil profile a Soil forming processes operate from the surface downward b Vertical differences are called horizons which are zones or layers of soil c O horizon organic matter d A horizon organic and mineral matter i High biological activity ii Together the O and A horizons make up the topsoil e E horizon little organic matter i Zone of eluviation and leaching f B horizon zone of accumulation g C horizon partially altered parent material 4 The O A E and B horizons together are called the solum or true soil Classifying Soil 1 Soil types a Groups consist of items that have certain important characteristics in common 2 Soils are classified using a system known as the soil taxonomy a Based on physical and chemical properties of the soil 3 Soil taxonomy a Includes six hierarchical categories of classification ranging from order broadest to series most specific b Especially useful for agricultural and related to land use purposes The Impact of Human Activity on Soils 1 The agricultural productivity of soils can be improved through fertilization and irrigation 2 Soils can be damaged or destroyed by careless activities a Soils are crucial for providing food fiber and other basic materials b Soils are one of the most abused resources 3 Clearing the tropical rain forest a case study a Tropical forests are cleared for logging and agricultural use b Soils in tropical forests are poor in nutrients and unsuitable for agriculture i Most of the nutrients in tropical rain forests are found in the trees c Clearing tropical rain forests also promotes soil erosion 4 Soil erosion a Soil erosion is a natural process in the rock cycle b Erosion rates are dependent on climate slope and type of vegetation i Human activities such as deforestation and farming practices can enhance soil erosion Weathering and Ore Deposits 1 Weathering creates deposits by concentrating metals into economically valuable concentrations secondary enrichment 2 Bauxite a Principle ore of aluminum b Forms in rainy tropical climates from chemical weathering and the removal of undesirable elements by leaching 3 Other deposits a Weathering processes concentrate metals that are deposited through low grade primary ore i Copper and silver ii Typically occurs in deposits containing pyrite


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UNCC GEOL 1200 - Earth’s External Processes

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