Ch 9. Weathering A. Mechanical Weathering B. Chemical Weathering C. Soil.
-
Upload
marcus-townsend -
Category
Documents
-
view
235 -
download
0
Transcript of Ch 9. Weathering A. Mechanical Weathering B. Chemical Weathering C. Soil.
Ch 9. Weathering
A. Mechanical Weathering
B. Chemical Weathering
C. Soil
Weathering
Mechanical Weathering Physical disintegration of rock
(with no chemical alteration) Chemical Weathering
Chemical alteration of minerals within the rock
Usually softening or dissolving the minerals
Forming clays, oxides and solutes
Weathering
Weathering, a geologic process:1. Separates out mineral grains2. Converts silicates to clay3. Oxidizes iron minerals (i.e., ferromagnesians) 4. Dissolves some ions (e.g, calcium) Erosion Transports Sediments
Quartz
Na Plagioclase
Biotite
Quartz1
Feldspar1
Clay2
Hematite3
Calcite4
Sediments of:
Weathering and Plate tectonics
Tectonic forces lift the land up Compression and uplift at
convergent boundaries Isostatic uplift due to crustal
thickening and buoyancy Stresses fracture rocks
Weathering and Erosion break the land down Mechanical and chemical
breakdown Erosion by gravity, wind, water,
ice moves material downhill
A. Mechanical Weathering Rocks that have been
exhumed Pressure release forms
sheet joints and exfoliation Frost wedging propogates
fractures and breaks rocks into smaller fragments
Crushing and Abrasion break rocks down further in tallus slopes, streams and beneath glaciers
An exfoliation dome in Yosemite, CA (Half Dome)
Mechanical Weathering
Talus Slopes
The result of Mechanical
weathering Rock falls and slides Crushing and
abrasion (more mechanical weathering)
Rock Avalanches Slopes of rock
fragments may let go and careen downhill as a very fast flow
Sediments of: Parent rock Mineral particles Angular fragments(Near source)
Sediments from Mechanical Weathering
Weathering
Mechanical Weathering Frost Action Pressure Release Crushing and Abrasion
Chemical Weathering Dissolving Dissolved ions Oxidation Iron in Ferromag. Minerals Iron
Oxides (e.g., Hematite) Formation of Clays from silicates (e.g., Feldspar)
Chemical Weathering (Table 9.1)
Oxidation: 4FeSiO3 + O2 + H2O FeO(OH) + 4SiO2
Hydration: CaSO4 + 2H2O CaSO4·2H2O
Hydrolysis
Soil Formation and Weathering Related to Climate
Weathering of Granite
1. Mechanical Weathering forms rock and mineral
fragments of parent rock
Chemical Weathering2. Converts silicates* to clay3. Oxidizes iron in minerals
(i.e., ferromagnesian minerals) 4. Dissolves some ions
(e.g, Ca, Na, K, Mg)
*except quartz
Quartz
Na Feldspar
Quartz1
Feldspar1
Clay2
Hematite3
Ions4
Sediments of:
Biotite
Rates of Weathering Stable at High
Temperatures Quickly Weathered
Slowly Weathered Stable at Atmos.
Temperatures
Con
vert
ed t
o cl
ay,
oxid
es,
and
ions
by
che
mic
al w
eath
erin
g
Olivine
PyroxeneGroup
AmphiboleGroup
MicaGroup
ClayGroup
FeldsparGroup
Quartz
IsolatedSilicateStructure
Single ChainStructure
Double ChainStructure
SheetStructure
FrameworkStructure
SilicateStructure Mineral
Mech. Weath.
Cool mountainous regions accelerate mechanical weath.
Warm moist climates accelerate chem. weath.
Increased slopes accelerate weathering Gravity caries sediments
away from slopes Exposing fresh surfaces
Other factors increasing
Rates of Weathering
Joints or fractures accelerate both mech. and chem. weath.
Other factors increasing Rates of Weathering
John Ford Point MonumentAlso See Fig. 13.7 of the
Colorado Plateau
Figure 3.8: Ship Rock ArizonaVolcanic Neck
Fig 5.3
Ship Rock, New MexicoVolcanic Neck
John Ford Point Monument, Arizona.
Differential Weathering
Weathering forms Soils
Acidic water percolates downward and A. Chemically weathers minerals in soil to form
• Clays• Iron Oxides• Dissolved Ions
E. Leach downward and B. Accumulate. C. Mechanical weathering breaks down bedrock
Sediment: Products of Weathering
Immature Mature
Transport Distance
Near source Far from source
History of Weathering
Short history of mechanical weathering
Long history of mechanical & chemical
weathering
Products of Weatheringi.e., types of
sediment
Forms rock and mineral fragments
Angular at first Rounded with time
and transport
Converts silicates to clays, iron oxides and dissolved ions which are transported to the ocean by streams
leaving stable minerals, eg. qtz. (& fld.)
cm2cm2
m2