GLG 362/598 Geomorphology

Weathering lecture notes


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:
    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).

Chemical weathering



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.)


Relative resistance: Two simple ways to anticipate resistance to chemical weathering

1) Rocks:
Igneous and metamorphic rocks
(least stable)
Sedimentary rocks
Weathering products
(most stable; environment most similar to surface)

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)
Amphibole & K feldspar
Pyroxene & Na-Plag
Olivine & Ca-Plag

Particular reactions

Many reactions tend to use up H+, producing more OH- in a solution, so Table 3.1 from Ritter et al. indicates "abrasion pH:  equilibrium PH after depletion of H+ by immersion of mineral powder in water."  It shows that many clays actually may liberate more H+, making the solution more acidic, but those more mafic minerals such as olivine have abrasion pH of 10, supporting Goldich's results.


Extract ions from crystal lattice and eventually the mineral falls apart.  The freed ions go into solution and may subsequently precipitate.

Most important rx:

2 Ca CO3 + 2 H20 + CO2 <->2 Ca2+ + 2 (HCO3-) + H2CO3
CaCO3 + H2O <->Ca2+ + 2 (HCO3-)

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)


Oxidation occurs rapidly above the water table (wet, but interstitial spaces are filled with air); red colors.
Element loses electrons to an oxygen ion.
"Weathering of iron-bearing minerals"
Reduction occurs below water table or in other conditions of low oxygen (anoxic); black colors.


4 Fe2+ + 3 O2 <-> 2 Fe2O3
Ferrous                      Ferric


4FeS2 + 14 H2O + 15 O2 <-> 4Fe(OH)3 + 8 H2SO4


2 Fe2+ + 4 HCO3- + 1/2 O2 + 2 H2O <-> Fe2O3 (hematite) + H2CO3


Reaction between cations which are replaced by H+ from disassociated water.
Important as basic way in which many silicates are attacked and decomposed.
See figure 3.2 from Ritter, et al.


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)

Ion exhange

Substitution for ions in solution for those held by mineral grains
-often onto surface of clay particles
-generates acids, and pulls some cations (Ca, Na, others) out of solution
See Table 3.3, Ritter, et al.


Controls extent of alteration

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.


Movement of water through weathering zone

1) removes dissolved minerals
2) adds fresh H+ (keeping things in solution)
3)  moves material within weathering zone possibly allowing precipitation of new minerals


H+ concentration: as pH goes down, mobility goes up


Normally immobile metal ions form more than one bond with molecules of the chelating agent resulting in the formation of a ring structure incorporating the metal ion.
Often a means of accelerated chemical weathering by plants.
Plants are capable of putting into solution or absorbing substantial portions of the total mass of the rock on which they grow.
See table 3-3 from Birkeland

Latitudinal distribution of chemical weathering:

Pelletier, UofA


Karst (Pelletier, UofA)

Chinese Karst (Pelletier, UofA)

Nice images of South China Karst

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

Physical or mechanical weathering

Disintegration and breakup of surficial materials without chemical changes.
Decrease particle size, thus helping to produce regolith available for transport
Increase surface area of particles and increase potential for chemical weathering.

Important processes

Expansion and contraction

driven by thermal processes may produce rock fracture or fatigue (many cycles) which will weaken and ultimately break rock.
1)  Rock is poor conductor, so most effects are concentrated near rock surface
2)  Different coefficients of thermal expansion for minerals sets up stresses in rock
3)  Daily thermal changes (can be up to 50C). Rock surface temperatures can be as high as 80C.
These effects are very strong with no other sources of stress (no confinement or burial).

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.
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

See Malin Antarctic Images

Unloading or exfoliation or sheeting

Change in confining pressure:  overburden is removed and rock expands.  Fractures form parallel to surface and the fractured shells spall off.
Fracture spacing decreases as we get closer to the free surface.

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.

Hydration and swelling

grus or saprolite (dissaggregated granite) and some soils:  Swelling: absorption by clays, especially bentonite which will absorb ~140x its own mass in water

Crystal growth

Freeze-thaw of water 9% increase in volume.
In a confined situation (like a frozen cap), you can get 30,000 lbs/inch
Frost cracking: with full saturation, rapid frezing, and frequent cycling, one can  move materials around by ice growth and even have frost heaving.

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 

Tafoni in Sandstone 

Papago Park tafoni ( )


Organic processes

trees open and hold open fractures and also root/tree/saguaro throw
animals: lots of churning
    termites and ants
    Burrowing critters

Root Wedging (Photo © Duncan Heron) 40 Acre Rocky, SC

Vegetation Growing in Joints (Photo © Duncan Heron) Stone Mountain State Park, NC


Relevant links

Images of weathering from Duke University

Landforms of weathering

Clay mineralogy basics from UNH

GLG 362/598 Geomorphology

Page maintained by
Ramon Arrowsmith

Last update August 30, 2000