Weathering and SoilsWeathering and SoilsLecture 5Lecture 5

Limestone dissolved by naturally acidic rainwater

Weathering and SoilsWeathering and Soils

• Weathering – Physical breakdown and Weathering – Physical breakdown and chemical alteration of rock at Earth’s surfacechemical alteration of rock at Earth’s surface

• Erosion – The transport of rock fragmentsErosion – The transport of rock fragments

Source: Tom Bean/DRK Photo

Preview of Mechanical Weathering

• Mechanical weathering – breaking of rocks into smaller pieces

• Four types of mechanical weathering

1. Frost wedging – freezing and thawing of water in cracks disintegrates rocks

Salt wedging is similar

Evidence of Evidence of Frost Frost Wedging in Wedging in Wheeler Wheeler Park, Park, NevadaNevada

Source: Tom Bean/DRK Photo

Rockfall caused by frost wedging ends up in a stream

ColluviumColluvium

Salt Weathering is similar, growing salt crystals in crackssplit the rock apart. At the shore and desert playas.

2. Unloading = Mechanical 2. Unloading = Mechanical ExfoliationExfoliation

Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure

Granite Batholith above subduction zone

Unloading = Exfoliation of a PlutonUnloading = Exfoliation of a Pluton

Batholith

Mechanical Exfoliation in Mechanical Exfoliation in Yosemite National ParkYosemite National Park

Source: Phil Degginger/Earth Scenes

3. 3. Thermal expansion and contractionThermal expansion and contraction

AAlternate expansion and contraction duelternate expansion and contraction due to heating and coolingto heating and cooling

Minerals expand and contract at different Minerals expand and contract at different ratesrates

Slide past one another, breaking any Slide past one another, breaking any intergrowthsintergrowths

Rock crumbles to individual mineral grainsRock crumbles to individual mineral grains

- - Important in desertsImportant in deserts

Tree Roots Growing in Rock Fractures

Animal Burrows Source: Runk/Schoenberger/Grant Heilman

BiologicalActivity

Mechanical weathering Mechanical weathering causes increase in surface areacauses increase in surface area

Same volume of rock, but many pieces have much more surface area than one piece

Joint-controlled weathering Joint-controlled weathering in igneous rocks in igneous rocks

Definition: Joints vs Faults

Increases surface area for chemical weathering

Chemical WeatheringChemical Weathering• Breaks down rock and mineralsBreaks down rock and minerals

• Important agent in chemical Important agent in chemical

weathering is weathering is waterwater (transports ions (transports ions and molecules involved in chemical and molecules involved in chemical reactions)reactions)

• The ions form the cements in The ions form the cements in Sedimentary RocksSedimentary Rocks

• These ions make the ocean saltyThese ions make the ocean saltyImportant for metals concentration

Three types of Chemical Weathering

DissolutionDissolution Weathered and Weathered and

Unweathered Unweathered Limestone Limestone BouldersBoulders

Source: Ramesh Venkatakrishnan

Dissolution of CalciteDissolution of Calcite

• DissolutionDissolution–By carbonic acid, COBy carbonic acid, CO22 in water in water

Soluble ions contained in Soluble ions contained in underground waterunderground water

H2O + CO2 <=> H2CO3 <=> H+ + HCO3-

CaCO3 +2H+ <=> H2O (l) + CO2 (g) + Ca++ (aq)

OxidationOxidation

• Chemical reaction where compound loses Chemical reaction where compound loses electronselectrons

• Usually while bonding with oxygenUsually while bonding with oxygen–Important in breaking down mafic Important in breaking down mafic

minerals minerals (contain Fe(contain Fe+2+2 or or +3+3))

–Rust- colored mineral (FeRust- colored mineral (Fe22OO33 ) from ) from

weathering of Basalt weathering of Basalt Which contains Pyroxene (Fe, Mg)Which contains Pyroxene (Fe, Mg)22 Si Si22OO66

http://www.geol.umd.edu/~jmerck/gal04/events/htmls/dauttl.html

Oxidation of Basaltic Lava FlowsOxidation of Basaltic Lava Flows

Rust-colored Iron Oxide forms

HydrolysisHydrolysis• Water makes H+ and OH- ions

– Hydrolysis is the reaction of any substance with water

– Water’s ions replace different ions

in a mineral– Feldspars, most abundant crust minerals,

become fine clay particles.– Clays are light weight, flat plates, easily

transported by streams

2222

Hydrolysis – Feldspar to Clay Hydrolysis – Feldspar to Clay expands in waterexpands in water

Mechanical fracture due to chemical weathering

Feldspars become

HydrolysisHydrolysisCleopatra’s Cleopatra’s Needle, Needle, (Egypt)(Egypt)

Source: New York Public Library, Locan History and Genealogy Division

Granite in aDry Climate

HydrolysisHydrolysisCleopatra’s Cleopatra’s Needle, Needle, (Central (Central Park, NYC)Park, NYC)

