Weathering and SoilsChapter 5

Earths surface processesFirst a definition: Weathering Physical breakdown and chemical alteration of rock at Earths surface

Source: Tom Bean/DRK Photo

Mechanical Weathering

Mechanical weathering breaking of rocks into smaller piecesFour types of mechanical weathering1. Frost wedging freezing and thawing of water in cracks disintegrates rocks

Frost Wedging

Evidence of Frost Wedging in Wheeler Park, Nevada

Source: Tom Bean/DRK Photo

Rockfall caused by frost wedging

Salt Wedging

Mechanical Weathering (continued)

2. Mechanical Exfoliation AKA Unloading exfoliation of igneous rocks at Earths surface due to erosion of overburden and reduction in pressure - Like peeling layers off an onion3. Thermal expansion and Contraction alternate expansion and contraction due to heating and cooling - important in deserts4. Biological activity disintegration resulting from plants and animals - root wedging, animal burrows

Mechanical Exfoliation of a Pluton

AKA Unloading

Mechanical Exfoliation in Yosemite National Park

Source: Phil Degginger/Earth Scenes

Mechanical Exfoliationof granite

AKA Unloading

Thermal Expansion and Contraction

Source: Tom Bean

Tree roots growing in rock fractures,plus animal burrows, expose deep rocks to water

Source: Runk/Schoenberger/Grant Heilman

Increase in surface area by mechanical weathering

Differential weathering controlled by jointing patternsDefinitions:Joints and Faults

Deep rock exposed to water

Joint-controlled weathering in igneous rocks

Joints in sandstone - Canyonlands Natl Park, Utah

Chemical WeatheringBreaks down rock and mineralsImportant agent in chemical weathering is water Water dissolves and transports ions and molecules The dissolved ions later bond & form the cements in sedimentary rocks

Source: Ramesh Venkatakrishnan

Chemical WeatheringMajor processes of chemical weathering

DissolutionBy carbonic acid, CO2 in water CO2 + H2O => H2CO3 H2CO3 => H+ + HCO3-Soluble ions contained in underground waterOxidationChemical reaction where compound loses electronsImportant in breaking down mafic mineralsRust colored weathering of Basalt Hydrolysis H+ or OH- ionsReaction of any substance with waterWaters ions replace different ions in mineralFeldspars, most abundant crust minerals, become fine clay particles. Easily transported.

Dissolution Weathered and Unweathered Limestone Boulders

Source: Ramesh Venkatakrishnan

Hydrolysis Feldspar to Clay

Mechanical fracture due to chemical weatheringFeldspars become

Angular Boulder Decomposes and Rounds

Angular Boulder Decomposes and Rounds

Source: Paul McKelvey/Tony Stone Images

HydrolysisCleopatras Needle, (Egypt)

Source: New York Public Library, Locan History and Genealogy DivisionGranite in aDry Climate

HydrolysisCleopatras Needle, (Central Park, NYC)

Source: Runk/Schoenberger/Grant Heilman

Granite in aWet Climate

Oxidation of BasaltRust (Iron Oxide) forms

Calcite (Limestone) No solids Ca ++ CO3--

WeatheringRates of weathering

Mechanical weathering aids chemical weathering by increasing surface areaOthers factors affecting weathering

Rock characteristics e.g. mineralsMarble and limestone easily dissolve in weak acidic solutions -Dissolution

Chemical weatheringby dissolutionLimestoneWeathering-resistant sandstone yields little soilSoilSoilSoilFeldspar-richgraniteIron-richbasaltChemicalweatheringby oxidationChemicalweatheringby hydrolysisFactors related to bedrock composition

Stability at SurfaceOthers factors affecting weathering

Rock characteristics continuedDiscontinuous series silicate minerals weather in the reverse order of their order of crystallization (Bowens Reactions)Hi T-P Olivine easiest to weather Lo T-P Quartz hardest, crystallizes closer to surface conditions

