Weathering and SoilsChapter 7

Weathering VocabularyWeathering Vocabulary

• Exfoliation: outer layers of rock are stripped away like an onion

• Hydrolysis: reaction of water with substance

• Frost wedging: repeated thawing and freezing of water in the cracks of rocks

• Oxidation: reaction of oxygen with a substance

Earth’s surface processesEarth’s surface processes

First a definition:• Weathering – Physical breakdown and

chemical alteration of rock at Earth’s surface

Types of weatheringTypes of weathering

• Chemical: breakdown of material using changes in chemical composition

• Physical: breakdown of material without chemical change.

Physical or Mechanical Physical or Mechanical WeatheringWeathering

• Physical weathering –Physical weathering is the disintegration of earth material without undergoing a chemical change.

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

Exfoliation– exfoliation of igneous rocks at Earth’s surface due to erosion of “overburden” and reduction in pressure

Biological activity – disintegration resulting from plants and animals

- root wedging, animal burrows

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

Salt Wedging

Mechanical Exfoliation in Mechanical Exfoliation in Yosemite National ParkYosemite National Park

Source: Phil Degginger/Earth Scenes

Mechanical Exfoliationof granite

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

Source: Runk/Schoenberger/Grant Heilman

Physical weathering increases Physical weathering increases the rate of chemical weathering the rate of chemical weathering

by increasing surface area.by increasing surface area.

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

Joints in sandstone - Canyonlands Natl Park, Utah

Chemical WeatheringChemical Weathering

• Breaks down rock and minerals• Important agent in chemical

weathering is water • Water dissolves and transports ions

and molecules • The dissolved ions later bond & form

the cements in sedimentary rocks

Chemical WeatheringChemical Weathering• Major processes of chemical weathering

• Dissolution– Soluble ions contained in underground water

• Oxidation– Chemical reaction where compound loses electrons– Important in breaking down mafic minerals– Rust colored weathering of Basalt

• Hydrolysis H+ or OH- ions– Reaction of any substance with water– Water’s ions replace different ions in mineral– Feldspars, most abundant crust minerals, become fine clay particles.

Easily transported.

DissolutionDissolution Weathered and Weathered and

Unweathered Unweathered Limestone Limestone BouldersBoulders

Source: Ramesh Venkatakrishnan

Angular Boulder Angular Boulder Decomposes and Decomposes and Rounds 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

Oxidation of BasaltOxidation of Basalt

Rust (Iron Oxide) forms

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

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

Soils

SoilSoil

• 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 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

Soil Horizons• Layers in Soil• Not Deposited, but Zones

of Chemical Action

Soil Profile• Suite of Horizons at a

Given Locality

Soil Horizons and ProfilesSoil Horizons and Profiles

An idealized soil profile

with horizons

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

• B horizon – zone of accumulation• C horizon – partly altered parent material

Mnemonic: Only Active Educators Become Champions

Soil ProfilesSoil Profiles

A mature soil profile with well A mature soil profile with well developed A, B, and C horizons developed A, B, and C horizons takes hundreds to thousands takes hundreds to thousands

of years to develop.of years to develop.Horizon “A”

Horizon “B”

Horizon “C”

Soils-3-6Image: NRCS

Remember the different horizons

O

A

E

B

C

Organic

Animal Activity

Exited

Back

Crushed Rock

Soluble Minerals

Soluble Minerals

Animal Activities in “A” horizon

Source: Runk/Schoenberger/Grant Heilman

• Soil types• The characteristics of each soil type primarily depend on the prevailing

climatic conditions

• Three very generic soil types• Pedalfer Moist All Year• Accumulation of iron oxides and Al-rich clays in the B

horizon. Brown B horizon– Best developed under forest landscapes

• Pedocal - High Evaporation– White calcium carbonate (caliche) in B horizon– Associated with dry grasslands and dry brush vegetation

• Laterite e.g. Monsoonal Climate– Alternating Hot dry and Cool Wet tropical climates– Intense chemical weathering– Red Iron oxide – horizons not distinct – Very thick soils but thin useful topsoil-Trees are Buttressed

pedalfer pedocal laterite

tropicsShortgrass Evergreen forests

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

O

A

E

B

C

Organic

Activity

Leached

Accumulation

Crushed Rock

CalicheCaliche

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

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

Balance Between: • Downward Lowering of Ground Surface • Downward Migration of Soil Horizons

If erosion rapid or soil evolution slow, soils may never mature beyond a certain point.

Extremely ancient soils may have lost everything movable

Limits of Soil DevelopmentLimits of Soil Development

Soil Developed on a Lava FlowSoil Developed on a Lava Flow

Source: Stanley Chernicoff/Patrick Spencer

End of Chapter 7End of Chapter 7