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Chapter 15 Soils & Mining SOIL: A RENEWABLE RESOURCE Soil – formed from mechanical (frost wedging) & chemical weathering (acid rain, oxidation) • Mature soils - arranged in a series of horizontal layers called soil horizons. SOIL: A RENEWABLE RESOURCE Figure 3-23 Layers in Mature Soils • O – leaf litter (leaves, twigs, wastes, fungi) • A – topsoil – humus & inorganic minerals – dark, loose, roots of plants • Some have E layer – eluvation (leached mineral layer) • B – subsoil • C – parent bedrock – unweathered • It takes 200 – 1000 years to get 1” of topsoil. Soil Profiles of the Principal Terrestrial Soil Types Figure 3-24 Some Soil Properties • Infiltration – water moves through pores in soil • Leaching – dissolved substances move to lower layers of soil Figure 3-25 More Soil Properties • Porosity – measure of the volume of pores & distance between spaces – finer = high water retention, coarser = higher air flow • Permeability – rate at which water & air move through soil • Structure – ways in which particles are clumped together • pH – affects uptake of nutrients by plants (too acidic – add lime/too basic – add S) Wearing Down and Building Up the Earth’s Surface • Weathering - external process that wears the earth’s surface down. Figure 15-6 GEOLOGIC PROCESSES • A very slow chemical cycle recycles three types of rock found in the earth’s crust: – Sedimentary rock – buried sediment under pressure (sandstone, limestone, shale, coal). – Metamorphic rock – pre-existing rock subject to high temps, pressure (slate, marble, quartzite, gneiss, schist, anthracite). – Igneous rock – from cooled magma (granite, pumice, basalt, obsidian, gabbro) Rock Cycle Figure 15-8 Erosion • Sheet erosion – water moves in wide flow, peels off sheets • Rill erosion – cuts channels in the soil • Gully erosion – rivulets join – cut wider & deeper until it forms ditches/gullies Soil Degradation • Desertification – productive potential falls by 10% or more – overgrazing, mining, irrigation, compaction • Salinization – irrigation water leaves behind salts • Waterlogging – applying lots of water to move salts down – makes water table move up Soil Conservation • No-till agriculture – cut slits into the soil for seeds – leave last year’s old growth • Terracing – levels across steep slopes • Contour farming – perpendicular to the angle of the slope – prevents gullies Soil Conservation • Strip cropping – plant corn with legumes – alternate rows • Alley cropping – intercropping – crops with trees/shrubs for fruit/fuel • Windbreaks – surround field with trees Soil Restoration • Organic fertilizer animal manure or green manure (plow clover under) • Compost • Crop Rotation ENVIRONMENTAL EFFECTS OF USING MINERAL RESOURCES • The extraction, processing, and use of mineral resources has a large environmental impact. Figure 15-9 4 Categories of Mineral Resources – Identified: known location, quantity, and quality or existence known based on direct evidence and measurements. – Undiscovered: potential supplies that are assumed to exist. – Reserves: identified resources that can be extracted profitably. – Other: undiscovered or identified resources not classified as reserves General Classification of Nonrenewable Mineral Resources • Examples are fossil fuels (coal, oil), metallic minerals (copper, iron), and nonmetallic minerals (sand, gravel). Figure 15-7 MINING – Surface mining: shallow deposits are removed. – Subsurface mining: deep deposits are removed. – Overburden – soil & rock on top – Spoil – discarded overburden – no topsoil, little plant growth – Tailings – waste separated from ore – Gangue - piles of tailings Open-pit Mining • Machines dig holes and remove ores, sand, gravel, and stone. • Toxic groundwater can accumulate at the bottom. Figure 15-11 Area Strip Mining • Earth movers strip away overburden, and giant shovels removes mineral deposit. • Often leaves highly erodible hills of rubble called spoil banks. Figure 15-12 Contour Strip Mining • Used on hilly or mountainous terrain. • Unless the land is restored, a wall of dirt is left in front of a highly erodible bank called a highwall. Figure 15-13 Mountaintop Removal • Machinery removes the tops of mountains to expose coal. • The resulting waste rock and dirt are dumped into the streams and valleys below. Figure 15-14 Dredging • Scrape underwater deposits Heap-Leach Extraction • Spray a pile of ore with cyanide – dissolves gold – removed by electrolysis Subsurface Mining • Room & Pillar – gouge & load – coal supports roof Subsurface Mining Longwall – steel props support roof – move down the line & the previous roof collapses Mineral Laws • Surface Mining & Control Act of 1977 – Mining companies must restore land to previous conditions • 1872 Mining Law – can stake claim on any public lands – buy land for $2.50 - $5/acre take any minerals for free – don’t have to actually mine – take $4 billion in minerals each year • Coal, oil, gas – pay 12.5% royalties for public land use Getting More Minerals from the Ocean • Hydrothermal deposits form when mineral-rich superheated water shoots out of vents in solidified magma on the ocean floor. Figure 15-17 Environmental Effects of Mining/Processing Mineral Resources • • • • Fires (coal mines) Chokes streams with placer mining Toxic chemicals (Acid-mine drainage) Tailings – left after separating ore from gangue • Smelting – separates metal from other elements in the other • Subsidence • Air pollution/land pollution