Bowens Reaction Series and Weathering

Our food grows in soils. Also very important in recognizing past climatesSoils

Soil

Soil - combination of mineral and organic matter, water, and air

It is that portion of the regolith (weathered rock and mineral) that supports the growth of plants

Components in soil that support plant growth

SoilFactors controlling soil formation

Parent materialparent material is the underlying bedrock - composition affects soil types

SoilFactors controlling soil formation

TimeSoils get better developed (Thicker, with greater differences between layers) with more timeClimateBiggest control on soil formationKey factors are temperature and precipitation

SoilFactors controlling soil formation

Plants and animalsOrganisms influence soil propertiesAlso furnish organic matter to the soil (especially plants)SlopeSteep slopes have poorly developed soils (due to faster erosion and downslope transportFlatter terrain accumulates soil faster

Variations in soil development due to topographyNote location of agriculture

Soil ProfileThe soil profile

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

An idealized soil profilewith horizons

SoilThe soil profile

O horizon organic matterA horizon organic and mineral matterHigh biological activity (animals live here)Together the O and A horizons make up topsoilE horizon little organic matterZone of leachingB horizon zone of accumulationC horizon partly altered parent materialMnemonic: Only Active Educators Become Champions

Remember the different horizonsO

A

E

B

COrganicAnimal Activity

Exited

Back

Crushed RockSoluble MineralsSoluble Minerals

Vegetation and Soil Development

1. Plants use Hydrolysis to get nutrient metals2. Humic acids and metals returned at death. Acids remove metals from E-layer

Animal Activities in A horizonSource: Runk/Schoenberger/Grant Heilman

Eluviation & Illuviation

Illuviation and EluviationLeaching and Precipitation of Iron

Source: Jens/Gutzmer/Rand Afrikaans University/GeologyIron Leached OutPure IronIron Oxide

Soil types

The characteristics of each soil type primarily depend on the prevailing climatic conditionsThree very generic soil types

Pedalfer Moist All YearAccumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizonBest developed under forest landscapesPedocal - High EvaporationWhite calcium carbonate (caliche) in B horizonAssociated with dry grasslands and dry brush vegetationLaterite e.g. Monsoonal ClimateAlternating Hot dry and Cool Wet tropical climatesIntense chemical weatheringRed Iron oxide horizons not distinct Very thick soils but thin useful topsoil-Trees are Buttressed

Rainfall seasonalLaterite

Equator to Poles Factors

Equatorialand tropicalrain forestsSavannahsLow-latitudedeserts andsemi-desertsGrasslands(steppes)Temperate regionsand mixedboreal forestsArcticandtundraregionsEquatorAnnual precipitationIncreasing depthof weathering1800mm600mm40C30C20C10CPrecipitationTemperatureEvaporationBedrock ator very nearsurfaceSoilSoilBedrockDeeply weatheredbedrock(~40 - 50 meters deep)Temperature30 degrees Latitude

pedalferpedocallateritetropicsShortgrass Evergreen forests

Pedocal with Caliche in the B horizonO

A

E

B

COrganicActivity

Leached

Accumulation

Crushed Rock

Caliche

Source: Teaching Collection/University of Washington, Department of Geological Science

Laterite in Sarawak, Borneo

Source: Fletcher & Baylis/Photo Researchers, Inc.

Earths surface processesErosion the physical removal of material by mobile agents like water, wind, ice, or gravity

Soil ErosionSoil erosion

Recycling of Earth materialsNatural rates of soil erosion depend onSoil characteristicsClimateSlopeType of vegetation

Erosion

Source: Ramesh VenkatakrishnanHeaded for the Sea

SoilSoil erosion

In many regions the rate of soil erosion is significantly greater than the rate of soil formationFarmers now level fields with lasers to slow loss of topsoilTerraces

Soil Developed on a Lava Flow

Source: Stanley Chernicoff/Patrick SpencerEnd of Chapter 5