Weathering

• the set of exogenic (physical, chemical and biological) processes that alter the physical and chemical state of rocks at or near the earth's surface
• intensity of most weathering decreases with depth, because variations in temperature and moistures decrease with depth
• therefore biochemical weathering is generally confined to the uppermost few metres of soil and rock
• occurs in situ (nontransported alteration), unlike erosion which removes soil and weathered rock; although the 2 sets of processes proceed simultaneously with positive feedback
• the 2 forms of weathering act simultaneously and affect the nature and rate of one another: disintegration produces an increase in rock surface area while changes in strength with changes in composition

Functions of weathering

1. gives rock lower strength and greater permeability, rendering it more susceptible to mass wasting and erosion; reduces strength (cohesion and friction) and increases permeability of rock and therefore decreases resistance to fluid and gravitational stresses; precursor to erosion
2. produces minor landforms, produces landforms in soluble rock (especially limestone) and otherwise creates microrelief (e.g. weathering pits)
3. releases minerals in solution (e.g. iron oxides, silica, carbonates) which become concentrated to form hard coatings on rocks and hard resistant layers in soil (duricrusts) that inhibit seepage and resist erosion
4. first step in soil formation; ultimately produces an unconsolidated mass of 1) minerals that resisted alteration (e.g. feldspar), 2) new minerals (e.g. bauxite), 3) organic debris

Physical weathering

• physical weathering is the disintegration of rock and soil aggregates, by physical (mechanical) processes acting primarily on pre-existing fractures (e.g. joints, cracks between mineral grains); reduces size of fragments according to rock and soil structure (producing grains, crystals, blocks, slabs, etc.), with no change in composition and

Processes

1. stress (pressure) release: disintegration of rock in parallel sheets as it expands in response to the removal of confining stress
   • most common mechanism of stress release is removal of overlying rock by erosion; thus this process is controlled by erosion but subsequently controls erosion
   • the dilation fractures conform to the surface topography and increase in spacing with depth (e.g. from a few cm at the surface to a few metres at 30 m in the Sierra Nevada Mountains of Yosemite National Park)
   • thermal contraction due to cooling counteracts expansion; therefore stress release is most pronounced near the surface where the rocks have already cooled and contracted
   • also most common in massive rocks: higher thermal conductivity causes heat loss and thus reduces counter influence of cooling and contraction, fractured or thinly bedded rock will expand with out sheeting, massive rocks store stress until overburden pressure is very low (i.e. about 100 m of overburden)
   • stress release causes exfoliation: the separation of concentric layers of rock
   
   
2. thermal expansion and contraction (insolation weathering)
   • the surface temperature of dark colored rock can vary from 0-50^o C between day and night, since rock (esp. jointed rock) has low thermal conductivity
   • the differential stresses of expansion and contraction of the outer 1-5 cm of rock causes separation of concentric shallow layers (another form of exfoliation) called spalling or spheroidal weathering when it effects boulders
   • controversy about effectiveness
     • re: the ability of solar radiation to generate sufficient heating and cooling
     • rocks disintegrate after fires, especially rocks composed of minerals with varying coefficients of volumetric expansion (e.g. granite: volume of quarts increases 3X more than that of feldspar; versus greater resistance of fine-grained rocks)
     • dry granite heated and cooled from 30 to 140o C for 89,400 cycles over 3 years (equivalent of 244 years of diurnal cycles) produced no perceptible change, even with microscopic examination (Griggs, D.T. 1936. The factor of fatigue in rock exfoliation. J. Geol. 44: 783-796)
     • but, 244 years is small amount of geologic time
   
   
3. growth of foreign crystals (salt weathering)
   • mainly hydrated salts which are water soluble at normal ranges of atmospheric temperature and humidity; they hydrate and dehydrate repeatedly generating considerable stresses in fractures and between grain boundaries in permeable rock
   • mostly granular disintegration
   • minerals are transported in solution and precipitate as soil and groundwater evaporate; thus most effective in desert landscapes where water tables are near the surface
   • origin of salt: sea water, chemical weathering of marine or evaporite sediments, dissolved in snow and rain, precipitates in lakes
   • e.g. gypsum: relatively insoluble except in acid rain (dilute sulfuric acid), thus enhanced weathering with acid rain
   
   
4. hydration (slaking)
   • wetting, swelling and disintegration of soil aggregates, layered and fine grained rocks
   • also pressure of air drawn into pores under dry conditions and then trapped as water advances into soil and rock; suction or -ve pore pressure (less than atmospheric) can exert considerable stress
   • e.g. biotite expands 40% by volume contributing to the weathering of granite

   Expansion of Clay Minerals by Volume
   Ca-montmorillonite	45-185%
   Na-montmorillonite (bentonite)	1400-1600%
   illite	15-120%
   kaolinite	5-60%

   
   

