Chemical Weathering and Soils

Fresh rocks and minerals that once occupied the outermost position on the Earth’s surface reached their present condition of decay through a complex of interacting physical, chemical, and biological processes, collectively called weathering.

1. new minerals created by the weathering processes,

2. minerals that resisted destruction, and

3. organic debris added to the weathering zone.

Chemical Weathering and Soils


Functions of weathering• reduces rock strength and increases 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

• produces minor landforms, produces landforms in soluble rock (especially limestone) and otherwise creates microrelief (e.g. weathering pits)

• 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

• 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

Decomposition can be viewed as a group of processes that attempt to create substances that are more nearly stable in that environment.

• Driving force for decomposition is water.

H20 + CO2 H2CO3

• How materials weather is related to how easily ions are released.

• Weathering of minerals often occurs at sites of excess energy on the mineral surface and relates to the internal properties of the mineral itself.


Processes of Decomposition

Oxidation and ReductionOxidation occurs when an

element loses electrons to an oxygen ion. The process tends to occur spontaneously above the water table where atmospheric oxygen is readily available; therefore, most elements at the earth’s surface exist in an oxidized state. Below the water table the environment is generally reducing; however, high concentrations of organic matter may cause local reducing conditions to occur above the water table



Oxidation and Reduction• iron is the most commonly

oxidized mineral element Fe+2 (ferrous iron) ——> Fe+3 (ferric iron) or 2FeO + O2 ——> Fe2O3

• other readily oxidized mineral elements include magnesium, sulfur, aluminum and chromium

• among the immediate chemical weathering processes

• gives altered earth material a characterisitic yellowish brown to red color


Oxidized soil has turned rusty red (red brown)


SolutionIs critical in chemical weathering because when atoms are dissolved from a

mineral, the structure becomes unstable. However, the precise way in which a mineral collapses varies with its crystalline structure and the mobility of its constituent ions.

• 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 limestone results in karst topography

• the insoluble minerals form soil



HydrolysisChemically, hydrolysis involves a reaction between a salt and water to

produce an acid and a base; it is probably the most important mechanism in breaking apart structures of the silicate minerals. During the process, metallic cations are separated from the mineral structure and replaced by H+, which is held in the original mineral complex. Most of the replaced cations are soluble in natural waters.

• 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)



Hydrolysis• 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 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


Ion exchangeIon exchange is the substitution of ions in solution for those held by mineral

grains - most effective in clay minerals. The ions to be exchanged are held on the surfaces of clays because unsatisfied charges, exposed hydroxyl groups (OH-) and isomorphic substitutions such as Al+3 for Si+4 have given the clays an overall negative charge.


Each clay species has a different propensity for adsorbing cations called its cation exchange capacity (c.e.c.). Ion exchange is governed by the composition and pH of the interstitial water as well as the type of ion in the exchangeable position.

Mobility of decomposed materialsThe extent to which chemical weathering will alter the parent mineralogy

depends largely on the relative mobility of the constituent ions. Some ions are easily removed (high mobility) from the weathering system under normal groundwater conditions, while others are relatively difficult to remove (high immobility).


1. Leaching - the significance of leaching is several fold:

• it removes in solution the constituents that have been separated from minerals by hydrolysis and ion exchange and, by providing new hydrogen, allows these processes continuously to alter the original material toward an ultimate degraded condition,

• it directly affects the pH of fluids surrounding minerals and thereby helps determine which elements will remain in solution, and

• it provides the mechanism by which dissolved ions and clays are transferred from higher levels to lower levels in the weathering zone.

Mobility of decomposed materials


2. Fixation and retardationSome cations, especially potassium, have a tendency to be retained or fixed in

the weathered zone, so that their mobility is considerably lower than might be expected. The precise mechanism involved is poorly understood, but is probably related to the fact that potassium silicates are more resistant to chemical weathering than are other minerals with similar lattices.

3. ChelationDefined as the equilibrium reaction between a metal ion and a complexing agent,

characterized by the formation of more than one bond between the metal and a molecule of the complexing agent and resulting in the formation of a ring structure incorporating the metal ion.

This means that metallic ions that are extremely immobile under normal conditions can be mobilized by reacting with complexing agents and be vertically transported as part of the compound.

Most complexing agents involved in chelation are organic compounds nurtured in soils by alteration of humus into a plant acid called fulvic acid.

Degree and rate of Decompostion


End products – what remains when no more weathering can happen?

If we have an idea what the end products are to be in any climate, we can estimate the degree of weathering (how far the material has progressed toward the steady state) by observing the mineral and chemical composition of the soil.



Mineral stability“What minerals will be most rapidly destroyed in a thoroughly leached open system?”

“Which types will remain if a steady state is attained?” Basically the guide to mineral stability is the inverse of Bowen’s reaction Series (with some exceptions). The minerals that are most out of touch with their formation conditions (hi temps and pressures for example) are the least stable at these surface conditions and are most susceptible to weathering.



Secondary mineralsThese secondary minerals, born within the weathered zone, are distinctly more

stable than their primary ancestors because they reach equilibrium in the temperature-pressure environment of the soil rather than in the magmatic or metamorphic conditions that created the original crystals.

