weathering and soils chapter 7. weathering vocabulary exfoliation: outer layers of rock are stripped...
DESCRIPTION
Earth’s surface processes Earth’s surface processes First a definition: Weathering – Physical breakdown and chemical alteration of rock at Earth’s surfaceTRANSCRIPT
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