Source: Runk/Schoenberger/Grant Heilman

Granite in aWet Climate

Vegetation and Soil DevelopmentVegetation and Soil DevelopmentPlants use Hydrolysis to get nutrient metals out of minerals

Bowens Reaction Series and Weathering

Also very important in recognizing past climatesSoils

• SoilSoil - combination of mineral and - combination of mineral and organic matter, water, and airorganic matter, water, and air

• It is that portion of the It is that portion of the

regolith (weathered rock and (weathered rock and minerals) that supports the minerals) that supports the growth of plantsgrowth of plants

Components in topsoil Components in topsoil that support plant growththat support plant growth

Chemical weatheringby dissolution Limestone

Weathering-resistant sandstone (mostly quartz) yields little soil

SoilSoil Soil

Feldspar-richgranite

Iron-richbasalt Chemical

weatheringby oxidation

Chemicalweatheringby hydrolysis

Factors related to bedrock composition (parent material)

Climate also very important, see below

Variations in soil development Variations in soil development due to topography: slope and time due to topography: slope and time

Equator to Poles Factors

Equatorialand tropicalrain forests

SavannahsLow-latitudedeserts andsemi-deserts

Grasslands(steppes)

Temperate regionsand mixedboreal forests

Arcticandtundraregions

EquatorA

nnua

l pre

cipi

tatio

nIn

crea

sing

dep

thof

wea

the

ring

1800mm

600mm

40ºC

30ºC

20ºC

10ºC

Precipitation

Temperature

Evaporation

Bedrock ator very nearsurface

Deep Regolith, shallow Soil

Soil

Bedrock

Deeply weatheredbedrock(~40 - 50 meters deep)

Tem

per

atur

e

30 degrees Latitude

Shallow nutrients

RainforestDesert

US & Europe

Factors related to Climate

Soil ProfileSoil Profile

• Soil forming processes operate from the surface downward as water descends

• Vertical differences are called horizons – zones or layers of soil

Soil HorizonsSoil Horizons

• The soil profile

• O horizon – organic matter

• A horizon – organic and mineral matter– High Biological Activity (animals live here)– Together the O and A horizons make up topsoil

• E horizon – little organic matter– Zone of leaching – soluble minerals removed

• B horizon – zone of re-precipitation

• C horizon – partly altered parent material

An idealized soil profile

ONLY

ACTIVE

EDUCATORS

BECOME

CHAMPIONS

Remember the different horizons

O

A

E

B

C

Organic

Animal Activity

Soluble minerals Exited

Soluble Minerals Back

Crushed Rock

ONLY

ACTIVE

EDUCATORS

BECOME

CHAMPIONS

Animal Activities in “A” horizon

Source: Runk/Schoenberger/Grant Heilman

Worms ingest mineral grains because they are covered with living organisms –their food. Their burrows, not their feeding, increase chemical weathering by

exposing the minerals to water and air

E and B: Eluviation & Illuviation

exited

back

Illuviation and EluviationLeaching and Precipitation of Iron

Source: Jens/Gutzmer/Rand Afrikaans University/Geology

E

B

Soil TypesSoil Types• Soil types

• The characteristics of each soil type primarily depend on the prevailing climatic conditions

• Three very generic soil types

•Pedalfer•Pedocal•Laterite

pedalfer pedocal laterite

tropicsShortgrass Evergreen forests

PedalferPedalfer

•Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon

•Best developed under temperate forest landscapes

PedocalPedocal

•White calcium carbonate (caliche) in B horizon

•Associated with dry grasslands and brush vegetation

Pedocal with Caliche in the B horizonPedocal with Caliche in the B horizon

O

A

E

B

C

Organic

Activity

Leached

Accumulation

Crushed Rock

LateriteLateriteHot and wet tropical climates

Monsoonal Climate

• Intense chemical weathering• Red Iron oxide - Topsoil not distinct from B horizon –bacterium responsible for dissolving soil

iron is not present

• Deep soil but usable nutrients shallow

Laterite in Sarawak, BorneoLaterite in Sarawak, Borneo

Source: Fletcher & Baylis/Photo Researchers, Inc.

The percolating rain water causes dissolution of primary rock minerals and decrease of easily soluble elements as sodium, potassium, calcium,

magnesium and silicon. This gives rise to a residual concentration of more insoluble elements predominantly iron and aluminum.

Earth’s surface processesEarth’s surface processes

• Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity

Natural rates of soil erosion depend on:

Source: Ramesh Venkatakrishnan

Headed for the Sea

Soil characteristicsSoil characteristicsClimateClimateSlopeSlope

Type of vegetationType of vegetation

SoilSoil

• Soil erosion• In many regions the rate of

soil erosion is significantly greater than the rate of soil formation

• Farmers level fields to slow loss of topsoil

• Farmers have been building terraces for thousands of years

http://engineering.missouri.edu/news/2009/02/03/mizzou-engineering-helping-build-better-farms/