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• Viscosity determines the ability of a melt to releases gasses coming out of solution
• Viscosity depends on silica content and• TEMPERATURE• Mafic (Basaltic) melts are extremely hot
and have low silica, do not explode much• Felsic (Granitic) melts are only moderately
hot and have very high silica. Explode violently
Weathering and SoilsWeathering and SoilsLecture 5
Limestone dissolved by naturally acidic rainwater
Earth’s surface processesEarth’s surface processes
• 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 weathering1. 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 beach and desert playas.
Unloading = Mechanical ExfoliationUnloading = Mechanical Exfoliation
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
Mechanical Exfoliation in Mechanical Exfoliation in Yosemite National ParkYosemite National Park
Source: Phil Degginger/Earth Scenes
3. Thermal expansion and contraction
AAlternate expansion and contraction duelternate expansion and contraction due
to heating and coolingto heating and cooling
- Important in deserts- Important 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
Dissolution of CalciteDissolution of Calcite
• DissolutionDissolution–By carbonic acid, COBy carbonic acid, CO22 in water in water
COCO22 + H + H22O => HO => H22COCO33
HH22COCO3 3 ==> H> H++ + H + HCOCO33--
–Soluble ions contained in Soluble ions contained in underground waterunderground water
CaCO3 +2H+ => H2CO3 + Ca++ H2CO3 => H2O + CO2
DissolutionDissolution Weathered and Weathered and
Unweathered Unweathered Limestone Limestone BouldersBoulders
Source: Ramesh Venkatakrishnan
OxidationOxidation
• Chemical reaction where Chemical reaction where compound loses electronscompound loses electrons
• 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 Olivine (Fe, Mg) SiO[which contains Olivine (Fe, Mg) SiO44
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
2323
Hydrolysis – Feldspar to ClayHydrolysis – Feldspar to Clay
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
• The soil profile
• Soil forming processes operate from the surface downward as water descends
• 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 – 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/
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