Weathering and Soils

Earth’s surface processesEarth’s surface processes

First some definitions:

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

• Erosion – The transport of rock fragments

Source: Tom Bean/DRK Photo

Preview of Mechanical Weathering

Mechanical WeatheringMechanical Weathering

• Mechanical weathering – breaking of rocks into smaller pieces

• Four types of mechanical weathering1. Frost wedging – freezing and thawing of water

in cracks disintegrates rocks

Frost Frost WedgingWedging

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

Thermal Expansion and Contraction and Salt Weathering

WeatheringWeathering

• Mechanical Weathering (continued)2. Unloading or Mechanical Exfoliation

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

3. Thermal expansion and contraction alternate expansion and contraction due to heating and cooling

- important in deserts

4. Biological activity – disintegration resulting from plants and animals

- root wedging, animal burrows

Unloading - Exfoliation of a PlutonUnloading - Exfoliation of a Pluton

Mechanical Exfoliation in Mechanical Exfoliation in Yosemite National ParkYosemite National Park

Source: Phil Degginger/Earth Scenes

Unloading-Exfoliationof granite

Thermal Expansion and Contraction

Source: Tom Bean

Tree Roots Growing in Rock Fractures

Animal Burrows

Source: Runk/Schoenberger/Grant Heilman

Increase in surface area by Increase in surface area by mechanical weatheringmechanical weathering

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 minerals

• Important agent in chemical

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

• The ions form the cements in Sedimentary Rocks

• The salt in the ocean

Dissolution of CalciteDissolution of Calcite

•Dissolution–By carbonic acid, CO2 in water CO2 + H2O => H2CO3

H2CO3 => H+ + HCO3-

–Soluble ions contained in underground water

DissolutionDissolution Weathered and Weathered and

Unweathered Unweathered Limestone Limestone BouldersBoulders

Source: Ramesh Venkatakrishnan

OxidationOxidation

•Chemical reaction where compound loses electrons–Important in breaking down mafic minerals (contain Fe)

–Rust- colored mineral (Fe2O3 ) from weathering of Basalt

[which contains Olivine (Fe, Mg) SiO4 ]

Oxidation of BasaltOxidation of Basalt

Rust (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

2424

Hydrolysis – Feldspar to ClayHydrolysis – Feldspar to Clay

Mechanical fracture due to chemical weathering

Feldspars become

Hydrolysis: Angular Boulder Hydrolysis: Angular Boulder Decomposes and RoundsDecomposes and Rounds

Hydrolysis: Hydrolysis: Angular Boulder Angular Boulder Decomposes Decomposes and Rounds and Rounds

Source: Paul McKelvey/Tony Stone Images

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

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

WeatheringWeathering

• Rates of weathering• Mechanical weathering aids chemical

weathering by increasing surface area

• Others factors affecting weathering• Rock characteristics e.g. minerals

– Marble and limestone easily dissolve in weak acidic solutions -Dissolution

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

WeatheringWeathering• Other factors affecting weathering

• Rock characteristics continued– Silicate minerals weather in the reverse order as their

order of crystallization (Bowens Reaction Series)

– Olivine least stable, conditions of formation least like surface.

– Quartz (sandstone) most stable

• Climate– Temperature and moisture most crucial factors

– Chemical weathering most effective in warm, moist climates

Bowens Reaction Series and Weathering

Also very important in recognizing past climatesSoils

SoilSoil

• Soil - combination of mineral and organic mater, water, and air

•It is that portion of the

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

Components in soil Components in soil that support plant growththat support plant growth

SoilSoil

• Factors controlling soil formation•Parent material

–parent material is the underlying bedrock - composition affects soil types

SoilSoil

• Factors controlling soil formation• Time

– Soils get better developed (Thicker, with greater differences between layers,

with more time

• Climate– Biggest control on soil formation

–Key factors are temperature and precipitation

SoilSoil• Factors controlling soil formation

• Plants and animals– Organisms influence soil properties

– Also furnish organic matter to the soil (especially plants)

• Slope– Steep slopes have poorly developed soils (due to

faster erosion and downslope transport

– Flatter terrain accumulates soil faster

Variations in soil Variations in soil development development

due to topography due to topography

Note location of agriculture

Soil ProfileSoil Profile

• The soil profile

• Soil forming processes operate from the surface downward

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

SoilSoil

• 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

• B horizon – zone of accumulation

• 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

Activity

Exited

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

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

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

Soil Soil

Bedrock

Deeply weatheredbedrock(~40 - 50 meters deep)

Tem

per

atur

e

30 degrees Latitude

Shallow nutrients

RainforestDesert

US & Europe

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

LateriteLaterite

Hot and wet tropical climates

•Intense chemical weathering

•Red Iron oxide - Topsoil not distinct from B horizon

•Deep soil but usable nutrients shallow

Laterite in Sarawak, BorneoLaterite in Sarawak, Borneo

Source: Fletcher & Baylis/Photo Researchers, Inc.

Earth’s surface processesEarth’s surface processes

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

Soil ErosionSoil Erosion

• Soil erosion• Recycling of Earth materials• Natural rates of soil erosion depend on

–Soil characteristics

–Climate

–Slope–Type of vegetation

ErosionErosion

Source: Ramesh Venkatakrishnan

Headed for the Sea

SoilSoil

• Soil erosion• In many regions the rate of

soil erosion is significantly greater than the rate of soil formation

• Farmers now level fields with lasers to slow loss of topsoil

• Terraces

Soil Developed on a Lava FlowSoil Developed on a Lava Flow

Source: Stanley Chernicoff/Patrick Spencer

End of Lecture 5End of Lecture 5