The Art of Breaking The Art of Breaking things…things…

Weathering and SoilWeathering and Soil

WeatheringWeathering

►Weathering is the physical breakdown Weathering is the physical breakdown (disintegration) and chemical alteration (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s (decomposition) of rock at or near Earth’s surfacesurface

►Mass wastingMass wasting:: the transfer of rock and soil the transfer of rock and soil downslope under the influence of gravitydownslope under the influence of gravity

► ErosionErosion: : the physical removal of material the physical removal of material by mobile agents such as water, wind, ice, or by mobile agents such as water, wind, ice, or gravitygravity

Types of weatheringTypes of weathering

►Mechanical weathering – breaking of Mechanical weathering – breaking of rocks into smaller piecesrocks into smaller pieces Frost wedgingFrost wedging – alternate freezing and – alternate freezing and

thawing of water in fractures and cracks thawing of water in fractures and cracks UnloadingUnloading – exfoliation of igneous and – exfoliation of igneous and

metamorphic rocks at the Earth’s surface metamorphic rocks at the Earth’s surface due to a reduction in confining pressuredue to a reduction in confining pressure

Thermal expansionThermal expansion – alternate expansion – alternate expansion and contraction due to heating and coolingand contraction due to heating and cooling

Biological activityBiological activity – disintegration – disintegration resulting from plants and animalsresulting from plants and animals

Types of WeatheringTypes of Weathering

► Chemical Weathering- chemically Chemical Weathering- chemically breaks down rock components and breaks down rock components and internal structures of mineralsinternal structures of minerals

*Most important agent involved in chemical *Most important agent involved in chemical weathering is water (responsible for weathering is water (responsible for transport of ions and molecules involved in transport of ions and molecules involved in chemical processes)chemical processes)

Processes of Chemical Processes of Chemical WeatheringWeathering

DissolutionDissolution Aided by small amounts of acid in the waterAided by small amounts of acid in the water Soluble ions are retained in the underground water Soluble ions are retained in the underground water

supplysupply

OxidationOxidation Any chemical reaction in which a compound or Any chemical reaction in which a compound or

radical loses electronsradical loses electrons Important in decomposing ferromagnesian mineralsImportant in decomposing ferromagnesian minerals

HydrolysisHydrolysis The reaction of any substance with waterThe reaction of any substance with water Hydrogen ion attacks and replaces other positive Hydrogen ion attacks and replaces other positive

ionsions

Alterations caused by Alterations caused by weatheringweathering

►Decomposition of unstable mineralsDecomposition of unstable minerals►Generation or, more correctly, Generation or, more correctly,

retention of materials that are stableretention of materials that are stable►Physical changes such as the rounding Physical changes such as the rounding

of corners or edgesof corners or edges

Rates of weatheringRates of weathering► These most important aspect of mechanical weathering is that These most important aspect of mechanical weathering is that

it aids chemical weathering by increasing the surface areait aids chemical weathering by increasing the surface area

► Others factors affecting weatheringOthers factors affecting weathering Rock characteristicsRock characteristics

► Rocks containing calcite (marble and limestone) readily dissolve in Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutionsweakly acidic solutions

► Silicate minerals weather in the same order as their order of Silicate minerals weather in the same order as their order of crystallizationcrystallization

ClimateClimate► Temperature and moisture are the most crucial factorsTemperature and moisture are the most crucial factors► Chemical weathering is most effective in areas of warm, moist climatesChemical weathering is most effective in areas of warm, moist climates

Differential weatheringDifferential weathering

►Masses of rock do not weather Masses of rock do not weather uniformlyuniformly Don’t forget different minerals have Don’t forget different minerals have

different propertiesdifferent properties

►This can results in many unusual and This can results in many unusual and spectacular rock formations and spectacular rock formations and landformslandforms

►Differential Weathering due to jointingDifferential Weathering due to jointing

ExampleExample

More than you ever wanted More than you ever wanted to know about Soilto know about Soil

► Soil is that portion of the regolith (rock and Soil is that portion of the regolith (rock and mineral fragments produced by weathering) mineral fragments produced by weathering) that supports the growth of plants… it is a that supports the growth of plants… it is a combination of minerals, organic mater, combination of minerals, organic mater, water, and airwater, and air Residual soil – parent material is the underlying Residual soil – parent material is the underlying

bedrockbedrock Transported soil – forms in place on parent Transported soil – forms in place on parent

material that has been carried from elsewhere material that has been carried from elsewhere and depositedand deposited

Factors controlling soil Factors controlling soil formationformation

►Greatest factor is the parent materialGreatest factor is the parent material Is the Parent material a very hard rock or Is the Parent material a very hard rock or

a soft rock?a soft rock?

Factors controlling Soil Factors controlling Soil FormationFormation

► TimeTime Important in all geologic processesImportant in all geologic processes Amount of time for soil formation varies for different Amount of time for soil formation varies for different

soils depending on geologic and climatic conditionssoils depending on geologic and climatic conditions► ClimateClimate

Most influential control of soil formationMost influential control of soil formation Key factors are temperature and precipitationKey factors are temperature and precipitation

► Plants and animalsPlants and animals Organisms influence the soil’s physical and chemical Organisms influence the soil’s physical and chemical

propertiesproperties Also furnish organic matter to the soilAlso furnish organic matter to the soil

► SlopeSlope Steep slopes often have poorly developed soilsSteep slopes often have poorly developed soils Optimum terrain is a flat-to-undulating upland Optimum terrain is a flat-to-undulating upland

surfacesurface

The soil profileThe soil profile► Soil forming processes operate from the surface downwardSoil forming processes operate from the surface downward► Vertical differences are called Vertical differences are called horizonshorizons – zones or layers of soil – zones or layers of soil

O horizon – organic matterO horizon – organic matter

A horizon – organic and mineral matterA horizon – organic and mineral matter► High biological activityHigh biological activity► Together the O and A horizons make up the topsoilTogether the O and A horizons make up the topsoil

E horizon – little organic matterE horizon – little organic matter► Zone of Zone of eluviationeluviation and leaching and leaching

B horizon – zone of accumulationB horizon – zone of accumulation

C horizon – partially altered parent materialC horizon – partially altered parent material

The O, A, E, and B horizons together are called the The O, A, E, and B horizons together are called the solumsolum, or “true , or “true soil”soil”