Last time Last time • 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
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Last time Viscosity determines the ability of a melt to releases gasses coming out of solution Viscosity depends on silica content and TEMPERATURE Mafic.
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Last timeLast time
• 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
• 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.
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
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