5. frost shattering and hydration shattering
   • freezing of water in pores and fractures
   • the specific volume (vol./unit mass) of water increases by 9% upon freezing producing stress that is greater than the tensile strength of all common rocks
   • therefore the stress generated by the crystallization of ice is the most pervasive mechanism of weathering, effects all rocks
   • however, the effectiveness of freezing water is influenced by
     • lack of confinement: if more than 20% of pore space is empty, then the tensile stress may be less than the tensile strength, thus frost shattering is most effective in saturated rock; ice under pressure deforms plastically and thus will extrude through daylighted fractures and pores
     • decreased freezing point with increasing pressure (supercooled water) and impurities (e.g. salt)
     • necessary frequency and magnitude: extent of frost shattering is a function of the combination of frequency, duration and intensity (rapidity and degree) of freeze-thaw cycles
   • hypotheses to account for apparent effectiveness of frost shattering; hydrofracturing:
     • thin (monomolecular) films of water do not freeze even at low temperatures, given strong capillary adhesion to rock; powerful molecular forces in these thin films of semicrystalline water draw water along microfractures opening and propagating them
     • shallow freezing forces films of capillary water along microfractures, disintegrating rocks well below the depth of freezing; e.g. hydrofracturing in the Allegheny Mountains (southern Appalachians of West Virginia and Pennsylvania) extends to 12-15 m, while frozen ground rarely extends below 1 m
   
   
6. plants
   • minor agent of weathering
   • maintain cracks created by other processes
   • roots may pry rocks apart when tall trees sway in a strong wind; root throw can break fragments away from bedrock
   • as lichens expand and contract or are removed by abrasion they can pull small rock fragments loose

Chemical Weathering

• chemical weathering is the decomposition of soil and rock (change in composition) by biochemical processes
• weathering pits form where water collects and accentuates rates of chemical weathering

Processes

1. oxidation
   • process by which an element loses an electron to dissolved oxygen
   • iron is the most commonly oxidized mineral element Fe⁺² (ferrous iron) ——> Fe⁺³ (ferric iron) or 2FeO + O₂ ——> Fe₂O₃
   • other readily oxidized mineral elevments include magnesium, sulfur, aluminum and chromium
   • among the immediate chemical weathering processes
   • gives altered earth material a characterisitic yellowish brown to red color
   • water table is boundary between oxidizing and reducing environments
   
   
2. hydroloysis
   • decomposition of minerals in water as hydrogen ions replace cations in minerals
   • pure water is a poor H+ donor, however CO2 dissolves in water to produce carbonic acid:
   • CO₂ + H₂O ——> H₂CO₃ (carbonic acid) ——> H⁺ + HCO₃^- (bicarbonate)
   • soil air is greatly enriched in CO₂ by decay of humus
   • up to 30% of soil air is CO₂ as compared to 0.03% of the atmosphere
   • biogenic CO₂ is the major source of carbonated groundwater
   • solubility of CO₂ increases as water temperature decreases (warm beer is flat)
   • hydrolysis is the most important process in the weathering of silicate minerals
   • the most common weathering reaction on earth is the hydrolysis of feldspars producing clay minerals
   • e.g. K-feldspar ——> kaolinte
   • 2KAlSi₃O₈ + 2H₂CO₃ + 9H₂O ——> Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2K⁺ + 2HCO₃^-
   • the other weathering products (silicic acid and ions) are in solution, so the residue is clay
   • the soil water solution becomes more basic as H+ is consumed
   
   
3. carbonation (solution)
   • dissolution of calcium carbonate in acidic soil and groundwater
   • CaCO₃ + H₂CO₃ ——> Ca⁺² + 2HCO₃^-
   • similar reaction as hydrolosis but the dissolution is congruent, that is, the products are ionic, there is no residue
   • bicarbonate represents the largest constituent of the dissolved load of most rivers
   • carbonation of limesotone results in karst topography
   • the insoluble minerals form soil
   
   
4. cation exchange
   • substitution of mineral cations in solution for those held by mineral grains and crystals
   • changes spacing in crystal lattice but not molecular structure
   • most effective in clay-textured sediments as cations adhere to the surface of negatively charged clay minerals
   • organic matter and sodium zeolites (watert softner salt) have a high cation exchange capacity (CEC)
   • CEC = f(temperature, content and chemistry of interstitial water, types and abundance of ions)
   • colloidal suspensions of clay and organic matter adsorb H⁺ creating acidic soil and weathering environment
   
   
5. chelation
   • minerals cations incorporated into hydrocarbon molecules (complexing agents or chelates)
   • chelating agents are produced by alteration of humus in plant acids and excreted by lichens
   • e.g. ethylenediaminetetracedic acid (EDTA) is a common food additive
   • chelates in solution are stable at pH under which the incorporated cation would normally precicpitate and thus they are leached in seeping soil water
   • H⁺ released during chelation from organic molecules is available for hydrolysis
   • thus plants contribute to the decomposition of soil and rock waste at depths to the base of the root zone
   • lab experiments with equisetum (horse tail) in crushed rock show silica uptake equivalent to removing the silica form 30 cm of basalt in 5350 years
   • the dissolved load of runoff from barren basalt plateaus in Iceland suggest that the rate of weathering is 1/3 as fast as on lichen covered surfaces

Climate and weathering

• weathering is an exogenic geomorphic process, i.e. climatically-controlled
• it is controlled by moisture, temperature and seasonality, the same parameters that define regional climates
• in tropical climates, high temperatures, large annual rainfall and continuous biological activity maintain high rates of chemical weathering
• water is the agent of all weatering, except stress release (although water erosion is a mechanism of unloading of rock)
• all chemical weathering occurs in solution