Estimates based on chemical analysesThe rate of chemical weathering is also commonly estimated by chemical analyses of water that has filtered through the rock and soil system. One assumes, however, that the chemistry of the water is solely a function of the chemistry of the parent material. Not realistic in most instances.

Soils


Weathering processes that continue over an extended period of time result in an unconsolidated mass of soil that is measureably different from the original rock/parent material in its physical and chemical properties.

Relationshipbetween bedrockand soil profile

Bedrock

Soil Profile


Soil Profile

B horizon

Soil Profile

O horizon

A horizon

Bedrock

E horizon

C horizon

Soil Properties

The vertical arrangement of the layers constitutes a diagnostic property of soils known as the soil profile.

Soil Properties


Texture is simply the relative proportions of different particle sizes in a soil horizon.

Soil Properties


Structure is a unique characteristic in that it designates the shape developed when individual particles cluster together into aggregates called peds. In clay-rich soils, the openings between peds may play an important geomorphic role by providing the only avenue for downward percolation through an otherwise impermeable soil.

Soil Properties


Organic matter in soils consists mainly of dead leaves, branches, and the like called litter, and the amorphous residue called humus, that develops when litter is decomposed.

Soil Classifications


The National Resources Conservation Service (Dept. of Agriculture) has developed a classification system that is nongenetic and requires no climatic interpretation. Soils are classified into ten major orders distinguished by the major horizons in their profiles.

Entisols - recentVertisols - inverts Inceptisols - inception Aridisols - aridMollisols - mollify (to lessen in

intensity, to softenSpodosol - podzol, oddAlfisol - pedalferUltisol - ultimateOxisol - oxideHistosol - histology (tissue)

Soil Classifications


(Bloom, 1998)

Pedogenic (soil forming) Controls


Of these, time is thought to be the most important:

• horizon development,• clay mineralogy,• total soil morphology,• iron oxide composition,• CaCO3 concentration.

Soil = ∫ (climate, biota, time, parent material and topography)



Recognize that the value of each property changes at a different rate and that given enough time, each will reach a condition where the property no longer changes or its rate of change is negligible.

Climate: The regional climate; because precipitation and temperature are not interdependent, they can be treated as separate functions.

Organisms of biotic factor: Because vegetation is most important, this factor is the summation of plant disseminules reaching the soil site or the potential vegetation; it is approximated by a list of species growing in the surrounding region that could gain access to the site under appropriate conditions.



Topography: Included are the shape and slope of the landscape related to the soil, the direction the slope faces, and the effects of a high water table, the latter being commonly related to the topography.

(Brady and Weil, 1999)



Parent Material or initial state of the system: Included are materials both weathered and unweathered, from which the soil formed. Parent material could also be a soil in the case where one wishes to study the effect of climatic change on a pre-existing soil.

Correlation of Geology and Soil Distribution for both El Dorado and Placer County

Geology Soil

(Similarities shown in same colored arrow)

Geology SoilSoil

25

miles



Time: Elapsed time since deposition of material, the exposure of the material at the surface or formation of the slope to which the soil relates; if a study is being made of the effect of climatic change on a preexisting soils, the time since the change.



Podzolization A process that involves a pronounced downward translocation of iron, aluminum,

and organic matter to form a eluvial E horizon overlying a tan illuvial B horizon. The process is not well understood. Parent materials controls the formation of spodosols, to some extent, because they are common in sandy materials.



Laterization Characterized by an oxic horizon (extreme chemical alteration of the original

parent material) which can extend tens of meters in depth. Usually common in high precipitation and temp, intense leaching conditions (tropics). One reason why tropical soils are not very good for farming, heavy chemical alteration.



CalcificationA general buildup of calcium carbonate in the soil profile. Occurs in sub-humid to

arid environments where the precipitation is inadequate to drive soil water to the water table so the Ca2+ ions re-precipitate as a CaCO3 rich B horizon.



Salinization occurs in warm and dry locations where soluble salts precipitate from water and accumulate in the soil. Saline soils are common in desert and steppe climates. Salt may also accumulate in soils from sea spray.

Geomorphic significance of soils


Because soils properties are altered by time and climate change, soil formation has even greater importance in deciphering the sequence of events in Quaternary history. In fact, one of the greatest uses of weathering and soils is to establish relative ages of glacial deposits and, by inference, the sequence of glaciations.

They are used primarily • in the subdivision of a local succession of deposits,



They are used primarily • to provide data on the lengths of time that separate periods of deposition,

• to facilitate short- and long-term correlation, and, of recent discovery,

• to date Quaternary faulting episodes.

• Other more recent applications include archeological studies where soils have been used to indicate the ages of certain layers, past moisture conditions, previous vegetation conditions.



The use of soils in Quaternary geomorphology is a dual-edged sword.

In areas that have experienced several climatic variations, the soil profile may reflect any or all of these variations. These polygenetic soils (complex soils) complicate the record because thickness and maturity of soils are reliable indices of age only when the conditions developing the soils have been maintained continuously.



Soils that form on a landscape of the past are paleosols.

They can be of three types: 1. buried soils are developed on a former landscape and subsequently

covered by younger alluvium or rock;

2. relict soils were not subsequently buried but still exist at the surface; and

3. exhumed soils were at one time buried but have been re-exposed when their cover was stripped by erosion.

buried soils

relic soils

exhumed soils

Chemical Weathering and Soils

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