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 WeatheringSpheroidal WeatheringRocks are rounded from a blocky shapeWeathering occurs at the edges and corners of rocks, smoothing out the rock

Increase in surface area by mechanical weathering

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

SoilFactors controlling soil formationParent materialResidual soil parent material is the underlying bedrockTransported soil forms in place on parent material that has been carried from elsewhere and depositedSoilFactors controlling soil formationTimeImportant in all geologic processesAmount of time for soil formation varies for different soils depending on geologic and climatic conditionsClimateMost influential control of soil formationKey factors are temperature and precipitationSoilFactors controlling soil formationPlants and animalsOrganisms influence the soils physical and chemical propertiesAlso furnish organic matter to the soilSlopeSteep slopes often have poorly developed soilsOptimum terrain is a flat-to-undulating upland surface Variations in soil development due to topography

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

SoilThe soil profileO horizon organic matterA horizon organic and mineral matterHigh biological activityTogether the O and A horizons make up the topsoilE horizon little organic matterZone of eluviation and leaching SoilThe soil profileB horizon zone of accumulationC horizon partially altered parent materialThe O, A, E, and B horizons together are called the solum, or true soil An idealized soil profile

A soil profile showing different horizons

SoilSoil typesThe characteristics of each soil type primarily depend on the prevailing climatic conditionsThree very generic soil typesPedalferAccumulation of iron oxides and Al-rich clays in the B horizon

SoilThree very generic soil typesPedalfer continuedBest developed under forest vegetationPedocalHigh accumulations of calcium carbonateAssociated with dry grasslands and brush vegetation

SoilThree very generic soil typesLateriteHot and wet tropical climatesIntense chemical weathering

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61 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 flow62 Controls and triggers of mass wastingLandslides without triggersSlope materials weaken over timeRandom events that are unpredictable

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64Classification of mass wasting eventsGenerally each event is classified byType of material involvedDebrisMudEarthRock

65Classification of mass wasting eventsGenerally each event is classified byType of motionFlow (material moves as a chaotic mixture)Slide (material moves along a surface as a coherent mass)Fall (free-falling pieces)The velocity of the movementFastSlow

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68Forms of flowsDebris flow (fast)Consists of soil and regolith with a large amount of waterOften confined to channels69Forms of mass wastingDebris flowSerious hazard in dry areas with heavy rainsDebris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars70

71A lahar from the 1980 Mt. St. Helens eruption

72Forms of mass wastingEarthflow (slower)Form on hillsides in humid regionsWater saturates the soilCommonly involve materials rich in clay and silt73 An earthflow on a newly formed slope

74Forms of flowSlow movementsCreepGradual movement of soil and regolith downhillAided by the alternate expansion and contraction of the surface material

75 Creep

76 Some visible effects of creep

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78Forms of flowSlow movementsSolifluctionPromoted by a dense clay hardpan or impermeable bedrock layerCommon in regions underlain by permafrostCan occur on gentle slopes79 Permafrost regions in the Northern Hemisphere

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81Ground subsidence in Alaska due to permafrost

82Forms of SlidesRockslide (rock avalanche)Blocks of bedrock slide down a slopeGenerally very fast and destructiveComes to rest at less steep terrain, filling in bottom of a valley

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84Forms of slidesSlump (rotational)Movement of a mass of rock or uncon-solidated material as a unit along a curved surfaceOccurs along oversteepened slopes

85A slump with an earthflow at the base

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87Forms 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)88

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91Old Man of the Mountain, New Hampshire, pre-2003

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