Weathering and Soils. Earth’s surface processes Earth’s surface processes First some...
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Transcript of Weathering and Soils. Earth’s surface processes Earth’s surface processes First some...
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
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
Evidence of Evidence of Frost Frost Wedging in Wedging in Wheeler Wheeler Park, Park, NevadaNevada
Source: Tom Bean/DRK Photo
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
Mechanical Exfoliation in Mechanical Exfoliation in Yosemite National ParkYosemite National Park
Source: Phil Degginger/Earth Scenes
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 ]
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: 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
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
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
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
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
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
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
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