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Transcript
Chapter 5 Weathering = breakdown of rock and minerals 2 Types of weathering Mechanical Chemical Mechanical Weathering = physical forces breakdown rocks into smaller pieces without changing the mineral composition Frost Wedging Unloading (exfoliation) Biological Activity Increases the surface area of the rock The mechanical breakup of rock caused by the freeze-thaw cycles of water in the cracks of rocks. When water freezes, its volume expands The freeze-thaw cycles break rock into pieces Cracks are formed from water freezing, expanding, and breaking the asphalt or concrete Sections of rock that are wedged loose may tumble into large piles called talus which typically form at the base of steep rocky cliffs Most common in mountain regions of middle latitudes Exfoliation- reduced pressure on igneous rock causes it to uplift and allows slabs of outer rock to break off in layers Plants , burrowing animals and humans affect the weathering of rocks. Roots Construction Animals digging into rock Occurs when the chemical and mineral composition of a rock have been changed WATER is the most important agent of chemical weathering Water absorbs gases from the atmosphere and chemically react with minerals : EX: Water + CO2 + Calcite = Carbonic Acid breaks down limestone EX: H2O + O2 + iron + = rust EX: H2O + sulfur gas OR nitrogen gas = Acid Rain Evidence of chemical weathering Mechanical weathering affects the rate of chemical weathering Mechanical weathering breaks rocks into several pieces, which INCREASES the surface area of exposed rock, allowing faster chemical weathering to take place. 3 things that affect the rate of weathering Rock characteristics (chemical composition) Climate Surface Area (amount) of exposed rock Rock’s mineral composition affects weathering Granite is resistant to chemical weathering Marble chemically weathers very quickly Marble is formed from calcite which easily dissolves in acid Affected by temperature and moisture Chemical weathering is most effective in areas with high temperatures and an abundance (lots) of moisture Tropical- high temperatures + high moisture = lots of chemical weathering Polar- low temperatures + low moisture = little or no chemical weathering Deserts- high temperatures + low moisture = very slow chemical weathering Climate with most favorable weathering conditions has high temperatures & lots of moisture Cracks allow water to seep into the rock The more the crack & crevices, the greater the weathering Soil Composition 4 major components of soil 45% Mineral Matter (mineral broken down rock) 25% Water 25 % Air 5% Organic Matter or dead stuff (decayed remains of organisms) Soil texture is determined by particle size sand (large) silt clay (small) Loam - a mixture of all three sizes Best suited for supporting plant life Parent Material Time Climate Organisms Slope Source of mineral matter in the soil TOPSOIL- PARENT MATERIAL IS THE MINERAL MATTER SUBSOIL- PARENT MATERIAL HAS BEEN DEPOSITED FROM ELSWHERE AND WEATHERS Important to ALL weathering the longer the soil has been forming the thicker it becomes Major effect on soil formation Variations in temperature and precipitation influence rate, depth, and type of soil *change weathering in your notes to “soil” ) Hot & wet climate may produce a thick layer of chemically weathered soil Dry & cold climates may produce only a thin layer of chemically weathered soil Influence the soil’s physical and chemical properties Plants are the main source of organic matter in the soil They also aerate the soil (provide air to the soil) Steep slopes erode quickly and have little or no soils Flat slope has little erosion and produces quality soils. North slopes are cooler and wetter (poorer soil) South slopes are warmer and drier (better soil) *** soil forms best in hot/moist, tropical climates *** Soil profile shows all of the soil horizons in a cross section Soil varies in composition, texture, structure, & color at different depths Variations divide the soil into zones called horizons Horizons Mature soils have three distinct soil horizons A Horizon B Horizon C Horizon A Horizon = known as topsoil consists of organic matter, insects, fungi & microorganisms B Horizon = known as subsoil Contains: fine clay particles washed out of A horizon, by water that filters through pore spaces Lower limit of most plant roots & burrowing animals C Horizon = falls between the B horizon and unaltered parent material Contains partially weathered parent material Unaltered parent material The bedrock that has NOT gone through any weathering Loss of fertile topsoil major problem in United States We are eroding much more soil now than we have in the past Cross out “water carries tiny pieces of _______ All the way to Rate of Erosion Depends on soil characteristics, climate & slope, and the amount of vegetation Accelerated by human activity Removal of natural vegetation by: Farming Logging Construction Water erodes soil faster than wind Exception with unprotected spoil. EX: 1930’s Dust Bowl Reservoirs fill up with sediment Sediments are contaminated by pesticides and fertilizers Planting rows of trees called windbreaks Prevents wind erosion Terracing hillsides Prevents water & wind erosion Soil erosion is accelerated by removing protective natural vegetation Human activities such as farming, logging, and construction increase erosion greatly = soil is being eroded faster than it can be created EX: rivers are becoming clogged with eroded sediment from farmland = interfere with river transportation, increase pesticide levels in water, fill up reservoirs, and increase flooding