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Weathering, Erosion, Soils, and Mass Wasting

Weathering, Erosion, Soils, and Mass Wasting

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Weathering, Erosion, Soils, and Mass Wasting. Weathering – the physical breakdown ( disintegration ) and chemical alteration ( decomposition ) of rock at or near Earth’s surface Mass wasting – the transfer of rock and soil downslope under the influence of gravity - PowerPoint PPT Presentation

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Volcanoes and Igneous Activity Earth - Chapter 4

Weathering, Erosion, Soils, and Mass Wasting

Weathering the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earths surfaceMass wasting the transfer of rock and soil downslope under the influence of gravityErosion the physical removal of material by mobile agents such as water, wind, ice, or gravity

WeatheringTwo types of weatheringMechanical weathering breaking of rocks into smaller piecesChemical WeatheringBreaks down rock components and the internal structures of mineralsMost important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

Mechanical WeatheringFour types of mechanical weatheringFrost wedging alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocksEx. Pot holes in roads

Frost wedging

WeatheringMechanical Weathering continuedUnloading exfoliation of igneous and metamorphic rocks at the Earths surface due to a reduction in confining pressureThermal expansion alternate expansion and contraction due to heating and coolingBiological activity disintegration resulting from plants and animals

Exfoliation of igneous rocks

WeatheringChemical WeatheringBreaks down rock components and the internal structures of mineralsMost important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)WeatheringMajor processes of chemical weatheringOxidationAny chemical reaction in which a compound or radical loses electrons4 Fe + 3O2 2 Fe2O3Important in decomposing ferromagnesian minerals rustHematite, limonite

WeatheringMajor processes of chemical weatheringDissolutionAided by small amounts of acid in the waterSoluble ions are retained in the underground water supplyHydrolysisThe reaction of any substance with waterHydrogen ion attacks and replaces other positive ions

WeatheringAlterations caused by chemical weatheringDecomposition of unstable mineralsGeneration or retention of materials that are stablePhysical changes such as the rounding of corners or edges

Effects of WeatheringDifferential weatheringMasses of rock do not weather uniformly due to regional and local factorsResults in many unusual and spectacular rock formations and landformsJoint-controlled weathering in igneous rocks

WeatheringRates of weatheringAdvanced mechanical weathering aids chemical weathering by increasing the surface areaOthers factors affecting weatheringRock characteristicsRocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions WeatheringOthers factors affecting weatheringRock characteristics continuedSilicate minerals weather in the same order as their order of crystallizationClimateTemperature and moisture are the most crucial factorsChemical weathering is most effective in areas of warm, moist climates

Rock Type and ClimateWarm, wet climates carbonates (limestone, dolomite, marble, etc) weather faster (mostly chemical) and form our valleysSandstones, quartzites, metabasalts ridge formers (subject to mechanical weatheringArid climates (western US) limestones the ridge-formers, sandstones valleysAcid RainEnvironmental application of chemical weathering.Rain water is normally slightly acidic, pH=5.6 as water mixes with CO2 in atmosphere.Pollution from NOx and SOx mixes with H20 in air and forms H2SO4 and HNO3, making rain more acidicDissolves marble statues, destroys trees, fish

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SoilSoil is a combination of mineral and organic mater, water, and airThat portion of the regolith (loose surface sediment of rock and mineral fragments produced by weathering) that supports the growth of plants (Upper part of the regolith.)

Typical components in a soil that yield good plant growth

SoilThe soil profileSoil forming processes operate from the surface downwardVertical differences are called horizons zones or layers of soil

36 Controls and triggers of mass wastingImportant triggers includeRemoval of anchoring vegetationGround vibrations from earthquakesMay cause expensive property damageCan cause liquefaction water saturated surface materials behave as fluid-like masses that flow37 Controls and triggers of mass wastingLandslides without triggersSlope materials weaken over timeRandom events that are unpredictable

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42A lahar from the 1980 Mt. St. Helens eruption

43 An earthflow on a newly formed slope

44 Some visible effects of creep

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46Forms of flowSlow movementsSolifluctionPromoted by a dense clay hardpan or impermeable bedrock layerCommon in regions underlain by permafrostCan occur on gentle slopes47 Permafrost regions in the Northern Hemisphere

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49Ground subsidence in Alaska due to permafrost

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51A slump with an earthflow at the base

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53Forms of FallsRockfallsBlocks of bedrock break off and freely falls or bounces down a cliffCliffs either naturally occurring or undercut by rivers, glaciers, or wave-actionWeakness due to fractures or joints, frost-wedgingTalus rock fragments that accumulate at base of cliff (Black Rock)54

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57Old Man of the Mountain, New Hampshire, pre-2003

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