Essen4als*of*Geology,
Weathering*and*Soils
Chapter*6
Stanley(Ha*ield
Southwestern(Illinois(College
Jennifer(Cole
Northeastern(University
(
Modified(by(G.(Smith

Earth’s External Processes
• !Weathering!–!the!physical!breakdown!and chemical!altera6on!of!rock!at!or!near!Earth’s surface
• Mass!was6ng!–!the!transfer!of!rock!and!soil downslope!under!the!influence!of!gravity "

Earth’s External Processes
• Erosion'–'the'physical'removal'of'material'by'mobile agents'such'as'water,'wind,'ice,'or'gravity '''

Weathering
• Two$types$of$weathering
• Mechanical$weathering$–$breaking$of$rocks into$smaller$pieces • Four$types$of$mechanical$weathering
• Frost$wedging$–$alternate$freezing$and$thawing of$water$in$fractures$and$cracks$promotes$the disintegra;on$of$rocks
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Frost Wedging

Weathering
• Mechanical)Weathering,)con0nued
• Unloading)–)exfolia0on)of)igneous)and metamorphic)rocks)at)the)Earth’s)surface)due)to a)reduc0on)in)confining)pressure
• Thermal)expansion)–)alternate)expansion)and contrac0on)due)to)hea0ng)and)cooling • Biological)ac0vity)–)disintegra0on)resul0ng from)plants)and)animals "
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Exfoliation of Igneous Rocks

Weathering
• Chemical)Weathering
• Breaks)down)rock)components)and)the internal)structures)of)minerals • Most)important)agent)involved)in)chemical weathering)is)water)(responsible)for transport)of)ions)and)molecules)involved)in chemical)processes) Weathering
• Major&processes&of&chemical&weathering
• Dissolu6on
• Aided&by&small&amounts&of&acid&in&the&water
• Soluble&ions&are&retained&in&the&underground water&supply • Oxida6on
• Any&chemical&reac6on&in&which&a&compound&or radical&loses&electrons "

Weathering
• Major&processes&of&chemical&weathering
• Oxida7on&con7nued
• Important&in&decomposing&ferromagnesian minerals • Hydrolysis
• The&reac7on&of&any&substance&with&water
• Hydrogen&ion&a>acks&and&replaces&other posi7ve&ions !

Weathering
• Altera'ons+caused+by+chemical+weathering
• Decomposi'on+of+unstable+minerals
• Genera'on+or+reten'on+of+materials+that are+stable • Physical+changes+such+as+the+rounding+of corners+or+edges Weathering
• Rates&of&weathering
• Advanced&mechanical&weathering&aids chemical&weathering&by&increasing&the surface&area
• Others&factors&affec:ng&weathering
• Rock&characteris:cs
• Rocks&containing&calcite&(marble&and&limestone) readily&dissolve&in&weakly&acidic&solu:ons Increase in Surface Area by Mechanical Weathering

Weathering
• Others'factors'affec-ng'weathering
• Rock'characteris-cs,'con-nued
• Silicate'minerals'weather'in'the'same'order'as their'order'of'crystalliza-on • Climate
• Temperature'and'moisture'are'the'most'crucial factors • Chemical'weathering'is'most'effec-ve'in'areas of'warm,'moist'climates "

Weathering
• Differen'al*weathering
• Masses*of*rock*do*not*weather*uniformly due*to*regional*and*local*factors • Results*in*many*unusual*and*spectacular rock*forma'ons*and*landforms Joint-controlled Weathering in Igneous Rocks

Soil
• Soil%is%a%combina,on%of%mineral%and%organic%mater, water,%and%air • That%por,on%of%the%regolith%(rock%and mineral%fragments%produced%by%weathering) that%supports%the%growth%of%plants
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Typical Components in a Soil that Yield Good Plant Growth

Soil
• Factors(controlling(soil(forma/on
• Parent(material
• Residual(soil(–(parent(material(is(the(underlying bedrock • Transported(soil(–(forms(in(place(on(parent material(that(has(been(carried(from(elsewhere and(deposited

Soil
• Factors(controlling(soil(forma/on
• Time
• Important(in(all(geologic(processes
• Amount(of(/me(for(soil(forma/on(varies(for