Geomorphic processes operate on surficial materials; not just bedrock
Definition: Weathering is the disintegration and decomposition of rocks and minerals at or near the earth's surface as a result of physical, chemical, and biological processes. No transport or entrainment is considered.
Surficial materials:
Bedrock
new materials created by weathering
resistant materials
organic debris
We divide weathering into two principal process types, although they
do not work independently:
Physical (or mechanical) weathering
-disaggregation with no change in chemistry: creates surface
area
Chemical weathering
-alteration to cause chemical or mineralogic changes: weakens
rocks
Primary controls on weathering: climate (temperature and precipitation) and geology (rock type and distribution).
Secondary controls: topography (relief and aspect) and vegetation
(changes chemistry and is vigorous physically).
First Stage of Chemical Weathering (Photo © Duncan Heron) Kershaw County, SC |
Corners Rounded by Weathering (Photo © Duncan Heron) NC Museum of Life & Science |
Forum, Rome Italy (Photo © Duncan Heron) |
|
Solution of Jefferson M. Marble (Photo © Duncan Heron) Washington, DC |
Balustrades at Organization of American States (Photo © Duncan Heron) Washington, DC |
-Decomposition by chemical processes to cause chemical and mineralogic
changes
-Disequilibrium response: surface of the earth is much different
than the environment in which most rocks form.
--Genesis of material to be weathered is most important. What are the
differences in physical and chemical conditions?
Occurs on mineral surfaces (see figure 3.3 from Ritter et al.)
2) Empirical formulation for minerals (Goldich, 1938; compared parent
and resulting minerals)
(inverse Bowen's reaction series; more shared Si-O bonds means more
resistance to weathering)
Quartz
Muscovite
Biotite
Amphibole & K feldspar
Pyroxene & Na-Plag
Olivine & Ca-Plag
Most important rx:
Variations in mineral solubility are strongly dependent on cystal structure (esp. crystalline versus amorphous) and pH (4-9 is typical for soil waters)
Remaining insoluble materials might be Fe3+ or Al2O3
(Bauxite)
Reactions:
and
4FeS2 + 14 H2O + 15 O2 <-> 4Fe(OH)3 + 8 H2SO4
and
2 Fe2+ + 4 HCO3- + 1/2 O2 + 2 H2O <-> Fe2O3 (hematite) + H2CO3
Reactions:
2 KAlSi3O8 (orthoclase-Kspar) + 2H+ + 9 H2O) <-> Al2Si2O5(OH)4 (kaolinite-clay) + 4H4SiO4 + 2 K+ + 2HCO3-
Will keep going until solvent is saturated with respect to cations
What keeps the reaction going? Leaching--movement of groundwater
H+ supplied and cations (K+, Ca2+,
Na+, Mg2+) removed.
Acids: often come from rainfall (CO2 in the atmosphere) and from organic material in the soil: humic acid
CO2 (gas) + H2O (water) <-> H2CO3
(carbonic acid) <-> H+ (hydrogen ion) + HCO3-
(bicarbonate ion)
less mobility:
3 KAlSi3O8 (orthoclase) + 2 H+ + 12
H2O <-> KAl3Si3O10(OH)2
(illite) + 6 H4SiO4 + 2 K+
more mobility:
2 KAlSi3O8 (orthoclase) + 2 H+ + 9
H2O <-> H4Al2Si2O9
(Kaolinite) + 4 H4SiO4 + 2 K+
K+ is a mobile ion, so this indicates that the fluid was
nearly saturated or that there was incomplete orthoclase breakdown.
1) removes dissolved minerals
2) adds fresh H+ (keeping things in solution)
3) moves material within weathering zone possibly allowing precipitation
of new minerals
Chinese Karst (Pelletier, UofA)
Nice images of South China Karst http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_7/GEO_PLATE_KL-1.HTML
Landforms often display one or more kinds of weathering in roughly equal
parts such as the landscape in the photograph below where limestone
dissolution and physical abrasion have both significantly influenced
the landscape.
Havasu Creek in the Grand Canyon--lots of disolved ions from chemical
weathering and dissolution of carbonates in groundwater.
Vasey's Paradise in the Grand Cayon--lots of dissolved ions in that
water from dissolution and chemical weathering of carbonates.
Travertine-cemented colluvium on Colorado River slope in the Grand
Canyon
Famous experiment:
Griggs, 1936 heat and cooled cubes of granite 140C to 30C for the equivalent
of 240 years of daily fluctuations, but nothing happened.
That was dry, with wetting, in ~2.5 years they fell apart.
Why?
Effect of the water for subcritical crack growth and corrosion in general.
The samples may have been very clean and thus flaw free
Thorough heating?
Really intense heat -> forest fires and lightning may crack rocks
Examples: Sierra Nevada domes like El Capitan and Half Dome
Enchanted Rock in Texas: Virtual
tour
Stone Mt Georgia Sheeting Dome (Photo © Duncan Heron) |
Sheeting (Photo © Duncan Heron) 40 Acre Rock, South Carolina |
Canyon parallel fractues in South Canyon--Grand Canyon
Canyon wall parallel sheeting fractures in South Canyon, Grand Canyon.
Salt crystal growth: sulfates, carbonates, chlorides
Salt crystals precipitate and they too can induce stresses in the material
(as well as induce chemical weathering) by changes in temperature and hydration.
Need arid to semi-arid conditions
Can produce tafoni:
Tafoni in Sandstone http://user.tninet.se/~bgb354w/home/Tunisien.htm#8 |
Tafoni in Sandstone http://user.tninet.se/~bgb354w/home/Tunisien.htm#8 |
Papago Park tafoni ( http://www-glg.la.asu.edu/~glg_intro/papago/papagostudent.htm ) |
Root Wedging (Photo © Duncan Heron) 40 Acre Rocky, SC |
Vegetation Growing in Joints (Photo © Duncan Heron) Stone Mountain State Park, NC |
Clay mineralogy basics from UNH
Last update August 30, 2000